How to Go Back in Time

Ever since Einstein, physicists have regarded the universe as four- dimensional. In addition to the three physical dimensions -- length, width and height -- there exists time, which is treated mathematically as though it were equivalent to the other three. But there is one important difference: while humans can travel freely in any physical direction -- up and down, left and right, back and forth -- they can go only forward in time, never backward.

Still, there is nothing in the laws of physics that says time cannot run backward. Einstein's equations of motion work equally well, mathematically, when the direction of time is reversed. Yet no one has ever been able to travel back in time. Theoretical physicists find the situation intriguing: if the laws that govern nature really permit time reversal, there should somehow be a way to achieve it. Now a theorist at Princeton University has come up with a way that travel into the past might, in principle, be accomplished, even if it may not be practical.

J. Richard Gott's calculations, which appear in the prestigious journal Physical Review Letters, create an imaginary time machine that takes advantage of an Einsteinian concept: that both space and time are distorted in the presence of very large masses or when objects are moving at speeds approaching the velocity of light. Gott is not the first to take this tack; in 1988 a Caltech physicist, Kip Thorne, and two colleagues constructed their own theoretical time machine and wrote about it in the same journal.

The Caltech machine involved travel through a wormhole, a bizarre object that physicists believe might exist at the core of a black hole. Under the infinite density and gravity at the black hole's center, space could be so profoundly warped that a tunnel would form, far narrower than a subatomic particle, that might reach to some distant part of the universe. Anyone or anything entering the tunnel would appear instantly at the other end and, under special circumstances, would essentially travel into the past.

It is hard to see how this particular time machine could be of much use. The time traveler would have to survive the crushing pressure inside a black hole and somehow squeeze through an opening smaller than a single atom. Moreover, since a wormhole tends to collapse a fraction of a second after it forms, some means would have to be found of propping it open.

Still, says Gott, "it is an ingenious concept, and it got me thinking about other ways you might achieve time travel." Gott's idea is simpler than Thorne's. No black holes, no wormholes -- just a spaceship traveling at near light speed, and a peculiar object called a cosmic string. Like wormholes, cosmic strings may or may not exist; they are at present just theoretical constructs.