"Perhaps
surprisingly, this turns out to be a subtle question. It is not obviously ruled
out by our current laws of nature. Recent investigations into this question
have provided some evidence that the answer is no, but it has not yet been
proven to be impossible."
Even
the slight possibility of time travel exerts such fascination that many
physicists continue to study not only whether it may be possible but also how
one might do it.
"Is
it possible to travel through time? To answer this question, we must be a bit
more specific about what we mean by traveling through time. Discounting the
everyday progression of time, the question can be divided into two parts: Is it
possible, within a short time (less than a human life span), to travel into the
distant future? And is it possible to travel into the past?
"Our
current understanding of fundamental physics tells us that the answer to the
first question is a definite yes, and to the second, maybe.
"The
mechanism for traveling into the distant future is to use the time-dilation
effect of Special Relativity, which states that a moving clock appears to tick
more slowly the closer it approaches the speed of light. This effect, which has
been overwhelmingly supported by experimental tests, applies to all types of
clocks, including biological aging.
"If
one were to depart from the earth in a spaceship that could accelerate
continuously at a comfortable one g (an acceleration that would produce a force
equal to the gravity at the earth's surface), one would begin to approach the
speed of light relative to the earth within about a year. As the ship continued
to accelerate, it would come ever closer to the speed of light, and its clocks
would appear to run at an ever-slower rate relative to the earth. Under such
circumstances, a round trip to the center of our galaxy and back to the
earth--a distance of some 60,000 light-years--could be completed in only a
little more than 40 years of ship time. Upon arriving back at the earth, the
astronaut would be only 40 years older, while 60,000 years would have passed on
the earth. (Note that there is no 'twin paradox,' because it is unambiguous
that the space traveler has felt the constant acceleration for 40 years, while
a hypothetical twin left behind on a spaceship circling the earth has not.)
"Such
a trip would pose formidable engineering problems: the amount of energy
required, even assuming a perfect conversion of mass into energy, is greater
than a planetary mass. But nothing in the known laws of physics would prevent
such a trip from occurring.
"Time
travel into the past, which is what people usually mean by time travel, is a
much more uncertain proposition. There are many solutions to Einstein's
equations of General Relativity that allow a person to follow a timeline that
would result in her (or him) encountering herself--or her grandmother--at an
earlier time. The problem is deciding whether these solutions represent
situations that could occur in the real universe, or whether they are mere
mathematical oddities incompatible with known physics. No experiment or
observation has ever indicated that time travel is occurring in our universe.
Much work has been done by theoretical physicists in the past decade to try to
determine whether, in a universe that is initially without time travel, one can
build a time machine--in other words, if it is possible to manipulate matter
and the geometry of space-time in such a way as to create new paths that circle
back in time.
"How
could one build a time machine? The simplest way currently being discussed is
to take a wormhole (a tunnel connecting spatially separated regions of
space-time) and give one mouth of the wormhole a substantial velocity with respect
to the other. Passage through the wormhole would then allow travel to the past.
"Easily
said--but where does one obtain a wormhole? Although the theoretical properties
of wormholes have been extensively studied over the past decade, little is
known about how to form a macroscopic wormhole, large enough for a human or a
spaceship to pass through. Some speculative theories of quantum gravity tell us
that space-time has a complicated, foamlike structure of wormholes on the
smallest scales--10^-33 centimeter, or a billion billion times smaller than an
electron. Some physicists believe it may be possible to grab one of these truly
microscopic wormholes and enlarge it to usable size, but at present these ideas
are all very hypothetical.
"Even
if we had a wormhole, would nature allow us to convert it into a time machine?
Stephen Hawking has formulated a "Chronology Protection Conjecture,"
which states that the laws of nature prevent the creation of a time machine. At
the moment, however, this is just a conjecture, not proven.
"Theoretical
physicists have studied various aspects of physics to determine whether this
law or that might protect chronology and forbid the building of a time machine.
In all the searching, however, only one bit of physics has been found that
might prohibit using a wormhole to travel through time. In 1982, Deborah A.
Konkowski of the U.S. Naval Academy and I showed that the energy in the vacuum
state of a massless quantized field (such as the photon) would grow without
bound as a time machine is being turned on, effectively preventing it from
being used. Later studies by Hawking and Kip S. Thorne of Caltech have shown
that it is unclear whether the growing energy would change the geometry of
space-time rapidly enough to stop the operation of the time machine. Recent
work by Tsunefumi Tanaka of Montana State University and myself, along with
independent research by David Boulware of the University of Washington, has
shown that the energy in the vacuum state of a field having mass (such as the
electron) does not grow to unbounded levels; this finding indicates there may
be a way to engineer the particle physics to allow a time machine to work.
"Then why do there seem to be no time machines? Two
different answers are consistent with our knowledge. The first is simply that
the classical theory has a much broader set of solutions than the correct
theory of quantum gravity. It is not implausible that causal structure enters
in a fundamental way in quantum gravity and that classical spacetimes with time
loops are spurious--in other words, that they do not approximate any states of
the complete theory. A second possible answer is provided by recent results
that go by the name chronology protection: One supposes that quantum gravity
allows microscopic structures that violate causality, and one shows that the
character of macroscopic matter forbids the existence of regions with
macroscopically large time loops. To create a time machine would require
negative energy, and quantum mechanics appears to allow only extremely small
regions of negative energy. And the forces needed to create an ordinary-sized
region with time loops appear to be extremely large.
