Basically scientists theorise you cannot go back in time because you’d inevitably change the future which leads to changing the present from where you just came.
Brain = malfunctioning.
Here's the paradox (thanks wiki):
Suppose a man travelled back in time and killed his biological grandfather before the latter met the traveller's grandmother. As a result, one of the traveller's parents (and by extension the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which means the grandfather would still be alive, and the traveller would have been conceived allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox.
MIND = BLOWN
To take the brain ache away here are some sweet pics.