yeah i think i agree with darth soup... time dilation and al that jazz is good and all, but it's really more descriptive than anything else, as all good science is. it describes reality so we can make predictions. that's really all it is. it doesn't have to be true, doesn't have to be close, just has to be useful. like if i said: staying out in the sun too long will cause photon fairies to infect your body and give you skin cancer. true? possibly. but it doesn't matter as long as the cause and effect are pretty good approximations of what really happens. for the more mathematically and physically inclined, i make a better example. when we say an object starts at rest and then begins moving with a s(t), we can take the derivative of that to get v(t) and a(t). but the fact that it starts at rest is the kicker. you see, if we get v(t) to be constant, we know that there has to be an acceleration present to start the motion. moreover, we know by extension that there has to be a jerk to cause that acceleration, and so it snowballs into infinity. so in science we do approximations, not truth. don't confuse the two.
furthermore, i personally believe that the flow of time is arbitrary and that it depends upon ratios between displacements of objects. in fact we only measure time based on movement. there is no other way. that is to say, what we basically do is say: let's let C be the distance it takes for X to move from Y to Z. then the time it takes for any other object to move from A to B is (B - A) / (Z - Y). i know that's probably confusing but it's true if you think about it. and perhaps in our physical universe light is the absolute measuring rod. oh and my equation isn't intended to be true, just give you an idea that time is a relationship amongst distances (and likely also masses and energy), not a property unto itself. in fact perhaps space is also a figment of our imaginations given by mass and energy. i wouldn't be surprised.
bottom line: i personally don't think time travel is possible. it doesn't matter if time is linear, hyperbolic, sinusoidal, coiled or cyclic; in any of these cases it's possible to back up. but i think time isn't something that really exists except inside our heads. we use it to make sense of things, and thus can't "rewind" it any more than we can imagine a hooker and bang her. time is a pure effect, causing nothing, and thus to get results we have to effect the cause[s] of time. just like you can't cure a cold by taking cold medicine. anyway... i hope at least one person's head explodes, i've got a headache now... too early...