[I’m pushing this 2014 post to the top because it is related to chapter 10, the time travel chapter, in my recent book ‘Philosophy of Time: A Contemporary Introduction. It concerns an issue I’d no room to post there, about why we would help a time traveller at all…]

In Edge of Tomorrow, a character played by Tom Cruise, computer-game-like, keeps going back to the same point in time every time he dies. And he dies a lot because he is in a battle against aliens. Generally, I’m not a fan of the ‘changing-the-past’ plotline of many time travel stories. It works best in Twilight Zone circumstances, where paranoia and Oedipal fate issues make it twisted and interesting. But the idea of, say, going back to save your now-dead husband just has a lot of problems. You go back and stop it happening. So does that mean that, once you stop it, it never happened? Then you don’t go back. So who stopped it happening? You didn’t — because you changed it so he survived and you didn’t go back.

This is often called the Grandfather Paradox, after a thought-experiment where someone goes back for a far less compassionate reason. They go back to kill their own grandfather. But in succeeding, they prevent their own birth. So, they don’t exist to go back. They don’t kill their grandfather. Their grandfather survives. They are born; they go back; they kill their grandfather; so they cease to exist.  So, they don’t exist to go back. They don’t kill their grandfather; their grandfather survives. They are born; they go back; they kill their grandfather. So they cease to exist.  So, they don’t exist to go back. They don’t kill their grandfather; their grandfather survives. They are born; they go back; they kill their grandfather; so they cease to exist.  So, they —

But the grandfather paradox doesn’t need all this killing. All you need is knowledge about the world, the will to go back to change the past, and then to succeed in doing so to get the same paradoxical situation.

If I knew that my housemate had eaten my mandarin oranges, then I travelled back to stop them, then stopped them, the same paradox results. Why did I go back if I succeeded? They didn’t eat them if I succeeded — thus wiping out my reason to go back. So I don’t go back (at least for that reason) and so my housemate eats my oranges, and …

Anyway, I talk about this elsewhere, calling it The Intention Paradox. For more, see this post Changing the Past: The Intention Paradox (it’s password-protected. I’m happy to provide it if you contact me).

Edge of Tomorrow highlights a different issue with time travel: When the protagonist goes back in time, where does everyone else go? And why the hell would they help him?

The answer I propose is an approach to time travel which allows both changing-the-past – or, at least, the appearance of it — and also explains why anyone else would help.

Why Help the Time-Traveller?

As an example of the issue, in Edge of Tomorrow, there is a training scene where one character ‘re-starts’ the main character deliberately so that he can come back in better condition. Protagonist A injures himself so badly he is out of action. B knows this about A and that A needs to go back in time to be better. A can only go back in time by dying. So B kills A. The story then continues from A’s perspective: A wakes up back at the beginning of events, ready to go again. But what about B’s perspective?

B’s Perspective

I think one can ask a number of questions about what happens from B’s perspective:

(a) What happens to B after B shoots A? What does B experience next?

(b) Does time just stop or cease to exist for B and everyone else like B, i.e., the rest of the human race and aliens (and trees, cats, dogs, the sun, etc. etc.)?

(c) Or does time go on? In that case, how does it go on?

If it just stopped, this would be effectively consigning everyone at that moment to oblivion. Why would you help that happen? Here’s why I wouldn’t help A:

If A’s death consigned everyone else to oblivion, I would do my best to keep him alive, not to kill him. I’d lock him up and put him in a coma. (He’d become the Red King in Alice Through the Looking Glass.) Even if that meant being defeated by the aliens, at least there’s a chance against them. But there’s no chance once he dies.

Yet, if time continues on, that would be equally complicated for one’s motivation. Consider the situation just after B has shot A. B now knows two things:

1. A is gone back to some past time with the knowledge of these events. B is not gone back. Instead, B is still stuck in this course of events.

2. Whatever happens after that past time to which A has gone is not what leads to this (A has gone back to avoid them).

So whatever happens after A goes back is — what? Not really happening? Is that comforting for B when they are stuck with A’s dead body (and having to explain why A and B made the body dead)?

Is it comforting when the aliens invade?

