Killing Grandpa: How to Make Decisions in the Past

Deciding in the past

Posted Feb 28, 2012

Time travel. We're all fascinated by the idea. Philosophers are interested in time travels into the past in part because they are interested in what our universe would need to be like in order for anyone to travel backwards in time, and in part because they are interested in how time travellers would make decisions and in what sorts of decisions they might make. You might wonder why that is at all interesting given that there is little prospect of there being any time travel into the past any time soon, and indeed, it may be that the laws of nature in our world mean that time travel is physically impossible. The interest lies in the fact that thinking about how decisions might be made in the past, draws our attention to various interesting features of how decisions are made at any time.

What do philosophers have in mind when they are considering time travelling scenarios? While there is debate, many philosophers think that we need to be very careful when think about travelling into the past. They think, for instance, that many of the representations of time travel that we see in film and on television are misleading. Here is a common one. Billy travels backwards in time to the 1930s. Billy has done some history and is aware that Hitler's coming to power in Germany in the thirties resulted in terrible events. So Billy decides to change what happened in the 1930s. He arrives in the past equipped with his modern pistol, he discovers a youthful Hitler before he has come to power and before he has much in the way of security, and Billy assassinates Hitler before the national socialists come to power under his sway. Thus the rest of history is changed: is no second world war. When Billy travels forward in time to the location from which he left - his present - the time he returns to is changed in various ways as a result of the changes he made in the past.

Most philosophers think that this sort of time travel scenario is impossible: that is, the scenario is inconsistent. There is no way our world could be, in which time travel like that occurs. What these philosophers object to is the idea that someone could travel into the past and change that time from being one way, to being some other way. This, they hold, simply makes no sense. Change is always change with respect to something: usually it's with respect to time (you and I change by being one way at one time, and a different way at some other time). But there is no dimension in which a moment in time can change.

If one accepts this widely held view about the past, then all sorts of interesting questions are raised about the nature of decision and agency. The first thought you are probably having is that if the past cannot be changed, then we can make no sense of time travel at all. After all, when a time traveller steps out of the time machine she is thereby changing the past: she is changing it from being a time when she was not there qua time traveller, to being a time at which she is there qua time traveller. Even stepping on an ant will amount to changing the past, even if only in a small way.

Here is where philosophers want to distinguish between changing the past, and causally interacting with the past. Changing the past is altering some moment in time from being one in which some event x happened, to being one in which x did not happen. Causally interacting with the past is being in the past, and being part of the events that unfold, and causing various things to happen in the past. Why is the second not the same as the first? Surely if I causally interact with the past, I thereby change it. No. Suppose that Billy travels back in time and meets the young Hitler and tries to assassinate him. Billy fails in his quest - we know he fails, because Hitler did, in fact, go on to become leader of Germany. This is a fact about the past, and as such is something that will not be changed. But Billy causally interacts with Hitler when he talks to him, and when he tries to kill him. Indeed, for all we know Billy tries to kill Hitler multiple times, and it may be that it is these repeated assassination attempts that significantly impair Hitler's psychology, to the point that he becomes the person he does and heads down the road that we all know he takes. If that were so, then Billy's time travelling would, as it turns out, be the cause of the second world war. Billy does not change the past, he merely makes the past the way we now know it to have been.

The crucial though is that when someone travels back in time, what they do in the past has just the same sort of causal effects as what you and I do in the present. If a time traveller drops a glass on a hard floor, it most likely shatters. It is just that the things that the traveller does makes the past the way that it is. If the time traveller shatters a glass, then it has always been the case, ever since that event, that a glass was shattered at that moment in time. The past is not changed from being one in which no glass shatters, to being one in which the glass does shatter.

Here is where the whole time travel scenario becomes interesting from the point of view of thinking about decisions and agency. What makes someone who has travelled into the past interesting is that he, unlike you and I, already knows a whole bunch of things about that past moment and about what will happen later in the future. Suppose that you remember a tragic day when you were six and you left the gate of your house open and your dog escaped and was lost forever. That was a terrible time for you, so you decide to travel back to that day.
On the assumption that you cannot change what happened in the past, there is an interesting question about what the older wiser time travelling you ought to decide to do when you travel back. You know that the gate was left open. So you know that there is no point deciding to go back in time and prevent yourself from leaving the gate open. That action is doomed to failure, since you clearly remember that the gate was open. So it would be irrational of you to travel back in time with the intent of closing the gate, unless you think that your memory from that time might be compromised in some way. You also know that the dog escaped, never to be seen again. So it would be irrational to try and prevent the dog from having escaped. But you do not know what happened to your beloved dog. You have always feared that something bad might have befallen the dog. So here is a perfectly rational plan. You travel back in time and wait until your younger self leaves the gate open and the dog escapes. Then you grab the dog as it decamps from the yard, and take it to live with a nice family in the next town.

That action is consistent with everything you know about the past. So when you travel back your actions bring it about that your dog led a happy life. This also explains why the dog was never seen again: it was living with a new family a town away. You did not change the past, you were responsible for the disappearance of the dog.
What is interesting about this case is that only some decisions you might make about what you will do in the past are rational. It would be silly of you to decide to prevent the dog from having disappeared, given that you know that the dog did disappear. You have good reason to think that the dog did disappear, because not only do you remember this happening, but so does everyone else in your family. But now consider a case in which your knowledge is rather less certain. The moa were large flightless birds that lived in New Zealand and were hunted to extinction by the Maori. That there are no more moa in New Zealand is undoubted, and that there were hunted by the Maori is beyond debate. But suppose you are a huge fan of the flightless bird, and you have a time machine, What can you do about their plight?

Certainly you cannot travel in back in time and explain to the Maori people that continued hunting will result in the birds becoming extinct. You cannot bring it about that the moa do not vanish from New Zealand. But no one really knows what happened the last of the moa. It is consistent with everything we know about the past, that someone collected a reasonable number of moa and left New Zealand with the birds. It is not reasonable to suppose that anyone moved a whole bunch of moa to someone else on the planet and that those birds survived there, since there is no evidence of any colony of those birds today. But it is perfectly possible that someone removed a whole bunch of moa from New Zealand and took them to a different time altogether. There is a way of saving the moa from extinction: you travel back in time, collect the moa and bring them back to our own time or to a time that is in the future relative to our location. Nothing about what we know rules this out, so it would be perfectly rational for someone to decide to save the moa from extinction in this manner.

In effect, what makes these cases interesting is that our knowledge of what did happen, changes the sorts of things that it is rational to attempt to do when in the past. The usual way to save a bird from extinction is to change its environment to one that is more conducive to its reproducing. The usual way to prevent the trauma of the loss of a dog is to make sure the gate is closed. But neither of these strategies would be rational ones, given what any time traveller knows about what did happen in the past.

Paradoxically then, the less a potential time traveller knows about some time in the past, the less he or she is constrained in terms of which projects it would be rational for her to plan to carry out in the past. Since we know comparatively little about times that are very far back in time, there are vastly more projects that it would be rational for a time traveller to attempt were she to travel back to any of these times. We know a great deal about what happened yesterday, so if someone travels back to yesterday there are comparatively fewer actions that such a person can rationally attempt. Ignorance, then, really is bliss if you are a time traveller.