Parallel Worlds (22 page)

While doing
research in cosmology, he became interested in "cosmic strings," a
relic of the big bang that is predicted by many theories. Cosmic strings may
have a width thinner than an atomic nucleus, but their mass may be stellar and
they may extend for millions of light-years in space. Gott first found a solution
to Einstein's equations which allowed for cosmic strings. But then he noticed
something unusual about these cosmic strings. If you take two cosmic strings
and send them toward each other, then, just before they collide, it is possible
to use this as a time machine. First, he found that if you made the round-trip
around the colliding cosmic strings, space was contracted, giving it strange
properties. We know that if we move around a table, for example, and return to
where we started, we have traveled 360 degrees. But when a rocket travels
around the two cosmic strings as they pass each other, it actually travels
through less than 360 degrees, because space has shrunk. (This has the topology
of a cone. If we move completely around a cone, we also find that we travel
less than 360 degrees.) Thus, by going rapidly around both strings, you could
actually exceed the speed of light (as seen by a distant observer) since the
total distance was less than expected. This does not violate special
relativity, however, because in your own frame of reference your rocket never
exceeds light speed.

But this also
means that if you travel around the colliding cosmic strings, you can take a
trip to the past. Gott recalls, "When I found this solution, I was quite
excited. The solution used only positive- density matter, moving at speeds
slower than the speed of light. By contrast, wormhole solutions require more
exotic negative-energy- density material (stuff that weighs less than
nothing)."

But the energy
necessary for a time machine is enormous. "To allow time travel to the
past, cosmic strings with a mass-per-unit length of about 10 million billion
tons per centimeter must each move in opposite directions at speeds of at least
99.999999996 percent of the speed of light. We have observed high-energy
protons in the universe moving at least this fast, so such speeds are
possible," he observes.

Some critics
have pointed out that cosmic strings are rare, if they exist at all, and
colliding cosmic strings are even rarer. So Gott proposed the following. An
advanced civilization may find a single cosmic string in outer space. Using
gigantic spaceships and huge tools, they might reshape the string into a
rectangular loop that is slightly bent (resembling the shape of a reclining chair).
The loop, he hypothesized, might collapse under its own gravity, so that two
straight pieces of the cosmic string might fly past each other near the speed
of light, briefly creating a time machine. Nevertheless, Gott admits, "A
collapsing loop of string large enough to allow you to circle it once and go
back in time a year would have to be more than half the mass-energy of an
entire galaxy."

TIME PARADOXES

Traditionally,
another reason physicists dismissed the idea of time travel was because of time
paradoxes. For example, if you go back in time and kill your parents before you
are born, then your birth is impossible. Hence you could never go back in time
to kill your parents to begin with. This is important, because science is based
on logically consistent ideas; a genuine time paradox would be enough to completely
rule out time travel.

These time
paradoxes can be grouped into several categories:

Grandfather paradox. In this paradox, you alter the past in a
way that makes the present impossible. For example, by going back into the
distant past to meet the dinosaurs, you accidentally step on a small, furry
mammal that is the original ancestor of humanity. By destroying your ancestor,
you cannot logically exist.

Information paradox. In this paradox, information comes from
the future, which means that it may have no origin. For example, let's say a
scientist creates a time machine and then goes back in time to give the secret
of time travel to himself as a youth. The secret of time travel would have no
origin, since the time machine the youthful scientist possesses was not created
by him but was handed to him by his older self.

Bilker's paradox. In this kind of paradox, a person knows
what the future will be and does something that makes the future impossible.
For example, you make a time machine to take you to the future, and you see
that you are destined to marry a woman named Jane. However, on a lark, you
decide to marry Helen instead, thereby making your own future impossible.

The sexual paradox. In this kind of paradox, you father
yourself, which is a biological impossibility. In a tale written by the British
philosopher Jonathan Harrison, the hero of the story not only fathers himself,
but he also cannibalizes himself. In Robert Heinlein's classic tale "All
You Zombies," the hero is simultaneously his mother, father, daughter, and
son—that is, a family tree unto himself. (See the notes for details. Unraveling
the sexual paradox is actually rather delicate, requiring knowledge of both
time travel and the mechanics of DNA.)

In
The End of Eternity,
Isaac Asimov envisions a
"time police" that is responsible for preventing these paradoxes. The
Terminator
movies hinge on an information paradox—a microchip recovered
from a robot from the future is studied by scientists, who then create a race
of robots that become conscious and take over the world. In other words, the
design for these super robots was never created by an inventor; it simply came
from a piece of debris left over from one of the robots of the future. In the
movie
Back
to
the Future,
Michael J. Fox struggles to avoid a grandfather paradox when
he goes back in time and meets his mother as a teenager, who promptly falls in
love with him. But if she spurns the advances of Fox's future father, then his
very existence is threatened.

Scriptwriters
willingly violate the laws of physics in making Hollywood blockbusters. But in
the physics community, such paradoxes are taken very seriously. Any solution
to these paradoxes must be compatible with relativity and the quantum theory.
For example, to be compatible with relativity, the river of time simply cannot
end. You cannot dam the river of time. Time, in general relativity, is represented
by a smooth, continuous surface and cannot be torn or ripped. It may change
topology, but it cannot simply stop. This means that if you kill your parents
before you are born, you cannot simply disappear. This would violate the laws
of physics.

