My time-travel literature review continues with Pastwatch: The Redemption of Christopher Columbus. It was, all in all, better than I was expecting, although at several points I feared it would be much worse. The pacing is off, the emotional arc stumbles, and Card’s trademark dialogues — veering clunkily between exposition and didacticism — remain eminently skimmable. But the plot is clever, the character of Columbus himself is vivid, and some of the ideas are quite striking.

In any event, if The Anubis Gates is the one kind of time-travel story — what Stanislaw Lem would call a “time loop” — Pastwatch is the other. As in the timeiverse of Back to the Future, mucking about in the past changes the “future,” so that different things happen. Only the time traveller is aware of these divergences, because only he or she remembers the original future. Indeed, thanks to the temporal rewriting, that original future is in a sense obliterated, replaced by the new future. The original now even exists only as a memory.

The thing that I like most about Pastwatch is that Card takes this latent paradox and mines it for its moral significance. On one level, time travel is thus a fairly obvious metaphor for playing God, a metaphor that Pastwatch takes slightly literally, if usually playfully. On another, the novel tees up some quite unsettling questions about the moral duties the time traveller owes to the residents of the two different timelines.

These questions are hard enough in a universe in which time flows n only one direction. Trying to resolve moral questions that involve potential people is extraordinarily difficult; as Derek Parfit has described, it forces us to confront quite possibily unresolvable issues about personal identity and morality through time. If I change the world slightly to delay a birth, is the baby born five minutes later the “same” moral agent as the one who would have been born five minutes earlier? Every action has the potential to cause some future people to exist and others not to exist. What moral principle can tell us how to balance one potential nonexistence against another? And in what sense is my moral duty towards other future people different from my moral duty towards my own future self? These questions are all real stumpers.

With future-altering time travel to the past, however, the problems just get worse. There are some oogly technical issues involving the time traveller’s own personal identity, issues like those involved in the teleportation hypotheticals. Since the point of the teleportation examples is supposed to be that of shedding some light on the problem of personal identity through time, one might think that time travel would be the simpler case. But a discontinuous movement through time, in the wrong direction, and which may causes the previous self not to have ever existed—well, that doesn’t satisfy very many of the traditional criteria for a stable identity. (For example, yes, the traveller has memories of his or her previous existence in the future, but those memories are now in a very real sense completely false, since that future, by hypothesis, will now never exist.)

Worse still are one’s moral duties to others from the futures. By travelling back to the past and changing it, you will arguably be causing the people who lived between the point of departure and the point of intervention never to have existed. Their timeline will be written out of existence and replaced with a new one. Have you harmed them? The problem is more complicated than one involving potential people in the future, because the innocents from the recent past whose existence you will obliterate by going back in time have already lived their lives. You are taking away not only the existence they might have, but the one they already had. Is that moral? Is it even possible? Does the difficulty of formulating the question suggest that “changing the past” will not actually undo those existences? If so, doesn’t that that undermine the whole project of going back in time to undo past suffering?

My brain is not designed to handle these questions. One might take this as an argument against the possibility of time travel of this second, future-rewriting sort. But then again, there are all sorts of things my brain is not designed to handle. Elliptic integrals, say, or the muscle movements involved in whistling. I don’t think I’d want to take my own bafflement as an argument against the possibility of either.