This is first of a series of posts on TimeTravel.
I'm interested in the philosophically interesting kind of time travel, by which I mean time travel to the one and only actual past, and, in particular, to your past, to the time when your parents were children, or to the time when you were a baby. I won't be talking about time travel to a parallel universe or to a different dimension of time. I will also be limiting my attention to cases of time travel at possible worlds that are as much like ours as is consistent with time travel; so, unless there is reason to suppose otherwise, I will be assuming that a time travel world is governed by laws much like our own laws.
I believe that this kind of time travel is logically and metaphysically possible.
This is now the majority view, at least among those who think and write about time travel . So this part of my view is not especially interesting.
However, I depart from the other defenders of time travel in insisting that time travel, while possible, is very odd. David Lewis, whose 1976 paper, "The Paradoxes of Time Travel", remains the classic defense of time travel, said that time travel is possible but also cautioned us that "a world where time travel took place would be a most strange world, different in fundamental ways from the world we think is ours". Lewis's paper succeeded in convincing many philosophers that time travel is possible. Indeed, he succeeded so well that some of the contemporary defenders of time travel argue that time travel isn't merely possible, it isn't that strange either.
I disagree. Time travel is possible, but I insist that it is also strange. Time travel worlds are backwards causation worlds – worlds where some causes happen after their effects – and this makes a huge difference to our common-sense view of the world and ourselves.
These posts are about one of the ways in which time travel is strange. I will be arguing that if our world turns out to be a time travel world, then we are not entitled to some of our common-sense assumptions about our abilities. If our world is a time travel world, then we have some abilities that common-sense says we do not have. And if our world is a time travel world, then we lack some abilities that common-sense says we have. But before I get to this, I want to point out another way in which time travel is very strange.
If time travel to the past is permitted by the laws, then Time-Slice determinism is false. Time travel to the past entails the nomological possibility of causal loops, and causal loops entail the falsity of Time-Slice determinism because, while every event in the loop might have a cause the loop itself has no cause (neither sufficient nor probabilistic). It follows, then, that facts about the present, together with facts about the laws, are not logically sufficient for facts about the future. And this means that Time-Slice determinism is false. More generally, Time Slice determinism is false at any possible world where the laws permit time travel to the past (and thus permit causal loops).
If Time Slice determinism is false, so too is that approximation of Time Slice determinism we use in everyday causal reasoning. Call this Local Time Slice determinism. We are used to assuming that we can predict what will happen next on the basis of what's going on right now, and nearby. We don't need to know what happened in the past, because the past causes the future only by way of the present. And we don't need to know what's happening in remote corners of the universe because there is no direct causation at a spatial distance. All this is very rough and ready, of course, but it works for our purposes, and in real life, and we are used to making predictions, and giving causal explanations, on the basis of Local Time Slice determinism.
Here is an example of the falsity of Local Time Slice determinism, at a time travel world. I'm in my kitchen. You are an atom-for-atom duplicate of me, in your kitchen, which just happens to be an atom-for-atom duplicate of my kitchen. If Local Time Slice determinism were true, it would be possible, at least in principle, to predict whether you and I will walk across our kitchen floors during the next minute – perhaps to get a cup of coffee. And the predictions would be the same. Either the prediction would be that we both do it, or that neither of us does it. What is there to distinguish us, after all? If you have a broken leg, so do I; if your floor is covered with landmines, so is mine. And so on.
But if we live at a time travel world, Local Time Slice determinism is false. The facts about me and my kitchen (you and your kitchen) at a particular time don't suffice to predict our respective futures. We need to also take into account facts about the future. If a time traveler turns up in my kitchen during the next few seconds, blocking my way, then I won't be walking across my kitchen floor. But you may still be walking across yours. So if we live at a time travel world, we can't use facts about the present to predict the future. And that's weird.
Weirdness is not the same as impossibility, and I don't know any good arguments for the claim that time travel is logically or metaphysically impossible.
So I'm going to assume that time travel, including the interesting kind, to our past, is possible. I'm not interested in arguing against it, or in defending it. I'm interested in something else. I'm interested in one of the stranger consequences of time travel. In a paper I wrote some years ago, I argued that one of the strange consequences of time travel is that whether or not a time traveler can kill a helpless unprotected baby depends on who the baby is. If the baby is her younger self, the time traveler cannot kill the baby.
