Is Time Travel Possible? Are We Close to Doomsday? and Other Big Deals…
Interview by Richard Marshall.
‘The correct moral (following David Lewis’s wonderful 1976 paper ‘The Paradoxes of Time Travel’) is that time travel into the past can be logically possible (without needing to invoke many-worlds or branching histories), provided that we are clear about what travellers in the past can and cannot do. There will be certain past outcomes that might seem possible relative to a restricted set of data but that turn out to be impossible once you look at the bigger picture. (Such local data might include e.g. Grandfather-to-be isn’t bulletproof; bigger picture might include: he survives to be a grandfather.)’
‘In both the Moore and Sorensen originals, the ‘lucky’ veiled immortal manages this feat by living assort of staccato existence, e.g. you live forty years of normal consciousness then twenty billion years of nothingness intervenes, then you live twenty years of normal consciousness followed by another twenty billion years of nothingness .. and so on. The intervals of personal consciousness get shorter and shorter but they do so in a Zeno sequence so that there is always some personal time ahead of you no matter how tiny it may eventually become, after any finite interval of external time. Now I love these examples and also like Roy’s idea that we should value personal time above external time.‘
‘The Doomsday Argument applies anthropic thinking to our place in history. It says (roughly), we should favour the prospect of imminent human extinction on the grounds that our location, qua randomly selected humans, is more probable if a large fraction of all humans there will ever be have already lived. In other words, the argument runs, if we apply anthropic reasoning to our location in history, we should increase our probability for history being close to its end.’
‘Bostrom defends a trilemma for functionalists who think advanced (‘posthuman’) civilisations would run lots and lots of simulated minds. Specifically, such functionalists must distribute their credences between the following three options: a) posthumans are rare, b) posthumans run few simulated minds or c) we’re probably simulated minds here and now. So, as a corollary, if we discount the possibility of our being computer-simulations run by posthuman civilisations, we must either expect that we never acquire posthuman technology or that posthumans choose to run few simulations.‘
Alasdair Richmond has published on constructive empiricism, the Anthropic Principle, Doomsday arguments, Descartes’ conception of immortality, time travel and the topology of time. Following research leave 2008-09 (part-funded by the AHRC), he is currently working on a book entitled ‘Time Travel for Philosophers‘ and a series of related articles. Besides teaching epistemology, metaphysics and philosophy of science, he was closely involved with the Higher Philosophy programme 1999-2003, conducting classes for pupils and Continuing Professional Development days for teachers from all over Scotland. Here he discusses time travel and the grandfather paradox, parahistories and Roy Sorensen, the John Titor Fiasco, hell, time travel and super tasks, Newtonian space and Newtonian Time, Spore Gods, Achilles and the Tortoise, the Doomsday Argument, the Ussherian Corollary, the Simulation Argument and Nick Bostrom, anthropic reasoning, and why we should heed philosophers.
3:AM: What made you become a philosopher?
Alisdair Richmond: Oh would that I had …. Not totally sure that I have become a philosopher. (Descartes and Elisabeth of Bohemia were philosophers; Nancy Cartwright and Rae Langton are philosophers – Al Richmond, one fears, has yet to win his spurs. One day, maybe …) How I did get into philosophy? Partly, the negative pressure of realising that what I originally wanted to do at university (which shall be nameless here) was alas something that I wanted to pursue instead in my spare time. Mainly (and much more enjoyably), the positive realisation of what a treasure trove of ideas about our selves, our world and our values was lying around in the works of philosophers. I had done practically no philosophy at all before going to university – at school, I’d tried some Bertrand Russell which I could barely grasp and made an interested but totally ill-equipped attempt at a book on Wittgenstein. However, I realised soon after I went to university (in Aberdeen) that there were a whole host of interesting problems and attempted solutions to be read and discussed that tackled things with clarity, precision and courage that I had only been aware of dimly and vaguely but which seemed to me, the more I heard about them, about as interesting as any things could be. In short order, I got to hear about (e.g.) Hume on induction, Berkeley on perception, St. Anselm on God, Plato on the Good, Kant on duty, Descartes on the soul, Sartre on freedom … and I couldn’t get enough of any of it. (Still can’t actually.) So my lecturers on my (undergraduate) M.A. at Aberdeen University 1989-1993 are who got me into this – unfailingly patient with my fumblings but also very quick to push, instruct and correct where it was (as it so often was) needed. Thank you for everything, Aberdeen Philosophy staff 1989-1998.
