Parallel Universes

http://www.youtube.com/watch?v=XmyrE9I8HNg&feature=related
Part 1 of a 4 part history special on parallel universes interesting stuff.
http://www.youtube.com/watch?v=RnkE2yQPw6s&feature=related
This is also an interesting take on my theory of on not being able to change the past see I said you can't travel back in time, in which I didn't realize what I was saying. You can go back in time but you can't change the past. Like he says if you change the past that universe, that timeline(along with you) will fork off into another timeline alternate to the one you spent your past in. This fork already occured in the past of course but you can't see a parallel dimension so this is why you wouldn't know. This would also open up the fact that all timelines and universes and parallel universes and whatnot, have to fork off infinitely at all points in time.
http://www.youtube.com/watch?v=X02WMNoHSm8&feature=related
This one alludes to the way you would accomplish time traveling if you were to try.