If, for whatever reason, by whatever means, you travel back in time, you absolutely must not find and kill your own grandfather before he has had a chance to procreate. Doing so would break the Universe – because if your grandfather never has offspring, you would not be born and could not travel back in time to kill him. Quite how the Universe would resolve this conundrum is unclear. Maybe the laws of physics would simply prevent it from happening in the first place. Stephen Hawking said so.