3:AM: Time travel has intrigued you. I remember sitting with a friend and saying time travel couldn’t happen because if it ever did happen we’d have heard about it by now. You’ve looked at how it’s dealt with in fiction as well as just a thought experiment. What are the central paradoxes related to time travel, and do you think that most fictions when dealing with it tend to cheat to overcome them? Are there any fictions you know of that are sophisticated about these issues?
AR: Ah well, the big one everybody knows is the Grandfather Paradox – that old chestnut that says that time travel must be impossible because if you could travel back in time, you could so interfere with history that your journey became impossible, e.g. assassinating your Grandfather-to-be before he’s fathered any children. The correct moral (following David Lewis’s wonderful 1976 paper ‘The Paradoxes of Time Travel’) is that time travel into the past can be logically possible (without needing to invoke many-worlds or branching histories), provided that we are clear about what travellers in the past can and cannot do. There will be certain past outcomes that might seem possible relative to a restricted set of data but that turn out to be impossible once you look at the bigger picture. (Such local data might include e.g. Grandfather-to-be isn’t bulletproof; bigger picture might include: he survives to be a grandfather.) A more mundane example: can I stop smoking? Well, there are local data that fit with that perfectly well, e.g. out there are nicotine patches, support groups, self-hypnosis tapes, etc., etc., all designed to help smokers quit. But logically, I can’t quit smoking because I don’t smoke. An outcome that looks possible relative to one data-set becomes clearly impossible relative to a bigger data-set – but there’s no paradox here.
The restrictions on time travel in the past may look very weird, but then again they may look perfectly commonplace, e.g. you get Grandfather-to-be in your sights but your gun jams, your nerve fails or you slip on a banana skin. Likewise, maybe you shoot down someone who you think is due to be your Grandfather but find that you’ve hit some innocent passer-by. I always try to stress that Lewis’s aim is merely to defend the logical possibility of backward time travel – not its physical or technical possibility – so any objections that he leaves time travel somewhere between merely unlikely and contrived rather miss the point. People have lavished enormous (if slightly tedious) ingenuity on depicting ever-more outlandish ways in which time-travel scenarios could play out weirdly but who’s to say that they must do so? So the real lesson of the ‘Grandfather Paradox’ argument is that arriving in one’s own history might pose a set of constraints – maybe epistemic, maybe ontological, maybe both – but not necessarily any offences to logic. Logic alone won’t settle for us whether or not time travel is possible – you need to appeal to something else, like physics for one thing.
The state of play with respect to causal loops (the other big time travel problem) is more complicated. In a causal loop, at least one event in a chain of events (at least partially) causes itself. So a causal loop is a case where an event is among its own causes e.g. through backwards causation, foreknowledge or (most popular in the current literature) time travel. I imagine a case in my lectures (adapting it from Anthony Burgess I think) where a time traveller from c. 2020 goes back to the late 16th century and gives the young Shakespeare a copy of a future-printed edition of ‘his’ (i.e. Shakespeare’s but note the care-quotes) complete works. So the young Will then copies the works down, see them printed and hand them on to posterity. The plays get transmitted in the normal way to the 21st century, whereupon the traveller comes along and takes a copy back to the 16th century … Now, this isn’t contradictory – but it is exceedingly weird. It gets weird especially once we start to press the question of just who (if anybody) actually writes Hamletin this (very far-fetched) scenario. Lewis effectively says: in this case, Hamlet isn’t written at all – the existence of Hamlet is not amenable to (causal) explanation. Now this may sound unsatisfactory but (I reckon) that Lewis is offering a parity argument. His claim is, in effect, that causal loops present no more ultimate (my emphasis) difficulty in explanation than any other causal chain. To put it another way: where does any efficient cause (or chain of efficient causes) come from – ultimately? Causal loops seem to pose a problem of origins, i.e. that of where such things (ultimately) come from. Every loop-event has its immediate causal ancestor but the causal chain leads back to the very event in question. However, Lewis’s (1976) offers some therapeutic suggestions for unease about causal loops: firstly, quantum physics countenances acausal events and causal loops pose no worse explanatory problems than they do; secondly, linear causal chains either extend infinitely or terminate in some uncaused event – both of which are just as hard to explain ultimately as a looped chain; thirdly, trying to explain the whole loop may simply mistake an illegitimate explanation-request for a legitimate one. We can explain many, many kinds of event very well – maybe we are less well-equipped to explain whole chains of events though.
