I found something on my work PC. I must have saved this text a while back I only just read it, here's a little bit of it.
(I AM NOT THE AUTHOR OF THE FOLLOWING TEXT)
"In his memoirs, Einstein wrote that he was disturbed that his equations contained solutions that allowed for time travel. But he finally concluded: the universe does not rotate, it expands (i.e., as in the Big Bang theory) and hence Goedel's solution could be thrown out for "physical reasons." (Apparently, if the Big bang was rotating, then time travel would be possible throughout the universe!)
Then in 1963, Roy Kerr, a New Zealand mathematician, found a solution of Einstein's equations for a rotating black hole, which had bizarre properties. The black hole would not collapse to a point (as previously thought) but into a spinning ring (of neutrons.) The ring would be circulating so rapidly that centrifugal force would keep the ring from collapsing under gravity. The ring, in turn, would act like the Looking Glass of Alice. Anyone walking through the ring would not die, but could pass through the ring into an alternate universe.
Since then, hundreds of other "wormhole" solutions have been found to Einstein's equations. These wormholes connect not only two regions of space (hence the name) but also two regions of time as well. In principle, they can be used as time machines.
Recently, attempts to add the quantum theory to gravity (and hence create a "theory of everything") have given us some insight into the paradox problem. In the quantum theory, we can have multiple states of any object. For example, an electron can exist simultaneously in different orbits (a fact which is responsible for giving us the laws of chemistry.) Similarly, Schrodinger's famous cat can exist simultaneously in two possible states: dead and alive. So by going back in time and altering the past, we merely create a parallel universe. We are changing someone ELSE's past by saving, say, Abraham Lincoln from being assassinated at the Ford Theater, but our Lincoln is still dead. But does this mean that we are able to jump into H.G. Wells' machine, spin the dial, and soar several hundred thousand years into England's future?
No. There are a number of difficult hurdles to overcome. First, the main problem is one of energy. In the same way that a car needs gasoline, a time machine needs to have fabulous amounts of energy. One either has to harness the power of a star, or to find something called "exotic" matter (which falls up, rather than down) or find a source of negative energy. (Physicists once thought that negative energy was impossible. But tiny amounts of negative energy have been experimentally verified for something called the Casimir effect, i.e., the energy created by two parallel plates.) All of these are exceedingly difficult to obtain in large quantities, at least for several more centuries!
Then there is the problem of stability. The Kerr black hole, for example, may be unstable if one falls through it. Similarly, quantum effects may build up and destroy the wormhole before you enter it. Unfortunately, our mathematics is not powerful enough to answer the question of stability because you need a "theory of everything" which combines both quantum forces and gravity. At present, superstring theory is the leading candidate for such a theory (in fact, it is the ONLY candidate; it really has no rivals at all.) But superstring theory, which happens to be my specialty, is still too difficult to solve completely. The theory is well-defined, but no one on earth is smart enough to solve it.
Interestingly enough, Stephen Hawking once opposed the idea of time travel. He even claimed he had "empirical" evidence against it. If time travel existed, he said, then we would have been visited by tourists from the future, ergo: time travel is not possible. Because of the enormous amount of work done by theoretical physicists within the last 5 years or so, Hawking has since changed his mind, and now believes that time travel is possible (although not necessarily practical.) Furthermore, perhaps, we are simply not very interesting to these tourists from the future. Anyone who can harness the power of a star would consider us to be very primitive. Imagine your friends coming across an ant hill. Would they bend down to the ants and give them trinkets, books, medicine, and power? Or would some of your friends have the strange urge to step on a few of them?
In conclusion, don't turn someone away who knocks at your door one day and claims to be your future great-great-great-grandchild. They may be right."
[This message has been edited by Rambler (edited April 18, 2000).]
