Time Travel to the Past

The film Back to the Future -- saw the actor Michael J Fox traveling back in time to ensure his parents get together and that he actually exists. But could someone travel back in time and alter the course of events? There are two theories of time travel to the past, the first suggests that there are many parallel world histories with events played out differently in each. A journey through time could switch between universes and therefore an individual could travel back in time, alter their life course (eg kill their grandmother) so that they are not then born in that universe but do exist there as a time traveler. The second theory suggests that you cannot alter the past because you have always been part of it -- the principle of self-consistency. Richard Gott is more sympathetic to this theory of past time travel where you can visit the past but you cannot change it.

If you want to see the past rather than visit it this is already possible. Because of the finite speed of light, when we look at a star 4 light years away we are seeing what it looks like 4 years ago because it takes the light and the image that long to travel back to us. In fact every time you look in a mirror you are seeing the past, again due to the speed of light. If you are 5 feet in front of your mirror the image you receive is of yourself 10 nanoseconds ago. But to visit the past means traveling faster than the speed of light, which is not possible within Einstein's theory of special relativity. Gott puts forward his own theory of �cosmic strings' for how this law of physics can be beaten, using Einstein's theory of curved space-time, providing the reader with simple cut out models to make to illustrate how the curvature of space-time could allow time travel. Gott also reviews many other theories for past time travel all based on the concept that if you can beat a light beam by taking a short cut then the possibility of time travel to the past opens up. Learning whether time travel to the past can be accomplished may require a full theory of quantum gravity, and this is one of the reasons time travel is so fascinating.

Time Travel to the Future

If you wanted to visit the earth 1,000 years from now all you would need to do is get in a spaceship, travel to a star a bit less than 500 light years away and return traveling both ways at 99.995% of the speed of light. When you came back earth would be 1,000 years older but you would have only aged 10 years. This is possible because time passes more slowly when traveling at such speeds. Of course there are huge technical barriers to overcome in achieving this but it is scientifically possible. In fact, time travelers to the future already exist. Because the Russian cosmonaut Sergei Avdeyev was in orbit a total of 748 days during three space flights, he's about 1/50th of a second younger than he would be if he hadn't gone on those trips. ie when he arrived home, he found the earth 1/50th of a second to the future of where he expected to find it.

Why Study Time Travel?

But why examine time travel at all? Gott suggests that the paradoxes associated with time travel pose great challenges and often provide a clue that some interesting physics is waiting to be discovered. Learning about whether time travel could occur in principle may give new insights into how the universe works -- and even how it got here.

The Beginning of the Universe

The beginning of the universe is relevant to the time travel debate as the extremes of this situation may be exactly those needed to allow time travel. Gott suggests that as the laws of physics allow curved geometries we can have a Universe that has a beginning without having an earliest event. In other words, at the beginning the Universe could have a small time loop allowing it to create itself -- just as a time traveler could in theory go back and become their own father or mother. Gott proposes that a time loop existed at the beginning of the Universe but then shut down so that after this point we all moved forward in time.

Predicting the Future

Gott has discovered a way of predicting how long something you are observing is likely to last. The idea is based on the Copernican Principle -- the idea that your location in time and place is not special. Gott discovered this idea in 1969 whilst looking at the Berlin Wall, which he correctly predicted (with 50% confidence) would have a future longevity of between 2-2/3 years and 24 years. If you observe something at a non-special or random time there is a 50% chance that it is in the middle two quarters of its period of existence. At one extreme the future will be three times as long as the past at the other extreme the future is one third as long as the past. There is a 50% chance you lie between these extremes so the future is between one third and 3 times as long as the past. Gott has refined this theory to ensure that the prediction is 95% reliable, the scientific standard, and has used it to predict everything from the future longevity of the human race (5,100 years � 7.8 million years) to the future longevity of the New York Times (3.8 years � 5,811 years) years.

The Future of the Space Program

Gott suggests that the goal of human space flight progammes should be to increase our survival prospects by colonizing space. However, he predicts that we will are not likely to heed this warning. As we were born on Earth, of all human beings to be born -- past, present, future -- a significant fraction must be born on Earth otherwise we would be special (going against the Copernican Principle above). This means it is not likely for humans to heed this warning and escape Earth in a big way. And this may well be the reason for our likely early demise as a species. Abandoning the human space flight program would be a tragic mistake yet a mistake we are likely to make.

The Likelihood of Time Travel

It would take a supercivilisation with enormous skills and technical abilities to attempt time travel. But the fraction of civilizations that turn into supercivilisations must be extremely low otherwise the likelihood would be that we were within one now. Gott suggests that this doesn't mean time travel is impossible just that it must be rare at best.

J Richard Gott III is a professor of astrophysical sciences at Princeton University. A recipient of the President's Award for Distinguished Teaching at Princeton, Gott has written for Time, Scientific American, New Scientist and American Scientist. He lives in Princeton, New Jersey where the neighborhood children think he has a time machine in his garage. Even colleagues behave as if he can travel through time and have sent invitations to speak six months after he has spoken suggesting that he can of course travel back in time to give the talk!

In Time Travel in Einstein's Universe Gott takes the reader on a journey analogous to climbing to the summit of Mount Everest. From the base camp of the science fiction of HG Wells, Gott takes you up by the easiest route possible, and the vistas are breathtaking.

• Published in hardback [in U.K.] by Weidenfeld & Nicolson. Available from Amazon.com for $17.50 plus shipping.