About time travel
|About J. Richard Gott, III
A small pre-school kid enters a library with his mom and asks for a book
about gravity. "For what age?," asks the librarian. "An advanced high
school level," replies the kid. He looks at the description of gravity
in the book and says: "There has to be more than that." That is how Dr.
Gott's interest for gravity began about 50 years ago. Since then, he has
turned this interest into profession and contributed to better
understanding of gravity as an astrophysicist.
Already in his early age, he has distinguished himself by his passion
for astronomy. During his high school years, he won several science fair
awards, including the prestigious Westinghouse Talent Search. Twenty
years later, he became the chairman of the Judges of this same
competition. He received a B.S. in mathematics from Harvard University
in 1969. Just three years later, he completed an award wining PhD thesis
in astrophysics from Princeton University, where he spent the most of
his career since then.
Dr. Gott has been working extensively on the problems of large
structures in the universe and the theory of creation of the universe.
His work on gravity led him to several important theoretical discoveries,
including a possibility for time travel to the past. Dr. Gott's
achievements extend to science education as well. Several times,
students at Princeton have voted Dr. Gott the school's outstanding
professor. He is actively involved in promoting science in to the public
and, especially, among high school students, recognizing the importance
of science education.
Q: Time travel has been described in many
books and movies. We usually call them science fiction. While they deal with
the fictional part, you are dealing with the science part. Therefore, here
comes the basic question: is it possible to travel through time?
Einstein showed in 1905 that the time travel to the future is possible
within his theory of special relativity. According to the theory, clocks
tick slowly and this has actually been observed. We observe that if we take
atomic clocks on an airplane trip around the world toward the East, where
the velocity of the plane adds to the rotation velocity of the Earth, these
clocks come back about 59 nanoseconds slow. In fact, we observe that
particles that are moving with speeds nearly the speed of light decay more
slowly. So, if you wanted to visit the Earth in the year 3000, all you have
to do is to get on a rocket ship going at 99.995% of the speed of light, go
to a star 500 light years away, turn around and come back at that speed.
When you get back, the Earth would be 1000 years older, but you would only
aged 10 years. The biggest time traveler so far is an astronaut named Sergey
Evdeyev, who was on the Mir Space Station for a total of 748 days on three
space flights. When he returned to earth he had aged about a 1/50 of a
second less that if he stayed home. That is, when he returned to earth he
found the Earth a 1/50 of a second to the future than what he expected it to
Time travel to the past is more difficult but it is a theoretical
possibility that seems to be allowed by Einstein's theory of general
relativity. There, space and time are curved, and you can have solutions to
those equations that are sufficiently twisted that allow you to circle back
and visit an event in your own past. This is in the same way that Magellan's
crew left from Europe, went steadily west, and circled the globe. There are
solutions like this to Einstein's equations of general relativity: wormhole
solutions, some that I found involve cosmic strings, and so forth. However,
the question is whether these possibilities can be realized in our Universe
or not. To understand that, we may need to understand the laws of quantum
gravity. That is why gravity is particularly interesting project. We take
solutions to Einstein's equations quite seriously because they have been
tested. For example, light bending around the Sun and so forth. Therefore,
the time travel to the past is a topic under current investigation.
About the Universe
Q: There are several universal questions that humankind has asked
itself since the beginning of its time. One of them is about the beginning
of everything, beginning of the Universe. You have an answer to this
We believe today that the Universe started after a state of very rapid
expansion called inflation, back at the Big Bang. The Universe inflated very
rapidly during this phase. This has been confirmed by recent observations of
the cosmic microwave background, as the theory of inflation is made definite
predictions of what we should have seen and they have been confirmed quite
nicely. Professor Linde at Stanford has shown that if you have an inflating
Universe like this that the quantum fluctuations cause it to create baby
universes, like branches growing out of a tree. Each branch grows up to be
as big as the trunk and it sprouts branches of its own. Thus, you have an
infinite fractal tree of branching universes in this theory of inflation.
