Time Travel and Philosophy
Richmond (Alasdair)
Source: Ward (Dave), Pritchard (Duncan), Massimi (Michela), Lavelle (Suilin), Chrisman (Matthew), Hazlett (Allan) & Richmond (Alasdair) - Introduction to Philosophy
Paper - Abstract

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Author’s Abstract

  1. In this final session, we will think about some issues in metaphysics: a branch of philosophy that investigates the ways that reality could intelligibly be. Our case study will be the possibility, or otherwise, of time-travel. Some have thought that the apparent possibility of creating a machine that we could use to transport a person backwards in time can be ruled out just by thinking about it. But is time-travel really logically impossible? What would the universe have to be like for it to be possible? And can we know whether our universe fits the bill?
  2. Author: Dr. Alasdair Richmond is a threefold graduate of Aberdeen University and joined Philosophy at Edinburgh in 2003. He has published on constructive empiricism, the Anthropic Principle, Doomsday arguments, Descartes’ conception of immortality, time travel1 and the topology of time. He is currently working on a book entitled ‘Time Travel2 for Philosophers’.

    Introduction – why a philosophy of time travel3? (p. 1)
  1. What might time travel4 be anyway? (pp. 2 – 3)
  2. Grandfather paradoxes (pp. 4 – 6)
  3. Two senses of change (pp. 7 – 8)
  4. Causal loops (pp. 9 – 11)
  5. Where next? (pp. 12 – 13)
  6. Appendix 1: Relativity, Kurt Gödel and the unreality of time (pp. 14 – 19)
  7. Appendix 2: Some notable time travel5 fictions and films (pp. 20 – 21)
  8. List of references / further reading (pp. 22 – 24)

  1. There’s a substantial hand-out available (Link) as well as the usual slides and draft transcripts for the first couple lectures (which I’ve ignored).
  2. Suggestions for further reading are divided between two sources:-
    • those on the lecture web-page itself, and
    • those given in the hand-out (see below)
    As usual, those on the web-page are mostly confined to the on-line Stanford Encyclopedia of Philosophy:-
  3. However, there are also references to some interesting websites:-
  4. Finally, there are two versions of a very technical paper:-
  5. The hand-out contains an extensive reading-list. For my own convenience, I list those items I already possess. I don’t want to get carried away digging out papers there’s no possibility of me ever reading, on a topic peripheral to my main research interests:-


Correspondent 1
  1. Although he stated Lewis thinks that time-travel is a logical possibility, I still think it isn’t! He touched on this, but as far as I can see, any change, no matter how tiny would affect the future, and therefore be contradictory, and as he said, things can’t be contradictory if they are to be logical. The only way you get around this is by having multiple histories, like Deutsch & Lockwood. To me, all of this is firmly in the realm of science fiction, which I really like to read, but surely has nothing to do with reality!
  2. Presumably you have never read "Niffenegger (Audrey) - The Time Traveler's Wife"? Probably not your cup of tea, but I must admit some of the things that came up in his lecture today reminded me of that book! It made me wonder whether Audrey Niffenegger maybe studied a bit of philosophy before she wrote it!!
  3. The argument by Lewis that a time traveller10 could make “counterfactual changes” in the past seems nonsensical to me. He is dealing with the hypothetical, so that actually, nothing is changed at all, because it was going to happen that way anyway. Because as he pointed out, we know the outcome cannot be changed, because it is fixed in history.
  4. His discussion of causal loops was interesting. I love the logic of “information simply exists”! It seems to be the same as “God simply exists”.

  1. I thought the lecturer downplayed the constraints on the time-traveller (though the hand-out is clearer). Lewis is definite that there can be only counterfactual changes to the past (or any other time, more on that later). So, nothing can change from what it is at a time – though obviously things can change from one time to the next. In fact, it’s not as though the past is being re-played, this time with the time-traveller in it when he was absent the first time around. There’s only one “playing” of the scene, and the time-traveller is there in that scene the one and only time it is played. This obviously would place a lot of constraints on the time-traveller, even before he decided to enter the time-machine11 – in that if he was present in the past, as a time traveller12 from the future, then while he might have some leeway as to when he makes the trip, he has no choice but to make it some time – to fail to do so would create a contradiction. And, when he does travel back in time, he must do exactly what he did when he was there. But, I doubt it’ll appear to him like he’s in a straight-jacket: he’ll have the same feeling of free will that we all have, but his will will either be frustrated or not depending on what he tries to do. The difficulty lies in that our time-traveller has some knowledge of how things turn out. If he tries to make things turn out how they didn’t turn out, he will fail – but will act just as he did in the one and only time the scene is played out. The hand-out (and maybe the lecture, I forget) makes a good point in that the trip to the past may be what makes things turn out just how they did – and you maybe wish they hadn’t – the example given is that when you go back into the past to kill Hitler, it’s your very attempt that saves his life. I suppose the very paradoxical case would be where the antics of the time traveller13 were very well documented and known to the time-traveller – in that case he would know exactly what he would do, and he would do it whether he wanted to or not.
  2. No – I’ve not read "Niffenegger (Audrey) - The Time Traveler's Wife" – though I’ve heard of it and have had a look at the chatter on Amazon about it. Marmite, by the sound of things.
  3. Counterfactual change is an interesting concept. The important thing is that the presence of the time-traveller in the past isn’t that of an epiphenomenal ghost – he actually does things and affects events! But these things have already happened, and if he hadn’t been there, things would have turned out differently to a greater or lesser degree. Lewis’s point is that none of us ever change anything at a time, only from one time to the next. So, Blucher “changed history” by turning up on time at Waterloo, ensuring Napoleon’s defeat. If he’d been late, Napoleon would have won. But the change that would have been brought about by his lateness is a counterfactual change, because he was, in fact, on time. I’m not quite sure of the terminology – and will read Lewis’s paper to check – that is, whether Blucher’s timeliness also instigated a counterfactual change at a time. Napoleon was going to win, but he changed the course of events so that he lost – but this isn’t a replacement change – there was only one battle of Waterloo, and Napoleon lost it – so Blucher’s timeliness must have instigated a counterfactual change from what would have happened (had he been late) to what did happen. Difficult.
  4. I’m less satisfied by Lewis’s unconcern about “information from nowhere” than you are – maybe because I’m not sure what information is (I should know, being an IT chap, and have some unread books on the topic, stemming from a book given me years ago – "Gitt (Werner) - In the Beginning was Information" – which refers you to "Shannon (Claude) & Weaver (Warren) - The Mathematical Theory of Communication". There’s also a chapter on it in "Andrews (Edgar) - Who Made God? Searching For a Theory of Everything". There are a number of processes – for instance causation14, explanation, belief justification – that leave us with a trilemma – infinite regress, foundationalism or coherentism – ie. things either go back forever, stop somewhere, or go round in a circle. Like you, I expect, I’m less happy about some “brute facts” than others – the complete works of Shakespeare isn’t one that I had in mind. The theistic line is a foundationalist approach – things don’t go back forever or round in circles, but stop somewhere – with God. So, it is said, it doesn’t make sense to ask “who created God” or “what explains God”. And fair enough, but I’d still like an explanation of where the complete works of Shakespeare came from, and a tight causal loop isn’t good enough.

