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The philosophy of time travel

Professor Andrew Brennan

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Matt Smith: Hello and welcome to a La Trobe University podcast. I'm your host and La Trobe University’s resident Time Lord, Matt Smith and with me today is Professor Andrew Brennan, a philosopher and Pro Vice-Chancellor of Research at La Trobe University. Thank you for joining me Andrew.
Andrew Brennan: Always a pleasure to be here Matt. Thank you.
Matt Smith: Now, we’re here today to discuss time travel and what I find when I start a podcast with an academic is everything that I think I know about a subject I actually don’t know. So to you, to a philosopher, what is time travel, and why aren’t you leaving that to the physicists?
Andrew Brennan: Oh, that’s an interesting question. Time travel is defined most of the time by saying it’s when you set out on a journey and for some reason or another, the duration of your journey doesn’t match the duration of the time that elapses when you get back to the starting point or when you get to the finishing point. Normally, if you go out and you fly somewhere, you know how long that journey’s going to take. You make adjustments for world time, so you’re flying from Melbourne to Sydney, there’s no time adjustment. You’re flying from Melbourne to Singapore, there’s a three hour time adjustment. When you add in the time adjustment and your flight time, you find out that you’re exactly the age you thought you were going to be when you arrived. Your journey, in your own personal time, has taken the same time as the journey measured by clocks, measured by other external devices.; But we know that up in the space station, there are astronauts who orbit the earth at thousands of miles an hour and it just follows from bits of physics that I don’t understand but which I believe because the physicists say they’re true, it follows that these astronauts, when they come back to earth after a few weeks in space, are actually a few seconds younger than they would have been if they’d actually stayed on earth. And this is a version of what’s known in relativity theory as the twins paradox, that if you have twins that are born at the same time, but one of them spends a long period of time in space, flying at very high speeds, say at close to the speed of light, then the twin that’s flying close to the speed of light seems to run his body processes much slower relative to the twin who stayed on earth. And so although the two twins might die at the age of 80, if one of them has spent forty years in space as part of their 80 years of personal life, they’ll die at a different date altogether from the date on which their twin dies.
Matt Smith: So wouldn’t that be an aspect of your biological processes change when you’re in space and you adapt to that. The actual concept of time and the way that time flows and time duration itself isn’t changing.
Andrew Brennan: Yes, it is. The twins paradox is an example of what physicists call an unproblematic kind of time travel, and it’s unproblematic because you’re actually travelling into the future, which is what we’re all doing, all the time, when we travel in time. You and I are sitting here travelling in time. If you measure the nanoseconds that go past as we’re speaking to each other, billions of them have gone past and we’re moving into the future. Now in the twins paradox in relativity theory, it’s not a paradox at all actually, because apparently the mathematics makes it work out okay. If you measure the particular time of the twin who’s in the space ship by things like their pulse rate or the ageing processes in their body, or the time it takes them to digest a meal, then by their particular time, they’re still experiencing minutes, hours, days. But these minutes, hours and days are not the minutes, hours and days as measured back on earth by the time that the other twin is being measured in. And that’s why in philosophy of time travel, and also in the physics of time travel, there’s often made a distinction between what’s called local or particular or personal time, and external time, and so you would say that in the case of the twins, the one that’s flying in the space ship, very very very fast, that twin is actually, in terms of his local time, ageing at just the same rate as the twin who’s on earth, but in terms of external time, it’s ageing much more slowly than the twin that’s left on earth. And that’s called sliding into the future. That is a kind of simple form of time travel.; It’s the same kind of time travel you get in other movies, if you ever see those movies like old aliens movies, when people were travelling long voyages to go on search for the aliens, they go into suspended animation machines, and again the idea of the suspended animation machine is you slow body processes right down so that the organism is just kept alive. That means that although the journey might take four years in external time, the person in the suspended animation state has maybe only aged by twenty minutes in terms of their body processes.; So that means they’re travelling into the future. They are going to see bits of the future that they would not have seen had they stayed on earth and aged at the normal rate.
