SOMANYMYSTERIES,SOLITTLETIME.NOWYOUCANUNRAVELMOTHERNATURE'SBEST-KEPTSECRETSWITHTHEASTOUNDING,EYE-OPENINGANSWERSTOMODERNLIFE'S

MOSTBAFFLINGQUESTIONS.

Howdoesaflameknowwhichwayisup?Ifthehumiditygetsto100%,willIdrown?Canafarmerreallysmellraincoming?Whydomirrorsreverseleftandrightbutnotupanddown?Whenmytireswearout,wherehasalltherubbergone?Whyisn'titcoldinspace?

ThankstoRobertL.Wolke,eventhemostcomplicated,unfathomablephenomena in our everydayworldhave clear, concise,well-I'll-be-darnedexplanations, from the weather to the food you eat to the reason yourshower curtain seems to have amagnetic attraction to you. Discover theamazingunbreakablelawsofscienceandNatureinthebookthatexplainsitall!

BOOKSBYROBERTL.WOLKE

Impact:ScienceonSocietyChemistryExplainedWhatEinsteinDidn'tKnow:ScientificAnswerstoEverydayQuestionsWhatEinsteinToldHisBarber:MoreScientificAnswers toEveryday

Questions

I dedicate this book to my late father, Harry L. Wolke, to whom fatedeniedtheopportunityofpursuinghisowninclinationstowardscienceandlanguage,orevenofseeinghissonbecomeascientistandanauthor.Thisone'sforyou,Pop.

Acknowledgments

Iwanttoexpressmydyinggratitudetoallofmyfriendswhosaid,“Hey,

Bob,Ithoughtofagreatquestionforyourbooktheotherday,butIforgotit.”I earnestly thank two nice guys with whom it has been a pleasure to

work: my agent, Ethan Ellenberg, and my editor, Mike Shohl. Ethanskillfully navigated my proposal through the “shohls” of contractnegotiation, while Mike wielded his blue pencil with understanding andrestraint,gettingallmyjokesandallowingthemtosurvive.DianaZourelias'sdelightfuldrawingsaddanevenlightertouchtowhat

wouldotherwisehavebeenarelentlesslygray-pagedvolume.AllIdidwasgive her a paragraph describing each situation to be illustrated, and herwhimsicalcreativitytookitfromthere.IamindebtedtoRichardE.Eckelsforguidingmetoatrueexplanation

ofhowairplanesfly.Thetwowomeninmylife,mydaughter,Leslie,andmywife,Marlene,

neverflaggedintheirencouragementorintheirregardformyworkdespitemy computer's often competingwith them formy time. For that and fortheirloveandadmirationIamgratefuleverydayofmylife.

Contents

Introduction

Movin’andShakin’Newtondiscoveredthreelawsofmotion

…butheneverconsideredthese:Why do we drive on the right? Why are cloverleaf intersections so

complex?CanEarth'sorbitbechangedifabillionpeoplejumpatthesametime? Can jumping up at the last instant save you in a falling elevator?Wheredoestherubbergowhenatirewearsout?Canabulletfiredintotheaircomedownandkill somebody?Whydoes theLoneRangerusesilverbullets?What really keeps an airplane up? Are astronauts weightless? Ifyoudriveyourcarfasterthanthespeedofsound,canyouheartheradio?…Andmore.

LookyHere!Seeingisbelieving

…butnotuntilweunderstandwhatwesee.WhatmakesDay-Glocolors sobright?Why is snowwhite?Whyare

there two sets of primary colors?Howdo fluorescent and halogen lampswork? Why do mirrors seem to reverse left and right, but not top andbottom?Whydostagecoachwheelsappeartoturnbackward?Whydowetthings look darker?Why is glass transparent?Why doWintOGreen LifeSaversmakesparks?…Andmore.

HotStuffIfyoucan'tstandtheheat,getoutoftheuniverse

…becauseheatistheultimateformofenergy.Is 100 degrees twice as hot as 50 degrees? What is temperature,

anyway?Howcoldcanitget?Whyisthebathroomfloorsocoldonyourbarefeet?Howhotcanitget?Howdoesaflameknowwhichwayisup?

Why is a candle flame tapered at the top? Could we counteract globalwarmingbyturningonallourairconditioners?What'ssodangerousabouthighvoltage?Whydoesn'titraindeadsparrows?…Andmore.

TheEarthBeneathOurFeetOhappyEarth,whereonthyinnocentfeetdoevertread!–SPENSER

…Andthyinnocentminddotheverstrivetounderstand.WhydoesEarthpulleverythingtowarditsexactcenter?Howdoeshot

airdefygravitybyrising?Ifhotairrises,whyisitcolderinthemountains?Doesitevergettoocoldtosnow?IfEarthisspinningsofast,whydon'tweflyoff?CantheastronautsseeEarthturningbeneaththem?Wouldapolarbear weigh less at the equator? Do toilets flush counterclockwise in thenorthern hemisphere and clockwise in the southern hemisphere?Can youstand an egg on end during the vernal equinox?Why is nuclear energyuniqueonEarth?Howdoesradiocarbondatingwork?…Andmore.

HeavensAbove!Theseearthlygodfathersofheaven'slights

ThatgiveanametoeveryfixedstarHavenomoreprofitoftheirshiningnightsThanthosewhowalkandknownotwhattheyare.

–SHAKESPEARESorrytodisagreewithyou,Will,butit'smuchmorefunifyouknowwhattheyare.Theair,thesky,themoonandthestarsareallupthereforustocomprehend.

Howdoodorsfindyournose?Canyouoperateavacuumcleanerinavacuum?Whydoesaliontamer'swhipmakesuchaloud“crack”?Whatisthesoundbarriermadeof?Whydoesthundersoundthewayitdoes?Whyis themoonsomuchbiggerwhen it'snear thehorizon?Whydo thestarstwinkle?HowdoesthemoonkeeponesidealwaysfacingEarth?Howdotheoceans' tideswork?Does themoon ever turnblue?Why is it cold inspace—orisit?…Andmore.

AllWetYou can lead a horse to water, but you can't make him

think.–WOLKE

We humans, however, can ponder the remarkable properties of the mostabundantchemicalonEarth.

What color is water?Why are the oceans blue?And salty? Preciselywhereissealevel?Whydoesspilledcoffeedrytoaring?Whydoesyourshowercurtainclingtoyou?Wheredoyoursocksgowhentheydisappearinthelaundry?What'sthemostexpensiveingredientinlaundrydetergents?(Advertising.) Is glass a liquid? What makes ice cubes cloudy? If thehumiditygottobe100percent,wouldwedrown?Howcanyouclearyourfogged-upwindshield?Canafarmersmellrain?…Andmore.

StuffandThingsHewhodieswiththemoststuffwins.–YUPPIEPHILOSOPHY

Wehavesurroundedourselveswithhundredsofmaterialthingsthatweusebutmaynotreallyunderstand.

Are airplanes safe? How does an eraser erase? Why does rubberstretch? Why are cars noisy? Why do clothes wrinkle? How does askateboardwork?Whathappenswhenyoushakeabottleofsoda?Canyougetelectricityoutofalemon?Aresmokealarmsradioactive?Canfertilizerexplode? How do sailors keep clean? If you stopped dusting, how longwouldittaketobeburiedindust?Canyouunburnamatch?…Andmore.

SomeTechspeakBuzzwords

Introduction

Iknowwhatyou'rethinking.You'rethinking,“DidEinsteinevenhavea

barber?”You'veseenhispictures,right?Andit'sperfectlyclearthatthegreatman

devoted a lot more time to cultivating the inside of his head than theoutside.Butthisbookisn'taboutbarbers,andit isn'tevenmuchaboutEinstein.

(Hisnamecomesuponlyfourtimes.)It isabookofscientificsmall talk,thekindsofthingsthatEinsteinmighthavetalkedaboutwithhisbarber—simplethingsthatmayhavebeentrivial to thegreatscientist,but that therestofusmaywonderabout.Therearemanyscience-is-funbooksforyoungreaders.Butitisn'tonly

childrenwhowonder“Why?”or“How?”Curiositydoesn'tendatpuberty,nor does the genuine fun of understanding why things happen. And yet,once we are “done with science” in school we encounter few books forpeople of any age who are simply curious about their everydaysurroundingsandderivepleasurefromknowingwhatmakesthemtick.Thisisthatkindofbook.Maybe you are convinced that science is “not for you,” that it is

inherentlydifficultstuff,andthatifyouweretoaskaquestiontheanswerwouldbetootechnicalandcomplicatedforyoutounderstand.Soyoujustdon'task.Youmayhavecometotheseconclusionsbecauseofunfortunateexperienceswithschoolscienceclassesorsimplyfromthesciencestoriesinnewspapersandmagazinesandontelevision.Thesestoriesarebytheirvery nature guaranteed to be technical and complicated, because they areabout the latest discoveries of leading scientists. If they weren't, theywouldn'tbenews.Youwon'tseeaTVspecialonwhythebathroomfloorfeelssocoldonyourbarefeet.Buttheexplanationofthatphenomenonisscience,everybitasmuchasadiscussionofquarksorneutronstars.

Scienceiseverythingyousee,hearandfeel,andyoudon'thavetobeanEinstein or even a scientist to wonderwhy you are seeing, hearing andfeelingthosethings,becauseinmostcasestheexplanationsaresurprisinglysimpleandevenfun.Thisisnotabookoffacts.Youwillnotfindanswersheretoquestions

suchas“Whodiscovered…?”“Whatisthebiggest…?”“Howmany…arethere?”or“Whatisa…?”Thosearen'tthekindsofthingsthatrealpeoplewonderabout.Collectionsofanswerstosuchcontrivedquestionsmayhelpyouwinatriviacontest,buttheyarenotsatisfying;theydon'tcontributetothe joy of understanding. The joy and the fun come not from merestatementsoffactbutfromexplanations—explanationsinplain,everydaylanguagethatmakeyousay,“Wow!Isthatallthereistoit?”Therearewelloverahundredexplicitquestionsaddressedinthisbook,

butthatbynomeanslimitsthenumberofthingsthatareactuallyexplained.Thephysicalworldisacomplexwebofgoings-on,andnothinghappensfora single, facile reason. In science, every answer uncovers new questions,andnoexplanationcaneverbecomplete.Nevertheless, I have written each question-answer unit to be self-

contained, tobereadandunderstood independentlyofall theothers.Thismustinevitablyleadtosomeoverlap—anessentiallinkinlogiccannotbeomittedsimplybecauseitistreatedingreaterdetailelsewhere.Butaseveryteacherknows,abitofrepetitionneverhurtthelearningprocess.Whenever another Q&A unit contains closely related information, you

willbereferredtothepagenumberonwhichthatunitappears.Thus,thereisnoneed to read thebook sequentially.Readanyunit that catchesyoureyeatanytime.Butdon'tbesurprisedifyouareluredintoawebofrelatedunitsbythepagereferences.Followthelark.Thatway,you'llbefollowingtrains of thought sequentially, as if they had been laid out in a (heavenforbid) textbook, which neither of us wants. You've been there, and I'vedone that. And whenever a complete explanation requires a little moredetail than you may be in the mood for, that detail is banished to aNitpicker'sCorner.There, youmayeither continue readingor just skip itandmoveontoanotherquestion.Yourcall.I have studiously avoided using scientific terms. I believe that any

concept that is capable of being understood should be explainable inordinarylanguage;that'swhatlanguagewasinventedfor.Butfortheirownconvenience, scientists use linguistic shortcuts that I call “Techspeak.”WhenaTechspeakwordisinescapable,orwhenitisawordthatyoumayhave heard and avoiding it might seem contrived, I define it in plain

language on the spot. You will find the definitions of some usefulTechspeakwordsinthebackofthebook.Iassumenopreviousscientificknowledgeonyourpart.Therearethree

ubiquitousTechspeakwords,however,thatIusewithouttakingthetroubleto define each time: atom,molecule and electron. If you're a bit skittishabout your familiarity with them, check them out in the Techspeak listbeforeyoubegin.Scattered throughout the book youwill find a number of Try Its—fun

thingsthatyoucandoinyourownhometo illustrate theprinciplesbeingexplained.YouwillalsofindanumberofBarBetsthatmayormaynotwinyouaroundofdrinks,butthatwillcertainlygetaspiriteddiscussiongoing.

When Albert Einstein was in residence at the Institute for AdvancedStudy at Princeton University, an eager young newspaper reporterapproachedhimoneday,notebookinhand.“Well,ProfessorEinstein,”heasked,“what'snewinscience?”Einstein lookedathimwithhisdeep,softeyesandreplied,“Oh?Have

youalreadywrittenaboutalltheoldscience?”What he meant was that science isn't to be characterized only by the

latestheadline-makingdiscovery.Scientificobservationhasbeengoingonforcenturies,andinthattimewehavelearnedatremendousamountabouttheworld around us. There is a vast heritage of knowledge that explainsordinary,familiarhappenings.That's the “old science.” Everyday science. That's what this book is

about.

Everythingismoving.You may be sitting quietly in your armchair, but you are far from

motionless. I don'tmeanmerely that your heart is beating, your blood iscoursingthroughyourveinsandyouarepantingattheprospectoflearningsomany fascinating things from this book. In short, I don'tmean simplythatyouarephysicallyandmentallyalive.Imeanthatwhileyouaresittingtheresopeacefully,Earthbeneathyour

feetisspinningyouaroundatabout1,000milesperhour(1,600kilometersperhour). (The exact speeddependsonwhereyou live).MotherEarth issimultaneously hauling you around the sun at 66,600 miles per hour(107,000kilometersperhour).Nottomentionthefactthatthesolarsystemandall thestarsandgalaxies in theuniverseare racingmadlyawayfromoneanotherinalldirectionsatincrediblespeeds.Okay,youknewall that.Exceptmaybe for theexactspeeds.Butwe're

stillnotdone.Youaremadeofmolecules.(Yes,evenyou.)Andallyourmoleculesare

vibrating and jiggling around to beat the band, assuming that your bodytemperatureissomewhereaboveabsolutezero.Inmotionalsoaremanyoftheatomsofwhichyourmoleculesaremade,andtheelectronsofwhichtheatomsaremade,andtheelectrons,atomsandmoleculesofeverythingelseintheuniverse.Theywereallsetintomotionabout12billionyearsagoandhavebeenquiveringeversince.Sowhatismotion?Inthischapterwe'llseehowevery-thingfromhorses

to speeding automobiles, sound waves, bullets, airplanes and orbitingsatellitesmovefromoneplacetoanother.

HorsingAroundontheHighway

Whydotheydriveontheleftinsomecountriesandontherightinothers?

Itgoesbacktothefactthatmosthumansareright-handed.Longbeforewehadmodernweaponssuchasgunsandautomobiles,people

hadtodobattleusingswordsandhorses.Nowifyouareright-handed,youwearyourswordontheleft,sothatyoucandrawitoutrapidlywithyourrighthand.Butwiththatlong,danglingscabbardencumberingyourleftside,theonlywayyoucanmountahorseisbythrowingyourfreerightlegoverhim.Andunlessyouare inaMelBrooksmovieandwant towindupsittingbackwardonyoursteed,thatmeansthatthehorse'sheadhastobepointingtoyourleft.Tothisdaywestilltrainhorsestobesaddledandmountedfromtheirleftsides.Nowthatyouaremounted,youwillwanttostayontheleftsideasyoustart

downtheroad,becauseanyonecomingtowardyouwillbeonyourright,andifthatsomeoneturnsouttobeanenemy,youcanwhipoutyourswordwithyourright hand and be in position to run the scoundrel through. Thus, prudenthorsemenhavealwaysriddenontheleftsideoftheroad.Thisleft-sideconventionwasalsohonoredbyhorse-drawncarriagesinorder

to avoid annoying collisions with horse-men.When horseless carriages madetheir appearance, some countries continued the habit, especially during theoverlapperiodwhenbothkindsofcarriageswerecompetingforroadspace.SowhydopeopledriveontherightintheU.S.andmanyothercountries?Whenswordswentthewayofbowsandarrows,theneedfordefendingone's

rightflankdisappearedandtrafficrulesweresuddenlyupforgrabs.Youngerorlesstradition-boundcountriesmigratedtotheright,apparentlybecausetheright-handedmajority feelsmore comfortable hugging the right side of the road. Itquicklyoccurredtoleft-handedpeoplethatitwasunhealthytoarguewiththem.SomecountriesthatI'vebeeninmusthavelargepopulationsofambidextrous

people,becausetheyseemtopreferthemiddleoftheroad.

Four-GriefClovers

Why do highway and freeway intersections have to be socomplicated,withallthoseloopsandramps?

Theyenhance the traffic flow—fromconstructioncompanies topoliticians'

campaignchests.Sorry.Theyallowus tomake left turnswithoutgettingkilledbyoncoming traffic.

It'samatterofsimplegeometry.Whenfreewaysandsuperhighwaysbegantobebuilt,engineershadtofigure

outhowtoallowtraffictomaketurnsfromonehighwaytoanintersectingonewithoutstoppingforredlights.Becausewedriveontheright-handsideof theroadintheU.S.,rightturnsarenoproblem;youjustveeroffontoanexitramp.Butaleftturninvolvescrossingoverthelanesofopposingtraffic,andthatcancauseconflictsthatarebetterimaginedthanexpressed.Enterthecloverleaf.Itallowsyoutoturn90degreestotheleftbyturning270

degreestotheright.Thinkaboutit.Afullcircleis360degrees;a360-degreeturnwouldtakeyou

rightbacktoyouroriginaldirection.Iftwohighwaysintersectatrightangles,aleft turnmeansturning90degreestotheleft.Butyou'dget thesameresultbymakingthreerightturnsof90degreeseach.It'sthesameaswhenyouwanttoturnleftinthecityandencountera“NoLeftTurn”sign.Whatdoyoudo?Youmakethreerightturnsaroundthenextblock.That'swhattheloopofacloverleafdoes; it takes you 270 degrees around three-quarters of a circle, guiding youeitheroverorundertheopposinglanesoftrafficasnecessary.The highway interchange is a four-leaf clover, rather than a two- or three-,

becausetherearefourdifferentdirectionsoftraffic—going,forexample,north,east,southandwest—andeachofthemneedsawaytomakealeftturn.ForreadersinBritain,Japanandothercountrieswheretheydriveontheleft,

just interchange thewords “left” and “right” in the preceding paragraphs, andeverythingwillcomeoutallright.Thatis,allleft.YouknowwhatImean.

Ready,Set…Jump!

If every person in China climbed to the top of a six-foot (two-meter) ladderandthenall jumpedoffat thesametime,could itnudgeEarthintoadifferentorbit?

No,butitsurewouldcreateawindfallforChinesepodiatrists.IsupposethateverybodypicksonChinawhentheyaskthisquestionbecause

Chinais themostpopulouscountryonEarth,containing2.5billionpotentiallysorefeet.There are really two questions here, aside from the question ofwhy people

whoaskthisquestiondon'thaveanythingbettertodo.(Justkidding;it'sfuntowonder about such things.) The first question is how strong the jump-thumpwouldbe,andthesecondquestioniswhetheranysizethumpatallcouldchangeEarth'sorbit.It'seasytocalculatetheamountofenergyfromagravitationalfall.(Anddon't

tellmethey'renotfallingbecauseChinaisupsidedown.)Assumingapopulationof1.2billionChineseweighinganaverageof150pounds(68kilograms)each,theircollectivepouncewouldhitthegroundwithanenergyof1.6trillionjoules.(Ajouleisjustaunitofenergy;don'tsweatit.)That'sjustabouttheamountofenergy released in a medium-sized earthquake measuring 5.0 on the Richterscale.Suchearthquakeshavebeenoccurringformillionsofyears,andthere isnoevidencethattheyhavenudgedEarthintodifferentorbits.But no amount of earthquake or footquake energy could change the orbit

anyway, so both earthquakes and Chinese ladders are irrelevant. Planet Earthcontinuescirclingthesunbecauseithasacertainamountofmomentum,whichmeans that it has a certain amount of mass and a certain velocity, becausemomentumisacombinationofmassandvelocity.Ourplanetcarriesalongwithit everything that is attached to it by gravity, including jumping Chinese andacrobatson trampolines.We'reallonebigpackageofmass,andnoamountof

jumping up and down can change Earth's total amount of mass. Nor can itchange the planet's velocity, because all the Chinese are being carried alongthroughspaceat the samespeedas the restof theplanet;we'reall inonebig,interconnectedspaceship.Youcan'tchangethespeedofyourcarbypushingonthewindshield,canyou?Norcanyouliftitbypushingontheinsideoftheroof.WemightputitintermsofNewton'sThirdLawofMotion,whichyoumust

haveheardamilliontimes(andwillagain,ifIhaveanythingtodowithit):“Foreveryactionthereisanequalandoppositereaction.”Pushonabrickwallandthewallpushesback.Ifitdidn't,yourhandwouldgostraightthrough.WhentheChineseland,theirfeethitthegroundwithacertainamountofforce,butatthesame time the ground hits their feet with an equal amount of force in theoppositedirection.Thus,(a)thereisnonet(unbalanced)forcethatcouldaffectourplanet'smotionand(b)theirfeethurt.

Jump…Now!

IfI'minanelevatoranditstartstofalltothebottomoftheshaft,canIjumpupatthelastinstantandcanceltheimpact?

Ho hum. I don't know howmany times this question has flashed into the

mindsofworrywartsinelevators,orhowmanytimesithasbeenaskedofeveryfriendly neighborhood physicist. It is easy to answer in one word (No), butthinkingaboutitdoesraiseawholebunchoffunquestions.First,here'sthequickanswer:Yourobjectiveistoarriveatthebottomofthe

shaftlikeafeather,withoutanyappreciabledownwardspeed,right?Thatmeansthatyouhave tocounteract theelevator'sdownwardspeedbyjumpingupwardwithanequalamountofspeed.Theelevator(andyou)mightbefallingat,say,50 miles per hour (80 kilometers per hour). Can you jump upward withanywhere near that speed? The best basketball players can jump at maybe 5milesperhour(8kilometersperhour).Endofquickanswer.Let'sconsidertheinstantbeforeyourelevator'scablesnaps.Intheseventeenth

century,longbeforeelevators,SirIsaacNewton(1642–1727)realizedthatwhena body exerts a force on another body, the second body exerts an equal andoppositeforceonthefirstbody.Today,that'sknownasNewton'sThirdLawofMotion.Whenyou're standingon theelevator floorandgravity (forcenumberone)ispullingyoudownagainstthefloor,thefloorispushingyoubackupwithanequalforce(forcenumbertwo).That'swhygravitydoesn'twinoutandmakeyoufalldowntheshaft.It'sthesamewiththeelevatorcaritself;inthiscaseit'sthecable'supwardpull thatcounteractsgravity'sdownwardpullonthecar.Soneither you nor the elevator falls down the shaft. You both move upward ordownwardataspeedthatiscontrolledbyamotor'sslowwindingandunwindingofthecablefromabigdrumatthetopoftheshaft.Whenthecablesnaps,boththeupwardpullofthecableandtheupwardpush

oftheflooraresuddenlygone,sobothyouandtheelevatorarefreetosuccumb

togravity'swillandyoubothbegintofall.Foraninstantyouareleftfloating—feeling “weightless” because the customary push of the floor on your feet isgone.Butfollowingthatinstantofblissfulsuspension,gravityhasitswaywithyouandyoufall,alongwiththeelevator.

NITPICKER'SCORNER

About that moment of “weightlessness” when the elevator begins to fall:

Obviously,youhaven'treallylostweight.Earth'sgravityisstillpullingonyouasitalwayshas,andthestrengthofthatpulliswhatwecallweight.Whatyou'velost is apparent weight. Your weight just isn't apparent because you're notstandingonascaleorafloorthatfeelsyourpressureandpressesbackuponyourfeet.Of course, this whole question of falling elevators is hypothetical because

elevator cables just don't snap. And even if they did, there are spring-loadedsafetydevicesthatwouldkeepthecarfromfallingmorethanacoupleoffeet.But, as roller coastersprove, somepeople seem to enjoy the contemplationofimminentdisaster.Ifyouhappentobeoneofthoserollercoasterfans,that“floating”feelingyou

getasthecarfallsfromoneofitshighspotsisexactlythesamethingyou'dfeelin a falling elevator. It's called free fall.Astronauts in orbiting spacecraft alsofeelit.

DeadTread

When my car's tire treads wear out, where has all the rubbergone?

Ithasbeenrubbedoff—andno,that'snotwhytheycallitrubber—ontothe

road,whence itwas scattered in the formof fine dust into that vast, complexeverywhere thatwe call the environment.Someof itwas thenwashedoff theroad and into sewers by rain, or else it was blown around by the wind andeventuallyfellorwasrainedoutoftheairontoanyandallsurfaces.Eventually,alltherubberjoinedthesoilandtheseasaspartoftheEarthfromwhichitwasborn.Likeeverythingelse,adeadtreadreturnsuntodust.We tend to thinkofautomobile tiresas rollingsmoothlyalong,withoutany

scuffingagainsttheroadthatmightscrapeawayrubber.Thatcouldbetrueonlyiftherewerenoresistancewhatsoeverbetweenthetire'ssurfacesandtheroad'ssurface.Andiftherewerenoresistance,yourtirescouldn'tgetagripandyou'dgonowhere.You'dgetaspectacularwarrantyonasetoftireslikethat,becausethey'dneverwearout.Betweenanytwosurfacesthatareattemptingtomovepasteachother—even

a tire and a road—there is always some resistance; it's called friction. Evenrollingwheelsexperiencefrictionagainsttheroad,althoughrollingfrictionisalot less than sliding friction. When necessary, you can roll your car straightaheadbypushing,butjusttrytoslideitsideways.Frictiongobblesupsomeof theenergyofmotionandspits itoutasheat. If

there were no diminishment of motion by the conversion of some of it tofrictionalheat,amachinecouldgoonforeverwithoutslowingdown:perpetualmotion.Becausetherealwaysmustbesomefrictionalheatloss,howeversmall,everydevicethathaseverbeentoutedasaperpetualmotionmachinehastobeafake,howeverwell-intentioneditsinventor.

TRYIT

If you don't think that tire-against-road frictionmakes heat, just feel yourtiresbeforeandafterdrivingforanhourorsoonthefreeway.Muchoftheheatyou'll feelcomesfromfrictionagainst theroad,butsomecomesalsofromthecontinualflexingandunflexingoftherubber.

Regarding the disappearing tread on your tires:Wherever there is frictionalresistancebetweentwomaterials,oneofthemhasto“give”—thatis,havesomeof itsmolecules scrapedoff by the other.Betweenyour soft tire and the hardhighway,it'snocontest;it'stherubberthatgivesandgetsrubbedoffgraduallyintinyparticles.If all of our roadsweremade of a substance that is softer than rubber, the

roadswouldwearout insteadof the tires. Instead,oursocietyhasdecided thatit's less trouble for car owners to replace their tires than for governments tocontinually replace road surfaces. Thenwhy, youmay ask, dowe continuallyhave to dance the orange barrel polka to get through interminable roadreconstructionzones?Unfortunately,Icanansweronlyscientificquestions,notpoliticalones.The squealing tires in movie car chases are the result of sliding friction:

rubber scraping, rather than rolling, on thepavement.Onamicroscopic scale,wewouldsee the tirealternatelygrabbingandslipping thousandsof timespersecond, producing a series of chattering vibrations that fall in the frequencyrange of a screech. It's easy to see that with all of this frictional dragging ofrubberagainst the road,a lotof rubberwillbe rubbedoff. In fact, the frictionmakesenoughheattomeltsomeoftherubber,whichpaintsitselfontotheroadasablackskidmark.

YouDidn'tAsk,but…

Whyare the tiresonracingcars so smooth?You'd think they'dneedallthetractiontheycouldget.

That's precisely why they're smooth. Regular tires waste a lot of their

potential road-grabbing surface by having grooves, which act like gullies tochanneloutrainandmud.Butracingcarsusuallycompeteingoodweather,sothe rain-and-mud grooves aren't necessary. They're justwasted space that canbetterbeusedtoaddmoreroad-grabbingrubberforbetterhandlinginturnsandbetter braking response.Toget evenmore road-grabbing surface, the tires aremade much wider than those on your family chariot. And they're made of asofter rubber thatwears off like crazy onto the track.You thinkyou don't getgoodtiremileage?Whydoyouthinkthey'realwaysstoppingtochangetires?

Ready,Aim,Scram!

Inmoviewesterns, and even inmany parts of theworld today,peoplefiregunsstraightupintotheairaswarningshotsorjusttomakenoiseduringacelebration.But thosebulletshave to comedown somewhere. How dangerous will they be if they hitsomebody?

Quitedangerous.Aswe'll see,physics tellsus thatwhen ithits theground

thebulletwillhavethesamevelocityithadwhenitleftthemuzzleofthepistol,whichcanbe700 to800milesperhour (1,100 to1,300kilometersperhour).Butthatignoresairresistance.Morerealistically,thebullet'slandingspeedcanbearound100to150milesperhour(160to240kilometersperhour).That'sfastenoughtopenetratehumanskin,andevenifitdoesn'tpenetrateitcanstilldoalotofdamage.Butjusttrytotellthattotheidiotswholiketoshoottheirguns“harmlessly”intotheair.Therearetwokindsofforcesthataffectthebullet'sspeedonthewayupand

on the way down: gravity and air resistance. Let's look first at the effects ofgravity,neglectingairresistanceentirely.Itwillbeeasier tounderstand thebullet's flight ifweconsider it in reverse.

That is,we'll start at the instant atwhich the bullet has reached the top of itsflight and is just starting to fall downward. Then we'll consider its upwardjourneyandcomparethetwo.Gravityisaforcethatoperatesonafallingobject—andisindeedwhatmakes

itfall—bypullingonit,attractingit towardthecenteroftheEarth,adirectionthatwecall“down.”Aslongastheobjectisintheair,gravitykeepsontuggingonit,urgingittofallfasterandfaster.Thelongeritfalls,themoretimegravityhastoworkonit,sothefasteritfalls.(Techspeak:Itaccelerates.)ThestrengthofEarth'sgravitationalfieldissuchthatforeverysecondofpull

—thatis,foreverysecondthatanobjectisfalling—theobjectspeedsupbyan

additional32feetpersecond(9.8meterspersecond)orabout22milesperhour(35kilometersperhour).Itdoesn'tmatterwhattheobjectisorhowheavyitis,becausethestrengthofthegravitationalfieldispurelyacharacteristicofEarthitself.Soforeverysecondofdownwardfall,thebulletgains22milesperhour(35kilometersperhour)ofspeed.Ifitfallsfortenseconds,itsspeedwillbe220milesperhour(350kilometersperhour),andsoon.Butgravitywaspullingonthebulletwiththesameforcewhenitwasonits

way up. That's what slowed it down somuch that it eventually reached zerospeedatthetopofitsflightbeforestartingtofall.Foreverysecondthatitwasonitswayup,gravity'spullremoved22milesperhour(35kilometersperhour)ofspeed.Thetotalamountofspeedremovedonthewayupmustbethesameasthe totalamountofspeedregainedonthewaydown,because thegravitationaleffectwas the sameall the time. If thatweren't true, thebulletwouldhave tohave acquired some speed or lost some speed because of some other outsideforce.Andtherewasnootheroutsideforce(exceptairresistance,andwe'llgettothat).Soweseethatwhatgravitytakethawayonthewayup,gravitygivethbackon

thewaydown.Onthebasisofgravitationaleffectsalone,then,thebulletwouldhavenomoreorlessspeedwhenithitsthegroundthanithadwhenit left thegun: itsmuzzle velocity, and that's how fast it will be goingwhen it hits theground.…Oraninnocentbystander.Uptonow,we'veignoredtheslowing-downeffectoftheair.Asyoucantell

by sticking your hand out thewindow of amoving car, the faster you go themoretheairtriestoholdyouback.Soasourbulletfallsfasterandfasterunderthe influence of gravity, air resistance tries to make it go slower and slower.Prettysoon,thetwoconflictingforcesbecomeequalandcanceleachotherout.Afterthat,nomatterhowmuchfarthertheobjectfallsitwon'tgoanyfaster.Ithasreachedwhatphysicistsliketocallitsterminalvelocity,whichisTechspeakforfinalspeed.(Because “terminal velocity” is such an impressive-sounding term,manyan

innocentphysicsstudent—Iwasone—getstheimpressionthatit'ssomekindoffundamental limitationofNature, likethespeedof light.But there'sabsolutelynothing sacred or fixed about it. The final speed of a falling object simplydependsonitssizeandshape,andonhowitcatchestheair.Ifyoufalloutofanairplane, your terminal velocitywill certainly be a lot less if you'rewearing aparachute.Teamsofskydiversadjusttheirairresistancebymakingtheirbodiesmorecompactormoreextended, so theycan rendezvousat the same terminalvelocityandfrolicaroundtogetherbeforepullingtheirripcords.)

If a shooter is fairly close to a target, there isn't much opportunity for airresistance toslowthebulletdownduring itsshort flight.Evenwhenfired intotheair,astreamlinedobjectlikeabulletdoesn'tsuffermuchairresistanceonthewayup,becauseitispointingstraightaheadalongitspath.Butduringitsfallitis probably tumbling, or evenmore likely falling base-first, because that's themost stable orientation for a bullet-shaped object. The air resistance on atumblingorbase-firstbullet isquiteabitgreater thanonastraight flyer, so itmay be slowed down substantially on the way down and end up quite a bitslowerthanitsmuzzlevelocity.Oneexpertestimatesthata.22LRbulletwithamuzzlevelocityof857milesperhour(1,380kilometersperhour)mightfalltothegroundwithavelocityof96to134milesperhour(154to216kilometersperhour), depending on how it tumbles. That's more than enough speed to doseriousorlethaldamagetoacraniallandingsite.Andbytheway,thejerkwhofiresthebulletisn'tverylikelytobehitbyit,no

matter how carefully he aims straight up. In one experiment, out of fivehundred.30-calibermachine-gunbulletsfiredstraightupward,onlyfour landedwithin 10 square feet (3 square meters) of the gun.Wind has a great effect,especially since.22- to.30-caliber bullets can reach altitudes of 4,000 to 8,000feet(1,200to2,400meters)beforefallingbackdown.

WarIs…Swell

Whydogunsputspinontheirbullets?

Aspinningbulletfliesfartherandtruerthanitwouldwithoutthespin.Andif

your favorite sport is football rather than shooting, just about everything I'mgoingtosayaboutspinningbulletsalsogoesforspiralingpasses.The fact that a spinning bullet or football goes farther may sound strange,

becauseyou'dthinkthattherangewoulddependonlyontheamountofenergyitgets from the gunpowder charge or the quarterback's arm. But bullets andfootballshavetoflythroughtheair,andairdragplaysanimportantpartinanyprojectile's trajectory, whether it is fired from a handgun, rifle, machine gun,howitzerorarm.First,let'sseehowagunmakesthebulletspin.Running the length of the inside of the barrel are spiraling grooves, called

rifling.Asthebulletpassesthroughthebarrel,thesegroovescutintoit,makingitrotatetoconformtothespiral.Somegunshavegroovesthattwisttotherightand some have grooves that twist to the left; it doesn'tmatter. (And no, theydon'ttwistonewayinthenorthernhemisphereandtheotherwayinthesouthernhemisphere.)Early bullets were round balls of lead, like miniature cannonballs. Bullet-

shaped (Techspeak: cylindroconoidal) bullets were developed around 1825,when it was found that they maintained their speed better in flight. That'sbecauseforagivenweightofleadanelongated,tapered-noseshapemeetswithlessairresistancethanaroundball;it'sstreamlined.Butthere'saproblemwithelongatedbulletsthatsphericalbulletsdon'thave.

Whenanelongatedbulletisfired,anytinyirregularitiesonitssurfacecancatchthe air and push it slightly sideways, so that its nose is no longer pointingstraight ahead. This slight misalignment increases the air resistance on theforwardside,whichturns thebulletevenmore.Prettysoonit is tumblingend-

over-end,which causes evenmore air drag, seriously shortening its range andpushingitoff-course.Thus,bothdistanceandaccuracysuffer.That'swheretheriflingcomesin.Ifthebulletisspinningproperlyaroundits

longaxisasitflies,itresistsanychangeinitsorientationordirectionofflight.Thereasonforthatisthataheavy,spinningobjecthasalotofmomentum.Notonlydoesithavemomentumalongitsdirectionoftravel(linearmomentum),butbecause of its spin it also has rotational momentum, or what physicists callangularmomentum.Andmomentum,whetherlinearorangular,ishardtoupset.Infact,themomentumofanobjectwillremainunchangedunlessanduntilitisdisturbed by some outside force. (Techspeak:Momentum is conserved.) Thespinningbullet,therefore,willmaintainitsangularmomentumbyspinningwithitsaxis in thesamedirection foras longas it is in theair,because there isnooutside force to disturb it. Those tiny surface irregularities are now peanutscomparedwiththebullet'ssubstantialamountofangularmomentum.With its nose pointed straight ahead, the projectile encounters less air

resistanceandthusfliesfartherandtruer.Whenitultimatelyhitsanobject, itsmomentum—both linear and angular—still won't disappear, but will betransferred to theunfortunate target—or in thecaseofa football, the fortunatereceiver.International law actually requires that bullets spin. Otherwise, a tumbling

bulletmighthit itsvictimsideways,doingmoredamage than if ithadmadeanice,clean,roundhole.It's justoneofthosenicetiesofwar:Ifyou'regoingtokillsomebody,pleasedoitneatly.The Geneva Convention spells out certain other niceties about how to kill

people.Forexample,becauseleadissoftanddeformable,itcangosplatwhenithits its target, again producing a very unsightly hole. So bullets have to bejacketedwithahardermetal,suchascopper.Theworld'smilitaryestablishmentsgladly complywith that requirement, but it's not because of any humanitarianmotives. It'sbecausemodernmilitaryassaultweaponsfire theirbulletsatsuchhigh speeds that if they weren't jacketed with high-melting copper the leadwouldmeltfromfrictionwiththeair,makingthemflyerraticallyandmisstheirtargets.Afterall, aclean, roundhole inanenemy is somuchpreferable tonoholeatall.

YouDidn'tAsk,but…

WhydoestheLoneRangerusesilverbullets?

Theyservemostlyasacallingcard,buttheydohaveaveryslightadvantage

overlead.Ordinarybulletsaremadeofleadbecauseleadissoheavy,ordense.Andit's

cheap.Wewantabullettobeasheavyaspossiblebecausewewantittohaveasmuchdamage-causingenergyaspossiblewhenithitsitstarget,andenergyisacombination of mass and speed. (Techspeak: Kinetic energy is directlyproportional to themass and to the square of the velocity.) It's easier to gainenergy by increasing the bullet'smass than by increasing its velocity, becauseincreasing the velocity would require a longer barrel in order to give theexplosion'sgasesmoretimetoacceleratethebullet.Asilverbulletisabout7.5percentlighterthanaleadbulletofthesamelength

andcaliber.Sinceagivenpowderchargeimpartsthesameamountofenergytoanybullet,thelightersilverbulletmusttravelfaster.Itworksouttobe4percentfasterthanaleadbullet.So the Lone Ranger's silver bullets get to their targets very slightly sooner

than a lead bulletwould. If the bullet's velocity is 1,000 feet per second (300meterspersecond)andanoutlawfiftyfeet(fifteenmeters)awayisdrawinghisgun,thesilverbulletgivesourheroatwo-millisecondadvantage—notevenlongenoughforTontotosay,“Ugh!”Also,becausesilverisalotharderthanlead,whentheLoneRangershootsthe

gunoutof abadguy'shand—henever shoots theguyhimself—itmust reallysting.Andwhenitstrikes,insteadofthedullthudoflead,asilverbulletmakesagreat“ping”soundforthemicrophonesthatseemalwaystobenearby.

BARBET

TheLoneRanger'ssilverbulletsflyfasterthanleadbullets.

HowtoStopanAirplane

When there's anairplane flyingoverhead,why is it thatwhen Iwalkintheoppositedirectionitlooksasifit'salmoststationary?Certainlymywalkingspeedispeanutscomparedwiththeplane'sspeed,sohowcanitbehavinganyeffect?

Whetherwerealizeitornot,wejudgethemotionofanairplaneinthesky

by its relation tocommon thingson theground, suchas trees, telephonepolesandhouses.That'stheonlywaymotioncanbedetected:inrelationtosomethingelse.There'snosuchthingasabsolutemotion;it'sallrelativetosomethingelse.Sothefastertheplaneappearstobepassingthetreesandhouses,thefasterwejudgetheplanetobemoving.Butwhen you yourself aremoving in relation to the trees and houses, you

upsetthissimpleassociationbecausethetreesandhousesappeartobemovingalso.Asyouwalkforward,theyappeartobemovingbackward,don'tthey?Ofcourse,youknowthatthey'renotreallymovingbackwardbecauseyourdaddytoldyousowhenyouweretwoyearsold.So as you walk forward (which, I trust, is your customary direction of

locomotion), but in the opposite direction from the airplane's, the trees andhousesalsoappeartobemovingbackwardwithrespecttoyourdirection;thatis,theyappeartomoveinthesamedirectionastheplane.Itappears,then,thattheairplane and the houses are moving together; the plane doesn't seem to beovertakingthem.Andanyairplanethatcan'tevenpassahousewouldseemtobeoneveryslowairplane.Want todo thepassengersa favorandget them to theirdestinationsooner?

Just walk in the same direction as the plane. As the trees and houses “movebackward”it'lllookasiftheplaneispassingthemevenfaster.

It'sTrulyNotBernoulli

I just can't bringmyself to believe that huge airplanes can fly,supportedastheyareonthinair.Howdotheydoit?

Join the club. Even though I know something about how airplane flight

works(andyouwilltoo,soon),itneverceasestoamazeme.Irememberlandingafter a transatlantic flight in a Boeing 747 and being directed by the crew todeplanedirectlyontothegroundandintoawaitingbus,insteadofthroughoneof those people tubes. I looked up in utter dis-belief at the four-hundred-tonmonsterthathadjustwaftedmeacrosstheAtlanticOceanatanaltitudeofmorethanfivemilesaboveEarth'ssurface.MyawewasmagnifiedbythefactthatbackwhenIwas“taught”whatmakes

airplanesfly,Iwasmisled.Inspiteofthefactthatmostflighttrainingmanualsattributeanairplane'slifttosomethingcalledBernoulli'sPrinciple,thatisnotthemain reasonairplanes stayup. It justhappens tobeaquick, easyexplanation,butlikeallsimpleanswersitismisleading,borderingondownrightwrong.1First,let'sputtheSwissmathematicianDanielBernoulli(1700–1782)onthe

witnessstandandseewhathehastosayforhimself.In 1738 Bernoulli discovered that as the speed of a moving fluid (gas or

liquid) increases, its pressure on adjacent surfaces decreases. For example, airthat is blowingbyas ahorizontalwinddoesn't have the timeor energy, so tospeak,topressveryhardupontheground.Howdoesthisaffectairplanes?Thetopsurfaceofaconventionalairplanewingishumpedupward,whilethe

bottomsurfaceisrelativelyflat.Astheplaneflies,airsweepsoverthesetwosurfaces.Onitswaytotheback

(trailing)edgeofthewing,theaironthetopsurfacehasfarthertogobecauseofits curved path. The Bernoulli-Makes-Planes-Fly advocates claim that the topandbottomairmustreachthewing'sbackedgeatthesametime—that'scalled

theequaltransittimeassumption—andthat inasmuchasthetopairhasfarthertotravelitmustmovefaster.AccordingtoMr.Bernoulli,then,thefastertopairexertslesspressureonthewingthantheslowerbottomairdoes,sothewingispushedupwardbyanetforcecalledlift.That'sallverywellexceptforonething:Thetopairandthebottomairdon't

have to reach the trailing edgeof thewing at the same time; the equal transittimeassumptionisjustplainwrong,inspiteofallthearm-wavingthatphysicsteachersandflight instructorsdo to try to justify it.YouandIcanboth forgetour embarrassment at never having understood that point in school. There issimplynogood reason that the top air has to arrive at the trailing edge at thesametimeasthebottomair.TheBernoullieffectdoescontributesomelifttoanairplanewing,butacting

byitselfitwouldrequireawingthatiseithershapedlikeahumpbackwhaleortravelingatanextremelyhighspeed.Thankyou,Mr.Bernoulli.Youmaystepdownnow.WenowcallSirIsaacNewtontothestand.Newton's three laws of motion are the ironclad foundation of our

understandingofhowthingsmove.Newtonianmechanics(asdistinguishedfromquantummechanicsandrelativity)canexplainthemotionsofallobjects,aslongas theyarenot too small (smaller thananatom)andarenot traveling too fast(near thespeedof light).Newtonfiguredouthis lawsfor themotionsofsolidobjects, but they can be applied as well to the interactions between airplanewingsandair.Let'sseehow.Newton'sThirdLawofMotion(again)says that foreveryaction theremust

beanequalandoppositereaction.Soiftheplane'swingisbeingpushedorliftedup,thenbygoshsomethingelseisbeingpusheddown.Itis.Theair.Thewingmustbewhooshingastreamofairdownwardwithaforceequaltotheliftitisgetting.We'llcallitdownwash.How?When a fluid such aswater or air flows along a curved surface, it tends to

cling to the surfacemore tightly than youmight expect. This phenomenon isknownastheCoandaEffect.(Seetheexplanation,butinsteadofwaterflowingoveracurvedglasssurface, thinkofair flowingoveracurvedairplanewing.)Because of this clinging, the air flowing over the surfaces of the wing isconstrainedtohugtheshapesofthewing;thetop-of-the-wingairclingstothetop surface and the bottom-of-the-wing air clings to the bottom surface. Thestreamsnotonlytakedifferentpaths,butasaconsequenceofthewing'sshapetheywindupflowingindifferentdirectionsatthebackofthewing.It'snotasifthewingwere simply cutting through the air like a flat knife blade, with the

airstreampartingtoletitthroughandthenclosingbacktoitsoriginaldirectionafterthewingpasses.As the top-of-the-wing airmeets the leading edge of thewing it flows first

upwardoverthesurfaceandthendownwardagainasitleavesthetrailingedge.But the shape of the wing leads it farther downward than where it began; itleavesthetrailingedgeofthewinginanetdownwarddirection.Inotherwords,the top-of-the-wingair is actuallybeing thrustdownwardby thewing's shape.AndaccordingtoNewton'sThirdLaw,thewingisthereforethrustupwardwithanequalamountofforce.Voilà!Lift!Doyouthinkthiscanbeonlyasmallamountofforce,comingasitdoesfrom

apushby“thinair”?Hah!Thinkagain.Evenasmallplane likeaCessna172flyingat110knots(204kilometersperhour)ispumpingthreetofivetonsofairdownwardeverysecond.Just thinkof thehundredsof thousandsof tonsofairthat an 800,000-pound (360,000-kilogram) Boeing 747 is pumping downwardeverysecondtogetoffthegroundandstaythere.WecangiveIsaacNewtonstillmorecreditfor liftingairplanes,becausethe

lift doesn't all come from downwash (with a slight assist by Mr. Bernoulli).SomeofitcomesfromyetanotherapplicationofNewton'sThirdLaw.Airplanewingsarenotparalleltotheground;theyaremadetobetiltedslightlyupwardinfront—usually about 4 degrees when the plane is in level flight. That makesmorepressureonthebottomsurfacethanonthetop,therebypushingthewingupward and contributing to the lift. The pilot can tilt the plane even fartherupwardinfront(Flyspeak:Hecanincreasehisangleofattack)togetevenmoreliftfromthiseffect.SirIsaac'sThirdcomesinbecauseastheplanemoves,thewing ispushing theairdown in frontof it, so theair respondsbypushing thewingsup.Wesee,then,thattwodifferentwingactionscreatelift:thewings'shape—the

“airfoil”—and their upward tilt, or angle of attack. Both must be used tomaximumeffect inorder togruntaheavyplaneoff thegroundduringtakeoff.That'swhy you see planes taking off from the airport at such steep angles ofclimb; thepilotsmust increase theirangleofattack togainextra liftwhile theplaneissoloadeddownwithfuel,nottomentionthatfatladyintheseatnexttoyou.And you thought the pilot was simply pointing the plane's nose in the

directionhewantsittogoin,asifitwereahorse.

BONUS: Have you ever wondered why ski jumpers bend over so farforwardwhenthey'reintheairthattheirnosespracticallytouchthetipsoftheir

skis? Two reasons. First, if they stood straight up they'd encounter more airresistance, which would slow them down. But second, their arched backssimulate an airfoil.Their upper surfaces are curved like an airplanewing, andtheyactuallygainsomeliftthatkeepsthemintheairlonger.

FlyingwiththeTopDown

If an airplane's wings are shaped to give it lift, how can anairplaneflyupsidedown?

Itcanbedonetowowthecrowdatanairshow,butitwouldn'tworkfora

commuter flight to Schenectady because, although it's theoretically possible,passengerplanesaren'tbuilttostandthestress.(Norarethepassengers.)A conventional airplane's wings are curved or humped on top, and that

produces lift for reasons thatare far fromsimple.But if thewingwereupsidedown,wouldn'tthatproducetheoppositeeffect,turningthe“lift”into“plunge”?Yes, if the pilotweren't partially offsetting that effect by changing the plane'sangleofattack,theangleatwhichthewingshittheair.

TRYIT

Stick your hand out the window of a speeding car—not above the speedlimit,ofcourse.Whenyouholdyourhandflat,palmparalleltotheground,youfeeltheair'spressureonwhatpilotswouldcalltheleadingedgeofyourhand—thethumbedge.Butthenifyoutiltyourhandslightlyupward,sothatyourpalmgets the brunt of the wind, your wing—uh, hand—is pushed upward. There'smorepushonthebottomthanonthetop,andthatmakeslift,nomatterhowyourhand—orawing—mighthappentobeshaped,aslongasit'sreasonablyflat.

Sowhenflyingupsidedown,thestuntpilotpointshisnose(theplane's,that

is) upward, so that thebottomsof thewings—whichused tobe the tops—aregettingthebruntofthewindandarebeingforcedupward.Asamatteroffact,stunt planes don't even have wings that are more curved on top; the top andbottom surfaces are the same shape, so it doesn't matter which side is up—everythingisaccomplishedbyangleofattack.As you saw from your hand-out-the-window experiment, increasing your

angleofattackproducesnotonlylift,butdrag—morewindresistancetryingtohold your hand back. Similarly, when the stunt pilot increases his angle ofattack,theplaneexperiencesmoredragagainstwhichtheengineshavetolabor.Stuntplanes, therefore,have tohavepowerfulengines,aswellascrazypilots.Well,crazylikeafox,perhaps,becauseit takesgreatstrengthandpresenceofmindtothinkinthreedimensionswhileyou'rebeingsubjectedtoforcesthatareeightor ten timesasstrongasgravity.Andstuntpilotsaren'tprotectedby“g-suits,”thosepressuresuitsthatfighterpilotsweartokeepthebloodfromleavingtheirheadsandblackingthemoutduringhigh-accelerationmaneuvers.Allthesame,I'lljustwatchfromtheground.

HowAstronautsLoseWeight

Does gravity peter out at a certain distance from Earth?Otherwise,howcanorbitingastronautsbeweightless?

Answertothefirstquestion:No.Answertothesecondquestion:They'renotweightless.There'sacompletely

differentreasonwhyastronautscandoallthosesillytricksforthecameras,suchas performing somersaults in midair or sitting upside down on absolutelynothing,lookingmorewitlessthanweightless.Earth's gravitational attraction, like all gravitational attraction, reaches out

indefinitely;itkeepsgettingweakerandweakerthefartherawayyougo,butitneverdiminishestozero.Everyatomintheuniverseisgravitationallypullingoneveryotheratom,nomatterwhere.Butofcourse,thebiggertheagglomerationof atoms you have, such as a planet or a star, the stronger will be theircumulativepull.That's all beside the point, however, because the paltry 250-mile (400-

kilometer)altitudeatwhichthespaceshuttlegoes’roundand’roundispeanutsas farasgravitationalweakening isconcerned.Afterall,Earthholdson to themoon pretty well, doesn't it? And that's 239,000 miles (385,000 kilometers)away.(Okay,sothemoonismuchmoremassivethananartificialsatelliteandthestrengthoftheattractionisproportionaltothemass,butyougetthepoint.)Ifthosefloatingfolksaren'tweightless,whatdowemeanbyweight,anyway?WeightisthestrengthofthegravitationalpullthatEarthexertsonanobject.

BecausethatstrengthdiminishesthefartheranobjectgoesfromthecenteroftheEarth,its“weight”diminishesalso.Butnevertozero.Okay, then. If orbiting astronauts aren't exactlyweightless, how come they

can float around in the shuttle like that? The answer is that their still-considerableweightiscounteractedbysomethingelse:aforcethatcomesfromtheirorbitalspeed.(Techspeak:centripetalforce.)

TRYIT

Tie a string firmly to a rock and swing it around in a circle (outdoors!),holding your hand as stationary as possible. The rock is the shuttle and yourhandistheEarth.Whydoesn'ttherockflyoff?Becausebymeansofthestringyou'repullingontherockwithexactlyenoughforce—animitationgravitationalforce—to counteract its tendency to fly away. Pull a little less hard (let somestringslipout)anditfliesoutward,awayfromyourhand.Pulltighterbypullingthe string in (imitating a stronger gravitational attraction) and the rock “falls”inwardtowardwhereyourhandusedtobe.

