Radioactivity and Man

8/3/2019 Radioactivity and Man

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Radioactivity and Man-Made Radioactive Decay

Radioactivity is the spontaneous decay of an unstable nucleus. An unstable nucleus may break

apart into two or more other particles with the release of some energy. This breaking apart can

occur in a number of ways, depending on the particular atom thats decaying.

You can often predict one of the particles of a radioactive decay by knowing the other particle.

Doing so involves something called balancing thenuclearreaction. (A nuclear reaction is any

reaction involving a change in nuclear structure.)

Balancing a nuclear reaction is really a fairly simple process. Before reading an explanation of the

process, you should know how to represent a reaction:

ReactantsProducts

Reactants are the substances you start with, and products are the new substances being formed.

The arrow, called a reaction arrow, indicates that a reaction has taken place.

For a nuclear reaction to be balanced, the sum of all the atomicnumberson the left-hand side of

the reaction arrow must equal the sum of all the atomic numbers on the right-hand side of the

arrow. The same is true for the sums of the massnumbers.

Heres an example: Suppose youre a scientist performing a nuclear reaction by bombarding a

particularisotopeof chlorine (Cl-35) with a neutron. You observe that an isotope of hydrogen, H-1,

is created along with another isotope, and you want to figure out what the other isotope is. The

equation for this example is:

Now to figure out the unknown isotope (represented by Pr), you need to balance the equation. The

sum of the atomic numbers on the left is 17 (17 + 0), so you want the sum of the atomic numbers

on the right to equal 17 too.

Right now, youve got an atomic number of 1 on the right; 17 1 is 16, so thats the atomic number

of the unknown isotope. This atomic number identifies the element as Sulfur (S).

Now look at the mass numbers in the equation. The sum of the mass numbers on the left is 36 (35

+ 1), and you want the sum of the mass numbers on the right to equal 36, too.

Right now, youve got a mass number of 1 on the right; 36 1 is 35, so thats the mass number of

the unknown isotope. Now you know that the unknown isotope is a Sulfur isotope (S-35). And

heres what the balanced nuclear equation looks like:

This equation represents a nucleartransmutation, the conversion of one element into another.

Nuclear transmutation is a process human beings control. S-35 is an isotope of sulfur that doesnt

exist in nature. Its a man-made isotope.

Alchemists, those ancient predecessors of chemists, dreamed of converting one element into

another (usually lead into gold), but they were never able to master the process. Chemists are now

able, sometimes, to convert one element into another.
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The Process of Natural Radioactive Decay

Certain naturally occurringradioactiveisotopes are unstable: Their nucleus breaks apart,

undergoing nuclear decay. Sometimes the product of that nuclear decay is unstable itself and

undergoes nuclear decay, too.

For example, when U-238 (one of the radioactive isotopes of uranium) initially decays, it produces

Th-234, which decays to Pa-234. The decay continues until, finally, after a total of 14 steps, Pb-

206 is produced. Pb-206 is stable, and the decay sequence, or series, stops.

The nucleus has positively charged protons shoved together in an extremely small volume of

space. All those protons are repelling each other. The forces that normally hold the nucleus

together sometimes cant do the job, and so the nucleus breaks apart, undergoing nuclear decay.

All elements with 84 or more protons are unstable; they eventually undergo decay. Other isotopes

with fewer protons in their nucleus are also radioactive. The radioactivity corresponds to theneutron/proton ratio in the atom:

If the neutron/proton ratio is too high (there are too many neutrons or too few protons), the

isotope is said to be neutron rich and is, therefore, unstable.

If the neutron/proton ratio is too low (there are too few neutrons or too many protons), the

isotope is unstable.

The neutron/proton ratio for a certain element must fall within a certain range for the element to be

stable. Thats why some isotopes of an element are stable and others are radioactive.

There are three primary ways that naturally occurring radioactive isotopes decay:

Alpha particle emission

Beta particle emission

Gamma radiation emission

In addition, there are a couple of less common types of radioactive decay:

Positron emission

Electron capture

Alpha emission

An alpha particle is defined as a positively charged particle of a helium nuclei. An alpha particle is

composed of two protons and two neutrons, so it can be represented as a Helium-4 atom. As an

alpha particle breaks away from the nucleus of a radioactive atom, it has no electrons, so it has a

+2 charge. Therefore, its a positively charged particle of a helium nuclei.