Matt Smith: I think a modern equivalence is maybe Event Horizon and if you ever saw that, they travelled through space in suspended ... but the suspended animation concept, I agree with you, with the movie, was a bit weird. Austin Powers, he was suspended from the sixties. So what about going back in the other direction then? That’s time travel in a forward way. Is time travel going back in time, from your perspective, is it possible, and if it is, wouldn’t it already have happened? Wouldn’t we know about it?
Andrew Brennan: Well, time travel back in time is a really interesting case, because time travel back in time actually doesn’t involve going back in the time traveller’s time. If you make that distinction between external time and particular time, then in terms of the particular time that’s local to the time traveller, they’re still going forward in time and that’s one of the first mistakes people often slip up into when they’re thinking about time travel, they think the time traveller is somehow going in reverse. They’re not. They’re going forwards in their own particular time.
Matt Smith: So their timeline is still locked and current, but they’re just physically ...
Andrew Brennan: They’re going forward into the past. And so how are they going forward into the past? They’re going forward in their own local time but they’re going into the past by external time measures, like the Greenwich Mean Time clock.; Now what’s interesting about that kind of time travel is of course it gives rise to puzzles and paradoxes that the sliding into the future doesn’t give rise to. And the common kind of paradox is they are the ones that are most often discussed, are things like, well, could you go back in time and kill your own grandfather, even before he met your grandmother? And in that case, what would happen to you? Would you by that way be committing suicide in an indirect way? Or the other thing is the visiting paradox. I mean, if you can travel in time, then maybe if I've got a time machine in my office, I can fix it so I go back and visit myself the week before, in my office. And so then the question is, is that possible? Can I go back and visit myself in my office the week before? The philosophical arguments about this are really fairly basic. For instance, if you were sitting in your office at ten past two on Monday, and then the next Monday you travel back to visit yourself and you stand and have a chat with yourself, then you are both sitting and standing. And the argument is, isn’t that contradictory? How can the same person be both sitting and standing at the same time, because it’s ten past two on Monday afternoon. The way out of that on the story that I've been telling in my time lectures, is by thinking of us as time worms that go through space time. We’re four dimensional worms. Our timeline is like a worm through the fabric of space time. So I've got three spatial dimensions and one temporal dimension.; So what happens when I travel back in time? Is it a later stage of my time worm meets an earlier stage of my time worm? So, although I'm talking to myself, if you think about the worm as a whole, it’s actually one temporal part of me that’s talking to a different temporal part of me, and that temporal part of me that’s standing, is different from the temporal part of me that’s sitting down. So there isn’t a contradiction. There’s no contradiction in saying that I'm standing and sitting at the same time, because I have one temporal part that’s standing and I've got one temporal part that’s sitting, and there’s no contradiction. There would be a contradiction if I was sitting and not sitting at the same time, or if I was standing and not standing, but I can be sitting and standing at the same time because I've got two temporal parts doing that. And that’s no more puzzling – if you unwind my timeline, my worm, and go forward, sometimes in my office I stand, sometimes in my office I sit. Is that a contradiction? That sometimes I'm sitting in my office and sometimes I'm standing in my office? No, because the slice of me that’s sitting isn’t the same temporal segment of my time worm as the one that’s standing. So long as you can slice the worm into segments, you can have standing segments and sitting segments.
Matt Smith: But one can influence the other at that point, can’t they?
Andrew Brennan: That’s right, and they can talk. And remember, if time travel is real and it happens, and we do it, it’s not that I was sitting in my office at ten past two one Monday with no one there and then somehow magically that scenario gets re-written by the time travelling, I'm sitting in my office that Monday talking to myself. And that’s really interesting. And I'm talking to another slice of myself who’s come back from the future to have a chat with me. And so long as you tell that story consistently, there’s going to be no contradiction in it.
Matt Smith: Yeah, that’s where the interesting travel starts when you’re trying to keep it consistent that way.
Andrew Brennan: And that’s the only thing that philosophy’s interested in. We don’t care about the physics, we don’t care about the mechanics of the time machine. What we do care about is that there’s not a contradiction in the story.
