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The Reality of the Quantum WorldEinstein held that quantum-mechanical descriptions of physicalsystems are incomplete. Laborato tests show he was probabwrong; the bizarre nature of the quantum world must be accepted

We live in a remarkabe era inwhich experimental resutsare beginning to eucidate

phiosophica questions. n no domain have the results been more dramatic than in quantum mechanics.

The theory has been conirmed magnicenty since the 920s as its predictions of atomic, moecuar, nucear, optical, soid-state an eementary-partice phenomena wereshown to be accurate. Yet in spiteof these successes the bizarre andcounterintuitive character of quantum mechanics has ed some investigators, incuding Einstein, to believequantummechanica descriptions ofphysica systems are incompete andin need of suppementation. Recentexperiments show that this opinion

is very ikely wrong. The experimental results revea more ceary thanever that we ive in a strange "quantum word that deies comfortabe,commonsense interpretation

Here are a few of the new, strangendings we must begin to acceptFirst, two entities separated by manymeters and possessing no mechanism for communicating with eachother nonetheless can be "entanged they can exhibit striking correlations in their behavior, so thata measurement done on one of the

entities seems instantaneousy to affect the result of a measurement onthe other. The inding cannot be explained om a cassica point of view,but it agrees completely with quantum mechanics. Second, a photon,the ndamental unit of light, can behave ike either a particle or a wave,and it can exist in an ambiguous stateunti a measurement is made. f a particeike property is measured, thephoton behaves ike a particle, and ifa waveike property is measured, thephoton behaves ike a wave Wheth

er the photon is wave- or particlelikeis indenite until the experimental

46

by Ab Shmy

arrangement is specied. Finay, thenotion of indeniteness is no ongerconined to the atomic and subatomic domains. nvestigators have foundthat a macroscopic system can undersome circumstances exist in a state

in hich a macroscopic observabehas an indenite vaue Each of theseindings aters drasticay the way weperceive the word.

n understanding of these exper im nts and their phiosophicalimpications requires some famiiarity with the basic ideas of quantummechanics. Essentia to any discussion of the theory is the concept ofthe quantum state, or wave nction.The quantum state species a thequantities of a physica system to the

extent that it is possibe to do so Thecaveat at the end of the precedingsentence is crucial, because according to quantum mechanics not alquantities of a system have simutaneously denite vaues. The famiiar Heisenberg uncertainty principe,which asserts that the position andthe momentum of a partice cannotbe simutaneousy denite, is perhaps the best-known instance of thispropoition.

What the quantum state of a system does provide unequivocay is

the probability of each possible outcome of every experiment that canbe done on the system f the probabiity is the outcome is certain tooccur if the probabiity is zero, theoutcome is certain not to occur f,however, the probabiity is a numberbetween zero and then it cannot besaid in any individua case what theoutcome will be. Al that can be saidis what, on the average, the outcomes of a speciied experiment carried out on a large number of repicasystems wil be

magine, for instance, that measurements are made on a photon. The

quantum state of the photon is ixedif three quantities are known thephotons direction, its frequency andits inear polarization (the directionof the eectric ed associated withthe photon) A suitabe apparatus for

measuring poarization is a sheet ofpoarizing m. The m is ideaizedso that it transmits a ight incidenton it at a right ane if the ight is ineary poarized aong a certain direction in the lm caed the transmission axis. The m bocks a ight incident on it at a right angle if the ightis ineary poarized perpendicuar tothe transmission axis

Various experiments can be performed by rotating the poarizingm in dierent ways. f the photonis ineary poarized aong the trans

mission axis, there is a probabiityof that it wi be transmitted. f thephoton is ineary poarized perpendicuar to the transmission axis, theprobabiity that it wi be transmittedis zero. A rther impication of quantum mechanics, going beyond whathas been said so far, is that if the photon is ineary poarized at some ange to the transmission axis betweenzero and 90 degrees, the probabiityof transmission is a number betweenzero and (speciicaly, the square ofthe cosine of that particuar ange) f

the probabiity is, say, one-haf, thenout of 00 photons inearly poarizedat the corresponding ange to thetransmission axis 50 wil be transmitted on the average