But electrons are basically free easy to lose and easy to gain. So normally, an alpha particle is

shown with no charge because it very rapidly picks up two electrons and becomes a neutral helium

atom instead of an ion.

Large, heavy elements, such as uranium and thorium, tend to undergo alpha emission. This decay

mode relieves the nucleus of two units of positive charge (two protons) and four units of mass (two

protons + two neutrons). Each time an alpha particle is emitted, four units of mass are lost.
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Radon-222 (Rn-222) is another alpha particle emitter, as shown in the following equation:

Here, Radon-222 undergoes nuclear decay with the release of an alpha particle. The other

remaining isotope must have a massnumberof 218 (222 4) and an atomicnumberof 84 (86

2), which identifies the element as Polonium (Po).

Beta emission

A beta particle is essentially an electron thats emitted from the nucleus. Iodine-131 (I-131), which

is used in the detection and treatment of thyroid cancer, is a beta particle emitter:

Here, the Iodine-131 gives off a beta particle (an electron), leaving an isotope with a mass number

of 131 (131 0) and an atomic number of 54 (53 (-1)). An atomic number of 54 identifies the

element as Xenon (Xe).

Notice that the mass number doesnt change in going from I-131 to Xe-131, but the atomic number

increases by one. In the iodine nucleus, a neutron was converted (decayed) into a proton and an

electron, and the electron was emitted from the nucleus as a beta particle.

Isotopes with a high neutron/proton ratio often undergo beta emission, because this decay mode

allows the number of neutrons to be decreased by one and the number of protons to be increased

by one, thus lowering the neutron/proton ratio.

Gamma emission

Alpha and beta particles have the characteristics of matter: They have definite masses, occupyspace, and so on. However, because there is no mass change associated with gamma emission,

you could refer to gamma emission as gamma radiation emission.

Gamma radiation is similar to x-rays high energy, short wavelength radiation. Gamma radiation

commonly accompanies both alpha and beta emission, but its usually not shown in a

balancednuclearreaction.

Some isotopes, such as Cobalt-60 (Co-60), give off large amounts of gamma radiation. Co-60 is

used in the radiation treatment of cancer. The medical personnel focus gamma rays on the tumor,

thus destroying it.

Positron emission

Although positron emission doesnt occur with naturally occurring radioactive isotopes, it does

occur naturally in a few man-made ones. Apositron is essentially an electron that has a positive

charge instead of a negative charge.

A positron is formed when a proton in the nucleus decays into a neutron and a positively charged

electron. The positron is then emitted from the nucleus. This process occurs in a few isotopes,

such as Potassium-40 (K-40), as shown in the following equation:
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The K-40 emits the positron, leaving an element with a mass number of 40 (40 0) and an atomic

number of 18 (19 1). An isotope of argon (Ar), Ar-40, has been formed.

Electron capture

Electron capture is a rare type of nuclear decay in which an electron from the innermost energy

level is captured by the nucleus. This electron combines with a proton to form a neutron. The

atomic number decreases by one, but the mass number stays the same.

The following equation shows the electron capture of Polonium-204 (Po-204):

The electron combines with a proton in the polonium nucleus, creating an isotope of bismuth (Bi-

204). The capture of the 1s electron leaves a vacancy in the 1s orbitals. Electrons drop down to fill

the vacancy, releasing energy in the X-ray portion of the electromagnetic spectrum.

Introduction to Radioactivity

Radioactivity, the process by which the nuclei (cores) of unstable atoms of an element emit

radiation (particles of matter and rays of energy), and in so doing become atoms of other

elements. It is a property of certain types of matter. Substances in which radioactivity takes

place are called radioactive. The particles and energy given off by these substances are

formed of nuclear radiation. In making the emissions, the nucleus of a radioactive atom is said

to decay. The radiation of a radioactive substance is harmful to life. Properly used, however,

this radiation is extremely useful in science, medicine, agriculture, and industry.

