Is Time Travel Possible?
- Playing fast and loose with time: a new study suggests that time
is not quite impossible.
by William F. Allman
Time travel has popped from the annals of science fiction into the
realm of scientific respectability at least in theory, that is.
By combining two well-established principles originally outlined by
Albert Einstein nearly a century ago in his theories of relativity,
three theoretical physicists have proposed a novel scheme that appears
to permit a limited sort of time travel. The scientists' result is
not antything close to a blueprint for building the sort of time
machine that science-fiction heroes are forever leaping into to
explore the lives of cave men or peek into the future. But it
suggests that either Einstein's equations are wrong or that the
universe is governed by some very different principles from what
physicists have long assumed.
The notion that the causes of an event taking place in the present
could be eliminated by traveling into the past and changing history
has always made physicists (not to mention stock speculators and
newspaper publishers) squeamish about the idea of time travel;
physicists have thus held that there must be laws that provide a kind
of "cosmic censorship" that prevents moving through time, and thus any
tampering with the sequence of events. Various theoreical scenarios
for time travel have been cooked up in the past, but in each case
physicists have found physical laws that blocked it.
The new study, however, appears not to violate any known physical
laws. "We're asking, if you take Einstein's equations far enough,
will they get you in trouble?" says Michael S. Morris of the
University of Wisconsin, one of the co-authors of the new study.
"We're suggesting that maybe they will, because at some level we may
have to abandon either Einstein or causality"-the' basic principle of
physics that one event
Time flies. The authors' hypothetical time "machine" starts with
Einstein's famous, and apparently paradoxical, discovery that time is
not constant throughout the universe, but rather varies depending on
the velocity at which the timekeeper is traveling. While to an
astronaut, a clock on board a speeding spaceship will appear to tick
at a normal rate, to a stationary observer on Earth, the clock will
appear to be marking time very slowly. This apparent slowing of time
applies not only to clocks but to everything in the spaceship,
including the astronauts, and has given rise to what is known as the
"twin paradox": When the speeding astronaut returns to Earth, he will
have aged less than a twin brother he left behind. This bizarre
time-slowing effect has been demonstrated by researchers who
synchronized two highly accurate atomic clocks and then flew one of
the pair aboard a high-speed jet for hours. When the two clocks were
reunited, the airborne clock had fallen slightly behind its stationary
The second Einsteinian principle that goes into the time machine is
the warping of space by gravity. This idea is often explained by
analogy to a worm crawling over a sphere. To the worm, the world is
flat. Crawling from one spot on the sphere to another in what seems
to be a straight line, the worm doesn't realize that it is actually
tracing out a path that curves in three-dimensional space to follow
the sphere's contour. The worm also doesn't realize that it could
take a shortcut by burrowing through the core of the sphere.
Similarly, what appears to us as three-dimensional space is, according
to Einstein's theory of relativity, bent in dimensions that we cannot
easily imagine, and a "wormhole" could connect two seemingly distant
points in space.
To turn a wormhole into a time machine, say the physicists,
technicians in some "arbitrarily advanced" civilization could harness
the gravitational forces necessary to make one end of the hole move
back and forth at extremely high speed while keeping the other end
stationary. Like the astronaut in the twin paradox, this rapidly
moving end of the its stationary counterpart. By entering the "older"
end of the wormhole and coming out the "younger" end, a person could
theoretically travel from the present to the past.
Keeping the door open. While the physicists' calculations suggest
there's nothing to prevent all this from happening, they admit that no
one has actually ever seen a wormhole. Calculations by Morris and
co-authors Kip Thorne and Ulvi Yurtsever of the California Institute
of Technology imply that a wormhole can be kept open only under some
strange and exotic physical conditions. Phenomena that under normal
circumstances exist only in infinitesimally small particles for
infinitesimally short periods of time would have to exist throughout
the wormhole for as long as it remained open.
Even if a wormhole time machine were possible, it wouldn't permit the
kind of time travel fancied by H. G. Wells and others who have mused
on the possibility of traveling far into the past or future. Even
though members of an advanced civilization might be able to cause one
end of a wormhole to age less slowly than the other, at best they
would only be able to make time stand still at the instant the tunnel
was created, not move it back any further into the past, and they
couldn't move the other end ahead into the future.
There's at least some circumstantial evidence that the more
far-reaching kind of time machines envisioned by sciencefiction
writers will never be created, even by the most technologically
advanced civilization imaginable. If such a machine were ever to be
built, a traveler from the future probably would already have shown up
here by now.