Another basic idea of quantum mechanics is the superposition principe, which asserts that from any twoquantum states of a system rtherstates can be formed by superposingthem. Physicay the operation corresponds to forming a new state that"overaps each of the states fromwhich it was formed The concept

can be iustrated by considering twoquantum states of a photon in which

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the direction of the photons poarization in the irst state is perpendicuar to the direction of the photon'spoarization in the second Thenany number of states can be formedin which the photons poarizationpoints at some ange between thetwo perpendicuar directions

F

rom these two basic ideas aone

indeiniteness and the superposition principeit shoud be cear aready that quantum mechanics conicts sharpy with common sense. fthe quantum state of a system is acompete description of the system,then a quantity that has an indeinite vaue in that quantum state isobjectivey indeinite its vaue isnot merey unknown by the scientistwho seeks to describe the system.Furthermore, since the outcome of ameasurement of an objectivey indefinite quantity is not determined by

the quantum state, and yet the quantum state is the compete bearer of information about the system, the outcome is stricty a matter of objective

chancenot just a matter of chancein the sense of unpredictabiity bythe scientist. Finay, the probabiity of each possibe outcome of themeasurement is an objective probabiity assica physics did not conict with common sense in these fundamenta ways.

Even more starting impicationsow from quantum mechanics if the

system consists of two correatedparts. Suppose two photons y apartin opposite directions. One possibequantum state of the pair of photonsis the state in which both photonsare ineary poarized aong a verticaaxis Another possibe state is theone in which they are both ineary poarized aong a horizonta axisThere is nothing particuary bizarreor surprising about either of thesetwo-photon quantum states, beyondthe pecuiarities of the inge-photonstates mentioned above. But if the su

perposition principe is brought intopay, strange efects can occur

In particuar, by using the superposition principe one can form a

quantum state that contains equaamounts of the verticay poarizedstate and the horizontay poarizedstate This new state wi igure prominenty in what foows, and so it wibe given a name, (since the Greeketter psi is commony used to represent a quantum state) The properties of are most pecuiar indeedmagine, for instance, inserting in the

paths of the photons poarizing imswith verticay oriented transmissionaxes Because contains equaamounts of the verticay and horizontay poarized states, there isa probabiity of onehaf that bothphotons wi be transmitted throughtheir respective ims and a probabiity of one-haf that both wi bebocked What cannot happen is thatone photon wi be transmitted andthe other wi be bocked. n otherwords, the outcomes of the inearpoarization experiments on the two

photons are stricty correatedThe resuts wi be the same if the

poarizing ims are oriented at anange of 45 degrees with respect to

EXPEIMENTA TET ae now seddin it on topics in

quanum mecanics at wee once conned o te eam of

piosopica debae In is expeiment, wic was done by

ain Aspec and is coeaues at e Institute of ptics of e

Univesity of Pais, e ases at eac side of e pictue excite

individua cacium atoms in te vacuum cambe (center Eacatom euns o its unexcied sate by emitin a pai of poons

(Te poton is te fundamenta unit of it) Te potons tave

in opposite diecions tou 65 metes of pipe, and tose ta

pass tou poaization anayzes impine on potodeecos

Quantum mecanics pedicts tee soud be deicate coea

tions in e poaizations of te oppositey dieced poons te

coeaion conics wit cassica teoies caed iddenva

iabes modes Te expeimen conmed quantum mecanics

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. . . .. -. _

INDINITN of a quanum sysem is iusaed fo a poon A see of poaizin

m ansmis a i inciden on i a a i ane if e i is ineay poaized

aon a ceain diecion in e m caed e ansmission s (hathing Tis poaizaion sae of e poon is epesened by e wavy cooed ine a e op Te mbocks a i inciden on i a a i ane if e i is ineay poaized pependic

ua o e ansmission as (wav gray line at top Now suppose a poon is ineaypoaized a some ane o e ansmission as beween zeo and 90 deees (bottomTen wee o no e poon wi be ansmied is indenie e pobabiiy of

