The Multiple universes hypothesis states that there are infinite number of universes, collectively known as the "multiverse". If a person is about to travel back in time, he will create his own parallel universe upon arrival in the past. So if he kills "his" grandfather, a paradox would not occur because the grandfather that he has killed is not his own grandfather from the universe from which he came, but that of the version of himself in the universe he is now in. I believe string theory also relates to this? What would happen if a person went back in time to meet themselves? Wouldn't this also mean our universe was created by a person who travelled back in time which we would surely have evidence of?