theoretical physics has always been one of my favorite subjects. I'm going to post some links that will explain it alot better then me.
http://www.mkaku.org/articles/phys_time_travel.shtml -pretty simple explanation of time travel and some of the paradoxs of it. There's also some other crazy physics on there.
http://www.ettnet.se/~egils/essay/essay.html#what -little more indebt then the last link. But still easy to understand.
http://homepage.sunrise.ch/homepage/...pace-time.html -this sites for the physics geek that knows einstiens equations, how they're derived and stuff like that.