The real reason that we’re not deluged in time travellers is this: the arrival of the time traveller is an event like any other and branches a new timeline off from the reality he went back to. So, it’s never crowded. And because his arrival creates a new branch he can do all those paradoxical things you hear about. He can kill his father as a little boy to prevent his own birth. He can meet himself. It won’t matter, because he isn’t affecting the timeline that leads to him, he is changing events on a new timeline.’
