Time travel has been a popular science-fiction
theme since H. G. Wells wrote his celebrated novel The Time Machine
in 1895. But can it really be done? Is it possible to build a machine
that would transport a human being into the past or future?
For decades, time travel lay beyond the fringe
of respectable science. In recent years, however, the topic has become
something of a cottage industry among theoretical physicists. The
motivation has been partly recreational--time travel is fun to think
about. But this research has a serious side, too. Understanding the
relation between cause and effect is a key part of attempts to construct
a unified theory of physics. If unrestricted time travel were possible,
even in principle, the nature of such a unified theory could be drastically
affected.
Our best understanding of time comes from
Einstein's theories of relativity. Prior to these theories, time was
widely regarded as absolute and universal, the same for everyone no
matter what their physical circumstances were. In his special theory
of relativity, Einstein proposed that the measured interval between
two events depends on how the observer is moving. Crucially, two observers
who move differently will experience different durations between the
same two events.
The effect is often described using the "twin
paradox." Suppose that Sally and Sam are twins. Sally boards a rocket
ship and travels at high speed to a nearby star, turns around and
flies back to Earth, while Sam stays at home. For Sally the duration
of the journey might be, say, one year, but when she returns and steps
out of the spaceship, she finds that 10 years have elapsed on Earth.
Her brother is now nine years older than she is. Sally and Sam are
no longer the same age, despite the fact that they were born on the
same day. This example illustrates a limited type of time travel.
In effect, Sally has leaped nine years into Earth's future.
Jet Lag
The effect, known as time dilation, occurs whenever two observers
move relative to each other. In daily life we don't notice weird time
warps, because the effect becomes dramatic only when the motion occurs
at close to the speed of light. Even at aircraft speeds, the time
dilation in a typical journey amounts to just a few nanoseconds--hardly
an adventure of Wellsian proportions. Nevertheless, atomic clocks
are accurate enough to record the shift and confirm that time really
is stretched by motion. So travel into the future is a proved fact,
even if it has so far been in rather unexciting amounts.
To observe really dramatic time warps, one
has to look beyond the realm of ordinary experience. Subatomic particles
can be propelled at nearly the speed of light in large accelerator
machines. Some of these particles, such as muons, have a built-in
clock because they decay with a definite half-life; in accordance
with Einstein's theory, fast-moving muons inside accelerators are
observed to decay in slow motion. Some cosmic rays also experience
spectacular time warps. These particles move so close to the speed
of light that, from their point of view, they cross the galaxy in
minutes, even though in Earth's frame of reference they seem to take
tens of thousands of years. If time dilation did not occur, those
particles would never make it here.
Speed is one way to jump ahead in time. Gravity
is another. In his general theory of relativity, Einstein predicted
that gravity slows time. Clocks run a bit faster in the attic than
in the basement, which is closer to the center of Earth and therefore
deeper down in a gravitational field. Similarly, clocks run faster
in space than on the ground. Once again the effect is minuscule, but
it has been directly measured using accurate clocks. Indeed, these
time-warping effects have to be taken into account in the Global Positioning
System. If they weren't, sailors, taxi drivers and cruise missiles
could find themselves many kilometers off course.
At the surface of a neutron star, gravity
is so strong that time is slowed by about 30 percent relative to Earth
time. Viewed from such a star, events here would resemble a fast-forwarded
video. A black hole represents the ultimate time warp; at the surface
of the hole, time stands still relative to Earth. This means that
if you fell into a black hole from nearby, in the brief interval it
took you to reach the surface, all of eternity would pass by in the
wider universe. The region within the black hole is therefore beyond
the end of time, as far as the outside universe is concerned. If an
astronaut could zoom very close to a black hole and return unscathed--admittedly
a fanciful, not to mention foolhardy, prospect--he could leap far
into the future.
My Head Is Spinning
So far I have discussed travel forward in time. What about going backward?
This is much more problematic. In 1948 Kurt Gödel of the Institute
for Advanced Study in Princeton, N.J., produced a solution of Einstein's
gravitational field equations that described a rotating universe.
In this universe, an astronaut could travel through space so as to
reach his own past. This comes about because of the way gravity affects
light. The rotation of the universe would drag light (and thus the
causal relations between objects) around with it, enabling a material
object to travel in a closed loop in space that is also a closed loop
in time, without at any stage exceeding the speed of light in the
immediate neighborhood of the particle. Gödel's solution was shrugged
aside as a mathematical curiosity--after all, observations show no
sign that the universe as a whole is spinning. His result served nonetheless
to demonstrate that going back in time was not forbidden by the theory
of relativity. Indeed, Einstein confessed that he was troubled by
the thought that his theory might permit travel into the past under
some circumstances.
Other scenarios have been found to permit
travel into the past. For example, in 1974 Frank J. Tipler of Tulane
University calculated that a massive, infinitely long cylinder spinning
on its axis at near the speed of light could let astronauts visit
their own past, again by dragging light around the cylinder into a
loop. In 1991 J. Richard Gott of Princeton University predicted that
cosmic strings--structures that cosmologists think were created in
the early stages of the big bang--could produce similar results. But
in the mid-1980s the most realistic scenario for a time machine emerged,
based on the concept of a wormhole.
