: Try this: Take of piece of paper and think of one : side as representing space (going to infinity) and : the other side as time (going to infinity). Now take : a pencil and put its point on the paper- it represents : you in normal space-time. Now, without picking : up the pencil, put you pencil somewhere else on the : paper. You have to travel in space-time to get : somewhere else. This is why time travel is impossible.

: You cannot pick your pencil off the paper because : it is impossible to just "drop" out of space-time : and travel about freely (after all, we exist in space, : and would not exist elsewhere). If you can figure : out how to "drop" out of space-time and still exist, : then you can time travel. If you can figure it out, : (with real Physics), you're a genious!