ansmission is a numbe beween zeo and 1 (e squae of e cosine of e ane)

the horizontal: either both photonswll be transmitted or both wll beblocked. It smply cannot happenthat one photon wil be transmittedand the other wll be blocked In fact,it does not matter what the orienta

tions of the ilms are as long as theymatch each other; the outcomes ofthe linearpoarization expermentsare strctly correlated for an innitefamly of possible experments (Ofcourse, no more than one of the exeriments can actually be carriedout) Somehow the second photon ofthe pair "knows whether to passthrough ts polarzing ilm n orderto agree wth the passage or nonpassage of the irst photon, eventhough the two photons are well separated and neither has a mechansm

for informng the other of its behavior. In ths kind of situaton, then,quantum mechaniC challenges therelativstc concept of locality, whichholds that an event cannot have effects that propagate faster than lght(and, n particular, instantaneous effects at a dstance).

It must be emphasized that all thepecular implcations that havebeen drawn so fa-objective indeinteness, objective chance, objectiveprobabilty and nonlocality-dependcucally on the pemise that a syste's quantum state is a complete de

8

scription of that system. A number oftheorists have maintaned, however,that the quantum state merely descrbes an ensemble of systems prepared n a uniform manne, and thatthis s why good predictions can be

made about the statistcal results ofthe same experment performed onall members of the ensemble. At thesame time, the argument goes, the indvidual members of the ensembledifer from one another in ways notmentioned by the quantum state, andthis s the reason the outcomes of theindvdual experments are diferent.The properties of individual systemsthat are not speCiied by the quantumstate are known as hidden varables

If hddenvaiables theorists arecorect, there is no objective ndef

niteness. There s only ignoranceon the part of the scentist about thevalues of the hidden variables thatcharacterze an individual system ofinterest Moreover, thee s no objective chance and there are no objective pobablities Most important,the quantum correlatons of wellseparated systems are no more surprsing than the concordance of twonewspapers prnted by the samepress and mailed to diferent cites.

In 196 John S. Bell of the European laboratory for particle physcs, showed that the pedictions oflocal hiddenvariables models are

incompatible wh the predictons ofquantu mechancs. Reection onsome hiddenvarables moels ofDavd Bohm of Birkbeck ollegeLondon and Louis de Broge led Bllto prove the important theorem thatno model that is local (n a carefullyspeCiied sense) can agree wth allth statistical predctons of quantummechanics. In other words, there are

physcal stuations n whch the predictons of quantum mechancs dsagree with those of every local hiddenvariables model [see "The Quantum Theory and Reality, by BernarddEspagnat SF MA November, 1979

The idea of Bels theorem can begrasped, at least n part, by returningto onsider the quantum state Asnoted above, the results of lnearpoarzation experments performed ona pair of photons n ths state must bestrictly correlated when the ange be

tween the transmsson axes of thetwo polarzng ims s zero degrees(as t is when both axes are algnedvertcally). It should not be surprising to learn, therefore, that for thestate there s always at least apartal correlaton between the outcomes, no matter what the anglebetween the transmisson axes is.(SpeCialy, f one of the photonss trnsmitted through its polarizngilm, then the pobablity that theoter photon will be transmittedthrough its ilm is the square of the

cosine of the angle between the twotransmsson axes.)

onsequently a hiddenvariablesodel that agrees wth all the statistical predictons of quantum mechancs must assign quantties to eachpair of photons in the ensemble in adelicate way n order to guaranteethe strict or partial correatons at every angle between the axes. But theconditon of localty requires that thequantties assgned to each photonn a par must be ndependent of theorientaton of the polarzng lm on

which the other photon impingesand independent of the other photons passage or nonpassage. It is thslocality condition that makes qutempossible the delcate adjustmentsthat would be necessary fo reproducing all the correlations, strct andpartial, mplied by

Bells theorem shows that n principle one can determne experimentally whch is corect: quantummechancs or the loca hiddenvariables models. It was mportant to dosuch a test because, in spite of the immense body of conirmng evidence


