wodmarach: The only true limit to time travel is that you can only go back to the point at which your machine was built no earlier.
Titanium: Hold on there, this is now a fact? It was said by one guy that was raising money for his theoretical time machine, and gave no proof whatsoever.
The thing with time relativism, the "higher speed - slowing time " thing, was in fact proven (two really good clocks, one on earth, the other in a space capsule, and then compared the times).
The only reason that you can't go back into the past, if you
had a "working" time machine, is that you could create a time paradox, like kill yourself actively or passively trough your actions. In that way, your "future" would not exist, or making an alternate universe.
The creation of a Paradox doesn't actually mean TT is impossible the main current theory is that any working time machine would actually prevent you from changing the outcome in some way (the guy you shoot wasn't your real GF etc)
But yes the no travel to before its made is the only theory that works right now (though theres a new quantum approach that requires quantum events to be non-linear), relativity is still a theory btw as is evolution they are not proven just meet the facts better than any other idea out there.