Yeah yeah, another post about time travel. I just cant get it out of my mind. there are so many things i want to go back and do, go back and change, but i just can tfigure out how?? Its a constant thought running through my head. I mean,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.
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.
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