I would emphasise that Lewis is not saying that causal loops are somehow okay, untroubling or unexceptionable – as though we could easily bolt them on to our existing understanding of the physical world without any upset ensuing at all. (I suspect I may be more pro-time-travel than most and even I have set my personal probability for actual causal loops very close to zero .. Not literally at zero but pretty close to it. The evidence needed dto make me change my mind would have to be very powerful. And no, YouTube film of people allegedly walking around in the 1920s with cell-phones and anachronistic-looking haircuts is nowhere near the evidence I have in mind, thanks.)
So Lewis’s view is perfectly consistent with causal loops being a profound offence to what we currently understand by way of physics or metaphysics. (All that Lewis claims about causal loops is that they can be logically possible. However, not necessarily and invariably giving causal loops a prior probability of zero is perfectly consistent with according them prior probabilities exceedingly close to zero or conditional probabilities of literally zero.) However, causal loops are one kind of causal chain. The three kinds of causal chain that Lewis discusses (and very likely the only three that are conceivable) are infinite linear chains, finite linear chains and closed (i.e. looped) chains. Where does the presumably highly ordered and technical information in Hamlet come from ultimately? From an infinite linear chain, a finite linear chain or a finite closed chain? Those are the three kinds of chain on offer but in any of these cases, where does the whole chain come from – ultimately? Yes, Lewis gives no account of the ultimate origin of the looped chains he discusses but what ultimate explanation have we for any causal chain whatsoever? Our intuitions may be offended when confronted with a finite but complete closed causal chain but perhaps they shouldn’t be, because it isn’t clear that we are in any worse case when it comes to explaining where a finite closed causal chain comes from then we are if asked to explain the origins of any linear causal chain (whether finite or infinite). While presumably it’s a well-formed explanation request to demand of a given event why it occurred, it’s much less clear that the same well-formedness extends to questions about the existence of entire causal chains. Of each event on a causal loop we can cite an earlier event which is that event’s cause. However, it’s maybe a category-mistake to ask why the whole loop came into being.
Apropos the lack of time travellers hereabouts: as above, I agree completely that there is no good evidence at all that travel into the past has been achieved (and travel into the future via time-dilation has only been achieved for humans – specifically astronauts – to the extent of mere nanoseconds at most), but this observed lack of time travellers hereabouts is not a knockdown objection to time travel per se. It depends very much on how the time travel is achieved. Physics imagines backward time travel achieved, not via some wonderful Wellsian transport-device that lets you go anywhere in history you please, but via regions of curved spacetime. Hence physically realistic time travel is rather restricted – a time ‘machine’ in physics is really best thought of as being a region of spacetime and not as a vehicle. Hence a physical way to achieve time travel can only let you access regions where it itself exists. Just as a moving staircase can only take you to places where it itself exists so curved spacetime doesn’t offer you unlimited access to all time and space.
So maybe backward time travel can occur but only in regions of spacetime that we haven’t reached yet. (Although I doubt it I stress.) There are possible worlds – often very unrealistic and counterfactual in their assumptions I stress – that are described by General Relativity that contain closed timelike curves (CTCs): paths that a physical object can follow into the local future but that wind up in the external past. (The family of model universes first discussed by Kurt Gödel in 1949 is the classic set of examples – these being the first explicit descriptions of a CTC-compatible model universe in the literature.) Now whether CTCs can thrive in more realistic spacetime models, and in particular whether they can survive in whatever theory of quantum gravity will survive the ongoing unification of General Relativity and quantum mechanics, are completely different questions. (Questions I am in no good place to try and answer I stress.) Right now, my bet is that the prospects for realistic CTC-generation are, and are going to remain, poor if not totally unrealistic – which I confess sort of disappoints me – but that bet is at least is a falsifiable conjecture …
Apropos fictions: while I yield to no one in my adherence to the law of non-contradiction in real life, I also defend an author’s write to depict (or pretend to depict if you will) an inconsistent world if that suits her/his purposes best. I overwhelmingly prefer my fictions consistent but I will countenance exceptions. In particular, there are many time travel stories I love where it seems the line of least resistance is simply to bite the bullet and accept that a single, inconsistent world is being depicted. (Or pseudo-depicted if you like.) The classic is Ray Bradbury’s 1952 luminous tale of butterfly-bothering existential terror, ‘A Sound of Thunder’, which, I reckon, would lose all its point and pathos if it is simply diluted down into a story about mere travel between worlds. Another lovely, and also harrowing, example of time travel fiction which I think plays best as deliberately paradoxical is John Crowley’s 1989 novella ‘Great Work of Time’. So I do think that time travel fictions can legitimately be inconsistent even though I strongly presume that reality cannot. On the other hand Philip K. Dick’s short story ‘A Little Something for Us Tempunauts’ derives all its (considerable) horror precisely from its refusal to be inconsistent. Another under-rated consistent short gem is Lester Del Rey’s ‘My Name is Legion’ (1942), which imagines a non-paradoxical but utterly startling fate for Hitler, achieved via time travel.