Some chemical elements, including uranium, radium, thorium, and polonium, are naturally

radioactive. Any element that is not naturally radioactive can be made radioactive in a nuclear

reactor or particle accelerator.

The principal particles emitted by radioactive substances are alpha particles and beta

particles. Most of the naturally radioactive elements emit only one of these two types of

particles. The emission of either alpha or beta particles is often accompanied by the emission

of gamma rays. Radium and certain other elements produce all three types of radiation.

The discovery of radioactivity late in the 19th century helped lay the foundations of modern

physical science. It put an end to the theory that atoms are indivisible and indestructible, and

led to great advances in the knowledge of the structure of matter, and of the relationship

between matter and energy. From the research inspired by the discovery of radioactivity have

come nuclear weapons on the one hand, and, on the other, nuclear power and the use of

radioactive substances for the benefit of mankind.

Causes of Radioactivity

To understand how radioactivity occurs, it is necessary to know the structure of an atom and

how it can change. An atom is built up of electronstiny negatively charged particlesthat

revolve around a heavy, positively charged nucleus. The nucleus of an atom is made up of

two basic kinds of particlesprotons, which carry a positive electrical charge; and neutrons,


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which carry no charge. Protons and neutrons are made up of even smaller particles called

quarks. A nucleus is said to be stable when the forces of attraction and repulsion acting upon

its particles are in balance. When these forces are out of balanceas occurs when there are

too many or too few of one kind of particlethe nucleus is unstable. An unstable nucleus

becomes stable by undergoing radioactive decay. Particles that have opposite electriccharges attract each other, and those that have the same electric charge repel (push away)

each other. That is why the negatively charged electrons are attracted to the positively

charged nucleus and stay within it, and the positively charged protons repel one another. The

protons and neutrons stay in the nucleus only because they are bound together by an

extremely powerful force, called the strong nuclear force or the strong interaction.

Nuclei are most stable when they contain even numbers of both protons and neutrons. Of

some 1,000 different kinds of nuclei with odd numbers of both protons and neutrons, only six

are not radioactive. Protons and neutrons make up the nucleus of every element except that

of the most common form of hydrogen. Only one proton makes up the nucleus of a hydrogenatom.

Protons and neutrons are arranged in layers, or shells, within the nucleus. Each shell can

hold only a certain number of particles. A nucleus is stable when all its shells are filled with the

proper number of particles. If there are too few or too many particles in one or more shells,

the nucleus tends to be unstable and therefore radioactive.

The number of protons and neutrons changes in an atoms nucleus when an atom absorbs

radiation (elementary particles or energy) or emits it. When the number of protons changes,

an atom of a different element is produced. A radioactive atom gives off radiation

automatically to become more stable. This process continues until it becomes stable and

nonradioactive. During this process, a radioactive atom changes into different elements or

different forms of the same element.

There is a relationship also between radioactivity and the atomic number of an element.

(Atomic number is the number of protons in the nucleus.) Every element with an atomic

number greater than that of lead (82) is radioactive. The nuclei of some of these elements can

decay by splitting in two. This process is called spontaneous fission.

Radioactive Decay

The process of giving off atomic particles by radioactive atoms is called radioactive decay. Inradioactive decay one element is changed into another. Its rate depends on the form of an

element and on the element. A group of radioactive elements formed by the decay of one

element into another is called a radioactive series. The rate at which elements decay is

expressed in terms of half-lives.

Transmutations

Radioactivity produces transmutations in elements, that is, it changes one element into

another. In the Periodic Table, elements are classified by their atomic numbers. When an

atom emits an alpha or beta particle, the atom becomes a different element, thereby changing

its place in the Periodic Table.


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An atom that emits an alpha particle loses two protons and two neutrons. The loss of the two

protons reduces its atomic number by two. This change is shown by the diagram Radioactive

Decay. For example, the first element in the diagram is uranium, which has an atomic number

of 92. When it emits an alpha particle, it becomes thorium, which has an atomic number of 90.