It'sthesamewiththeshuttle.Thefactthattheshuttlekeepsgoingaroundina

stablecircleratherthanflyingoffintospacemeansthatitstendencytoflyawayfromEarthisbeingexactlycounterbalancedbyEarth'sgravitationalpull,whichholds it down. In otherwords, gravity is continuallymaking the shuttle “fall”towardEarth,exactlyenoughtokeepitfrom“rising”fartheraboveEarth.The same thing is happening to the astronauts inside. Their tendency to fly

awayfromEarthisexactlybalancedbyEarth'spull,sotheyneitherflyawaynorfall to Earth; they stay suspended in midair, not knowing which way is up.Which isperfectlyokay,because there isno“up.”“Up”hasalwaysmeant“inthe opposite direction from gravity's pull,” and gravity's pull is no longerdiscernible.That'swhyit'ssomuchfunforthemtoposeforthecamerawithone

guyupsidedown.Orisitdownsideup?Incidentally,thefactthatEarth'sgravitationalforceisbalancedbytheorbiting

astronauts' centripetal force doesn't entirely exempt them from the effects ofgravity.It'sonlyEarth'sgravitythatisbalancedout.Themoon,theplanets,theshuttle and the astronauts themselves still attract one another because they allhavemass.Butbecausethemoonandplanetsaresofaraway,andbecausetheastronauts and their equipment don't have much mass, all these gravitationaleffectsdon'tamounttomuch.They'restillthere,however,andthat'swhyspacescientistsnevertalkaboutzerogravity;theysaythattheastronautsareoperatinginanenvironmentofmicrogravity.

Up,Up…andAround!

How high does a rocket have to go before it can orbit aroundEarth?

It'snothowhigh—it'showfast.There is acertain speedcalled theescape

velocitythatanobjectmustachievebeforeitcankeepcirclingEarthinastableorbitandnotfalldown.Letmetakeyououttotheballgame.Supposethatacenterfieldertriestothrowarunneroutathomeplatewitha

singlemightythrowinsteadofrelayingitviathesecondbaseman.Hethrowstheball horizontally or slightly higher than horizontally, straight at the catcher. Ifthere were no gravity (and no air resistance), the ball would continue in astraight line and go on forever. Or as Isaac Newton said in his First Law ofMotion,“Anobjectwillcontinuemoving ina straight lineataconstant speedunless some other force screws it up.” (Hemay not have said it exactly thatway.)Butinthiscasethereisanotherforce:gravity,whichispullingcontinuously

down on the ball whether it ismoving or not. The combination of horizontalmotion from the throw and vertical motion from gravity results in the ball'sfollowingacurvedpathortrajectory.Unfortunately,fewout-fielderscanthrowasfastandfarasisnecessarytopickoffarunnerathomeplate,sotheballwillhitthedirtwellinfrontofthecatcher.Nowlet'saskSupermantothrowabaseballhorizontallyoutoverthePacific

Ocean.(Andagainwe'llignoretheair'sresistance.)Ifhethrowstheballat,say,1,000milesperhour (1,600kilometersperhour), its curvedpathwillbea lotlongerandbroaderthaninthecaseoftheoutfielder,buteventually,gravitywillstillbeabletobringitdownafterperhapsafewmiles.Embarrassedbythispipsqueakperformance,ourherothenwindsupandhurls

anotherbaseballoutovertheoceanat25,000milesperhour(40,000kilometers

perhour).Thistimetheball'strajectoryissuchabroad,shallow,flatcurvethatit matches the curvature of Earth's surface itself, so it just keeps going at aconstantheightabovethesurfaceandneverfallsdown.Ithasgoneintoorbit.So you see, putting a baseball or a satellite into orbit is purely amatter of

throwingorshootingitfastenoughthatitstrajectorywillmatchthecurvatureofEarth.Thatspeed,theescapevelocity,is6.96milespersecond(11.2kilometersper second) or just about 25,000miles per hour (40,000kilometers per hour).AnyslowerthanthatandgravitywillbringtheobjectdownbeforeithasgonefullcirclearoundEarth.Anyfasterthanthatanditwillstillgointoorbit,butitwillreachahigheraltitudeabovethesurfacebeforegravitywinsoutandbendsitstrajectorytothecurvatureofEarth.Inaveryrealsense,theorbitingbaseballorsatelliteneverdoesstoptryingto

fall to thesurface; it's just that it isgoingfastenough“outward” tocounteractgravity'sinwardpull.That'swhyphysicistsandspacescientistssaythatanorbitingsatelliteorspace

shuttleisincontinuousfreefall,fallingfreelytowardthecenteroftheEarth,justasifithadbeendroppedfromaheight.Andthat'swhytheastronautsinsideanorbitingshuttlefloatfreelyintheair,justastheywouldiftheywereinafallingelevatorwhosecablehadsnapped.

YouDidn'tAsk,but…

IfEarthisspinning,whydoesn'ttheatmospheregoflyingoffintospace?

Inordertoleavethisplanet,theair—justlikeanythingelse—wouldhaveto

be moving at a speed equal to the escape velocity. That would amount to ahumongous wind. While Earth's motion does affect the winds, the effect isnowherebigenoughtogetthemtoblowasfastastheescapevelocity.Individualairmoleculesmayreachescapevelocity,however,andsomeofthe

lightestatomssuchashydrogenandheliumdoindeedgointoorbitatthetopoftheatmosphere.

EavesdroppingontheLake

Sometimes when I'm in my summer cabin on the lakeshore atnight, I can hear actual conversations of people on the oppositeshore,eventhoughit'shalfamileormoreaway.Howcome?

It'sasifthelakemagnifiesthesoundsomehow,isn'tit?Butitisn'tactually

magnifyingthesound,asamicrophoneandamplifyingsystemwoulddo;it'sjustthatmoreofthesoundisbeingfunneledtowardyourears.Soundconsistsofvibrationsoftheair.Theguyontheothersideofthelake

makessoundsbyforcingairfromhislungsoverhisvocalcords,whichmakesthemvibrate.They, in turn,make theair exitinghismouthvibrate.Heshapesthesevibrationsintowordswithhislipsandtongue,andthemodifiedvibrationsare transmitted through the air to you as air-pressurewaves, similar to ripplesmovingacrossthesurfaceofwater.Asyou can seebydropping a rock into a quiet pool ofwater,waterwaves

spreadoutequallyinalldirections.It'sthesamewithsoundwaves,butinthreedimensions; they spread out through the air in all directions: up, down, north,east,southandwest.Naturally,whenyou'reatsomedistancefromthespeakeryouwillbeabletohear—thatis,yourearswillintercept—onlyasmallfractionofthespreadingwaves.Thefartherawayyouare,thesmallerthefractionofthetotal sound energy your earswill be able to intercept, becausemost of it hasgone in other directions and the farther away you are, the more “otherdirections”thereare.Athalfamileaway,thefractionthatreachesyourearsisusually so small thatyoucan'thear theguyat all ifhe's speakingat anormalconversationallevel.The unusual effect that you're describing has to dowith the fact that sound

travelsslightlyfasterinwarmairthanincoolair.That'sbecauseairmoleculescantransmitvibrationsonlybyactuallycollidingwithoneanother,andwarmermoleculescollidemorefrequentlybecausethey'removingfaster.Sowehaveto

take a close look at the temperature of the air above the lake, to see whattemperatureeffectstheremightbeandhowtheymightaffectthesound.During the day, the sun had been beating down on the air and water. But

comparedwithair,water isveryhardtoheatup,so thewaterremainedcoolerthantheair.(Youmayevenhavejumpedintothelaketocooloff,right?)Thecoolwater cools the layer of air immediately above it, so that there is now alayerofcoolairbeneaththeupperlayersofwarmerair.Andifthereisnowindtoscrambleuptheairlayers,they'llstaythatwayintotheevening.You, at the edge of the lake, are prettymuch in the cool layer. The sound

comingfrombigmouthacrossthewatertravelsmostlythroughtheupper,warmlayer, but when it gets to you it encounters cooler air and slows down. Thissuddenslowingdownofthesoundwavesmakesthembenddownward;theyarerefracted, just as lightwaves are bent downwardwhen they are slowed downwhilegoingfromair intowater.Youcan thinkof itas thefaster,uppersoundwaves overtaking the slower, lower soundwaves and tumbling over them, sothat the sound spills downward.Thus, anunusual numberof soundwaves areaimeddownwardtoyourearsandyouhearmorethanyouhaveany“right”tohear,basedsolelyuponyourdistance.Ofcourse,thisworksbothways.Sowhenyou'resittingonyourcabinporch

in the early evening hours on a calm, summer day, watch what you say—especiallyaboutthatjerkontheothersideofthelake.

ListenFast!

IfIcoulddrivemycarfasterthanthespeedofsound,wouldIstillbeabletoheartheradio?

As your question implies, this is purely an exercise in “What if?”

Automobiles,ofcourse,aren'tbuiltsleekenoughorstrongenoughtoexceedthespeedofsoundortowithstandthephysicalstressesofthesoundbarrier.Butit'sfuntothinkabout.Theanswerissimple:Yes.Or,Icouldhaveposedadifferentquestionthatwouldsettletheissue:Onthe

supersonic Concorde airliner, can the passengers converse? At those pricesthey'dbetterbeableto.Buthow,iftheyaretravelingfasterthansound?Evenifyouweredrivingfasterthanthespeedofsound,youandthecarand

theradioandyourterrifiedpassengerswouldallbemovingatexactlythesamespeedrelativetothecountryside.You'reallinthesameboat,sotospeak.Asfarassoundisconcerned,theimportantthingtorealizeisthatyouandtheradioandtheairinbetweenaren'tmovingrelativetooneanother.Theradiohasthesamespatial relationship to you as if the car were standing still. It emits its soundwaves through the car's air to your earswith the speedof sound as if nothingunusual were happening, because inside the car, nothing is. In fact, if thespeedometerandwindowswereblackedout(Godhelpyou),youwouldn'tevenknowyouweremovingexceptforthenoiseandvibrationfromthewindandthetires.Whatifyouweredrivingasupersonicconvertiblewithnowindshieldandthe

radiospeakerintheback?Couldyoustillhearit?No.Notevenconsideringtheeffectsofthewindonyourpoor,batteredearsandbrain,youwouldn'tbeabletoheartheradio.Thesoundwavesfromthespeakerarebeingtransmittedthroughthe air toward you at the speed of sound, but the air itself—the transmissionmediumforthesound—ismovingbackwardawayfromyouevenfaster.Sothe

soundwillneverreachyou.Thesoundislikearowboatrowingupstreammoreslowlythanthewaterisflowingdownstream.By theway, the radio receives its signalsby radiowaves,not soundwaves,

andradiowavestravelatthespeedoflight,whichisamilliontimesfasterthanthespeedofsound.Soanymotionofyourcariscertainlynotgoingtohaveanyeffectontheradio'sabilitytoplay.Nowwhataboutthesoundsthatescapefromyourcar?Whatwouldaroadside

cowhear?(You'renotdoingthisoncitystreets,Ihope.)Your car noises,whether from radio, tires, engine or screaming passengers,

are being sent out in all directions at the speed of sound. But you areapproaching the cow faster than that; you are actually outrunning your ownsound.Asyourcarapproachesthecow,then,shecanhearnoneofthecarnoisesthat are trailing behind you until shortly after you pass,when shewill hear asonicboomandallthecarnoise.Notethatifyouareoutrunningsound,youwon'tbeabletohearanysounds

comingfrombehindyou,becausetheycan'tcatchupwithyou.That'swhyyoucanseetheflashinglightsonthatpolicecarthat'schasingyou,butyoucan'thearthesiren.Idoubt,however,thatthetrooperwillacceptthatasanexcuse.

1 The following treatment of airplane flight is based upon DavidAnderson and Scott Eberhardt's article “How Airplanes Fly: A PhysicalDescription of Lift” (Sport Aviation, February 1999), which was pointedouttomebyRichardE.Eckels.

… And God said, “Let there be ultraviolet, visible and infrared

radiation.”Well,that'snotanexactquote,butitcertainlywasagooddecision.The

Lord'sLightbulb,thesun,isthesourceofnotonlylight,butalltheenergyweuseonEarth,withtheexceptionofenergyfromnuclearreactors,whichhumans invented in1942,andEarth'sowndeep-downheatenergy,whichweareonlynowbeginningtotapforpracticalpurposes.ButthemostapparentrolethatOldSolplays—theonlyone,infact,that

mostpeopleever thinkabout—is that itprovides the lightwe seeby, thepurifyinglightofdaythatbrightensandilluminatesallofEarth.Whenanylight—solarorartificial—strikesanobject,someofitbounces

off (is reflected), some of it is absorbed and transformed into heat, andsome of itmay even go straight through, as in the case—fortunately—ofair,waterandglass.Thischapterisabiographyoflight—whatitismadeof,whereitcomes

from and goes to at its incredible speed of 186,000miles per second (3million kilometers per second), and how it can entertain us, trick us andburn us. Aswe follow this path of enlightenmentwe'll have occasion toplay in the snow, go to the movies, watch television with a magnifyingglass,coolourselveswithanelectricfan,foolaroundwithmirrorsandeveneatsomecandythatmakessparksinthedark.

BrighterThanBright

ThosebrilliantDay-Glocolors—they'reunreal!Howcantheybeso much brighter than anything else? They look as if they'reactuallygeneratingtheirownlight.

Theyare.InaDay-Glo-coloredobjectthere'sachemicalthattakesinvisibleultraviolet

radiationoutofthedaylightandconvertsitintovisiblelightofthesamecolorastheobject.Thus, theobject isnotonlyreflecting itsnormalamountofcoloredlight,butisactivelyemittingsomelightofthesamecolor,whichmakesitlook“extra-colored”anduptofourtimesbrighter.The Day-Glo Color Corporation of Cleveland is only one manufacturer of

what are called daylight fluorescent pigments. As the self-proclaimed world'slargest supplier, itmakes a dozen different colors, from aurora pink to saturnyellow. It sells thepigments to companies thatput themand similardyes intoeverythingfromorangesafetyvestsandtrafficconestoyellowtennisandgolfballsandhighlightingpens.What'sgoingonis fluorescence,anaturalprocessbywhichcertainkindsof

moleculesabsorbradiationofoneenergyandre-emit itasradiationofalowerenergy.Themoleculesinthepigmentareabsorbingultravioletradiation,akindofshort-wavelengthradiationthathumaneyescan'tsee,andre-emittingitasalonger-wavelength light that human eyes can see. The radiation is, in effect,shiftedfrominvisibletovisible.How domolecules absorb and re-emit radiation?Molecules contain lots of

electrons that have certain specific amounts of energy characteristic of theparticular molecule. But these electrons are always willing to take on certainamountsofextraenergyfromoutside.(Formoreonthispoint,meetmeintheNitpicker'sCorner.)Amoleculeofa typicalpigmentmaycontainhundredsofswirlingelectrons

ofvariousenergies.Whenabulletofultravioletradiation(Techspeak:aphoton;)hitssuchamolecule,itmaykicksomeofthoseelectronsuptohigherenergies.(Techspeak: The electrons become excited; honest, that's what scientists say.)Buttheycanholdontotheiroverabundanceofenergyforonlyafewbillionthsofasecond(afewnanoseconds)beforespittingitbackoutasradiationagain—usuallyasseveralphotonsof lowerenergiesor longerwavelengths. It'ssortoflikespittingoutbuckshotafterstoppingabullet.Nowthe“buckshot” radiation,havingsomewhat lessenergy thanultraviolet

radiation, falls into the region of radiations that human eyes can see: coloredlight.Thenetresultisthatthepigmentmoleculehasabsorbedinvisibleradiationandre-emitteditasvisibleradiation.Aslongasthepigmentmoleculesarebeingexposedtoultravioletradiation—

anddaylightcontainslotsofit—theywillbeabsorbingitandemittinglightofavisiblecolor.Ifthepigmenthappenstobeorangetobeginwithandtheemittedlight is also orange, the dyed object will be an unnatural super-orange—“oranger”thanyouthinkithasanyrighttobe.

TRYIT

Shine an ultraviolet lamp—a so-called black light lamp—on a Day-Glofluorescentobject,suchasapaperwithafewstreaksoffluorescenthighlighteron itor, ifyou're the typewhowears them,yourDay-Glo-printedT-shirt.Thefluorescentdyewillglowverymuchbrighterthanitdoesindaylightbecausethelamp puts out much more ultraviolet radiation. If you don't want to buy anultravioletlightbulb,takeyourstreakedpaperorT-shirtintooneofthosetackystoresthatselltastelessgiftsandfluorescentposters,andusetheirblacklightforfree.

By the way, if you use a fluorescent yellow highlighter on your books ornotes, remember that it is brightest in daylight, which contains plenty ofultraviolet. Ordinary household incandescent lightbulbs give off very littleultraviolet light;moreover, their light is somewhat yellowish, and thatwashesout the yellow highlighter color. So when reviewing your highlighted bookpassagesornotesbylamplightthenightbeforeanexamorapresentation,youmayfindtoyourchagrin thatyourhighlightingisallbut invisible. It'ssafer tousethestrongerhighlightercolors:orange,greenorblue,whetherfluorescentornot.

Inmyworkasaprofessor,justabouttheonlyexcuseIhaven'theardfroma

student who did poorly on an exam is that the highlighting on his notesdisappeared.

YouDidn'tAsk,but…

Whydoesawhiteshirtglowbrightlyunder“blacklight”?

It'sthesamefluorescencephenomenonastheDay-Glocolors.Mostlaundry

detergents contain “brighteners” that absorbultraviolet radiation fromdaylightand re-emit the energy as a bluish light thatmakes the shirt look “whiter andbrighter.”Moreover, the blue cancels out any yellowish cast.When stimulated by an

ultravioletlamp,whichisevenricherinultravioletradiationthandaylightis,thefluorescence becomes bright enough to see as an actual glow-in-the-darkluminescence.

YouDidn'tAskThisEither,but…

Howdothoseluminouslightstickswork?

You mean those plastic rods full of liquid chemicals that are made by

Omniglowandothercompaniesandaresoldatstreetfairs,festivalsandconcertsandthatstartglowingwithgreen,yelloworbluelightwhenyoubendthem,andthatgraduallylosetheirlightafteranhourorso?Neverheardofthem.Okay,seriously.Bynowyouknowthatafluorescentdyeneedstobestimulatedbyabsorbing

energybeforeitcanre-emitthatenergyaslight.Butthestimulatingenergyneednot be visible light or ultraviolet radiation; it can also be heat, electrical orchemical energy. In the case of the light sticks, the stimulating energy ischemical.Whenyoubendthestick,youbreakathinglasscapsulecontainingachemical, usually hydrogen peroxide, that reactswith another chemical in thetube.Thereactiongivesoffenergy,whichistakenupbyafluorescentdyeandre-emittedas light.As thechemical reactiongraduallyplays itselfoutbecausethechemicalsareusedup,thelightfades.

NITPICKER'SCORNER

At several places in this book I talk about a substance's absorbing certain

colorsorwavelengthsoflight.Youmaybewonderinghowmoleculesactuallyabsorb light, and what determines which wavelengths they absorb. If thatproblem is not exactly keeping you awake at night, Nitpicker's Corners aredesignedtobeskippable.Amoleculehascustodyofalltheelectronsthatbelongtotheatomsthatitis

madeof. (Moleculesarenothingbutatomsglued together.)Butelectrons,andforthatmatterallsubatomicparticles,haveapeculiarproperty:Theycanhaveonly certain amounts of energy and no others. (Techspeak: The electrons'energiesarequantized.)Forexample,theelectronsinacertainkindofmoleculecan have energies A, B, C or D, etc., but never A-and-a-half or C-and-two-thirds.TheycanchangetheirenergiesupordownamongthevaluesA,B,C,D—thatis,fromAtoBorfromDtoCandsoon—buttheycanneverhavevaluesanywhereinbetween.Nobodycangiveyouareasonforthis;that'sjustthewayit is.Whenyougetdownto thingssmaller thananatom, it'sadifferentworldfromtheoneweseeeverydayuphereinbig-land.Now inasmuch as each unique substance is made up of its own unique

molecules, it will have its own unique collection of electrons with their ownuniquesetsofallowableenergies.Whenlightenergyfallsuponthesubstance,itselectronswillabsorbonly thoseenergies thatcorrespond to itsallowedenergyjumpsfromAuptoBorC,etc;itwillrejectandreflecttherest.Thismeansthatthe substance is actually picking out the light energies (wavelengths) that itprefers,leavingtheotherstobouncebackasreflectedlight.Andthat'swhyeverycoloredsubstancehasitsowncolor:thecolorofthose

wavelengthsthatitcannotabsorbandthatitreflectsbackforoureyestosee.

SnowWhiteandtheSevenHues

Whyissnowwhite?It'smadeofwater,andwateriscolorless.Sohowcomeitturnswhitejustbyfreezing?

First,wehavetolookatwhat“white”is.You'veheardpeoplesaydozensoftimesthatwhitelightisthepresenceofall

colors.Butotherpeopletellyouthatwhiteisn'tacoloratall,thatit'stheabsenceof color. You use bleach to remove all color from your laundry andmake itwhite,don'tyou?Sohowcanwhitebebothallcolorsandnocolor?Theansweristhatthesetwogroupsofwell-meaningpeoplearetalkingabout

twodifferentthings:whitelightandwhiteobjects.Whitelight,asitcomestousfromthesun,isindeedamixtureofallpossible

colors—all visible wavelengths. Because we “grew up” as a species withsunlightasournatural,neutral,everydaylight,wenamedit“white,”awordthathasitsoriginsinanIndo-Europeanwordmeaning“bright”or“gleaming,”withnocolorimplicationsatall.Sowhitelightiscolorlesslight—tohumaneyes.Butin1666SirIsaacNewtondiscoveredthatthisneutrallightcanbebroken

down into a rainbow of component colors, simply by passing it through atriangularchunkofglass—atriangularprism.Hethenprovedtohimselfthatallthesecolorswereindeedpresentintheoriginalwhitelightbyrecombiningthem:Heprojectedoverlapping rainbowsontoawallandsaw that theycombined toformwhitelight.Newton thought it would be a nice idea to divide the entire rainbow or

spectrumofcolors(andheinventedtheword“spectrum”for thepurpose) intoseven categories that would be analogous to the seven musical tones in anoctave.Forhiscolorcategories,hechosered,orange,yellow,green,blue,indigoand violet. Unfortunately, more than three centuries later we are still beingtaughtinschoolthattheseare“thesevencolorsoftherainbow”—eventhoughnobodyseemstoknowwhat“indigo”is.SirIsaachadtofudgeabittoekeout

sevencolornames.Inreality,thereareaninfinitenumberofcolors—bothvisibleandinvisibleto

humaneyes—inthesun's light, justas thereareaninfinitenumberofpossiblemusical tones. Change the wavelength of light or sound by an infinitesimalamount and you've got a brand-new color or tone, irrespective of whetherhumans can detect the difference. For example, there are dozens of differenthues that we lump under the term “red,” limited only by our eyes' ability todistinguish them. It is said that the human eye can distinguish as many as350,000differenthues.(Whoseeye?Iwonder.)Awhiteobject,asdistinguishedfromwhitelight,iswhitebecausewhenwhite

lightfallsuponit,itreflectsallthosezillionsofcolorsbacktooureyesequally,withoutchangingthecompositionofthemixtureatall.Itsmoleculesjustdon'thappentobeabsorbersofvisible light,so itappears tobe thesame“color”asthelightthatfelluponit:whatwechoosetocall“white.”Theobjectcontributesnocolorofitsown.But colored objects are indeed contributing colors of their own. Their

moleculesareselectivelyabsorbingandretainingcertainofthesunlight'scolors,reflectingbacktheothersasanalteredmixture.Thinkofanactoronthestage,wearingaredcapeoverawhiteshirt.Ifyou

shinearedspotlightonhim,hewillappearredallover,theshirtaswellasthecape.That'sbecausetheonlylightthatanypartofhiscostumecanreflectbacktous isred.Nopartofhimcanreflectbackanygreenorblue lightbecause itsimplyisn'treceivingany.Nowshineawhitespotlightonhim.Theredcape isstill red,because that's

thenatureofthedyeinit;thatparticularchemicalwaschosenbecauseitabsorbsallothercolorsinthewhitespotlight,reflectingbackonlythered.Butthewhiteshirtdoesn'tabsorbanyofthecolorsinthespotlight;ithasnoreddyeinit(untiltheactorsurreptitiouslyappliessomeinthestabbingscene).Theshirtjustsendsthewholemixtureofspotlightcolorsbacktous,lookingjustaswhiteaswhentheycameoutofthecan.Nowlet'sgetbacktothesnowbeforeitallmelts.Snowiswhite,younowknow,becauseitsmoleculesreflectbacktousallthe

colorsinthesunlight.Itdoesn'tselectivelyabsorbanyparticularcolors.“Butwaitaminute,”you'rethinking,“neitherdoesliquidwater;it'smadeof

the same H2O molecules. So why isn't liquid water as white as the drivensnow?”Becauseliquidwaterisapoorreflector.Whenlighthitsitsquarely,almostall

of it goes straight in—penetrates—rather than bouncing back. In otherwords,

liquid water is transparent. And if practically no light bounces back, it can'tdisplaymuchcolor,notevenwhiteness.Snow,ontheotherhand,isanexcellentreflectoroflight—whateverkindof

lighthitsit.Youwantgreensnow?Hey,Sammy!Turnonthegreenlights!It'sanexcellentreflectorbecause,unlikeliquidwater,whichpassivelyallowslightto penetrate it, snow consists of zillions of ice crystals, each one a tiny jewelwithdozensof sparkling facets that reflect light likemirrors.Allof thiswhitelight bouncing back to our eyes, with its full complement of original colorsintact,iswhatmakessnowappearevenwhiterthantheactor'ssweatyshirt.

YouDidn'tAsk,but…

Whatisblack?Isitacolor?

A black surface is one whose molecules are absorbing all visible

wavelengthsofthelightthatisfallinguponit,andreflectingvirtuallynoneofitback. So black isn't really a color, becausewe define a color in terms of thespecificcombinationoflightwavelengthsthatreflectbackintooureyes.But,ofcourse,youcanseeablackobject,soitmustbereflectingsome light

backtooureyes.Hey,who'sperfect?Thelightthatablackobjectisreflectingcomes from the fact that its surface has a small but unavoidable amount ofshininesstoit.Soitreflectsbacksomeofthelightthathitsitataglancingangle.That'swhy there are “light black” and “darkblack”objects, dependingon themicroscopicglossinessoftheirsurfaces.Gotoahardwarestoreandlookattheblackpaints;they'reallequallyblack,butthey'llrangeinreflectivityallthewayfromflattoglossy.

NITPICKER'SCORNER

I said thatwhen lighthits liquidwatersquarely,mostof itgoesstraight in

withoutbeingreflected.Theemphasiswasonthatword“squarely.”AsI'msureyou'veobserved,thesurfaceofwatercanbeaverygoodreflectoroflightthatishitting itobliquely—ataglancingangle.When the sun is lowovera lake, forexample,itsreflectiononthewatercanbealmostblinding.It'sthesamewithsnowflakes.Yes,they'remadeoftransparentcrystalsofice,

but because there are zillions of them, all with complicated shapes, scatteredhelter-skelterwiththeirsmoothreflectingfacetsfacinginalldirections,thelight

isalmostinvariablystrikingthefacetsobliquelyandbeingreflected.That'swhysnowissuchagoodreflector—sogood,infact,thatskiersandothermasochistswho like to frolic in frigid weather have to wear very dark glasses to avoid“snowblindness.”Afinalpoint. I letyougetawaywith thinking that liquidwater iscolorless.

It'salmostcolorless,butnotquite.

SchoolColors

In science class they told us that the primary colors are blue,greenandred.Butinartclasstheytoldusthattheprimarycolorsareblue,yellowandred.Whycan'tartistsandscientistsagree?

Becausetheythinkofcolordifferently.ScientistsdescribeobjectivelywhatNatureprovides.Theythereforethinkof

color as a fundamental characteristic of light itself. To a scientist, light ofdifferentcolorsisradiationofdifferentwavelengths.Artists,ontheotherhand,createtheirowninterpretationsofNature.Theythereforetendtothinkofcolorsubjectively, as something tobemanipulatedwithpaints anddyes, rather thanacceptinglightinitsnaturalstate.Why,then,dothesetwocampshavetousedifferentprimarycolors—triosof

colorsthatcanbecombinedindifferentamountstoproduceallothercolors?Inanutshell,it'samatteroftheprimarycolorsoflightversustheprimarycolorsofpigments. As we'll see, they can be called the additive primaries and thesubtractiveprimaries,respectively.Light-minded (not light-headed) scientists claim that they canmake lightof

anyperceivedcolorbycombiningblue,greenandredlightofvariousintensities.Ontheotherhand,pigment-mindedartistsclaimthattheycantintanobjectanycolor by combining blue, yellow and red pigments in various amounts. Andthey'rebothright,becausethereisafundamentaldifferencebetweenthecoloroflightandthecolorofanobject.Colored light is a certain color because it ismade up of amixture of light

wavesofvariouswavelengths.Thedifferent-colored componentsadd togethertoproducethenetcolor.Ithappensthat,becauseofthewayoureyeswork,blue,greenandredlightcontainallthenecessarywavelengthsthatneedtobemixedinorder toproduceanyperceivedcolor.Soblue,greenand redare called theprimarycolorsoflight.(Understood:forhumaneyes.)

A colored object, on the other hand, is a certain color because of thewavelengthsthatitabsorbsfromthelightthatisfallingonit.Inotherwords,itsubtracts certainwavelengths from the lightand reflects the restback tousasthe color thatwe see.Variousmixtures of blue, yellow and red pigments arecapableof absorbingalmost anycombinationofwavelengths.Soblue, yellowandredareconsideredtheprimarycolorsformixingpaintsanddyes.(Butseebelowforalittlehedgingaboutthesethreecolors.)Thelight-basedsystemofprimarycolorsiscalledadditivebecausedifferent

combinationsofwavelengths add together to producedifferent colors of light.The pigment-based system of primary colors is called subtractive becausedifferent combinations of wavelengths are absorbed or removed from light toproducedifferentcolorsofpaintsanddyes.Let's look at the light primaries first, then the primaries for objects or

pigments.Light:Thehumaneye—evenanartist'seye—worksontheadditiveprinciple.

Ithasthreekindsofcolor-sensitivecells(so-calledconecells)ontheretina:Oneismost sensitive toblue light, one togreen andone to red.Ourperceptionofvariouscolorsdependsontherelativedegreesofstimulationofthesethreetypesof cells by the incoming light; the brain adds them together to producesensationsofvariouscolors.That'swhyscientists—humanchauviniststhattheyare—choseblue,greenandredastheprimarycolorsoflight.(Muskratscientistsundoubtedly use a different set of primary colors.) Our eyes react only tostimulationsof those three color receptors, so theyare allweneed inorder toproduceallhumanlydiscerniblehues.Andthat'swhythereare three,andonlythree,“primary”orfundamentalcolorsoflight.Notethateachkindofconecellisnotsensitiveexclusivelytopureblue,green

orredlight;eachoneissensitivetoalesserdegreetotheothercolorsaswell.That'swhywecanseepureyellowlight,eventhoughwedon'thaveanyyellow-sensitiveconecells.Theyellowlightslightlystimulatesboththegreenandredcells,andourbrainsperceivethatcombinationasyellow.Your color TV and computer screens take advantage of the three-color

idiosyncrasy of human vision. They contain blue, green and red phosphors(chemicals that glow when stimulated by electrons), glowing with varyingbrightnesses.Theglowsalladdtogethertoproducethecolorsthatweperceive.

TRYIT

LookatthepictureonyourcolorTVorcomputerscreenwithamagnifying

glass. You'll see that it's made up of tiny blue, green and red rectangles—noother colors—that are being stimulated to glowwith varying brightness.Youreyeblends themall togetherbecause the individual rectanglesare toosmall toseeatnormalviewingdistance.Addedtogetherinthisway,theprimary-coloredrectanglesmakethehundredsofdifferenthuesthatyouperceive.

Pigments:Thecolorfilminyourcamera,ontheotherhand,makesitscolorsbytheartist'ssubtractivesystem.Itcontainsthreelayersofdyesthatabsorborfilteroutblue,greenandred.Andtheabsorbersorfiltersthatbestabsorbblue,green and redhappen to be yellow, red andblue, respectively.Soyellow, redandbluearethethreesubtractivecolorsincolorfilm.Butare thesecolor-filmfilters thesameoldyellow,redandbluecolors that

yourartteachertoldyouaretheartist'ssubtractiveprimarycolors?Sortof,butnotexactly.Here'sthehedgingthatIpromisedyou:Thethreecolorsthatarereallybestat

absorbingtheblue,greenandredthatourconesaremostsensitivetoareyellow,apurplishredcalledmagentaandagreenishbluecalledcyan.Yellow,magentaandcyanarethereforethethreerealprimarysubtractivecolorsthatareusedinconcoctingtheentirespectrumofink,photographyandpaintcolors.

Allartists,fromkindergartenkidswithcrayonstothesubtlestwatercolorists,

couldcreatetheirentirepalettesbymixingvariousamountsofyellow,magentaandcyan.Butit'saloteasiertobuypaintsandcrayonsalreadyblended.

LetThereBeFluorescence!

How do fluorescent lampsmake somuch light without a lot ofheat?Andwhenoneburnsout,canIreplaceitwithanytubethatfits,oraretheredifferentkinds?

Fluorescentlightswereinventedforonepurpose:toconfuseyou.I'mgladto

seethatthey'redoingtheirjob.When an ordinary incandescent lightbulb burns out you can just screw in a

new one with the help of a certain number of friends, depending on yourvocationorethnicity.Butwhenafluorescentlightburnsoutyoulookatthetubeto find out what kind to replace it with and you see markings that looksomethinglike“F20CW-T12.”Ifyoureplaceitwiththe“F15W-A10”thatyousawinthestore,willitexplodewhenyouturniton?Cheer up. 'Tis better to light a candle and read this book than to curse the

darkness.First,let'sdecipherthosehieroglyphicsonthetube.They'reasecretcodethat

divulges everything about thebulb.Not toyou, thepoor consumer, of course,buttothepeoplewhomakeandsellthem,whoapparentlyhaveaneedtoappearsmarterthanyouare.I'mgoing to tellyouhowthesecretcodeworks. (I suppose thatnowthey'll

havetokillme.)Anygivenfluorescenttubeiseitherstraight,U-shapedorcircularinshape;it

has a certainwattage; it gives off a certain color of light; and it has a certaindiameter.Thelettersandnumbersonthetubegivethisinformationinthatorder:shape,wattage,color,diameter.Theonlytroubleisthatyouhavetoknowhowthisinformationiscoded.Forshape, it'saUoraCforU-shapedorcircular,andno letteratall if it's

straight.Thencomesthewattage:4,5,8,13,15,20,30,40orwhatever. (Thewattage is generally lower than for comparable light-producing incandescents,

because fluorescent lighting is from two to four times more efficient.) Thencomes the color code:W forwhite,CWfor coolwhite,WWforwarmwhite,plus abbreviations for other exotic colors that we needn't bother with. Lastcomesthetube'sdiameter,butitisgiven—wouldyoubelieve?—ineighthsofaninch:T8means a tube that is eight-eighths of an inch in diameter,which anysanehumanbeingwouldcalloneinch.AT12tubeistwelve-eighthsorone-and-a-halfinchesindiameter,andsoon.Pop Quiz: Describe the properties of an F40CWT10 fluorescent bulb.

(Answerattheendofthissection.)Oh,Iforgottotellyou:ThecodesalwaysbeginwithanFfor“fluorescent,”

presumablytokeepyoufromtryingtoscrewthemintoanordinarylampsocket.(Howmanyidiotsdoesit taketoscrewafluorescenttubeintoanincandescentlampsocket?)Asanalertconsumer,youmayhavenoticedthatyoucan'treplacean18-inch-

longtubewithonethatis24incheslong.Themanufacturersgraciouslygiveyouenough credit tomake that decision on your own, so youwon't find a lengthcodeonthetubes.Okay,now.Howdothethingswork?Youknowthatordinary incandescent lamps, includinghalogenlamps,make

light by electrically heating a filament towhite heat. The outside of the lampbulbcangetup to temperaturesofseveralhundreddegrees.Fluorescent lampsworkonanentirelydifferentprinciple.Thefluorescenttubeisfilledwithasmallamountofinertgas(usuallyargon)

plusadrop'sworthofmercury.Ateachendisasmallfilamentthatisheatedbytheelectriccurrentsothatitemitselectrons.(Youdon'tknowwhyahotfilamentemitselectrons?Gostandinthecorner.TheNitpicker'sCorner,thatis.)Theelectronsemittedfromthefilamentsflythroughthegasinthetubetoget

from one filament to the other, and in the process they collide with mercuryatoms,which have been vaporized by the filaments' heat. Themercury atomsabsorbthecollisionenergyandspititoutagainaslightenergy.Butwecan'tseethat light because it's in the ultraviolet region ofwavelengths, so it has to beconverted into light that humans can see. This is accomplished by that whitecoatingontheinsideofthetube.Itconsistsofchemicals(calciumandstrontiumphosphates and silicates) that absorb ultraviolet light and re-emit it as visiblelight;thiswavelength-shiftingprocessiscalledfluorescence.Fluorescentlampsarecoolerthanincandescentlampsbecausetheyhaveonly

thosetwolittlemildlyheatedfilamentsattheendsandthefluorescenceprocessitselfdoesn'tproduceanyheat.But they'rehard tostart,because thefilaments'electronsfirsthavetoblasttheirwaythroughthegasintheentirelengthofthe

tube.Thatrequiresseveralhundredvoltsofpush,butourhouseholdvoltageisonly 115 volts. So something has to provide an initial voltage kick to theelectrons.That'swhat the starter does—or the ballast. And here'swhere it gets really

confusing, because there are several kinds of fluorescent lamp systems andcircuits. Some have ballasts, those heavy little iron transformers,while othershavestarters,thoselittlealuminumcans.Andsomehaveboth.Fuhgeddaboudit.Youdon'thaftaknow.Whattodowhenyourfixtureofunknownbreedwon'tlightup?First,replace

thetubewithonethathasidenticalcodenumbersonit.Youcan'tevensubstitutea different wattage, as you can with incandescent bulbs; that can causedangerousoverheatingoftheballast,whichwasdesignedfortheotherwattage.Theonly freedomyouhave is to swapa coolwhite for awarmwhiteor viceversa,ortosubstituteoneofthemanyother“deluxe”colors.Ifyourfixturehasone of those little starter cans in it, youmay aswell replace that too; they'recheapandtheysimplytwistinandoutofthesocket.If you're still in the dark, both literally and figuratively, buy a whole new

fixture.Oh, and an F40CWT10 is a 40-watt, cool white, one-and-a-quarter-inch

straightfluorescenttube.

YouDidn'tAsk,but…

Whydo small fluorescent tubes cost somuchmore than thebigfour-foot-long“shoplights”?

You can buy the common, 48-inch shop-light tube in home centers for a

couple of dollars,whereas a small, thin fluorescent tube for under the kitchencabinetmight cost up to five times that amount. The answer is that the four-footers,usedby the thousands inschools, factoriesandofficebuildings,vastlyoutsell the smaller, more specialized tubes and are mass-produced at a muchlowerunitcost.It'saclassictextbookcaseofsupplyanddemand.

NITPICKER'SCORNER

Why do the filaments in a fluorescent tube emit electrons when they're

heated?Almost anything will emit electrons if heated hot enough. Atoms contain

negatively charged electrons, which are held on to with various degrees ofstrength,dependingonwhichatomswe'retalkingabout.Metalatomsholdontotheirelectronsveryloosely.Whenyouheatametal,someofthoseelectronsgainenoughenergytodetachthemselvescompletelyfromtheiratomsandgoflyingoff.Inafluorescenttubetherearetwofilaments,oneateachend,bothgettinghot

becauseoftheirresistancetotheflowofasixty-cyclealternatingelectriccurrent(a current that continually reverses its direction). At any given instant, onefilamentisnegativelychargedwithrespecttotheother,butahundred-twentieth(halfasixtieth)ofasecondlater itbecomespositivelychargedwithrespect to

theother.Atanyinstant,theelectronsfromthenegativefilamentareattractedtothepositivefilament,andtheonlywaytheycangetthereistoplowthroughtheinterveningmercuryvaporinthetube,makingitemitultravioletradiation.

Star Light, Star Bright, Which Bulb Should I UseTonight?

What'ssospecialabouthalogenlightbulbs?

They contain a gas called a halogen, which makes them brighter, whiter,

moreefficientandlonger-lasting.And,ofcourse,muchmoreexpensive.A halogen lamp is a variation on the standard incandescent, as opposed to

fluorescent, lamp.An incandescent lampcontainsa tungstenfilamentenclosedin a glass bulb filled with gas. An electric current heats the filament toincandescence—awhite-hotglow.Itmaylookverybright,butinrealityonly10to 12 percent of the energy it emits is visible light; about 70 percent of it isinvisibleinfraredradiation,whichheats,ratherthanilluminates.Inaregularbulb,thegasinsideisaninert(unreactive)onesuchasargonor

krypton with some added nitrogen. These inert gases keep the tungsten fromoxidizing, or “burning up,” as it would in air. Some smaller bulbs solve theproblembybeingcompletelyevacuated;there'spracticallynogasinsideatall.In a halogen bulb, the gas is usually iodine or occasionally bromine, two

highly reactive chemical elements in the family that chemists call halogens.Theyperforma two-stepchemicaldance thatmakes the filament last twiceaslong.Butfirst,wehavetounderstandhowthestandardbulbworks.Thefilament isacoilof thin tungstenwire.Tungsten isusedbecause ithas

thehighestmeltingpointofallmetals—6200degreesFahrenheit(3400degreesCelsius)—and it stays strong even at white-hot temperatures of 4500 degreesFahrenheit(2500degreesCelsius)orhigher.Moreover, ithasthelowestvaporpressureofallmetals,meaningthatitevaporateslessthananyothers.Yes,evenmetalsevaporateafewatomsnowandthen,butsoslowlythatwenevernoticeitexceptatveryhigh temperatures. (Never fear;yourgold jewelry isn'tgoing todryup.)Whenitiswhite-hot,eventungstenwillevaporateenoughsothatthefilament

gets thinner and thinner as the bulb burns, until it finally breaks apart andinterrupts the electric circuit.That'swhenyour bulbburnsout.For some timebeforethisdisasterstrikes,youcanseetheevaporatedtungstenasadarkcoatingon the inside of the glass, where it has condensed because of the glass'srelatively low temperature.Thisdarkening,ofcourse,progressivelycutsdownontheamountoflightthatthebulbputsoutasitages.Sometimesabulb'sfilamentwillhavedevelopedsuchathinspotthatitwill

blowoutsuddenlywhenyouturnontheswitch.Theblueflashthatyouseeisanelectric arc, leaping across the widening gap as the thin spot evaporatescompletelyundertheheatstressofthepowersurge.Tip:Whenabulbburnsout,trytappingorshakingitgentlywhilethepoweris

on.Sometimesyoucangetthebrokenendscloseenoughtogethersothatanarcwill flow between them and weld them back together, rewarding you withperhapsanhourorsooflife-after-deathexperience.What halogen-filled bulbs do is to cut down the evaporation rate of the

tungsten in a very interesting way. First, the iodine vapor reacts with theevaporated tungsten atoms before they can condense out on the glass andconvertsthemtotungsteniodide,agaseouschemicalcompound.Themoleculesof tungsten iodide then float around inside the bulb until they happen toencounter thewhite-hotfilament,whereuponthehightemperaturebreaks thembackdownagain into iodinevaporandmetallic tungsten,whichdeposits itselfbackonthefilament.Thereleasediodineisthenfreetoapprehendanddelivermoretungstenatoms,andthecyclecontinues,withtheiodineatomscontinuallycapturing evaporated tungsten atoms and returning them to the filament. Thisrecyclingprocess approximatelydoubles the life of the filament, andhenceofthebulb.The halogen process allows the lamp to be operated at a much higher

temperature without excessive deterioration of the filament, and that makes abrighter,whiterlight.Infact,thetemperatureofthebulb'sinsidewallhastobehigh—aboveabout480degreesFahrenheit (250degreesCelsius)—tokeep thetungstenatomsfromcondensingonitbeforetheiodinevaporcangrabthem.Halogen bulbs are made of quartz, which withstands much higher

temperatures—and is more expensive—than ordinary glass. They are usuallytube-shaped and closely surround the filaments to stay hot. In fact, tungstenlampsburnsohotthattheycanbeafirehazardifusedtooclosetoflammablematerialssuchascurtains.

YouDidn'tAsk,but…

Whydon'tlightbulbslastlongerthantheydo?

Lightbulbsareverycarefullyengineeredtolastforacertainlengthoftime.

Asuspiciouspersonmightbetemptedtosaythattheyarecarefullyengineeredtoburn out after a certain length of time. There is no reason that a lightbulbcouldn'tbedesignedtolastalmostindefinitely.Butyouprobablywouldn'tlikeit.As with most devices, there is a trade-off among several conflicting

considerations.More than anything, the life of a bulb depends on the runningtemperatureof thefilament.Foragivenwattage(theamountofelectricpowerconsumption), the higher the temperature and light output, the shorter thelifetime.“Long-life” lightbulbs have filaments that are designed to glow at a lower

temperature. But the lower temperature doesn't produce as much light. Also,sincehigher temperaturesproduceabluer,whiter light, the long-lifebulbscanhaveaslightlyyellowishcastbycomparison.Long-life bulbs achieve their lower temperatures by using a filament that

allowslesselectricalcurrenttopassthrough.Lesscurrentflowmakeslessheatandlesslight,soyougetnotonlyayellowerlight,butlessofit.Ifyoubuylong-lifebulbs,youhavetobuyahigherwattagethanusualtogettheamountoflightyouexpectfromanormalbulb.By law, the packaging of standard lightbulbs must tell you the number of

hours they are intended to last and the amount of light they put out in alldirections:thenumberoflumens.Comparethenumbersofhoursandlumensona long-life package with the numbers on a regular package of comparablewattage. If you'rewilling toputupwith the lesser amountof light andhigherpricefortheconvenienceofnothavingtochangethebulbforalongerperiodoftime,buythelong-lifer.

On the other hand, if you're a compulsive discount shopper for standardlightbulbs, takeyourcalculator to thestore.Foragivenwattage,youwant themost lightfor thelongest timeat thelowestprice.Dividethepriceincentsbythe number of lumens, and then divide the result by the number of hours ofexpectedlifetime.Thesmallestnumberisthebestbargain.Andspeakingofsavingmoney,adimmerswitchreducesthevoltageapplied

to the bulb, which reduces the current flowing through the filament, whichreduces the temperature, which reduces the evaporation of tungsten, whichconsiderablyincreasesthelifetimeofthebulb.Thenextbestthingtoturningoutthelightswhenyouleavearoomistodimthem.

Mirror,Mirror, on theWall,HowComeYouDon'tInvertatAll?

When I look in themirror and raisemy right hand,my imageraisesitslefthand.Andyetbothourheadsarestillontop.Whydoesamirrorreversethingsrighttoleft,butnottoptobottom?

This isoneof those loadedquestions that candriveyoucrazybecause the

question itself ismisleading. It startswith amistaken assertion and asksus tocarryonourreasoningfromthatpoint.Butyoucan'tpursuetheroadtotruthifsomebodystartsyouoffinthewrongdirection.Amirrordoesnotreversethingsrighttoleft.Itreversesthingsfronttoback;

itreversesinandout.Readthatagain.Andthinkaboutit.Allamirrorcandoisreverseadirection.Itcan'trotateanything.It'syouthat

imaginesyourselfrotated.Themirrordidn'tdoit.Stand in front of a full-length mirror. Let's name the person in the mirror

Egami.Now how do you thinkEgami got thatway,with his left arm towardyourrightandhisrightarmtowardyourleft?I'llbetyousevenyearsofbadluckthatyouthinkEgamigotthatwaybyyourturningaround—byyourrotatinghalfa turn, executinganabout-face.That'swhyyou think right and lefthavebeenreversed.Youdidityourself,byturningyourselfaround—inyourimagination.Butthat'snotwhatthemirrordid.Allthemirrordidbytakingitsincominglightandshootingitbackatyouwas

toreversethedirectionofthelight.Egamiissimplyyouwithyour“toward”and“away” directions reversed. You are, of course, in the habit of looking awayfromyourself,butEgamiislookingtowardyou;ifyou'refacingnorth,Egamiisfacingsouth.Andwheneverapersonisfacingintheoppositedirectionfromyouandlookingtowardyou,hisleftarmwillbeonyourright,no?What'ssounusual

aboutthat?Norotationsorright-to-leftswapsareneeded.Notice that thewords“up,”“down,”“top”and“bottom”appearnowhere in

theforegoing.They'recompletelyirrelevanttotwopeoplewhoarefacingeachother.“Up”and“down”meanexactlythesamethingtobothofthem.Unless,ofcourse,oneofthemisstandingonhishead.Howcanwegetoneofthemtostandonhishead?Easy.Holdthemirrorhigh

aboveyourheadandparalleltothefloor.Orelseputthemirroronthefloorandstand near (not on!) it. Egami is now standing on his head, isn't he? Whichproves that themirror reversesonly its inandoutdirections,which fromyourcurrentviewpointjusthappenstobeupanddown.Youcanseethesameup-downreversal in themirrorlikesurfaceofasmall,

calmlakeorpond.Lookatthereflectionofthetreesontheotherside.They'reupsidedown,aren'tthey?And by the way, I've referred to Egami with masculine pronouns to avoid

“him-or-her”-ing all over the place in an explanation that you may think isalready complex enough. If you're female, please don't think I'm saying thatmirrorsreversegender.(Anddon'twearaskirtwhenyouputthatmirroronthefloor.)Oh, thename? Ifyouhaven'tyet figured itout,Egami is “image,” reversed

fromrighttoleft.

YouDidn'tAsk,but…

When I look into the bowl of a shiny spoon, my image is bothreversed right to left and invertedupsidedown.Howdoes it dothat?

Ijustfinishedexplainingthattheimageisn'treallyreversedrighttoleft,so

let'sputthataside.Butindeed,howabouttheupside-downinversion?Thespoon'sinnersurfaceisconcave—thatis,itishollowlikeacave.(That's

agoodwaytorememberthedistinctionbetweenconcaveandconvex.)Whenyoulookintothespoon,you'llnoticethatthetoppartisshapedsothatitreflectsitslightslightlydownward,likeamirrorheldhigh.Atthesametime,thebottompart is shaped so that it reflects its light slightly upward, like amirror on thefloor. These “high” and “low” reflectors give you a stand-on-the-head image,exactly as the above-your-head and on-the-floor mirrors did in the precedingexplanation.

Mirror,Mirror,ontheWall,Who'stheSharpestOneofAll?

I'mnearsighted.WhenIlookinthebathroommirrorwithoutmyglassesonIcanseemybeautifulfacequiteclearly,buteverythingelseintheroomisblurred.Shouldn'teverythingbeequallyclear,becausealltheimagesinthemirrorareequallyclosetomyeyes?

Thedistancefromyoureyestothemirrorisirrelevant.It'sthedistancefrom

your eyes to a given object that counts, just as it wouldwhen you look at itwithoutthemirror.The light reflected fromanobjecthas toget toyoureyes somehow,or else

youwouldn'tseeit.Thelightcomingfromthingsbehindyourbackwouldnevergettoyoureyesifthemirrorweren'ttheretoturnitaround.That'sallthemirrordoes: It takes light thatwould have passed you by and shoots it back at youreyes.Supposeyou'refacingthemirrorandlookingatanobjectbehindyou.Instead

ofcomingstraightfromtheobjecttoyoureyes,thelighthastopassyouby,gotothemirrorandthencomebacktoyoureyes.That'sagreaterdistancethanifyou had been facing the object, so it is even blurrier than if you had turnedaroundandlookedatitdirectly.Theimageofyourbeautifulfaceisalsoblurrierthanifyouwerelookingatitfromthepositionofthemirror.Thelighthastogofromyourbeautifulfacetothemirrorandbacktoyourbeautifuleyes—twiceasfarasifyouwerelookingatyourbeautifulfacefromthepositionofthemirror.This isallbasedon the fact that the fartherawayanobject is, the fuzzier it

willappeartonearsightedeyes.That'sgenerallytrue,andhere'swhy.Nearsightedeyesaregoodatfocusinglightraysthatarediverging,radiating

outinalldirections,astheyarefromanearbyobject.Butnearsightedeyesarenotsogoodatfocusinglightraysthataremoreorlessparallel,astheyarefrom

adistantobject.It'snotthatnearanddistantobjectsareshootingtheirlightoutdifferently; everyobject reflects light inmanydirections. (Rememberhowwedrew a shining sun in kindergarten, with all those rays coming out in alldirections?)Butwhenyou'refarawayfromanobject,youreyesareinterceptingonlyasmallfractionofthose“alldirections”rays.It'sasifalltheraysarenowcomingfromthesame,severelylimiteddirection,likeabundleofparallelsticks,all pointing from the object straight at you.And that's the situation—focusingparallelrays—thatnearsightedeyescan'thandlewell,sotheobjectisblurred.

