“You could consider the idea of the multiverse, and think of it as something like a tree—that is, the universe we live in is one of an uncountable number of branches of possible universes, created by random chance and the decisions of sentient beings. So, for instance, when I rang you up in the morning, there was a possible future universe in which you answered the phone, and another in which you did not, and by answering the phone you put us in one universe and not the other. In that instance the time traveler doesn't just move from the future to the past and back to the future: he moves down one branch of the universe, toward the root that's back at the beginning of time, and back up another branch.”