There are three main types of beta decay. In the most common type, a neutron in the nucleus

of an atom changes into a proton. A negative beta particle (an electron) and a particle called

an antineutrino are formed in the process and are emitted from the nucleus. This type of beta

decay results in the atomic number of the nucleus increasing by one. Each case of beta

decay shown in the diagram is of this type. For example, thorium (atomic number 90)

becomes protactinium (atomic number 91). In the other two types of beta decay, the atomic

number decreases by one. In both, a proton changes into a neutron and a particle called a

neutrino is formed and emitted from the nucleus. In one, a positive beta particle (a positron) is

also formed and emitted from the nucleus; in the other, an inner electron of the atom is

captured by the nucleus and destroyed.

The weight of both neutrons and protons is roughly equal to one unit of atomic weight. In

comparison, the weight of electrons, positrons, neutrinos, and antineutrinos is negligible. Thus

when an atom emits an alpha particle, its atomic weight decreases by four units, but when an

atom emits a beta particle, its atomic weight remains the same. For example, when thorium

230 decays, it emits an alpha particle and becomes radium 226. On the other hand, when

lead 214 decays, its atomic weight remains at 214, although it becomes an atom of a different

element (bismuth).

Isotopes are atoms with the same atomic number but differing atomic weights because they

have unequal numbers of neutrons. Isotopes of the same element have identical chemical

properties but slightly different physical ones. Many of the isotopes of naturally occurring

elements are radioactive. The decay properties of radioactive isotopes of the same element

usually differ greatly.

The Radioactive Series

As certain radioactive isotopes decay they produce other isotopes of other elements that are

also radioactive. These daughter isotopes produce still other radioactive daughters, and so on

until a stable isotope of some element is formed. Radioactive isotopes of this kind form

radioactive series.

Three radioactive series occur in nature: the thorium series, the uranium series, and the

actinium series. (The diagram Radioactive Decay shows the uranium series.) The thorium

series begins with thorium 232, the uranium series with uranium 238, and the actinium series

with uranium 235. Each series ends in a stable lead isotope. The neptunium series is

produced artificially in nuclear reactors. It begins with neptunium 237 and ends with bismuth

209.

Half-lives

Every radioactive isotope decays at its own fixed, unchangeable rate. As time passes, more

and more of the isotope changes into an isotope of another element. Eventually half the


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parent isotope changes. In an equal length of time, half of what now remains of the parent

isotope will change, and so on. The time it takes for half of a radioactive isotope to change

into another isotope is called the half-life of the parent isotope.

Uranium 238 has a half-life of 4.5 billion years. This means that in 4.5 billion years, half of a

given amount of uranium 238 will have changed to thorium 234. In 24.1 days, half of a sample

of thorium 234 will have become protactinium 234. The half-lives of other radioactive

substances range from a very small fraction of a second to more than 100,000,000,000 years.

Radioisotopes

A radioactive isotope is called a radioisotope. Scientists produce more than 900 different

kinds of artificial radioisotopes for use in science, medicine, agriculture, and industry. Some

are valuable because of the kind of radiation they emit. Cobalt 60, for example, emits high-

energy gamma rays that can be used for the same purposes as x-rays. Other kinds of

radioisotopes are valuable because their radioactivity makes it possible to trace them with

radiation detectors when, for example, they pass through a living body. Radioisotopes used in

this way are called tracers, or tagged elements.

An artificial radioisotope is produced by making a stable isotope radioactive. Some artificial

radioisotopes are produced by smashing atoms in particle accelerators, such as the cyclotron.

Most are produced by bombarding atoms with particles and rays emitted by radioactive

elements in a nuclear reactor.

A number of radioisotopes are obtained by fission, the splitting of an atom into two

approximately equal parts. Others are produced by spallation, which occurs when a

bombarded atom loses a number of particles but the nucleus does not split. Still otherradioisotopes are formed when the nuclei of atoms capture particles with which they are

bombarded. The captured particles unbalance the nuclei and make them radioactive.

Studying Particles and Rays

The particles and rays given off in radioactivity are much too small to be seen. They can be

studied only through their properties. One of these properties is that of producing

fluorescence (glowing) in certain substances. Another is the property of producing

ionizationthe stripping of electrons from molecules or atoms to give them an electric charge.