TheyWent…Which-a-way?

Inwesternmovies,whydothestagecoachwheelssometimesturnbackward?

This is the only remaining artificiality in today's remarkable, computer-

drivenmovieeffects,whichcanmakeanythingimaginablelookreal,nomatterhow bizarre—except, ironically, an old-fashioned stagecoach wheel. You canalsoseetheeffectwithautomobilewheels,inthosetelevisioncommercialsthatshowthecarsspeedingalonganopenroad.Ifyouwatchcarefully,you'llsee that thewheelsgobackwardonlysomeof

thetime;atothertimestheylookasifthey'rerollingforwardratherslowly,andat still other times they seem to stop entirely, making the coach look like asleigh.It'sallamatteroftiming—thespeedoftherollingwheelcomparedwiththespeedofthecamera'spictures.Amoviecameratakesaseriesofstillpicturesattherateof24persecond,or

24“framespersecond.”FortunatelyforHollywood,ourslowhumanbrainscan'tassimilatesomanyseparatepicturesandweperceivethemasallruntogether,asiftheobjectsinthemwereprogressingsmoothlyfromonepositiontothenext.(Actually, it's our eyes that can't separate the images if they come too closetogether:Our brains are fast enough.But it still takesmy brainmore than anhour—ifthen—tounderstandwhat'sgoingoninsomemovies.)Let's say thatoneof thewagonwheel's spokes ispainted red.And let's say

that when the camera snaps picture number one, the red spoke is pointingstraightup,atthetwelveo'clockposition.Dependingonthespeedofrotationofthe wheel, when picture number two is snapped a twenty-fourth of a secondlater,theredspokemighthappentobecaughtintheoneo'clockposition—evenifithadmadeacoupleofcompleteturnsintheinterim.Thatmakesitlookasifithadmovedtotheright,orclockwise.Or,itmighthappentobecaughtintheeleven o'clock position, making it look as if it had moved to the left, or

counterclockwise.Asthecameracontinuestotakeits24picturespersecond,thered spoke—together with the rest of the wheel—will look as if it is movingcontinuously,eitherclockwiseorcounterclockwise.Forextracredit,asweprofessorsliketosay,canyoufigureouthowfastthe

wheelappears toberotating in thisexample?(Theanswercanbefoundat theendofthissection.)Sodependingonthenumberofspokesinthewheelandtheactualrotational

speedof thewheelcomparedwith the24 framesper secondatwhich the filmwasshot,thewheelcanappeartobemovingforwardorbackwardor—whenthespokespeedjusthappenstobesynchronizedwiththecamera'sshootingspeed—notmovingatall.Thislastisahighlyspecificcoincidence,soitdoesn'thappenoften.Butifyoulookclosely,youcanseethewheel“stop”brieflyasitpassesfrom“forward”motion,whenthespokeisslightlyaheadofthecameraclicks,to“backward”motion,whenitisslightlybehind.Inreality,ofcourse,thewheelsdon'thaveoneredspoke;theyalllookalike.

Anyspokeisadoubleforanyother.Therefore,anyspokeatallmightbeintheone o'clock or eleveno'clock positionwhen the camera's shutter clicks, and itwillstilllookasifthewheelisturningrightorleft.Whenthewheelisgoingfastenough,thespokesaremovingtoofastforthe

camera's shutter speed to stop their motion. They therefore degenerate into ablur,andthewholeeffectofbackwardorforwardmotiondisappears.You can see exactly the same effects inmovies that depict a latermode of

transportation:propeller-drivenairplanes.Whentheplane'sengineisstarted,thepropellerlooksasifitisalternatingbetweentheclockwiseandcounterclockwisedirections.As its speed increases, thebladespass through successive “slightlyahead”and“slightlybehind”positionswithrespect towhenthecameraclicks.Astheirspeedbecomesfastenough,thebladesbecomeablur.Want to see the same effects at home, but you don't have a stagecoach or

airplanehandy?Trythis.

TRYIT

Ifyouhaveaportableelectricfan,takeitintoaroomthatisilluminatedwithfluorescent light.When you turn the fan on and it speeds up, the bladeswillappear to be rotating first in one direction and then the other. That's becausefluorescent lights flicker on and off 120 times a second (yes, 120; visit theNitpicker'sCorner),andthat'sfivetimesfasterthanaprojectedmovie,soweareunawareoftheflickering.The“on”flickersarewhatyouseeby,soit'sjustthe

same as if youwere beingpresentedwith a series of rapid frames in amovietheater.

NITPICKER'SCORNER

Fluorescent lights run on alternating current (AC). That means that the

electricityflowsinonedirectionforhalf thetimeandintheoppositedirectionthe rest of the time. In theU.S., theAC frequency is sixty cycles per second,meaningthatonefullcycletakesasixtiethofasecond.

Let's say that thecurrent is“positive” for the firsthalf-cycleand“negative”

fortheotherhalf-cycle.Thatmeansthatitis“positive”forahundred-twentiethof a second (half of a sixtieth) and then is “negative” for the next hundred-twentiethof a second, and soon.Thus, there are twocurrent surges (albeit inoppositedirections)duringeachsixtiethofasecond,foratotalof120surgespersecond.Afluorescentlightis“on”onlyduringthecurrentsurges,soyoumightsaythatitbehaveslikeamoviecamerathatissnapping120picturespersecond.

YouDidn'tAsk,but…

Whydideverybodylookasiftheyweremovingsofastinmoviesfromtheearlydaysofthelastcentury(thetwentieth,thatis)?

Photographicfilmwasn'tassensitiveasitistoday,sotheexposureshadto

belongerandthereforefurtherapartintime.Thecamerasshotonly16picturespersecond,ratherthan24.Inthat longeramountoftimebetweenpictures, thepeople moved farther, so in a second's worth of pictures they seem to havecoveredmoredistance.Moredistancepersecondequalsfaster.Answer to theextra-creditquestion:Thereare twelvepositionson theclock

andthecameraiscatchingtheredspokeatthenextpositioneverytwenty-fourthof a second. The spoke thereforemakes one full revolution in twelve twenty-fourths of a second, or half a second. One revolution per half-second is tworevolutionspersecond,or120revolutionsperminute(rpm).

DamnedSpot!

Whydoesthewetspotonafabriclookdarker?

I'll assume that you're in the dining room, concerned about soup on your

necktie,althoughyoumayhavenoticedthisphenomenoninotherroomsunderdifferentcircumstances.We see anobject because light is coming from that object and enteringour

eyes. Themore light coming from the object, the brighter it appears. And ofcourse, thereverse isalso true:Anobject that issending less light tooureyesappearsdarker.Soourjobistoexplainwhythereislesslightcomingfromthewetspot.Where does an object get the light that it sends to our eyes? If it is not

inherentlyluminous,likethesun,alightbulborRudolph'snose,thenitmustbereflectingsomeofthelightthatitreceivesfromelsewhere.Butnothingreflectsallofthelightthatfallsuponit;everysubstanceabsorbssomelightandreturns,or reflects, the rest. So the wet spotmust be reflecting less light because forsomereasonitisabsorbingmore.Let'stakeahighlymagnifiedlookatthewetfabricasitwouldbeseenbyan

incomingrayoflight.Afabricisalatticeworkofinterwovenfibers.Whenitgetswetandsoaksup

water by capillary action, the spaces between the fibers become filled withwater. Many of the incoming rays of light will then be falling upon a watersurfaceinsteadofstrikingafiber.Nowwhen a ray of light enters awater surface at an angle—and by sheer

statistics most of the rays will be hitting the water at an angle, rather thanperfectlyperpendicular toitssurface—afunnythinghappens:Theraychangesdirection.(Techspeak:Itisrefracted.Whydoesitchangedirection?MeetmeintheNitpicker'sCorner.)Insteadofcontinuingthroughthewaterinthedirectioninwhichitentered, the lightrayveersawayfromthesurfaceandplunges into

the watery depths at an even steeper angle than its entry angle. This steeperangleofpenetrationmeansthatthelightraypenetratesdeeperintothedepthsofthefabric,whereithasanincreasedchanceofbeingabsorbed,nevertobeseenagain.Thus,thereismore“lostlight”insideawetspotthaninadryone,thereislesslightreflectedandthespotappearsdarker.Similargoings-onexplainwhywetrocks,leavesandgrassappeartobemore

intenselycoloredwhenthey'rewet—whythecountrysidelooks“fresher”afterarain. These objects have colors in the first place because they absorb certainwavelengthsoflightfromthemulticoloreddaylightandreflecttherestbacktooureyes.Whentheyarecoatedwithafilmofwater,theincidentlightraysarerefracted deeper into theirmicroscopically rough surfaces. The refracted lightthenbouncesbackandforthoffthesesurfaces,whichprovidesthemwithmanymore opportunities for their absorbable wavelengths to be absorbed. Theremaining reflected light is thus even more depleted in these absorbedwavelengths than it ordinarilywould be, and it therefore looksmore intenselycolored.

NITPICKER'SCORNER

Whyislight“bent”whenitenterswater?Wheneverascientisthas toexplainsomethingabout light,heorshehas the

choiceofexplainingitonthebasisoflightwavesorlightparticles(Techspeak:photons), because light behaves as if itwerebothor either a particle and/or awave.Explaining refractionon thebasisof light'sbeingawavewould requiremy drawing a diagram and using such terms as “wave front” and “phasevelocity,”whichwouldmakethislooktoomuchlike(heavenforbid)asciencebook.So I'll take theeasywayoutand talkabout refractionas if the light raywereaphotonbullet.Betteryet,anarrow.If you stand at the edge of a swimming pool (DO NOT TRY THIS AT

HOME!)andshootanarrowintothewateratanangle—notstraightdown—youwon'tbesurprisedtoobservethatthearrowlosesspeedasitentersthewaterandswervesdownward,awayfromthesurface.That'sbecausethearrowmusttravelmore slowly inwater than in air, and the drag slows down its forward speed.Well,thesamethinghappensifthearrowisastreamofphotons.Astheyenter

thewatertheyslowdownandchangetheirdirectiontoasteeperanglethantheoneatwhichtheyentered.Thelightstreamhasbeenrefracted.(Notethatifyouhad shot straight down, the arrow would have been slowed, but its directionwouldn't have been changed. It's the same with light; if it enters the waterperpendicular to thesurface, itsdirection isn'tchanged.)Did I say that light issloweddownwhenitentersthewater?Yes,indeed.Butisn'tthespeedoflightalwaysthesame?Indeed,no.When people talk about “the speed of light” as being 186,000 miles per

second(3millionkilometerspersecond),theyshouldalwaysbecarefultoadd“inavacuum.”Becausewhenlightentersatransparentmediumitslowsdown,anddifferenttransparentmediaslowitdowntodifferentdegrees.Thespeedoflightinwater,forexample,isonlythree-quartersasfastasitisinair.Andthatslowingdownleadstothebendingofthelightwhenitenterswaterfromair.Thebending—refraction—oflightisevengreaterwhenitentersglassfrom

air,becausethespeedoflightinvarioustypesofglassisonly50or60percentof its speed in air.Which is justgreat, because that allowsus touse speciallyshapedpiecesofglass—lenses—toreallybendlightalotandmakeallsortsofclevergadgetssuchastelescopes,microscopesandeyeglasses.

SpurnThatBurn

MydermatologisttoldmethatasunscreenlotionlabeledSPF30doesnotblockouttwiceasmuchharmfulradiationasonelabeledSPF15.Whatgives?

Yourdoctoriscorrect.TheSPFnumbersaren'tsun-filteringfactors—they're

sun-protectingfactors.SPFstandsfor“sunprotectionfactor.”Thenumbersarenottellingyouhowmuchradiationtheyblockout,buthowmuchtimeyoucanspend in thesunbeforeyourskin turns red,aconditiondoctorscallerythema.Andthat'squiteanothermatter.WithanSPF15onyou,youcanstayoutinthesunfifteentimeslongerthan

withbareskin.WithanSPF30,youcanstayoutthirtytimeslongerthanwithbareskin.That'stwiceaslongaswithanSPF15.AndyetanSPF30blocksoutonlyabout3percentmoreoftheharmfulradiationsthananSPF15does!I'mwell aware that the foregoing is probably themost confusingparagraph

youhaveeverreadoutsideofanIRSpublication.ButI'llshowyouthatit'sallquitelogical.First,though,whatarethosemenacingradiationsthatraindownuponusfrom

our life-giving star? The sun's atoms, being as hot as they are (about 9800degreesFahrenheitor5400degreesCelsiusatthesun'ssurface),arecontinuallygivingoffradiationsofalmosteveryenergy…uh,underthesun,rangingfromradiowaves toX rays. The dangerousX rays are prettymuch filtered out byEarth's atmosphere,while the sun's radiowaves are substantially less harmfulthan those emanating from a hard-rock radio station. That leaves only visiblelightandtwotypesofinvisibleradiations:infrared,whichwarmsusbutdoesn'tburnus,andultraviolet.Thislastoneisthevillain.Ultraviolet(UV)radiationisusuallysubdividedintothreeregionsofenergy,

which scientists have imaginatively labeled A, B and C. We can eliminateultravioletC (abbreviatedUVC) fromour fears, because it is absorbed by the

atmosphere'sozonelayer,which,thoughthreatenedbyhumanactivities,isstillprettymuchup there.So theonly thingswehave toworryaboutat thebeachbesidesourpaunchesandcelluliteareUVAandUVB,whichcancausenotonlysunburn,butpermanentskindamageandcancer.Sunscreensareamixtureofactivechemicalsinacosmeticallyappealingbase.

Themoleculesofanychemicalselectivelyabsorbradiationsofspecificenergies.The sunscreen chemicals have prodigious appetites for absorbing ultravioletradiation, even when in extremely thin layers on the skin. On the labels ofsunscreencontainers,you'll seeUVAabsorbers suchasavobenzoneorParsol;UVB absorbers such as octyl methoxycinnamate and other cinnamates,homosalate, octyl salicylate and padimate O; and double-threat UVA-UVBabsorbers such as oxybenzone and other benzophenones. A chemical calledPABAused to be popular, but it irritated somepeople's skin and is no longerused.Okay, chemistry class dismissed. But I thought you'd like to be able to

interprettheingredientlistsontheproductlabels.Nottoworryaboutthenames,however.Mostproductsarecarefullybalanced

witches'brewsofchemicalsdesignedtoabsorbtheentirerangeofharmfulUVenergies. But remember that they are tested primarily for burn prevention,whereasresearchcontinuestofindcertainUVenergiestobeworsethanothersatcausingprematureskinagingorcancer.It'sbesttochoosea“broadspectrum”sunscreentocoverbothyourbackandyourbets.Nowback to those trickySPFnumbers. It'sall in thearithmetic.Watchme.

Nothingupmysleeve.Suppose that Brand X sunscreen cuts out half—50 percent—of the burn-

producingUVrays.Obviously,youcouldstayouttwiceaslongasusualwithoutburning.Ifyou'dordinarilyburninonehourwithnoprotection,youcouldstayoutfortwohours.Inotherwords,theSPFis2.NowsupposethatBrandYcutsout75percentoftheUVrays,whichmeans

thatyou'rebeingexposedtoonly25percentoftheburningraysinsteadof100percent. You'd be able to stay out four times as long as with no protection,wouldn't you? (100 ÷ 25 = 4.) The SPF then is 4.Brand Y cuts out only 25percentmoreoftheUVraysthanBrandXdoes,yetitsSPFistwiceashigh:4insteadof2!I won't go through the algebraic derivation (do I hear a release of bated

breath?),butifyouwanttofigureoutthepercentofabsorbedburningraysfroman SPF number, here's how: subtract 1 from the SPF, multiply by 100, anddividetheresultbytheSPF.Forexample,foranSPFof20:20‒1=19;times100=1,900;dividedby20=95percentabsorption.

Inthatway,youcanfigureoutthatanSPFof15absorbs93.3percentoftheUVrays,whileatwice-as-bigSPFof30absorbs96.7percent,only3.4percentmore.Youseethatbypayingmoremoneyforahigher-SPFproduct,you'reblocking

only a small amount of additional radiation. It's a classic case of diminishingreturns. Even if you're a creamy-skinned redhead whose skin tends to matchyourhairafteranhour in the sun,youdon't reallyneedanSPFofmore than,say,30.Whatmakesyouthinkyou'regoingtobeoutdoorsformorethanthirtyhours,anyway?Thesundoeshaveahabitofsetting,youknow.

BARBET

AsunscreenratedatSPF30allowsyoutostayoutinthesuntwiceaslongasanSPF15,yetitcutsoutonlyabout3percentmoreofthesunburningrays.

Wrong,Wrong,Wrong!

Those “light windmills” that we see spinning around in thewindowsofnoveltystores:Whatmakesthemwork?

They'recalledradiometersandaregenerallysupposedtoillustratethatlight

haspressure.Buttheydon't.Ifamachinecouldbeaconartist,thisgadgetwouldtakethecake.You've seen them. They look like a lightbulb on a stand. Inside the bulb,

whichhashadmostoftheairpumpedout,arefourthin,metalvanes,mountedlike a pinwheel on a low-friction pivot. One side of each vane is shiny (orsometimeswhite),whiletheothersideisblack.Theshinysideofonevanefacestheblacksideofthenext,andsoon.Whenexposedtosunlight,thevanesspinmerrilyaround,away from theirblacksidesand in thedirectionof their shinysides.Peoplehavebeentryingtofindoutwhatmakestheradiometerturneversince

1873,whenitwasinventedbySirWilliamCrookes(1832–1919).Hethoughtitwaspressure from the light,whichwas somehowpushingharderon theblacksurfacesthanontheshinysurfaces.SirWilliam,whowasasmartmanbutwaswrong about the light-pressure effect, launched a scientific quest that hasn'tstopped yet. Even today's encyclopedias give a popular, but demonstrablywrong,explanationofhowCrookes'sradiometerworks.Warning:You are about to encounter one of only two places in thiswhole

book (I hope) in which the answer to a question will be somewhat less thansatisfying.Thebestcurrentexplanationoftheradiometer,whichIpromiseIwillgiveyouattheend,isabithardtoswallowandisstillbeingdoubtedbysomescientists, includingme. The other less-than-satisfying explanation is why theshowercurtainissuckedinwardduringyourshower.(Forthatone.)First, let'sdebunksomeoftheobviouslywrongradiometerexplanationsthat

arecirculatingasrecklesslyasaradiometerinhell.

Radiationpressure

Light,aseveryoneknows, iselectromagneticradiation.Andelectromagnetic

radiation, as you either know or can quickly find out, is a stream of tinypackagesofenergycalledphotons.Photonsactlikelittlebullets,insofaraswhentheyhitsomething,theycanhaveaphysicalimpact.Forexample,lightphotonscan actually knock electrons out of many solid substances. That's called thephotoelectriceffect,andthephotonexplanationthatIjustgaveyouwonAlbertEinsteinaNobelPrize.(HeexplaineditinalittlemoredetailthanIjustdid.)So,onemight think, it's thestreamofphotonbulletshittingtheradiometer's

vanes that spins them around, just like when you—DONOT TRY THISATHOME!—shootamachinegunataweathervane.Whileradiationpressuredoesindeedexist,wenowknowthatitismuchtooweaktobepushingthosevanesaround.Moreover,radiationpressureshouldmaketheradiometerturntheotherway!Here's why. Light is absorbed by black surfaces and reflected by shiny

surfaces.Theblacksurfacesofthevanessimplyswallowthephotons,whereastheshinysurfacesspitthemrightbackoutagain,gettingabackwardrecoilkickjust as agungetswhen spittingout abullet.Thatwouldmake thevanes spinaway fromtheirshinysidesand toward theirblacksides—just theoppositeofwhatweseehappening.

Gaspressure

Thisappearstobethebest-lovedofallthewrongexplanations.Itisdispensed

by theEncyclopædiaBritannica and other encyclopedias, aswell as bymanyscienceteachers.Thestorygoesthattheblacksurfacesofthevanesabsorbmorelightenergy

thantheshinysidesdoandaretherebyslightlywarmer.(Correctsofar.)Theairadjacent to the black sides—there's still a small amount of air in the bulb—iswarmedbythisenergy(stillcorrect),whichmakestheairpressurehigherontheblack sides (wrong!). This supposedly increased pressure pushes on the blacksides,makingthevanesmovetowardtheirshinysides.But let's ask the following question:When the air is heated, which indeed

makes its molecules move faster, why should those faster-moving moleculesdashthemselvesagainstthevanesanymoreoftenthantheydartoffinanyotherdirection? There can be no net directional force from the molecules' motion.Putting it another way, the air's pressure can't increase, because it is notconfined.Itisfreetoexpandandrelieveanyincipientpressureanywhereitlikeswithinthebulb,sothereisnomorereasonforittoexpandagainstthevanesthanin anyother direction.Thus, there is nonet vane-pushing force causedby thewarmerair.

Outgassing

Someconspiracytheoristswouldhaveusbelievethattheblackcoatingonthe

vanes containsadsorbed (surface-bound) gases, and thatwhen the black sidesare heated by absorbing light, those gas molecules are expelled, sort of likepopcornfromafryingpan.Theleapinggasmoleculeswouldexertaforceontheblacksurface,justasabasketballplayerexertsaforceonthecourtfloorwhenhe jumps, and this force pushes the vanes around. But if this were true, theradiometerwouldeventuallywearoutasalltheadsorbedgaseswerereleased.

Photoelectriceffect

Whatifthephotonsoflightareejectingelectronsfromtheblacksidesofthe

vanes and, in departing, the electrons give a backward kick to the vanes?Nocigar on that one, either, because you can make radiometer vanes out ofmaterials thatdon't exhibit thephotoelectriceffect; theirelectronsareheld tootightly for visible light to be able to knock them out. Also, the photoelectriceffect would still occur even if the bulb were completely evacuated, but theradiometerwon'tworkwithoutsomeairinthebulb.

Convectioncurrents

Theheatedblacksurfacesetsupaircurrentsbyconvection,and themoving

airblows thevanes around.Theonly troublewith thisone is thatnobodycaninventanyaircurrentsthatblowmainlyinonedirection:againsttheblacksidesofthevanes.

Thebestscoop

In1881,aBritishmechanicalengineernamedOsborneReynolds(1842–1912)

published a paper that explained the radiometer in away thatmany scientistsnowgrudginglyaccept.Thereasonitisn'tmorewidelyknownisprobablythatitisn'teasytodescribeortounderstand.Butheregoes.Ithassomethingtodowiththetemperaturedifferencebetweenthewarmerair

adjacent to theblack sidesof thevanes (due to their energy-absorbingnature)andthecoolerairadjacenttotheshinysides.Apparently,whenthisairflowsouttotheedgesofthevanes,thewarmer,fastermoleculesstriketheedgesatamoreoblique angle than the cooler molecules do, and that pushes the vanes in adirectionawayfromtheblacksides.Exactlywhythisshouldbetrueisburiedincomplexmathematics,which I shallnotattempt todecipher foryou(orme). Iconfessthatit'shardformetobelievethatit'stheedgesofthoseskinnyvanes,rather than their broad surfaces, that push them around. But that's what Mr.Reynolds says, and none of the other explanations stands up under closeexamination.Iwarnedyou,didn'tI?

YouDidn'tAsk,but…

Ifscientiststodaycanunravelthemysteriesoflifeitself,whycan'tthey explain the simple little radiometer after more than ahundredyearsoftrying?

Themainanswer is that theyhaven't reallybeen trying.Therehasbeenno

vastfederalprogramtoinjectbillionsofdollarsintoradiometerresearchastherehavebeenfortheManhattan(atomicbomb)Project,thespaceprogram,geneticresearchandotherhealth-relatedenterprises.Not thatmoneyalonecansolveascientificproblem,butscientistsarelikeeverybodyelse:Theytendtodowhattheygetrewardedfor,andnobodyisgoingtogetaresearchgrant,apromotionoraNobelPrizeforfiguringouthowatoyworks.

Window,Window,intheWall,HowComeYouBlockNoLightatAll?

Why are air, water and glass transparent, when practically noothermaterialsare?

Well,whatdoes“transparent”mean?Itmeansthatanylightbeingreflected

inourdirection fromanobjectoutside aglasswindow, forexample,canpassrightthroughtheglassunobstructedandcomeouttheotherside,whereoureyescandealwith it.We therefore see the object through thewindow.That'swhypeoplewhohave little regard for theEnglish languageuse the term“see-thru”(invariablyspelledthatway)insteadoftheperfectlygoodwordtransparent.Ingeneral,whenatravelingrayoflightencountersanewsubstance,itmaybe

reflected backward from the surface or it may penetrate the surface and beabsorbed. If itmanages toescapebothof these fates, itcancontinue travelingthrough themedium; it will be transmitted. So our job is to explainwhy air,waterandglassdon'treflectand/orabsorbverymuchofthelighttheyreceive.Almost all other substances—except some waterlike liquids and glasslikeplastics—absorbsomeofthelightandreflectmostofit,leavingpracticallynonetobetransmitted.Let's get air out of the way first. Under ordinary conditions, the spaces

between air molecules are around ten times bigger than the moleculesthemselves. So air is almost completely empty space, containing virtuallynothingthatcouldinterferewiththepassageoflightexceptforaveryoccasionalmolecule.Dittoforallgases.Water and glass are quite a different ball game, however, because their

molecules are very close together—close enough to do a fair amount ofreflecting. Remember the glare from that pond's surface or from that car'swindshieldona sunnyday?Soeven from themost transparent liquidor solidsubstances,somelightisreflected.Itdependsontheangleatwhichthelighthits

thesurface.Ofthelightraysthatdosucceedinpenetratingair,waterorglass,very,very

fewof themare absorbed; almost all the light gets through.Molecules absorblightbecausetheirelectronshavecertainpreferredenergies,andbytakingontheextraenergyofalightparticle(aphoton),theycanreachanother,higheroneoftheirpreferredenergies. Ithappens that noneof themolecules in air,water orglasscanabsorband“use”anyoftheenergiesinvisiblelight;theenergiesthattheycanabsorbarecertainradiationsthathumanscan'tsee,suchasultravioletandinfraredradiations.Dittoforalcohol,keroseneandotherfamiliartransparentliquids.Soifverylittlelightisabsorbedandtheangleisn'trightforreflecting,almostallofthelightwillgostraightthroughbydefault.

NITPICKER'SCORNER

Thereare,ofcourse,coloredglasses, liquidsandevengases.What'sgoing

on there is that theyselectivelyabsorbsomeof thewavelengthsorenergies inwhite (or colorless) light and transmit only those that they can't “use.” Thetransmitted light therefore has a different composition of wavelengths fromwhitelightandhenceaperceivedcolor.

YouDidn'tAsk,but…

Whyisamirrorsuchagoodreflector?

Mirrorsarethebestreflectorsoflightthathumaningenuityhasbeenableto

devise.Notice,however,thatthelightisreflectedonlyfromthebackingofthemirror,afterpassingthroughthefrontlayeroftransparentglass.Whatisthereaboutthebackingthatmakesitsuchagoodreflector?It'sathin,

smooth layer of silvermetal.Allmetals are shiny, or reflective, because theiratoms are held together by a sea of loose, swarming electrons that have noaffiliationwith anyparticular atoms. (That'swhymetals conduct electricity sowell—because electricity is just a movement of electrons.) The swarm offootloose electrons in the silver, belonging as they do to no particular atoms,havenoparticularpreferenceforabsorbinganyspecificwavelengthsoflight,sotheyrejectandreflectbackallwavelengths.Ofcourse,asheetofshinysilvermetalwouldmakeafinemirrorwithoutthe

glass,butitwouldquicklytarnish.

ALightBite

WhydoWintOGreenLifeSaversmakeflashesoflight?

Yourquestionmay sound silly to thosewhohaven't heard about it before,

butchompingonthoselittlecandiesreallydoesmakeflashesoflight.Itmaynothelp you at all to know that the phenomenon is called triboluminescence, butthere,I'vesaiditanddonemydutyasascientist.LifeSavers,itwillnotsurpriseyoutoknow,arelittlemorethandonut-shaped

crystalsofsugar.Certaincrystals,includingcanesugar,havelongbeenknowntoexhibitthispropertyoftri…whatever.Infact,waybackin1605,theEnglishphilosopherSirFrancisBacon (1561–1626) reported thatwhenhechoppedupblocksof sugar in thedark (sugarwas sold in bigblocks and candlelightwasdim), he observed flashes of “a very vivid but exceedingly short-livedsplendour.” Mineralogists have long known that certain mineral crystals alsogiveofflightwhensubjectedtosuddenshock.Here'swhat'sgoingon.Acrystal is anorderly, geometric arrangementof atoms, all bound together

intoasortofthree-dimensionallattice-workstructure.Examplesthatyoumaybefamiliar with are sugar (sucrose), salt (sodium chloride), quartz (silica) anddiamond(anoverpricedformofcarbon).Ithasbeenfoundthatcrystalswhosemoleculararrangementsarenotsymmetrical—that is,whosemoleculesarenotsituatedidenticallyintwooppositedirections—arethebestflashers.When such a crystal is cracked open, the atoms are torn apart from one

anotherandsomeoftheirelectronsaretornoffintheprocess.CrystalfragmentAmaywindupwithmoreelectrons thanitdeserves,whilecrystalfragmentBmaynothaveenough.Astheybegintoseparate,theextraelectronsonfragmentA are attracted strongly back to where they belong, and they zap across thewidening air gap between A and B, exactly like a bolt of lightning zappingthroughtheairbetweenacloudandtheground.

These miniature lightning bolts make tiny blue flashes because the air'smoleculesareenergizedbytheswiftrushofelectronsthroughthem,followingwhich they throw off their extra energy in the form of light.Hard, fracturingwhacksonthecrystalcanthereforeproduceweakflashesoflight.That's all that happens inmost triboluminescent crystals.But in the case of

WintOGreen Life Savers, that's not all that happens. There's an almostinstantaneoussecondstepthatmakesthelightmuchbrighter.Muchof the“lightning” that theelectron-zappedairgivesoff is invisible to

humans; it isultraviolet radiation,which isofhigherenergy thanvisible light.ButWintOGreenLifeSavers contain a chemical calledmethyl salicylate, alsoknownasoilofwintergreen;it'stheflavorintheleavesofthewintergreenplant,asmall,creepingevergreensometimesknownasteaberry.Thischemicalhastheproperty of being fluorescent. That is, its molecules absorb the ultra-violetradiationandre-emititasvisiblelight.It'sthatvisiblelightthatismainlywhatyouseewhensomeonechompsaWintOGreenLifeSaverinadarkcloset.Can'twaittotryit?

TRYIT

TakearollofWintOGreenLifeSaversintoadarkclosetwithahandmirrororaclosefriend.(Ifyou'realreadyinthecloset,somuchthebetter.)Makesuretheclosetiscompletelydark;waituntilnighttimeandplugthecrackunderthedoorwithatowelifnecessary.Thinkpurethoughtsforabouttenminutes,whileyour eyes become thoroughly dark-adapted. Now pop a Life Saver into yourmouth and, in spite ofwhat yourmother taught you, quickly crunch it noisilybetweenyour teethwithyourmouthopen.Yourmirrororyourfriendwillseesurprisinglybrightflashesoflightinsideyourmouth.BabydragonsaretrainedonWintOGreenLifeSavers.

Youmayalsowanttoplayaroundwithsugarcubes.Inthecloset,clashthemglancingly against each other, as if striking amatch. You'll see theminiaturelightning flashes, but they won't be brightened by the fluorescence ofwintergreen'smethylsalicylate.

Everythingishot.Thatis,itcontainssomeheat.Andasaconsequence,

it has a temperature. Even an ice cube contains heat. “Hot” is strictly arelativeterm.Heatistheultimateformofenergy,theformintowhichallotherforms

ultimately degenerate. There is energy of motion (Techspeak: kineticenergy), there is gravitational energy, chemical energy and electricalenergy. They can all be converted into one another with the rightequipment. We can convert gravitational energy into kinetic energy bypushing a boulder off a cliff.We can convert awaterfall's kinetic energyinto electrical energyby connecting awaterwheel to a generator.Wecanconvertchemicalenergyintoelectricalenergywithabattery,andsoon.But no conversion can be 100 percent complete. Some of the energy

must inevitablybe“wasted”—turned intoheat.When theboulderhits thegrounditheatsitupabitandwelosethatamountofheatenergy.Whenthewaterwheel turns, its bearings get warm from friction and we lose thatamountofheatenergy.Whenabatterydeliverscurrentitgetshotfromthechemicalreactionsinsideandwelosethatamountofheatenergy.Inshort,wecanconvertandreconvertenergyasmuchaswelike,buteachtime,wewilllosealittleintheformofheat.Canwecollectthat“wasted”heatandconvertitbackintoanotherform?

Afterall,weseemtoberecyclingeverythingthesedays;can'twerecycleheat energy? Sure, but not completely. That's because heat is a chaoticmotionofatomsandmolecules ,andtorestore themtoorder takeswork:energy.Wemustspendenergy to recover thatheatenergy,so thebottomlineontheenergybalancesheetwillalwaysshowanetdeficit.TheprecedingideasareembodiedinwhatisknownastheSecondLaw

ofThermodynamics,whichisoneofthemostprofoundsetsofrealizationsevertodawnuponthemindofman.

Butalthoughwecan'tuseitwith100percentefficiency,heatisfarfroma minor player in the energy game. The world thrives on heat. It is thecommon currency, the euro of energy, if you will, that we humansmanipulatetosuitourenergeticobjectives.Weaddittoourovensandweremoveitfromourrefrigerators—afterfirstconvertingitintoelectricity,ofcourse,whichissomucheasiertohandlethanfire.Like unfettered physical objects, heat can travel from one place to

another as long as it is going “downhill”: from someplace at a highertemperaturetosomeplaceatalowerone.Inthatsense,flowingheatisverymuchlikeflowingwater.But does the heat flow because the higher-temperature object contains

moreheatthanthelower-temperatureobject?Notnecessarily.Peopleoftenconfuseheatwithtemperature—peoplewhohaven'treadthischapter, thatis.Usingwaterflowasananalogytoheatflow,trythisriddleonforsize.

Thenreturntoitafteryou'vereadthesectionthatbeginsonpage79.

Ifawaterfall flows spontaneouslydown from lakeA into lakeB,does that mean that there is more water in lake A than in lake B?(Note:Heatisanalogoustotheamountofwater,whiletemperatureisanalogoustothealtitude.)

Thischapter,then,isaboutheatandtheelectricitythatwemakeoutofit.

It's about global cooling (yes, cooling), cold feet, cold steel, hot fire,sparrows,refrigerators,thermometersandbathtubs.WhoisthisguyLewisCarroll,withhisshoes,shipsandsealingwax?

DoubleTrouble

IliveinMiamiandmytwinsisterlivesinTucson.Onedayonthetelephone I mentioned that it was 80 degrees Fahrenheit (26degreesCelsius)inMiami,andshejokinglysaidthatitwas“twiceashot”inTucson.Ifthatwerereallythecase,whattemperaturewoulditbeinTucson?

It certainly wouldn't be 160 degrees Fahrenheit (71 degrees Celsius). But

that'snotbecause160degrees is toohot; it'snothotenough.The temperaturethatis“twiceashot”as80degreesFahrenheit,believeitornot,is621degreesFahrenheit!Here'swhat'sgoingon.Firstofall,wemustrealizethatheatandtemperaturearetwodifferentthings.

Pleaserepeatafterme:Heatisenergy,whiletemperatureisjustourhumanwayoftellingoneanotherhowdenselyconcentratedthatheatisinanobject.Let'stakeheatfirst.Theamountofheatenergyanobjectcontainscanbecountedupincalories,

just as if it were a donut. (A calorie is just an amount of energy, right?) Butyou'll grant that a big donut containsmore calories than a small donut,won'tyou?Well, it's the samewith the energy content of any substance.Aquart (aliter)ofboilingwatercontainstwiceasmuchheatenergyasapint(halfaliter)ofboilingwater,eventhoughthey'rebothatthesame212-degreetemperature.Anotherexample:There'salotmoreheatinabathtubfullofwarmwaterthan

in a single glassful that you might scoop out of that same bathtub, simplybecause there are more hot molecules in the tub. In short, the more of asubstanceyouhave,themoreheatenergyitcontains.(Righthere,youmaywishtotaketimeouttotakeacrackattheriddle.The

answercanbefoundattheendofthissection.)Soyoursister'sproblem,whethersherealizeditornot,wastofigureouthow

muchheatthereactuallywasintheoutsideair—let'ssayacubicyard(oracubicmeter) of it. Then if there was twice as much heat per cubic yard (or cubicmeter)intheoutsideairasinyourMiamiair,shecouldreallysayitwas“twiceashot.”How can we determine the amount of heat in an object? Taking its

temperaturewon'tdothejob,becausethatdoesn'taccountforhowbigtheobjectis.Aswediscoveredinthebathtub,abigobjectcontaininglotsofheatcanbeatthesametemperatureasasmallerobjectcontainingmuchlessheat.Moreover,temperatures, whether expressed as Fahrenheit or Celsius, are nothing butarbitrarynumbersinventedbythosetwoeponymousgentlemen.They'remerelyconvenientlabelsforpeopletotalkabout—numbersthateveryonehasagreedto,as if proclaimed on Mount Sinai: “Whensoever thine ice melteth, it shall becalled32degreesFahrenheitorzerodegreesCelsius.Andwhensoeverthywaterboileth, it shall be called 212 degrees Fahrenheit or 100 degrees Celsius.”TheseproclamationsweremadenotbytheLord,butbyMessrs.FahrenheitandCelsius.But theamountofheat thatanobjectcontainscannotbesubject tohumans'

monkeying aroundwith numbers.Weneed an absoluteway of expressing theheatcontentofthings.The crux of the problem is that on either of our temperature scales, zero

temperature does not mean zero content of heat. Zero degrees Celsius, forexample,ismerelythetemperatureofmeltingice.Doesthatmeanthattherecanbenothingcolderthanmeltingice?Ofcoursenot.Or lookat it thisway:Howcanyouuseascale tomeasuresomething if its

zero doesn't reallymean zero? Picture a yardstick (ormeter stick)with “zeroinches” (or“zerocentimeters”)somewhere in themiddle, insteadofat the leftend.Justthinkofthecrazymeasurementsyou'dget.Soifwe'reevergoingtobeabletomeasuretheamountofheatinanobject,or

in theair for thatmatter,we'llhave tohaveascaleofnumbersonwhichzeroactuallymeansnoheatatall.Andthat'swheretheLordreallydoescomein.No,notthatLord.LordKelvin,aBritishnoblemanandscientist(1824–1907),whosestreetmonikerwasWilliamThomson.Kelvin set up a scale of temperatures that begins at “no heat at all”—a

temperatureofabsolutelyzero,wherethingsareascoldastheycanpossiblyget:“absolutezero.”Then,heborrowedthesizeofMr.Celsius'sdegreeandstartedcounting upward from there. When you do that, the temperature of freezingwater,zerodegreesCelsius,turnsouttobe273degreesaboveabsolutezero,andthe temperature of boilingwater—100 degreesCelsius—is 373 degrees aboveabsolutezero.Humanbodytemperature(37degreesCelsius)turnsouttobe310

degreesontheabsolutescale.(Tellthattoyourdoctorwhenheaskswhatyourtemperatureis.)Youcanseethattheabsolutetemperature,measuredinKelvinsinhonorofLordKelvin,istheCelsiustemperatureplus273.Nowwe'rereadytoansweryoursister'sriddle.IfTucsonaircontainstwiceas

muchheatpercubicyard(orpercubicmeter)asMiamiair,thenwhatwemustdouble is theabsolute temperatureof theMiamiair.First convertingyour80-degree Fahrenheit temperature into Celsius (for how to do that), we get 27degreesCelsius.Adding273givesus300Kelvins,whichisnowarealmeasureof the heat content of the air. Doubling it to get twice the heat, we get 600Kelvins,whichconvertsto327Celsiusor621degreesFahrenheitasyoursister'soffhandestimateoftheTucsontemperature!Yeah,weknow,Sis:Youdon'tfeelitbecausethehumidityissolow,right?Similarly, inside your house, if your thermostat is set on 70 degrees

Fahrenheitandyouwanttwicetheheat,you'dhavetoturnitupto599degreesFahrenheit.Youcan takemyword for thatordo thecalculationyourself. InaCelsiuscountry,you'dhavetoturnthethermostatupto313degreesinordertomakea20-degreehousetwiceashot.

BARBET

If your room's temperature is 70degreesFahrenheit andyouwant it to betwice as hot, you'd have to turn your thermostat up to about 600 degreesFahrenheit.

Towinthisbet,there'snoneedtoscribblecalculationsonanapkin.JustpointoutpolitelytoyourbuddiesasyoupickupthemoneythattheFahrenheitscaledoesn't read zerowhen there's a complete lack of heat; there's an awful lot ofunaccounted-forheatbelowzero.Thus,thenumber“70”doesn'taccountforalltheheat,anddoublingitwon'tgetyouanywhereneartwicetheamountofheat.

YouDidn'tAsk,but…

Whyistherealimittohowcoldanythingcanget?

Heatisenergy.Whatkindofenergy?It'snotelectricalenergyornuclearenergyorthekindofenergythatyourcar

hasasyoubarreldownthehighway.It'stheenergythatanobjectcontainswithinitself, because the particles that it's made of, its atoms and molecules, areactuallyvibratingandbouncingaroundwithintheirlimitedspaceslikeabunchofmaniacsinpaddedcells.Themorevigorouslythoseparticlesaremoving,thehotter we say the stuff is: the higher its temperature. Even at the sametemperature, though,abiggerchunkof thestuffwillcontainmoreheatenergybecauseitcontainsmoremovingparticles.Whenwecoolsomethingdownbytakingheatenergyoutofit,theenergyis

lost by those moving particles, which will then be moving more slowly.Ultimately, ifwe cool it down far enough,we should reach apointwhere theparticles stop moving altogether. We will have reached the lowest possibletemperature:absolutezero.Andby theway,whenyouwant to tellyourdoctor thatyouhaveno fever,

please don't say that you have “no temperature.” That would mean that yourbodyisatabsolutezero,inwhichcaseadoctorwouldbeofnohelpwhatsoever.

NITPICKER'SCORNER

Atomicandmolecularmotiondon'tstopdead-stillatabsolutezero.Theory

saysthattherewouldbeatinybitofresidualenergyleft.Butabsolutezeroisn't

basedonmolecularmotionanyway. It's the temperature atwhichagaswouldshrinksomuchfromthecold that itwoulddisappearentirely.Nobodyhasyetsucceeded in cooling a substance down to precisely absolute zero—in fact,theory says that it can never actually be reached—although experiments havegottentowithinseveralbillionthsofadegreeofit.Foronething,you'dhavetokeepasubstanceinsideanabsoluteinsulator,throughwhichnotasingleatom'sworth of heat can penetrate.And that's not exactly a job for aKmart thermosbottle.Answer to theriddle:Ofcourse thewaterfalldoesn'tcarehowbig the lakes

are.Butjustaswaterwillflowfromahigheraltitudetoaloweronenomatterhowmuchwaterisinvolved,heatwillflowfromabodyatahighertemperaturetoabodyatalowerone,nomatterhowbigorsmallthebodiesmaybeorhowmuch actual heat they therefore contain. It's the temperature difference thatcounts—thedifferenceinenergybetweenthefast,hotmoleculesandtheslower,cooleronesthattheycollidewithandtransmittheirenergyto.

ColdFeet

Whydoes the tile floor inmybathroomfeel socoldonmybarefeet?

Assuming that you haven't forgotten to pay your gas bill, it's because

porcelain tile conducts heat better than that cozy bathmat does, even thoughthey'reatthesametemperature.It's a common experience that certain things feel colder than others. People

talkabout“coldsteel”asifthebladeofaswordweresomehowcolderthanitssurroundings.Bakersliketorollouttheirpastrydoughonamarbleslabbecause“it's colder.” Just toucha steelknifebladeoramarble slabandyou'llhave toagree:Theydofeelcolder.Butthey'renot.Thesteel,themarbleandthefloortilesaren'tonebitcolder

thananythingelseintheroom.Theyjustfeelasiftheyare.If they have been in the same room for any reasonable amount of time, all

objectswillbeatthesametemperatureaseverythingelseintheroom,becausetemperaturesautomaticallyeven themselvesout.Hotcoffeecoolsoffandcoldbeerwarmsup.Letacupofhotcoffeeandaglassofcoldbeerstandsidebysideon the table longenoughand they'll eventuallycome to thesame temperature,the prevailing temperature of the room. (Nevertheless, you'll still think of thecoffeeas“cold”andthebeeras“warm,”won'tyou?)The reason is that heat spontaneously flows fromwarmer to cooler. That's

becausethemoleculesinawarmobjectaremovingfasterthanthemoleculesina cool object; that'swhat temperature is: ameasure of themolecules' averagespeed. Sowhen a warm object comes into contact with a cool one, its fastermoleculeswill collidewith the slowermolecules and speed them up—that is,makethemwarmer.If an object should happen to be initially colder than its surroundings, then

heat will automatically flow into it until its temperature is the same as its

surroundings. Or if an object happens to be initially warmer than itssurroundings, heatwill flowout of it into the surroundings.Wehave found itusefultothinkofheatflowasifitwereflowingwater.Wateralwaysflowstoalowerlevel,whileheatalwaysflowstoalowertemperature.Wemightevensaythattemperatureseeksitsownlevel.It's not just steel, marble and tile that will feel cool to your touch. The

temperature of your skin is a bit below 98.6 degrees Fahrenheit (37 degreesCelsius),whileeverythingelseinyourroom(exceptahotradiator,perhaps)isatthe room's prevailing temperature—around 70 degrees Fahrenheit (21 degreesCelsius).Sowhenyoutouchanobjectintheroom,itwillfeelcooltoyourskinbecauseitreallyiscoolerthanyourskin.Heatwillthereforeflowfromyourskinintotheobject,andyourheat-deprivedskingivesyouthesensationofcoolness.

TRYIT

Pickupvariousobjects in theroomyou're in,other thanobviouslycoolorwarmobjectssuchasabottleofpop,acupofcoffeeorthedog.Pressthemonebyoneagainstyourforehead.Theywillallfeelslightlycooltoyou.

But as you have discovered to your discomfort, some things do feel colderthanothers;thetilefloorfeelscolderthanthebathmat,eventhoughwe'veseenthattheymustbeatthesametemperature.Howcome?

Theansweristhat,whileallobjectsintheroomarecoolerthanyourskinand

willthereforestealsomeheatfromit,somematerialsarebetterheatthievesthanothers.Somematerialsarebetterheatconductors— they are better at carryingthe stolen heat away. And the faster a material conducts the heat away, thecooleryourskinisgoingtofeel.Ithappensthatporcelaintileisamuchbetterheatconductorthanthecottonorsyntheticfiberbathmatis,soheatflowsfasteroutofyourtootsieswhenthey'reonthebarefloorandtheyfeelcolder.Differentsubstances,beingmadeofmoleculeswithdifferentproperties—and

that,ofcourse,iswhytheyaredifferentsubstances—willtransmitthisheatwithdifferentdegreesofspeedandefficiency.Substancesmadeofbig,unwieldyorrigidlyfixedmoleculeswon'tbeableto

jostletheirneighborsaseasily,sotheywon'tbeabletotransmitheatasquickly.That'sthecasewithsubstanceslikecotton,woodandrubber,forexample.Yourwoodenfloordoesn'tfeelascoldasthetilefloor,doesit?That'sbecauseitsbigmoleculescan'tstealheatawayfromyourskinasfast.Among all typesofmaterials, gases are theworst conductors of heat.Their

moleculesaresofarapartthattheycanbarelyfindothermoleculestobumpupagainst.Almosteverythingconductsheatbetter thanair,andthat'swhyalmosteverythingyoutouchfeelscooltosomeextent;you'recomparingitwiththeairthatyou'renormallysurroundedbyandaccustomedto.Airinsulatesyou.Metals,ontheotherhand,arethebestconductorsofheatamongallmaterials,

because of their unique structure. They contain loose electrons that can drifteasilyfromoneatomtoanother.That'swhymetalsconductelectricitysowell,butit'salsowhytheyconductheatsowell.Thosetinyelectronscancarryheatenergyfromoneplacetoanothermuchmoreefficientlythanbig,jostlingatomsormoleculescan,becausethey'resomuchmoremobile.Asthebestthievesofheatfromyourskin,metalswillfeelcoldestofall.Engineersandphysicistshavemeasuredhowwellmanysubstancesconduct

heat(Techspeak:their thermalconductivities).Here, in roundnumbers, ishowthethermalconductivitiesofsomefamiliarmaterialsstackupcomparedwithair,towhichI'veassignedthenumber1.Thefartherdownthelistyougo,thecolderthesubstancewillfeelwhenit'sreallyatroomtemperature.

HOWVARIOUSMATERIALSCONDUCTHEAT,RELATIVETOAIR

Themoralof thestory:Neverbuildahousewithsilverbathroomfloors.Or

toiletseats.

Gabriel,YouBlewit!

Why are the temperatures of freezing and boiling such oddnumbersas32and212degreesFahrenheit?

Theyare indeedstrangenumbersforsuchcommon,everydaygoings-onas

the freezing and boiling of water. We're stuck with them because a GermanglassblowerandamateurphysicistnamedGabrielFahrenheit(1686–1736)madeacoupleofbaddecisions.Gadgetsformeasuringtemperaturehadexistedsinceabout1592,eventhough

nobodyknewwhattemperaturewas,andnobodyhadtriedtoattachnumberstoit.Thenin1714Fahrenheitconstructedaglasstubecontainingaverythinthread

of mercury—a nice, shiny, easily visible liquid—that went up and down byexpansion and contraction as it got hotter and colder. But Fahrenheit'sthermometer,likeallthatprecededit,waslikeaclockwithoutaface.Hehadtoputnumbersonthething,orelsehowcouldanyonecomplainabouttheweather?SoFahrenheithad todevisea setofnumbers to inscribeonhisglass tubes,

suchthatthemercurywouldrisetothesamenumberonallthermometerswhentheywereatthesametemperature.Andthat'swhenGabrielblewit.Historiansstill speculate about what must have been going through his mind, but thefollowingmightbeagoodguess.First,hedecidedthatbecauseafullcirclehas360stepsordegrees,itwould

beniceiftherewerealso360steps—andwhynotcallthemdegrees—betweenthetemperaturesoffreezingwaterandboilingwater.But360stepswouldmakeeachdegreetoosmall,sohechose180instead.Thatfixedthesizeofthedegree:exactly1/180thofthedistanceonthetube

between the freezing and boiling marks. But what, he wondered, should theactualnumbersbe?Zeroand180?180and360?Or,heavenforbid,32and212?(212‒32=180,right?)

Well,hestuckhisthermometerintothecoldestconcoctionhecouldmake—amixture of ice and a chemical called ammonium chloride—and called thattemperature“zero.”(Whatarrogance,Gabriel!Wouldnobodyinhumanhistoryeverbeabletomakeacoldermixture?Why,twocenturieslater,wecanmaketemperaturesalmost460degreesbelowyourzero.)Whenhe tookhisowntemperature, the thermometerwentup toaround100

degrees. (Okay,98.6,but see the following forhow thatnumber cameabout.)ThatwasatouchthatFahrenheitliked:Humans,hefelt,shouldscore100onhistemperaturescale.Next,hestuckhisthermometerintoanice-watermixture,andfoundthatthe

mercurywentup32degreeshigher than inhis zero-temperaturemixture.Andthat'showthefreezingpointofwatercametobe32degrees.Finally,ifboilingwaterwastobe180degreeshigherthanthat,itwouldwindupat32+180,or212.EndofGabrielFahrenheit'sstory.Six years after Fahrenheit's body temperature became equal to that of his

surroundings, a Swedish astronomer named Anders Celsius (1701–1744)proposed the centigrade scale of temperature, which we now call the Celsiusscale.“Centigrade”means100degrees;hesetthesizeofadegreesothatthereare 100 of them, not 180, between the freezing and boiling points of water.Furthermore,hedefinedhis“zerotemperature”atthefreezingpointofwater,areferencepoint thatanyonecouldeasilyreproduce.Andthus, theboilingpointofwaterfellat100degrees.(Curiously,forreasonsknownonlytoeighteenth-centurySwedishastronomers,Celsiusoriginally tookthefreezingpointas100andtheboilingpointaszero,butpeopleturneditaroundafterhedied.)Andwhataboutthatnumber98.6asthe“normal”humanbodytemperature?

It'sjustafluke.People'stemperaturesvaryquiteabitdependingonthetimeofday, the time ofmonth (forwomen) and just plain differences inmetabolism.Butitwaversaroundanaverageof37degreesCelsiusformostpeople,sothat'swhat doctors have adopted as “normal.” And guess what 37 degrees CelsiusconvertstoinFahrenheit?Right—98.6,anumberthatlooksforalltheworldasif it were more precise than it really is. That extra six-tenths of a degree isnothingbutanaccidentoftheconversionarithmeticandhasnosignificanceatall.Speakingofconversions,Ican'tresisttheopportunity—IdoiteverychanceI

get—topublicizeaneasyway toconvert temperatures. I don't knowwhy theycontinue to teach those complicated formulas in school, with all their 32s,parentheses and improper fractions, when there is a much simpler way that'sabsolutelyaccurate.Here'show:

ToconvertCelsiustoFahrenheit,add40,multiplyby1.8,thensubtract40.ToconvertFahrenheittoCelsius,add40,divideby1.8,thensubtract40.

