Episode 1: Time Travel / August 28, 2020

With Dr Steph Rennick

Hosted by Hamish Stewart

Edited by Constantinos Stylianou

Posted in Metaphysics Main feed

Hello, and welcome to the first ever episode of Glasgow University Philosophy Department’s Podcast, Thoughts! Today we’ll be discussing time travel: an oft discussed, but rarely properly examined, topic. Joining us is Dr Stephanie Rennick, Glasgow University researcher, and hosting today is Hamish Stewart.

This episode covers all things time travel. Steph defines time travel and a popular paradigm of time, and then discusses causal loops, the ‘predestination paradox’, and how time travel might threaten free will.

You can follow Steph on Twitter @EpicureanCure, or check out her website.

01:18 – Why Steph likes time travel, and why she thinks it’s important.
03:40 – What time travel is, philosophically speaking
07:45 – Time travel paradoxes

Further reading:

Rennick, Steph. "The Epicurean Cure"
Lewis, David. "The Paradoxes of Time Travel"


Hamish Stewart: I’ll keep you in the loop, in the... in the causal loop…
Dr. Steph Rennick: Haha, nice, thanks.
HS: Aha! Aw, I should have made that joke in the…
Dr. SR: You can just edit it in; it’ll be fine, it’ll be seamless.
HR: Yeah, yeah, I’m not sure I quite have the editing skills for that yet. Anyway, yeah.


HS: Hello everybody, it’s Hamish Stewart here, and you are listening to Thoughts: a new, student run, Glasgow University philosophy department podcast. Whilst I’ve got you, remember to follow us if you’re listening on Spotify, and remember to subscribe if you’re on Apple Podcasts. Let’s get on with the show.

HS: Today on the show we have Dr. Steph Rennick who is a researcher at Glasgow and who works on time travel and foreknowledge. She also founded the Epicurean Cure, which is a website for philosophers and pop culture fans alike. Thank you so much Steph for coming on the show!

Dr. SR: You’re very welcome. I’m delighted to be here.

HS: Thank you. And I would just like to say, from a personal perspective, she’s the perfect guest for episode one because on her website she writes, she believes in “making philosophy accessible and engaging to a wider audience” which is literally our mission too. Ok, right, let’s jump into time travel. Steph: why is talking about time travel even important?

Dr. SR: That’s an excellent question. There are loads of reasons why it’s worth talking about time travel philosophically, but I’ll give you three. The first is that we’re obsessed with it. There are countless films and video games and poems and webcomics and books thinking about time travel, exploring time travel. And, insofar as philosophy and academia generally should engage with issues of perennial folk interest, I think that that is a good reason to worry about time travel. The second is that it helps us shine a light on other issues that are difficult but philosophically important. So, things like free will, causation, luck and coincidence, foreknowledge, intentions, a whole range of gnarly philosophical problems, can be…

HS: Is that an Australian word there?

Dr. SR: Gnarly, yeah.


HS: I love it! Ok.

Dr. SR: So, that’s the second reason. And the third is that we’re time travelling right now. It’s not very exciting; we’re time travelling at a rate of one second per second into the future. Probably nobody’s going to write stories about it but given that we’re all doing it we should probably understand what’s going on.

HS: Ok. Why, though, are YOU personally interested in time travel?

Dr. SR: That can be a very long story or a very short one, but the short one is: I like stories, and I like puzzles. And time travel brings those two things together in a really wonderful and compelling way. And it also gives me the opportunity, because there are so many time travel texts of which people are serious fans, it gives me a really nice way of engaging with people and giving them the tools to think robustly and critically about stuff that’s hard but fun.

HS: Ok. Now, you mentioned time travel texts there. Could you give us some examples of some of those?

Dr. SR: So, my favourites are: Bill and Ted’s Excellent Adventure…

HS: Yep, love it.

Dr. SR: …fantastic, philosophically consistent time travel text, with two very genre-savvy main characters who realise they’re in a time travel film and make use of that, which I thoroughly enjoy.

HS: That’s funny, yeah.

Dr. SR: Harry Potter and the Prisoner of Azkaban is one of the best for dealing with really interesting, seemingly paradoxical type circumstances in a way that’s philosophically consistent. And more recently…ooh actually, maybe I don’t actually want to say that because I don’t want to spoil it.