For me, the best picture of time travel in recent fiction – maybe in fiction full stop – and the most concerned to be consistent (those two points are maybe not unrelated) is Audrey Niffenegger’s 2005 novel The Time Traveler’s Wife. Professor Niffenegger lavishes a remarkable fictional talent on depicting what time travel constraints might look like if they were actually lived. A very remarkable book indeed – often gruelling but absolutely thorough in its consistency. (In this story, the ability to time travel is far more of an affliction than a gift.) For what I am sure were good reasons, the film version had to omit some of the more disturbing implications and incidents from the original but the original book is quite unflinching. Another, slightly more recent, novel I can thoroughly recommend is Christopher Priest’s 2016 The Gradual, which, besides being full of Priest’s compelling trademark investigations of memory and identity, also manages to give far more of a flavour of what time travel in a physically-realistic world might be like than almost any other story I know. (See also Christopher Priest’s 1979 delightfully eerie novella ‘Palely Loitering’, which comes pretty close to inventing an Alcubierre warptube some years early. Can never make up my mind whether this one is consistent or not though … Either way, it’s brilliant.)
3:AM: One element of time travel you’ve discussed are the philosophical problems that might occur with the notion of parahistory and past artefacts. Can you say what a parahistory is and what is the dilemma? Why do you say past artefacts will not be as valuable as one might have thought?
AR: Parahistory is a (lovely) idea due to the great and powerful Roy Sorensen. In discussing how one might apply Hume’s notorious arguments concerning testimony to miracles to (alleged) cases of time travel, Roy coined the term’ parahistory’ as a sort of analogue of parapsychology: just a parapsychology studies data (supposedly) derived from non-standard sensory channels, so parahistory would study objects retried form other times. In my original paper on parahistory, I argued that there is a problem about making an artefact-based parahistory, because past-derived artefactual evidence will either disintegrates under the impact of contradictory indicators of sage and period or it just collapses into testimonial evidence. (Inevitability a time-travelling object will have misleading indicators of its age as opposed to its date of origin, e.g. its varnish won’t have aged as much as it should, its pigments won’t have faded as much as a conventionally-aged painting from the same period, etc.) Although I have had a change of heart about the prospects of past-artefact –derived-parahistory in general, thanks in no small part to suggestions from two students of mine, and I am basically working on a paper that will qualify / roll-back pretty much everything I said about parahistorical artefacts and their value in the first place.
3:AM: Does this line of thought link with time travel testimony? What philosophical lessons do you draw from what you call the ‘John Titor Fiasco’?
AR: The Titor thing interested me because here we had some extraordinary claims being circulated (e.g. that around 1998, the world had been visited by a man from post-nuclear-war 2036) and apparently believed. However, to be frank I had (and still have) problems really accepting that many of these supposed Titor devotees actually believed what it was that they claimed to believe. Not least because once various dates started coming round and apparently falsifying Titor’s claims (e.g. no Y2K disaster, Olympic games persisting after 2004, no American civil war starting 2004-05, et.), Titor fans started claiming that he had not come not from the (one-and-only) future but rather from a future, i.e. one of a slew of alternative futures, which makes falsifying his claims that bit harder. It all seemed a textbook illustration of Hume’s remarks about the tendency to believe extraordinary claims and especially those that give a special role to the person sharing the (alleged) information. I sort of suspected that there was a large element of wishful thinking in there, even though Titor’s story implied that the then-near future (now the recent past) would be nightmarish for billions of us. This wishful thinking may also have combined with that slightly guilty urge to be the person who has the Big Secret about the future known only to a select (elect?) few.