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CELATIN between te poaizations of two potons oc

cu wen te potons ae in a specia state caed (afte teette psi in te Geek apabet)_ Te state can be fomed by su

peposin te state in wic bot potons ae ineay poaized

aon a vetica as wit te state in wic tey ae bot ineay

poaized aon a oizonta as Te state contains equaamounts of te veticay poaized state and te oizontay

poaized state Now imaine tat poaizin ms wit oizon

tay oieted tansmission axes ae inseted in te pats of te

potons ince '0 contains equa amounts of te two states

tee is a 50 pecent pobabiity tat bot potons be tans

mitted tou tei espective ms and a 50 pecent pobabi

ity tat bot wi be bocked Wat cannot appen is tat one

poton wi be tansmitted and te ote wi be bocked te

outcomes of te ineapoaization expeiments ae sticty

coeated In fact it does not matte wat te oientations of te

ms ae as on as tey matc eac ote; someow te second

poton of te pai knows wete to pass tou its poa

izin m in ode to aee t te passae o non passae of

te st poton even tou te potons ae we sepaated

for qatm mechacs at the tmee proved hs theorem the verpots where qatm mechacs swthot eqvocato rrecocabewth commo sese had ot et beeprobed

I 969 Joh F aser the at omba Uverst chae A Horeof osto Uverst Rchard A Hotthe at Harvard Uverst ad I proposed a desg for the reqste testPars of photos wth correated

ear poarzatos were to be obtaedb exctg atoms to a approprateta state the atoms wod sbseqet retr to the excted stateb emttg two photos Fters adeses wod esre that whe thephotos ew o opposte or vrta opposte drectos oe photowod mpge o a poarzato aazer ad the other wod mpgeo aother aazer. swtchgbetwee two oretatos of each aazer ad recordg the mber ofphoto pars trasmtted each ofthe for possbe combatos of oretatos of the two aazers measremets of correatos of trasmssos of the photos of a par codbe made

We sggested that ether cactecrstas or pes of gass pates serveas the poarzato aazers sceeach of them s mch more ecet tha a acta poarzg m bockg photos poarzed perpedcar to the trasmsso axsPhotodetectors paced behd theaazers wod detect a xed fracto of the photos passg throghthe aazers If two photos, oe at

each detector, were regstered wth20 aosecods (boths of a secod) of each other the probabtwod be qte hgh that the wereemtted b the same atom Sce theeses wod coect the two photosover a e age the qatm statewod ot be exact the state dscssed above bt a moded state [,whch aso eads to correatos thatcaot be reprodced b a ocahddevarabes mode

he expermet was doe b Start] Freedma ad aser at theUverst of afora at erkee 972, b dward S Fr ad Rada hompso at exas A & Uverst 975 ad b other gropsaer that ost of the expermetarests agree wth the correatopredctos of qatm mechacsad dsagree wth the hddevarabes modes oreover the reabt of the dssetg expermets ssspect becase of sbte weakesses ther desg

Yet t ver recet a the expermets had a oophoe that aowed stach defeders of hddevarabes modes to matather hopes the poarzato aazers were kept ther respectve oretatos for tervas of a mte orso whch s ampe tme for the exchage of formato betwee theaazers b some hpothetcamechasm As a rest the defederscod coted that the speca theorof reatvt does ot mp the vadt of es ocat codto hephsca stato of the expermets t the these expermets

wod ot serve as decsve tests betwee qatm mechacs ad theoca hddevarabes modes

To bock ths oophoe, Aa Aspect Jea Dabard ad GrardRoger of the Isttte of Optcs of theUverst of Pars dd a spectacarexpermet whch the choce betwee the oretatos of the poarzato aazers s made b optca swtches whe the photos are

ght I ther expermet whchreqred eght ears of work adwas competed o 1982, eachswtch s a sma va of water whch stadg waves are perodca geerated trasoca. hewaves serve as dfracto gratgsthat ca deect a cdet photowth hgh ecec If the stadgwaves are tred o, the photo wbe deected to a aazer thats oreted oe wa f the stadgwaves are tred o, the photo wtrave straght to a aazer that soreted aother wa

he swtchg betwee the oretatos takes abot 10 aosecods.he geerators that power the twoswtches operate depedet athogh (fortate for the compete detveess of the expermet) the operato s perodc rathertha radom The dstace betweethe aazers s 3 meters so that asga movg at the speed of ght(the hghest speed aowed b thespeca theor of reatvt) takes