HS: Ok…

Dr. SR: If you want more recent ones, tweet at me, or send me an email or something and I’ll…

HS: Ok, right, listen to that, you have to tweet Steph if you want some more recent ones. Ok. I need, though, to start simple. So, what is time travel?

Dr. SR: So, when philosophers talk about time travel, we mean the sort of thing that people mean in science fiction. So, at its heart time travel is a discrepancy between time and time. Right, so one way to cash that out as David Lewis does… I’ll probably mention his name quite a lot, his 1976 paper The Paradoxes of Time Travel is, sort of, THE big original interesting piece of work on time travel. It’s not the first but it’s the most popular. So, he distinguishes between external time which is time as we know it, time proper, and personal time which usually for the average person they go along at the same rate, right? And we might measure personal time as the time going by on your wristwatch or the time measured by the aging of your cells. When a time traveller gets in a time machine and moves into the future or past, it might be in external time that they’ve travelled 50 years, but from their perspective in the time machine let’s say it’s 15 minutes, right? That kind of discrepancy is what’s at the heart of time travel.

HS: Ok, perfect. And I want to know also how time’s working.

Dr. SR: Ok. So, there are lots of different theories of time, both in physics chat and in philosophy chat.

HS: Yeah.

Dr. SR: And, it’s at least plausible that time travel could work in several of those. But the one that most philosophers of time travel assume, when talking about time travel, and the one that’s at the heart of all the texts I just recommended is four-dimensionalism which is a variety of eternalism. And the real kind of quick and dirty version, the bits you need to know are just this: it treats the universe as having four dimensions. Three spatial dimensions – the ones we’re used to, height, breadth, width, right? And also a temporal dimension. And so just like Australia is spatially far away from us here in Glasgow, but equally real, so too the future and the past are temporally far away from this moment which feels to us like the present, but there’s no kind of objective specialness about this moment, they’re all equally real. H. G. Wells’ The Time Machine, which is one of the first time travel texts, the time traveller talks about a cube that is extended in four dimensions and he says, you know, if you have a cube that lasts no time at all then it doesn’t exist. Right, things need to have duration, they need to be stretched through time as well as through space. The only other thing you really need to know about four-dimensionalism to make sense of what’s going on in a lot of these time travel texts is that events only happen once. So, you might see them from multiple perspectives. The time traveller might travel to that time and see it multiple times, but when that time occurred it always occurred that way. So, if I was to time travel to the first of January 1920 it was always the case, that on the first of January 1920, that I was there.

HS: Ok.

Dr. SR: So that’s the quick and dirty

HS: And you talking about the cube makes me think of a description one of my tutors once gave. It was like a metaphor for how to visualise individual people’s lives in four-dimensional time. And she said something like – imagine a noodle snaking through a transparent soap bar.

Dr. SR: Haha, yep!

HS: It’s very random, but something like that, do you think that’s a good…?

Dr. SR: That’s perfect, that’s lovely, I love the noodle. So usually people talk about spacetime worms.

HS: Yeah, ok.

Dr. SR: But it’s the same idea, right? Erm, this reveals my age, but for anybody who had an older computer, and it used to lag and you dragged the mouse and you’d get shadows of the mouse across the page, that’s how I kind of like to think of it, right? That we are these, we are stretched out through time in the same way we are stretched out in space.

HS: Yeah…I love how you’re talking about only old computers lagging after our half an hour fiasco!

Dr. SR: Haha, sure, but sadly our lag didn’t result in us having slices of ourselves spread out across the screen. That would have been a better visual metaphor.

HS: Haha, yeah that would have been good. Ok, so, that makes perfect sense, but I want to talk now in the second half of the episode about the paradoxes of time travel. Now, there’s one which is about causal loops. Could you explain that to me?

