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.

What we are proposing,' Alicia said, 'is that the laws of physics are such that causality violation is subject to a form of version control, one that prevents a forking of history. That instead of causality violation creating an alternate universe, one version of history is outright overwritten by another. One past is replaced with another future. Which means that the memories of the past of the people in that future are replaced with memories of a different past.'

Carson interrupted. 'Including the memories of any—'

'Purely hypothetical—'

'—time travelers.'

'So take our time traveler from the traditional story,' Carson continued. 'He leaves his utopian future for the past. He kills the butterfly. The Magna Carta is never written. He returns to the dystopian future that his misstep created. But he doesn't see it as a dystopia: he sees it as home, the world he grew up in, the world he left to go back in time. Because he doesn't remember that first future, and has no other world to which he can compare this one. Maybe he even sees it as a utopia. Maybe everyone does. Maybe everyone in this dark place believes that they live in the best of all possible worlds.

First, the idea of the multiverse is essentially the fantasy of preserving perfect information. One of the hard things to deal with in life is the fact that you destroy potential information whenever you make a decision. You could even say that's essentially what regret is: a profound problem of incomplete information. If you select one thing on a diner's menu, you can't know what it would have been like to taste other things on it, right then, right there. When you marry one person, you give up the possibility of knowing what it would have been like to have married any number of others. But if the multiverse exists, you can at least imagine there's another version of you who's eating that other thing you thought about ordering, or who's married to that other man you only went on two dates with. Even if you'll never see all the information for yourself, at least you'll be able to tell yourself that it's there.

'The second reason the multiverse seems like such a neat idea is that it gives human beings just an incredible amount of agency, which they can exercise with the least effort. Why, Carson here created an entire alternate universe when he ordered hash browns on the side of his French toast instead of bacon—'

'Ah, I should have gotten bacon, how could I forget,' Carson said, and attempted to hail the waitress.

'But the history of science shows that any theory that covertly panders to the human ego like that, that puts humans at the center of things, is very likely to be found out wrong, given enough time. So, just for the sake of argument, let's assume that there's just this one universe, and we're stuck with it. What happens to our time traveler then?

Then you would have acted to change history, but you would have replaced one history with another. You would not be able to know what past version of history you would have altered, because that history would never have happened. Nor would you be able to know what you had done in the past, or that you had done anything at all in the past, or that you had even been to the past. You would not be able to compare histories in your mind; if you entered the causality violation device knowing this, then you would realize that you would be trading one set of memories for another, and that there would never be any evidence, in your mind or in the world, that you had done this.

But I cannot accept a vision of You as an engineer who spends His days maintaining the machine of morality. I cannot take the idea of You as an optimizer, introducing evil into human affairs in an attempt to create the best of all possible worlds. I cannot bear this cold mathematician's God who sees all the universe as nothing more than an elaborate problem to be solved. Such a world is a world with no meaning, one in which one history is no more or less preferable to any other.

But we think that if a human were to violate conventional causality—'

'By time traveling—'

'Please, please don't call it that. If a human were to violate causality, the experience from her point of view would be similar. You would act while in the past, but not be able to recall your actions later, because that period of time for you would be lost between histories: the old one you left and the new one to which you would return. It would exist outside of the normal course of events. It would be, in a very real sense, lost time.