Time Travel has long intrigued us, it's enough to spur a whole sub-genre of science fiction. It's mind-boggling to consider all of the implication of traveling through time and having free will. It's easy to see that the universe as we know it would be rather unstable if, for instance, you could travel backwards in time and kill your own grandfather.But is it possible? To begin to understand the posiblities of time, we first need a brief introduction of spacetime. We're all familiar with our three-dimensional world, but we need to consider a fourth dimension as well-time. Therefore,you can sit still in a chair not travelling in three-dimensions, but traveling in spacetime. we think of times as passing forward. stephen Hawking explains this as three "arrows of time". The thermodynamic arrow of time points from a time of low entropy (high organization) to a time of high entropy (low organization/high choas). It passes from a glass of water sitting on a table to a shattered glass and a pundle of water on the floor. There's a psychological arrow of time: we remember the past, but not the future. Finally, there's a cosmological arrow of time. The universe is expanding (though this arrow could reverse in the future). We tend to think of time as an absolute: there are 60 seconds in every minute, and my 60 seconds should be the same as your 60 seconds. However, Einstein's theory of relativity defies this with time dilation. When a body approaches the speed of light, time efficency slow down. Therefore, if observes traveling at different fractions of the speed of light were to hold clocks, the clocks would be ticking at different speeds. Tie is relatives to the observer, rather than absolute. Consider what's reffered to as the twin paradox: one identical rwin aboard the spaceship will experience time dilation, and thus will age less quickly than the twin at home. Furthermore, if the twin's journey were long enough, the twin could return to earth to find that everyone he or she once knew was long dead. Essentially, this would be travelling into the future. This isn't the exciting kind of time travel from the sci-fi books and movies, though. We would prefer our time travel to be instanteous and not limited to the future. So far, we've examined only linear time travel in the form of slowing time down. But does time have to paas linearly? Is it possible that there could be loops in spacetime leading to the past and future? ONe possible candidate for such travel is the presence of wormholes. While wormholes also seem the stuff of sci-fi, their basis is actually in a paper written by Einstein and Nathan Rosen, where they refer to "bridges"in spacetime. They believed bridges to be extremely unstable and thus only temporary. The idea behind these bridges/wormholes is that there are theoretical tunnels between two far apart locations in spacetime. The distance of the wormhole wouldn't necessarily have to correspond to the distance between the two locations. There is evidence that such wormholes could theoreticaly exist, but that is beyond the scope of this paper. We'll suffice it to say that even if we find such wormholes, we'd have to figure out how to stabilize them in order to utilize them for time travel. Stephen Hawking currently believes time travel into the past to be impossible for many reasons. One less than scientific reason is that humans tend to love "spilling the beans." If someone in the future had figured out how to time travel into the past, he or she likely would have traveled back and told us! So will we eventually be able to time travel without limit? I can't say, my psychological arrow of time doesn't point that way. Oh well, it's all relatives anyway. Q. As used in line 10, the word "passing" most closely means