That's all there is to it. It works because (a) 40 below zero is the same

temperature on both scales and (b) aCelsius degree is 1.8 times larger than aFahrenheitdegree.(180÷100=1.8.)Afinalpoint:Thermometersmeasureonlytheirowntemperatures.Think about it. A cold thermometer registers a low temperature; a hot

thermometer registers a high temperature. A thermometer doesn't register thetemperatureofanobjectthatyoustickitintountilititselfwarmsupto,orcoolsdown to, that object's temperature. That's why you have towait for the feverthermometertowarmuptoyourbody'stemperaturebeforeyoureadit.

BARBET

A fever thermometerdoesn'tmeasureyourbody's temperature; itmeasuresitsowntemperature.

Hot,Hotter,Hottest?

If absolute zero is the lowest possible temperature, is there ahottestpossibletemperature?

Yes.Butlet'sstartoffatmerelywarmandgraduallyturnuptheheat.Heatistheenergythatasubstancecontainswithinitself,duetothefactthat

its atoms andmolecules aremoving.But temperature is aman-made concept,invented so that we can converse among ourselves about how much of thatenergyasubstancehasandactuallyassignnumberstoit.Whenwesayweare“raising the temperature”of anobject,weare addingheat energy to its atomsandmoleculesandmakingthemmovefaster.Theultimatelimittocoolingandslowingthemdownhastobewhenthey'renotmovingatall;that'sabsolutezero.Ourcurrentquestion,then,comesdowntowhetherthereisanylimittohowfastthoseatomsandmoleculescanmove.But long beforewe reach any such speed limit, several thingswill happen.

First, if the substance is a solid it will melt into a liquid. Then at a highertemperature the liquid will boil and become a vapor or gas—a condition inwhichtheatomsormoleculesareflittingaroundfreelyinalldirections.Asthetemperaturegetshigherandhigher, theyflitfasterandfaster.Forexample, thenitrogen molecules in the air in your 350-degree-Fahrenheit (177-degree-Celsius) oven are flitting about at an average speed of 1,400 miles per hour(2,300kilometersperhour).If thesubstanceismadeofmolecules(clustersofatomsgluedtogether), the

molecules will eventually be knocked to pieces—broken apart into smallerfragments or even into their individual atoms by the shattering forces of theirviolentcollisions.Inotherwords,everymolecularcompoundwilldecomposeatahighenoughtemperature.Willtheindividualatomsthemselveseverbebrokenapart?Yes,indeed.Ata

high enough temperature the atoms' electrons will be torn off, resulting in a

seething,fluidinfernooffreeelectronsandchargedatomicfragments,calledaplasma.This is thestuffof the interiorsofstars,at temperatures in the tensofmillionsofdegrees.Still higher temperatures? Why not? There would seem to be nothing to

preventusfromheatingaplasma'selectronsandatomicfragmentstofasterandfaster speeds, except for one thing. There happens to be a speed limit in theuniverse: the speedof light in a vacuum,which is 671millionmiles per hour(1.08billionkilometersperhour).AlbertEinsteintoldusthattheelectronsinaplasma—oranyobject,forthat

matter—mayapproachthespeedoflight,butcanneverachieveit.Healsotoldus that as a particle goes faster and faster, it gets heavier and heavier. Forexample,whencruisingalongat99percentofthespeedoflight,anelectronhas7timesitsnormalmass;at99.999percentofthespeedoflight,itis223timesheavierthanwhenit'snotmoving.There must be an ultimate temperature limit, then, lest the particles in a

plasma reach the speed of light and become infinitely heavy. Theoreticalconsiderations peg this temperature at around140,000,000,000,000,000,000,000,000,000,000degrees—FahrenheitorCelsius,takeyourpick.Thenexttimesomeonesaystoyouonablazingsummerday,“Whew!How

hotcanitget?”tellhim.Butdon'tworry.Globalwarmingstillhasalongwaytogo.

If Flames Always Go Upward, Why Do BuildingsBurnDown?

Howdoflamesalwaysknowwhichwayisup?

Lightamatchand,whileit'sburning,twistitintoavarietyofpositions.The

flamekeepspointingunerringlyupward,regardlessoftheorientationofitsfuel.How,indeed,doesit“know”?Youarewellawarethathotairrises.(Ifyouwanttoknowwhy,checkoutp.

107.)A flame,whatever it is,must therefore be carried upward by the risingcurrentofhotair.Andthat'sallweneedtoknowaboutwhyflamesgoupward.Butamorechallengingquestionis,Whatisaflame?Isittherisingairitself,

glowingfromtheheat?Nope.A flame is a region of space in which a chemical reaction is going on:

combustion—areactionbetweentheoxygenintheairandaflammablegas.DidIsaygas?Yes.Butdon'tsolidsandliquidsburnwithflamesalso?Yes.

(AndwhenwillIstopaskingquestionsofmyself?)Wood and coal are solids, and they are indeed flammable; gasoline and

kerosene are liquids, and they are indeed flammable. But none of them willactuallyburnuntiltheyhavebeenconvertedintoagasorvapor.It'stheirvaporsthat burn, because only vapors canmix into the air intimately enough to rubelbows—rubmolecules,thatis—withtheair'soxygen.Moleculescan'treactunlesstheyactuallycomeintocontactwithoneanother.

Theoxygengasintheaircan'tpenetratethesolidorliquidfuel,sothefuelmustvaporizeandgoouttomeettheoxygen.That'swhywehavetolightafire.Wehavetogetthefuelhotenoughinatleastonesmalllocation,soitwillvaporize.Oncethevaporstartsburning,theheatofcombustion—thecombustionreactionreleasesheat—keepsvaporizingmoreandmorefuelandkeepstheprocessgoinguntil all the fuel is gone. (Provided that there is an inexhaustible supply ofoxygen.)

Afuelthatisalreadyavapor,suchasthemethaneinourkitchengasranges,hasnotroublemixingwiththeair,soitcanbeignitedbyamerespark.Propane-burninggasgrillsandbutane-burningcigarettelighterscontainthosefuelsintheliquidform,underpressure.Butassoonastheyarereleased,theyvaporizeintogasesandmixintotheair,whereupontheycanalsobeignitedeasilybyaspark.Whenwelightacandlewithamatch,thematchfirsthastomeltabitofthe

wax, the liquefiedwaxmust travelup thewickbycapillaryattraction,and thematchmustvaporizesomeofthatliquid.Onlythencanthewaxvapormixwiththeairandignite.Withoutawicktoconducttheliquefiedwaxuptowherethereisagoodairsupply,acandlewon'tburn.But if a flame is simply two invisible gases reacting with each other, how

comewecanseeit?Inthecaseofacandle,theflameisvisiblebecauseoxygencan't flowinfastenough to reactcompletelywithallof therapidlyvaporizingwax.Sosomewaxremainsunburnedastinyparticlesofcarbon,glowingyellowfromtheheatandsweptupwardbythecurrentofhotair.As the crowd of glowing carbon particles rises higher, the oxygen nibbles

away at its outer edges, burning particles up completely into invisible carbondioxidegas.Thecrowdofglowingparticlesisthusdepletedmoreandmoreasitrises.That'swhyacandleflametapersofftowarditsupperend.

GlobalCooling?

Could we counteract global warming if everybody in the worldturned on their air conditioners and refrigerators full blast andleftthedoorsopen?

Unfortunately,no,forseveralreasons.First of all, the world's supply of air conditioners and refrigerators isn't

anywherenearwhatyoumightthinkbylookingaroundyourneighborhood.Butevenifeverycitizenoftheless-developednationswereprivilegedtoenjoycoolbedroomsandfrozenpizzas,theamountofavailablecoolthwouldn'tamounttoan ice cube on a glacier. (Yes, I know there is no suchword as coolth.Untilnow.)Ofcourse,youexpected thatanswer,butmaybenot thisone:Whatyouare

proposingwouldactuallyheatuptheworld.As you know too well from your electric bills, air-conditioning and

refrigeration don't come free, in either money or energy. Someone has toproducetheelectricitythattheyuse,andtheproductionprocessitselfgivesoffalot of heat; it's part of the overall energy equation, and therefore part of theenvironmentalproblem.Inmostcases, thefirststepinelectricityproductionis tomakeheat through

theburningofcoalorbynuclearfission.Thentheheatisusedtoboilwatertomakehigh-pressuresteam,thesteamisusedtoturnthebladesofaturbineandtherotatingturbineshaftdrivesanelectricitygenerator.Thatisaremarkablyinefficientchainofevents,andthere'stherub.Oroneof

the rubs. Only about one-third of the fuel's inherent energy ever winds up asusableelectricity.Theother two-thirdsgoesup thesmokestackashotgasesordowntheriverashotcoolingwater,orelseislostwhiletheelectricityisbeingtransmitted through thewires to your house, because power lines are slightlywarmedbytheirresistancetotheelectricityflow.That'swhybirdsperchtherein

coldweather.More than anything else, then, what power plants really do is heat up the

countryside.Themoreelectricityyoudemandforcoolingyourfoodandbrood,themoreheatthepowercompaniesmustflingintotheenvironment.Insteadofthinkingaboutopeningthedoorofyourrefrigerator,you'dbedoingtheworldafavorbyturningtheapplianceoff!Okay, you say, but all of that wasted heat is already part of the global

warmingpicture.Turningour refrigerators andair conditioners looseupon theoutdoorswouldhaveaneffectoverandabovethat,wouldn'tit?Again,unfortunately,no.Considerhowa refrigeratororair conditionerdoes its job. It takes inwarm

air, removes heat from it and discharges that heat somewhere else. Therefrigeratorremovesheatfromtheair insidetheboxandthrowsitout intothekitchen via coils located behind or beneath the box, while the air conditionertakesinairfromtheroom,extractsheatfromitandthrowsitout thewindow.But—and here's the main reason your scheme won't work—these machinesthrowoff evenmoreheat thanwhat they remove from the air.Youmight saythatrefrigeratorsandairconditionersmakemoreheatthancoolth.Here'swhy.We know that the natural direction for heat to flow is “downhill” from a

highertemperaturetoalowerone.Inordertoreversethatnaturaltendencyandforceheattogo“uphill”fromacoolinteriortoawarmerexterior,thefridgeorAChas touseelectrical energy. (That'swhyyouhave toplug it in.)And thatelectrical energy, after it's done its job, turns into heat. You can feel it bytouchingtheoutsideoftherefrigeratororairconditioner;it'swarm.When you add it all up, then, there is more heat—usually about one-third

more—comingoutof the“cooling”machine than theamount it removedfromtheboxor the room.Thebottom lineon theenergybalancesheet tellsus thatthesemachinesareactuallyheatingdevices.Thefinalnail inyourcool-the-worldcoffin is this:Even if fridgesandACs

could operate without using any electric power, the best you could hope forwould be to break even: one calorie of heat discharged somewhere for everycalorie removed from somewhere else. And that wouldn't change the world'soverallquotaofheat.Allyou'dbedoingismovingitaround.

NITPICKER'SCORNER

Converting coal or nuclear energy into electricity is, aswe've seen, a very

inefficientprocessthatputsalotofwasteheatintotheenvironment.Butwhatifyour refrigerator and air conditioner ran on electricity from clean, non-fuel-burningsourcessuchashydroelectric(water),windorsolarpower?Whiletheycertainly aren't aswasteful as theprocesses of burning coal andnuclear fuels,these energy sources still can't be converted to electricitywith anywhere near100 percent efficiency. And the waste energy inevitably shows up asenvironmentalheat.

BARBET

Arefrigeratorisreallyaheatingmachine.

Shocking!

How high does “high voltage” have to be before it's a serioushazard?

Voltage in itself isn't dangerous. A 10,000–volt shock can be no more

disturbing than a pinprick, but you can get a serious jolt from a 12–voltautomobilebattery.What'sdangerousistheamountofelectriccurrentthatflowsthroughyourbodyasaresultofthevoltage.Acurrentofelectricity,asyouundoubtedlyknow,isaflowofelectrons.The

voltageistheamountofpushthaturgesthemtoflowfromoneplacetoanother.Iftheyaregivennoplacetoflowto,noamountofurgingbyavoltagewillmakethemflow.Voltageislikeheight:Nomatterhowhighyoumaybeonacliff,theheightisharmlessaslongasyoudon'ttakeadirectroutetothegroundbelow.Electricalsafetyissimplyamatterofmakingsurethattheelectronscangettothegroundbyarouteotherthanthroughyourbody;theycan'thurtyouifthey'renotflowingthroughyou.That'swhythebirdsaresafeperchingonhigh-voltagetransmissionlines.But it's high timewe focusedon those electrons that I talk about in several

placesthroughoutthisbook.Electronsarethenegativelychargedparticlesthatmakeuptheentirebulkof

allatoms.Everyatomofeverysubstanceisessentiallyablobofelectronswithan incredibly tiny, incredibly heavy, positively charged nucleus buriedsomewhereinthemiddle.Theelectronsinatomshavecertainenergiesthatarecharacteristicofthetype

of atom they're in.Whatmakes a flow of electricity possible is thatmany oftheseelectronsareeasilydetachablefromtherestoftheiratomsandwilltravelelsewhereundertheinfluenceofavoltageshove.Inmostcasesit takesonlyafewvoltstoevictatleastsomeofthemfromtheirhomeatoms.Someelectronsaresoloosethatyoucanjustrubthemoff.Scuffyourshoes

acrossacarpetonadrydayandsomeelectronswillberubbedoffyourshoes'atoms onto the carpet. Because your feet are presumably firmly connected toyourshoes,yourentirebodynowhasadeficitofelectrons,whilethecarpethasasurplus.Normally,allatomsareelectricallyneutral,becausetheyhavejustasmuch positive charge in their nuclei as they have negative charge in theirelectrons.Butnow,yourbodyhasfewerelectronsthanyouratomsrequire.Ifyounowtouchanelectronconductorsuchasametalradiatororwaterpipe,

electrons from the huge supply in the rest of the world—the ground—willeagerly leap to your finger even before it touches the metal, lighting up theintervening air with a crackling blue spark and inspiring you to utter anexpletive. Insteadof awater pipeyoumay even touch anotherperson,who isunlikely to be as electron-deficient as you are, and some of his electronswilljumptoyourfinger,elicitinganexpletivefromhim.But here's the thing: The voltage that urged the electrons to flow into your

finger from the water pipe or your shocked friend may have been severalthousandvolts,butyou'renotdeadbecause thenumberofflowingelectrons—theamountofcurrent—wasmuchtoosmalltodoanyharmtoyourbody.Afterall,yourshoesolesaren'texactlyelectricgenerators, like theonesdownat thepowerplantthatpushgazillionsofelectronsthroughtransmissionlinestoyourhouse.Athome,wherethevoltagehasbeenreducedto120or240volts,ifyoutouch

a“live”wirewhilesomeotherpartofyouisconnectedtotheground,thepowercompany will blindly supply as many electrons as can possibly flow throughyourbody—thatis,aslargeacurrentascanflowthroughyou,givenyourbody'sresistancetotheflow.Andyou'readeadduck.Inshort,thedangerofelectricityliesnotinhowmanyvoltsyouaresubjected

to,butinhowmuchelectriccurrentflowsthroughyourbody.Thetroubleisthatweneverknowwhatthecurrentcanorwillbeinanygivensituation,sowemuststayawayfromanyvoltageabovebatterylevelsatalltimes.

YouDidn'tAsk,but…

If it's thecurrent,not thevoltage, thatcanelectrocuteaperson,howmuchcurrentisnecessaryto“dothejob”?

Electriccurrentismeasuredinamperes.Anampereisahugeunitofelectric

current,equivalentto6billionbillion(6followedby18zeros)electronspassingby every second. So you often hear talk of milliamperes or milliamps—thousandthsofamperes.Onemilliamppassing throughyourbodywillcauseamildtinglingsensation.Tentotwentymilliampscancausemusclespasmsthatmaypreventyoufromlettinggoofthe“hot”object.Twohundredmilliamps,ortwo-tenthsofanampere,maketheheartfibrillate(beatuncontrollably)andcanbefatal.Largercurrentscanstoptheheartentirely,buttheymaynotalwaysbelethalbecausetheheartcansometimesberestartedtobeatnormallyagain.A typical automobile battery is capable of delivering a hundred amperes or

more;ittakesthatmuchcurrenttodothejobofturningoveranengine.Theonlyreasonthatautomechanicsaren'tdroppinglikefliesistheirelectricalresistance;every substance resists the flow of electricity to a certain degree, and theresistanceofhumanbodiesisquitehigh.That'swhyittakesasubstantialvoltagetoforceenoughelectronsthroughapersontoelectrocutehimorher.A12–voltautobatterydoesn'thavethatmuchforce.Wemayencounterdangerouselectricityinmanydifferentcircumstances.I'll

assume that you are not terribly concerned about being formally electrocutedwhileseatedinaspecialchair.Butwhataboutlightning?Thesurgeofelectronsbetweenacloudandtheground,orbetweentwoclouds,ispoweredbytensofmillionsofvolts,andthatcanforcetensofthousandsofmilliampsthroughtheair,whichordinarilywon'tconductelectricityatall.Getintheway,andalotofthosemilliampscangothroughyou.Howdoyou“getintheway”?Bybeingclosetoanobjectthatisofferingthe

lightning'scurrentaneasypath to theground. Ifever therewereasituation in

whichtheexpression“pathofleastresistance”applies,thisisit.Thelightning'selectrons will flow through the best conductors—materials having the leastelectrical resistance—that they can find. If youoffer theman attractivedetourthroughyourbody,they'lltakeit.Of allmaterials,metals are the best conductors of electricity; they have the

lowestelectricalresistance.That'sbecausetheelectronsinmetalatomsareveryloose and can flow right along as part of the current. So when suddenthunderstormshavecomeuponthegreens,abagofmetalgolfclubshasbeenmanyaduffer'stickettothatgreatfairwayinthesky.Because air is such a poor conductor of electricity, the lightning will take

almostanyotheravailablepathratherthanplowingthroughtheairforthoselastseveral yards to the ground. Trees, with their nice, juicy sap inside, offerlightningapreferredalternative,so takingshelterfroma thunderstormunderatreemayalsoearnyouatriptotheultimatenineteenthhole.Butevenifyou'reoutontheseventhgreenwithnotreesnearbywhenastormcomesup,littleoldyou, sticking up only six feet off the ground, can be the lightning's preferredroute.Yourbestlie,sotospeak,isflatontheground,awayfromyourclubsandcart.

WhyDoesn'tItRainRoastedSparrows?

Whydon'tbirdsgetelectrocutedwhenperchingonhigh-voltagepowerlines?

Thisquestion isasoldaselectricpower itself. Ithasbeenaskedalmostas

oftenas“Doyouloveme?”andwithequallyunconvincingreplies.The common answer—“The birds aren't electrocuted because they're not

grounded”—doesn't get to the root of the question.Does everyonewhowalksaway after that explanation really know what “grounded” means? What's sospecialabouttouchingtheground?Asyouknow,anelectriccurrentisaflowofelectrons.Thekeywordhereis

“flow.”Unless theelectronscan flowfromoneplace toanother, theycan'tdoanythinguseful,orharmful,anymore thana streamcan turnawaterwheelbystandingstill.Togetelectriclight,forexample,wemakeelectronsflowthrougha thin tungsten filament, in one end and out the other. In forcing their waythroughtheverythintungstenwireundertheinfluenceofa115–voltpush,theyheatitsomuchthatitglowswhitehot.Noticethatthevoltageisthepush;that'swhatvoltageis:aforcethatpushes

electronsfromoneplace toanotherso theycandoworkforus.Butnomatterhowhighthevoltage,theelectronscan'tdoanythingunlesstheyaregivenapathtotraverse.Thepowertransmissionwiresarethatpath.Undertheinfluenceofahigh-voltagepush, they conduct electrons all theway from thepowerplant toourhouses,wheretheymaybetappedofftoflowthroughalightbulb,atoasteroratelevisionset.Where do the electrons go after they pass through our electric appliances?

TheyreturntoMotherEarth,whichiswheretheelectriccompanygotthemfromin the firstplace.Whereelse, forheaven's sake,could theyhavegotten them?Themoon?SoMotherEarth,whomwefamiliarlyrefertoas“theground,”istheoriginal source of electrons at the power company and their final destination

whenwe'redonemakingthemworkforus.Earthismadeofgazillionsofatomscontaining multigazillions of electrons. By rough estimate, the number ofelectrons on Earth is 1, followed by 51 zeros. That's what I'd call aninexhaustiblesupply.Now,back to thebirds.Their little feet are certainly in contactwith lots of

electronsthatarewaitingtobedrainedoffandreturnedtothegroundviayourelectrictoaster.Butfortunatelyforthebirds,theirbodiesoffernowayofleadingtheelectronstotheground.Thebirdsjustaren'tconnectedtoanything;they'reablindalley,anelectrondeadend.Theelectrons thushavenowayofusing thebirdsasaconduit to theground,andnoelectricity flows through them.That'swhywedon'texperiencearainofelectrocutedsparrows.And by theway,what are those birds doing on the power lines in the first

place,besidesbefoulingyourautomobile?Inthewinter,atleast,theyaretherebecausetheelectriccurrentgoingthroughthewiresgeneratesasmallamountofheat thatkeeps their tootsieswarm.Andwhilewe're at it, howcan they sleeptherewithoutfallingoff?Whentheirfootmusclesarerelaxed,theytightenup,rather than loosen likeoursdo.Sonever fallasleepwhilehangingfroma treebranch.Youmay have seen an electric company lineman, raised from a truck in a

“bucket,”workingonelectricwireswithhisbarehands.He'sassafeasthebirds,because the bucket is completely isolated—insulated—from the ground.Electrons can't find a path through the lineman's body to the ground, so theycan'tmakehimglowlikeawhite-hottungstenfilament.

Thereareotherplanetsintheuniverse,butwehaveafirmattachmentto

ourveryownMotherEarth.It'scalledgravity.2Gravitynotonlylimitsourgolfdrivesandmakesourbodypartssagwith

age, but serves a number of useful functions, not the least of which iskeepingtheatmospherefromflyingoffourspinningEarthlikespitfromarollercoaster.Gravitymakesdust settle andhot air rise. It does innumerablebig and

little jobs for us, such as keeping themoon up and skirts down. It evenallows us to make electricity from water. Gravity is ubiquitous. Evenastronautsdon'tleavehomewithoutit.This chapterwill attempt to tell youhow thismostwide-rangingof all

forces—its effects are felt across the breadth of the universe—operates,even though we can't yet explain what makes it tick, or should I say“stick”?Earth is, of course, spinning atmore than 1,000miles per hour (1,600

kilometersperhour)asitsailsaroundthesunatmorethan10,000milesperhour(16,000kilometersperhour).Andwe'renotevendizzy(mostofus).Butinspiteofthefactthatwearetotallyoblivioustothem(andI'lltellyouwhy), these motions have crucial consequences in our daily lives. Theyaffect hurricanes, ocean currents and ocean tides. They affect—no, theycause—everyday,nightandseasonofourlives.In examining the Earth beneath our feet, we'll visit the center of the

planet, the North and South Poles, Mount Kilimanjaro in Tanzania, aswirlinghurricaneandatoiletbowlthesizeofNorthAmerica.Andfinally,lestweoverlookthefactthatlivingthingsconstitutearather

important component of our planet, we'll see how we use radiocarbon

datingtoexplorethepastlivesofplants,animalsandhumans.

AMatterofSomeGravity

WhydoesgravitytrytoattractallthingstothecenterofEarth?Whytothecenter?WhynottoMecca,orDisneyWorld?

BecausethecenteroftheplanetisthecenterofEarth'sgravity:itscenterof

gravity.You'veheardtheexpression“centerofgravity”before,andnow'syourchance

tounderstandwhatitreallymeans.Butfirst,whatisgravity,or,moreproperly,gravitation?Gravitation isoneof the threefundamental forces inNature. (Theother two

are the strong nuclear force, which holds atomic nuclei together, and theelectroweak force,whichdrives certain radioactive changes and is responsibleforallelectricandmagneticeffects.)Andwhatisaforce?Aforceiswhatmakesthingsmove, and nobody can define it any better, despite pages and pages ofequations.The gravitational force acts between any two pieces of matter and tries to

bringthemtogether.Everyparticleofmatter intheuniverseisattractingeveryother particle of matter, simply because gravitational attraction is an inherentpropertyofmatteritself.(Andnobodyknowsexactlywhy.)Butliketwopeopleonanidealdate,gravitationisn'taone-wayattraction.It's

mutual;eachbodyattractstheother.Andthemoremassabodyhas—themoreparticlesofmatteritcontains—thestrongeritsaggregateattractiveforcewillbe.That'swhywhenyoujumpoffaladder,Earthdoesn'tfallupwardtomeetyou.Its superior mass wins out, andMohammed falls toward the mountain, so tospeak.(If you think you don't know what mass is, pay a visit to the Nitpicker's

Cornerattheendofthissection.Ifyouthinkyoudoknowwhatmassis—andifyouthinkyoudo,youdo—thenreadon.)Ifmassiswhatattractsothermassesby gravitation, then Earth should attract objects toward wherever most of its

mass is concentrated—toward someplacewithin the body of the planet, ratherthantosomeplaceonthesurface.Butstill,whythecenter?Considerthis:Everyparticleofmatterinthebodyofourplanetisattracting,

andbeingattractedby,alltheotherparticles.Aparticlethat'sonlyafewmetersdeepbeneath thesurface isbeingpulleddownbya lotmoreparticles thanarepullingitupward,becausetherearealotmoreparticlesbelowitthanaboveit.Itthereforefeelsanetdownwardpull.Thesamethingcanbesaidforallparticlesthat havemore stuff below them than above them, and they are therefore allattracteddownward.Downward towardwhere?Toward theoneplace that hasequalamountsofstuffarounditinalldirections:thecenterofEarth.Thus,Earthactsasifithasonlyonepointtowardwhichitattractseverythingbygravitation:thecenterofitsgravity.Thecenterofgravityof abowlingball, then, is at itsgeometriccenter.But

everyobject,nomatterhowcomplicated its shape,hasacenterofgravity. It'sthe one spot that is the center of all itsmasses, which is not necessarily thecenterofitsshape.MotherEarthisn'taperfectsphere;she'sslightlysquashedfromnorthtosouth

and,liketherestofus,shebulgessomewhataroundherequator.Herdiameterthrough the North and South Poles is 26 miles (42 kilometers) shorter thanthrough the equator. We could still find the geometric center of this slightlyunsphericalshapeandcallitthecenterofgravity,exceptthatEarth'smassisn'tdistributeduniformly throughout,and it's thecenterofall themass thatcountswheregravityisconcerned.Forexample, if therewereahugemassof leadburieda fewhundredmiles

belowFrance, Earth's center of gravitywould be shifted in that direction.Anobject dropped inNorthAmericawould fall slightlymore towardFrance thanwhatisnow“straightdown.”Moreover,FrancewouldbeclosertoEarth'scenterofgravitythanitisnowandeverythingwouldbeheavier.Thesoundoffallingsouffléswouldbedeafening.Analmostincredibleapplicationofthisprincipleisthemappingoftheoceans'

floorsbythemeasurementofslightchangesingravitycausedbyunderseapeaksand trenches. Wherever there is a concentration of mass due to an underseamountain, the gravitational attraction to thewater above is stronger, so watermoleculestendtogravitatetowardthatlocation,asiftoastrongermagnet.Thatmakesaslightpileupofwaterandabulgeintheocean'ssurfacethat,believeitornot,canbedetectedbyanoverheadsatelliteshootingradarbeamsdownattheseaandwatchinghowtheyreflectbackup.Conversely,wherethereisadeep-sea trench, thewater's surfacemightbedepressedbyasmuchas200 feet (60meters). In thisway, scientists havemade detailedmaps of the oceans' floors

without even gettingwet.Geology books can show you astoundingly realisticpictures of the world's ocean bottoms, as if the waters had been parted by amodern-dayMoses.

NITPICKER'SCORNER

Anobject'smassistheamountofstuffor“matter”thatitcontains.Hereon

Earth, we measure the mass of an object by seeing how strongly Earth'sgravitationpulls it downonto a scale.Themoremass, themorepull.Wecallthatamountofpulltheobject'sweight.Of course, the scale is measuring the amount ofmutual attraction between

Earthand theobject.ButbecauseEarth'spull isalways thesame,weattributethescale'sreadingtotheobject'sownattractiveforce:itsmass.Sowhenyourbathroomscaleshowsahigherreading,youcan'tattributeitto

anincreaseinEarth'smass.It'sthatburdensomemassofyours.(Don'tsaythataloudtoofast.)

ALottaHotAir

Everybodysaysthatheatrises.Butforheaven'ssake,why?

Whydotheysayit,orwhydoesitrise?Theysayitbecausethey'respeakingcarelessly.Thestatementisjustalotof

hotair,becauseheatdoesn'trise.Whattheymeantosayisthathotairrises.Heat is one of many forms of energy; it is energy in the form of moving

molecules. But it's meaningless to say that any form of energy rises, falls orcreeps along sideways.True, energy is always going places and doing things;that'sitsmission.Butitisn'tpartialtoanyparticulardirection,except,ofcourse,forgravitationalenergy,whichonEarth showsadistinctpreference fordown.(But that's only because the center of Earth—lies beneath our feet, whichwechoosetodefineas“down.”)Wespendour livesengulfedinaseaofair,sowhenwethinkofsomething

risingwemean that it's rising through theair.Onlyairorothergasescanrisethroughtheair;solidsandliquidscan'tbecausethey'rejusttooheavy,ordense.Thatlastword,“dense,”isthekey.Thedensityofasubstancetellshowheavy

agivenvolumeorbulkofitis.Forexample,acubicfootofwaterweighs62.4pounds (a liter of water weighs 1 kilogram), while a cubic foot of room-temperature, sea-level airweighs about an ounce (a liter of it weighs about agram).InAmericanTechspeak,wewouldsaythat thedensityofwater is62.4pounds per cubic foot and the density of air is about 1 ounce per cubic foot.Since there are about 1,000 ounces in 62.4 pounds,we could say loosely thatwateris1,000times“asheavy”(strictlyspeaking,“asdense”)asair.Now everyone in theworld except theUnited States of America and three

othergreatpowers(Brunei,MyanmarandYemen)usestheInternationalSystemofMeasurement (called “SI,” forSystèmeInternational in French),which isapprehensivelyreferredtointheUnitedStatesastheMetricSystem.InSIunits,thedensitiesofwaterandairareverysimple:1kilogramperliterforwaterand

1gramperliterforair(atsealevel).Butthat'sroom-temperatureair.Likemostotherthings,airexpandswhenit's

heated, because at higher temperatures its molecules are moving faster andrequiremoreelbowroom,sotheyspreadout,leavingmoreemptyspacebetweenthem.Moreemptyspacemeans thatacubic foot (ora liter)of thewarmerairwillweighless.Itisnowlessdensethanitwas.But the 62.4–dollar question is,Whatmakes that warmer, lighter airmove

upwardthroughtheheavier,coolerair?Well,whatdoes“heavier”mean?Itmeansthatgravityispullingdownonthe

cool air more strongly than on the warm air. (There are more molecules percubic footor liter topullon.)Sowhereverwarmandcoolair find themselvesnexttoeachother,thecoolairwillbepulleddownpastthewarmair.Thewarmairhasnoalternativebuttogetoutofthewayandbedisplacedupward.Lo!Itisrisen.When one of those beautiful hot-air balloons takes off into the blue sky,

peoplegawkingupwardfromthegroundmaywonderwhatforceis“pushingitup.” Now you know that there is no upward force. That bubble of hot air ismerely being subjected to a lesser downward force, comparedwith the coolersurroundingair.Andthathasprecisely,exactly,absolutelythesameeffect.

NITPICKER'SCORNER

Whenhotairrisesthroughtheatmosphere,theveryactofrisingcoolsitoff

somewhat.Iknowthissoundsparadoxical,butdon'tturnthepagequiteyet.Whenawarmmassofairrises,it,ofcourse,gainsaltitude.Massesofaircan

gainaltitude,evenifthey'renotwarm,perhapsbydriftingupagainstamountainandbeingforcedtoswoopupwardalongitsslope.Whateverthereasonforairmasses' moving upward, there must always be equal masses of air movingdownwardtoreplacethem.Theresultisthattherearerisingandfallingmassesofairallovertheworld.Let'sseewhathappenstoaparticularpasselofrisingairasitgainsaltitude.At higher altitudes, the atmosphere is thinner. That's because there's less

atmosphereaboveit,soit'snotunderasmuchcompressionbygravity.(Gravitypulls air down just as it does everything else; airmaybe light, but it still hasweight.) In other words, at higher altitudes there is less pressure from the

atmosphere,andthatallowsourrisingpasselofairtoexpand.But in order to expand, the passel's molecules have to elbow aside the air

molecules thatarealreadyoccupying thatspace.Andthatusesupsomeof thepassel's own energy.What kindof energy?Theonly energy the air has is theconstantflitting-aroundmotionofitsmolecules.Soinelbowingasidetheothermolecules,theexpandingair'sownmoleculeswillbesloweddown.Andslowermoleculesarecoolermolecules,becauseheatitselfisnothingmorethanmovingmolecules.(Thefaster itsmoleculesaremovingthehotteranystuff is,andtheslowerthey'removingthecooleritis.)Therefore,asourpasselofairrisesandexpands,itgetscooler.The higher a mass of warm air rises through the thinner and thinner

atmosphere,themoreitexpandsandthemoreitcools.Thisisonereasonthatit'scolderuponamountainthandowninthevalley.(Butseep.112for themainreasonit'scolderathigheraltitudes.)You have undoubtedly experienced the automatic cooling of an expanding

gas, whether you paid attention to it or not, because there isn't anybodywhohasn't used an aerosol spray can for paint, hair spray, deodorant orwhatever.Grabthenearestone,andtrythis:

TRYIT

Pointanaerosolspraycaninaharmlessdirectionandsprayforthreeorfourseconds.Notice that the can gets cold. It contains a compressed gas—usuallypropane,nowthatchlorofluorocarbons(CFCs)havebeenbanishedbecausetheychewuptheozonelayer.Whenyoupressthevalvetospraytheliquid,thegasisallowedtoexpandandpushtheliquidoutthenozzle.Duringthatexpansion,thegasbecomescooler.

YouDidn'tAsk,but…

Is there any way to tell whether a barbecue grill's propane isgoingtorunoutinthemiddleofacookingsession?

It'sprettyhardtolookinsidethatsteeltankandseehowmuchpropaneisleft

beforeyoufireupthegrill,isn'tit?Notallgrillshavepressuregauges.Buthardwarestoressellaningeniouslittleindicatorthatlookslikeastripof

plasticbecauseit is.Youstickitontotheoutsideofthetankand,bychangingcolor,itshowsyouexactlywherethepropanelevelisinsidethetank.Itworksby detecting the cooling of the propane gas as it flows out through the valveduringuse.Thepropane inside the tank isunderpressure,so it isactuallymostly in the

formofliquid,withsomegasaboveit.(Youcanheartheliquidsloshingaroundifyoujostlethetank.)Whileburningyourhamburgers,youaretappingoffsomeof thegas,andmoreliquidevaporates toreplaceit.Thisevaporationcools thegas,soyouhavealayerofcoolgasabovealayerofwarmerliquid.The strip of plastic contains liquid crystals, which have different optical

propertiesatdifferenttemperatures.Whatitshowsyou,then,isonecolorabovethe liquid's surface, reflecting the temperature of the cool gas, and a differentcolor below the surface, reflecting the temperature of the warmer liquid. Theborderlinebetweenthecolorsiswheretheliquid'ssurfacelieswithinthetank.You'llfindthatthegaugeworksonlywhileyou'rebleedingoffgas.Afteryou

shutdown the tankand itwarmsup, there isno temperaturedifference inside,andtherearenodifferentcolorsonthegauge.

WhereIt'sHilly,It'sChilly

Howdoes amountaintop, even in the tropics, stay coveredwithsnowallyear‘round?

Obviously,becauseit'salwayscolderupthere.Butwhy is it alwayscolderup in themountains thandownat theseashore?

Afterall,doesn'thotair rise?Shouldn't it thereforebehotterup there?There'scertainlyplentyofhotairinequatorialTanzania,butKilimanjaro,whichthrustsits peak 19, 340 feet (5,895 meters) into the tropical atmosphere, is alwayscappedwithsnow.Itallstartswiththesun.Andwhatdoesn't?Withthesoleexceptionofnuclear

energy, the sun is the source of all the heat and all other forms of energy onEarth.AsthesunshinesdownonEarth,itslightpassesquitetransparentlythrough

theatmosphere,asyoumusthaveconcludedfromthefactthatyoucanseethesun.Notmuchhappenstothelightuntilitstrikestheplanet'ssurface.Then,thevarious typesofsurfaces—oceans, forests,deserts,car roofs,GeorgeHamilton—absorb the sunlight and arewarmed (and in some cases tanned) by it. ThismakestheentiresurfaceofEarthagiant,warmradiator,andanythingnearby—suchastheairaboveit—willalsobewarmed,justasyouarewarmedwhenyoustand near the radiator in an old house. (A radiator, not surprisingly, issomethingthatradiatesheatradiation..)Itstandstoreason,then,thatthecloseryouaretotheheat-radiatingsurfaceof

Earth, themore heat youwill be getting from it, just as if youwere standingclosertoahouse'sradiator.SotheairnearestEarth'ssurfaceiswarmedthemost,and the higher you go away from it, the colder the air will be—cold enoughaboveabout10,000feet(3,000meters)thatallprecipitationwillbeintheformofsnowanditwillalmostnevermelt.(Alesserreasonwhyit'scoldinthemountainsisthatasairmassessweepup

themountainside, theyexpandbecauseof the loweratmosphericpressure,andwhengasesexpandtheygetcooler.)ExactlyhowdoesEarth'ssurface,oncewarmedbythesun,transmititsheatto

theairaboveit?Mostlybyradiation—thesamewayaradiatorwarmsyou.Butradiationisn'ttheonlywaythatheatcanbetransmittedfromawarmsubstancetoacoolerone.Itcanalsomovebyconductionandbyconvection.Let'stakeaquicklookateachmechanism.Conduction:Whenyougrab a hot fryingpanhandle (DONOTTRYTHIS

ATHOME!),theheattravelsintoyourhandbyconduction.Theheatenergyisbeing conducted, or transmitted, by direct molecule-to-molecule contact. Hotfryingpanmoleculesknockupagainstyourskinmoleculesandpasstheirheatenergydirectlytothem.Yelpingandreleasingyourgripbreaksthismolecule-to-moleculecontact.(Actually, theyelpdoesn'taccomplishmuch.)Unfortunately,theheatwillalreadybeinyourskin,continuingtodoitsdamageandreplacingyouryelpwithamoreleisurelystringofexpletives.(Tip:That heatwill stay in your skin, continuing to hurt for amuch longer

time than youmight expect, because flesh is a poor conductor of heat. For aminorburn,getthatheatoutasquicklyaspossiblebyholdingitunderthecoldwaterfaucet.)Convection:Whenyouopenyourovendoorquicklytopeekinatyourturkey

andyoufeelablastofhotaironyourface,it'stheairthatiscarryingtheheattoyou.That'sconvection:heatbeingcarriedonthewingsofamovingfluid,suchasairorwater.Inthiscase,theheatismovingbyhitchhikingontheair.Whenhot air rises, the heat is moving upward by convection. So-called convectionovens are ordinary ovens with fans in them that circulate the hot air around,whichspeedsupthecooking.Radiation:Thenext timeyou're inablacksmith'sshop(okay,soimagineit)

noticethatyoucanfeeltheheatofhisred-hotfurnaceonyourfaceclearacrosstheroom.You'renot touchinganythinghot,so it'snotconduction.Andthere'sno moving air, so it's not convection. The heat is reaching you by radiation:infraredradiation.Infraredisatypeofelectromagneticradiation,likevisiblelightexceptthatit

hasalongerwavelengthandhumaneyescan'tseeit.What'suniqueaboutit isthat it is of just the right wavelength that most substances can absorb it,“swallowing”itsenergyandbecomingwarmedbyit.Theinfraredradiationisn'theatperse,inspiteofwhatmanybooksmaytellyou;Icallit“heatintransit.”Itisemittedbyhotobjectsand travels throughspaceat thespeedof light,but itdoesn'tactuallyturnintoheatuntilitstrikessomesubstanceandisabsorbedbyit.Onlyasubstancecanbehot,becauseheatisthemovementofmolecules,and

onlysubstances—notradiations—havemolecules.

YouDidn'tAsk,but…

Doestheairkeepgettingcolderandcolderwithoutlimitaswegohigherandhigherinaltitude?

No, but it does keep getting colder—by an average of about 3.6 degrees

Fahrenheit for every thousand feet (6.5 degreesCelsius per kilometer)—up toaround33,000feet(10,000meters)abovesealevel.That'sjustabithigherthanthe cruising altitude of large commercial jet aircraft.Youmay have heard theairliner'scaptain try to impressyouwhenflyingat thataltitudebyannouncingthatthetemperatureoutsideyourflimsy-lookingwindowwassomethinglike40degreesbelowzeroFahrenheit(−40degreesCelsius).Goodthingthewindowisdouble-pane-insulatedplastic.Above about 33,000 feet (10,000meters), you're in the stratosphere,where

theairstopsgettingcolderasyougohigher; itstaysroughlyconstantatabout−55 degrees Fahrenheit (−48 degrees Celsius) for the next 12 miles (20kilometers) or so, and then starts gettingwarmer. Above the stratosphere, thetemperature does a couple of other flip-flops, getting first colder and thenwarmeragain.What'sgoingon?For one thing, the air has somewhat different chemical compositions at

differentaltitudes.Theheaviermolecules(carbondioxide,argon)tendtosettleouttowardthebottomoftheatmosphere,whilethelighterones(helium,neon)tendtorisetothetopofthepile.Becausethosedifferentmoleculesabsorbthesun'senergiesindifferentways,theymayheatupdifferently.Thestratosphere,forexample,iswheremostoftheozonemoleculeslive.Ozoneabsorbsalotofthesun'sultraviolet(veryshort-wave)radiation,whichheatsitupandmakesthestratosphere warmer than it otherwise would be. Earth's atmosphere is reallyquiteacomplicatedsystem.Beyondtheatmosphere?You'veheard that the temperature inouterspace is

extremelycold,haven'tyou?Well,itisn't.

It'sNottheCold,It'stheHumidity

I'veoftenheardpeoplesaythatit'stoocoldtosnow.Isthereeveranytruthtothat?

It's true that when it's very cold it won't snow, but the statement is

misleading.Once the temperaturegetsbelowfreezingandotherconditionsareright for snow, the will-it-or-won't-it question is purely a matter of theavailabilityofwatervapor.Inmostcases,inorderforittosnowtheremustfirstbetinydropletsofliquid

waterintheairthatcanfreezeintosnowflakes.Butwhentheaccessiblesupplyofwaterisverycold,itstronglypreferstostaywhereitis,namelyintheliquidform, so it doesn't contributemuchwater vapor to the air. Thus, at very lowtemperaturestherejust isn'tenoughwater in theair toformthose tinydropletsthatcouldfreezeandfallassnow.Of course, if it has been very cold for some time,most of the local water

supplies will be inaccessible for vapor production anyway because they'refrozen.In thoseNationalGeographic picturesofblinding,whiteoutblizzards in the

Antarctic,it'snotsnowing—it'sblowing.Verystrongwindsareblowingaroundloose,already-fallensnow.Andwhendidthatalready-fallensnowfall?Duringperiodsofmildertemperatures(butstillbelowfreezing),whenwatervaporwasmoreabundant.

YouDidn'tAsk,but…

Whichpoleiscolder,theNorthortheSouth?

TheSouthPole,where the average temperature is about 56degrees below

zero degrees Fahrenheit (−49 degreesCelsius).At theNorth Pole the averagetemperatureisarelativelybalmy20degreesbelowzero(−29degreesCelsius).Antarcticaisactuallyacontinent,withtheiceandsnowlyingontopofahuge

landmass,whereasthesmallArcticicepackfloatsatoptheArcticOcean.TheSouthPole itself isatanelevationofsome12,000feet(3,700meters),andit'salways colder at higher altitudes. Moreover, the much bigger ice and snowsurface at the South Pole radiates heat awaymore quickly as soon as the sungoesdown.Yetanotherfactoristhatwaterdoesn'tgetheatedorcooledaseasilyasland

does,soitkeepsthetemperatureattheNorthPolefromgoingtoextremes.

BARBET

It'smuchwarmerattheNorthPolethanattheSouthPole.

Wheeeeee!

If the whole Earth is spinning at 1,000 miles per hour (1,600kilometers per hour), why don't we get dizzy, feel the wind orsomehownoticethemotion?Isitjustbecausewe'reusedtoit?

No,it'sbecauseEarth'srotationisauniform,unvaryingmotion,andwecan

feel only changes in motion (Techspeak: acceleration). Any time a movingobjectisdivertedfromitsmotion,eitherbyachangeinitsdirectionorachangeinitsspeed,wesaythatithasexperiencedanacceleration.Accelerationdoesn'tjustmeangoingfaster.Sayyou'reapassengerinacarthat'smovinginastraightlineandisoperating

oncruisecontrol—thatautomaticspeedgovernorthatkeepsthecarmovingataconstant speed.You don't feel any forces pushing your body around, do you?But as soon as the road changes from straight to curved your body becomesawareofit,becauseyouarethrustslightlytowardtheoutsideofthecurve.Orifthe driver suddenly steps on the gas (the “accelerator”), your body becomesawareof itbecauseyouarethrustagainst thebackof theseat.Orif thedriversuddenlyhitsthebrakes(anotheraccelerator,butaslowing-downoneinsteadofa speeding-up one), your body becomes aware of it because you are thrustslightly toward the frontof thecar.Butas longas thecardoesn't speeduporslowdownorgoaroundacurve(Techspeak:angularacceleration),yourbodyfeelsno forces trying topush it around. In effect, yourbodydoesn't know it'smoving,evenifyourbraindoes.Well,yourbrainknowsthatEarthisspinning,butyourbodydoesn'tbecause

themotion is smooth, uniform and continuous.As IsaacNewton put it in hisFirstLawofMotion,abody(includingyours)thatismovingataconstantspeedinastraight linewillcontinuemovingthatwayunlesssomeoutsideforceactson it.Without such an outside force, the body (including yours) doesn't evenrealizeit'smoving.

But, you protest, we're certainly being carried around a curve, aren't we?We'refollowingthecurvatureofEarth'ssurface.Itmaybeaconstantspeed,butitisn'tastraightline.Sowhyaren'twebeingthrustoutward?Well,weare.Butthecurvatureissogradual—Earthissobig—thatthecircularpathisvirtuallyastraight line, so that theoutward force isminuscule.Whenyou thinkabout it,even your car on that perfectly straight road was going around the same bigcurve: the curvature of Earth. If you continued in that “straight line” longenough,you'dgetrightbacktowhereyoustarted.Thisisallverydiscouragingtothediabolicaldesignersofamusementparks(I

callthemabusementparks),whowantustoexperiencemotiontothemax.Theydeliberatelymakeus feelunbalanced,unstable,precarious,disoriented,pushedaroundandinsecure.That'swhynothinginthewholeplacemovesataconstantspeedinasingledirection,exceptperhapstheoutwardflowofmoneyfromyourwallet.Everyrideeitherspinsyouaround,hurlsyoufirstupandthendownorslingsyouthroughsomecrazycombinationofup,downandaroundatthesametime. The best (?) roller coasters are those that combine ups and downswithspeedups, slowdowns, twists and curves. These changes ofmotion, whichwecertainlycanfeel,allfallintothecategoryofaccelerations.Eventhemerry-go-roundisacceleratingyou,becauseitiscontinuallydivertingyoufromastraightline,forcingyoutoturninacircle.Oh, you asked why we don't feel the wind as the cosmic merry-go-round

namedEarthspinsusaround?It'sbecausetheairisbeingcarriedaroundatthesame 1,000-mile-an-hour (1,600-kilometer-per-hour) speed as ourselves, ourcars,ourhousesandevenourairplanes.Sothereisnorelativemotionbetweenusandtheair.

ThisDizzyWorld

If Earth is rotating at around 1,000 miles per hour (1,600kilometersperhour),whycan'tIseeitmovingbeneathmewhenI'minanairplanethat'sgoingalotslower?

Becauseevenwhenyou'reflyingofftoaremoteislandtogetawayfromit

all, you can't escape being part of “it all.” Your airplane is attached to Earthalmost as tightly as the mountains below are, except that the airplane is (wehope)atahigheraltitude.Yourpilotwouldbethefirsttoassureyouthattheplaneisfirmlyattachedto

theair.AndsincetheairisattachedtoEarth,youmightsaythatwe'reallinthesame boat, sailingmerrily eastward alongwith the surface of Earth at around1,000milesperhour(1,600kilometersperhour).(The ground's speed is actually 1,040miles per hour [1,670 kilometers per

hour] at the equator; that's the circumference of 24,900 miles [40,100kilometers]dividedby24hours.Butit'ssloweraswegonorthorsouthontheglobebecausethecircularpathsgetsmaller.)Youdo,ofcourse,seetheground“moving”beneathyouasyoufly.Butit's

yourownairplane'smotionthatyou'reseeing,nottheground's.It'sthesameasseeingthetrees“speedbackward”asyouspeedalongthehighwayinyourcar.That's a very important point to realize: There is no such thing as absolutemotion.Allmotionisrelative.Nothingcanbesaidtobemovingornotmovingwithout specifying “relative to what?” Motion is motion only when it iscompared with some independent reference point (Techspeak: a frame ofreference).Tothetrees,youandyourcararemoving,buttoyouandyourcar,thetrees

aremoving.Who'sright?Ifyouhadbeenborninyourcarasecondago,you'dswear that itwasthetrees thatweremoving, intuitivelyandegotisticallyusingyourselfasthereferencepoint.Itisonlywithexperiencethatwelearntoaccept

referencepointsoutsideofourselves.Ifeachdrivertookhimselforherselfasthereferencepoint, the treeswould be “moving” everywhichway at all kinds ofspeeds,becauseeveryself-centeredperson'sreferencepointwouldbemovinginadifferentdirectionatadifferentspeed.Stationarytrees,however,aresomucheasier to dealwith, sowe humans have agreed to take the trees and the landthey'reattachedtoasourstationaryreferences.Butlet'sstandbackandtakeabiggerviewofEarth.Whenwesaythatapalm

tree on the equator ismoving alongwith the ground at 1,040miles per hour(1,670 kilometers per hour), we have to ask, “Relative to what?”Well, howaboutrelativetothecenterofEarth?That'stheonlypointonorinsidethewholeglobethatisn'tmovingaroundincircles.Inotherwords,we'retakingthecenterofEarthasour“stationary”referencepoint.Butwhoa!Let'sstandbackevenfarther.Thewholeplanetismovingaround

the sun at 10,600miles per hour (17,100 kilometers per hour) relative to thecenterofthesun,whichwecannowtakeasournewreferencepoint.Butthesunitself ismovingrelativetootherstars.Andthestarsaremoving

relativetothecenterofourgalaxy.Andourgalaxy…Andonandon,literallyadinfinitum.Beforewegettoodizzy,let'sgetbackintotheairplane.Sittingthere,anyplace

on the plane is your assumed reference point, so you see Earth “movingbackward”withthe(forward)groundspeedoftheplane.Butrememberthatyouandyour little bagof peanuts and that screamingbaby across the aisle are allmovingtogetheratapproximatelyEarth'srotationalgroundspeed,relativetothecenterof theEarth. I say“approximately”because ifyou're flyingeastward inthesamedirectionasEarth'srotation,theplane'sspeed(relativetothecenterofEarth) is added to Earth's rotational speed; if you're flying westward in theoppositedirectionofEarth'srotation,theplane'sspeedissubtractedfromEarth'srotational speed. Ifyou're flyingnortheastor southby southwest, consultyourhighschooltrigonometryteacher.Canyousay“vector”?

NITPICKER'SCORNER

I said that the plane is firmly attached to the rotating Earth because it is

firmlyattachedtotheairandtheair, in turn, isfirmlyattachedtoEarth.Well,notexactly.

Air is a fluid,meaning that it isn't rigid; it flows.So asEarth turns, the aircan't precisely keep up; it drags and slops around a bit like a puddle in arowboat.Althoughtheplaneisindeedfirmlyheldbytheair,theairissomewhatlooselyheldbyEarth.That'snot to say thatwe're in anydangerof losingouratmosphere;gravityholdsthatwholelayerofairdownquitefirmly.Butwithinthat layer, theair isachurning,blowing,movingmass,and local irregularitiescan still kick your airplane around with tail winds, head winds and coffee-splattering bumps that make you feel as if you're not very well attached toanything.

YouDidn'tAsk,but…

IfIcan'tseeEarthturningfromanairplane,cantheastronautsseeitturningwhentheylookdownfromtheirorbitingshuttle?