Dr. SR: Yes, of course. So, first thing I want to say is that paradoxes get used in two different ways in philosophy. There are strict paradoxes which are contradictory, and they’re really bad and we don’t want them, right? Contradictions can’t occur so anything that would lead to a contradiction, if time travel would result in contradictions, that would suggest that time travel is impossible. That’s the first reading of paradox. The second, weaker, kind of more common-sensical account of paradox, less technical account of paradox, just means something like a tricky puzzle, where it doesn’t seem like you can hold all the bits together in your head. All of the paradoxes of time travel, they claim to be the first kind but at best they’re the second kind, right? So, I don’t think there is anything paradoxical in the strict sense about them. But causal loops are a strange artefact of time travel, or could be. So, here’s a really simple version of a story with a causal loop. This is from Lewis: a time traveller goes back in time and gives his younger self the plans to build a time machine. That person grows up, builds the time machine, and then goes back in time to give his younger self the plans to build the time machine. Where did the plans come from in the first place? Right? So usually when we track our causal histories, which, in a really really simplistic way let’s think of it as a chain of dominoes, right? So, you have a line of dominoes, starting with your birth as the first domino, and then each domino in turn as it tumbles causes the next event, causes the next event, causes the next event, right? But in this time travel case, because we have somebody going back in time, that chain of dominos bends back on itself and what you’ve got is a circle where the last domino starts off the first domino going. So, some of this looks really puzzling in that stuff seems to come from nowhere, and in our everyday chat we tend to think things don’t just come from nowhere. That strikes me as less problematic than some people have worried, not least because pretty much everything either just starts or comes from nothing or goes on forever, right? When we talk about the universe, when we talk about any atom, the history of anything, it will either just come to be or it will have lasted forever or…there’s always a kind of strange first cause. So, that doesn’t worry me too much. The other thing that I guess might be puzzling is that it works. So, that this thing that came from nowhere in the blueprints case, the time traveller goes back, has these blueprints that work, that give you a functioning time machine. That’s strangely coincidental. So, Richard Hanley points that out, and other philosophers have pointed out, that that’s not a possibility issue, it’s a probability issue. It seems wildly improbable that these things work, but, again, not a contradiction.

HS: Interesting. Ok, and something you’ve been particularly interested in is called the pre-destination paradox.

Dr. SR: Yeah, that’s what I called it, it maybe has other names…

HS: Ok, well that’s what everyone at Glasgow will be calling it from now on.

Dr. SR: Ha, yeah! One of the things I’m interested in is knowing the future, which has come out of my interest in time travel because it seems like if I could travel back in time now, then from the perspective of the past that I find myself in I know stuff about the future, right? I know what’s going to happen. And, if I were to time travel into the future, and then come back, likewise, right? I might know stuff about what’s going to happen. Now, there might be other ways we can know things about the future. Maybe you think that there’s a god, or that there’s prophets or there’s some supercomputer that can predict the future if we live in a deterministic universe, or whatever, there might be all different ways we can know the future, but, time travel seems to give us one of those ways. And, so, thinking about that, thinking about what difference it makes for people to know stuff but also for different events at these different times to depend on each other in these interesting ways made me think about this thing that I’m calling the pre-destination paradox. So, to give you a really easy example: imagine an archaeologist discovers a skeleton, right? He’s an archaeologist, he’s interested in the origins of the skeleton so he gets into the time machine, goes back in time, finds himself in a really inhospitable environment, dies, and becomes the skeleton. Now, that’s a causal loop in that his discovering the skeleton is what spurred him on to go back in time, that causes him to die, that causes him to decompose and become the skeleton, and that skeleton is what spurs him on to travel in time. So we’ve got that issue. It’s not a causal loop in which something comes from nothing, it’s more like a loop of a rollercoaster; it has a beginning and a bit that comes kind of off the circle but nonetheless we’ve got this causation folding in on itself. But we also have this strange predestination where it seems like, given that the archaeologist is the skeleton, given that he went back and died, at the moment he gets in the time machine doesn’t he have to get in the time machine, right? Doesn’t it seem kind of fated that he does? Now, as with these other paradoxes, I think no, right? He DOES get in the time machine - we know that he does because if he didn’t get in the time machine there wouldn’t have been the skeleton. Or perhaps there would have been the skeleton but it wouldn’t have turned out to be him. So it’s not the case that at the moment that he gets in the time machine it was necessary for him to get in the time machine, it’s just the case that because he does, these other things happen. Nonetheless, this has really worried people. And we can get these sorts of predestination paradoxes coming out with foreknowledge as well. So, imagine that a different time traveller comes back in time and says to me “you’re going to wear a red dress next Friday” and…I think “well, that doesn’t sound like me, I don’t wear red, I just wear black and occasionally grey; I’m not very exciting”. But then a whole bunch of stuff seems to happen where it’s like if it’s true, if I believe the time traveller or even if I don’t believe it but independently it is true that I will wear a red dress on Friday – do I suddenly not have a choice? Right, what is it that brings about…either, I might try really really hard not to wear a red dress but a bunch of fantastical and improbable thing happen to mean that I am ina red dress. Or, do I give in and comply feeling like I don’t have the freedom to do otherwise. But all of these artefacts are kind of psychological, or probability stuff. They’re interesting, they’re weird, they’re puzzling -  but they’re not contradictions.

