There are several theories out there about time travel:

- Whether or not someone can actually travel into the past verses solely in the future,

- The argument of a parallel universe

- The existence of alternate dimensions and realities

...All interesting theories...

Then, of course, there is the argument of whether reality can be altered or unchanged:

The Butterfly Effect vs. Predetermination.

The butterfly effect is based on chaos theory which states that any event that is not rational or predictable can drastically change the events surrounding it. With such a theory in practice, if someone were to travel in the past and 'change' the events (however minute the change) the consequences of his actions could be extraordinary. Examples of chaos theory or altered realities are...The Butterfly Effect, Back to the Future, and The Terminator trilogy.

Predetermination or determinism is a concept where every event, including human cognition and action, is predestined and cannot be altered. With this particular theory in place, if a time traveler were physically able to travel into the past, then there is no possible way to change the history already set in place. Something or someone would prevent the person from changing the predestined timeline. Examples of predetermination or unaltered realities are movies such as...Time Machine, Fetching Cody, and Twelve Monkeys

I believe in the concept of free will. Meaning, everything 'we' do matters. Our lives are not written or set in stone, and we have the power to determine our own outcome.