No,eventhoughthey'realothigherandmovingalotfasterthananairplane,

the situation is still the same.To them,Earth's surface appears tobe “movingbackward”attheirspeedof18,000milesperhour(29,000kilometersperhour),justasitappearstoyouwhenyou'reinacaroranairplane.Theonlydifferenceis that because of their higher speed, they can see awhole continent “movingby”inlesstimethanitprobablytakesyoutodrivetowork.Youmayhaveseenmotion pictures of space-walking astronauts with the continents “moving”westwardinthebackground.Butwhywestward?Aha!That'saninterestingstory.HaveyoueverwonderedwhytheKennedySpaceCenterwasbuiltontheeast

coastofFlorida, rather thanon thewestcoastofCalifornia?Afterall,MickeyMouseisequallyaccessibleonbothcoasts.Firstofall,wewanttoshootourrocketsoutoveranocean,ratherthanover

anypopulatedareas,sothatboosterrocketscanbesafelyjettisoned.Butsecondandmoreimportant,wehavetolaunchourshuttlesandsatellitesintotheirorbitsaround the globe by shooting them eastward, in the same direction Earth'ssurface is moving. That way, we get a free, 1,000-mile-per-hour (1,600-kilometer-per-hour) shove from Mother Earth. And that means the eastwardAtlanticOceanratherthanthewestwardPacific.Aftertheshuttleisinorbit,itcontinuestoflyeastward,andlookingdown,the

astronautsseeEarth'ssurfaceapparentlymovingwestward,justasiftheywereinanairplaneflyingfromLosAngelestoNewYork.Butwithalotmorelegroom.

HowtoLoseWeight

I'm not sure if this is science or a riddle, but my ten-year-olddaughteraskedmeifapolarbearwouldweighlessattheequatorthanitdoesattheSouthPole.

It's both.The riddle part is that polar bears live at theNorthPole, not the

SouthPole.Furthermore, a polar bear at the equatorwouldn't be a polar bear,wouldhe?He'dbeanequatorialbear.Butlet'stakethequestionatfacevalue;namely,willabear—oranythingelse,

for thatmatter—weighlessat theequator thanitdoesateitherpole?Allotherthings being equal (and, of course, they never are), the answerwould be yes.Slightly.Firstofall,becauseEarthbulgesoutsomewhataroundtheequator, thebear

willbeabitfartherfromthecenterofEarthandgravity'spullwillthereforebeabitweaker.Butwhatyourdaughterundoubtedlyhadinmindwastheeffectofourplanet's

rotation,which isonecomplete turnevery twenty-fourhours. (Isn't thataneatcoincidence?Ofcoursenot.That'showwehumansdefinedtwenty-fourhoursinthe first place.) At the equator, which is 24,900 miles (40,070 kilometers)around, thatworksout toagroundspeed—palmtrees,bearsandall—of1,040milesperhour(1,670kilometersperhour).BackhomeattheexactNorthPole,however,thebearwasn'ttravelingaroundatall;hewasjustrotatinginplace,atthecenterofthemerry-go-round.BecauseoftheEarth'srapidrotationalspeed,bears(andeverythingelse)are

subjected to an outward centrifugal force tending to fling themoff the planet,justasadog flingswateroffhisbackby rotatinghimself rapidlyafterabath.ButthereasonthatthespacearoundEarthisn'tfilledwithflyingbearsisthattheplanet'smuchstrongergravitationalforceholdsthemfirmlytotheground.Nevertheless,theoutward-flingingcentrifugalforcedetractsslightlyfromthe

Earth-holdinggravitational force, so thatanequatorialbear'sweight is slightlydiminished—byalittlemorethanthree-tenthsofapercent.An800-pound(360-kilogram)bearwouldweighabout3pounds(1.4kilograms)lessattheequatorthan he does at the North Pole. In human terms, a 150-pound (68-kilogram)personwouldweigh half a pound (200 grams) less at the equator than at theNorthPole.Ofcourse, thesearetheextremes.Anywhereinbetweentheequatorandthe

poles, the rotational speed of the planet is somewhere between zero and theequatorial speed, because the distance around is shorter. So there is a graduallossofweightasonemovestowardtheequatorfromanywhereinthenorthernorsouthernhemisphere.Ifyouweigh150pounds(68kilograms)atthelatitudeof Washington, D.C., and Madrid, Spain, for example, you'd weigh about 5ounces(14grams)lessattheequator.That'snotaveryeffectiveweight-lossstrategy,however,unlessyougetthere

bywalking.

YouDidn'tAsk,but…

WouldIweighlessatthebottomofadeepmineshaftthanIdoonthesurface?

Boy,youmustreallywanttoloseweight!Yes,you'dweighveryslightlyless.You're probably thinking that your weight is a consequence of Earth's

gravitationalpullonyourbody,andthatifyou'rebelowthesurfacegravityhasalready done part of its job, so there is slightly less pull left. Well, there issomethingtothat,althoughI'dputitdifferently.Thegravitationalforcebetweentwoobjectsactsasifitwerecomingfromthe

centersofgravityoftheobjects.Thatis, itactsasifall themassofeachbodywereconcentratedatthoseprecisespots.Forauniform,regularlyshapedobjectsuchasasphere,thecenterofgravityisthesameasitsgeometriccenter.WhileEarth isn'taperfect sphere, it iscloseenough thatwecansay itsgravitationalattractionispullingyoutowardthecenterofEarth.Nowwhenyou'reonthesurface,youare(ontheaverage)3,960miles(6,371

kilometers) from Earth's center, the seeming location of all its pull. At thebottom of a mile-deep shaft, you are being pulled toward the center by lessEarth-mass than before, because someofEarth'smass is above you and is nolonger contributing to the center-directed pull. (Actually, it's pulling youupward.)IfthereislessmasspullingyoutothecenterofEarth,yourweightisless,becausethat'sthedefinitionofyourweight:thestrengthofEarth'scenter-directedpullonyourbody.Howmuch lesswouldyouweigh?At the bottomof a ten-mile shaft, you'd

weighaboutseven-tenthsof1percentlessthanatthesurface.Notcountingalltheweightyou'dlosedigging.Oddlyenough,thehigheryougoabovethesurface the lessyouweighalso.

Youwouldweigh lesson topof amountain thandown in thevalley, because

you'refartherfromthecenteroftheEarth.Butwait!Putdownthatmountain-climbinggearthatyouboughtfromthose

mail-orderweight-losshucksters.Ifyouweigh150pounds(68kilograms)atsealevel,you'dweighonly7ounces(200grams)lessontopofMountEverest,thehighestpointonEarth.Hardlyworththeclimb,isit?Exceptfortheexercise,ofcourse.

FlushedwithKnowledge

Do toilets really flush counterclockwise in the northernhemisphereandclockwiseinthesouthernhemisphere?

No. It's just another one of those urban legends, probably started by an

overenthusiasticphysicsteacher.Butit'sbaseduponagrainoftruth.MovingfluidssuchasairandwaterareslightlyaffectedbyEarth'srotation.

The phenomenon is called the Coriolis effect, after the FrenchmathematicianGustaveGaspardCoriolis(1792–1843),whofirstrealizedthatamovingfluidonthe surface of a rotating sphere (Earth, for example) would be deflectedsomewhatfromitspath.And by theway, it's theCoriolis effect, not the Coriolis force, as somany

booksandevensomeencyclopediasrefer toit.AforceissomethinginNaturethatcanmovethings,suchasthegravitationalforceoramagneticforce.ButtheCoriolis effect doesn't move anything; it is purely a result of two existingmotions—themotionofairorwater,asmodifiedbythemotionofPlanetEarth.TheCorioliseffectissoweak,however,thatitshowsuponlyinhugemasses

of liquids and gases such as Earth's oceans and atmosphere, where it affectswindsandcurrentsquitesignificantly.Butevenif itweremuchstronger, theCorioliseffectwouldn'tshowupina

toiletbowlanyway,becausethewaterswirlsaroundforaverydifferentreason:waterjetsbeneaththerim.Thetoiletdesignersshootthewaterinonatangent,so as to start it swirling.Of the two toilets inmyhouse,one shoots thewaterclockwise,while theother shoots it counterclockwise.And they're in the samehemisphere.(It'sasmallhouse.)Ontheotherhand,therearenojetsinasinkorbathtub,sowhenwatergoes

down the drain the direction of its swirl is up for grabs.Drainingwatermusteventuallymakeawhirlpoolthatturnsinonedirectionortheother,becauseasits outer portions move inward toward the drain opening, they can't all rush

straighttoitscenteratthesametime.Awhirlpoolisthewater'swayofliningupandtakingturns,whilestillleavingaholeinthemiddlefortheairtocomeupoutofthepipes.Iftheairdidn'thaveanyspaceforrisingtothesurface,itwouldblockthewaterfromgoingdown.But is there any hemispherical preference, no matter how small, for the

directionoftheswirlinasinkorbathtub?

TRYIT

Fillyourbathroomsinkorbathtubandletthewaterquietdownforaboutaweeksothattherearenocurrentsortemperaturedifferencesthatcouldpossiblyfavor one direction over another. Now open the drain without disturbing thewater in the slightest. (Good luck.) The water will begin to drain and willeventuallyformawhirlpool.Repeatthisexperimentathousandtimesandrecordthenumberoftimesitgoesclockwiseandcounterclockwise.

Youdon'thavethetimeorpatiencetodothis?Good.Forgetit.Yoursinkandbathtubaredoomedtofailureanyway,becausethedrainisn't inthecenterandthe water currents wouldn't be symmetrical. Whirlpools are supposed to becircular.

Scientistswhoapparentlyhadlittleelsetodohaveperformedthisexperiment

with the biggest,most carefully constructed, temperature-controlled, vibration-

free,automatic-central-drain-opening“bathtub”youcanimagine,andhavebeenunabletodetectanyconsistentpreferenceforonedirectionortheother.Inotherwords, it wasn't the Coriolis effect that determined the direction, but variousother uncontrollable factors. That's hardly surprising, though, because we cancalculatethemagnitudeoftheCorioliseffecttobeexpected.Inanormal-sizedbathtub it would be so weak that at most it could push the water around toproduceabout1revolutionperday—nowhereenoughtoovercometheeffectsofinadvertentlycausedcurrents.Here's the nitty-gritty on how the Coriolis effect works. Picture Earth as a

globe,withNorthAmericafacingyou.NowreplaceNorthAmericawithagianttoiletbowl.ItsdrainopeningwillbecenteredsomewhereinSouthDakota.(Nooffense,Dakotans.)And let's say that it has nowater jets, so that its flushingdirectioncanbedeterminedentirelybyMonsieurCoriolis.Theglobe,toiletandall,isrotatingfromyourlefttoyourright—fromwestto

east; that's the way Earth turns. But Earth's surface is moving faster at theequatorthanitisfarthernorth,justasahorsieattherimofamerry-go-roundisgoingfasterthanonenearthecenter.That'sbecauseapointontheequatorhasmuchfarthertotravelduringeachrotationthanapointneartheNorthPoledoes.Thus, when you drive your car northward from anywhere in the northern

hemisphere, the farther north you go, themore slowly the surface of Earth iscarryingyoueastward.Youdon'tnotice this,of course,becauseyouandyourcararefirmlyattachedtothesurfaceofEarthandaremovingalongwithit.Airand water, however, are different; they're only loosely attached to Earth'ssurface,andarefreetosloparoundsomewhat.That'swhytheCorioliseffectcanaffectonlyairandwater.Now suppose that you are in theNorthAmerican toilet bowl, floating in a

rowboat somewhere south of the drain opening—say, in Texas. As you startrowingnorthwardtowardthedrain(awayfromtheequator),Earthunderyouiscarryingyoueastwardmoreandmoreslowly.ButyourTexaninertiakeepsyoumovingeastwardat the fasterTexas speed;youareoutrunningEarth's surfaceandgettingslightlyaheadofit.Neteffect?RelativetoEarth'ssurface,youhaveedgedeastward.Youhavebeenforcedintoveeringslightlytoyourright,fromnorthboundtoslightlyeastbound.Similarly(proveittoyourself),aboatfloatingsouthwardfromCanadawould

alsobedeflectedtoitsright:slightlywestward.Sonomatterwhichdirectionthewater(andyourboat)startsoutinonitswaytothedrain,ifit'sinthenorthernhemisphereitwillalwaysbecoaxedintoveeringtotheright.Andrightturnsgoclockwise.(Butdon'tgoawaybeforevisitingtheNitpicker'sCorner.)I'llspareyouseveralmoreparagraphsoftoiletmechanics,butletmejustsay

that in the southern hemisphere every-thing works the opposite way. Largebodiesofmoving air andwater receive a leftward twist, and therefore tend toswirlcounterclockwise.Butremember:Thebodyofwaterhastobehugebeforeyoucanseemucheffect.Oceans,yes;toiletsandbathtubs,no.

NITPICKER'SCORNER

Okay, so tornadoes and hurricanes really rotate counterclockwise in the

northern hemisphere and clockwise in the southern hemisphere—exactly theoppositeofwhatIjustledyoutobelieve.Justholdyourhorsesandeverythingwillturnoutright.Orleft.Whatever.Lemme’splainittoya.Andlet'sstayinthenorthernhemisphere,okay?Hurricanesforminareasof

lowairpressure.Thatmeansthattheairthereisdistinguishablylessdense,lessheavy than the air surrounding it; it's sort of like a hole in the air. Now if,because of the Coriolis effect, all the heavier air surrounding the “hole” isglancingoff it to the right, thatwouldmake the“hole” itself rotate to the left.Thus,theresultinglow-pressurehurricanespinscounterclockwise.No?Well,howaboutthis?Thelow-pressurezoneisaroulettewheelandyou

arethehigher-pressureair.Whilethrustingyourhandtotheright,youbrushitagainstthewheel'sedge.Won'tthatmakethewheelspintotheleft?Or this:You're pushing somekids aroundon one of those little playground

merry-go-rounds,carousels, roundabouts,whirligigsorwhatever they'recalled.Youpushittotherightandthekidsspintotheleft.Right?Or—oh,hell.Justlookatthediagram.Andwhataboutthesouthernhemisphere?Justinterchangeallthe“lefts”and

“rights” in the last fourparagraphs andall the “clockwises”will run theotherway.BONUS:Here is your reward for reading all of the foregoingwithout your

headspinningeitherclockwiseorcounterclockwise: I'mgoing to tellyouwhyallourclocksrunclockwise.

It's because the first mechanical clocks were invented in the northern

hemisphere.Notobvious?Considerthis.Toanobserver in thenorthernhemisphere, the sun is always somewhere in

the southern sky. Looking southward toward the sun, a northern-hemisphereobserverseesitmovingacrosstheskyfromeasttowest,whichtohimisfromlefttoright.Thehourhandsonearlyclocks—andatfirst therewereonlyhourhands—were intended tomimic this left-to-rightmovementof the sun.Hence,theyweremadetomoveacrossthetopofthedialinthedirectionthatwenowcall“clockwise.”Whentherefinementofminutehandscamealongtowardtheend of the sixteenth century, they, of course, were made to go in the samedirection.Canyou imagineaclockwith thehourhandgoingonewayand theminutehandgoingtheother?

BARBET

Ifmechanical clocks had been invented inAustralia, they'd all be runningcounterclockwise.

TheInfernalEquinox

Isittrue,assomepeopleclaim,thatduringthevernalequinoxitispossibletostandaneggonend?

Absolutely.Andduringtheautumnalequinoxaswell.AndonTuesdaysin

February, and anytime during the fourth game of theWorld Series when thecountisthreeandtwoonaleft-handedbatter.Getthepicture?The point, of course, is that equinoxes have nothingwhatsoever to dowith

balancingeggs.Butoldsuperstitionsneverdie,especiallywhenperpetuatedyearafteryearbykookswholiketochantandperformpixiedancesinthemeadowsonthedayofthevernalequinox.Youcanbalanceaneggonendanytimeyoufeellikeit.

TRYIT

Takeacloselookatanegg.Itisn'tglassysmooth,isit?Ithaslittlebumpsonit.Go throughadozenandyou'resure to findseveral thatarequitebumpyontheirwideends.

Nowfindatabletoporsomesuchsurfacethat isrelativelysmooth,butnotglassy smooth. With a steady hand and a bit of patience, you'll be able toaccomplishthismiraculousastronomical(moreappropriately,astrological)featwithout any contribution fromMother Earth, except for supplying the gravitythatmakes the taskchallenging. If thebalancingsurface is ratherrough, likeaconcrete sidewalk, a textured tablecloth or a low-pile rug, for example, it's apieceofcake.Anoldafter-dinnertrick—onanydayoftheyear—wastoconcealawedding ringunder the tableclothand,with feigneddifficulty,“balance” theeggonit.

Somuchfortheoldegggame.Butwhatisanequinox,anyway?PictureEarth,circlingthesunattherateof1revolutionperyear.Thecircle

madebyEarth'sorbit around the sun lies in aplane, just as a circledrawnonpaper lies in the plane of the paper; it's called the plane of the ecliptic. NowMotherEarthwearsanothercirclearoundhermiddle;it'scalledtheequator,anditalsoliesinaplane,calledtheequatorialplane.Wecanimaginetheequatorialplane being extended beyond Earth, way out toward the sun. Funny thing,though: It misses the sun. You usually won't find the sun anywhere in theequatorial plane. That's because Earth is tilted, so its equatorial plane passesaboveorbelowthepositionof thesun.(Theequatorialplaneis tiltedfromtheplaneoftheeclipticby23½degrees.)

AsthetiltedEarthmovesaroundthesun,therewillbetwotimesintheyear

when the twoplanes intersect—that is, two timeswhen the sun, in its eclipticplane,isalsointheequatorialplane,meaningthatitisdirectlyovertheequator.Forhalfoftheyear,thesunisnorthoftheequatorandthenorthernhemispherehas spring and summer; for the other half of the year the sun is south of theequator and the southern hemisphere has spring and summer. The two“crossover” instants usuallyoccuronMarch21 andSeptember23.Those twoinstants are how we define the beginnings of spring and fall in the northernhemisphere; they are called the vernal (spring) equinox and the autumnal(autumn)equinox.The word equinox comes from the Latin meaning equal night, because at

thoseinstantstheperiodsofdaylight(thedays)anddarkness(thenights)areofequaldurationallovertheworld.Youcanseethatfromthefactthatthesunisdirectlyover theequator, favoringneithermoredaylight in thenorthnormoredaylightinthesouth.Without knowing all of this, primitive people found the equal-light-and-

darknessdates tohavespecial significance,ushering in,as theydo, seasonsofwarmthandgrowthorcoldandbarrenness.Soallsortsofsuperstitionsgrewuparound these dates. You can see, though, that there is no “alignment of theplanets”oranyotherpossiblegravitationaleffectsof theequinoxesthatwouldmakeeggsdoanythingweird.Theonly things thatareweirdare thenutswhostillclaimthatthesedateshavemagicalpowers.Oh,yes,thenthere'sthematterofthesolstices.Theyoccurhalfwaybetween

theequinoxes.Thesummerandwintersolsticesaretheinstantsatwhichthesungets as far north or south of the equator as it ever gets during the year. Fornorthernhemispherians,thesummersolsticefallsonJune21or22andthehoursofdaylightarelongest;youmightcallit“maximumsummer”or“midsummer.”It has nomore mystical power over eggs than the equinoxes do, although inScandinavia, where thewinters are long and dark andMidsummerDay is anexcuse for great revelry, it does seem to have amysterious effect on alcoholconsumption.

O,SolarMio

Whentheworldrunsoutofcoalandpetroleum,couldwegetallourpowerfromsolarenergy,whichisinexhaustible?

Probablynot,ifyoumeanmakingelectricityfromsolarpanels.There certainly is lots of sunshine, but capturing it and converting it

efficientlyistheproblem.Let'sdothearithmetic.Every day, the sun shines down upon the surface of Earth an amount of

energyequaltothreetimestheworld'sannualenergyconsumption.Thatmeansthattokeepupwithconsumptionwewouldhavetocaptureandconvertallthesunlightfallingonaboutone-tenthofapercentofEarth'ssurface.Thatmaynotsoundlikemuch,butit'sabout180,000squaremiles(470,000squarekilometers)of solar panels, or about the size of Spain. Double that to take care of theinescapablefactthatit'salwaysnighttimeinhalftheworld.Andoh,yes:Thereareclouds.Butifyouthinkaboutit,allofourenergysourcestodaycomefromthesun,

with only one exception: nuclear energy, which we discovered how to makeaboutsixtyyearsago.Nuclearenergy,intheformofnuclearfusion,iswherethesungetsitsenergyinthefirstplace.Sospeakingcosmically,thereisreallyonlyonesourceofenergyintheuniverse,andit'snuclear.EvenEarth'sinternalheat,thesourceofvolcanosandhotsprings,isfedbynuclearenergyfromradioactiveminerals.Butuntilwelearnedhowtomakesomeofourownnuclearenergydownhere

onEarth,wehad toprocureourshareofcosmicnuclearenergy throughago-between:OldSol.Thesunconverts itsownnuclearenergy intoheatand lightforus,andallofourcurrentenergysourcescomefromthatheatandlight.Theyarethereforesolarenergyinarealsense.Let's look at our “solar energy” sources one at a time. Fossil fuels: Coal,

natural gas and petroleum are the remnants of plants and animals that lived

millions of years ago. But what created those plants and animals? The sun.Plants used the energy of sunlight to grow by photosynthesis and the animalscamealongandatetheplants(and,alas,oneanother).AlllifeonEarthowesitsexistencetothesunandso,therefore,doestheenergywegettodayfromfossilfuels.Waterpower:Hydroelectricpowerplantssuckthegravitationalenergyoutof

fallingwaterbyenticing it intoplummetingdownonto thebladesof turbines,our modern version of the waterwheel. Instead of your having to have awaterwheel or turbine in the kitchen to grind your coffee beans, the turbine-drivengeneratorsconvertthewater'sgravitationalenergyintoelectricalenergy,whichisthenpipedtoyourwalloutletthroughcopperwires.ThewatercascadesdownNiagaraFallsorspillsoverHooverDambecausein

deferencetoSirIsaacNewton(thefalling-appleguy),itistryingtogetclosertothe center of Earth. Then isn't water power really the power of gravitationalattraction?Isn'titEarth-provoked,ratherthansun-provoked?Yes, but hold your horsepower.Howdid thatwater get so high in the first

placethatitcanthenfalldownundertheinfluenceofgravity?It'sthesunagain.Thesunbeatsdownontheoceans,evaporatingwaterinto

theair,whereitisblownaroundbythewinds,formscloudsandeventuallyrainsorsnowsbackdown.Sowithoutthesun'swater-liftingpower,wewouldn'thavewater-falling power. We wouldn't have waterfalls or rivers, because withoutbeingreplenishedfromabovebysun-raisedrainandsnow,they'dallrundry.Windpower:Windmillscaptureenergyfrommovingair.Butwhatmakesthe

airmove?Youguessedit:thesun.The sun's rays shine downuponEarth's surface, a little stronger here and a

littleweaker there, depending on the seasons, the latitudes, cloud cover and anumberofotherthings.Butthelandiswarmedupbythesun'sraysmuchmorethan the oceans are, and that creates unevenly heated air masses around theglobe.Asthewarmermassesriseandthecoolermassesrushinatgroundlevelto replace them, the air flows, producing everything from balmy breezes tomonsoons.Becauseallofthesewindsareultimatelytraceablebacktothesun'sheat,windpoweristrulysun-provoked.

NITPICKER'SCORNER

All right, so all of ourwinds aren't caused by the sun. Some of them arecausedbyEarth,withoutanyoutsidehelp.Earthisrotating,andasitrotatesitcarriesalongathinsurfacelayerofgas—

theatmosphere.Nowgasesandliquidsarewhatwecall fluids, substances thatflow easily, unlike solids. (Most people use the word “fluid” to mean onlyliquids, but gases also flow, and are therefore fluids.) Any fluid will have atough time staying in place when the solid body it's trying to hang on to ismoving.Inanairplane,forexample,thecoffeeinyourcupslopsaroundwhentheplanehitsbumpyairthemomentaftertheflightattendantpoursit.Inthesameway,therotationalmovementofEarthmakestheairsloparound

to a certain extent, like the coffee in the cup.Andwhat is air that's sloppingaround? Wind. So some of our winds are Earth-provoked, rather than sun-provoked.OnewayinwhichEarth'srotationaffectsairmovementsisquiteinteresting.

It'scalledtheCorioliseffect.

HowtoDateaMummy

Canradiocarbondatingtellushowoldanythingis?

Itwon'thelpyoutodeterminetheageofanythingthatisstillalive,suchasa

twelve-year-old posing as a twenty-five-year-old in an Internet chat room.Radiocarbondatingisusefulfordeterminingtheagesofplantoranimalmatterthatdiedanywherefromaroundfivehundredtofiftythousandyearsago.EversinceitsinventionbyUniversityofChicagochemistryprofessorWillard

F.Libby(1908–1980)inthe1950s(hereceivedaNobelPrizeforitin1960),theradiocarbon dating technique has been an extremely powerful research tool inarchaeology,oceanographyandseveralotherbranchesofscience.Inorderforradiocarbondatingtotellushowoldanobjectis,theobjectmust

contain some organic carbon, meaning carbon that was once part of a livingplantoranimal.Theradiocarbondatingmethodtellsushowlongagoitlived,ormoreprecisely(aswe'llsee),howlongagoitdied.Radiocarbontestscanbedoneonsuchmaterialsaswood,bone,charcoalfrom

anancientcampfireoreventhelinenusedtowrapamummy,becauselinenismadefromfibersoftheflaxplant.Carbon is the one chemical element that every living thing contains in its

assortment of biochemicals—in its proteins, carbohydrates, lipids, hormones,enzymes and soon. In fact, the chemistryof carbon-based chemicals is called“organicchemistry”becauseitwasatonetimebelievedthattheonlyplacethatthesechemicalsexistedwas in livingorganisms.Today,weknow thatwecanmake all sorts of carbon-based “organic” chemicals from petroleum withouthavingtogetthemfromplantsoranimals.But the carbon in living things does differ in one important way from the

carbon in nonlivingmaterials such as coal, petroleum andminerals. “Living”carbon contains a small amount of a certain kind of carbon atom known ascarbon-14, whereas “dead” carbon contains only carbon-12 and carbon-13

atoms.Thethreedifferentkindsofcarbonatomsarecalled isotopesofcarbon;theyallbehavethesamechemically,buttheyhaveslightlydifferentweights,or,properlyspeaking,differentmasses.What'suniqueaboutthecarbon-14atoms,besidestheirmass,isthattheyare

radioactive.Thatis,theyareunstableandtendtodisintegrate—breakdown—byshooting out subatomic particles: so-called beta particles.All living things aretherefore slightly radioactive, owing to their content of carbon-14. Yes,including you and me; we're all radioactive. A typical 150-pound personcontains amillion billion carbon-14 atoms that are shooting off 200,000 betaparticleseveryminute!

BARBET

Youareradioactive.

If theworld's nonliving carbon isn't radioactive,where do living organismsget their carbon-14? And what happens to it when the organisms die? Theanswers to those questions iswhere the radiocarbon story really gets exciting.Professor Libby, working right down the hall from my laboratory at theUniversityofChicago,wasabletorecognizetherelationshipsamongaseriesofseeminglyunconnectednaturalphenomena that,whenput together,gaveusaningeniousmethod for looking intoour ancientpast and into thehistoryofourentireplanet.Followthissequenceofevents.(1) Carbon-14 is continuously being manufactured in the atmosphere by

cosmicrays,thosehigh-energysubatomicparticlesthatareshootingthroughoursolarsysteminalldirectionsatvirtuallythespeedoflight.(Someofthemcomefromthesun,buttherestcomefromouterspace.)WhenthesecosmicparticleshitEarth'satmosphere,someofthemcrashintonitrogenatoms,convertingtheminto atoms of carbon-14. The carbon-14 atoms join with oxygen to becomecarbondioxidegas,whichmixesthoroughlyaroundintheatmospherebecauseofwinds.Sotheentireatmospherehasacertainamountofcarbon-14init,inthechemicalformofcarbondioxide.Thisprocesshasbeengoingonforeons,andthecarbon-14intheatmospherehassettledintoafixedamount.(2)TheradioactivecarbondioxideisbreathedinbyplantsonEarth'ssurface

andusedtomanufacturetheirownplantchemicals.(Youknow,ofcourse,thatplants take in carbon dioxide to use in photosynthesis.) All plants on Earththereforecontaincarbon-14.Theyallwindupwithabout1atomofcarbon-14

forevery750billionatomsofcarbonthattheycontain.(3) For as long as a plant is alive, it continues the process of taking in

atmosphericcarbondioxide, thusmaintaining its1-in-750-billionatomratioofcarbon-14.(4) As soon as the plant dies it stops breathing in carbon dioxide and its

accumulation of carbon-14 atoms, no longer being replenished by theatmosphere,begins todiminishby radioactivedisintegration.As timegoesby,then, there are fewer and fewer carbon-14 atoms remaining in the dead plantmaterial.(5) We know the exact rate at which a number of carbon-14 atoms will

diminish by radioactive disintegration (visit the Nitpicker's Corner). So if wecounthowmanyof themare left in someoldplantmaterial,wecancalculatehowmuchtimehasgonebysinceithaditsfullcomplementof1in750billionand, hence, how long ago the plant died. In the case of a piece ofwood, forexample,wewillknowwhenthetreewascutdown;inthecaseofamummy,wecanmeasureitslinenwrappingandcalculatewhentheflaxplantwasharvestedtomakethelinen,andsoon.Neat,huh?But what about animal relics such as bones and leather? How can we tell

when an animal lived and died? Well, animals eat plants. Or else they eatanimals that have eatenplants.Or in the caseof human animals, both.So thecarbonatomsthatanimalseatandfromwhich theymanufacture theirownlifechemicals have the same ratio of carbon-14 atoms as the plants do: 1 out ofevery750billion.Whentheanimaldies itstopseatingandexchangingcarbonatomswithitssurroundings,anditsloadofcarbon-14beginstodiminishinitsprecisely knownway. Bymeasuring howmuch carbon-14 remains today, wecan calculate howmuch time has elapsed since the relic was part of a livinganimal.Therehavebeenseveralspectacularapplicationsofradio-carbondatinginthe

past few decades. One of these was the dating of the Dead Sea Scrolls, acollection of some eight hundredmanuscripts that were hidden in a series ofcavesonthecoastoftheRedSea,tenmileseastofJerusalem,byEsseneJewsaround 68 B.C. They were discovered by Bedouin Arabs between 1947 and1956.Thelinen-wrappedleatherscrollscontainauthentic,handwrittenportionsoftheOldTestamentthatweredeterminedbyradiocarbondatingtohavebeenwrittenaround100B.C.Another triumph of radiocarbon dating was the finding that the Shroud of

Turin, believed by some to be the burial cloth of Jesus, is a medieval fakeconcoctedsometimebetween1260and1390A.D.,which isveryA.D. indeed.This unambiguous scientific result, obtained independently in 1988 by three

laboratories in Zurich, Oxford andArizona, continues to be rejected by thosewhoprefertobelievewhattheyprefertobelieve.

NITPICKER'SCORNER

Howdoweknowpreciselyatwhatratetheamountofcarbon-14diminishes?Everyradioactive,orunstable,atomhasacertainprobabilityofdisintegrating

within a certain period of time. Some kinds of radioactive atoms are moreunstablethanothersandhavehigherprobabilitiesofdisintegrating.Wecan'ttellwhenanysingleatomwilldisintegrate,butaveragedoverthezillionsofatomsin even a minute speck of radioactive matter, the statistics are completelypredictable.It'sliketossingcoins:Youhavenoideawhetheranysingletosswillbeheadsortails,butyouknowforsurethatifyoutossthecoinazilliontimes,therewillbehalfazillionheadsandhalfazilliontails.In the case of radioactive atoms, the statistics are such that one-half of the

atomsdisintegratewithinacertainamountoftimecalledthehalf-life.Andthat'struenomatterhowmanyofthoseradioactiveatomsyoustartwith.Thehalf-lifeofcarbon-14hasbeenmeasuredtobe5,730years.Startoutwith

azillioncarbon-14atoms,and5,730yearslaterthere'llbehalfazillionofthemleft.Afteranother5,730years,there'llbeonlyaquarterofazillionremaining,and so on. So ifwe count the number of carbon-14 atoms in a sample of oldwoodandfindthatitcontainsexactlyhalfasmanyasinasimilarpieceoflivingwood,thenweknowthatitwascutfromthetree5,730yearsago.Andsoitgoesforanyamountofcarbon-14andanyamountoftime,althoughthemathisn'tassimple.Calculusandlogsandstuff.Youdon'twanttoknow.

2Thenameofthegravitationalforceisgravitation,notgravity;gravitysimplymeansheaviness.ButeverybodyoutsideofthePhysicists'Clubcallstheforcegravity,andwheneverIfeellikeitthroughoutthisbook,sodoI.

Onedifferencebetweenhumansandanimals is thatanimalsnever look

upat thesky.All theirfoodlieswithinathinlayeronornearthegroundthatbiologistscallthebiosphere.Andfoodisalltheyneed.Butourhumanneedfornourishmentisspiritualandintellectualaswell

asphysical.Fromthefirstmomentwebegantowonder“how”or“why,”wehavealwayslookedtotheheavensforanswerstoourwondering.The heavens—the greatup there— have always held for us amystical

attraction.Theheavensareaconceptualsublimationofeverythingthatliesbeyond our comprehension. Earliest man looked up there and wonderedwhat the stars were. Then we invented gods, and where else should weestablish their home offices butup there?Heaven, the ultimate unknownbeyonddeath,couldbeplacednowhereelse.Laterinhumanhistory,inanattempttobuildamoretangiblebridgeto

the heavens, astrologers concocted an intricate web of supposedassociationsbetweenthemotionsofthestarsandplanetsupthereandallofourmotionsandemotionsdownhere.Incredibly,inthetwenty-firstcenturytherearestillthosewhobelievethataplanetabillionmilesawaycantuckawinninglotteryticketintotheirpockets.Today,havingexploredeverythingfromthegroundonupas faras the

outeredgesofEarthandbeyond,wefindmuchlessmysteryremainingintheup there.We can fly not only to the top of the sky, but beyond it toother planets. We now have to focus on a more distant realm of theunknown,theoutthereofspace,awholeuniverseofunimaginablesecretsthatwillcontinuetoevadeus,perhapsforever.Wecontinuetolookupwardandwonder.In this chapter we will first explore the lowest level of sky, the

atmosphere,whichnotonly sustainsall lifewith itsoxygen (for animals)andcarbondioxide(forplants),butconveysalllighttooureyes,soundto

ourearsandscentstoournoses.We'llseethemoonturnblue,we'llhearasonicboomemanatingfroma lion'scageandwe'll smellsomeabsolutelydisgusting stuff. Thenwe'll turn out the lights and look at the night sky,whichhas never ceased to enchant humankind.Doyou really knowwhythestarstwinkleandthemoondoesn't?And finally, we'll leave Earth and venture into outer space, where it's

really,reallycold.Orisit?

P-U!

WhenI'msmellingsomereallydisgustingstuff,arelittlepiecesofthatstuffactuallyenteringmynose?

Sorry,butyes.Notactualfragments,but individualmole-cules—molecules

thathaveevaporated from the“stuff”andhave floated through theair toyournose.Butdon'tgetsickatthethought.Ittakesonlyanincrediblysmallnumberof

moleculestobedetectedbyhumansasanodor.Andthemoleculesaren'tevenmoleculesof“thewholestuff.”Virtually everykindof stuff you can imagine (andothers that youmaynot

even care to imagine) are complex mixtures of many different chemicalsubstances.Eachof thesechemicalshasacertain tendencytosendsomeof itsmoleculesoffintotheairasavapor.Themoleculesthatenteryournosearenotagross(punintended)representationoftheentire,disgustingstuff,butonlythemoleculesof itsmostvolatile (easilyevaporated)chemicalcomponents.Whenyousay“IsmellstuffX,”it'sbecauseyouhavelearnedtoassociatethesmellofthose few volatile chemicals with the entire, chemically complex stuff X.Individually, any particular one of these chemicals in its pure form, removedfromitsdisgustingcontext,maybequiteinnocent,eventhoughsmelly.Nevertheless, several unpleasantly odoriferous chemical compounds have

been named for the “stuff” that they are found in, and that they are largelyresponsible for the odor of. Caproic acid is so named because it smells likegoats. (Caper is Latin for goat.) Cadaverine is a chemical component ofputrefyingflesh.Andskatolesmells like…well,skatos is theGreekwordforexcrement.Most astounding fact of the week: Skatole is used in perfumes. Yes, it's a

fixative,whichkeepsperfumes fromevaporating too fast—butnot frombeingdescribedinimpassioned,romantictermsbyadvertisingcopywriters.

Iftheyonlyknew.

VacuumCleanersSuck!

WhatwouldhappenifIoperatedavacuumcleanerinavacuum?

You'dgetanexceedinglycleanvacuum.But seriously, I don't know why you'd want to imagine a thing like that,

because there is nothing cleaner than a true vacuum; it is the epitome ofnothingness. I'll assume, however, that you ask the question out of scientificcuriosity,ratherthanbecauseit'sfunny.What is a vacuum?Peopleuse thewordvery loosely todescribe any space

that contains something less than its normal complement of atmospheric airmolecules. In normal air at sea level, a cubic inch of air contains about 400billion billion molecules. (That's 27 billion billion molecules per cubiccentimeter.)Sucksomeof themoutbyanymeansatyourdisposal—asippingstraw, a vacuum cleaner or a vacuum pump—and you're allowed to call thespaceavacuum.Butit'sreallyonlyapartialvacuum;there'sstilllotsofairinit.Avacuumcleanercan'tevenpumpouthalftheairinacontainer.Aperfectvacuum,arealvacuum,ontheotherhand,isaspacethatcontains

absolutelynothing,notevenasinglemolecule.Butaperfectvacuumisonlyanabstractconcept,likeaperfectlytrustworthypolitician.Itjustdoesn'texistintherealworld.Why?Becauseevenifyoucouldinventa100percentefficientvacuumpump

thatcouldsuckeverylastmoleculeofairoutofacontainer—andyoucan't,forareason that will very soon become apparent—the container itself would besloughing off molecules of itself into the pumped-out space. That's becauseabsolutely every substance in the world has a vapor pressure—a certaintendency for itsmolecules to flyoff intospaceasvapor.That's truenomatterhowsolidandGibraltar-likethesubstancemayappeartobe.Ascientistwouldsay(andIwill)thatthereisanequilibrium—abalance—betweenthesubstanceinthesolidformandthesamesubstanceinthevapor,orgaseous,form.Every

moleculeonthesurfaceofasolidhastheoptionofstayingattachedtothesolidorflyingoffintospaceasagasmolecule.AllI'msayingaboutsolidsiswhatyoualreadyknowtobetrueofliquids:that

moleculesofaliquidcangoflyingoffintospaceasavapor.Water,forexample,evaporates (becomes vapor) at a pretty good clip; its vapor pressure is fairlyhigh.Oils,ontheotherhand,don'tevaporateverymuch;theirvapor-producingtendencies,orvaporpressures,arelow.Much,much,much lower than any liquid are the evaporating tendenciesof

solids.You've never seen a piece of iron “dry up” and disappear into the air,have you?But that doesn'tmean that, now and then, an occasional iron atomisn'tbreakingitsattachmenttoitssolidbuddiesandsailingoffintothewildblueyonder.To put things in perspective: The tendency of liquid water to evaporate is

500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 timeshigher than thatof solid iron.Butthat still doesn'tmean that you could build a perfect vacuum chamber out ofiron.There'llalwaysbeafewironatomsfloatingaroundinit.Moreover,whatwould you use to seal it up airtight? Rubber gaskets? Rubber has a verysignificantvaporpressureandtherewillbelotsandlotsofrubbermoleculesinyour“vacuum”space.Andsoon.Evenifyoucouldbuildavacuumchamberentirelyoutoftungsten

metal,whichhasthelowestvaporpressureofanyknownsubstance—somethinglike one or two atoms flying around in the entire universe—you still couldn'tpump it out completely because the vacuum pump itself is made of stuff likegaskets,oilandgrease,etc.,allwiththeirownsignificantvaporpressures.All this hasn't prevented scientists from trying to produce the best possible

vacuum.Thebestthey'vebeenabletodosofarisaspacethatcontainsonlyafewmillionmoleculespercubicinchorcubiccentimeter,ascomparedwiththe27 billion billion molecules in ordinary air. That's emptier than a wallet justbeforepayday.Butyourquestionimpliedthatyouwantedtostandinacompletelyevacuated

room(ifyoucouldsurvivethere)withavacuumcleanerinyourhand,andyouwanted to know what the vacuum cleaner would suck in. Nothing. The fanwouldjustgo’roundand’roundwithoutsuckingorblowinganything,becausethere'snothingtosuckorblow.Butyouknewthat,didn'tyou,yourascal.

TheCrackofBoom

Whenaliontamercrackshiswhipitmakesaveryloud“crack.”Buthe'snothittingthelionandit looksasifthewhipisn'teventouchingtheground.Whatmakestheloudnoise?

The crack of a bullwhip is actually a miniature sonic boom, produced

becausethetipof thewhipis travelingthroughtheairfaster thanthespeedofsound.When the cat master snaps his whip sharply, he's putting a great deal of

energyintothehandleend.Thatenergyhasnoplacetogoexcepttotraveldownthelengthofthewhipasawaveofmotion.InTechspeak,energyofmotioniscalledkineticenergy,andit'safunctionofbothweightandspeed(actually,massandvelocity,butlet'snotquibble).Agivenamountofkineticenergycancomefromaheavyobjectmovingrelativelyslowlyoralightobjectmovingrelativelyfast.Forexample,inordertomatchthekineticenergyofaten-tontruckmovingat50milesperhour(80kilometersperhour),aone-tonautomobilewouldhavetobetravelingat158milesperhour(254kilometersperhour).(The mathematically unchallenged will immediately recognize that those

speedsaren'tinverselyproportionaltotheweights.That'sbecausekineticenergyisproportionaltothesquareofthevelocity.)Astheenergymovesdownthelengthofabullwhipithaslessandlessmass

toworkwith, because thewhip is tapered. The energy has to staywithin thewhipbecauseithasnoplaceelsetogo,soasthethicknessandweightdecreasethevelocityhastoincrease.Haveyoueverplayed“crackthewhip”oniceskates?Alonglineofskaters

travels in unison, and when the lead skater makes a turn a wave of turningenergyacceleratesdownthelineuntilthelastguyisyankedaroundsofastthathecanbarelyholdon.Inalongbullwhipsnappedhard,thespeedatthetipcaneasilyexceedthespeedofsoundandcreateasmallsonicboom.

Whathappenstotheenergywhenitgetstothetipofawhip?Ifyouexamineawell-usedone,you'llseethatmanyofthe“guysattheend”haveactuallybeensnappedoff;thetipisfrayed.Butmuchoftheenergyhasgonedirectlyoutintotheairassound,whilesomeofitisreflectedbackupthelengthofthewhip.Thereflection turnaround at the tip is incredibly fast, and that fast-reversingwavealsocontributestothenoise.Nowallweneedtounderstandiswhyliontamerseverdecidedtousechairs.

You'd think they could find something more sophisticated and professional-lookingintheTamers“R”Usstore.

YouDidn'tAsk,but…

Whatcausesasonicboom?

There's a lot of nonsense out there about sonic booms. The Columbia

Encyclopedia5thedition(1993)says,“Anobjectsuchasanairplanegeneratessound.Whenthespeedoftheobjectreachesorexceedsthespeedofsound,theobjectcatchesupwith itsownnoise” (Iwish somepoliticianswoulddo that),whichcauses“piled-upsound.”Ridiculous!Willsomebodypleasetellmewhatapileofsoundissupposedtobe?On theotherhand,manypeoplebelieve that there is a tangible thingcalled

“the sound barrier,” and that when an airplane passes through it it makes acrashing sound,as if crashing throughan invisiblewallofglass.That'swrongtoo. I guess people have been led to think that way because of the word“barrier.”Itwasnevermeanttoimplythat therewasaphysicalobstructionupthere in the air, but only that the speed of sound posed an obstruction to thedevelopmentoffasterandfasterairplanes.Itwasanaeronauticaldesignbarrier,notaphysicalone.Nevertheless,whenanairplane“crosses” the soundbarriertherecertainlyisalotofphysicalstressontheplanebecauseoftheshockwave,aswe'llsee.Theactualbarriertosupersonicflightisimposedbythespeedofsounditself.

(Andby theway,supersonicmeans faster than the speedof sound;ultrasonicreferstosoundofahigherfrequencythanhumanscanhear.)Uniquethingsdoindeedhappenwhenanobjectapproachesthespeedofsoundinair.Here'swhatgoeson.Air, of course, consists of molecules: molecules of nitrogen and oxygen,

mainly. Inallgases, themoleculesare flitting frenetically throughspace inalldirectionslikeaswarmofmaniacalbees.Atroomtemperature,forexample,theoxygenmolecules in the air are zipping around at an average speed of 1,070milesperhour(1,720kilometersperhour).Thehotterthegasis, thefasterthe

beesareflying.Anairplaneflyingthroughtheairatapaltryfewhundredmilesorkilometers

perhourgivesthesesprightlymoleculesplentyoftimetogetoutofthewayandlet it through; it's likeapersonwendinghisway slowly throughacrowd.Butwhentheplane'sspeedbecomescomparabletothemolecules'ownspeed,theydon'thavetimetogetoutoftheway;theyjustpileuponthefrontedgesoftheplane and get pushed along in front of it like snowbefore a plow.This rapidpileupofcompressedairconstitutesan“airshock”orshockwave,whichis,ineffect, a loud noise. The soundwaves radiate out in all directions and can beheardasa“boom”onthegroundbelow.Theplanecarriesits“circleofboom”alongwith it, so that people on the ground along the plane's pathwill hear itwhentheplanepassesoverthem.Thisexplainsawaythepopularmisconceptionthatthereisasingleboomastheplanecrossesthesoundbarrier.Itisatravelingboom.Whatdoesallthathavetodowiththespeedofsound?Well,soundisnothingbutaseriesofcompressionsandexpansionsintheair.

Iftheair'smoleculesareflittingaroundatsomeparticularspeed,therewillbealimit to how fast that air can be compressed and expanded, because themoleculescan'tbecompressedandexpandedanyfaster than theycanadvanceand retreat to and from one another. Thus, the speed of the air's moleculesimposesalimitonhowfasttheywillpermitsoundtopassthrough—alimitonthespeedofsoundthroughthatparticularair.Sound will travel faster in warm air than in cool air, because warmer

moleculesaremovingfasterandcancollidewithoneanothermoreeffectively.Example: The speed of sound at sea level is 947 miles per hour (1,524kilometersperhour)at80degreesFahrenheit(27degreesCelsius),butonly740milesperhour(1,200kilometersperhour)at32degreesFahrenheit(0degreesCelsius). Sound also travels faster in dense high-pressure air because themoleculesareclosertogetherandcanbettertransmitcompressions.Puttingitalltogether,then,thespeedofsoundisfastestinwarm,sea-levelair

andslowestincold,thinair.That'swhysupersonicaircraftoperatebestatfrigidhighaltitudes,wheretheydon'thavetogoquitesofast toexceedthespeedofsound.At30,000feet(9kilometers)abovesealevel,theairiscoldenoughandthinenoughthatthespeedofsoundisonly680milesperhour(1,100kilometersperhour).

OnDonnerundBlitzen

Why does thunder sometimes sound like a sharp crack, andsometimeslikealowrumble?

It depends on how far you are from the lightning.The closer you are, the

higherthepitchof thesoundyouhear; thefartherawayyouare, thelowertherumble.First,wehavetoremindourselvesofwhatthunderis.Astroke of lightning is extremely fast; it occurswithwhatmight be called

lightningspeed.Itssuddenheatmakesthesurroundingairwhitehot—heatedtotensofthousandsofdegrees.Theairexpandsattremendousspeed,afterwhichitrapidly cools and contracts back to its normal temperature and pressure. Airmoving so suddenlymakes huge vibrations, and that'swhat soundwaves are:shudders, or pressure waves, moving through the air. Hence, the noise ofthunder.Itwillnotsurpriseyoutolearnthatthundertravelsatthespeedofsound.But

light travels almost a million times as fast as sound. Obviously, then, you'regoing to see the lightning flashalmost instantaneously,butyouwon'thear thethunderuntilittravelsfromthelightningstriketoyourears.

TRYIT

Thenexttimeyouhavetheprivilegeofwitnessingabang-upthunderstorm,countthenumberofsecondsbetweenalightningflashandthebeginningoftheassociatedthunderclap.Dividethatnumberofsecondsby4tofindoutroughlyhowmanymilesawaythelightningwas.Ormultiplythenumberofsecondsby400 toget theapproximatedistance inyards. (But see theNitpicker'sCorner.)Youmaybeshocked—sorry,Imeansurprised—tofindhowclosemanyofthe

lightningstrikesare.Andwhileyou'reatit,noticethatthecloserthelightningis,thehigher-pitched“crack”youhear.Readon.

Sounddoesn'talways travelat thesamespeed. Itdepends, forone thing,onwhatmedium it is traveling through. The pressurewaves can't be transmittedfromoneplace toanotherunless the transmittingsubstancehasmolecules thatcancollidewithoneanothereffectivelyandpasstheenergyon.Supposewehavetwotrainsonthesametrack,collidinghead-on.(DONOT

TRY THIS AT HOME!) The impact energy will be transmitted, car by car,downthelengthsofthetrains,fromtheirenginesall thewaytotheircabooses(unlesstheyderail,ofcourse).Eachcartransmitsitsshocktothenextcarinlinebycollidingwithit;thatcartransmitsittothenextoneinlinebycollidingwithit,andsoon,and theshockenergy travelsdown the trains likeawave.That'show the pressure waves of sound are transmitted through materials, but bycollisionsofmolecules,ratherthanrailroadcars.

Youcan see that if the railroadcarsweren't coupledvery tightly together it

wouldtakemoretimefortheshockwavetotravelallthewaytothecabooses,because timewould be lost by each car's having tomove toward the next carbeforeitcouldcollidewithit.Inthesameway,it takesmoretimeforasoundwave to be transmitted through a substance if themolecules of that substancearen'tveryclosetogether.In air, as in all gases, the molecules are very far apart, so sound travels

relatively slowly through air: about 900miles per hour (1,400 kilometers perhour)atsealevelandroomtemperature.Inwater,themoleculesaremuchclosertogether;soundtravelsthroughwaterat3,300milesperhour(5,300kilometers

per hour). In a dense solid such as steel, it travels at 13,000 miles per hour(21,000kilometersperhour).Somuch for how fast sound travels.Now let's look at how it changes as it

travels.Asyoucanimagine,theclose-upsoundoflightningisasharp,high-pitched

crackle—justwhat you'd expect from a huge spark.But by the time a distantthunderclapreachesyou,itmaybealow-frequencyrumble.Theconclusionwedrawfrom that is that low-frequencysounds travel longerdistances thanhigh-frequencysounds,whichtendtopeteroutwithdistance.Evernoticethatwhenyouridiotneighborplayshisstereoloudenoughtopeelthepaintoffthewallsyouhearprimarilythebassnotes?Thetreblenotesjustdon'tcarryasfarandarealsoabsorbedbetterbythewalls.Thereasonisthatthehigher-frequencysoundsaremakingtheairandthewallsvibratemoretimespersecond,sotheyareusinguptheirenergyfasterastheygo.That'swhythelowfrequenciesofthethunderclapcarryfartherthanthehigh-

pitchedpopsandcrackles,andthefartherawayyouarefromtheactualelectricalevent the lower the sound pitchwill be. That's anotherway of comparing thenearnessorfarness(whyisn'tthataword?)oflightningstrikes.Thefartherawaythestrikeis,thelaterandlowerwillbethesound.Youmusthavenoticedthatthunderisn'tsimplyhigh-orlow-pitched,butisa

mixture of high- and low-frequency sounds.That's because the lightning itselfhappensatamixtureofdistances fromyou.Theboltmaybemiles long,withhuge branches spreading out from the main stroke, so various parts of it arevariousdistances fromyou,and that spreadsout the frequenciesof thesoundsyouhear.Youhavealsonoticedthatthunderrumblesandrollsforanextendedperiod

of time. There are two reasons for that. One, the sound is traveling variousdistances from the various branches of the bolt, and two, it is echoing off thegroundasittravels.Nowyoumaycrawlbackunderthebed.

NITPICKER'SCORNER

Sound waves aren't transmitted through air simply by making the air

moleculescollidewithoneanotherinastraightline,likeastringofrailroadcars

in a crash.Sound energy converts “smooth air” into a series of zones that arealternately compressed and expanded. That is, sound forces the air intoalternatingregionsofhighand lowdensity. It is thesedensityalternations thathityoureardrumattherateofacertainnumberofcompressionsandexpansionsper second. The more of these compressions and expansions that hit youreardrum per second, the higher the frequency, or pitch, of the sound that youhear.Thespeedofsoundinairvariesquiteabitdependingontheair'stemperature

and pressure. The rule of thumb I gave above for timing how far away alightning bolt struck is only a rough guide, because we can't know thetemperature and pressure of the airwhere the bolt createdmost of its thundernoiseor theairconditionsbetweenthereandus.Ichosefoursecondsforeachmileofsounddelay,butyou'llseefivesecondssuggestedinotherbooks.Don'tsweat it. As mentioned above, lightning bolts are long, and they may createthunder all along their paths in air that has a variety of temperatures andpressuresandisatvariousdistancesfromyou.That'swhyyoumayhavetroubletimingthethunderanyway;doyoutimefromtheflashtothebeginningof therumble, or the end? It's far fromanexact science,unlessweknowa lotmoreaboutthelightningboltthanweusuallydo.

