@inkican Going back in time would require more energy than is available in the universe, for going faster than the speed of light, which is impossible. That's the basis in the theory (should be called 'law' by now) of relativity.
Backwards time travel would obviously interfere with causality in creating an alternative past, which would lead to a different present. So it's a good thing that it's physically impossible, despite what a desperate astrophysicist believes.