A widely used instrument for detecting and measuring radioactivity is the Geiger-Mller

counter, which detects the particles emitted by a radioactive substance by recording the

ionization they produce in passing through a gas-filled chamber. Similar instruments are

designed to record the ionization the particles produce in passing through a solid material.

The material is connected to an electric circuit and acts as an electrical insulator until some of

its atoms are ionized. It then briefly generates an electrical signal that is recorded

electronically. In semiconductor radiation counters, the material used is a semiconductor,

usually silicon or germanium. In crystal counters, the material is a crystal with high electrical

resistance.


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Scintillation counters measure radioactivity through the flashes produced by a fluorescent

substance. In a common type of scintillation counter, the energy of the light flashes is

converted into electrical energy, which is then amplified to operate the counter.

The bubble chamber is a clean, smooth-walled container in which a liquid can be heated

above its boiling point without boiling. When an alpha or beta particle passes through a

superheated liquid in a bubble chamber, the particle ionizes molecules of the liquid. The

ionization energy triggers boiling along the path of the particle, producing a trail of tiny

bubbles.

Other detectors include photographic film or plates and cloud chambers. One photographic

technique is called autoradiography, or radioautography. It consists of placing photographic

film against an object containing a radioactive substance, usually a tracer. The particles

emitted by the substance expose a pattern on the film. The pattern reveals the distribution of

the radioactive substance in the object. A cloud chamber, like a bubble chamber, makes

visible the paths followed by alpha and beta particles.

The unit of radioactivity is the curie. One curie is equal to 37,000,000,000 emissions a

second.

Radioactivity Discoveries

Radioactivity was discovered in 1896 by Antoine Henri Becquerel, a French physicist. His

discovery led to the important research of Pierre and Marie Curie and others. Radium and

other radioactive elements were discovered, and much was learned about the nature and

effects of radioactivity. About 1900 Ernest Rutherford discovered alpha and beta particles in

radioactive emissions.

In 1934, Frdric and Irne Joliot-Curie discovered that when aluminum foil was bombarded with

alpha particles, the foil continued to emit radiation after the bombardment was stopped. The

foil was the first known artificial radioisotope. In the late 1930s, as the result of the work of a

number of scientists, it was discovered that uranium fissioned (split into two approximately

equal parts) when bombarded with neutrons. This discovery led to the atomic bomb and, later,

to nuclear power.

Introduction to How Time Travel Will Work

There may be no other concept that captures the

imagination more than the idea oftime travel --

the ability to travel to any point in the past or

future. What could be cooler? You could jump into

your time machine to go back and see major

events in history and talk to the people who were

there! Who would you travel back to see? Julius

Caesar? Leonardo da Vinci?Elvis? You could go

back and meet yourself at an earlier age, go

forward and see how you look in the future... It's

Photographer:Nicemonkey| Agency:Dreamstime.com

The dream to travel through time

has existed for centuries.
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these possibilities that have made time travel the subject of so many science fiction books

and movies.

It turns out that, in some sense, we are all time travelers. As you sit at your desk, doing

nothing more than clicking yourmouse, time is traveling around you. The future is constantly

being transformed into the past with the present only lasting for a fleeting moment. Everythingthat you are doing right now is quickly moving into the past, which means we continue to

move through time.

Ideas of time travel have existed for centuries, but when Albert Einstein released his theory

ofspecial relativity, he laid the foundation for the theoretical possibility of time travel. As we all

know, no one has successfully demonstrated time travel, but no one has been able to rule it

out either.

In this edition ofHow Stuff Will Work, we will learn about the concept of time and the

different theories surrounding the viability of time travel.

Understanding Time

Back to the Future

Noel Vasquez/Getty Images

The DeLorean Time Machine

The spacetime continuum was

frequently mentioned in themovie "Back

to the Future." Doc Brown and Marty

travel from 1985 to 1955 in a DeLorean

that has been converted into a time

machine.

What Marty and the Doc soon realize is

that interactions with Marty's parents in

1955 threaten to unravel the fabric of

the spacetime continuum. Check out

more information aboutDeLorean DMC-

12

AstronomerCarl Sagan had it right when he said that time is "resistant to simple definition."