40 aosecods to trave betweethem oseqet the choce oforetato for the rst poarzaton


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DETECTOR

COINCDENCECOUNTNG

DTCTOR

SEARCH FOR CORRELATIONS between members of pairs of

photons was carried out in the 1970's by a number of investiga

tors_ The photon pairs were emitted in energy-state transitions

of calcium and mercury atoms; each photon impinged on a po

larization analyzer Quantum mechanics predicts there must be

delicate correlations in the passage or nonpassage of the pho

tons through their analyzers. even tough the photons ave no

apparent means of communicating with eah other The experi

ments mainly conrmed quantum mecanics, but they had a

loophole: the orientations of the two analyzers were xed before

the photons were emitted Consequently it was possibe that

information was someow exchanged between the analyzers

zer ogh o o ece hero of he eco phoohrogh he eco zer he choce of oreo for he eco zer ogh o o ecehe ro of he r phoo hrogh he r zer heepere rrgee hepece o e occoo foow hccorgo e heorehere ho beoe voo of he qechc preco of correo he epere ocoe

po of fc however he epere ele he oppoe re he correlo gree wh eperel error wh he qechc preco h recce o he b of he q e . oreover he gree b ore h ve r evo wh he eree owe ccorg o el heoreb of he oc hevrbeoe

Eve hogh he epere of Apec h colege o coplee eve o peope beeve he propec of overhrowg he re b re eperere eree ee like h he f of oc hevrble oe c be vgeThe rge propere of he q wor-obecve eeeobecve chce obecve probb oloc-wo pper o

be perel ereche i phscl heoryOe of he srages propere of

50

he q wor ooc he fc h er oe crcce eree o oe phoo ppre eo ffec he rel of eree ooher phoo be cpze o oe ege fer h he peeof lgh? Fore for he pecheor of rev he wer o heqeo o A erg po of h heorh o g c rve fser h ighpreerve

Here s bref rge hhow wh Sppoe wo peopew o coce b e of evce ir o he oe for egocl hevrbe oe ewee he oberver sorce eispr of corree phoo Ech oberver prove wh porzo lzer phooeecorhe oberver re ee o ore herso e of her zer w hey chooe

Sppose he oberver gree og he ro e verc he ever e pr of phoos ee here w be rc correo i he oucoe eher bohphoo wll p hrogh he zer or boh wl be bocke uhe rc correo of o value forech oberver soo fro heoher he rs oberver w oeh hlf of he e phoos phrogh he rs zer o he

verage hf of he e here bocke The eco oberverw oe he e hig for he ec

o zer I oher wor echoberver ioo ees ol ro per of rssio blockge

Now gie h he r oberverre o ecoe oe foro e o he eco oberver bchgg he oreo of he rpolrzo lzer Depeg ohe oreo of h lzer herewi be eher rc or prl correo bewee he ocomes of heeve ech eecor Oce ghowever ech oberver w oeh o he verge hlf of he ephoo ps hrogh he zer hf of he e he are blockeI geer o er wh he oreo of he lzer re ech oberver soo ee ol ro ( icl ieic) paer of rso blockgeThe qu correo beweehe phoo c be checke o b

coprg he accule he wo eecor Hece he epo expo he q correioo se ege fser hn lighcno ccee

I h ee here pecefulcoeece bewee qm echic relivi heor pieof quechc oloclyFor hi reso i wo be miseg ( wrog) o s ha noloc n he quumechnicese is reverson o co a isce s i he prerevsic gravi

oa heor of Newo I s epg o characerize qunummechancal onlocay as "pssion a a


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ce o w y preeoo prove epo or ere correo oy o epze e correo coe epoe o eer coroe ece ore rpy c e e