FoolMoon

Why is the moon so much bigger when it's rising and setting,comparedwithitssizewhenit'shighinthesky?

Practicallyeverybodyhasnoticed thisoddityatone timeoranother.When

themoonislow,nearthehorizon,it lookshugecomparedwithhowit looksafewhours laterwhen it is higher overhead.The effect is especially noticeablewhenitisabig,beautiful,fulldisk—afullmoon.Butyoucannoticetheeffectatanyphase.People have been wondering about this curiosity for at least two thousand

years, since longbefore they evenknewwhat themoonwasor how itmovesaroundEarth.(Butyouknow,don'tyou?Anydoubts?)Nowwouldyoubelievethatintoday'sso-calledspaceagewecanplayhop-scotchonthemoon,butwestilldon'tknowtheanswertothepuzzleaboutitsapparentsize?Asyoucanimagine,peoplehavecomeupwithdozensof“explanations”over

theyears.Butallsaveafewofthemcaneasilybeshowntobewrong.AdefinitiveexplanationoftheMoonIllusion—andthat'swhatitis,anoptical

illusion—continues to evade science. If itwere amatter of physical science, Iassure you we'd know what's going on by now, because physics is a highlyadvanced science. But apparently it's a matter of human perception, and ourunderstanding of our own psychology isn't nearly as advanced as ourunderstandingoftheworldaroundus.Ifthereisonethingwearesureof,it'sthatasitorbitsaroundEarth,themoon

certainlydoesnotyo-youpanddowninsizelikeafatladyonafaddiet.Earth'soriginal satellite isn't one whit bigger when it's rising and setting near thehorizonthanitiswhenit'sdirectlyoverhead.Soit'sgottobesomethingaboutthewayitappearstoourhumaneyesandbrains.Butwhat?Beforewe shoot down some of thewrong theories and add our support to

some of the more plausible ones, let's prove to ourselves that it is indeed an

illusion—that when we think we're seeing bigger and smaller moons, we'rereallynot.

TRYIT

Checkyourdailynewspaperforthedateofthenextfullmoon;it'srighttherewiththeweathermap.OrcallyourlocalTVmeteorologist.Onthatfatednight,gooutassoonasit'sdarkandsinkyourfangsintothecreamy,whitethroatofabeautifulyoung…Oh,sorry.Wrongbook.Onthatfatednight,gooutassoonasit'sdarkandlocatethemoonwhileit's

stilllow,nearthehorizon.Ifyouhaveto,gotothenearesthilltop.NowtakeouttherulerthatIforgottotellyoutobringalong,holditatarm'slengthagainstthemoon and measure its apparent size. It will span about a half-inch (12millimeters). Write down its “size” to the nearest sixteenth of an inch (ormillimeter).Now wait a few hours until the moon is high in the sky and measure its

apparentsizeagain.WhatdidItellyou?It'sexactlythesame,isn'tit?

or…

Takeseveralpicturesofthefullmoonwhenit'snearthehorizonandlater,asitclimbshigher in thesky.Usea telephoto lens tomakea large imageon thefilm.Ifyouhaveazoomlens,makesureyou'reshootingallpixatexactly thesamefocallength.Useseveralshutterspeedstogetatleastonegoodexposureateach position. You'll find that the moon is exactly the same size in all thepictures!

Sorulersandcamerasaren'tfooled,butweHomosapiensare.Humbling,isn'tit?Nowtoshootdownsomeofthetheoriesthathavebeenadvanced.

“When the moon is low, you're unconsciously comparing it with trees,

buildings andmountainson theground, and it looksbig comparedwith them.Butwhenit'sallaloneupintheskythere'snothingtocompareitwith,soyoudon'tthinkit'ssobig.”Well,maybe.Butevenoutontheprairie,wherethere'snothingatallonthe

horizon,itstilllooksbiggerwhenit'slow.“Whenthemoonislow,you'reseeingitthroughalotmoreairthanwhenit's

directlyabove.Allthataircanactlikealens,refracting(bending)thelightrayslikeamagnifyingglass.”Sorry,Charlie,butanysuchrefractioneffectissmallandcanmakethemoon

lookslightlydistortedinshape,butnotinsize.“Whenthemoonislowyou'relookingstraightahead,butwhenit'shighinthe

skyyourheadistiltedupwardandyoureyeballsareslightlysquashed,andthatmakes…yada,yada,yada.”Nonsense.So what's the answer? Psychologists who study human perception have a

coupleoffairlyconvincingtheories.Theory number one: All our experience since the day we opened our eyes

(somepsychologiststhinkit'seveninborn)hastaughtusthatwhenanobjectiscomingtowardusitgetsbigger.Thinkofanapproachingairplane,orevenaflyball coming toward you in the outfield. But the “moon ball” seems to bebreakingalltherules;asitmovesoverheaditisn'tgettinganycloseranditisn'tgetting any bigger. So your brain interprets it as being unnaturally small, andthat's theconclusionyoudraw.It'snot that thehorizonmoonlooksbigger, it'sthattheoverheadmoonlookssmaller.I'd bemore inclined to believe that theory if themoon rose in amatter of

seconds.When I glance at it high in the sky, I'm reallynot comparing itwithwhatitlookedlikeseveralhoursearlier.Theorynumbertwo:Lookupat thesky. Ifyoudidn'tknowbetter,wouldn't

youthinkitwasahuge,overheaddome?Ancientastronomers,infact,thoughtthatitliterallywasadome,intowhichthestarsandplanetsweresetlikejewels.Eveninthisspaceage,westillseemtohaveabuilt-inimpressionoftheskyasadome.Wecan'tgrasptheideaofinfinity,sowetacitlyimaginethatithasfinitelimits.Pictureitconsciouslyforamoment.NowwhatifIaskedyouhowfaraway

thesky-domeis?You'reverylikelytofeelthattheedgeofthedomethattouchesthehorizonisfartherawaythanapointonthedomethat'sstraightoverhead.Inotherwords, we think of the sky as a somewhat shallow dome; it just seemsmore comfortable that way. Why? Our experience has always told us thathorizonsarefaraway,butthereisnothinginourexperience,andnovisualcuesorclues,totellusthatthe“topofthesky”isalsofaraway.Thus,whenthemoonisnearthehorizon,wesubconsciouslybelievethatitis

fartherawaythanwhenit isoverhead.Butallofourvisualexperiencetellsusthatfarther-awaythingslooksmaller.SowhentheManintheMoonthumbshisnoseatourexpectationsbyremaininghisusualsizeevenwhenhe's“faraway”onthehorizon,ourbrainsays,“Wow!Thatguymustbereallybig.”Andthat'stheimpressionweget.Mymoneyridesonthislastexplanation.

NITPICKER'SCORNER

EverythingI'vesaidaboutthemoon(exceptthatitorbitsEarth)goesforthe

sun too. It also looks bigger when it is near the horizon, and for the samereasons. Haven't you noticed those spectacular sunsets with those absolutelyhuge suns? Now you know why your pictures of sunsets are always sodisappointing.(“Icouldhaveswornitwasmuchbiggerthanthat!”)

Twinkle,TwinkleLittle…Planet?

Whydothestarstwinkle?

The answer that you see everywhere is that the twinkling is caused by

turbulenceintheatmosphere,whichdistortsthelightcomingfromthestar.Butthat doesn't explain why “atmospheric turbulence,” whatever that is, shoulddistort light in the first place, or where the on-and-off blinking effect comesfrom,orwhyonlystars,butnotplanets,twinkle.(That'sright.Ifthatdotoflightintheskyisn't twinkling,it'saplanetoranairplane.Theonlystarthatdoesn'ttwinkleisthesun.Why?Readon.)Mere turbulence in the air, more commonly known as wind, has no effect

whatsoeveronlightwaves.Thelightistravelingat671millionmilesperhour(morethanabillionkilometersperhour),anditcouldn'tcarelessif theair it'spassingthroughispokingalongevenathurricanespeedsof100milesperhour(160 kilometers per hour). What does distort light waves is the varyingtemperaturesoftheair,notitsvaryingspeeds.Obviously, the temperatureofEarth'satmosphere isn't thesameeverywhere.

Notonlyaretherevaryingclimates,buttheair'stemperaturevariesagreatdealwithaltitude.Andthat'snotevenconsideringthecrazyquiltofhot-airpatternsfrom sun-heated land, from factories and from politicians' promises that thestarlightmustpenetratebefore itcan reachoureyesdownhereon theground.Thelightfromastarhastotraverseaveritableobstaclecourseofairatdifferenttemperatures. Turbulence is involved only insofar as thewinds are constantlyscramblingthepatternsofdifferent-temperatureair.Sowhat?Well,whenlightentersatransparentmediumsuchasair,wateror

glass, it generally changes direction. (Techspeak: It is refracted.) That's howcomethosechunksofglassorplasticinfrontofyoureyescancorrectthewayinwhichlightisfocusedonyourretina.Buttheamountthatanygiventransparentmediumwill bend light depends on its atomic constitution. Air, for example,

refractsorbendslightlessthanglassdoes.Buthere'sthepunchlineforallyoutwinkle fans: Warm air bends light to a lesser degree than cool air does.Althoughtheatomsinwarmandcoolairarethesame,theyarefartherapartinthelighter,thinner,warmair,sotheycan'tdotherefractingjobaswell.It'sverysimilartohowwarmandcoldairbendsoundwaves.Nowanystar(except thesun) issofarawaythatweseeitasonlyasingle,

perfectdotinthesky,ageometricpointwithnoapparentsizeatall,evenwhenviewedthroughthemostpowerfultelescopes.Itlooksasifitissendingusonlyasinglerayoflight.Asthatraycomesdowntousthroughtheatmosphere,itisscatteredhitherandyonasitpassesthroughairofmanydifferenttemperaturesandbendingpowers.Wheneveritisscatteredawayfromoureyes,thestarseemstodisappearforaninstant.Thatis,itblinksoff.Whentherayhappenstoscatteragain into our eyes, it blinks on again. This on-and-off flickering is whatromanticsliketocallatwinkle.Forabig-appearingobject like the sunor themoon, all that light scattering

doesn'tmatter,becausetherearesomanylightrayscomingtowardusthatjustasmanyof themarebeingscattered intooureyesasarebeingscatteredawayfromthem,andtheimageappearssteady.Planetsmaylookasifthey'reabsolutepointsoflightlikethestars,butthey're

not. Even a pair of binoculars will show them to you as disks. So they don'ttwinkle for the same reason that the sunandmoondon't:While someof theirlight rays are being scattered away from our eyes, there are enough otherscomingtowardustokeeptheimagesteady.Andbesides,“Twinkle,Twinkle,LittlePlanet”doesn'thavetherightrhythm.

YouDidn'tAsk,but…

Whydodistantobjectsseemtorippleandshimmyonahotday?

Formuch the same reason that stars twinkle, except that there are enough

lightrayscomingfromtheobjectthatnomatterhowmuchtheyscatter,someofthemwillalwaysbereachingyoureyes.Sothere'snoactualtwinkling.Whenyoulookdowntheroadonahotday,youmayseeshimmering“linesof

heat”or“heatwaves,”andadistantcarwillappearwavy.Whatyou'reseeingisthe effects of light refraction: the bending of light rays when they leave onetransparentmediumandenteranother.Inthiscase,thelightraysfromthecaryou'relookingatarepassingthrough

various regionsofairon theirway toyoureyes—airofdifferent temperaturesanddifferentlight-bendingabilities,dependingonjusthowhoteachsectionoftheroadhappenstobe.Alightraycomingatyoufromonepartofthecarmaybe traversing a different combination of air temperatures—and hencemay bebentbyadifferentamount—thanthelightfromsomeotherpartofthecar.Andthatlookstoyouasifthecaritselfisbent.Butwhydoes thedistortedimagekeepwavering?Becausetherisinghotair

andotheraircirculationskeepchangingthepatternsofairtemperaturesthroughwhich the light is traveling. If the consequent amount of ray-bending keepschanging,sodoesyourimageofthecar.

ManinMoonMoonsEarth!

HowdoesthemoonmanagealwaystokeepitssamefacetowardEarth?

Sounds odd, doesn't it? Either it's themost colossal coincidence that ever

occurred, or there's something real fishy going on. Well, even the fishiest-seemingcoincidencescanhaverationalexplanations.Your first guessmight be that themoon isn't spinning on its axis the way

Earthis,andthatitjustgoesaroundus,maintainingthesameorientationtowardus.Butitisspinning.Andevenifitweren't,wewouldstillbeseeingallsidesofitasitcircledEarth.Here'swhy.

TRYIT

Let's say that you're themoon andyourbuddy isEarth.Stand several feetaway,facinghim.Nowkeepstaringatthesamespotonthewall—thatis,don'tspinonyouraxis—andcirclearoundhim.(Insquaredanceparlance,performado-si-do.)Noticethatatsometimeduringyourcircling,youcan'thelpshowinghim your backside. To avoid that, you'd have to keep facing him all thewayaround,andthatrequiresthatyourotateonefullturn.

Then if themoon is indeed rotatingwhile circlingEarth, and yet keeps thesamesidealwaysfacingus,itmustbeturningataperfectlysynchronizedrate:exactly onemoon turn for each circle that itmakes aroundEarth.How in theworldcanthathappen?

Well, you know that the moon and Earth are tugging on each other

gravitationally.Youalsoknowthatthemoon'stugonEarthpullstheoceansupintobulgescalledtides.ButwhatyouprobablyneverthoughtofisthatEarthisalsopullingupbulgeson themoon—notbulges in its nonexistent oceans, butbulges in the moon's very ground. Slight bulges, to be sure, but bulgesnevertheless.Callthemtidesintheground,ifyouwish.RememberingthatEarth'spullonthemoonismuchstrongerthanthemoon's

pull on Earth because Earth is so much more massive, you will realize thatEarth's pull candeform themoon a lotmore than themoon's pull candeformEarth.This deformation of the moon by Earth's gravity acts like a brake on the

moon'srotation.It'sas ifEarth'sgravityweretryingtoholdonmoretightlytothemoonbumpsbecausethey'reatinybitcloser.Andthathasaslowing-downeffect.Soevenifthemoonwasspinninglikeatopbillionsofyearsago,Earth'sgravity has slowed it down to its present crawl.We have grabbed themoon,tameditandmadeitpirouettetoourowntune.Andbytheway,themoonisdoingthesamethingtoourhomeplanet,albeit

to a much lesser extent because its gravitational pull is weaker. That is, bytuggingontheoceans,themoonhasbeenslowingdownEarth'srotation,makingourdays longer.About900millionyears agoanEarthdaywasonly eighteenhourslong.Backthen,thelaborunionswerejoyful,becauseanythingmorethansix hours of work was considered overtime. White-collar workers weren't sohappy,though,becausetheirannualsalarieshadtostretchovera487-dayyear.

NITPICKER'SCORNER

Let'scleanupacoupleofpointsaboutthemoon.Firstofall,themoondoesn'tshowpreciselythesamefacetousallthetime.

Althoughitkeepsspinningataconstantrate,itwobblesalittlebitfromlefttoright, teasingusperiodicallywithaglimpseof itsbackside.Mooningus,so tospeak.Moresurprising,perhaps,isthefactthatifyoureallywanttobepickyabout

it, themoondoesn'torbitaround thecenterofEarth.That is, thecenterof themoon'sorbitdoesnotlieatEarth'scenter.Thereasonforthatisthatgravitationisnot a one-way street,withEarth holding themoon in orbit. Themoon alsoholds Earth, but not as strongly, of course, because of its smaller mass. Youmight say, then (and Iwill), that Earth is trying to orbit themoon to a slightextent. The result is that they're each orbiting the other in a sort of whirligigdance.It's like two square dancers, a heavy man and a light woman, executing a

swing-your-partnermaneuver.Eachisorbitingaroundtheother,butthewoman,being lighter,doesmostof theorbiting.Somewhere inbetween them there'safixedpoint that isn't going in circles at all; it is thenonmovingcenterofbothorbits.Thatpointwill be closer to theman than to thewoman,becausebeingheavier,heisabetteranchorforthewholewhirlingconfiguration.Wecallthatstationarypointthecenterofmassofthecouple.It'sthesamewiththedanceofMarilynMoonwithErnieEarth.Thecenterof

bothorbits—thecenterofmassoftheEarth-moonsystem—willbemuchclosertoMr.EarththantoMs.Moon.Infact,Earthissomuchheavierthanthemoonthat the center ofmasswill actually lie somewherewithin Earth—somewhereoutwardfromitsgeometriccenter.Tosumup:InsteadofsayingthatthemoonorbitsEarth,weshouldreallysay

thattheEarth-moonsystemrevolvesarounditscenterofmass.

It'stheMoon,Stupid

What makes the ocean tides? I know, it's the moon. But how?Andwhy are there two high tides and two low tides every day,whenthereisonlyonemoon?

Wheneversomeonepompouslyproclaimsthattheocean'stidesarecausedby

the moon, everybody mutters, “Uh, okay,” and goes away just as puzzled asbefore.“It's themoon” is a cop-out, because a real explanation requires a lotmore

thanthat.Oceantidesarethenetresultofseveralforcesproducedbymotionsofthemoon,thesunandEarthitself,allinteractinginacomplexway,butallverythoroughlyunderstoodbyoceanographersandgeologists.Comewithme,andwe'llsortitallout.Well,mostofit,anyway.PictureEarthandthemoonastwoballs,withthesmallermoon-ballcircling

theEarth-ballmoreorlessaroundtheequator.ButstopthemotionsofEarthandmoon for amomentwhile themoon is to the right of Earth.Got the picture?Earthleft,moonright.Themoon'sgravitationalforceistryingtopullthecenterofEarthtowardits

owncenter—toward the right. (Why the centers?).Let's call this attraction the“center-to-center pull.” But on Earth's right-side oceans, the pull is slightlystrongerthanthecenter-to-centerpull,becausetheright-sideoceansareclosertothe moon than Earth's center, and gravitational force is stronger at closerdistances.Thisslightlystrongerpullraisestheoceanswithrespecttotherestoftheplanet,makingthembulgeoutward,andwehaveahightideonthesideofEarthfacingthemoon.Meanwhile,onthesideofEarthoppositethemoon(theleftside),theoceans

areslightlyfartherfromthemoonthanEarth'scenter,andtheythereforefeelaslightly smaller rightward pull than the center-to-center pull. The strongercenter-to-centerpullpullsEarthslightlyawayfromtheleft-sideoceans,andthe

oceansare leftbulgingoutwithrespect to therestof theplanet.Thatmakesasecondhightideontheoppositesideoftheworldfromthemoon.Thus,therearealwaystwobulgesofoceanwateronoppositesidesofEarth—

onthesidefacingwhereverthemoonhappenstobeatthemoment,andonthedirectlyoppositeside.Now let's permitEarth to rotate.As it spinsmerrilybeneath the twobulge-

makingforces,eachlocationonEarthpassesthroughahigh-tidesituationtwiceinitstwenty-four-hourrotation,givingeachlocationtwohightidesperday.Andin between the high tides, what else? Two low tides. After all, the high-tidewaterhastocomefromsomeplace.Andbytheway,ifyou'renottooselectiveaboutwhomyoulistento,youmay

haveheardsomeonesaysomethinglikethis:“Humansaremorethanhalfwater,andsincethemoonactsonwater tomaketides, thephasesof themoonaffecthumanbehavior.”Wellnow, lookhere.Theoceansof theworldweigh1.5billionbillion tons

andaremovedonlyafewyards(meters)bythemoon'sgravity.Ahumanbodymightcontainafewhundredthsofasingletonofwater.Gravitationalforcesareproportionaltomass;figureitout.Anyonewhobelievesthatthemoon'sgravitycanaffecthumanbehaviormusthavewateronthebrain.

NITPICKER'SCORNER

The two daily high tides aren't exactly twelve hours apart. At any given

locationonEarth,they'retwelvehoursandfiftyminutesapart.Why?Becausethebulgesarecausedbythemoon'sattraction,andtheymove

along with the moon in its travels around Earth. While Earth makes its fulleastward rotation in a period of twenty-four hours, the moon is also movingeastward,soitgetsslightlyaheadofanygivenlocationonEarth.Earththenhasto rotate an extra fiftyminutes in order for that location to catch upwith themoon—thatis,tocatchupwiththenexthigh-tidebulge.Another important nit to pick: The tides aren't caused only by the moon.

There'sanotherbigthingouttherewithanawfullotofgravity—thesun.It's27milliontimesmoremassivethanthemoon,butit's397timesfartheraway.Theway gravitationworks, distance reduces the force a lotmore powerfully thanmassincreasesit.(Techspeak:Theforceof

gravity increases in direct proportion to the mass, but it decreases inproportiontothesquareofthedistance.)It works out that the sun's gravitation affects the tides about 46 percent as

stronglyasthemoon'sdoes.Tracingthesubtleeffectsofthat46percentonthetideswouldbealotmoreworkthaneitheryouorIcaretodo.Ignoringthesun'seffectsstillgivesusaprettygoodunderstandingofthetides.

TimeandTideWaitforNewMoon

Whyarethehightideshigherwhenthemoonisfull?

It'seasytofoolyourselfintothinkingthatthemoonisbiggerwhenit'sfull,

andthatitthereforepullsontheoceansmorestronglytomakehighertides.ButthemoonisalwaysthesamesizeanddistanceawayasitcirclesEarth.Itisjustlitupdifferentlybythesunatdifferenttimesinitsjourney.That'swhyitlookstouslikeawholedisk(afullmoon),apartialdisk(asemicircleoracrescent)ornodiskatall(anewmoon).Inotherwords,itgoesthroughphases.Whenthemoon,sunandEarthhappentobealllinedup,weseeeitherafull

moonoranewmoon.ThemoonlooksfullwhenEarthisinthemiddle,betweenthemoonandthesun.ThinkofitasifwearesittinginTheaterEarth,withtheMan in theMoonon the stageandSpotlightSunbehindus.We'll see the fullfaceoftheManintheMoon.Ontheotherhand,whenthemooncirclesaroundbehindusEarthlings,gettingbetweenusandtheSpotlightSun(turnaroundinyour theater seat and look at the moon behind you) we see the moon as adarkeneddisk—thatis,anewmoon.Ineitheroftheselined-uparrangements—fullmoonornewmoon—thesun's

andmoon'sgravitationalforcesarepullingalongthesamelineofdirection,andthey reinforce each other to produce an extra-high tide: a “spring tide.” Thenamehasnothingtodowiththespringseason;it'scalledthatbecauseit“springsup”twiceineverymooncycle:abouteverytwoweeks.

BlueMoon,PartOne

Howoftenis“onceinabluemoon”?Doesithaveanythingtodowiththerealmoon?

Therearetwoanswerstothelatterquestion:NoandYes.TheNo answer: The expression “when themoon turns blue” was used for

hundreds of years to mean “when hell freezes over” or “fat chance.” “Bluemoon”firstappeared inprint in thenineteenthcentury,butwasprobablyusedeven before that because it's a quirky idea and almost rhymes. There was nointentiontoconnecteitheroftheseexpressionswiththemoon'sactualbehavior.(Butpeoplemightonceinawhilehaveseenareal,blue-tingedmooncausedbysmokeintheair.)TheYesanswer:Whenever thereare twofullmoonsinthesamemonth, the

secondone isoften referred toasabluemoon.Calling it that isavery recentdevelopment.ItdatesfromaMarch1946articleintheastronomymagazineSkyand Telescope, based on an article in theMaine Farmer's Almanac that hadappeared ten years earlier. The editors of Sky and Telescope have recentlyadmitted,however,thattheymisinterpretedtheMaineFarmer'sAlmanacarticleandthatthetitle“bluemoon”wasreallymeanttobebestoweduponthefourthfull moon in any season. Seasons are three months long, so they wouldordinarilyhaveonlythreefullmoons.That makes a big difference. The fourth full moon in a season is not

necessarilythesamefullmoonasthesecondoneinamonth;itmighthappentofallinamonthallbyitself.Butthefourth-one-in-a-seasonconceptisnotaseasyfor people to grasp as simply counting the number of fullmoons in amonth(anybody can count up to two), so I predict that the second-full-moon-in-a-month“bluemoon”willneverdie,nomatterwhattheastronomerssay.Itisn'tveryunusualfortwofullmoonstofallinthesamemonth;ithappens

about four times a year, much more frequently than a fourth full moon in a

season,which reallydoeshappenonlyonce inabluemoon—every twoandahalfyearsorso.Here's how two fullmoons canoccur in a singlemonth.Asyouknow,our

calendar contains eleven 30- or 31-day months plus February. But the lunarmonth, the time it takes themoon tocircleEarth (youknow,ofcourse, that itdoes)andreturntothepositioninwhichitistotallyilluminated—full-looking—isonlyabout29½days.So twoof those29½-day illuminationscaneasily fallwithinthesame30-or31-dayperiod.ItcanneverhappeninFebruary,though,becauseatonly28or29days,Februaryisshorterthanthelunarmonth.

BlueMoon,PartTwo

C'mon,now.Doesthemooneverreallyturnblue?

Yes,butonlyonceina…greatwhile.Therehastobeexactlytherightkind

ofsmokeordustintheair.It happened most spectacularly in 1883, when the Indonesian volcano

Krakataublewitstop,spewingdustallaroundtheglobe.ThebluestmoonsinceKrakatauwascausedbyaseriesofforestfiresinwesternCanadain1951.Whenthesethingshappen,themoonitselfdoesn'tchangecolor,ofcourse;it'sjustthewayitappearswhenviewedthroughthesmokyair.Understanding this will take us a bit away from astronomy, but the

explanationinvolvessomefundamentalideasaboutthenatureoflightthatwillserveuswellinmanyothersituations.Soevenifyoudon'tcareafigaboutsad-lookingmoons,stickaround.What'sbehindablue-appearingmoon—andalotofotherthingsthatwesee—

isthefactthatlightscatters.Idon'tmeanthatitreflects,suchaswhenitbouncesback off the bathroom mirror to remind you that you're getting older. By“scattering,” scientists mean that individual particles of light bounce offindividualmoleculesandothertinyparticles,likebilliardballsbouncingoffoneanother.DidIsayparticlesoflight?Yes,indeed.Andyouthoughtthatlightconsisted

ofwaves?Waves of energy, rather thanparticles of energy?Well,we're bothright.Let'sgetthatlittleproblemoutofthewayfirst.Light—andallotherso-calledelectromagneticradiations,fromradiowavesto

Xrays—areindeedwavesofpureenergy,travelingthroughspaceatthespeedof,uh,light.Wecanmanipulatelightwavesbyputtingthemthroughspeciallyshaped pieces of glass or other transparent materials: lenses and prisms.Practitionersofthescienceofoptics,whobringusourmicroscopes,telescopesandeyeglasses,havenoproblemtreatinglightraysasiftheywerepurewaves,

makingthemreflectandrefract(changedirection)toperformavarietyofusefulopticaltricks.But certain other things that light does, such as knocking electrons out of

atoms,canonlybeexplainedif lightconsistsofastreamoftinyparticles, likebulletsfromamachinegun.Wecallthoselightbullets—andthebulletsofotherelectromagneticradiations—photons.Soisabeamoflightastreamofwavesorastreamofparticles?Perhapsthe

most astounding and unsettling discovery in human historywas that light andotherelectromagneticradiationsbehaveasiftheyarebothwavesandparticles.Or, ifyouprefer, theybehaveaseitherwavesorparticles,dependingonwhatyoucatchthemdoingatanyparticularmoment.WhenamannamedAlbertEinstein(1879–1955)proposedin1905thatlight

canknockelectronsoutofatomsasifitwereastreamofbulletlikeparticles,heearnedaNobelPrize.(Hisprizewasawardedforthiswork,forexplainingthisso-calledphotoelectriceffect,notforhistheoriesofrelativity,whichcamemuchlater.)Itwasalmostashardforphysiciststoswallowthistwo-facedideaasitisforyou.Butplentifulevidencehassinceprovenbeyondanydoubtthatit'strue.Not only that, but (are you ready?) the reverse is also true: Honest-to-Godparticlessuchaselectronscanactasiftheyarewaves.Physicistsarenowquitecomfortablewiththisweirdsubatomicschizophrenia

andrefertoitaswave-particleduality,orsimplyduality.Noamountoffurtherpalaveronmypartwillmakeitseemanymorereasonabletoyou.That'sjustthewayitis,andifyoudon'tlikeit,movetoanotheruniverse.Didn'tmeantobebrusquethere,butwe'vegot tomoveonandexplainblue

moons.Isaidthatthey'recausedbythescatteringoflightphotons,presumablyafter

collidingwithsomething.Well,whatwouldcauseaparticleoflighttoveeroffinadifferentdirectionafteracollision?Obviously,acollisionwithsomeotherparticle that isat leastasbigas it is.Becausecertainly,abaseballwouldn'tbescatteredbyacollisionwithamosquito,wouldit?Butifitcollidedwithanotherbaseballduringitshome-runflightoutoftheballpark,itwouldbedeflectedintosomemuchlessfortuitousdirection.So we must conclude that a photon of light will be scattered best when it

collideswithsomethingthatisapproximatelyitssize.Butwhatisthesizeofaphoton?Howdoyoumeasureit,whenitwon'teven

standstill,oscillatinglikeawavewheneveritfeelslikeit?Well,iflightcanbeschizophrenic,socanphysicists,whotakerefugeinthewavedescriptionoflightwhenever they feel like it. They consider the “size” of a photon to be itswavelengthwhenit'sactingasawave.(Asawaveoscillatesupanddown,which

iswhatwavesdo,thewavelengthisthedistancebetweentwosuccessive“ups”or twosuccessive“downs.”)Ourconclusion, then:Lightwillbescatteredbestfromobjectsthatareapproximatelythesamesizeasitswavelength.Holdon,now.Themoonisabouttoturnblue.The light that comes to us from the sun is a mixture of all colors—all

wavelengthsfromred,thelongest,toviolet,theshortest.Whenallthedaylightcolorsaremixedtogether,astheyarewhenwereceivethemhereonEarth,oureyesandbrainsinterpretthelightasnocoloratall:whitelight.That'sthelightthat we can see. But there are other “colors”—infrared and ultraviolet, forexample—thatourhumaneyesaresimplyinsensitiveto.In the light thatwe can see, blue has just about the shortestwavelength; it

consists of the “smallest” photons. It will therefore be scattered best by thesmallestparticlesthatitmayencounterinitstravelsthroughtheair,namelythemoleculesofnitrogenandoxygenthat theair itself ismadeof.ItwasEinstein(again) who figured out exactly how molecules scatter light of differentwavelengths:Theshorterthewavelength,themorethescattering.Nowwhat if theaircontainssomebigger-than-moleculesizedparticles,such

as particles of dust or smoke? Then the other colors of light, the longerwavelengths,canbescatteredmorethanusual.If—andit'sabigif—aforestfireorvolcanoshouldhappen tomakesmokeordustparticles thatareexactly theright size to scatter longer-wavelength red light, then the light coming downfrom themoonwill have a lot of its red scattered away before it reaches theground.Andlightthatisdeficientinredlooksbluishtous.Hence,sodoesthemoon.

YouDidn'tAsk,but…

Whyistherealwaysabluehazearoundsomemountains?

Evergreen trees give off vapors of resinous chemicals. These vapors can

reactwithozoneintheairtoproduceextremelytinysolidparticlesofjustabouttherightsizetoscatterbluelight.Sobluelightphotonsarebeingscatteredandrescattered all around, while the other colors are passing by in straight lines.Thus,morebluereachesoureyesthantheothercolors.

YouDidn'tAskThisEither,but…

Isthatwhytheskyisblue?

Pretty much, yes. But the sky isn't blue because the blue light is being

scatteredbydust, aswasoncebelieved, andasmanypeople still believe.Theblue light is being scattered by the nitrogen, oxygen and othermolecules thatmakeuptheair.Thesemoleculesarebestatscatteringtheshortestwavelengths,withbluelightbeingscatteredabouttentimesmorethanred.Whenyoulookupatthesky,we'reseeingallthatextrabluelightthatmaynothavestartedoutinyourdirection,butthathasbeenscatteredandrescatteredintoit.

NITPICKER'SCORNER

Photons of light will also bounce off much bigger things—things much

bigger than their wavelengths. That soaring baseball a couple of pages backwould certainly be deflected (and its batter dejected) if it encountered theoutfieldwallinitsattempttobecomeahomerun.Sowavelengthdoesn'tmatterwhen the scattering object is bigger than all the wavelengths in visible light;they'll all bounce off. That's what happens when all the colors of light arereflectedequallyfromasolidsurfacesuchasamirror.Nochangeoccursinthecolorbalance.

IsItColdUpHere,orIsItMe?

Whyisitsocoldinspace?

Itisn't.Satellitesandspaceshuttlesdoindeedgetcoldupthere,butit'snot

becauseit'scoldupthere.Firstofall, there'sreallynosuchthingascold,nomatterwhatthepenguins

tell you. Cold is a linguistic concept, not a scientific one. Our cavepersonancestors needed aword for “not hot,” and “cold” (or its grunt equivalent) iswhattheycameupwith.It'slikelightanddark,wetanddry.Lightandwateraretangible things, but dark and dry denote the lack of light and water. They'renegativeadjectives,ifgrammarianswillpermitme.

Okay,thatwassemanticfun,buteveryoneknowswhatwemeanby“cold.”Soexplainwhyspaceisn'tcold,already.

Okay,okay.Heatisenergy.It'stheenergythatanobject'smoleculeshavebyvirtueofthe

factthatthey'reinmotion.Whyaretheyinmotion?Becausearound12billionyearsagoanincomprehensibleamountofenergyaroseinthevoid(orwhatever)via the Big Bang—thatmind-boggling blast that scientists believe ignited theuniverse—and all the atoms are still quivering. Some, the hotter ones, arequiveringmorethanothers;werefertothoseothersascolder.Some forty years ago, when we left the cuddly atmosphere of our native

planet to venture into the vast beyond, we encountered for the first time anenvironment inwhich there isnoheat tocompareanythingwithbecause thereare no (or precious few) molecules up there to quiver, and the word “cold”becameevenmoremeaningless.Spacecanbeneitherhotnorcoldinanysenseofthewords,becauseitisemptyofmatter.

Thenwhydosatellitesandspacecraftgetso…frigid?PartsofNASA'sspaceshuttle do get down to a couple of hundred degrees below zero Fahrenheit(around−130degreesCelsius).Here'swhat'shappening.Aspaceshuttleoranyotherobjectcangainorlose

heatnotonlybybeingincontactwithstuffthat'shotterorcolder—andthat'soutbecause there'snostuffup there—butalsobyradiation.Thesunandstarsareputting out all sorts of radiation—waves of pure energy, both visible to thehumaneye(light)andinvisible(ultraviolet, infraredandothers).Thisradiationtravelsthroughspacewithoutbeingdiminishedbecausethere'snothingtheretoabsorbit.Butwhenitstrikesanobject,forexampleaspaceshuttle,someofitwillbounceoffandcontinueonitswayinadifferentdirection.Someofitwillbe absorbed, however, and its energywill dissipate into heat. Thus, the spaceshuttleisreceivingradiatedheatfromthesunandstars.Thesun,ofcourse,isbyfarthechiefheatradiatorbecauseitissomuchcloserthantheotherstars.Butatthesametimetheshuttle,stillcarryingitsburdenofearthlywarmth,is

radiatingsomeofitsownenergyaway,becauseanythingthathasanywarmthatall sends out infrared radiation—“heat radiation”. That's how night-visiondevices can “see”people in thedark: by the infrared radiation they're sendingout. And that's how old-fashioned radiators work: They radiate heat into theroom,ratherthanblowinghotairaroundthehouse.Theshuttle,then,isreceivinglotsofradiatedheatonthesidefacingthesun

whileradiatingheatrapidlyawayontheotherside,whichthengetsexceedinglycold.Note, then, that the shuttle itself can be said to be cold because it is a real

object,buttheenvironmentitisflyingthroughisnotcold,eithersemanticallyorphysically.

BARBET

It'snotcoldinouterspace.

It'stheonesubstancethatisindispensabletoalllivingthings.Itmakesupmorethanhalfofourownbodyweights.It is the most abundant chemical on Earth, with more than a billion

billion tonsof it covering71percentof theplanet's surfaceandprobablyanother billion tons in those little plastic bottles that everyone carriesaroundthesedays.Whenevenalittlebitofitisdiscoveredonanotherplanet,astronomers

growgiddywithspeculationabouttheexistenceofextraterrestriallife.It is water, H2O, one of the simplest and most stable of all chemical

compounds.We normally think of water as a liquid, because that's what it is

throughout our most comfortable range of living temperatures: between,say,40and80degreesFahrenheit (4and27degreesCelsius).Butasyouknow,atanytemperaturebelow32degreesFahrenheit(0degreesCelsius)itpreferstoexistasthesolidthatwecallice.Andatanytemperatureabove212degreesFahrenheit(100degreesCelsius)itpreferstoexistintheformofvapor—an invisiblegas, just like thenitrogenandoxygengases in theair.(Watervaporisn'tsteam.Steamisacloudoftinydropletsofliquidwater

thataretoosmalltofalloutoftheair.)Water doesn't have to reach its boiling temperature in order to turn at

leastpartiallyintovapor.Whereverthereiswaterthereiswatervaporintheair around it. We sometimes call it humidity, and it has far-reachingconsequences in many aspects of our lives, far beyond making usuncomfortableinthesummertime.Inthischapterwe'lllookatsomeoftheamusingthingsthatwaterdoes

when in its liquid form, such asmaking coffee stains,making theoceansbothsaltyandblueandmakingyourwetshowercurtainslapyouright in

the…shower.We'lltakeasmalldetourthroughthePanamaCanalonourway to the kitchen, where we'll play with some ice cubes and lollipopsbefore going to the laundry to find out what's inside all those detergentbottlesandboxes.Thenwe'llexaminehowwatervaporaffectscosmetics,clothesdryersandthatolddevilhumidity.And as usual, we'll explode a fewmisconceptions along the way, this

timeconcerningthecolorofwater,theflowofglassandwhetherwarmaircanreally“holdmoisture.”

TheWateryBlues

Whyistheoceanblue?Isitjustareflectionofthesky?

No.That'sacommonbeliefthatjustdoesn'tholdwater,sotospeak.Firstof

all, theocean'ssurfaceisn'texactlywhatyou'dcallamirror.Andsecond,howcomeit'samuchdarkerbluethanthesky?No, theworld's oceans really and truly are blue—many different shades of

blue (ask any sailor), depending on several factors, a few ofwhichwe'll talkaboutbelow.Buthere'sasurprise:Evencrystalpurewater—withoutthesalt,siltandfish—

isblue.That'sinspiteofthefactthatalmosteverydictionarydefineswateras“acolorless, odorless liquid.”All youhave to do is fill your bathtub and see foryourselfthatitisn'tcolorless.Sowhy iswater blue?Becausewhen daylight,which contains all colors of

lightmixedtogetherhitsthewaterandpenetratesit,certaincolorsinthedaylightare absorbedby thewatermolecules.The light that is reflectedback from thebathtubandreachesyoureyeafterpassingthroughthewateristhendiminishedin those particular colors, so it has a different color composition from theoriginaldaylight.

TRYIT

Fillyourbathtubandlookatthewater.You'llseethatit'sapalebluecolor.(Iassumethatyourbathtubiswhite.)Theonlyreasonthatyoudon'tseeblueinaglassofwateristhatyou'renotlookingatenoughwater.Thecolorbuildsup,oraccumulates,asyoulookintoorthroughthickerandthickerlayersofit.Ifthewindowpanes inyourhousewere ten timesas thickas theyare,you'dsee thatthe“colorless”glassisreallygreen.

Specifically, water molecules show a slight preference for absorbing theorangeandredportionsofthesun'slight.Lightthatisdiminishedinorangeandred looks tousas if ithas toomuchblue,comparedwithwhatwecall“whitelight.”Sothewaterappearsblue.But an ocean is a muchmore complicated kettle of fish than just H2O. In

addition to the obvious salts andminerals, it containsplankton, for one thing:tinyplants(phytoplankton)andanimals(zooplankton)thataretoosmalltosettleout and that float perpetually about until decomposed by bacteria or eaten byanythingbiggerthantheyare.(It'sacruelworld.)

Seawater also contains a lot ofmiscellaneous dissolved organicmatter that

scientists call by its German name, gelbstoff. Loosely translated, it means“yellowcrap,”becausethat'swhatitlookslikewhenit'sdry.Whendaylight enters seawater, thephytoplankton absorbsmostlyblue light

plusalittlered,whilethegelbstoffabsorbsmostlybluelight.Theseabsorptionsshift the balance of the remaining light from the pale blue of purewater to adeeper,purplishblue.That'swhy theoceansaredarker than thewater inyourbathtub,whichIcertainlyhopeisdevoidofgelbstoff.Unfortunately,themanyfacesthattheseaspresenttousindifferentweather

andindifferentpartsoftheworldarenotthateasytoexplain.Foronething,it'snot just the absorption of light that gives seawater its color, it's also thescatteringoflight.Certaincolorsoflightarescatteredbymicroscopicparticlesofmatterinthewater.Whenaphotonoflighthitsoneofthoseparticles,whichmightbeanythingfromasinglemoleculeonup, itcanricochetoff inanotherdirection.Thischangesthedistributionofcolorsthatreachesoureyes.Itisjustsuchascatteringoflightfromairmoleculesthatmakestheskyblue,

becauseairmoleculesscatterbluelightmorethanothercolors.Somescientists

havetriedtoexplaintheoceans'bluenessasbeingdueentirelytothesamekindofscattering,butthey'veapparentlyneverpeeredintotheirbathtubs.Phytoplanktonisanespeciallygoodscattererofgreenandyellowlight,soin

general themore phytoplankton there is, themore greenish thewaterwill be.That'swhatislargelyresponsibleforthebeautifulgreenishturquoisecolorofthewaterssurroundingtheCaribbeanandSouthPacificislands.Thetropicalclimateandabundantsunlighttherecreatelushbreedinggroundsforplankton.Andhoneymooners.

AllThatSaltandNoPopcorn

Whyaretheoceanssalty?

When you say “salty,” you're undoubtedly thinking of sodium chloride,

common table salt.But to a chemist, a salt is anymember of a large class ofchemicals,andtherearedozensofthemintheoceans.Toput theword“salt” inperspective,please indulgeme inaone-paragraph

chemistrylesson.A“molecule”ofsalt(it'snotreallyamoleculeinthestrictsense,butIwon't

tellanybodyifyoudon't)consistsofapositivelychargedpartandanegativelychargedpartthat,beingoppositelycharged,attracteachother.Thepositiveandnegativepartsarecalledions.Inthecaseofsodiumchloridethepositiveionisachargedatomofsodiumandthenegativeionisachargedatomofchlorine.Buta salt's positive ion can be a charged atom of anymetal, and there are someeighty-fiveknownmetals.Also, therearemanynegativeionsbesideschloride,soyoucanseethatthereisaverylargenumberofpossiblesalts.Endofchemistrylesson.The main metal ions in seawater are sodium, magnesium, calcium and

potassium,while themain negative ions are chloride, sulfate, bicarbonate andbromide.Yourquestion,then,ishowallthisstuffgotintotheoceansinthefirstplace. The short answer is that it was washed out of the land by rain-water,whichthenflowedasriverstotheseas.Seawateriscontinuallybeingrecycled.Eachyear,thetopmeter(3feet)orso

oftheoceansevaporatesintotheair,movesaroundinvariousweathersystemsandfallsbackontotheoceansandlandasrainandsnow.Ofthisprecipitation,76percentfallsontheoceansand24percentfallsonthecontinents.Thewaterthat lands on the continents flows down in streams and rivers, eventuallyreturning to the seas. In the process of washing down, these waters pick upanything that will dissolve, mostly the salts that exist in the soils, rocks and

minerals.Anychemistwilltellyouthatsodiumsaltsdissolvemorereadilyinwaterthan

dosaltsofpotassium,magnesium,calciumormostothermetals.Morethananyothers, then, it's thesodiumsalts thatdissolveandwashdowninto theoceans.There are approximately equal amounts of sodium and potassium in the soils,rocksandminerals,yetthereistwenty-eighttimesmoresodiumthanpotassiuminseawater.Allofthesedissolvedsaltsmakeup3.47percentofseawater,byweight.Only

six elements make up more than 99 percent of those salts: chlorine, sodium,sulfur (in the form of sulfates), magnesium, calcium and potassium, indecreasingorder.Anothersourceofseasaltsisvolcaniceruptions,bothonlandandunderthe

sea, which spew out enormous amounts of solids and gases. Among theprominentvolcanicgasesarechlorineandsulfurdioxide,whichmayaccountforthe fact that chlorine is themost abundant element in seawater,makingup55percent of the salts'weight,while sulfates are second only to chlorides as thenegative-ionportionsofthesalts.Puttingallthistogether,sodiumandchlorinemakeup86percentofthesalts

intheoceans.Soifyouwanttosaythattheoceansaresaltybecauseofsodiumchloride,nobodywillgiveyoumuchofanargument.

YouDidn'tAsk,but…

Whyaretheoceanssalty,butnotthestreams,riversandlakes?

Rainwater washes down from the land into the streams, rivers and lakes,

carryingdissolvedsalts justas itdoeswhen itwashes into theoceans.But thedifferenceisthattheoceansaremucholderthantheotherwaters—4or5billionyearsold,comparedwithmeremillions.Overthosebillionsofyearstheoceanshave been recycling theirwater—evaporatingwater that rains out on the landandflowsback,returningeachtimewithafreshloadofsalts.Thesecycleshavecontinuallyincreasedtheloadofsaltintheoceans.

WheretheDevilIsSeaLevel?

I understand that the Panama Canal has locks because theAtlanticandPacificOceansaren'tatthesamelevel.Thenwhatdowemeanwhenwetalkaboutelevationsabove“sealevel”?Whichsea?

Whoa!That'snotwhythePanamaCanalhaslocks.Thelocksaretherefor

the purpose of lifting ships over the hump of land known as the Isthmus ofPanama.Togetoverthathump,theshipshavetobefloatedupward85feet(26meters) above the entrance ocean and then lowered back down into the exitoceanontheotherside.Thatgoesforbothdirections.Whydidn't theyjustdigaflatditchfromoneoceantotheother,aso-called

sea-levelcanal?Mostlybecause itwouldhavemeantexcavatinga tremendousamountofdirtattremendousexpense.Butalso,therewouldhavebeentorrentsofwatergushingthroughasea-levelcanal.That'snotbecauseofanypermanentdifferenceinlevelofthetwooceans,however;theiraveragesealevelsarejustaboutthesame.It'sbecauseofthetides.At thePacificendof thecanal thetidescanriseandfallasmuchas28feet

(5.5meters),whereasattheAtlanticendthetidesvarybyonlyabout2feet(60centimeters).SotherewouldbeperiodicsurgesofwaterthroughthecanalfromthePacifictotheAtlantic—thatis,fromeasttowest.DoyouthinkIgotthatbackward?Isn'tthePacificOceanatthewesternend

ofthecanal?Nope.BecauseofthewaytheIsthmusofPanamasnakesaround,thePacificentrancetothecanalis27miles(43kilometers)eastoftheAtlanticentrance.Checkamap.

BARBET

A ship passing through the Panama Canal from the Pacific Ocean to theAtlanticOcean sails from east towest. (It's actually southeast to northwest, ifyoumustquibble.)

Nowbacktowhatwemeanby“sealevel.”Obviously,becauseofthetides,wecantalkonlyabouttheaverage,ormean,

level of any ocean at any location.While themean levels of theAtlantic andPacific Oceans may be approximately the same at the Panama Canal, forexample,thatdoesn'tmeanthatalltheoceansintheworldhavesettleddowntoacommonlevel.Youmightexpectthemtobethatway,becauseifyoulookataglobe you'll see that they're all connected; Earth's oceans are one giganticswimmingpoolwithchunksoflandscatteredabout.Butevenwhenyouaverageoutthetides,therearereasonswhytheoceansdifferintheirlevels.Foronething,becausegravitationaleffectsarebiggerforbiggermasses,the

biggeroceanswillbe raised intohigher tidesby themoon'sgravitationalpull.(Onlythebiggest lakeshavetides.)Weatherpatternsalsoaffect thesealevels.When the air pressure over an ocean is low, the water will actually expand.Moreover, prevailing westerly winds can actually make the water pile upsomewhat toward the east.And finally, differences in ocean depth can have agravitationaleffectonthewater'slevel,becausethedeeperanoceanis,themoretightlyitswatersarepackeddownbygravity,andtheloweritssurfacelevelwillbe.Theseareallsmalleffects,butwhenworkingonsuchhugebodiesofwater,

theycanmakesignificantdifferencesinthesealevelatdifferentlocationsintheworld. Since they are all connected, the waters do try, of course, to seek acommonlevel,buttheyarejusttooslowtokeepupwithallofthesechangingconditions.Sowhatismeansealevel?It'sacarefullycompiledaverage,measuredovera

periodofnineteenyearsatmanytidalstagesinmanyplacesaroundtheworld.Wheneveryouhearthatsomethingissomanyfeetormetersabove“sealevel,”orthattheatmosphericpressureat“sealevel”issomanyinchesormillimetersof mercury, it's understood that they're talking about mean sea level: aworldwide,long-termaverage.

Why-ingOverSpiltCoffee

Whenspilledcoffeedriesonmykitchencounter,itformsabrownring,withalmostnothinginside.Whydoesallthecoffeegototheedgestodry?

Foryears,peoplehaveobservedthisphenomenonwithoutgivingitasecond

—or even a first—thought. Hundreds of less-than-fastidious, coffee-sippingscientistshaveprobablyglancedat thering,mumbledsomethingaboutsurfacetensionandtoldtheirlabassistantstocleanitup.But it wasn't until 1997 that six scientists at the University of Chicago

ponderedthisearthshakingquestionandpublishedtheirresultsintheprestigiousinternationalscientificjournalNatureforthebenefitofallmankind—oratleastforthoseslobsamonguswhodon'twipeuptheirspillsbeforetheydry.Here's what they concluded after producing reams of mathematical

calculations,undoubtedlysupportedbylotsofcaffeine.Whenacoffeepuddlefindsitselfonaflat,levelsurface,ittendstospreadout

in all directions. In anygivendirection, the liquidwill stop spreadingwhen ithitsabarrier,anyslight irregularity in thesurface that itcan'tcross, suchasamicroscopic ditch. Depending onwhere the barriers happen to be, the puddlewill take on a certain shape: longer in this direction, shorter in that, like anamoeba.Asevaporationtakesplace,thepuddlewillstarttodryfirstwhereit'sthinnest:

attheedges.Thatwouldhavetheeffectofmakingthepuddleshrink,pullingitsedgesback,butitcan'tdothatbecausethey'restuckintheditches.Soaswaterevaporates from the edges, it has to be replenished from somewhere, and theonlyplaceitcancomefromistheinteriorofthepuddle.Thus,there'samovementofwaterfromtheinteriorofthepuddletotheedges,

where it evaporates. That flow of water carries alongwith it themicroscopicbrown particles that give coffee its color. The brown particles then find

themselvesstrandedattheedgeswhenthepuddlefinallyrunsoutofwater.

TRYIT

First,cleanyourkitchencounter;nogreasefilmsallowed.Ifyourcountertopislight-colored,spillaboutaquarter-teaspoon(amilliliter)ofcoffee—black,nosugar—on it and let it dry overnight. You'll see the brown ring. If yourcountertopisdark,theeffectismuchbetterifyouusesaltwater.Dissolveabouthalfateaspoon(afewgrams)oftablesaltinhalfacup(250milliliters)ofwaterand make a few quarter-teaspoon (milliliter) puddles on the counter. Whenthey'redry,you'll seewhite ringsofsalt.Thesaltcrystalsarecoarser than thecoffeeparticles,sotheringswillbemoreirregular.

ShowerPower

When I'm taking a shower,why does the curtain sneak up andslapmeontheleg—orsomewhere?

You're lucky you asked that question today, because today is bargain day.

I'm going to give you four answers for the price of one. It's not because I'mfeelinggenerous,butbecauseIcan'tmakeupmyownmindaboutwhichonetobelieve.To my knowledge, the National Science Foundation has not yet funded a

comprehensive university research project designed to solve this perplexingproblem, so scientists have been left to debate their theories over coffee andbeer.Herearefourcontendingsolutionstothegreatshowercurtainmystery.Yapaysyermoneyandyatakesyerchoice.(1)Hotairrising.Thestorygoes that theair inside theshower isheatedby

thewaterand,aseverybodyknows,hotairrises.(Butifyoudon'tknowwhyhotair rises.) If the hot air in the shower is rising, then cold air has to rush in toreplaceitatthebottom,andintheprocessitblowsthecurtaininward.Thisisanice,simple,appealingandwrongexplanation.Justtryitwithcoldwaterinsteadofhot,andyou'llseethecurtainmoveinwardjustasmuch.(Youdon'tactuallyhave to be in the cold shower; you can do the experiment while standingoutside.)(2)Electrostaticcharge.Whenwaterstreamsoutofanarrowopeningsuchas

the hole in a showerhead, it can pick up a static electric charge. It's not toodifferent from scuffing your feet on a carpet, whereupon some electrons arescraped off your shoes onto the carpet and you develop a positive charge.Electrons can also be scraped off—or onto—the water by the showerhead,dependingonwhatit'smadeof.Butif thewater'smoleculesweretopickup,say,anegativechargeontheir

wayoutof theshowerheadbypickingupsomenegativeelectrons, thoseextra

electrons would repel some electrons from the surface of the shower curtain,becausesimilarchargesrepeleachother.Thatwouldleavethecurtain'ssurfacewith a deficiency of negative charge, and its inherent positive charges woulddominate.The negativewater and the positive curtainwould then attract eachother,asisthewontofoppositecharges,andthecurtainwouldmovetowardthewater.

