Well, here's the riposte which is usually trotted out by the physicists to say that time travel, at least into the past, is impossible: put simply if it were possible, wouldn't the present be inundated with people from the future? Surely someone would have gone back and assassinated Hitler, wouldn't they? Or Stalin? Or Pol Pot? Or people who acquire wisdom through experience and realise that their parents were right all along, and want to have the opportunity to tell them, Mike Rutherford style?
As Naomi says, Einstein's equations at least allow for travel into the future.