Another e ce e eye choiceepere wc wpropoe 78 y o ArcWeeer e Prceo very o reve e ree o e wor e c ppro e epere ereroeer wc e c ep recoe A pe o ro er re e e per wc oree c wy o e pe roe per reece r e o e reco o e ce pe e ro e wop eey recoe

ererece per c e eece wc eore e wveey o

ow ppoe e pe o er ee o everey y e ere oy oe poo e ereroeer cewo ere eo c e eo e poo Doe e pooe ee roe o eerre or reece y e eper erey e prcee propery? Or e poo

DETECTOR

DEECTOR

oe ee eoy re reece o ererew e erey ow wvee propery?

A wer w recey ppey rro O Aey Oe G owcz W Wce oe very o ry oeePr epeey y He H Wer Arr G Z

oc o e very o co ro o poo eve e prce we prcee propere re ere eve e wve wewvee propere re eree rere ovey o e re e epere w rre o e eco o ere prcee or wvee propere w e er ec poo erce w e e per oeey e poo co ve ee ore e cr

c oe o erco w ee per weer o eve e prce e ee roe oro eve e wve propeo wo roe

e e o o roe eereroeer w o 43 eer wc poo c rvere roy 5 oeco Ovoy oe o ow eo eor orry ecc evce owc ewee er prce wvee propere e e

CONCDENCECOUNTNG

w e poe w wcce Poce ce wc c e ce e oeco or e APoce ce co cry ecoe rere we voe ppe cro porzeo oe o e cry prope veocy ere o o porze o e perpecr reco Gve e proper

coce o voe coroeoery porze oe reco we eer e ce weere porze e perpecr reco Te Poce ce w ere oe o e wo roe epoo co e er ercw e e per [see illustrationon next page].

A piece o porz w e.other ee eee eeeo wc ewee eree oprcee wvee propere

L eer o e Poce cepe o e ce wo e porzo o e wc e porz reecee pooeecor erey wer e eo o wcroe cor e pooprcee propere e ce wo e porzo o e wc e porz re e wc e w coe w e coro coro e oer roe ererece e

DETECTOR

DETECOR

RAPID SWITCHING between orientations of poarization ana

yzers as photons ew was the hamark of the experiment done

by Aspect and his coeagues (see iusraion on page 47 whicwas competed in 982 When a switch was "on a photon was

deected to an anayzer that was oriented one way; when the

switch was "o the photon traveed straight to an anayzer that

was oriented another way The time reuired for ight to trav

e between he anayzers was greater than the time reuired

to switch between orientations, so that the choice of orienta

tion for each anayzer cou not inuence the observation made

at the other anayzer (Unfortunatey for compete denitive

ness however, the switching was periodic rather than random)

The experiment conrmed uantum mechanics; it woud appear

that te strange impications of the theory must be accepted


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DELAYED-HOIE EXPERIMENT is another test that reveals the strangeness of the

quantum world A photon impinges on a beam splitter Two questions about the photon

can be asked Does the photon take a denite route so that it is either transmitted or re

lected by the beam splitter thereby exhibiting a particlelike property? Or is the pho

ton in some sense both transmitted and relected so that it interferes with itself exhib

iting a wavelike property? To nd out a stch is positioned in one of the two paths the

photon can take after interacting with the beam splitter (her path A) If the switch ison the light is delected into a photodetector (path B), thereby answering the questionof which route and conrming the photon's particle like properties If the switch is o,

the photon is free to interfere with itself (paths A and A) and produce an interferencepattern demonstrating the photon's wavelike properties Results from the experiment

show that a photon behaves like a wave when wavelike properties are measured and

behaves like a particle when particlelike properties are measured Remarkably the

switch was triggerd after the photon had interacted with the beam splitter so that the

photon could not have been "informed whether to behave like a particle and take a

denite route or to behave like a wave and propagate simultaneously along two routes

fects conirmed the photon's wavelike aspect

Both groups of investigators havereported results that are in excellentagreement with quantum mechanics. Their work shows that the choicebetween the two questions can bemade after an individual photon hasintracted with the beam splitter ofan interferometer

