time travel is impossible based on the principle of conceservation of energy, there is only so much energy and matter in teh universe, so tirod is on the right track he's just not being the most clear in getting the point, but anyways, if you were to travel back in time you would have to displace teh matter that you are sending back in time (which i'd love to see you to figure out how to do that), the other possiblity is that you can ignore the law of conservation of energy for brief periods of time, but that's beyond the physics most of you have taken at this point (or will take), but in short it's impossible if it would ever be possible it would be many lifetimes beyond ours, the only possiblilty is you can manipulate the passage of time by traveling speeds near that of the speed of light