Lots of us think we know what time is, but it is hard to define. You can not literally see or touch

time, but you can see its effects. The evidence that we are moving through time is found in

everything -- our bodies age, buildings weather and crumble, trees grow. Most of us feel the
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pressure of time as we are pushed to meet deadlines and make appointments. Our lives are

often dictated by what time we need to be somewhere.

Ask most people to define time and they are likely to look at theirwatchor aclock. We see

time as the ticking of the hands on these devices. We know that there are 60 seconds in a

minute, 60 minutes in an hour, 24 hours in a day and 365 days in a year. These are the basicnumbers of time that we all learned in grade school.

Time is also defined as being the fourth dimension of our universe. The other three

dimensions are of space, including up-down, left-right and backward-forward. Time cannot

exist without space, and likewise, space cannot exist without time. This interconnected

relationship of time and space is called the spacetime continuum, which means that any

event that occurs in the universe has to involve both space and time.

Up Next

How Time

Works

How Special

Relativity Works

Science

Channel: Time

Travel Quiz

According to Einstein's theory ofspecial relativity, time slows as an object approaches

the speed of light. This leads many scientists to believe that traveling faster than the speed of

light could open up the possibility of time travel to the past as well as to the future. The

problem is that the speed of light is believed to be the highest speed at which something can

travel, so it is unlikely that we will be able to travel into the past. As an object nears the speedof light, its relativistic mass increases until, at the speed of light, it becomes infinite.

Accelerating an infinite mass any faster than that is impossible, or at least it seems to be right

now.

But time travel in the other direction is not as difficult, and the future may one day be a

possible destination...

Black Holes

While writers have produced some great ideas fortime machines over the years, a real-life

time machine has yet to be built. Most theories of time travel don't rely on machines at all.

Instead, time travel will likely be done by way of natural phenomena that will transport us

instantly from one point in time to another. These space phenomena, which we are not even

sure exist, include:

Rotating black holes

Wormholes

Cosmic strings

When stars that are more than four times the

mass of oursunreach the end of their life and

have burned up all of their fuel, they collapse

under the pressure of their own weight. This

Photo courtesy NASA
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implosion creates "black holes," which have gravitational fields so strong that

evenlight cannot escape. Anything that comes in contact with a black hole'sevent

horizon will be sucked in. The event horizon is the boundary of a black hole at which nothing

can escape.

You can think of the shape of a black hole as similar to an ice cream cone. It is large on topand tapers into a point, called a singularity. At the singularity, the laws of physics cease to

exist and all matter is crushed beyond recognition. This kind of non-rotating black hole is

called a Schwarzschild black hole, named after the German astronomer Karl

Schwarzschild.

Another type of black hole, called a Kerr hole, is also theoretically possible. Kerr holes are

rotating black holes that could be used as portals for time travel or travel to parallel

universes. In 1963, New Zealand mathematician Roy Kerr proposed the first realistic theory

for a rotating black hole. In his theory, dyingstars would collapse into a rotating ring of

neutrons that would produce sufficient centrifugal force to prevent the formation of a

singularity. Since the black hole would not have a singularity, Kerr believed it would be safe to

enter it without being crushed by the infinitegravitational forceat its center.

If Kerr holes do exist, it might be possible to pass through them and exit out of a "white" hole.

A white hole would have the reverse action of a black hole. So, instead of pulling everything

into its gravitational force, it would use some sort of exotic matter with negative energy to

push everything out and away from it. These white holes would be our way to enter other

times or other worlds.

Given the little we know about black holes, Kerr holes may possibly exist. However,

physicist Kip Thorne of the California Institute of Technology believes that the laws of physics

prevent such a formation. He says there is no such way to enter and exit a black hole, andthat anything attempting to enter a black hole will be sucked in and destroyed before it even

reaches the singularity.

We'll take a look at some otherspace phenomena in the following sections.

Wormholes
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Imagining space as a curved, two-dimensional plane, wormholes like this could be

formed by two masses applying enough force on spacetime to create a tunnel

connecting distant points in the universe.