“Oh, in some other universe/timeline/whatever, this isn’t happening. I’m sure glad I shot that guy.” says A

as the sky darkens overhead with the still overwhelming threat.

Reverse Physical Processes, not Time

Here is the way to solve this problem. Now, I don’t think this solution works for the actual world, but it is at least possible and it also makes sense of character motivations. (It also makes sense of the stupid thing Superman did in the 1970s film. Not sure about similar ideas, such as those in Looper — because why was he wearing shoes???).

What we need to do is distinguish between the arrow of time and what I’ll call here the physical sign or appearance of the arrow of time (something I discuss in my latest book’s chapter on physics).

The arrow of time is the actual order of events in time. But the physical sign of time’s arrow is the typical physical pattern that maps onto such order. I mean however these are only typical sequences of physical events (actually, I don’t like this term ‘physical sign’ but am not sure of a better term –).

Here is an example of the difference: Imagine an egg smashing onto the ground. Now imagine a smashed egg spontaneously leaping off the ground and reassembling into the shape of a solid egg.

At the very least, I think the following is the case: the order of the first situation is a physical sign of time’s arrow — because it is a typical sequence of events. The second is a physical sign of reversed time.

I say at the ‘very least’ they are a sign. They might be more: they might actually be time reversing: when an egg reassembles, it is a case of time reversing. But I don’t think this has to be the way of putting it; this depends on how you understand temporal order (the arrow of time)  and its relationship to the typical sequence of events in time.

Reducing Time’s Arrow

There is an issue here with whether or not one should or can reduce time’s arrow to something else to do with physical events. As I discuss in my book, over the years, attempts have been made to reduce temporal order to causal order and to increases of entropy.

If you think that the arrow of time just is — or just is a product — of typical sequences of events, then you might also think the arrow of time follows the typical sequence even if it seems to be the opposite of the typical sequence, i.e., if we have a reverse of a typical sequence, then we also have a reversed arrow of time. E.g., when the smashed egg reforms into a whole egg, the arrow of time reverses (for that egg at least),

In that case, time does reverse for one process relative to other processes: time runs backward for the smashing egg relative to other processes so that, to other processes, it seems to reassemble (also, it reverses for the other processes relative to the one process; for the smashing egg, the rest of the universe runs backward).

I’ll put aside whether or not there is a reducible arrow of time here. As said, this is not a situation for the actual world. In any case, for an observer watching the processes in the imagined situation, one of these processes seems to reverse, while the other does not. This does not require a reversal of time’s arrow; it only requires that one of the processes reverse (albeit, if that’s all there is to time’s arrow, then it is enough for the arrow to reverse).

Were you to witness both the reassembling and the smashing of the egg, the sequence of events would be of the same temporal order for both. It’s just that one — the re-assembling egg — is unusual. It’s still the case that, for you, in the strange event, the egg is smashed and then the egg is whole; for the typical event, the egg is whole and then the egg is smashed. Temporal order is captured by the ‘and then’: Smashed egg followed by whole egg.

What Happens in Edge of Tomorrow

One can make sense of the plot of Edge of Tomorrow if one does the following: When A ‘goes back in time’, A doesn’t necessarily go back in time at all. What happens is that everything in the universe reverses processes back to how it was at a certain point in time. Time itself doesn’t reverse – at least, if temporal order is more than typical physical order. All that happens is that physical signs of time reverse (again, this might be all you need for temporal order reversing — but I’m not assuming that).

The process is like a fishing line: you throw it out then rewind it back in. Then throw it out again.

Let’s develop the egg example: At the restart moment, to which A keeps going back and waking, an astronaut accidentally drops a raw egg in space. That egg floats away out of reach and tumbles throughout the day until, several hours later, it smacks another astronaut’s visor. Say A dies just as the egg smacks the astronaut. Then what happens is that all physical processes, including the egg-smack, reverse. The egg’s disparate parts are pulled off the mask toward each other, and reassemble back into an egg. Following that, the whole egg floats back the way it came, eventually into the fumbling hand of the original astronaut.