Correspondent 2
  1. My take on lecture 7 is that if one were to go back in time then just your mere presence alone would cause a 'contradiction'. It's the way that the most innocent of actions can often snowball, given time, rather than any individuals intended actions. Take, for example, if you went back in time and that just your presence alone caused a fly to be diverted from its original path and it then got caught in a spider’s web. That spider was then caught by a bird that would otherwise have gone elsewhere but whilst the bird was flying off with its lunch a cat suddenly darted across a road to catch the bird which caused a car to swerve onto the pavement killing a young Albert Einstein or Charles Darwin. History would be changed forever. It's the butterfly beating its wings in South America ultimately causing a tornado in Kansas type scenario.
  2. Anyhow, why wouldn't the time machine15 operator (and/or the machine) grow younger, or older? I don't see how they would be disconnected from the direction of time travel16.
  3. And I guess that as history still records Hitler (he does seem to get such a very bad press!) and other terrible events in history
  4. I can't see that time travel17 will ever happen in the future, but the philosophical arguments are very interesting.

  1. Thanks for your thoughts on time travel18. They are quite perceptive, though I think the first, long, comment is misguided. As I'd tried to explain in my response to Correspondent 1, there can be no changes at all to what happened in the past - simply because a time happens only once – so whatever you do when you go back into the past has already happened, including any knock-on effect of what you do or cause to happen. Your exposition of the butterfly effect eloquently explains why this must be so in order to avoid contradictions.
  2. As for why reverse time-travel isn’t just time reversed, or something analogous … well, that’s another story altogether. Have you read "Amis (Martin) - Time's Arrow or the Nature of the Offence"? Recommended. I occasionally wonder why personal time continues to go forward in reverse time travel19. I suppose assuming it goes backwards is just changing the subject – we’re trying to work out the logical consequences of reverse time travel20 as traditionally understood. We could come up with thought experiments21 where time went backwards for you – but if the time machine22 just reversed time for a bit, you’d just hop out of the machine backwards, time would go forward again and you’d be in a very tight loop (“Groundhog Minute”). If you were somehow isolated from the rest of spacetime – as in the traditional time-machine23 idea – then if weird “getting younger” events happened to you there would be problems with conservation of mass/energy as you got smaller / bigger as you got younger – though I don’t know what’s supposed to be happening in this regard as a time-machine24 just pops into existence as a time – where have its mass-energy come from? I don’t know – but your idea is just another thought-experiment25 which would need filling out. While there’s some argument about whether reverse time travel26 is physically possible, we’re not really trying to work out what would actually happen in practice – we’re trying to work out what is logically possible, given some fairly well-defined premises.
  3. Your third point is fair enough – if we could change the past, you’d have thought someone would have kindly sorted out some of the nastiest things that have happened, so that they didn’t. But this isn’t logically possible, as we’ve noted – a time traveller27 can’t change anything – indeed he can’t do anything other than what’s been done – including what’s been done by him, then. Your idea is not quite the same objection as that raised by Stephen Hawking – ie. “where are all the time-travellers”. It seems that it’s likely that in a physically-realisable time-machine28, you can never go back in time to before the time the machine was built – so the answer to Hawking’s question is that no-one’s built one yet; and – as far as it goes – that might answer your objection. But if and when someone does build a time-machine29, time-travellers will just be part of life, and won’t seem extraordinary, except maybe the first time one is encountered. But what they will never be able to do is change the past. They will be active in the then present, which just happens to be an earlier time to that in which they had previously been active.


In-Page Footnotes

Footnote 9:

Text Colour Conventions (see disclaimer)

  1. Blue: Text by me; © Theo Todman, 2020
  2. Mauve: Text by correspondent(s) or other author(s); © the author(s)

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