You still might ask yourself the question, though, where did the trunk come
from. Li-Xin Li and I proposed that simply one of the branches circles back
around and grows up to become the trunk. This is a model where the Universe
is its own mother. It is a model with a little time loop at the very
beginning. This is a model that has quite interesting properties.
Q: Nevertheless, is it possible to test your theory of self-creating
|The model of self-creating
universe explains why we are constantly moving toward the future and
never toward the past. On this sketch, our three dimensional world is
represented as a two dimensional surface. The third dimension is used
for showing the direction of time. An infinite number of universes grow
out of the time loop. In that loop, a signal (red arrows) can travel
only in one direction of time. This direction allows the signal to
escape into the trunk with branches made of universes. A signal
traveling in opposite direction would always end up in the loop, and
would not be able to escape it. This would eventually destroy the loop.
Therefore, a signal is not allowed to travel in the wrong direction in
the first place. (
When we made this model we found that it had one
interesting very testable prediction. It predicted that there would be an
arrow of time. One of things that we notice in the Universe is that if I
shake charges, electrons, here, light waves would go out with the speed of
light and they would get to Alpha Centauri, which is 4 light years away, 4
years later. However, Maxwell's equations of electrodynamics equally allow
light beams to go toward the past and intersect Alpha Centauri 4 years ago.
We never see that, we never see light beams going to the past, even though
Maxwell's equations are time symmetric, they make no difference between the
future and past. We see that the light beams are always emitted toward the
future. This explains the cause and effect that we usually see in the
Universe today. Since it is not the laws of electrodynamics that are causing
this, we would suppose that this must something to do with the initial
conditions of the Universe.
In our model, there is a very simple explanation. The only way to have a
self-consistent model in our case is if the light beams are only allowed to
go toward the future. Any light beam going toward the future would go out
one of these branches and just continue run out, no problem. But, a light
beam going to the past go back down that branch, back down the branches,
back into the trunk, and then it would circle this time loop an infinite
number of times which would cause it to blow up and destroy it. This would
not be the geometry you started with. In this time travel solution, you have
to have a self-consistent solution where you are not killing your
grandmother. The only way how to get a self-consistent solution is to have
an arrow of time pointing away from the time loop at the beginning.
That was a very interesting and testable prediction that it had. In the
future, one will have to see how this picture of our fits in whatever future
superstring theory of everything. We think that it already fits in it very
well conceptually because these branches originally had circumfereces that
were very small and the superstring theory tells us that the big dimensions
of space that we see used to be very tiny and curled up. And there are some
curled up microscopic extra dimensions today that we don't see. The
macroscopic dimensions that we see today are dimensions that used to be tiny
as well. Thus, all of the dimensions in our Universe, all of the spatial
dimensions were curled up in time. In our model, the time dimension is
curled up in time as well. This is a little time loop at the beginning. We
think it fits well into the superstring theory, but we will have to wait and
see how that superstring theory close out and whether it allows solutions of
this kind. We will have to see other predictions of the superstring theory
that would convince you that it is the right theory.
About science fiction
Q: You often use examples from the science
fiction literature to explain complicated physics and mathematics behind
your theories. Since many often scoff this genre, what do you think about
I think that science fiction is very interesting because a lot of times
ideas are explored first in science fiction and then later by scientists as
well. For example, H.G.Wells wrote the book "The Time Machine" in 1895. This
is 10 years before the special relativity. At that time, with the Newton's
theory, time travel looked impossible. But with the Einstein's theory of
special relativity moving clocks tick slowly, and later, with the general
theory of relativity, space and time are bendable. That is how a possibility
of traveling in time came up. The same thing is true with the Carl Sagan's
book "Contact" about wormholes. He asked Kip Thorne to examine whether his
wormhole physics make sense. Then Kip Thorne looked into the wormhole
physics seriously and found that circumstances and solutions like this might
be possible. If you had wormholes like this, you might use them to make a
time machine to visit the past. Thus, I think science fiction has often
sparked interesting science investigations.