My argument was simple and based on counterfactuals It requires you to consider a particular time traveler – I called her 'Suzy' – who has managed to travel back to the time of her own childhood, and who now stands, gun in hand, in front of the unprotected sleeping baby who is her younger self.
It seems that Suzy can kill the baby. What's stopping her?
But my argument says that she can't.
It was a very simple argument, with only two premises.
The first premise links the abilities of agents to counterfactuals. It says that Suzy can kill that baby - the one in front of her - only if it isn't true, on every occasion, that:
(Would Fail) If she had tried (again) to kill that baby, she would have failed.
The second premise says that Suzy fails this counterfactual requirement on ability because it is true, on every occasion -- both the times she tries and the times she doesn't try -- that if she had tried to kill that baby, she would have failed.
(Let her try as many times as you like. Let her return on a different day with a different gun. It isn't just that she will always fail. It's also true that if she had tried again, she would have failed again. And again, and again.)
Therefore, my argument concluded, Suzy cannot kill that baby.
And because my argument was perfectly general, I concluded that no time traveler can kill the baby who is her younger self.
This conclusion is surprising, even shocking. If it is correct, then some of our common-sense assumptions about what a person can do are false. We ordinarily assume that the facts about what a person can do consist of facts about what she is like, on the one hand – facts about her skills and her physical and psychological capacities – and facts about her more or less immediate surroundings, on the other hand. And we assume that these two categories of facts are independent of each other, in the following sense. We can tell whether a person has "what it takes" to do a certain kind of thing (to swim, to ride a bike, to shoot a close-range target) without asking whether she is able to do it in the particular surroundings in which she currently finds herself. We've seen Suzy at target practice and this, together with her unbroken limbs, steady nerves, and unwavering murderous intent, is all we need to know that Suzy has "what it takes" to kill babies in general and this one in particular. And we can tell whether a person's surroundings are favorable for the doing of a certain kind of thing without asking whether she has "what it takes" to do that thing. We know the kinds of things that protect babies - bulletproof vests, locked doors, bodyguards - because we've seen the kinds of things that protect other babies against various and sundry attempts to kill them. In my story of Suzy and the baby who is her younger self, none of these protections obtain. If we relied on our common-sense assumptions, then, we would say that Suzy can kill that baby. She's 'got what it takes' and her surroundings are as favorable as they can possibly be.
But if my argument is sound, we cannot rely on these common-sense assumptions. Even though Suzy has what we would ordinarily call the ability, the means, and the opportunity, she cannot kill the baby who is her younger self.
My argument has received some attention in the literature.d The responses are from philosophers who defend the possibility of time travel and they all say something like this:
"If Vihvelin's argument succeeds, it succeeds too well. It establishes the fatalist's conclusion – that no time traveler is ever able to do anything she fails to do. But if that's right, then time travel is impossible. But time travel is possible, so Vihvelin must be wrong. The time traveler is able to kill her younger self, and, more generally, the time traveler can do whatever the rest of us can do."
There are different diagnoses of how my argument goes wrong, but the consensus is that I am making some kind of mistake about counterfactuals or counterfactual reasoning or about the link between ability and counterfactuals.
Since my area of expertise is the free will/determinism problem and, more specifically, the metaphysics of the free will/determinism problem (and related questions about ability, laws, causation, and counterfactuals) it would be very bad if these criticisms were cogent. But after taking a serious look at these criticisms, and another look at my paper, I have decided that my argument was not mistaken. I have also come to understand my argument better.
In this series of posts, I will argue the following: First, my argument entails no fatalist collapse of 'will not' and 'cannot'; second, even if it did, this wouldn't show that time travel is not possible; third, my argument makes no counterfactual or fatalist mistake; it is my opponents who are confused about counterfactuals and ability; and fourth, my argument shows that a principle about ability that I call Snapshot does not hold at time travel worlds. I will propose a principle -'Vihvelin's Principle' -- that tells us why we should expect Snapshot to fail in the way that I say it fails.
There is also a bonus result. Investigating the constraints on our abilities at time travel worlds will shed light, by contrast, on the abilities we actually have. Some of the distinctions that I draw here will, I think, provide us with the diagnostic tools for a more precise articulation of the free will/determinism problem.