There seems now (or at least there was fairly recently) to be a real ‘will to believe’ about time travel, which floats pretty free of any realistic scientific thinking on the topic. (Not unlike the ‘will to believe’ in UFO abduction, ancient astronauts, etc., that was such a feature of my 70s boyhood television viewing.) I thought to myself: if you really thought a nuclear war was impending, and (especially if) you thought you could either avert it or escape from it, wouldn’t you do that – rather than sit there blogging? On the other hand, if you acquire a belief for strange reasons but then act on that belief consistently, e.g. you flee from a Martian ‘invasion’ that turns out to be a convention-defying dramatization by Orson Welles, then your belief-forming is maybe flawed but at least you have integrated your beliefs with your actions correctly. So one of the lessons I drew is that you can fail to follow an epistemic norm by not integrating your (alleged) beliefs properly into your behaviour.
3:AM: You’ve drawn up a model of hell using time travel and super tasks haven’t you? Can you sketch for us how this version of hell which is spatio-temporally finite and yet eternal for those in it?
AR: Yes, I have and I’ll try to sketch it here. Partly, I blame all that on my coming from Calvinist stock and my maternal Grandmother dropping the prospect of eternal damnation into conversation when I was at an impressionable age. (“Hell mend ye” as she used to say – which must mean something like ‘May your justly-earned eternal damnation teach you the error of your frivolous ways’. I have never, ever been blasé about the idea of Hell ever since.) However, also partly it grew from the same idea that prompted the Achilles and the Tortoise paper (see below), i.e. that a time machine operated arbitrarily becomes a kind of supertask machine too but applied this time to eternal life and (sad to say) unending punishment. It also partly sprang from reading Hud Hudson’s brilliant book The Metaphysics of Hyperspace, which explores with real imagination and attack a range of extraordinary possibilities for eternal life, Heaven, Hell, judgement, etc. in terms of entirely physical model worlds. Anyway, between all these issues, I started thinking about spacetime and eschatology – what sorts of spacetime could support physical lives that offered infinitely-long (personal) time and I realised that spacetime might be finite overall but still have room for these (I confess rather horrible) infinite-duration existences. The idea is to imagine a region of spacetime where the time co-ordinate is closed, so it’s like a spacetime cylinder. Anything that enters such a region, and grows both shorter and slower in the right ratios, can spend infinite personal time confined to a finite spatiotemporal region – a sort of pocket Hell. So one cannot rule out (at least in theory) the prospect of eternal punishment just by pointing to the (likely) finite future duration of the universe as a whole – in the right kind of (time-travel) spacetime, you can live an infinitely-long life in an external time that is itself finite. (So you could – again in theory – in some real sense live longer than the universe you inhabit.)
3:AM: Through a discussion of time travel, hyperspace and Cheshire cats you argue that for a time traveller the problem isn’t Newtonian space but Newtonian Time. Can you show us why you reach this conclusion?
AR: Actually, I think I argue exactly the opposite.
3:AM: Yes you do. Just testing!
AR: Haha. Well, I think Newtonian time can be time-travel compatible but you’ve got to have a lot of freedom of movement with respect to what you can do to space. Counter-intuitive as it sounds, backward time travel can be performed against a background of Newtonian absolute time without the (very) weird spatial problems diagnosed by William Grey and Robin Le Poidevin (the ‘double occupancy’ self-collision problem and the ‘Cheshire Cat’ fading-away problem respectively) but the cost is that space has to be very flexible indeed – flexible in ways that make it thoroughly non-Newtonian. If you want to travel through time in the way made familiar by H. G. Wells, you have to be able to warp space as and when you wish.
3:AM: You’ve also looked at Roy Sorensen’s notion of Spore Gods, creatures adapted from David Lewis’s ideas regarding time travel, whose finite personal lives are distributed across infinite external time. Can you say whether you think these are logically and metaphysically possible and why you fault Sorensen in presenting them in Eleatic terms?
AR: Spore gods are beings, first discussed by Thomas Nagel and I think inspired by a suggestion from Robert Nozick, whose lives can be started arbitrarily earlier: they can lie around in spore-like form waited to be activated, so they could have the kind of same germination / development but located in different times. Roy imagined them as capable of having their lives placed ‘on hold’, awaiting activation like spores, and hence capable (in theory) of starting their lives at earlier times with no significant loss of material identity-conditions. I certainly think these beings are logically possible – even metaphysically so – but their physical implementation seems very hard indeed.