The important point is this: time itself does not reverse. It continues on, filling up the hours. It is still several hours later when the egg finally reaches the original location in the clumsy astronaut’s hand. When it does, at that moment, A wakes up. The astronaut drops the egg, and it begins its slow fall toward smacking the other unwitting astronaut several hours later (unless A dies in the meantime).

Over and over and over again, this happens without anyone remembering the repeated sequences. Except A: A remembers the unreversed sequence of physical events (but, so far as the story goes, not the reversing of the sequence).

Answers

So, we can answer the questions as follows:

(a) What happens to B after B shoots A? What does B experience next?

B, like everything, reverses physical processes: the bullet flies out of A and back into the gun, and so on and so on.

What does B experience next? I have really no idea. But what we do know is that, in the story, only A remembers what happens. So I guess B, like, everyone else, doesn’t remember anything up until the restart — including the reversal (again, A doesn’t remember that part of the process).

(b) Does time just stop or cease to exist for B and everything else like B, for example, the rest of the human race, aliens, trees, cats, dogs, and the sun?

No, time continues on. There is just a period of time where physical processes reverse. If A spends a day before dying, then given the reverse takes as long as going forward, when A wakes up again, two days have actually passed. But the physical effects of those two days cancel each other out.

(c) Or does time go on? In that case, how does it go on?

It does go on. It goes on but without there being a noticeable change for anyone except A.

In that case, I can make sense of B killing A. The physical events to that point are leading to disaster. By killing A, B literally reverses events back to the way they were at an earlier time. But what doesn’t happen is that anyone actually goes back to an earlier time. This isn’t time travel — it’s the appearance of time-travel.

You can see more on why this is the case in my chapter on time travel.

Conclusion

I don’t think this could happen in the actual world. But putting aside even the idea of reversing physical processes, given how our world works, I don’t think it could happen here.

First, I don’t think you could have an observably identical universe after the reverse. The problem is time-lag. I’m not sure it could work given some of the observed processes (distant galaxies) are too far away to look reversed in a day. Astronomers might be confused by what they see (“hey….this star is in the wrong place”; it might make an interesting plot point earlier on in a mystery story with a plot like this).

The second problem is the account presupposes a single moment for everything, i.e., a kind of universal simultaneity, from which processes reverse (or time reverses, so far as the characters can tell). When A dies, everything at that time reverses. But universal simultaneity is disputed in the current model of time in physics (see again my chapter on physics).

I think that, if this involves a universal reversal, this needs a substantial or independently existing time — or at least time which, with respect to the physical world, is substantial or independent. This kind of time, a time not just derived from relations between physical events and physical things — the kind of time discussed by Shoemaker in his paper on time without change (jstor; sorry I can’t find an open access copy), is needed because:

A. The two periods in which change occurs and then gets cancelled out are indistinguishable from each other given only their physical processes; if it’s universal, then the processes which reverse are all the physical processes. What distinguishes the forward and the backward sets of physical processes is only that: – that one is forward (whole egg, smashed egg), the other backward (smashed egg, whole egg).

It looks like we need something physically independent for the temporal order by which one distinguishes forward and reverse.

B. No-one except A remembers it, and A’s physical body doesn’t ‘remember it’ either (being no longer dead). After each new reverse, (excepting A) we have the equivalent of the unobserved and unremembered period of time in which no change occurs. And even with A, given the story, the reverses are unobserved and unremembered. This makes them like frozen regions in Shoemaker’s paper. Whatever happens during that time is traceless afterwards; there might as well have been no change. What happens is the equivalent of a tree falling in the forest when no-one is around, except that, before anyone turns up, the tree gets back up again.

For more on this, see the chapter on epistemology of my latest book.

There is one more thing this story needs: that it is possible for psychological or conscious processes, e.g., memory, etc. to survive independent of physical processes. Otherwise, A would have their psychological/conscious processes reversed along with the physical processes. They’d lose their memory as well. They might have the power but it would do them no good. In fact, it might just curse them to an eternal loop — one very similar to the original situation in an episode of Star Trek (‘Cause and Effect’).

In any case, I suppose all that matters is if the characters believe that this physical reversing is what is going on. if they do believe this — or at least A and B believe it — we can make sense of why B would shoot A, or help A at all.