Q: What is your favorite science fiction book or movie?
I would have to say it is H.G. Wells' "The Time Machine". It is an amazing
book and, as I said, 10 years or so ahead of its time.
About understanding science
Q: However, even the most of physicists cannot grasp the complexity
behind your work. What can then an ordinary person expect?
I wrote a book "Time travel in the Einstein's Universe" about this. In the
book, I am explaining my theory to the lay audience, too. Everyone should be
able to understand this. Einstein's theory is about curved space-time, and
this is very visual. You can understand the concept that space and time
might be bent. You can visualize bending of a piece of paper. You can
visualize a curved surface because you see the surface of the Earth. Many
concepts of this can be explained rather simply. However, working out the
equations for general relativity is difficult. Nonetheless, many of the
solutions in general relativity that solved those equations are rather
simple. The black hole solutions, for example, are beautiful. I think that
the average person can understand what the implications of this theory are.
I also go to some trouble to try to describe how Einstein came to some of
Q: While everybody can appreciate and admire your work, most of
humankind is struggling with very basic problems of daily survival. From
their point of view, what would you say it is the importance of your work,
except satisfying the human curiosity?
The first of all, the human curiosity is very important, because, as a
species, we have been around for 200 thousand years. That is not remarkably
long. Tyrannosaurs Rex existed for two and half million years, for example.
However, in that very short time, one of the amazing things that we have
accomplished is we really understood where we are in the Universe, we
understood something about the laws of physics. That is what everybody
should be proud of. Also, I think it is important to say that you never can
tell quite what the implications of understanding science at a deep level
would be. Einstein was trying to understand the basic laws of physics and
the Universe. Well, one of the things that came out of that is E=mc2,
which is nuclear energy. One cannot often foresee that understanding science
may have profound applications for the future of the human race. And I think
one other thing that we should be doing as a human race is going to other
planets, colonizing every world. This would be a benefit to our survival.
This is something that you need science and technology to do. If we stay on
the Earth we are subject to extinction events as other species are, and our
survival chances to spread out into the Universe would improve.
About teaching science
Q: You served as the chairman of the judges
of the National Westinghouse and Intel Science Talent Search, the premier
science competition for high school students. How do you describe this
complex theoretical physics to high school students?
The important thing that is stressed in that kind of competition is that
high school students can actually do research. They do research projects,
and they enter these research projects. We think that one of the best ways
to find science talent is to actually encourage high school students to do
their own research. Students in high school can actually do many frontier
research projects. They may work for the scientists at a university, but
many projects people can do in their own basement. A lot of science is
learning about it by doing it. I teach a course in general relativity for
undergraduates, and I think the Einstein's theory of general relativity,
should be something that is on the undergraduate curriculum for physic
students. It shouldn't necessarily be put off to graduate school. I think
high school students should certainly be exposed to the ideas of special
relativity and general relativity because TIME magazine picked Einstein as
the person of the century. Thus, if you want to be an educated person, you
should understand what Einstein did, why he is there.
Q: Why do kids need these scary science classes if just a small number
of them will become scientists?
First of all, I would describe science classes as fun, not scary. I think
that exposure of high school students to science is important. Science is a
part of our modern world. It is one of the things that human beings have
discovered. Just as in high school, you ought to read some Shakespeare
because he is one of the important people who have written wonderful plays.
That is why I think you should know something about science. You should know
something about science to understand our modern world. If you are going to
be a politician, you may have to decide scientific questions. If you are
going to run a business, scientific considerations may come into some
decisions that you have to make on the future of your company, whether to
use some technology or not and why. Therefore, I think science education is
a part of our educational system. That is something that people would find
Dejan Vinkovic (Copyright: Vjesnik and znanost.org) 2002.
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