How are th results of he delaydchoice experiment to be intrprted? t is worthwhile rst to discaimone extravagant interpreation thathas somimes been advanced: thaquantum mechanics allows a kindof "reaching into the past. Quantum mechanics does not cause something o happen that had no happened previously SpeCically, in thdeayed-choice expriment quantummechanics does not cause the photon o take a denie route a imzero if 12 nanoseconds laer thePockels-cel switc is turned on, andit does not cause the photon to takeboth roues, in waveli fashion, ifthe swich is o

A more naural inerpration istha the objctive sae of the photon

in the interferomee eaves manypropeties indenit If the quanumstate givs a complt account of the

52

photon, then ha concusion is notsurprising, since in every quantumstate here are properties tha areindenite. Bu the conclusion doesraise a rther question How andwhen does an indeinite proprty become deinite? Wheeler's anser isha "no elemenary quantum phenomenon is a phenomenon until it isa registrd phenomenon In otherwords, the transiion from indeiniteness to deinieness is not completeunti an "irreversibe act of amplication occurs, such as the blackeningof a grain of phoographic emulsionStudents of h foundations of quanum mechanics disagree about theadequacy of Whelers answer, however The next experiment showswhy the question is still open.

In 1935 Erwin Schrdinger proposeda famous thought experiment Aphoton impinges on a half-siveredmirror The photon has a probability of on-haf of passing throughthe mirror and a probability of onehalf of bing releced If the photonpasses though the mirror, i is deteced and the decion acuates a

device that braks a botte of cyanide, which in un kils a ca in abox I cannot b detemined wheh-

er the cat is dead or alive until thebox is opened

There would be nothing paradoxical in this state of afairs if thepassage of the photon through themirror were objectively denitebut merely unknown prior to observation The passage of the photonis, however, objectively indenite.Hence the breaing of the bottle is

objecively indeinit, and so is thealiveness of the cat In oher wordsth cat is suspnded between lif anddeath until it is obsrved The conclusion is paradoxical, but at astit concerns only the results of athought experiment.

It is now more dicult to dismiss the paradoxical nature of theconclusion, because something similar o Schrdingers thought xperiment has recently been achieved bya number of groups of investigatorsincluding Richard F Voss and Rich

ard A Webb of the IBM Thomas ]Watson Research enter in Yorktown Heights, Lawrence D Jackel ofthe AT&T Bel Laboratories, MichaelH Devor of Berkeley and DanielB Schwartz of the State Universityof New York at Stony Brook Theirwork has relied to a certain extenon caculaions that were done byAnthony]. Leggett of he Universityof Illinois at Urbana-hampaign andSudip hakravarty at Stony Brook,among other investigators.

The experimental apparaus consists of an almost closed superconducting ring A thin slice of insulatingmaterial (called a Josephson junction) interrupts the ring, but an elctric curren can circulate around thering by "tunneling hrough the insuator. The curren generats a magnetic eld

The quantity that is of interstin the system is the magnetic uxhrough th ring, which (when theed is uniform) is equal to the area ofhe ring mutiplied by the componentof the magnetic eld prpendiculao the plane of the ring If the ringwere uninterupted, th ux wouldbe trapped within the ring, but the insulator allows he lux to slip fromone value to another With modernmagnetometers the lux can be measured with fantasic accuracy Thefact tha he lux arises om themotion of normous numbers ofelectrons (on the order of 10) inthe superconducing ring justiiesspeaking of he lux as a macroscopic quantity There is now good evi

dence tha sates of the superconducing ing can be prepared inwhich the ux does not have a def-


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ite valea qantmmechanicalfeatre that had previosly been established only for observables of microscopic systems.