I didn’t so much want to fault Roy’s presentation as to offer an alternative way of thinking about these cases. For example, Roy (following A. W. Moore) offers a ‘veiled immortality’ thought experiment wherein the recipient gets finite personal distribute across infinite external time. In both the Moore and Sorensen originals, the ‘lucky’ veiled immortal manages this feat by living assort of staccato existence, e.g. you live forty years of normal consciousness then twenty billion years of nothingness intervenes, then you live twenty years of normal consciousness followed by another twenty billion years of nothingness .. and so on. The intervals of personal consciousness get shorter and shorter but they do so in a Zeno sequence so that there is always some personal time ahead of you no matter how tiny it may eventually become, after any finite interval of external time. Now I love these examples and also like Roy’s idea that we should value personal time above external time. (Hence Roy concludes that ‘veiled immortality’ isn’t much good in practical terms, i.e. it would feel just like being mortal but in a rather funny way.) However, I did think that it might be interesting to frame these cases without the staccato Zeno-style stops and starts, just to see how close to physical plausibility these cases could come. Which brings me to …
3:AM: Why do you say they’re nomologically accessible within General Relativity?
AR: Because of the possibility within General Relativity of a fabulous class of spacetime called Hogarth-Malament (‘H-M’) spacetimes, named after David Malament and Mark Hogarth. These are a whole family of different solutions to General Relativity which are united by the (fascinating) property that therein, an observer can circumnavigate in a finite amount of observer-time a process that takes a literal eternity to complete for its worker. M-H regions don’t oblige travellers to exceed the speed of light or travel backward in time but they do allow an amazing divergence between personal time and external time: the one can be finite and the other infinite. (One of my all-time favourite facts about General Relativity.) So I thought these might be a useful way to model spore gods and hence (I hoped) to help put Roy’s fascinating ideas about immortality and death into a framework that comes closer to physical possibility than one might think.
3:AM: And what role does a time machine play in your recent version of Achilles and the Tortoise?
AR: The time machine basically lets me play around with iterating journeys in an unbounded space but finite time, so that a journey that should otherwise be completable as a matter of logical necessity turns out to be uncompletable. This paper was partly a product of musing about using unrestricted time travel to do supertasks, and partly it was a bit of fun I confess. I’d always wanted to write a dialogue on time travel. Plus, the characters of Achilles and the Tortoise had been used so splendidly by Lewis Carroll that it was just irresistible for me to try to have Achilles as the (actually rather gullible) time-travel sceptic and the Tortoise as the (plausible but really rather cruel and arch) inventor of an H.-G.-Wellsian time machine itself. (It didn’t hurt either my realising that Carroll’s original paper and Wells’ The Time Machine in book form were close contemporaries, both dating from 1895.) The dialogue is cast as a wager about the unperformability of what sounds like a very straightforward task, but it also allowed me to say several things about (e.g.) time travel, freedom, counterfactuals and even logic that I happen to think may actually be true but which have some plausible deniability for me if put into the mouth of Lewis Carroll’s Tortoise. (It also allowed me to name-check the aforementioned 1942 Lester Del Rey story ‘My Name is Legion’.)
3:AM: Another area of interest is the Carter and Leslie’s Doomsday Argument. So, what is this Doomsday Argument and what is it supposed to prove?
AR: It’s an extremely interesting (I reckon) off-shoot of the anthropic principle – that actually very useful and sensible but often maligned and misunderstood principle that says (roughly): being a context-sensitive observer makes it overwhelmingly likely you will only find yourself inhabiting a very circumscribed range of possible environments. For one thing, we’re carbon-based (I presume) and so we aren’t having this discussion inside a neutron star or inside a quasar’s accretion disk. We breathe oxygen and need liquid water so it’s not surprising that we evolved somewhere these things are available – and hence in an atypical environment, compared to say interstellar space. (Carter was always very clear that the anthropic principle could be used by any physically-sensitive observer and was not in any way necessarily anthropocentric or inevitably tied up with our purposes – a point that a fair few critics seem to have missed.)
Anyway, the Doomsday Argument applies anthropic thinking to our place in history. It says (roughly), we should favour the prospect of imminent human extinction on the grounds that our location, qua randomly selected humans, is more probable if a large fraction of all humans there will ever be have already lived. In other words, the argument runs, if we apply anthropic reasoning to our location in history, we should increase our probability for history being close to is end. Our current location appears quite likely assuming imminent human extinction but looks less likely assuming long-term survival. (Put it a slightly different way: if humanity endures and many more humans live after us than before, we would be atypically early in history. We shouldn’t, all else being equal, plump for explanations that make us improbable if there are explanations available that make us probable instead.) I have a lot of sympathy with Carter and Leslie’s general methods but I do think that there is merit in the criticism that say that Doomsday reasoning asks us to discount selectively data about our location in history and treat ourselves as randomly-drawn humans when the whole notion of a ‘random’ draw seems hard to finesse here.