Matt Smith: But now we could solve this now. In two minutes’ time I'll come in through that door, okay, an older version of me, and then the problem will be solved.
Andrew Brennan: That’s right.
Matt Smith: But that’s the thing. We would already know because humans, if they did ever achieve time travel, would straight away stuff things up because that seems to be human nature. We wouldn’t be able to travel back in the past and remain discrete about it.
Andrew Brennan: Yeah, you say we go back and stuff things up. And that’s a bit like the plot of Back to the Future, if you remember Marty McFly who nearly stuffs things up by substituting himself for his father in the car accident. And then what happens is, pictures start to grey out and hands start to disappear and so on. That’s the kind of fantasy. But back in the ‘50s, physicists had already worked out the physics of time travel and some of them had argued that in fact you wouldn’t be able to change very much in the past if you did travel into the past, even if you tried to change it. And that’s one of the ways that people find a solution to the grandfather paradox. I know from my own experience of first thinking about this many years ago ...
Matt Smith: I thought you were going to say, from my own experience of time travel ... Sorry.
Andrew Brennan: That would be good. I'm just thinking from my own experience of thinking about this, the first time I thought about it, I thought, we’re free agents. So when you’re travel back into the past, of course you’re just as free as if you travelling forward into the future. So if you go back in the past, you get into the TARDIS or some other time machine and you crank off into the vortex and emerge some time in the past, there’s no limit on what you can do. And so the notion that you cannot change what’s there seems really odd. But then if you think about it, it’s not so very odd. There are all sorts of things that we can do that we don’t do. There’s all sorts of things that are possible for us that we don’t do. And one of the things about the past is, whatever choices have been made in the past, whatever happened in the past, is done. It’s finished, and it’s fixed, and that’s why Aristotle, the Greek classical philosopher, Aristotle said, and the past cannot be changed. The past is fixed and necessary, so whatever you do when you go back in the past, you can’t change it.; Now that doesn’t mean you can’t tell stories that are really quite good fun. The Cambridge physicist John Barrow for instance, has a very nice story he tells in a recent book about the grandfather paradox. The idea of the grandfather paradox is Tim’s actually not wanting to have lived the life he’s lived. So he wants to go back into the past and shoot himself. He wants to kill himself. So he travels back to when he was a baby and he was a baby actually being held by his grandmother at the time. So he’s going to arrive and just wipe himself out. He’s got a little pistol and he’s just going to hold it up and he’s a few yards away and he’s going to try and shoot the baby. And he doesn’t care if he shoots his grandmother either. He’s just going to try and shoot the baby. The time machine works. He’s back in the past, and he’s ready to shoot himself as a baby. He holds out the gun and fires. But he misses, because of a shoulder injury that always makes it very difficult for him to get his arm lined up when he’s holding a gun. His grandmother was startled by the noise of a gun going off, and dropped the baby and the baby got an injured shoulder.; Now here’s a coherent time travel story in which all the events fit. There’s no contradictions. And you’ve got a nice coherent story in which time travel plays a part.
Matt Smith: It also kind of goes with the argument, so if anything that is meant to have happened, has happened, so if you time travel then that was always meant to be that way.
Andrew Brennan: I think that’s determinism and I don’t think you want to be a determinist. There are two different claims that you’ve got to separate out very clearly here. On the one hand, the thought is if something has happened, then it’s happened. Now that’s a necessary truth. If I had a cup of tea this morning, I had a cup of tea this morning. There’s no getting round that. Now maybe I wished I'd had a cup of coffee this morning, but I can’t do anything about the fact that I did have a cup of tea this morning. Now I didn’t have to have that cup of tea. It just so happened that I was in a hurry to come to my lecture, actually, I didn’t have time to make coffee so I made myself a quick cup of tea. If I'd gotten up earlier, I could have had coffee. So it was possible for me to have coffee, I just didn’t get up in time to do it. So here’s the two claims you’ve got to distinguish. You’ve got to say, if I had tea, then I had tea. That’s absolutely necessarily true. But it’s not the case that since I had tea, I had to have tea. That’s not a necessary truth. It’s not the case that the fact that I had tea was that that was the only possibility, and that’s what the determinist’s claim is. The determinist’s claim is the past has to be the way it is, the future has to be the way it is, but it’s not like. It’s necessary that if I had tea, I had tea. But given that I had tea, it was possible I had something else. I could have gotten up earlier and had coffee. It’s just that, as it happened, I chose tea.