This isn't quite as far-fetchedas itmay sound. (Wait'll you see someof the

otherexplanations.)Inducedelectrostaticcharge,whichiswhatthephenomenoniscalled,doeshappenandiswell-known.Haveyoueveropenedacartonpackedwith those styrene foam packing peanuts, especially when some of them arebroken into small fragments? Just try to keep those maddening motes fromjumping around or sticking to your hands as you try to brush them away. It'sbecauseofinducedelectrostaticcharges.

TRYIT

On a dry winter day, put a few small fragments of styrene foam packingpeanutsonatable.(Ifyouhaven'treceivedapackagelately,youcanusetorn-upscraps of lightweight paper.)Nowwalk across a nearby carpetwhile scuffingyourfeettoacquireabodycharge.Goquicklytothetableandtrytotouchtheplastic peanuts. Even before you touch them, theywill jump up tomeet yourhand.Yourbody'sstaticchargeinducedanoppositechargeintheplasticandtheresultingattractionofoppositechargeswasenoughtomakethemleapuptowardyou.

Whether or not induced electrostatic attraction is strong enough to move ashowercurtain,however,isupforgrabs.(3) Bernoulli's Principle. The water is carrying along some entrained air,

making an air current near the inside surface of the curtain.According toMr.Bernoulli,thefasteragasmovesacrossasurface,theloweritspressureagainstthatsurface.Since there isnospeedingairstreamontheoutsideof thecurtain,theairpressureontheinsideislowerandthecurtainmovesinward.

(4)TheCoanda Effect. Fluids have a tendency to stick closely to a curved

surfaceoverwhichtheyareflowing.ThisphenomenonisknownastheCoandaEffect in honor of Henri Coanda (1886–1972), a Romanian aeronauticalengineerwhofirstcalledattentiontoit.

TRYIT

Hold a drinking glass horizontally in the stream of water from a slowlyrunning faucet, so that the stream falls onto one side of the glass.Notice thatwhen thewatergets to thebottom itdoesn't fall straightdown. It sticks to theglassandfollowsitscurvedsurfacebeyondthebottombeforefallingoff.

Intheshower,ifthecurtainisalreadycurvedinwardsomewhat,perhapsfromone of the other effects, thewater flowing over its surfacemay pull it farther

inwardbecauseofCoandastickiness.

NITPICKER'SCORNER

Figuring out exactly why a flowing fluid sticks to a curved surface took

Coanda and other aerodynamic engineers more than twenty years. Here's theultimateexplanation.Themolecules of a fluid exhibit some stickiness toward one another; what

somemoleculesaredoingaffectstheirneighborsbecausetheyaresortoftiedtoone another. (Techspeak: Fluids have a certain viscosity.) If one layer of aflowingfluid'smoleculesshouldhavesomeadherencetoasurfaceoverwhichitisflowing,therestofthemoleculeswillbedraggeddowntothesurfacealongwiththemtosomeextent,andthefluidasawholewilltendtostickmorethanwemightexpect.Inthecaseofthewaterontheglass,thefirstlayerofwatermoleculeswants

tostickbecausewaterwetsglass.(Itdoesn'twetwax,forexample.)Thesecondlayerwants tosticktothefirst layer,soit isalsoweaklyattachedtotheglass.The third layer sticks to theglass through the first two layers and soon,witheachsuccessivelayerstickinglessstronglythantheprecedinglayer.Manyotherlayersaredraggedalongforaslongasthestickinessexceedsthepullofgravity,and then the water finally falls off the glass, having gone farther around thecurvethanwewouldhaveexpected.Theattractionthatairmoleculeshaveforoneanotherisalotsmallerthanin

thecaseofwater(Techspeak:Theviscosityofairisalotlessthanthatofwater),so itwill stick to the shower curtain's surface a lot less, but the effect is stillthere.Boththewateranditsentrainedairprobablycontributetotheattractionoftheshowercurtain.That is, if you believe that the Coanda Effect is the true cause of your

flapping,slappingcurtain.Me,Ifavortheelectrostaticexplanation.

PsychoticPsocks

Unless I use one of those fabric-softening dryer sheets, all myclothescomeoutof thedryer fullof staticelectricity, sticking toone another. What do fabric softeners have to do with staticelectricity?

Notmuch,exceptthatthestuffinthedryersheethappenstobegoodatboth

jobs.Youcanobtainthestatic-eliminationfunctionallbyitselfasaliquidinaspray can, so you can de-static your clothing even while you're wearing itwithout your having to—DO NOT TRY THIS AT HOME!—climb into thedryer.Themainingredientinbothtypesofproductsisasurfactant,achemicalthat

ismadeofwhatmightbecalledbisexualmolecules;theyareattractedtobothoilandwater.Mostotherchemicalsshowastrongpreferenceforoneortheother.Forexample,commonsalt(sodiumchloride) ismadeofelectricallycharged

atoms (Techspeak: ions), and charged atoms like to mix into—dissolve in—waterbecausewatermoleculeshaveelectriccharges thatattract them.Butsaltwon'thaveanythingtodowithfatsandoilsbecausetheirmoleculesdon'thaveanyattractive chargedparts. Just try todissolve some salt inoliveoil and seehowfaryouget.Surfactants,however,arepeculiarinthatoneendofeachmoleculeisafatty

materialthatisattractedbyoils,whiletheotherendischargedandisattractedby water. Soap and detergent molecules are surfactants; their oil-loving endslatch on to oily dirt and drag it into thewater bymeans of theirwater-lovingends.Orlookingatittheotherway,theirwater-lovingendsdragwaterintooilyplacesthatitwouldn'tordinarilyinvade,therebymakingthewaterwetter.Nowlet's impregnateapapersheetwithasoapy-feelingsurfactantchemical

and throw it into the dryer along with our wet clothes. As they tumble, theclothesrubagainstthesheetandbecomecoatedwithsurfactant.Theratherhefty

fattyendsofthesurfactantmoleculesimpartaslippery,waxyfeeltotheclothes,“softening”them.Thenwhentheclothesbegintodry,theirfrictionagainstoneanotherrubsoff

someelectronsandstaticelectricitybeginstobuildup.Thechargescan'tbuildupaslongastheclothesarewetbecausewaterconductselectricitywellenoughto conduct the rubbed-off electrons back towhere they came from.When thewaterisgone,thechargedendsofthesurfactantmoleculestakeover,conductingthechargesawayandkillingany“staticcling”thatmightresult.Deprivedoftheirstaticcling,socksfindthemselvesunabletobondwiththeir

partners andmay suffer a severe separation-anxiety syndrome. In fact, a sockmay become so depressed and emotionally unraveled that it will slink awaythrough the vent tube in search of psychiatric help. That's why you willsometimes find a sockmissingwhen you put awayyour laundry. I knowyouhavewonderedaboutthat.

NITPICKER'SCORNER

Therearethreekindsofsurfactantswhosenamesyouwillseeasingredients

on the labels of dryer sheets, clothes softener liquids, antistatic sprays andsyntheticdetergents (see thefollowing).Theymaybe listedascationic (CAT-eye-ON-ic), anionic (AN-eye-ON-ic) or nonionic (NON-eye-ON-ic). Thecharged ends of the molecules can be either positively charged (cationic) ornegativelycharged (anionic).Thenonionic surfactantmoleculesaren't chargedatall,sotheymaybegoodatclothessofteningbutareofnouseforkillingstaticcling.Awidely used cationic surfactant is dimethyl ditallow ammonium chloride,

andacommonnonionicsurfactantispolyethyleneglycolmonostearate.Laundrydetergents(seethefollowing)commonlycontaintheanionicsurfactantsodiumalkylbenzenesulfonate.As if you cared, right? But now you can have fun decoding the fine-print

ingredient listsonall thoseproduct labels.Run rightdown to the laundry andcheckthemout.

WashdayWonders

Every laundry detergent claims to be “New,” “Improved,”“Unique”andbetterthantheothers.Aren'ttheyalljustsoap?

No, those detergents aren't soap, although soap is a detergent. The word

“detergent” simplymeans a cleansing substance, from the Latin detergere, towipeoff.Aftermorethantwothousandyearsofusingsoap,whichiseasytomakeby

boiling upwood asheswith animal fat (don't youwonder how that discoverywasmade?),humans finallycreatedsynthetic detergents,which inmanycasesworkevenbetterthansoap.Todaywereservetheword“detergent”exclusivelyfor those artificial chemical concoctions that take up so many acres of shelfspaceinoursupermarkets.Alldetergents,includingsoap,aresurfactants,chemicalcompoundsthathave

the knack of bringing oil and water together.Most dirt adheres to our skins,clothing,dishesandcarsbymeansofasticky,oilyfilm.Coaxtheoilyfilmintothewaterandyouhavesucceededinremovingthe“glue”thatstuckthedirttotheobjects.But all those colorful bottles and boxes on the store shelvesmay contain a

mad scientist's laboratoryfulofother chemicalsbesides surfactants.Otherwise,howcouldthemanufacturerskeepclaimingthattheirproductsareanydifferentfromorbetterthanalltheothers?Here is a list of what may be hiding in your laundry products, household

cleaners, soaps, window cleaners, dishwashing detergents and the like, inaddition to surfactants. And don't forget themost expensive ingredient of all:advertising.Lotsandlotsofadvertising.Acidsandalkalis:Acidshelptoremovemineralbuildup,whilealkalisattack

fattyandoilysoils.Examples:aceticacid,citricacid,ammonia.Antimicrobial agents: Kill disease microorganisms. Examples: pine oil,

tricloban,triclosan.Antiredepositionagents:Onceyougetthedirtoff,youhavetokeepitfrom

going right back to where it came from. Examples: carboxymethyl cellulose,polyethyleneglycol,sodiumsilicate.Bleaches:Removestainsand“whitenandbrighten”yourclothes.Examples:

sodiumhypochlorite(chlorinebleach),sodiumperborate(“color-safe”bleach).Builders: Counteract hard water, which interferes with the surfactant's

performance. Examples: sodium carbonate (washing soda), sodiumtripolyphosphate.Thelatterisoneofthenotoriousphosphatesindetergents.Ifphosphatesget into thesewersand then intostreamsand lakes, theycanharmtheenvironmentbydisruptingtheecologicalbalance.The phosphates make algae grow in profusion, and when the waters can't

sustainanymorealgae theydieoff,whichprovidesafeast forbacteria,whichuseupoxygeninthewaterandkillthefish,whichmakesevenmoredeadbodiesfor the bacteria to feedon, etc.Becauseof this, phosphates havebeen largelyeliminatedfromdetergents.Corrosion inhibitors: Protect the metal parts of your washing machine or

kitchenutensils.Example:sodiumsilicate.Enzymes: Enzymes are natural chemicals that speed up natural chemical

reactions.Inlaundryproductstheyspeedupthedestructionofspecifickindsofstains,suchasgrass.Examples:protease,cellulase.Fabric softening agents: Soften fabrics and control static electricity.

Example:quaternaryammoniumcompounds.Fragrances:Cover up the smells of all the other ingredients andmakeyou

thinkyourlaundryis“fresh,”whateverthatmeans.Opticalbrighteners:Makeclotheslookbrighterbyconvertingyellowlightor

invisible ultraviolet light into bluish or whitish light. Example: stilbenedisulfonates.Preservatives:Protecttheproductfromoxidation,discolorationandbacterial

attack.Examples:butylatedhydroxytoluene,EDTA.Solvents: Keep all the ingredients dissolved in liquid products. Examples:

ethylalcohol,propyleneglycol.Sudscontrolagents:Controltheamountofsudsormakethesudskeeptheir

“heads.”Examples:alkanolamidesand—guesswhat?—soap.Lifeinthelaundryisn'tassimpleaswhenallwehadtodowasboilupanice

kettleofgoatfatandashes.

GlassDismissed

My son's teacher told the class that glass is really a very thickliquid,andthatgivenenoughtimeyoucouldseeitflowundertheinfluenceofgravity.Really?

That'sacommonlyquoted“amazingfact”thatissimplynottrue.Liquidsdogetsomewhatthickerastheyarecooled,andbecauseallglasswas

oncealiquidwhileitwashotandbeingformedintoshape,somepeopleliketothinkthatitgetsthickerandthickerasit“supercooled,”untilitgetssothickthatitactslikeasolid.Well,thetruthisthatitisasolid.Ifglassflows,itsmotionapparentlyrequiresmorethanfourthousandyearsto

becomedetectable,because that'show longglasshasbeenaroundandnobodyhasyetcomeupwithanyconvincingevidenceofitsmotion.That'soneholeinthe “supercooled liquid” theory.But the bigger, absolutely gaping hole is thatglassisnotasupercooledliquid,despitethefactthatquiteafewtextbooksandencyclopediassayitis.The“supercooledliquid”fablehasbeenaroundatleastsinceIwasinschool

and accepted everything my teachers said as gospel. But science and I havemarched many a mile since then, and there is no longer any excuse forperpetuatingthemyth.If you've ever observed glassblowing or the process of molding glass into

shapes,youknowthatwhenit'shotenoughtheglasscertainlydoesflowlikeavery thick (viscous) liquid. But as it cools down, we observe no suddentransition from liquid to solid, aswedo, forexample,whenwatercoolsdownandturnsintosolidice.Thathasledmanywell-meaningscientiststoconcludethat theglassmuststillbea liquidevenat roomtemperature,because itdidn'tturnabruptlyrigid.Moreover,theargumentgoes,solidsareusuallycrystalline,meaningthattheiratomsormoleculesoccupyprecisegeometricpositionswithrespect to one another, and glass's molecules don't. Examples of crystalline

solids are ice, table sugar, salt or almost any mineral you can think of. If atypical solid's atomsandmoleculesweren't fixed inplace, they could slip andslideoverandaroundoneanother;inotherwords,theywouldflowlikealiquid.Butasubstance'smoleculesdon'thavetobeintheformofahighlyorganized

crystal inorder for it tobea solid.Therearesuch thingsasamorphous solids(from theGreek,meaning“without form”), inwhich themoleculesare indeedfixed inplace,but inamoreor less randomarrangement.That's thecasewithglass.It'sasolid,allright;it'sjustnotacrystallineone.(Techspeak:Itsstructuredoesnotexhibitlong-rangeorder.)Whenglassiscooledfromthemoltenstate,its molecules can't find a repeatable, orderly arrangement into which to fitthemselves.It'sthesamewithmanyotheramorphoussolidssuchasplasticsandlollipops. Translucent lollipops are sugar (sucrose) in an amorphous, glassyform,asdistinguishedfromitscrystallineforminthesugarbowl.

TRYIT

Melt somesugaroververy lowheat ina smallpan. If somesugarcrystalsstayontopwithoutmelting,stirtheminwithafork.Whenitisallmelted,butbefore it gets too dark, pour it out onto a cool, flat surface such as a tilecountertoporthebottomofametalfryingpan.Thesugarmoleculeswillcoolsofastthattheydon'thavetimetoarrangethemselvesintocrystalsandwillwindupinaglassyform.Afteritcools,youcaneatyourcaramel-candyglass.

By theway,youcan forget theword“crystal” thatglasswaremanufacturersapply to their better-qualitymerchandise; scientifically speaking, it's just plainwrong. A “crystal chandelier” or “crystal goblet” is made of glass that is asamorphousasanyother.It'sjustaparticularlyclearandbrilliantqualityofglass,usuallycontainingleadoxide.Okay,nowit'stimetoaddresstheurbanlegendthatsimplywillnotdie:that

the windowpanes in several-hundred-year-old buildings are thicker at thebottom,havingfloweddownsomewhatovertheyearsasanygoodsupercooledliquidshould.Ifyouexamineoldcathedralwindowsthatstillhavetheiroriginalglassinthem,you'resuretofindmanythatareindeedthickeratthebottom.Thetrouble is that nobody has ever done measurements on enough panes todetermineifsignificantlymoreofthemarethickeratthebottomthanatthetop,middleorsides.

Butevenifasignificantfractionofoldpaneswerefoundtobethickestatthe

bottom,itwouldn'tprovethattheyhadflowed.Earlywindowglasswasmadebymethodsthatwerequitecrudecomparedwithourmodernplateglassprocesses,and uneven thicknesses were tolerated as being preferable to more seriousdefectssuchasbubblesandscratches.Nowifyouwereaworkmanassemblingwindowsfrompanesofuneventhickness,wouldn'tyoubeinclinedtosettheminwiththeirthickestpartsatthebottom?

YouDidn'tAsk,but…

Theremust be some relatively high temperature at which glassdoesbegintoflow.Whatisthattemperature?

Glassexpertstalkabouta“transitiontemperature”atwhichrigidglassdoes

indeed become slightly plastic. For ordinary window glass, the transitiontemperature isabout1000degreesFahrenheit (550degreesCelsius).Everyonemustagreethatthewindowpanesinoldbuildingsnevergotthathot.

Equal-ibrium

Ifwater freezesat exactly zerodegreesCelsius, and icemeltsatexactlyzerodegreesCelsius,whatwouldhappentoabowloficeandwateratexactlyzerodegreesCelsius?

Absolutelynothing,asfarasyouwouldbeabletotell.Theiceandthewater

woulddwellinpeacefulcoexistence.Butdownatthemolecularlevel,achaoticdancewouldbegoingon.ZerodegreesCelsius(32degreesFahrenheit)isindeedboththefreezingpoint

ofliquidwaterandthemeltingpointofsolidice.Youareundoubtedlypicturingapoorlittlezero-degreewatermoleculethatcan'tmakeupitsmindwhethertoflowor float, tobe liquidorsolid.Well, that's reallyagoodway to lookat it,becausetheindividualmoleculesdoindeedgettomakedecisions,inamannerofspeaking.Let's consider first what goes on when liquid molecules freeze. There are

someratherstrongattractionsbetweenwatermoleculesthattendtomakethemsticktogether.(InTechspeak,theyarecalledhydrogenbondsanddipole-dipoleattractions.) In liquid water, the molecules are moving fast enough that theattractionscan'treallytakehold.Butaswater—oranythingelseforthatmatter—cools, its molecules move more and more slowly. Zero degrees Celsiushappens to be the temperature atwhich themolecules aremoving just slowlyenough that they cangrabhold of one anotherwith their attractive forces andsettledownintotheuniquefixedpositionsthatcharacterizeice.Ice'smoleculesare tied rigidly in place; they can't go swimming around the way liquidmoleculesdo.Nowlet'sputanicecubeintoliquidwater.Someoftheicemoleculesatthe

surface of the ice will break their attachments to their fellows and join theirfreelyswimmingbrethren.Inotherwords, theywillmelt.Meanwhile,someoftheliquidmoleculesneartheice'ssurfacemaybemovingmoreslowlythanthe

capture speed (they're not allmoving at the same speed), and theywill freezeonto the ice.Sobothmeltingandfreezingcanbe takingplacesimultaneously,somemoleculesgoingonewayandsomegoingtheother.AslongasthewaterisslightlywarmerthanzerodegreesCelsius,therewill

bemoremelting going on than freezing, because therewon't be enough slowwater molecules to be captured onto the ice. Conversely, if the water'stemperature is slightly lower than zero degrees, there will be more freezinggoingon thanmelting,because therewillbemoreslowwatermolecules tobecaptured. At exactly zero degrees, there will be just as many ice moleculesmelting as there are liquidmolecules freezing.Millions of tinymolecules aregoing each way, but from our relatively gigantic human perspective, we seeabsolutelynothinggoingon.The iceandwater just sit there—until,ofcourse,theybegintowarmup,andthenmeltingtakesover.In a sense, then, zero degreesCelsius is neither the “meltingpoint” nor the

“freezingpoint”ofwater.It'sthetemperatureatwhichmeltingandfreezingarehappeningequally.Scientistscallthisexact-balancepointanequilibriumpoint.They would say that at zero degrees Celsius, ice and liquid water are “inequilibrium.”Equilibriumisaveryimportantconceptinchemistrybecausetherearemany

situations in which, down at the molecular level, two opposing processes aregoingonatequal rates,so thatuphereat thehuman levelweseenoapparentchange.Formore, look up “equilibrium” in the index of any chemistry book.But I

warnyou:Theremaybelotsofequations.Melting and freezing are so closely interrelated that just by touching an ice

cubeyoucankicksomewatermoleculesfromliquidtosolid.

TRYIT

Wet your fingers and touch the ice cubes in your freezer. The cubesmaysticktoyourfingerssotightlythatyoucanliftthemup.Theicecooledthewateron your fingers down to its own temperature, which is obviously below thefreezingpoint.Whenthewateronyourfingersfroze,itgrabbedontotheridgesofyourfingerprintsandheldontothemwhilesimultaneouslyfusingitselfontotheicecubes,thereby“gluing”yourfingerstothecubes.

Help!We'reTrappedinanIceCube!

Whyaremyicecubescloudierinthemiddlethanattheedges?

Thecloudiness isamassof tinyairbubbles—air thatwasdissolved in the

water and expelledwhen thewater froze.You can see the individual bubblesthroughamagnifyingglass.Thereisalwayssomeairdissolvedinanywaterthathasbeenexposedto…

well, the air. For this, theworld's fish are truly grateful.They are particularlygrateful for the fact that, even though air is only about 21 percent oxygen,oxygen dissolves in water twice as readily as the other 79 percent, which ismostlynitrogen.When water freezes, the loosely moving water molecules settle down into

rigid positions. In doing so, they squeeze out the dissolved air molecules,becausethereissimplynoroomforthem.Whenthewaterbeginstofreeze,theouterportionsfreezefirstbecausetheyareinthebestpositiontohavetheheatsucked out of them. As the dissolved air molecules are squeezed out, theybecome trapped within the encroaching casing of ice. The air molecules areforcedcloserandclosertogetherasthegrowingwalloffreezingwaterclosesinonthem.Eventually,theyarepackedsoclosetogetherthattheycongregateintobubbles.Andtheretheyremain,trappedwhentheinteriorwaterfinallyfreezes.

Help!I'mBreathing!

My girlfriend is worried that if the humidity gets to be 100percent, we'll be breathing pure water and drown. Of course,that'ssilly,butIcan'texplainwhy.

Askyourgirlfriend,“Ahundredpercentofwhat?”Chicken Littles who fear drowning in air are forgetting that “humidity” is

purely relative. Everybody goes around talking about “the humidity” as if it'ssomethingabsolute,butit'sreallytherelativehumiditythatthey'retalkingabout—relativetosomemaximum,butstillsmall,amountofwatervapor in theair.Andmindyou,that'svapor,notliquid.Evenwhentherelativehumiditygetstobe 100 percent at room temperature (we'll see that humidity varies withtemperature),thereisonlyonewatervapormoleculeintheairforeveryfortyorfiftyairmolecules.“Vapor”isafunnyword.Allitmeansis“gas”—theformofmatterinwhich

the molecules are flying freely around with huge spaces between them. Anysubstance can be transformed into a gas if we heat it hot enough to drive itsmoleculescompletelyawayfromoneanother.It'sonlywhenthegasinquestionrecentlyarosefromaliquidthatwerefertoitasavapor.Wecalltheoxygenintheairagasbecausewe've(mostofus)neverseenitasaliquid.Butwedon'tgenerally refer to gaseouswater as a gas becausewe know that it came fromliquidwater.Wecallit“watervapor.”Why does water choose to go into the air as vapor, anyway? At every

temperature,waterfindsauniquebalancepointbetweenitstendencytoexistinthe form of liquid and its tendency to exist in the form of vapor. At warmertemperatures,thebalancefavorsvapor,becausethemoleculesaremovingfasterand can escape more easily. So the higher the temperature, the higher thetendencyforwatertobeintheformofvapor,ratherthanliquid.Ifyouputsomewaterofacertaintemperatureintoaclosedbox,itwillfillthe

boxwiththeamountofvaporthat ischaracteristicof its temperature,andthenstop. It stopswhen thereare asmany liquidmolecules leaving the liquideachsecond as there are vapor molecules hitting the liquid's surface and sticking.Whenthesetworatesareequal,thereisnofurthernetchange.(Techspeak:Theliquidandthevaporareinequilibrium;.)Alotofpeople,includingsomescientists,wouldliketosaythattheairinthe

boxisnowsaturatedwithwatervapor,asif theairwereawetrag,holdingasmuch water as it can. But that's a misleading way to look at it. We'll put itanotherway:Theamountofvapor in thebox is100percentof themaximumamount that there can be at that temperature. In other words, the relativehumidityis100percent.Iftherewereonlyhalfthatamountofwatervapor,wewouldsaythattherelativehumidityis50percent,andsoon.Ifwelivedinaclosedboxwithsomeliquidwaterinit,therelativehumidity

wouldalwaysbe100percent—100percentofthemaximumamountofvaporforthatwatertemperature.Butofcourse,wedon't.Weliveinaconstantlyshiftingsea of winds bearing warm air, cold air, high- and low-pressure air andeverything else that theweather can devise to blowwater vapor around fromplace toplace.That'swhy the relativehumidity isn't always100percent,evenwhenit'sraining,orevenovertheocean.Frighten your timid friendwith this fact: In a steam bath orwet sauna the

relativehumidity is100percentand thensome.Firstofall, the temperature isdeliberatelykepthightogetasmuchwatervaporintotheairaspossible.Butinadditiontothatmaximumamountofwatervapor,thereareactuallytinydropletsof liquidwatersuspendedin theair.Wecall itsteamorfog.Inasteamroom,you'reactuallybreathingliquidwater.Butnobodyeverdrownedfrombreathingfogor steamat a reasonable temperature because there is still plenty of air inbetweenthesuspendeddroplets.(Caution: Steam can be dangerously hot, depending on how it has been

producedandatwhatpressure.Thesteaminasteambathis“coolsteam”andisnohotterthantheairintheroom.)

YouDidn'tAsk,but…

What is the “dew point” that weather reporters are alwaystalkingabout?

Meteorologistslovetotellusthedew-pointtemperature,eventhoughfewof

usknowwhatitisandevenfewerofuscare.ButaslongasI'mthisdeepintowatervapor,Imightaswellexplainthattoo.Thedewpoint,ordew-pointtemperature,isthetemperaturebelowwhichthe

liquid-vaporbalanceofwatershiftstofavortheliquidsideofthescales.Thatis,condensationwinsoutoverevaporation.Ifthetemperatureisabovethedewpoint,liquidwaterwillcontinuetochange

intovaporuntilitisallevaporated;wetthingswilldry.Butifthetemperatureisbelowthedewpoint,thebalanceshiftsinfavorofliquid,andvaporwilltendtocondense.When that happens up in the atmosphere, the vapor condenses intomassesofmicroscopicdropletsofliquidwaterthataretoosmalltofallandstaysuspendedintheair.Wecallthesemassesoftinywaterdropletsclouds.Amoreearthboundexample:Ifthegroundgetsanycolderovernightthanthe

dew-point temperature,watervapor in theairwillcondenseoutonto thegrassand leavesasdropsof liquiddew.That's important to farmersbecausedewis,afterall,freewaterforthecrops.Also,thereareecosystemsintheworldwhereit almost never rains andwhere small animals depend on dew for theirwatersupply.

NITPICKER'SCORNER

I wish that people wouldn't use the wordmoisture to mean water vapor.

“Moist”meansslightlywetordamp,andmoistureisthewater—liquidwater—that makes an object moist. Yet people use “moisture” to mean water vapor,which is water in its gaseous form. Air can't hold moisture, meaning liquidwater;itcanonlybesaidto“hold”watervapor,buteventhatisn'treallycorrect.Cosmetic advertisements, especially for “moisturizing” skin creams and

lotions,justlovetousetheword“moisture”whentheymean“water.”Althoughmoisture is nothing butwater, that's considered too common aword for suchelegantproducts.Sothenexttimeyou'reinafancyrestaurant,besuretoaskthewaiterforaglassofmoisture.Andbytheway,whatdocosmetic“moisturizers”do,anyway?Dotheyadd

moisture,orwater,orwhateveryouwant tocall it?No.If thatwere true,whycouldn't you just put water on your dry skin? “Moisturizing” lotions andcosmetics coat your skin with a concoction of oils and other water-blockingchemicals, so that your skin's supply of natural water stays in instead ofevaporating.Itsoundsparadoxical,butthecosmetic'soilproduceswater.

FireMakesDrier

Whydoesahairdryerhavetobothheatandblow?

This is one of those questions that seems so natural thatwe forget to ask

them.Butthat'swhatI'mherefor:tomakeyouwonderaboutthingsyoutakeforgranted,andthentoreplaceyourcomplacencywiththesmugnessofknowing.The water in your hair or clothes must first be converted from liquid into

vaporbeforeitcanbespiritedoffbyastreamofair.Blowingawayliquidwaterisn'teasy,asyoucan tellby thehurricane-forcewinds theyhave touse todryyourcaratthecarwash.Warmingtheliquidwaterinyourhairorclothes—andthat'swhatthewarmairdoes—speedsupthewater'smolecules,sothatmoreofthem can fly off into the air. (Techspeak:Warmer water has a higher vaporpressure;.)Theheat therefore speedsup evaporationof thewater, andonce ithasevaporatedintotheformofvaporitcanbesweptawaybytheblowingair.But how much vapor can the air-heated water produce? How fast can it

evaporate?Liquidwatermolecules can keep flying off and becoming vapormolecules

onlyuntil the space above the liquid is so crowdedwith vapormolecules thatjustasmanyarebouncingbackintothewaterasareflyingoutofit.(Techspeak:until the liquidandvaporare inequilibrium.)That'swhere theblowingcomesin. The moving air from the dryer blows away some of those water vapormolecules so that they can't go back into the liquid. This “makes room” formore,andtheevaporationcontinues.That'swhyclothesandhairdryersdobothheatingandblowing.Onewithout

the otherwouldn't do the job nearly aswell.What if your hair dryer's blowerblewout, so that itonlyheatedyourhair,or if itsheaterblewoutand itblewcoldair?Ifthereisalreadyalotofwatervaporintheair—forexample,ifthebathroom

is already steamed up and humid from your shower—the water in your hair

won't be able to evaporate as fast. It will require a much longer heating andblowingtimetodryyourtressestothatsilky,sexy,slow-motionslinkinessthattheyshowonTV.

ToHave,butNottoHold

Why is it that warm air can holdmoremoisture than cold aircan?That'swhyit'smorehumidinthesummer,isn'tit?

No.It'susuallymorehumidinthesummerbecausethere'smorewatervapor

available.I don'tmean that the oceans, lakes and rivers somehowexpand in the heat.

(Well, maybe a tad.)More precipitation? Perhaps. But it's not the amount ofwater itself; thehumiditycanbequite lowoverthemiddleof theocean.Whatcountsishowmuchwaterisbeingconvertedtovapor(orgas).Itismorehumidinthesummerbecausethewatersupplies—theoceans,lakes,riversandrains—are warmer, and water has a greater proclivity for making vapor when itstemperatureishigher.Notice that I have said nothing at all about the air or its ability to “hold

water.”Humidityisthewatervaporthatcomesfromwater,whetherthereisanyairpresentornot.Whenwesay,“It'shumidtoday,”weassumethatthe“it”inquestionistheairbecause,afterall,whatelseisthere?Buttheairplaysnorolewhatsoever in humidity; like Mount Everest, it is simply “there.” It is abystander.Thinkofitthisway:Wehappentobeimmersedinaseaofair,justasfishare

immersedinaseaofwater.Ifsomebodysuddenlydumpsaloadofredinkintotheoceanafishmightsay,“My,butit'sredtoday.”But“it”isn'tthewateritself;“it” is the ink thathasbeenmixedinto thewater.Well,humidity iswater thathappenstobemixedintotheair.Nevertheless, you'll hear scientists andmeteorologists explain humidity and

otherweatherphenomenabytalkingaboutthe“amountofmoisturethattheaircanhold”andsayingthatwarmaircan“holdmoremoisture”thancoldaircan.That'samistakenandmisleadingnotion.Theairisn'tholdingontowatervapor;ithasnosuchholdingpower.

Here's why. Air and water vapor are both gases, and in gases the spacesbetweenthemoleculesaresovastthatanytwogasescanmixinanyproportionswithouteitherone“knowing”—orcontrolling—howmuchoftheotheristhere.Alltheaircandoisacceptthewatervapor—whateveramountthewaterchoosestogiveoffaccordingtoitstemperature.It ispurelywater'sdecisionas tohoweagerlyitwantstobeintheformofvaporinsteadofliquid.NowIsupposeyouwanttoknowwhywarmwaterproducesmorewatervapor

thancoldwaterdoes,right?That'sscienceforyou:Everyanswergeneratesmorequestions.Water—likeallliquids—hasacertaintendencyforitsmoleculestoleavethe

surfaceoftheliquidandflyoffintotheair.That'sbecauseallthemoleculesaremoving with various speeds, and there will always be some of them at thesurface of the liquid that have enough energy to go flying right off as vapor.Becausemoleculesmove faster on the average at higher temperatures than atlowertemperatures,therewillbemorepotentialescapeesfromwarmwaterthanfrom cold. For example, at 86 degrees Fahrenheit (30 degrees Celsius) waterproduces seven timesmorevapormolecules inagivenspace thanwaterat32degreesFahrenheit(zerodegreesCelsius).Thereisalwaysasortoftug-of-wargoingonamongthemoleculesofaliquid.

Their speedwants them to fly off as vapor, but their attraction to their fellowmoleculeswants them to stay in the liquid.At every temperature,watermuststrikeabalancebetweenthesetwotendencies(Techspeak:Thewaterattemptstocometoequilibriumbetweenthesetwostates).Atlowertemperatures,theliquidtendstowinout;athighertemperatures,thevaporgetstheedgebecauseofthehighermolecularspeeds. (Theultimate limit iswhen the liquidboilsand turnscompletelytovapor.)At a given temperature, every liquid has its own preferred balance point

between vapor and liquid, because its molecules have their own degree ofstickiness toward one another. A liquid whose molecules are stuck tightlytogetherwill not formvapor easily, so its balancepointwill tend to favor theliquidformoverthevaporform.Gasoline'smolecules,ontheotherhand,don'tsticktooneanotherverymuchatall,sotheirbalancepointfavorsthevaporandgasolineevaporates(vaporizes)muchfasterthanwater.Thetendencyofliquidmoleculestoescapeandflyoffasvaporiscalledthe

vaporpressure of the liquid. In Techspeak,wewould say that gasoline has ahigher vapor pressure than water, and that warm water has a higher vaporpressurethancoldwater.Let'ssaywe'reinaclosedboxcontainingsomewater.Thewaterwouldsoon

strike a balance between liquid and vapor, according to the temperature. (The

liquidandvaporwouldbe inequilibrium.) Ifourboxsuddenlygotcolder, thewater would have to strike a new balance (find a new equilibrium balancebetween liquid and vapor) based on that new, lower temperature. The newbalancewouldbeinthedirectionoflessvaporandmoreliquid,sosomeofthevaporwouldhave tocondenseout andbecome liquid.Therewouldbe rainordewinourbox.Othersmayclaim that it rainedbecause“therewasmorewatervapor in the

coldairthanitcouldhold.”ButIneverevensaidtherewasairinthebox,didI?It rained solelybecause thewater shifted its liquid-vaporbalance all by itself.“It”wouldbejustashumidorjustasdryintheboxiftherewerenoairinitatall,butsomeothergaswithadifferentreputed“holdingpower.”

ICan'tSeeWhereI'mGoing!

Whenthewindshieldofmycargetsallfoggedup,howcanIclearitmostquickly?

Yourcarisaboxofhomemadeweather,producedbyyourairintakes,your

heater, your air conditioner and your passengers. But sometimes, because ofyour passengers' irrepressible habit of breathing, the car fills upwith a lot ofwatervaporandsomeof it condensesonto thecoldwindshield, fogging itup.Whatdoyoudo?

TRYIT

When it's very humid in your car and the wind-shield fogs up withcondensedmoisture on the inside, turn on the air conditioner, nomatter howcolditmaybeoutside.(Youcanalwaysturnontheheater,evenwhiletheACison.)Directtheconditionedairontothewindshieldanditwillclearupinajiffy.

What happenedwas that the air conditioner took in thewater vapor (alongwith theair that it'smixedwith)andcooled itdown toa lower temperatureatwhichthewaterwouldmuchratherbeliquid.ItcondensedintoliquidattheAC,where it was thrown away outside the car. There was then not enoughwatervaporinthecarforthetemperature,andtheliquidonthewindshieldrestoredthebalancebyturningitselfintovapor.Voilà!Adrywindshield.

Butwhatabouttherearwindow?Whenitfogsup,there'snowaytoblowair-

conditionedairontoit;thecoolairallcomesoutofductworkupfront,wherethedriverneeds it, and there isnoblower for the rearwindow.Sowhatdid thoseclevercardesignersdo?Theyembeddedheaterwiresintherearglass.Insteadofblowingdried-out,coolaironit,youjustheat theglass.Thatraisestheglass'stemperature above the point atwhichwater prefers to be liquid, so it turns tovaporandthefogdisappears.Odd,isn't it?Todefogthewindshieldyoucooltheair,buttodefogtherear

windowyou heat the glass, and the end result is the same.Whydon't the carmanuals ever explain this to you?Howmany people are driving aroundwithfogged-upglass,nothavingthefoggiestnotionofwhattodoaboutit?Now how about your bathroommirror back home? After your shower, it's

foggedupworsethananywindshieldevergot inthesteamiest jungle,andjustwhenyouwanttoshaveorputonyourmakeup.I'llbetyouhaveneitheranairconditionerinyourbathroomnorheatingwiresembeddedinthemirror,soyoucan'tuseeitherofthecarwindowtricksonit.Butyouprobablyhaveahairdryerhandy. Just sweep across themirrorwith it, as if youwere painting the glasswithhotair.Thedryer'sairwillheat thecondensedwaterontheglassenoughthat itwillprefer tobevapor rather than liquidand itwillevaporate, justas itdoesonyourelectricallyheatedrearcarwindow.

TheSmellofRain

A farmerneighbor tellsme thathe can smellwhen it's going torain.Ishejoshin’me?

Probablynot.It'snottherainitselfthathesmells,butjustabouteverything

else.Almosteverythingsmellsalittlestrongerwhenit'sabouttorain.Stormy weather is usually preceded by a drop in atmospheric pressure, or

whattheTVweatherpeopleliketocall“barometricpressure.”(Isthatwhatyoufeel when you're struck by a falling barometer?) That is, before it rains thepressure exerted by the air drops, and it presses down less heavily on thecountryside.Meanwhile, all the trees, grass, flowers, crops and, yes, even livestock are

emittingtheircharacteristicodors.Odorsaretinyamountsofvaporsemittedbysubstances, and we smell them when the vapor molecules happen to migratethroughtheairtoournoses.Whentheairpressureislowandisn'tpressingdownso hard, it allowsmore of these vapors to escape into the air, and everythingsmellsalittlebitstronger.Also,whentherain-bearinglow-pressurefrontmovesin,itisaccompaniedby

awindthatcarriesalongdistantsmellsthatarenotordinarilydetected.Of course, a farmer gets to be pretty good at readingweather clues, so he

mightbecheatingabitbyconsultingthesky,thewinds,andevenhisarthritis.Bytheway,doctorsusedtobelievethatpeoplewitharthriticjointscouldfeel

raincomingonbecausetherearetinygasbubblesinthejoints,andwhentheairpressuredecreases,thosebubblesexpandandcauseinternaljointpressure.Nicetheory,butIunderstandthatit'snolongerinvogue.

Oursisamaterialisticsociety.Wemaytalkaboutthebirds,thebees,thetrees,themoonandthestars,

butwhatwe surround ourselveswith is an accumulation of goods—stuffandthingsthathavebeenmanufactured,sold,bought,usedandultimatelythrownaway.Evenwhentrekkingthroughthewildernesswemusthaveasleeping bag, a canteen, a knife and, customarily, some clothing.(Mosquitoescanbefierce.)Manufacturedgoodsall.Science resides within everything. Every artifact has perfectly good

reasons,oftenunsuspectedandfascinatingreasons,forbeingpreciselywhatitisandnothingelse.I'mnottalkingaboutitsinventionorthetechnologyof its manufacture. Invention and technology are not science; they areapplications of science. I'm talking about the fundamental principles thatendoweachsubstanceorobjectwithitsuniqueindividualityasastufforathing.Inthischapterwe'llexaminethematerialsandarticlesthatweusedaily,

oralmostdaily—everything fromsoap tosodapop,erasers toexplosives,rubber to radioactivity, airplanes to automobiles, aluminum foil toskateboards.And then we'll end this book of answers by tackling the Most

FundamentalQuestion in theWorld:Why do some things happen andotherthingsnothappen?Believeitornot,thereisageneralanswer.

Extra!SharkBitesAirplane

Every time I get jittery about flying, someone tells me that thechancesofbeinginafatalairplanecrasharemuchsmallerthanthe chances of being attacked by a shark. What I don'tunderstand is howa lot of sharkattacksmakemyairplane anysafer.

Congratulations. You've put your finger on the most flagrant example of

figuresthatlie,withthepossibleexceptionofbreastimplants.Let'stakealookatsomestatistics.From 1994 to 1997, the average number of shark attacks per year in U.S.

waters was thirty-three. In the same four years, the average number of fatalairplane crashes was three. So you're eleven times less likely to be in a fatalcrashthantobeattackedbyashark,right?Wrong. It is utterly meaningless to compare two such completely different

sets of circumstances. Why on earth would anyone want to mention sharkattacks and plane crashes in the same breath, except to push a predeterminedpointofview?Anyonewhoiscomfortedbysuchafakeargumentismorelikelytodieofgullibilitythanfromeitherasharkoranairplane.Butevenifitwererelevanttocomparesuchtotallyunrelatedsetsoffigures,

they would still be meaningless without a lot of other information. Did thepeoplewhowerekilledinaircrashesflyalotmore,oralotless,thanyoudo?Ifso, their odds were different from yours. And how many of America's 250millioncitizensevenwentintothewater?DidtheydoitinFlorida,wheremostsharkattacksoccur,orinNewYork,whereallthedangerousanimalsareinthezoosandsubways?What people fail to understand is that when you're in an airplane, your

chancesofdyinginacrashareinfinitelyhigher—notlower—thanyourchancesofbeingattackedbyashark,becauseexceptforlawyers,therearenopredatory

beastsonairplanes.Worryaboutsharkswhenyou'reinthewater;worryaboutairplaneswhenyou'reintheair.Theonlypossibleconnectionwouldbeifyourplanecrashesintoshark-infestedwaters,inwhichcaseyourstatisticalassisupforgrabs.But a legitimate question remains: How much should you worry about

airplaneswhenyou'reintheair?Whataretherelevantstatistics?Ifeelsafewhenflyingbecauseofonestatisticonly,andithasnothingtodo

withthenumbersofdeathsbysharkattacks,drowning,accidentalfalls,suicides,auto accidents or lightning strikes—numbers that are frequently quoted toassuage the fearsofwhite-knuckledpassengers,andallequally irrelevant.TheonestatisticthatIkeepquotingtomyselfhastodowithmyparticularflight,theoneI'mactuallyon.Andtheprobabilityofanyoneflightendinginafatalcrashisabout1in2.5million.That'splentygoodenoughforme.Exceptwhenmyflightgetsbumpy,ofcourse.

Here'stheRub

Howdoesanerasererasepencilmarks?

It doesn't work like a chalkboard eraser, which wipes an accumulation of

chalkoffasmoothsurface.Paperisn'tthatsmoothandapencilmarkisn'tallonthesurface;mostofitisembeddedamongthepaperfibers.If you look at a pencil mark under a microscope, you'll see that it's not

continuous;itismadeupofindividualblackparticles,afewten-thousandthsofaninch(severalthousandthsofamillimeter)insize,clingingtothepaperfibersandtangledamongthem.Theeraser'sjobistopluckoutthesetinyparticles.Itcando thatbecause (a) it is flexibleenough to reachdownbetween the fibersand(b)itisjuststickyenoughtograbontotheblackparticlesandpullthemout.Butwhile the eraser is rubbing the paper, the paper's fibers are rubbing off

pieces of the rubber, which is now the rubbee, so to speak. The rubbed-offshredsofrubberrollup theircollectedblackparticles into thosepeskycrumbsthat you have to brush away. Under the microscope, those crumbs look likeenchiladasrolledincoaldust,tocoinaratherunappetizingsimile.The black particles are made of graphite, a shiny, black mineral form of

carbonthatbreaksaparteasilyintoflakes.Initspureformgraphiteismuchtoocrumblytomakesharp,detailedlines,soitismixedwithclayasahardenerandwaxasabinder,inordertomakepencil“leads”which,itbehoovesmetopointout,containnoleadatall.Real lead isa soft,graymetal thatwill leavedarkmarkswhen rubbedona

smoothsurface.Itwasusedforwritinguntilthemid–sixteenthcentury,whenaSwissnaturalistnamedConradvonGesner(1516–1565)putsomegraphiteintoa wooden holder and made the first pencil. Up until that time, graphite wasthought tobea formof lead,andwithanastoundingdisplayof inertiawearestillcallingtheblackstuffinpencils“lead”morethanfourcenturieslater.EvenHerrGesnerwasapparentlyunable toprevent theGermanword forhispencil

inventionfrombecomingbleistift,whichmeans“pegoflead.”Andwhilewe'reat it, the English word “pencil” comes from the Latin penicillus, meaning abrush.And ifyoumustknow,penicillus is thediminutiveofpenis,meaningatail.Don'tblameme.

BARBET

Leadpencilscontainnolead.

So-calledsoftpencils,likethenotoriousNo.2sthatwehaveallhadtouseonmachine-scored tests, are not darker because the particles of graphite are anyblacker. It's thatmoreof themare depositedon thepaper becauseof a higherratioofsoftgraphitetohardclayinthepencil.Thelargerproportionofgraphiteallowsmoreandbiggerblackparticlestobescrapedoffontothepaper,whichmakesabroaderanddensermark.Graphiteisashinymineral,andthedensemarksthatareleftonthetestsheets

whenwefillinthespaceswithourNo.2pencilsreflectlight.Thesheetsarerunthroughamachinethatscansthemwithlightbeamsandsearchesforreflections.If light is reflected from the right places but not from the wrong ones,congratulations.Yougetagoodgrade.And finally, didyou everwonderwhy rubber is called rubber?Because it's

usedtoruboutpencilmarks,ofcourse.In 1752, a member of the French Academy first suggested that coagulated

caoutchouc,or latex, agummysap fromcertainSouthAmerican trees thatweturn into rubber, could be rubbed on lead marks to erase them. In 1770 theEnglishchemistJosephPriestley(1733–1804)wasapparentlysoimpressedthathe named the substance “rubber,” and it stuck. Today the name makes littlesense, because rubber automobile tires, gaskets and so on aren't used to rubanything. Even the name “graphite” makes little sense today, because it wasderived from the Greekword graphein, meaning “to write,” and graphite hasmanyotheruses,initemsrangingfromlubricantstogolfclubs.All of which shows that writing has always been of more importance to

mankind thanautomobilesorgolf, although inourpresent societyyou'dneverknowit.

UnderstandingRubberIsaSnap

Whydoesrubberstretch?

If there is one statement that will make you a passionate believer in

molecules,it'sthis:Rubberstretchesbecauseitismadeofstretchymolecules.Arubberbandstretchesbecauseeachofitsmolecules,allbyitself,isbuiltlikeaminiaturerubberband.Rubbermoleculesare shaped like long, skinnyworms,all coiledandcurled

up,butcapableofbeingstraightenedoutbyappropriatetugsontheirheadsandtails.Apieceofrubberislikeacanoftheseworms,alltangledtogether.Butthink:Youcouldn'tstraightenoutawholesnarlofwormsbygrabbinga

headandatailatrandomandtuggingthemapart;theywouldjustslidepastoneanother (unless theybelonged to the sameunfortunate critter).The twowouldhavetobetiedtoeachotherinsomeway,sothatatugononewormwouldgettransmittedtoitsneighbor,andthentoitsneighbor'sneighbor,andsoon.What we need is a can of worms that are spot-welded to one another in

variousplaces:Siameseworms,joinedatthehipinvariouslocationsalongtheirlengths.(Okay,sowormsdon'thavehips,butyougetthepoint.)That'swhatthemoleculesofrubberarereallylike.Butnotatfirst,whenthe

latexsapdripsoutoftherubbertreeandiscongealedandpressedintoastickyglob.Itsmoleculesaren'tspot-weldedtogetherverymuch,andespeciallywhenwarmed theycan slideeasilyoveroneanother and the stuffbecomes soft andgooey.Aballmadeoutofrawrubberwouldhitthefloorwithadullthud.Anddon'teventhinkofmakingtires.Humans therefore have to come along and accomplish the spot-welding

themselves. They use a simple process called vulcanizing: heating the rubbertogetherwithsulfur.Thesulfuratomsformbridges,orcross-links,betweentherubber molecules, which allow them to stretch out to a certain extent, butunremittinglyurgethembacktotheirinitialpositions.That'swhytreatedrubber

iselastic:Themoleculeswillstretch,butthecross-linkswillalwaysbringthemback.Vulcanizationmakeswimpy, sticky raw rubber tough enough to be used in

tires.Theprocesswasdiscoveredin1839byCharlesGoodyear(1800–1860)—yes,thatGood-year—whohadbeentryingfortenyearstofindawayofmakingrubbertougheruntilheaccidentallyspilledsomerubbermixedwithsulfurontoahotstoveanditbecametoughandelastic.Hisdiscoverymadehimfamousbutnotrich,andhedieddeepindebt.That'sthewaytheballbounces.

TRYIT

Find a sturdy rubber band, at least a quarter of an inch (about half acentimeter)wide, and touch it briefly toyourupper lip to test its temperature.Nowstretchitquicklyandwhileit'sstretched,touchitagaintoyourupperlip.It'swarmerthanitwas.

Whatmadeitheatup?Whenyoustretchedtherubberband,youputenergyintoit,didn'tyou?That

energywarmeditup.Theenergycamefromyourmuscles,andyou'llhavetoeatacalorieorsooffoodtoreplenishit.When a rubber band is stretched, its stretched-outmolecules are in amore

orderly arrangement—more lined up— than they were in its unstretched,jumbled-upstate.Asevery-bodyknowsfromdoinghousework,orderlinesscanbeachievedonlybyputtingenergyintothejob.Sothestretched-outmoleculesmust contain more energy—they're hotter—than the relaxed, unstretchedmolecules.

I have just sneaked in on you one consequence of the Second Law of

Thermodynamics.Thislawofnatureexpressestherelationbetweenenergyandentropy, thedegreeofdisorderlinessof anarrangement.That'sdisorderliness:thedegreeof random,haphazardscrambling.TheSecondLawrecognizes thatNature'snatural tendenciesare (a) forenergy todecrease—things tend to slowdownandcooldown—and(b)forentropyordisorderlinesstoincrease—thingstendtorandomizeandscatter.Ifyouwanttocounteractthetendencyforhigherdisorderliness (higher entropy), you have to increase the energy of thearrangement. That's an inevitable fact of nature; everything that happens is atrade-offbetweenenergyandentropy.

LifeintheContactPatch

Whatmakesmynewsetoftiressomuchnoisierthantheoldones,especially when I drive fast? Sometimes I can hardly hear thepolicesirenbehindme.

I'llignoretheimplicationsofthatlastpart.Oneobvious factor is that yourold tiresmayhavebeenpretty smooth, and

smoothertireswillbequieter.Tire noise depends on the tread pattern, the roughness of the road and the

roundnessofthetires.Really,tirescanbeout-of-round,andthehighspotswillthump on the road during every revolution. But assuming that your tires aremoreroundthansquareandthatyou'reonarelativelysmoothhighway(thatis,you'renot inthestateofPennsylvania), therealquestioniswhyrollingrubbershouldmakeanynoiseatall;you'd think therewouldbenothingquieter.Andindeed,tiremanufacturersputalotofeffortintomakingtheirproductsoperateasquietlyaspossible.Herearesomeofthethingstheyhavetoconsider.As you can guess from the complexity of the sound—it's hardly a pure

musicaltone—itcomesfromacombinationofseveralfactorsthatmakethetiresvibrate.When the walls of a tire vibrate, it makes the air inside and outsidevibratealso,andthat'sexactlywhatsoundis:vibrationsofair.The source ofmost of the vibrations is the “contact patch”—the constantly

changingflattenedareathatisincontactwiththeroad.Aseachsegmentofthetire comes around in turn, it slaps against the road and is flattened into thecontact patch. That constant slapping makes noise. But your new tires aren'tperfectly smooth (unless you're a race driver). They have crosswise treadgroovesthatdividethemintoseparateblocksofrubber,andthoseblockshittheroad in a rat-a-tat-tat machine-gun sequence. More noise. Moreover, as eachblock of rubber reaches the back end of the contact patch it snaps back intoshape,againmakingtheairarounditvibrate.Stillmorenoise.