Thorne believes there could be another type of tunnel-like structure existing in the universe

that could be used for a timetravel portal. Wormholes, also called Einstein-Rosen Bridges,

are considered to have the most potential for time travel if they do exist. Not only could they

allow us to travel through time, they could allow us to travel many light-yearsfrom Earth in

only a fraction of the amount of time that it would take us with conventional space travel

methods.Wormholes are considered possible based on Einstein's theory of relativity, which states that

any mass curves spacetime. To understand this curvature, think about two people holding a

bed sheet up and stretching that sheet tight. If one person were to place a baseball on the

bed sheet, the weight of the baseball would roll to the middle of the sheet and cause the

sheet to curve at that point. Now, if a marble were placed on the edge of the same bed sheet

it would travel toward the baseball because of the curve.

In this example, space is depicted as a two-dimensional plane rather than the four

dimensions that actually make up spacetime. Imagine that this sheet is folded over, leaving a

space between the top and bottom. Placing the baseball on the top side will cause a

curvature to form. If an equal mass were placed on the bottom part of the sheet at a point that

corresponds with the location of the baseball on the top, the second mass would eventually

meet with the baseball. This is similar to how wormholes might form.

In space, masses that place pressure on different parts of the universe could eventually come

together to form a tunnel -- this is a wormhole. We could then travel from Earth to another

galaxy and back relatively quickly (within a lifetime). For instance, let's picture a scenario in

which we would want to travel to Sirius, a star that's seen in the Canis Major constellation just

below Orion. Sirius is about 9 light-years fromEarth, which is about 54 trillion miles (90 trillion

km). Obviously, this distance would be far too great for space travelers to traverse and returnin time to tell us about what they saw there. So far, the farthest people have traveled into
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space is to the moon, which is only about 248,548 miles (about 400,000 km) away from Earth.

If we could find a wormhole that connected us to the space around Sirius, then we could cut

the time considerably by avoiding the trillions of miles that we would have to cross with

traditional space travel.

So how does all of this relate to time travel? We'll find out in the next section.

Cosmic Strings

Yet another theory for how we might travel back and forth through time uses the idea

ofcosmic strings, proposed by Princeton physicist J. Richard Gott in 1991. These are -- as

their name suggests -- string-like objects that some scientists believe were formed in the early

universe. These strings may line the entire length of the universe and are under immense

pressure -- millions upon millions of tons.

These cosmic strings, which are thinner than anatom, would generate an enormous amount

ofgravitational pull on any objects that pass near them. Objects attached to a cosmic string

could travel at incredible speeds, and because their gravitational force distorts spacetime,

they could be used fortime travel. By pulling two cosmic strings close together, or one string

close to a black hole, it might be possible to warp spacetime enough to create closed time-like

curves.

Aspacecraft could be turned into a time machine by using the gravity produced by the two

cosmic strings, or the string and black hole, to propel itself into the past. To do this, it would

loop around the cosmic strings. However, there is still much speculation as to whether these

strings exist, and if they do, in what form. Gott himself said that in order to travel back in time

even one year, it would take a loop of string that contained half the mass-energy of an entire

galaxy. And, as with any time machine, you couldn't go back farther than the point at which

the time machine was created.

Time Travel Physics

As we discussed earlier, the theory of relativity states that as the velocity of an object nears

the speed oflight, time slows down. Scientists have discovered that even at the speeds of

the space shuttle, astronauts can travel a few nanoseconds into the future. To understand

this, picture two people, person A and person B. Person A stays on Earth, while person Btakes off in a spacecraft. At takeoff, their watches are in perfect sync. The closer person B's

spacecraft travels to the speed of light, the slower time will pass for person B (relative to

person A). If person B travels for just a few hours at 50 percent the speed of light and returns

to Earth, it will be obvious to both people that person A has aged much faster than person B.

This difference in aging is because time passed much faster for person A than person B, who

was traveling closer to the speed of light. Many years might have passed for person A, while

person B experienced a time lapse of just a few hours. Find out more about this twin

paradox in How Special Relativity Works.
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Wormholes could allow you to travel into the past and the future.

Radioactivity and Man

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