Matt Smith: Is this where theories of things like parallel universes come into play as well, where there’s a parallel universe, one where you had coffee and the other you had tea?
Andrew Brennan: Well of course in philosophy terms, we call them possible worlds, because we don’t worry about the physical possibilities of parallel universes the way physicists do. And of course when I say, if I'd got up earlier, I could have made coffee, what I mean in philosophy talk is, there’s a possible world in which I got up ten minutes earlier and I had time to make coffee.
Matt Smith: Is it a likely world?
Andrew Brennan: That world is quite like the real world, but it’s not entirely like it. That was a good question about how, was it a likely world, because what we do when we construct possible world stories and think about possible world theory, is we distinguish worlds according to how close they are to the actual world. We’re living in the actual world. Other possible worlds can be very close to the actual, or they can be just like the actual world, and only different because of one cup of coffee. Other possible worlds are so bizarre and remote that they are different in billions away from the actual world. And there’s a whole branch of logic devoted to studying possible worlds and what’s necessary and what’s possible in different systems of understanding possible worlds.
Matt Smith: If we ever could travel through time, do you think it’s something that we should do? Or should it be left to certain people? Or should it not be approached at all?
Andrew Brennan: That’s very interesting. That’s not a question that philosophers ask, because they’re always worried about just the consistency of the very story of travelling in time. We’ve never advanced to the phase of thinking, should we actually travel in time? Or should we not? I don’t think it would do any harm to travel in time. It would perhaps be a bit disappointing for some time travellers if they did think like Marty McFly thinks for a while, that they can change the past, because that’s the one thing according to the theories I hold, that’s not going to be possible for them to do. So in a way, time travel would be an interesting exercise but it wouldn’t be interesting because of what you’re doing, it would be more interesting because of the things you could see and things you could learn that you would otherwise not know about the past. So it would be an observer’s mission, not an action mission.
Matt Smith: What is your favourite time travel movie?
Andrew Brennan: I like time travel movies that suit my philosophical tastes, so I like both the book and the movie in the Harry Potter series where Hermione has a Time-Turner, and that is a very good example of a very consistent time travel story. She remembers, she gets the Time-Turner because she’s actually wanting to take extra classes, and by using the Time-Turner, she’s able to go to one class, travel back in time and then take the next class. And so she’s able to take an extra subject. And that would be a perfectly good way of using time travel and the story that J K Rowling tells in the novel, is completely consistent. They actually also managed to, oh I've forgotten the name of the animal, the hippogriff, Buckbeak the hippogriff they managed to rescue and they rescue it by using the Time-Turner again, and that’s in a perfectly consistent time travel story.
Matt Smith: That wouldn’t have been the one that I thought you would have come up with.
Andrew Brennan: Oh, what were you going to suggest?
Matt Smith: Oh, no. Well, there’s any number of them. I just didn’t pick you as Harry Potter.
Andrew Brennan: Oh no, I only read it because my kids were reading it but the thing about those stories that appeal to me as a philosopher is the way they avoid contradictions and they’re consistently told, and quite a lot of movies involving time and lots of Star Trek episodes involving time, tell stories that I just get turned off by because they don’t seem to be consistent.
Matt Smith: That’s all the time we have today for the La Trobe University podcast. I'd like to thank my guest today, Professor Andrew Brennan. Thank you for your time.
Andrew Brennan: You’re very welcome. Thank you Matt.
Matt Smith: And you can subscribe to his lectures on iTunesU. He’s teaching a course there on philosophical problems with Associate Professor Jack Reynolds. If you have any questions, comments or feedback about this podcast or any other, then send us an email at podcast@latrobe.edu.au.

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The philosophy of time travel

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