In science fiction, wormholes are sometimes
called stargates; they offer a shortcut between two widely separated
points in space. Jump through a hypothetical wormhole, and you might
come out moments later on the other side of the galaxy. Wormholes
naturally fit into the general theory of relativity, whereby gravity
warps not only time but also space. The theory allows the analogue
of alternative road and tunnel routes connecting two points in space.
Mathematicians refer to such a space as multiply connected. Just as
a tunnel passing under a hill can be shorter than the surface street,
a wormhole may be shorter than the usual route through ordinary space.
The wormhole was used as a fictional device
by Carl Sagan in his 1985 novel Contact. Prompted by Sagan,
Kip S. Thorne and his co-workers at the California Institute of Technology
set out to find whether wormholes were consistent with known physics.
Their starting point was that a wormhole would resemble a black hole
in being an object with fearsome gravity. But unlike a black hole,
which offers a one-way journey to nowhere, a wormhole would have an
exit as well as an entrance.
In the Loop
For the wormhole to be traversable, it must contain what Thorne termed
exotic matter. In effect, this is something that will generate antigravity
to combat the natural tendency of a massive system to implode into
a black hole under its intense weight. Antigravity, or gravitational
repulsion, can be generated by negative energy or pressure. Negative-energy
states are known to exist in certain quantum systems, which suggests
that Thorne's exotic matter is not ruled out by the laws of physics,
although it is unclear whether enough antigravitating stuff can be
assembled to stabilize a wormhole [see "Negative Energy, Wormholes
and Warp Drive," by Lawrence H. Ford and Thomas A. Roman; Scientific
American, January 2000].
Soon Thorne and his colleagues realized that
if a stable wormhole could be created, then it could readily be turned
into a time machine. An astronaut who passed through one might come
out not only somewhere else in the universe but somewhen else, too--in
either the future or the past.
To adapt the wormhole for time travel, one
of its mouths could be towed to a neutron star and placed close to
its surface. The gravity of the star would slow time near that wormhole
mouth, so that a time difference between the ends of the wormhole
would gradually accumulate. If both mouths were then parked at a convenient
place in space, this time difference would remain frozen in.
Suppose the difference were 10 years. An astronaut
passing through the wormhole in one direction would jump 10 years
into the future, whereas an astronaut passing in the other direction
would jump 10 years into the past. By returning to his starting point
at high speed across ordinary space, the second astronaut might get
back home before he left. In other words, a closed loop in space could
become a loop in time as well. The one restriction is that the astronaut
could not return to a time before the wormhole was first built.
A formidable problem that stands in the way
of making a wormhole time machine is the creation of the wormhole
in the first place. Possibly space is threaded with such structures
naturally--relics of the big bang. If so, a supercivilization might
commandeer one. Alternatively, wormholes might naturally come into
existence on tiny scales, the so-called Planck length, about 20 factors
of 10 as small as an atomic nucleus. In principle, such a minute wormhole
could be stabilized by a pulse of energy and then somehow inflated
to usable dimensions.
Censored!
Assuming that the engineering problems could be overcome, the production
of a time machine could open up a Pandora's box of causal paradoxes.
Consider, for example, the time traveler who visits the past and murders
his mother when she was a young girl. How do we make sense of this?
If the girl dies, she cannot become the time traveler's mother. But
if the time traveler was never born, he could not go back and murder
his mother.
Paradoxes of this kind arise when the time
traveler tries to change the past, which is obviously impossible.
But that does not prevent someone from being a part of the past. Suppose
the time traveler goes back and rescues a young girl from murder,
and this girl grows up to become his mother. The causal loop is now
self-consistent and no longer paradoxical. Causal consistency might
impose restrictions on what a time traveler is able to do, but it
does not rule out time travel per se.
Even if time travel isn't strictly paradoxical,
it is certainly weird. Consider the time traveler who leaps ahead
a year and reads about a new mathematical theorem in a future edition
of Scientific American. He notes the details, returns to his
own time and teaches the theorem to a student, who then writes it
up for Scientific American. The article is, of course, the
very one that the time traveler read. The question then arises: Where
did the information about the theorem come from? Not from the time
traveler, because he read it, but not from the student either, who
learned it from the time traveler. The information seemingly came
into existence from nowhere, reasonlessly.
The bizarre consequences of time travel have
led some scientists to reject the notion outright. Stephen W. Hawking
of the University of Cambridge has proposed a "chronology protection
conjecture," which would outlaw causal loops. Because the theory of
relativity is known to permit causal loops, chronology protection
would require some other factor to intercede to prevent travel into
the past. What might this factor be? One suggestion is that quantum
processes will come to the rescue. The existence of a time machine
would allow particles to loop into their own past. Calculations hint
that the ensuing disturbance would become self-reinforcing, creating
a runaway surge of energy that would wreck the wormhole.
Chronology protection is still just a conjecture,
so time travel remains a possibility. A final resolution of the matter
may have to await the successful union of quantum mechanics and gravitation,
perhaps through a theory such as string theory or its extension, so-called
M-theory. It is even conceivable that the next generation of particle
accelerators will be able to create subatomic wormholes that survive
long enough for nearby particles to execute fleeting causal loops.
This would be a far cry from Wells's vision of a time machine, but
it would forever change our picture of physical reality.