In the matter of classical physics, the position taken is one in which it is stated that an individual arriving in the past must do the things that history records them doing. For it is so stated that there is only ONE history, so try as one might, one is constrained to act out ones part in it.

In quantum mechanics, however, there exists an idea referred to as the 'many universes' interpretation. First posed by Hugh Everett in the mid- twentieth century, it basically infers that if something physically can happen, it will - in some universe.

When one considers the scope of this interpretation, the possibility of time travel exists, for the paradoxs that classical physics deem as insurmountable fall away. Thus it is possible to alter the past [and thus ones future] without necessarily doing so to anyone else [this being because, for the others, this point in time is the correct one and always has been].

For quantum mechanics demonstrates that an individual traveling backward through time could alter it, but that having done so, would return to a universe that was not his or her original. And thus, neither the grandfather paradox, nor the knowledge paradox would apply.

If one considers the point that when the universe was created, its existence was not singular, but multiple, then the possibility exists for time travel. Of course, one must first reach a speed at which the movement through time becomes possible. For now, it is nothing more than an exercise in theory, but one that might come true.