To nderstand how this indeiniteness is demonstrated experimentally it is necessary to know that foreach vale of the lx the ring has acertain potential energy Ordinarilyone wold not expect that the lx

throgh the ring cold change spontaneosly from one vale to anotherbecase a potential-energy barrierseparates adjacent vales of the xfrom each other. lassical physicsforbids the transition between twosch vales of the x nless someexternal sorce of energy typically thermal is spplied to traversethe barrier between them. In qantm mechanics on the other handthe barrier can be tnneled throghwithot any external sorce of energy The grops of investigators

mentioned above have shown thatthe lx does change between twovales and that the change cannotbe entirely acconted for thermally;the observed tnneling mst be atleast partially qantm mechanicalparticlarly at very low temperatres. Bt qantm-mechanical tnneling rests essentially on the indeiniteness of the lx which ths cannot be localized deinitely at or closeto one vale or another.

The experimental demonstrationof qantm indeiniteness in a mac

roscopic variable does not ipso factocontradict the statement by Wheelerqoted above bt it does show thatampliication from a microscopic to amacroscopic level does not in itselfexorcise qantm-mechanical indeiniteness. The emphasis in Wheeler'sstatement abot an "irreversible actof ampliication mst be placed onthe word "irreversible The conditions for the occrrence of an irreversible process are far from settledin contemporary theoretical physicsSome stdents of the sbject (inclding me) believe new physical principles mst be discovered before wecan nderstand the pecliar kind ofirreversibility that occrs when anindeinite observable becomes deinite in the corse of a measrement.

The strangeness of the qantmworld contines to be explored.Still other experiments have recently been performed or are now nder way; two classes of these experiments shold be mentioned hereeven thogh there is no room to discss them in detail In the netroninterferometer experiments of Hel-

mt Rach and Anton Zeilinger ofthe Atomic Institte of the AstrianUniversities Samel A. Werner ofthe University of Missori at olmbia and liord Shll of the Massachsetts Institte of Technologyand their associates the wave fnction of a netron is split b a sheetof crystal and recombined by one ortwo other sheets The interference

eects exhibited in the recombination demonstrate a nmber of remarkable properties inclding theindeiniteness of the netrons rotethrogh the interferometer

Finally R. hambers of the University of Bristol M611enstedt ofthe University of Tbingen and AkiraTonomra of Hitachi Ltd have conirmed by electron interferometrythe remarkable Aharonov-Bohm effect in which an electron "feels thepresence of a magnetic ield that is ina region where there is zero probabil

ity of nding the electron This is astriking demonstration of a kind ofnonlocality dierent om althoghremotely related to the nonlocalityexhibited by correlated photon pairsA thorogh nderstanding of the relation between the two kinds of nonlocality as well as the many otherperplexing isses raised by experiments probing the natre of theqantm world awaits rther work

MACROSCOPIC SYSTEM can under some

circumstances est n a state in which

a macroscopic varabe has an ndenite

vaue; indeniteness s not imied to mi

croscopic systems, such as the photon

The system shon here is a supercon

ducting ring that does not qute bend

back on tself A thn sce of nsulatng

matera separates the two ends of the

ring from each other and an eectric current is made to crcuate arond the ring

by "tunneling through the insulator The

current generates a magnetic ed If the

rng wer contnuous, the magnetic fthrough the rng (the area of the ring mul

tipd by the component of the magnet

ic eld perpendcuar to the plane of the

ring) woud be trapped at ed value,

but the nsulator aows the f to slipfrom one vaue to another Surprsngly,

the f does not have a denite vaue

INDEFINITENESS in the system sho at the top of the page s depcted schematicay

Each value of the f through the superconducting ring has a certan potenta energyassociated with it Ordnary one woud not expect that the f through the rng coudspontaneousy change from one vaue to another because a potenta-energy barrer

separates the adjacent vaues of the f from each other The barrers can be thoughtof as hills, and the state the system is n can be represented as a ba resding a vaey

among the hs Accordng to cassica physcs, a transton between two vaues sepa

rated by a barrier requres outside energy (to push the ba over the hl) Quantum mechancay, however, the barrer can be tunneed through wthout any externa source

of eer Tunneing s essentaly a manfestation of the ndeniteness of the f

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