Despite what science fiction writers and other forward thinking people believe, time travel is not possible for the simple fact that time is a human construct, and as such, our notion of time has no meaning in the grand scheme of things.

As an example, no one knows exactly how long a second is, because we base our measure of time on how long it takes the sun to go from east to west. Unbeknownst to us, over the course of its history, the earth's rotational speed has actually slowed, so that original "standard" is no longer useful.

We use atomic clocks that are based on highly precise oscillators, but we still don't know how long a second is because there is no naturally occurring standard for measuring something that exists only in the minds of human beings.