How to Go Back in Time

(2 of 2)

In this case the theories are those that describe the energy fields of the very early universe, shortly after the Big Bang. Under the right circumstances, physicists believe, very long, very thin strings of pure energy might have survived in their original state rather than cooling off with the rest of the universe. These cosmic strings would be infinitesimally thin but unbelievably dense, with a thousand trillion tons of mass for every inch of length. The enormous mass would warp the region around a cosmic string so that space itself would act like a distorting lens. Two light rays from a single source -- a star, for example -- could travel by two totally different paths, one on each side of the string, and still end up at the same place. The significant part of this theory is that these two paths could be of different lengths, depending on the position of the light source. And because light always travels at the same speed, one of the light rays would thus take longer than the other to reach its goal.

It is this difference in travel time that sets up Gott's time machine. Imagine a rocket ship moving at 99.9999% of light speed and taking the shorter of the two paths. In principle it could reach the far side of a string at exactly the same moment as a light ray traveling the longer path. In essence the ship would be moving faster than light, and under the peculiar logic of special relativity, it would thus go backward in time. For complex reasons, the ship has to make a complete loop around the string, and thus a single string will not do; there must be two strings -- passing each other at nearly the speed of light -- for the trick to work. But work it apparently does. Says Gott: "I've gotten enormous interest from other physicists and astrophysicists about this idea."

The reason is not that physicists really believe time travel can ever actually occur. But the fact that it appears possible in principle challenges the very foundations of physics. What does it mean if an effect can theoretically precede a cause? What if, to use a theme from science fiction, a person could go into the past and kill his or her grandmother at an early age? Such a concept appears to make no sense, yet it must have some meaning if Gott's and Thorne's ideas are correct, as they appear to be. Says Gott: "At some point physics will have to find some mechanism by which these things are forbidden, or else learn to live with them." With two examples in hand, the paradox can no longer be ignored.