Alessobvioussourceofnoiseinvolvestheescapeofcompressedair.Asthetireturns,agrooveenteringthecontactpatchcantrapsomeairandcompressitagainsttheroad.Thenwhenthegrooveleavesthecontactpatchthetrappedairis released to the rearwith a sudden—if you'll excuseme—fart. Experimentshave been done with porous road surfaces that can significantly reduce thissourceofnoisebyallowingtheairtobleedoffdirectlyintotheroad.All of these effects depend on the groove pattern in your tires and the

characteristics of the road surface.And the faster yougo, of course, themoretimespersecondallofthesenoise-producingprocessesaretakingplace.Drivingatslowerspeedswillnotonlyreducethenoisecomingfromyourtiresbutwillcompletelyeliminatethatannoyingsiren.

X-RatedWindows

Adirty film always seems to build up on the inside ofmy car'swindshield.Idon'tsmokeorallowanybodytosmokeinmycar.Whatmakesthatfilm?

One good film deserves another, so I'll borrowmy answer from a famous

line in the 1967movieTheGraduate: “Oneword: ‘plastics.’” It'smainly theplasticsinyourcarthatproducethecoatingonyourwindshield.Rememberhowyourcar smelledwhen itwasbrand-new? Itsmelledbrand-

new.Anewcar'ssmellisapotpourriofthemanyvolatilechemicalsusedinitsmanufacture, from paint and cement solvents to chemicals used in treatingrubber,plasticsandfabricsand,ifyou'reaffluent,that“richCorinthianleather”on the seats. In fact, every substance in the world is continually evaporatingsome of its molecules into the air to a greater or lesser degree. (Techspeak:Everysubstancehasacertainvaporpressure.)Wesmellasubstancewhensomeof its evaporatedmolecules reach the olfactory nerve cells in our noses. Andthosethatdon'twindupinournosesmightlandanywhereelseinthecar.Most of these volatile substances evaporate completely and dissipate long

beforeyou'vepaidofftheloan,andwhenthathappensyourcarnolongersmellsnew. But other substances, smelling less noticeably of new debt, are releasedmoreslowlyoveralongperiodoftime.Plastics,inparticular,arethebiggestlong-termemittersofchemicals:mainly

plasticizers,whicharewaxychemicalsthatgivethemflexibility.Whenyourcarisout in the sun, the intense radiationbeatingdown through thewindshield—modernautomobileshavealmosthorizontalwindshields for streamlining—hitsthe plastic dashboard cover and drives out plasticizer vapors, which thencondense on the slightly cooler glass. The resulting, sticky film of waxyplasticizerthencollectsdustparticlesthatblowinthroughtheairductsaroundthewindshield,whereuponyoumayeithercleanitorbuyanewcar.

Wrinkle,Wrinkle,LittleBar

Why do my clothes get so wrinkled, and how do pressing andironinggetthewrinklesout?

Your clothes getwrinkled because you insist on putting them on awarm,

moist, moving body. If you were cold, dry and motionless, you'd have noproblem.Noclothingproblems,anyway.It'sheatandmoisturethatputthewrinklesin,andit'sheatandmoisturethat

aregoing toget themout.Dry,coldpressing,evenwith tonsofpressure,willaccomplish little;youneedboth theheatand themoisture thatdid theoriginaldamage.It's hard tomakegeneralizations aboutwrinkling and ironing,because there

are somany different kinds of fibers that our clothes aremade of these days.There are the synthetic (man-made) fibers, including nylon and a variety ofpolyesters and acrylics with various trade names. The synthetics are allchemicals known as polymers:materials that aremade up of hugemolecules,eachofwhich consists of thousandsof identical, smallermolecules, all strungtogetherintoenormouslylong(foramolecule)chains.Manyofthesesyntheticfibersaresensitivetoheat.Thatis,whenheatedtheybendandwhentheycooltheyretain thebend.If that isdonetoagarmentat thefactory, itwillkeepitsshape.Ontheotherhand,therearethenaturalfiberstakenfromplants,animalsand

insects, including cotton from the cotton plant, linen from flax, wool fromvariousanimalsandsilk fromworms. I'llconcentrateononeof theoldestandmostwrinkle-proneofallthefibers:cotton.Asfaraswrinklingisconcerned,it'sonebadactor.Cottonfibersarefilamentsofcellulose,anaturalpolymerthatoccursinplant

cells.(Tosplithairs,sotospeak,afiberisasingleunitofcottonthatisatleastahundred times as long as it iswide,while a filament is an extra-long fiber, a

strand ismade up ofmany filaments and a thread is made ofmany twisted-togetherfilaments.Iknewyou'dwanttoknow.)Acottonfiberactssomewhatlikealong,thinbarofmetal,inthatwhenbent

slightly itwill springback to its original shape, but it canbebent only so farbeforeitwillstaybent.Theamountofbendingitcanstandbeforeretainingthecrimpdependsonthetemperature.Thereisacertaintemperaturebelowwhichitwillspringbackandabovewhichitwillstaybent.Fordrycotton,thisso-calledtransitiontemperatureisabout120degreesFahrenheit(49degreesCelsius).Sofar,you'relucky,becauseyourbodytemperatureisonlyaround99degrees

Fahrenheit(37degreesCelsius),wellbelowthecrimpingtemperature.Butthenthere'stheeffectofmoisture.Water,intheformofperspiration,for

example, can lower the transition temperature of cotton down to around 70degreesFahrenheit (21degreesCelsius).Andwhetheryouknowitornot,youare always perspiring. You don't usually notice it because the perspirationevaporatesfromyourskinasfastasitisproduced—unless,ofcourse,theairisvery humid, in which case it doesn't evaporate and you say that you are“sweating.”Sonow ifyouactuallyhave theaudacity to sitonyourpantsor skirt,or to

stressyourotherapparelbysharplybendingyourwarm,moistlimbs,thefiberscan be bent into new, crooked shapes. Then when you stand up, yourperspiration evaporates and the fibers cool down below their transitiontemperatureandstayinthosenewshapes.Yourclothesarewrinkledandyouarerumpled.How do you get the fibers back to their original, straight shapes? Just give

themheatandmoistureagain,togetthemabovethetransitiontemperaturewhileyouholdthefabricinitsoriginalflatshapewiththesoleplateofasteamiron.Thesteam lowers the transition temperaturewaybelow the temperatureof theiron,sotheoriginal,straightshapescanreform.Aneasywaytothinkofallthisisthatheatandmoisture“melt”thestructure

of the fibers, andwhen they cool their shapes “freeze,”whether those shapeshappentobestraightorcrooked.In the laundry,always try to removeyourclothes fromthedryerwhile they

arestillwarmandslightlydamp.Inthatconditionyoucanlaythemoutflatandthey'll cool into that flat shape.Or you can hang themup and let gravity pullthemflat.Butifyouleavetheminthedryertoolongafteritstops,theclotheswillcooldownintheirjumbledpositionsandthewrinkleswillbeset.

NITPICKER'SCORNER

Whyisthereacertaintemperatureabovewhichcottonbeginstowrinkle?Cotton ismade of cellulose, a natural polymer whosemolecules consist of

thousandsofsugar(glucose)molecules,joinedtogetherintolongchains.Cottonfibersarebundlesofthesecellulosemolecules,alllyingalongsideoneanotherinthedirectionofthefiber.Here and there, the cellulose molecules are weakly bonded to one another

sideways by so-called hydrogen bonds, which tie them together like a loosebundleofsticks.Thetrickistokeepthemthatway,becauseifthoseweakbondsarebrokenandreformedwhilethefibersarebent,theywillstaybent.Hydrogenbonds can be brokenby a combination of heat,whichmakes the

molecules jiggle, and water, which swells the fibers by getting between themolecules.Waterlowersthetransitiontemperaturebecausewhenthefibersareswollenthemoleculesarefartherapartandeasiertoseparate.That'swhyasteamironworkssomuchbetterthanadryone.

OllieOop!

I'veseenkidsdothingsonskateboardsthatseemtodefythelawsof physics. As they jump over an obstacle the board rises withthem,eventhoughit'snotattachedinanywaytotheirfeet.Howcanthatbe?

What you're describing is a maneuver called an ollie, named after its

inventor, Allen Ollie Gelfand. Gelfand was one of a number of southernCaliforniasurfersinthelate1950swhojustcouldn'twaitforgoodsurftocomeupanddecidedtosurfthesidewalks.That'swhatstartedtheskateboardcraze.An ollie is a jump into the airwithout the loss of one's skateboard. It does

indeedlookasiftheboardsimplyfollowsthefeet,asifinamagician'slevitationtrick,andagoodskateboarderdoesitsofastthatyoudon'tseehowit'sdone.Itdependsonthefactthataskateboardisn't justaflatboardonwheels—ithasabent-uptailattherearend,andthat'sthesecrettohowitgetslaunchedupward.

DON'TTRYIT

Learning to do tricks on skateboards takes lots of practice, not tomentionantiseptics,bandagesandsplints.The followingdescriptionmaysound logicalbutisnotintendedtobealesson.

Here'showaskateboarderdoesanollie.As he approaches an obstacle that hewants to jump over, the skateboarder

placesonefootinthemiddleoftheboardandtheotheroneatthetipofthetail.Hethenstompshardonthetailwithhisrearfoot,whichmakesthetailhit thegroundandthefrontendoftheboard(thenose)flipupliketheoppositeendofa

seesaw. Simultaneously—and timing is critical—he jumps upward, hopefullyhighenoughtocleartheobstacle.Ashebecomesairborne,theboard'snosewillstillbepressingupwardagainsthis front footwithmomentum that it receivedfromthetail-stomp.Hequicklyslideshisfrontfootforwardtopushtheboard'snosedownlevelwiththetail.Heisnowinmidaironalevelskateboard,sailing—againhopefully—withenoughforwardmomentumtoclearthefarendoftheobstacle(which,ofcourse,requiresthathehadenoughforwardspeedwhenhebegan the jump). Finally, as gravity begins to win out, he and the board falltogether,withhisfeetstillincontactwiththeboard.

Theimportantthingfortheskateboardertorealizeisthathecangonohigher

thanhecouldby jumpingstraightup fromastandingposition.Theamountofvertical travel he achieves is completely independent of any forward travel,because gravity doesn't know or care about any motion parallel to Earth'ssurface;itcaresonlyabouthowfarheisfromEarth'scenterandwhetheritcanpullhimdownanycloser.Soifaskateboarderwantstosailoverapicnictable,hemustfirstmakesure

that he can jump straight up and onto the table before he tries it with askateboardunderhim.Andtheboarddoesaddtotheheightthathemustjumpinorderforittoclearthetablealongwithhim.Notethattheskateboarderandhisboardreceivedtheirupwardflightenergies

from twodifferent sources:he fromhis leg-powered jumpand theboard fromthe tail-kickhegave it,whichshot thenose into theair. (Infact,evenwithoutanyoneonit,askateboardonthegroundwouldleapintotheairwhenitsbent-uptail is stompedon.)There'snothingmagic, then, about the fact thatboard andridergoupwardtogether,inspiteofnotbeingfastenedtoeachother.Anexpert

olliemeisterallowsnocrackoflighttoshowbetweenhisfeetandtheboard,soitreallydoeslookasifthey'regluedtogether.

Onceapersonmasterstheollieandisreleasedfromthehospital,hecanuseit

asthebasisforanynumberofotherskateboarders'tricks,allofwhichseemtoinvolvelifeinmidair.Whattricks?Howaboutanollie,grind,heelflip,kick-flip,ollieflip,popshov-it,shov-itkickflip,casper,melloncollie,McTwist, tailslide,wheelslide, lipslide, indygrab orwallride?Many of these tricks are performednotonthestreetbutinskateparkswithartificialslopes,wallsandslidesinwhichcompetitionsareheld.

TRYIT

To seehowa skateboardwill flyupward froma stompon its tail, place aspoononthetable,hollowsideup.Theturned-upbowlisliketheturned-uptailof the skateboard. Now tap the end of the bowl sharplywith a finger, as theskate-boarderwould stomponhis board's tail.The spoongoes flyingupward,handlefirst,inaseesaweffectandthencontinuestoflipend-over-end.Ifitwerea skateboard, the rider's front foot would be holding down the handle end,shiftingitsmomentumbackwardtothebowlend,whichwouldthenrisetothelevelofthehandle.

I'llsticktogolf.

Soda…POP!

WhydoesthechampagnegushoutallovertheplacewhenIopenthebottle?Thatstuff'sexpensive,andIhatetowasteit.

You mean you actually want to drink it? Judging by what we see on

television,you'dthinkthatthemajorroleofchampagneinAmericancultureistohosedownSuperBowlwinnersinlockerrooms.Somewhatyoungerchildrendo the same thingwith soda pop,making sure to shake the bottlewell beforemovingthethumbpartiallyasideonthetopinordertoaimbetter….Well,youknowtherest.(DONOTTRYTHISATHOME!)If I said that shaking a bottle of champagne, beer or pop raises the gas

pressure inside, ninety-nine out of a hundred people, even chemists andphysicists,wouldagree.Butit'snottrue.Whenyoushakeanunopenedbottleorcanofcarbonatedbeveragethepressureinsidedoesnotchange.Itcertainlydoesseemasifthepressureisincreasedbyshaking,andit'seasy

to dream up smug theories as to why that should be. But I won't muddy thewatersbyquotingthosetheorieshere,becausethey'veturnedouttobeallwet.Thenwhy does the liquid squirt out with somuch force when you open a

shakenbottle?It'sonlybecauseshakingmakes iteasierforgas toescapefromthe liquid, and in its eagerness to escapewhen the bottle is opened it carriessomeliquidalongwithit.ItwastwochemistsnamedDavidW.DeamerandBenjaminK.Selingeratthe

AustralianNationalUniversityinCanberrawhoin1988settledthequestioninthesimplestpossibleway:bymeasuringthegaspressureinsideabottleofpopbefore and after shaking it. They adapted a standard pressure gauge, not toodifferent froma tiregauge, so that it couldbe screwedonto the topof a sodabottle.Theirresults(whichwouldhavebeenthesameiftheyhadsplurgedandused

champagne): If an unopened bottle has been standing quietly at room

temperature foradayorsoand is thenshaken, thepressureofcarbondioxidegasintheheadspace(thespaceabovetheliquid)doesnotchange.Thereasonis that thegaspressureisdeterminedbyonlytwothings:(a) the

temperatureand(b)howmuchcarbondioxidecandissolveintheliquidatthattemperature(Techspeak:thesolubilityofthegasintheliquid).Thereisonlysomuchcarbondioxidegasinthebottle;someofitisdissolvedintheliquidandsome of it is loose in the head space.When an unopened bottle of soda hasremainedatthesametemperatureforsometime,theamountofgasdissolvedinthe liquid—andmore important, theamountofgas that isnot dissolved in theliquid—settles down to whatever the appropriate proportions are for thatparticulartemperature.(Techspeak:Thesystemcomestoequilibrium.)Youcan'tchangethoseproportionsbydoinganythingshortofchangingthetemperatureoraddingmorecarbondioxide.(Ifyouputthebottleinthefridgefortwenty-fourhoursorso,moreofthegas

willdissolve in the liquid,becausegasesdissolve toagreater extent incolderliquids.Therewill thenbe lessgas in theheadspace,and thepressurewillbeless.That'swhyyouget lessofanoutburstofgaswhenopeningacoldbottlethanwhenopeningawarmone.)The point is that shaking alone can't change the pressure because it doesn't

change the temperature or in any other way change the amount of force orenergy that is available inside thebottle.Sonever fear thatmanhandlingyourbeer,sodaorchampagneonthewayhomefromthestorewillmakethebottlesexplode.Ontheotherhand,makesurenottoletthebottlesheatupinthetrunkofyourcar,becausethehighertemperaturewillindeedraisethepressureofthegas.Nowwecantakeamoreeducatedlookatwhatcausestheexplosiveemission

whenweopenarecentlyshakenbottle.Itiscausedbyanincreaseintheamountofgasthatissetloose—notbyheating,butbythemechanical“outing”ofsomedissolvedcarbondioxidefromtheliquidwhenthebottleisopened.Here'show.Firstofall,abunchofdissolvedcarbondioxidemoleculescan'tjustdecideto

gather together inonespotand formabubble.Theyneedsomething togatherupon—amicroscopic speck of dust or even a microscopic irregularity on thesurface of the container. These congregation spots are called nucleation sites,becausetheyserveasthenuclei,orcores,ofthebubbles.Onceasmallgangofcarbon dioxide molecules has gathered at a nucleation site and formed thebeginningsofabubble,itiseasierformorecarbondioxidemoleculestojoinup,andthebubblegrows.Thebiggerthebubblegets,theeasieritisforevenmoremoleculestofinditandthefasteritgrows.

Nowwhenyoushakeaclosedbottleofpop,you'remakingmillionsof tinybubbles of gas from the head space that become trapped in the liquid. There,they serve as millions of nucleation sites upon which millions of brand-newbubblescangrow.Ifthebottleisthenlefttostandforalongtime,thenewbabybubbleswillbereabsorbedandall thecontentswill returntonormal, inwhichconditionitisnolongerathreat.But those new nucleation sites and their newly hatched bubbles don't

disappearveryquickly; they remain for some time ina recently shakenbottle,justwaiting for some unsuspecting soul to come along and open it.When hedoes,andthepressureintheheadspacesuddenlydropstoatmosphericpressure,themillionsofbabybubblesarefreetogrow,andthebiggertheygetthefastertheygrow.Thelargevolumeofreleasedgaseruptsabruptlyintoagiganticblurpthatcarriesliquidoutofthebottle.

BARBET

Shakingabottleor canofbeeror sodapopdoesnot increase thepressureinside.

Oh, the champagne? Same thing. The best way to handle it is to leave itundisturbedintherefrigerator longenoughfor it to“cometoequilibrium”—atleasttwenty-fourhours.Thenbecarefulnottoeitherwarmoragitateitbeforeorduringopening.Afterremovingthewiretwist,easethecorkupwardwithyourthumbs.Allofthechampagnewillstayinthebottleandthecorkwon'tbecomealethalmissile.

DietCokeLosesWeight

MybuddyclaimsthathecantellacanofdietCokefromacanofClassic (regular) Coke without opening them or reading thelabels.Canhe?

Probably.Itisn'tdifficult,anditworkswithPepsitoo.It'sbasedonthefact

thatacanofthedietdrinkisslightlylighterthanacanoftheregulardrink.Regular Coca-Cola is sweetened with sugar (sucrose) or corn sweeteners,

whichareothersugars,usuallyfructose,maltoseand/orglucose.DietCoke,ontheother hand, is sweetenedwith aspartame, an artificial sweetener.Gram forgram,aspartameis150to200timessweeterthansucrose,soonlyatinyamountofitisneededtoproducethesamesweetnessasinthesugaredproduct.Whiletheamountofsugarintheregulardrinkis2or3percent, thereareonlyafewhundredthsofapercentofaspartame in thedietdrink.Therefore,acanof thedietdrinkisveryslightlylighterinweight.Yourbuddycan'ttellthedifferencejustbyheftingthetwocans.Butifhefills

a sink with water and places the unopened cans in it, the diet can will floathigherinthewaterthantheregularcan,whichmightevensink.

BARBET

I can tell a can of dietCoke from a can of regularCokewithout openingthemorreadingtheirlabels.

SourPower

InanoveltycatalogIsawa“fruit-poweredclock.”Yousticktwowiresintoanorangeorlemon,anditrunsasmalldigitalclockon“the natural energy found inside a fresh fruit or vegetable.”What'sthescoop?

“Natural energy” is a favoritebuzz-phraseofhucksters andkookspushing

everythingfromarthritiscurestocommunicationwiththedead.Thereseemstobethisideathat“naturalenergy”iseverywhere,tobepluckedoutoftheairbysuchmagictrinketsascopperbracelets(forarthritis)orbythosecrystalamuletsthat you wear around your neck or fondle in your pocket to ward off whatsupposedly lesssophisticatedsocietieswouldcall“evilspirits.”Ifanyof thesethings provided one-thousandth of the energy that their boosters expend inpeddlingthem,we'dneverhavetoburncoalorpetroleumagain.Asfarasfruitsandvegetablesareconcerned,theironly“naturalenergy”isin

theformof thecalories thatyougetbyeating them—theenergy thatyougainwhen youmetabolize, or “burn,” the food, just as you can release energy byburningapieceofcoal.Eatingcoal,however,doesn'tworkbecauseourbodieshavenomechanismfordigestingandmetabolizingit—thatis,forextractingitschemicalenergy.Orangesand lemonscontainprecious little foodenergy,asyoumightguess

fromthefactthattheydon'tburnworthadamn(exceptfortheoilsintherind).Even if you could convert all its nutritional energy into electricity instead ofmusclepower,thefifteencaloriesinalemonwouldkeepa7½-wattnight-lightburningforonlyabouttwohours.Otherthanthat,theonlywaytogetusefulenergyoutofalemonwouldbeto

dropitfromatallbuilding.Doesthefruitclockactuallywork?Amazingly,itdoes.Itwillrunforweeks

or months with its wires thrust into a fruit or vegetable—almost any fruit or

vegetable. “Potato-powered” clocks are quite popular, presumably becausethere'snothingquitesodumbandlifelessasapotato,andgettingenergyoutofitappealstopeople'ssenseoftheridiculous.Here'showtheveggieclockswork.The wires that you thrust into the fruit are made of two different metals,

usually copper and zinc. Together with the fruit juices in between, these twometalsmakeagenuineelectricbattery(moreproperlycalledavoltaiccell,butwe'll call it what everybody else does). All it takes to make a battery is twodifferentmetalswithsomesortofelectricity-conductingliquidinbetween.Youknowthatanelectriccurrentisaflowofelectronsgoingfromoneplace

toanother—throughawire,throughalightbulb,throughamotororinthiscasethroughanelectronicdigitalclock.Thequestionis,Howdoyouenticeelectronsintotravelingfromoneplacetoanothersotheycanrunaclockalongtheway?Abatteryinduceselectronstotravelbecauseitcontainstwodifferentkindsof

atoms that hold on to their electrons with different degrees of tightness. Forexample,copperatomshugtheirelectronsmoretightlythanzincatomsdo.Soifyougivezinc'selectronsachance,they'llleavehomeandmigratetothecopper,wheretheyfeelmorewanted.Cleverhumansthatweare,weoffertheelectronsonlyoneroutefromthezinc

tothecopper:throughourdigitalclock.Iftheywanttogettothecopper,they'llsimplyhavetoforcetheirwaythroughourclock,operatingitastheygo.Then why is the fruit or vegetable necessary? The juice inside it is what

chemists call an electrolyte: a liquid that conducts electricity. It completes thecircuit of electrons, restoring themand their charges to the zinc,whichwouldotherwisequicklybecomesodepletedofelectronsthatthewholeprocesswouldstop.Sowheredoes the“natural energy”actuallycome from? It's inherent in the

constitution of the zinc and copper atoms—in their natural difference ofelectron-holdingpowers.A battery is so easy tomake that at least onemay have been built by the

Parthians, apeoplewho lived two thousandyears ago inwhat is now Iraq. In1938aGermanarchaeologistdescribedasmallclayjarfromthatperiod,thenintheNationalMuseuminBaghdad.Thejarcontainedanironrodinsideacoppercylinder;oneneededonlytofillitwithfruitjuice(orwine)forittohaveenoughkicktopoweranancientParthiandigitalwristwatch.Okay,sonobodyreallyknowswhatitwasusedfor.Ifindeeditwasabattery.Ifitwasn'tahoax.If…

YouDidn'tAsk,but…

Whydoesthe“Two-PotatoClock”needtwopotatoes?

Forthesamereasonthatyourflashlightneedstwobatteries.A set of zinc and copper metals will move electrons with only so much

oomph.That'sbecause there'sonlyacertainamountofdifferencebetween theelectron-holding powers of zinc and copper. But if you need more electron-moving force—to light a bulb, for example—you can connect a second set ofzincandcoppermetalsafterthefirst,givingtwiceasmuchkicktotheelectrons.The technical word for electron kick is voltage: the force with which the

electronsaremadetomove.Thezinc-coppercombinationmakesabout1voltofkick.Ifaparticularclockneeds2volts torun,you'llneedtwopotatobatteriesconnectedtogether.

ThereAreNoSmokeAlarmsinHell

WhilechangingthebatteryinmysmokealarmIdecidedtoreadthe fine print on the label. It says that it contains radioactivematerial:americium-241.Whatdoesradioactivityhavetodowithdetectingsmoke?

Whatyouhaveisanionization-typesmokedetector.Itdetectssmokebythe

factthatsmokeinterfereswithair'sabilitytoconductatinyelectriccurrent.Under ordinary conditions, air doesn't conduct electricity at all; it's an

excellentinsulator.That'sbecausethenitrogenandoxygenmoleculesintheairhavenoelectricchargeoftheirown,nordotheycontainanylooseelectronsthatcouldcarrycharge fromoneplace toanother, asmetalsdo. If thatweren't thecase, electricity from thosehigh-tensionpower linesoverheadwouldzap rightthrough the air to the ground, passing through anything—including us—in itsway.Air molecules—nitrogen, oxygen and a few others—don't have any net

electricchargebecausetheatomsofwhichtheyaremadecontainequalnumbersofpositiveandnegativechargesthatcanceleachotherout.Thepositivechargesresideintheatoms'nucleiandthenegativechargesareintheformofelectronsorbiting around the nuclei. But radioactivity can make air into an electricalconductorbyknockingelectronsoutofthemolecules, leavingthemwithsomeuncanceled positive charge. These electron-shy, charged molecules are calledions, andwe say that the radioactivityhas ionized theair.Because ionizedaircontainselectricallychargedmolecules,itwillconductelectricity.Howdoesradioactivityionizetheair?The nuclei of radioactive atoms are unstable, and they spontaneously

disintegrate by shooting out some of the particles of which they are made atspeedsclosetothespeedoflight.Thenucleiofamericium-241choosetoshootoutalpha

particles,whichcomparedwithotherradioactivelyemittedparticlesareasabaseballistoaBB.Aheftyalphaparticlecandoalotofdamagetoanatomthatithits,soitisverygoodationizingairmolecules.Atinyamountofamericium-241ispackagedinsideyoursmokedetectorand

itsalphaparticleskeepasmallregionofairarounditcontinually ionized.Thebattery provides a very small electric current that flows through that air. Butwhensomesmokeparticlesgetintothatair,theionscancollidewiththemandlose their charge. Less charge in the air means that less current can flow. Acircuitdetectsthisdropincurrentandtriggersanear-piercingalarm.The amount of radioactive americium-241 in a smoke alarm is extremely

small:usuallynine-tenthsofamicrocurie,whichcorresponds toaquarterofamicrogram. Even though that quarter of a microgram is emitting more than30,000 alpha particles every second, they're nothing to worry about, becausealphaparticlesaresuchweaklingsatpenetratingmatterthattheycanbestoppedbyasheetofpaper.Noalpha-particleradiationwhatsoevergetsoutofthesmokealarmbox.

NITPICKER'SCORNER

Whenever an atom of americium-241 (or any radioactive material)

disintegrates, it isno longer the samekindof atomanddoesn'thave the sameradioactive properties. So as time goes by, the remaining radioactive atomsdecreaseinnumberandso,therefore,doestheamountofradiationtheyemit.Inthe case of americium-241, its number of atoms decreases by half every 433years. (Techspeak: Its half-life is 433 years.) So 433 years from now, theamericium-241 in your smoke alarmwill be emittingonly about 15,000 alphaparticlespersecond.Butdon'tthrowitawayyet,becauseafteranother433yearsitwillstillbeworkingfairlywellwhileemittingonly7,500alphaparticlespersecond.I'dadviseyoutoreplaceitaround433yearsafterthat,however,becauseby theyear3300 theelectriccurrentwillbegettingprettyweakand thealarmmightgooffevenwithoutanysmoke.Andthosealarms,youknow,canmakeenoughnoisetowakethedead.Of course, if by then you're where I expect to be, smoke alarms aren't

permittedbecausethey'dbegoingoffallthetime.

FertilizerGrowBoom!

Newspaper accounts of terrorist bombings have said that achemical fertilizer was used as an explosive. How can a singlechemicalhavesuchJekyllandHydeuses?

It's one of those coincidences that aren't really accidental when you dig a

littledeeper.Aswe'llsee,thegood-guyandbad-guypropertiesbothstemfromthefactthatnitrogengasismadeupofmoleculesthatstronglyresistbeingtornapart.First,thefertilizerrole.Every gardener knows that nitrogen is one of the threemain elements that

fertilizersprovide,alongwithphosphorusandpotassium.Nitrogenisextremelyabundant; it makes up about 78 percent of the air we breathe. Its moleculesconsist of pairs of nitrogen atoms bound together into two-atom molecules,whichchemistssymbolizeasN2.Those two nitrogen atoms are tied together so tightly that plants can't split

them apart to get their nitrogen fixes. They need the help of lightning,whichundeniablyhas enoughpower to do the job as it cracks through the air.Also,there are certain so-called nitrogen-fixing bacteria and algae that can splitnitrogenmolecules,buttheyhaven'ttoldusexactlyhowtheydoit.We humans must resort to our powerful chemical technology in order to

convert those nitrogen molecules into more plant-usable forms, such asammoniumcompoundsornitrate compounds.The fertilizer ammoniumnitratecontainsnitrogenatomsinbothoftheseforms,whichmakesitadoublypotentfertilizer.Now what if the two separated nitrogen atoms in ammonium nitrate were

suddenly given the chance to pair up again into strongmolecules of nitrogengas?Theywouldgrabthatopportunityeagerly.Afterall,ifnitrogenatomsloveoneanothersomuchthatwhenpaireduptheystronglyresistbeingsplitapart,

wouldn't they want to break out of the ammonium nitrate to reestablish theirtight pairings and become nitrogen gas again? Theywould do that with sucheagernessthattheywouldliterallyexplodeoutoftheammoniumnitratetorejoineachotherandflyawayintotheairinblissfulgaseousfreedom.Ihavejustdescribedanexplosion:anytimeasolidturnsintoagaswithgreat

suddenness.Thewaveofreleasedgases,whichareexpandingrapidlybecauseoftheheatthatisalsobeingreleased,isthepressurethatdoesallthedamage.Inthecaseofammoniumnitrate,whichcontainsoxygenandhydrogenatoms

aswellasnitrogen, it'snot just thenitrogenatoms thatcombinesuddenly intotight gas molecules. Oxygen and water molecules are almost as tightly heldtogetherasnitrogenmoleculesare,sotheoxygenatomspairupintooxygengas(O2),whilethehydrogenandoxygenatomsjoinuptoformwatervapor(H2O).If given the chance, then, solid ammoniumnitratewill suddenlybreakup andturnintoanenormousvolumeofgases:nitrogen,oxygenandwatervapor.Allittakesforammoniumnitratetodecomposeviolentlyinthiswayisheat:

enough to reacha temperatureofat least570degreesFahrenheit (300degreesCelsius).Even at temperatures as lowas 340degreesFahrenheit (170degreesCelsius), ammoniumnitrate can explode, turning somewhat less violently intonitrousoxidegasandwatervapor.Keepyourpowderdry,certainly.Butalsokeepyourfertilizercool.

FoiledAgain

Whyisonesideofmyaluminumfoilshinierthantheother?

It'sbecauseofatime-andspace-savingshortcutthat'susedinthefinalstage

ofthemanufacturingprocess.Aluminum, likeallmetals, ismalleable; that is, itwill squishwhenenough

pressureisapplied.That'sindistinctiontomostothersolidmaterials,whichwillcrackunderpressure.Sometalscanberolledoutintoextremelythinsheets.Metalsaremalleablebecausetheiratomsareheldtogetherbyamoveablesea

ofcommonlyownedelectrons,ratherthanbyrigidbondingforcesbetweentheelectronsofoneatomandtheelectronsofthenext,asisthecaseinmostothersolids. In effect, then, it doesn't matter much where a metal's atoms are withrespecttooneanother,andtheyarethereforefreetobepushedaroundwithintheelectronsea.In the aluminum foil factory they roll sheets of aluminum through pairs of

steelrollersthatgetprogressivelyclosertogether,whichsqueezesthealuminumdown to progressively thinner sheets. Household aluminum foil is less than athousandthofaninch(two-hundredthsofamillimeter)thick.Tosavespaceinthefinalrolling,theyfeedasandwichoftwosheetsatatime

throughtherollers.Thetopandbottomsurfacesareincontactwiththepolishedsteelrollersandcomeoutniceandshiny.Buttheinnersurfacesofthesandwichare pressed against each other—aluminum against aluminum. Becausealuminum is so much softer than steel, these surfaces press into each othersomewhat,leavingarougher,dullersurfacewhenthey'reseparated.Itmakesnodifferencewhatsoeverinhowyou'reabletousethefoil.Andbytheway:Ihopeyou'renotoneofthosepeoplewhosometimescallit

“tinfoil.”Afoilisaverythinsheetofmetal—anymetal.Aluminumfoilisathinsheet of (surprise!) aluminum metal and tinfoil is a thin sheet of an entirelydifferentmetal:tin.Tinisaratherheavy,nontoxicmetalwhosefoilswereused

as food and medicine wrappers before aluminum became cheap and widelyavailable.Buthabitsdiehard,andmanypeoplestillcallaluminumfoiltinfoil.Someoneshouldalso let itbeknownthat“tincans”aren't tin,either.A“tin

can”usedtobeasteelcanlinedwithrelativelynoncorrodingtinontheinside.But these days the linings of steel and aluminum cans aren't even tin; they'replasticorenameledcoatings.

Avast,YeSlob…uh,Swabs!

While sailing on a friend's boat, I didn't want to use up freshdrinking water, so I tried to wash my shirt in seawater. But Icouldn't get any lather at all. Why doesn't soap work in saltwater?

It'soneof life's little ironies.Sailorsdohard,oftendirtywork,yetwithall

that water around they can't bathe or wash their clothes with soap. Not withordinarysoap,anyway.Thereisaspecialsoapcalled“sailors'soap”thatworksinsaltwater.Butfirstlet'sseewhytheordinarystuffdoesn't.Itwillnotsurpriseyou to learn that seawatercontainsa lotofsalt—sodium

chloride. Averaged over the world's oceans, every quart (liter) of seawatercontains more than half a tablespoon (10 grams) of sodium chloride. It's thesodiumthatmessesup thesoap,becausesoapmustdissolve inwaterbefore itcandoitsjobanditwon'tdissolvewellinwaterthatcontainsalotofsodium.Soapmoleculesaremadeofsodiumatomsattachedtolongtailsofwhatare

knownasfattyacids.Thewaysoapworksisthatitsfattytailgrabsontotheoilyor greasy part of the dirt,while its sodium end drags it into thewater.But iftherearealreadytoomanysodiumatomsinthewater,theentryofstillmoreofthemintheformofsoapmoleculesisinhibited.(InTechspeak,chemistsrefertothis situation as the common ion effect, because the sodium atoms, which arecommontoboththesaltandthesoap,areactuallypresentasions,orelectricallychargedatoms.)Thismeansthatasodium-containingsoapwon'tdissolveenoughinsaltwater

todoitsjobofdraggingstickyoiloffthesailorandintothewater,whereitcanberinsedaway.But soaps don't have to be made with sodium. Potassium is a very close

chemicalrelativeofsodium's,andittoocancombinewithlongfattyacidtailstomakesoapmolecules.Comparedwith sodium, there isvery littlepotassium in

sea-water, so potassium soaps aren't inhibited from dissolving. So-called“sailors'soap”isapotassium-basedsoap.

FromDusttoDust

Housecleaning is a never-ending round of dusting, dusting,dusting. If I stopped dusting,wouldmyhouse eventually fill upfloortoceilingwithdust?

You think you've got trouble? In China there are 2-million-year-old

accumulationsofdust(calledloessbygeologists)thataremorethan1,000feet(300meters) thick.But it'snotdue tosloppyhousekeeping.ThedusthasbeensweptupbywindsfromtheGobiDesert. Incertain locationswhere thewindsdiedown, theydrop their loadsofdustparticles.The resultinghugedustpilesbecame compressed from their ownweight over the years, and some of themhaveactuallybeenhollowedoutintocavedwellings.Butneverfear.Attherateatwhichthedusthasaccumulatedin theChinese

loesscliffs,youcouldstopdusting inyourhouse forahundredyearsandstillhavealayerthatisnomorethananinch(2centimeters)thick.Unlessyou livenear theGobiDesert, youmaybewonderingwhere all the

dustinyourhousecomesfrom.The dust in our atmosphere hasmany sources.Winds blow over dry earth,

such as plowed fields, dirt roads and deserts. Plants give off pollen and otherparticulatematter.Forestfiresandvolcanoscanspewdustandsmokeparticleshigh into theupperatmosphere,where theymayblowaround foryearsbeforesettling.Thereislessdustovertheoceansthanoverland,butstilltherearetinybitsofdriedsaltsprayandevenashparticlesfallingfrommeteoritesthatburnupintheatmosphere.And,you'rethinking,itallwindsuponyourbookshelves,right?Well,we're

not done yet. Let's take a close look at the household dust that you generateyourself.Notice thatdustsettlesonlyonhorizontalsurfacessuchassills,shelvesand

thetopedgesofpictureframes.(Forgotaboutthose,didn'tyou?)Therefore,the

stuffmustbefallingoutoftheairundertheinfluenceofgravity.Thatmeansthattheparticlesofdustmustbebiggerthanacertainsize;iftheywereanysmaller,theconstant,agitatedmotionoftheairmoleculeswouldkeepthempermanentlysuspended. That's the case with cigarette smoke, for example—the individualparticles are so small that thebombardmentof airmoleculeskeeps them fromfalling.Ontheotherhand,ifdustparticlesweretoobigtheywouldn'thavebeenwafted into theair in thefirstplace, later tocome to restupon thatuglychinafig-urinethatAuntSophiegaveyou.Butit'snotallamatterofparticlesize.Arelativelybigtuftoflintfromyour

clothing will float on the air because of its feathery shape, and this too willeventually finda landingpadsomeplacewhereyou'd rathernothave it.Thosedustbunniesthattakerefugeinthewindlessclimateunderyourbedaremadeuplargely of fibers from clothing and other fabrics, often tangled together withfallenhumanorpethairsandflakesofskin.(Ineversaidthiswouldbepretty.)Everything that moves inside your house has the potential for sending out

microscopicbits thatarewornoffandcarried into theair.Whenahigh-trafficareaonyourcarpetwearsout,wheredoyouthinkallthosecarpetfiberswent?Motebymote, theywoundup scatteredaround thehouse, tobedealtwithoncleaningday.Whichbringsupthequestionofhoweffectivedustingreallyis.Itdependsa

lotonhowyoudoit.Adrydustclothmightjustredistributethedust,movingitperhapsfromtheshelftothefloor,demonstrating“todustshallitreturn”intheliteral,ratherthanthebiblical,sense.Rubbingwithadustclothcanactuallybecounterproductive, because it can produce an electrostatic charge on the dustparticles. Once charged, they can adhere tenaciously to any nearby object, sotheywillsimplyhavebeentransferredfromoneobjecttoanother.Adustclothwithadownynapthattrapsthedustparticlesisonegoodidea.

Another is touseoneof thosecommercialdustingsprays.Theycontainanoilthatnotonlymakes thedustparticles stick to thecloth,butcoats themwithathininsulatinglayersotheycan'tadhereelectrostaticallytonearbyobjects.Forasurprisingglimpseofhowmuchdustthereactuallyisintheair,lookup

at the beam of light coming from the projector the next time you go to themovies.Thereasonyoucanseethebeamatallisthatthelightisbeingscatteredsidewaysbyordinarily invisibledustparticles thatareapproximately the samesizeasthelight'swavelength.

ToGo,orNottoGo?ThatIstheQuestion

Thismaybeastupidquestion,butwhatmakesthingshappenornothappen?Imean,waterwillflowdownhill,butnotup.Sugarwill dissolve in my coffee, but if I put in too much I can'tundissolve it.Icanburnamatch,butIcan'tunburnit.Is theresome cosmic rule that determines what can happen and whatcan't?3

There'snosuchthingasastupidquestion.Youhaveaskedwhatisperhaps

themostprofoundquestioninallofscience.Nevertheless,itdoeshaveafairlysimpleanswereversinceageniusbythenameofJosiahWillardGibbs(1839–1903)figureditalloutinthelatenineteenthcentury.The answer is that everywhere in Nature there is a balance between two

fundamentalqualities:energy,whichyouprobablyknowsomethingabout,andentropy,whichyouprobablydon't (but soonwill). It is thisbalancealone thatdetermineswhetherornotsomethingcanhappen.Certainphysicalandchemical thingscanhappenallbythemselves,but they

can't happen in the opposite direction unless they get some outside help. Forexample,wecouldmakewatergouphillbyhaulingitorpumpingitup.Andifwe really wanted to, we could get that sugar back out of the coffee byevaporatingthewaterandthenchemicallyseparatingthesugarfromthecoffeesolids. Unburning a match is quite a bit tougher, but given enough time andequipment,asmallarmyofchemistscouldprobablyreconstructthematchoutofalltheash,smokeandgases.Thepointisthatineachofthesecasesagooddealofmeddling—energyinput

fromoutside—isrequired.Leftentirelytoherself,MotherNatureallowsmanythings tohappen spontaneously, allby themselves.Butother thingswillneverhappen spontaneously, even if we wait, hands-off, until doomsday. Nature's

grandbottomlineisthatifthebalancebetweenenergyandentropyisproper,itwillhappen;ifitisn't,itwon't.Let'stakeenergyfirst.Thenwe'llexplainentropy.Ingeneral,everythingwilltrytodecreaseitsenergyifitcan.Atawaterfall,

thewatergetsridofitspent-upgravitationalenergybyfallingdownintoapool.(Wecanmakethatcast-offenergyturnawaterwheelforusonthewaydown,ifwelike.)Butoncethewatergetsdowntothepool,itis“energy-dead,”atleastgravitationallyspeaking;itcan'tgetbackuptothetop.A lotof chemical reactionswill happen for a similar reason:Thechemicals

aregettingridoftheirstored-upchemicalenergybyspontaneouslytransformingthemselvesintodifferentchemicalsthathavelesschemicalenergy.(Theburningmatch is one example.) But they can't get back up to their original energyconditionsbythemselves.Thus, other things being equal, Nature's inclination is that everything will

loweritsenergyifitcan.That'srulenumberone.Butdecreasingenergyisonlyhalfthestoryofwhatmakesthingshappen.The

other half is increasingentropy. Entropy is just a fancyword fordisorder, orrandomness, the chaotic, irregular arrangement of things. At the scrimmage,footballplayersarelinedupinanorderlyarrangement—theyarenotdisorderly,and they therefore have low entropy. After the play, however, they may bescatteredalloverthefieldinaverydisorderly,higher-entropyarrangement.It'sthesamefortheindividualparticlesthatmakeupallsubstances:theatomsandmolecules.Atanygiventime,theycanbeeitherinanorderlyarrangement,orinahighlydisorderedjumble,or inanykindofarrangement inbetween.That is,theycanhavevariousamountsofentropy,fromlowtohigh.Other things (namely, energy) being equal, Nature's inclination is that

everythingtends tobecomemoreandmoredisorderly—that is,everythingwillincrease its entropy if it can. That's rule number two. There can be an“unnatural” increase in energy as long as there is a more-than-compensatingincreaseinentropy.Or,therecanbean“unnatural”decreaseinentropyaslongasthereisamore-than-compensatingdecreaseinenergy.Gotit?So the question of whether or not a happening can occur in Nature

spontaneously—withoutanyinterferencefromoutside—ispurelyaquestionofbalancebetweentheenergyandentropyrules.The waterfall? That happens because there is a big drop in (gravitational)

energy; there isvirtuallynoentropydifferencebetweenthewatermoleculesatthetopandthoseatthebottom.It'sanenergy-drivenprocess.Thesugar in thecoffee? Itdissolvesprimarilybecause there'sabigentropy

increase;sugarmoleculesswimmingaroundincoffeearemuchmoredisorderly

thanwhentheyweretiedneatlytogetherinthesugarcrystals.Meanwhile,thereisvirtuallynoenergydifferencebetweenthesolidsugarandthedissolvedsugar.(Thecoffeedoesn'tgethotterorcolderwhenthesugardissolves,doesit?)It'sanentropy-drivenprocess.The burning match? Obviously, there's a big energy decrease, a sudden

exodusofenergy.Thestored-upchemicalenergyinthematchheadisreleasedas a burst of heat and light. But there is also a huge entropy increase; thebillowing flame, smokeandgasesaremuchmoredisorderly than thecompactmatch head was. So this reaction is doubly blessed by Nature's rules, beingdrivenbybothenergyandentropy.That'swhyitproceedswithsuchgustotheinstantyousupplytheinitiatingscratch.Whatifwehaveaprocessinwhichoneofthequantities,energyorentropy,

goesthe“wrongway”?Well,theprocesscanstilloccuriftheotherquantityisgoing the “right way” strongly enough to overcome it. That is, energy canincreaseas longas there isabigenoughentropyincreasetocounterbalanceit;and entropy can decrease as long as there is a big enough energy decrease tocounterbalanceit.WhatJ.WillardGibbsdidwastodeviseanequationforthisenergy-entropy

balance.IftheGibbsequationshowsthataftercounteractingany“wrong-way”entropychangesthereisstillsomeenergyleftover,thatenergy(Techspeak:thefreeenergy)canbeusedtomakethingshappenandtheprocessinquestionwilltakeplaceautomatically.If,ontheotherhand,theamountofavailable(“free”)energy is inadequate to counteract any “wrong-way” entropy changes, theprocesswillnotandcannottakeplaceunlesssomeadditionalenergyisobtainedfromoutside.By adding enough energy, then,we can always overpower nature's entropy

rulethateverythingtendstowarddisorderliness.Here'sanexample.Thereareabout10milliontons—60trilliondollars'worth

—ofdissolvedgolddistributed throughoutEarth'soceans, just sitting there forthe taking.With enough effortwe could collect it all, atom by atom.But theatoms are dispersed through 336 million cubic miles (1.4 billion cubickilometers) of ocean in a completely chaotic arrangement that hasextraordinarilyhighentropy.Theenergythatwewouldhavetoexpendinorderto reduce its entropy by collecting it all in one place would cost enormouslymorethanthevalueofthegold.Inafitoffervoroverthelawsofmechanics,Archimedes(287–212B.C.) is

reputedtohavesaid,“Givemealeverlongenoughandaplacetostand,andIwillmove theworld.” Ifhehadknownaboutentropyandapplepie,hemighthaveadded,“GivemeenoughenergyandI'llputthischaoticworldintoapple-

pieorder.”

3 This question is so fundamental to the way in which the universeworksthatI'mrepeatingtheanswer,slightlymodified,thatIgaveinWhatEinstein Didn't Know: Scientific Answers to Everyday Questions (Dell1999).Allotherquestionsandanswersinthepresentbookarenew.

SomeTechspeakBuzzwords

Wordsthatareseparatelydefinedappearinitalics.

Acceleration:Anychangeinthespeedordirectionofamovingobject.It canbea speedingup,a slowingdownoranydeviation froma straightline.

Atom:A“buildingblock”outofwhichallsubstancesaremade.Everyatomconsistsofanextremelytinyandextremelyheavynucleussurroundedby a number of “whirling” electrons. There are over a hundred differentkindsofatoms,distinguishedfromoneanotherbythedifferentnumbersofelectronstheycontain.Atomsjointogetherinvariouscombinationstoforma vast number of differentmolecules, making a vast number of differentsubstanceswithdifferentproperties.

Density:Ameasureofhowheavyagivenbulkofasubstanceis.Aliterofwaterweighs1kilogram.Thedensityofwater is therefore1kilogramper liter.The density of gold is 19 kilogramsper liter. Peoplewould saylooselythatgoldis“nineteentimesheavier”thanwater.

Dipole: A molecule whose two ends are slightly charged, one endpositively and the other end negatively. The molecule therefore has twoelectricpoles,analogoustothetwomagneticpolesinamagnet.Waterisacommonexample.Theoppositelychargedendsofwatermoleculesattracteach other, making water difficult to boil and vaporize compared withsimilarliquids.

Electrolyte: A liquid that conducts electricity because it containselectrically charged particles (ions) . Salt water is the most commonelectrolyte.

Electromagnetic radiation: Pure energy in wave form, travelingthrough space at the speed of light. Electromagnetic radiation ranges inenergyfromlow-energyradiowavestomicrowaves,lightrays(bothvisibleandinvisible),Xraysandhigh-energygammarays.

Electron: A tiny, negatively charged particle. Its native habitat isoutsidethenucleusofanatom,butelectronsareeasilydetachedfromtheir

atomsandundertheinfluenceofavoltagecanbemadetomovethroughagasorametalwirefromoneplacetoanother.

Equilibrium: A situation in which nothing is changing because allforcesarebalanced.Insomeequilibriumsituationswemaynotbeabletoseeanychange,butdownatthemolecularleveltwooppositeprocessesaretakingplaceatequalrates.

Excitation: An atom or molecule is said to be excited when it hasreceivedsomeenergyinexcessofitsnormal“resting”state.Itwillusuallyemitthatexcessenergywithinaveryshortperiodoftime.

Gravitation,orgravity:Aforceofattractionbetweenanytwoobjectsthathavemass.Thestrengthoftheforceisproportionaltotheamountsofmassintheobjectsandbecomesweakerthefartheraparttheyare.Earthhasahugemassandisthereforetheprimarysourceofgravitationalattractionthatwenormallyexperience.

Half-life:The amount of time it takes for an amount of a radioactivesubstance to diminish to one-half of that amount.The amount diminishesbecause the atoms of a radioactive substance are unstable, and arespontaneouslyconvertingthemselvesintodifferentkindsofatomsthataremorestable.

Halogen:A family of chemical elementswith similar properties.Themembersofthisfamilyarefluorine,chlorine,bromine,iodineandastatine.

Heat:A formof energy that is exhibitedby themotionofatomsandmolecules.

Hydrogen bond: A weak attraction between certain molecules thatcontainhydrogenatoms.Hydrogenbondsareveryimportantindeterminingtheuniquepropertiesofwaterandmanybiologicallyimportantchemicals,includingDNA.

Ion:Anatomorgroupofatomsthathasacquiredanelectricchargebygainingorlosingoneormoreelectrons.

Kineticenergy:Theformofenergythatamovingobjecthas.Energyofmotion.

Mass: The quality of “heaviness” that is possessed by all things, allobjects,allmatter,allstuff,fromsubatomicparticlestogalaxies.Allmassexerts a gravitational attractive force on all other mass. The effect ofEarth'sgravitationalforceonanobjectistheobject'sweight.

Molecule: A conglomeration of atoms, all bound together. Allsubstancesaremadeofmolecules(exceptafewthataremadeofunboundatoms). Different substances have different properties because theirmolecules contain different collections of atoms bound in different

arrangements.Momentum:Ameasureofhowmuchdamageamovingobjectcando

in a collision with another object. Momentum is a combination of theobject'smassanditsspeed.Theheavieritisandthefasteritismoving,themoremomentumithas.

Nucleus: The incredibly tiny and incredibly heavy central core of anatom. It is thousands of times heavier than all the atom's electronscombined.

Photon: A “particle” of light or other electromagnetic radiation.Pressure: The amount of force being applied to an area of surface. Allgasesexertapressureonallsurfacesthattheyareincontactwith,becausetheirmoleculesareinconstantmotionandarebombardingthesurface.

Quantum: A “piece” of energy. Energy and momentum aren'tcontinuous, but exist in tiny, discrete quantities called quanta (plural ofquantum).

Refraction:Thebendingoflightorsoundwaveswhentheyleaveonemedium(suchasglassorair)andenteradifferentmediuminwhichtheirspeedisdifferent.

Solubility:Thequalityofdissolvingorbeingdissolved.Chemistsusethe word to mean the maximum amount of a substance that can bedissolvedinaliquidunderanyparticularconditions.Thesolubilityoftablesalt(sodiumchloride)inwaterat32degreesFahrenheit(0degreesCelsius)is357gramsperliter.Temperature:Anumberthatexpressestheaveragekineticenergyofthemoleculesinasubstance.Thehotterasubstanceis,thefasteritsmoleculesaremoving.

Terminalvelocity:Afancyexpressionforfinalspeed.Whenanobjectfallsthroughtheairfromahighplace,itwillfallfasterandfasterduetotheaccelerationofgravityuntiltheairresistancebuildsupenoughtostoptheacceleration,afterwhichtheobjectwillfallnofaster.Itwillhavereacheditsterminalvelocity.

Vaporpressure: In every solid or liquid substance, but especially inliquids, there is a certain tendency for themolecules to becomedetachedfrom their fellowsandgooff into theairasavapor.Thestrengthof thattendencyiscalledthevaporpressureofthesubstance.

Viscosity:The“thickness”ofa liquid; itsresistancetoflowingfreely.Astheoldsayinggoes,“Bloodismoreviscousthanwater.”

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