Currently,
physicists are congregating around two possible solutions to these time
paradoxes. First, Russian cosmologist Igor Novikov believes that we are forced
to act in a way so that no paradoxes occur. His approach is called the
self-consistency school. If the river of time smoothly bends back on itself and
creates a whirlpool, he suggests that an "invisible hand" of some
sort would intervene if we were to jump back into the past and were about to
create a time paradox. But Novikov's approach presents problems with free will.
If we go back in time and meet our parents before we are born, we might think
that we have free will in our actions; Novikov believes that an undiscovered
law of physics prevents any action that will change the future (such as killing
your parents or preventing your birth). He notes, "We cannot send a time
traveler back to the Garden of Eden to ask Eve not to pick the apple from the
tree."

What is this
mysterious force that prevents us from altering the past and creating a
paradox? "Such a constraint on our free will is unusual and mysterious but
not completely without parallel," he writes. "For example, it can be
my will to walk on the ceiling without the aid of any special equipment. The
law of gravity prevents me from doing this; I will fall down if I try, so my
free will is restricted."

But time
paradoxes can occur when inanimate matter (with no free will at all) is cast
into the past. Let's suppose that just before the historic battle between
Alexander the Great and Darius III of Persia in 330
b.c.,
you send
machine guns back into time, giving instructions on how to use them. We would
potentially change all subsequent European history (and might find ourselves
speaking a version of the Persian language rather than a European language).

In fact, even
the tiniest disturbance into the past may cause unexpected paradoxes in the
present. Chaos theory, for example, uses the metaphor of the "butterfly
effect." At critical times in the formation of Earth's weather, even the
fluttering of the wings of a butterfly sends ripples that can tip the balance
of forces and set off a powerful storm. Even the smallest inanimate objects
sent back into the past will inevitably change the past in unpredictable ways,
resulting in a time paradox.

A second way to
resolve the time paradox is if the river of time smoothly forks into two
rivers, or branches, forming two distinct universes. In other words, if you
were to go back in time and shoot your parents before you were born, you would
have killed people who are genetically the same as your parents in an alternate
universe, one in which you will never be born. But your parents in your
original universe will be unaffected.

This second
hypothesis is called the "many worlds theory"—the idea that all
possible quantum worlds might exist. This eliminates the infinite divergences
found by Hawking, since radiation does not repeatedly go through the wormhole
as in Misner space. It only goes through once. Each time it passes through the
wormhole, it enters a new universe. And this paradox goes to perhaps the
deepest question in the quantum theory: how can a cat be dead and alive at the
same time?

To answer this
question, physicists have been forced to entertain two outrageous solutions:
either there is a cosmic consciousness that watches over us all, or else there
are an infinite number of quantum universes.

I think I can safely say that nobody understands quantum
mechanics.

—Richard Feynman

Anyone who is not shocked by the quantum theory does not
understand it.

—Niels Bohr

The Infinite Improbability Drive is a wonderful new method of
crossing vast interstellar distances in a mere nothingth of a second, without
all that tedious mucking about in hyperspace.

—Douglas Adams

In the
Hitchhiker's Guide
to
the Galaxy,
the
bestselling, irreverent, wacky science fiction novel by Douglas Adams, the hero
stumbles upon a most ingenious method of traveling to the stars. Instead of
using wormholes, hyperdrives, or dimensional portals to travel between
galaxies, he conceives of harnessing the uncertainty principle to dart across
the vastness of intergalactic space. If we can somehow control the probability
of certain improbable events, then anything, including faster-than-light
travel, and even time travel, is possible. Reaching the distant stars in
seconds is highly unlikely, but when one can control quantum probabilities at
will, then even the impossible may become commonplace.

The quantum
theory is based on the idea that there is a probability that all possible
events, no matter how fantastic or silly, might occur. This, in turn, lies at
the heart of the inflationary universe theory—when the original big bang took
place, there was a quantum transition to a new state in which the universe
suddenly inflated by an enormous amount. Our entire universe, it appears, may
have sprung out of a highly unlikely quantum leap. Although Adams wrote in
jest, we physicists realize that if we could somehow control these
probabilities, one could perform feats that would be indistinguishable from
magic. But for the present time, altering the probabilities of events is far
beyond our technology.

I sometimes ask
our Ph.D. students at the university simpler questions, such as, calculate the
probability that they will suddenly dissolve and rematerialize on the other
side of a brick wall. According to the quantum theory, there is a small but
calculable probability that this could take place. Or, for that matter, that we
will dissolve in our living room and wind up on Mars. According to the quantum
theory, one could in principle suddenly rematerialize on the red planet. Of
course, the probability is so small that we would have to wait longer than the
lifetime of the universe. As a result, in our everyday life, we can dismiss
such improbable events. But at the subatomic level, such probabilities are
crucial for the functioning of electronics, computers, and lasers.

Electrons, in
fact, regularly dematerialize and find themselves rematerialized on the other
side of walls inside the components of your PC and CD. Modern civilization
would collapse, in fact, if electrons were not allowed to be in two places at
the same time. (The molecules of our body would also collapse without this
bizarre principle. Imagine two solar systems colliding in space, obeying
Newton's laws of gravity. The colliding solar systems would collapse into a
chaotic jumble of planets and asteroids. Similarly, if the atoms obeyed
Newton's laws, they would disintegrate whenever they bumped into another atom.
What keeps two atoms locked in a stable molecule is the fact that electrons can
simultaneously be in so many places at the same time that they form an electron
"cloud" which binds the atoms together. Thus, the reason why
molecules are stable and the universe does not disintegrate is that electrons
can be many places at the same time.)