HS: Ok, and that brings us beautifully to our last topic. Which is - why do you think some people think that time travel could threaten free will?

Dr. SR: So there’s a couple of different reasons. I think it’s easy to make a mistake, a particular kind of reasoning mistake that leads a lot of people when they’re first thinking about it to think that there’s a problem for free will with time travel. And that goes like this: it’s just the same as the archaeologist case, it’s just the same as any of these cases. Let’s take Harry and Hermione going back in time. We know that in the Prisoner of Azkaban – it’s an old enough book, if you haven’t read it now I’m spoiling it for you – we know that they’ve gone back in time and that Harry saves himself and we know that he’s already saved himself in the moment he decides to go back in time, so it seems like he has to go back in time. And so people worry about the fact that it is true that he goes back in time and they move from that to the idea that it’s necessary that he went back in time. It is true that they went back in time but they might have done so freely. They didn’t have to go back in time because they went back in time they saved themselves and other events ensued. Likewise, it’s true now that I ate an oat bar for breakfast. I didn’t have to, nobody made me, right. I freely did it. But it is nonetheless true that I ate the oat bar. There might be truths about the future, they don’t have to happen, but they will because of the way certain things will occur. So it’s really important, I think, that people separate this idea from something being true and that it will happen, to its being necessary which means it has to happen. So, that’s, I guess the first answer. I think a lot of the worries of free will and time travel are based on this modal fallacy, you might call it, that’s the technical term but really this error of reasoning. But I think separately there’s some stuff about time travel that at least seems weird and the neighbourhood of free will but might not really be about free will. So, if I go back and time and speak to my younger self, right, it must be a really strange experience for my older self, thinking “wait a second, I’ve already had this conversation. Oh, I know what I’m going to say next”, right? Psychologically that seems a bit strange. The newest work on time travel and free will – because there wasn’t a lot of it for a while and there’s a few people doing some stuff on it right now – is sort of starting to explore, you know, it seems like knowing stuff that’s going to happen, it seems like going back in time might inhibit the sorts of things that we can usually do, but where exactly are the boundaries of that? Where are the limitations? And are they freedom inhibiting limitations?

HS: Yeah, those are very interesting questions. We are coming to the end of the show, though, but before we do finish is there anything you would like to plug?

Dr. SR: So as you mentioned, I founded a site called the Epicurean Cure which brings together academics and consumers and creators of pop culture. We’re interested in thinking robustly and critically about the fiction that people love. So, if anyone wants to contribute to that either by creating something, maybe writing a time travel story that subverts some of these tropes that we’ve been talking about, that does something a bit different, we’re just at epicureancure.com. And yeah you can find me on Twitter and Facebook and all the usual.

HS: I’m also going to plug now. I want to thank the Parr Centre and Chapel Phil. Chapel Phil is a student run ethics podcast coming out of the University of North Caroline at Chapel Hill. They inspired me to do this and so I would encourage you to check them out at chapelphil.com. The other two people I want to thank are Ruriadh Fleck and Thais Juhel for their music and art respectively, and you can find both R and T on Facebook and Instagram and Soundcloud.

Dr. SR: Thank you so much Hamish for having me on, especially for the first episode, it’s a complete delight!

HS: Thank you so much Steph, thank you so much. And thank you everyone else for hanging out with us. We will see you soon!

Transcript written by Frances Darling