3:AM: What’s Timothy Chambers’ ‘Ussherian Corollary’ and why doesn’t it work?
AR: As I understand it, the ‘Ussherian Corollary’ (named after Bishop Ussher’s ingenious dating of the Creation to 4,004 BC) tries to parody the Carter/Leslie Doomsday model by saying: if we should make our location appear as likely as possible by updating our beliefs in favour of impending human extinction, we should also update our beliefs in favour of recent human creation. The idea being that a short past makes us more likely just as (allegedly) Carter-Leslie Doomsday urges us to accept that a short future makes us more likely. My worry about this is that it assumes that the aim of the Doomsday Argument is to make our location appear likely no matter what, or rather to make our location appeal as likely as possible regardless of our evidential background. So even if the Doomsday Argument and the Ussherian Corollary are formally parallel (which is itself disputable but let that go), the two arguments are evidentially highly dissimilar – we have population data from the past but not the future, in a nutshell. The moral (I take it) is that Doomsday reasoning, like any anthropic argument, must involve probabilistic reasoning from an assumed evidential base – it isn’t some a priorimagic aid to making locations probable at any cost whatsoever.
3:AM: You’ve also engaged with Nick Bostrom’s discussion of the Doomsday Argument and his Simulation Argument. To begin with, can you sketch for us what this argument is and what it’s supposed to show?
AR: The Simulation Argument (about which I have my reservations be it said) essentially says this: if functionalism is correct and being conscious basically just is running the correct sort of programme, then advances in computing might one day allow computer-simulated minds to out-number carbon-based minds. In particular, Bostrom defends a trilemma for functionalists who think advanced (‘posthuman’) civilisations would run lots and lots of simulated minds. Specifically, such functionalists must distribute their credences between the following three options: a) posthumans are rare, b) posthumans run few simulated minds or c) we’re probably simulated minds here and now. So, as a corollary, if we discount the possibility of our being computer-simulations run by posthuman civilisations, we must either expect that we never acquire posthuman technology or that posthumans choose to run few simulations. Note Bostrom is not claiming that we should believe that we are simulated minds; merely that believing we are probably simulated minds is one of three options that confront technologically-optimistic functionalists about the mind. So the upshot is: functionalists should note that any evidence that simulating minds is easy / getting easier actually supports the conclusion that we are simulated minds ourselves. All else being equal, we should favour the hypotheses that makes our location probable over explanations that make it improbable – of which, more anon …
3:AM: You suggest that anthropic reasoning can lead from this argument to what you say are ‘odd conclusions about computation and our place in reality’. Can you say what you take ‘anthropic reasoning’ to be, and where it takes us?
AR: At its most straightforward (and I would say its most plausible) anthropic reasoning asks us to take account of the kind of observers we are when we’re investigating the physical world. We are highly context-sensitive creatures, it seems, who require a very wide range of conditions to be met before we can function. (Gentle reader, you didn’t grow up on a neutron star, I’ll wager …) Hence, says Carter, we should be able to infer something about our location in the universe (in time as well as space) by paying attention to the sort of conditions we need to help us survive. Carbon-based observers are overwhelmingly more likely to evolve where temperatures, pressures, radiation-levels, etc. are amenable to carbon forming (and maintaining) complex bonds, etc. So the conditions you will observe may be typical for those of observers of your kind but highly atypical of the universe as a whole. Earth-like planets seem to be a very small volume of the observable universe compared to intergalactic space, yet we live on a planet and not floating freely in vacuo.
Now of course there may be, for all we know, forms of life out there that owe nothing to carbon chemistry at all and can survive in conditions that would destroy life like us well-nigh instantaneously, but pending observation of such alternative life, we have to assume that we are typical of life as it exists. The key idea is the good old Copernican ‘Principle of Mediocrity’ idea that we should not assume ourselves atypical unless we have good reason to the contrary. (Note the Anthropic Principle is not really opposed to the Copernican Principle at all – it simply cautions us to take account of the bias introduced into our observations by the circumstances in which it is likely we have evolved.) Likewise, there may be very unlikely lifeforms out there, eking out a living by sheer fluke, but again, we shouldn’t prefer explanations that make us unlikely over explanations that make us likely. So really anthropic reasoning is about using the kinds of observers we are to distribute probabilities over different hypotheses about life and how it is distributed. So the Simulation Argument and the Doomsday Argument can both be read as extensions of anthropic reasoning, although I confess I don’t think the upholder of anthropic ideas is obliged to follow either argument terribly far.
