Think about this:
If someone went back into time then everything that they were doing would be altering the past and the future simultaneously. Therefore, if someone were to go back into the past they would've been, obviously, in the past rather than in the future... so if they weren't in the future in order to go back into the past how could they be in the past in the first place? Either, A: They would vanish from existence; or B: They could not go into the past at all; or C: They would be in a completely different reality all together.
I would bet with B. But what if an infinite amount of realities do exist, where an infinite amount of actions/combinations occur? Then, theoretically, no two realities could be the same. So then, if you did go in to the past, and you changed something... then maybe you just actually inserted yourself into a different reality rather than changing the one that you were already in.