3:AM: For some people the paradoxes about time travel and the issues discussed here may strike them as being no more than clever party tricks. They may generalise and say that’s all philosophy really is. How would you defend philosophy from those who don’t think it’s worth heeding anymore?
AR: I have (perhaps surprisingly) a lot of sympathy with the view that holds that philosophers’ thought experiments are in real danger of vanishing up their own noses, (shall we say). But also, firstly, I like to quote an apposite remark from Richard Dawkins, namely that “Thought experiments are not supposed to be realistic – they are meant to clarify our thinking about reality” (The Extended Phenotype). Secondly, the view I have of time travel (which I think could be attributed to David Lewis) is that the ‘paradoxes’ (so called) of time travel are really party tricks – the paradox arguments aren’t decisive and if we want to know whether or not time travel is possible, we need to leave our (philosophical) armchairs and try to engage with what physicists have to say.
While there are really good scientific reasons to be sceptical about the physicalpossibility of time travel, philosophers’ arguments against the logical possibility of time travel (or arguments that time travel is necessarily nonsense or meaningless etc.) strike me as signal failures each and every one. (No names, no pack-drill but one still sees the occasional paper in whose laborious course we learn merely that some aspect of time travel conflicts with some readily-dispensable intuition of the author’s, etc.) But then, a thing may be logically possible and yet still fail to occur in our world, or in any world like the one we think we live in. As I say to my students, it’s logically possible that I could take six marker pens, throw them at the wall and watch agog as they just happen to write out what reads like a ten-line proof of Goldbach’s Conjecture before they hit the ground. But it ain’t going to happen – I assume.
Because logical (im)possibility is such a powerful weapon in the philosopher’s armoury, it’s often worth emphasising how big a gap can separate the logically possible from the physically possible or technologically feasible. David Lewis though is very clear that a time travel world might be logically possible and yet still weirdly unlike the world that we think we live in. So “Back to David Lewis”, say I.
3:AM: And for the readers here at 3:AM, are there five books you can recommend that will take us further into your philosophical world?
AR:
David Berman, George Berkeley: Idealism and the Man, (Oxford, Clarendon, 1994). My favourite book about Berkeley: a book which resembles its author in being wise, articulate, learned and humane.
Craig Bourne and Emily Caddick Bourne, Time in Fiction, (Oxford, Oxford University Press, 2016). This is a modern classic – a thorough-going look at the metaphysics of time as it is reflected, portrayed, handled and (sadly sometimes) mishandled in fiction, that takes in everything from the scope of the witches’ foresight in Macbeth, via of course some notable time-travel cases, to the eerie stop-motion monstrosities in films like Clash of the Titans (especially the Ray Harryhausen version) and The Babadook. I’ve never seen fiction and metaphysics linked together to such detailed and fascinating effect. It also has some tremendously suggestive remarks about quantum theory and ontological incompleteness. (Should its authors read this, love your work and I apologise for my remarks above about ‘A Sound of Thunder’ and inconsistency.)
Lisa Bortolotti, An Introduction to the Philosophy of Science, (Cambridge, Polity, 2008). Although presented as a textbook (which it is too), this is a tremendously reflective and original take on many aspects of the philosophy of science and scientific method. In particular, I recommend the sections on the ethics of scientific research – topics rarely, if ever, covered even in otherwise very good books on these topics.
Nancy Cartwright, How the Laws of Physics Lie, (Oxford, Clarendon, 1983). Wonderful collection of essays, mainly on scientific laws and the tension (says Cartwright) between generality and truthlikeness. Every single item a gem.
Bas C. van Fraassen, The Empirical Stance, (Yale University Press, 2002). Bas van Fraassen has always been a tremendous philosopher of science but this volume shows him applying his unmatched gifts to questions of how we should approach belief in science, and more generally. How (and when) to adopt a theory, what adopting a theory means and how we should conceive of science, belief and even life generally – all viewed through a fascinating empiricist lens. (Also contains some of the best writing on Pascal’s Wager I have ever seen – in my view, some of the best since Pascal actually.)
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Richard Marshall is still biding his time.
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