Can We Change the Past? Time Travel Paradoxes (Philosophy Lecture)

[00:00:07]hi everyone my name is Alex Casman I'm one of the philosophy fellows here at failure and today I want to talk about time travel so time travel of course is a concept that's fascinated philosophers and non philosophers alike for many centuries and it's not difficult to see why just imagine all of the wonderful things that you could do if you had a time machine you could travel back in time and see the building of ancient civilizations you could talk to famous historical people you could even find out where you left your keys last night but if you look at most time-travel stories what you'll see is hopes not just obscene the pastor witnessing it but also hopes of changing the past of killing dictators before they come to power or warning your past self about various tragic events that happened in your youth and so on so what I want to ask in this lecture is as such hopes really coherent is it possible to change the past now time travel is a narrative device of course is completely ubiquitous in in popular culture from you know science fiction stories like Star Trek and Doctor Who and to romantic fiction like the Time Traveler's Wife silly comedies like Bill & Ted's Excellent Adventure and Hot Tub Time Machine about which probably the less said the better most histories of time-travel stories will trace this popular fascination with time travel back to HG Wells is the time machine published in 1894 it's often incorrectly described as the first time travel story in fact it wasn't even the first time travel story by HG Wells himself that honor goes to the cronic Argonauts published five years before in the science school's journal one year before that Henrique Gaspar published a Crona nautical journey wonderful novel that includes to my knowledge the first ever discussion of what has become known as the grandfather paradox and the first science fiction time travel story which I define as a time travel story where the time travel is achieved by means of a man-made object is the age of science by Francis pericarp who is writing under the pseudonym Merlin Nostradamus rather wonderful pseudonym the the man-made object in question is a sort of temporal version of a telegraph machine which takes pictures of newspaper clippings in the future and sends them back in time - to the 19th century Bailey all classicists among you may be amused to learn that the printed matter of this gigantic periodical the the photographs that are sent back in time is said to equal at least in bulk the whole of Gibbons history will mr. Jarrett's edition of Plato so there you go a nice little Baylor connection for you that um the first ever story in which a human is sent back in time to my knowledge is the story of a beer bong azam which was published as a novel in in the 1850s but published before that in 1936 in the family magazine music if community of artists and before all of that in 1733 we have what I think is the first example of backwards time travel at least memoirs of the 20th century by Samuel Madden a satirical account of the 20th century which are ostensibly men was sent back in time to the author by an infallible guide this book had something of a publication history four days after its publication all 890 unsold copies were recalled by the author and subsequently destroyed and no one not even his publisher to whom we know this account knows why he chose to do this he was clearly spooked by something but what I guess we'll never know before that I'm afraid things get a bit murkier so it's controversial whether there there is any time travel in fiction before the 1700s some have argued that this time travel going all the way back to the mahabharata and sacred Arabic texts it's controversial partly because it's controversial what's going on in these stories but also partly because it's controversial what exactly should count as time travel in the first place so let's now turn to that question of what time travel actually is and maybe we can approach this question by starting with what time travel is not so there's a few things that time travel definitely isn't firstly calendar tomfoolery in other words changing time zones this is not time travel okay some authors surprisingly seem to be a bit confused by this so one nice example is that pan cronic on a novel by Harold Steele McKay in which the protagonist reasons as follows what is it that makes the days go by ain't at the daily revolution of the Sun so if a fellow was the world clear around the world and cut up all the meridians in the same direction as the Sun and he made the whole trip around just as quick as the Sun did wouldn't time change not a might for him and if a fellow we'll just take a group of the North Pole and go whirling round it he'll be cutting meridians as fast as I hate other and if the Sun goes the other way around ain't it shorter unwind all the time that to be rolling up to which the answer is of course no the reasoning here goes wrong at the very first step because the Sun the daily revolution of the Sun is not what makes time go by so you can't go back in time just by changing your position relative to the Sun and Dasom and a philosopher Dave batsman from whom I got this example is fond of describing this as the nigel Tufnell fallacy so changing time zones won't make you go back in time any more than making the dials on your amplifier go up to 11 you will make the amplifier louder that's one of the spinal tap fans there okay so calendar tomfoolery not time-travel here's another thing that time travel isn't visions or premonitions of the future or the past if you have a vision over the past you haven't literally traveled to the past again that might seem straightforward but sometimes in time travel stories are all reported examples of time-travel stories it's not clear whether someone is merely having a vision of the past or literally there one nice example of this is Dickens A Christmas Carol at one stage in the book Scrooge is said to be walking along a road recognizing every gate and post and tree until a little market town appeared in the distance with this bridge inist Church in its winding river so here it seems for all the world like Scrooge has has literally traveled back in time to the time of his youth but later in the book the ghost named me the ghost of Christmas past in this case says these are but shadows of the things that have been they have no consciousness of us and that seems to suggest that really what's going on is that Scrooge is merely having a vision of his past from a third person point of view as it were and not really traveling to the past well it's controversial there are other bits in the book which are less easy to reconcile with that interpretation in any case visions of the past or the future is not time travel thirdly long sleeps and here's where things get a little bit trickier so consider for example the sort of pre-christian story of the Seven Sleepers this is a story where a group of people go to sleep in a cave and they emerge upon waking to find that it's 300 years after they went to sleep so they just slept for a very long time basically on the face of it this doesn't seem like time-travel all that's happened you might think is that these people's lifetimes have been sort of artificially extended and that's not time-travel same thing going on and sleeping beauty of course and whiff and Winkle and lots of other sorts of stories on reflection though it's not completely clear that this shouldn't be counted as time travel and one way of seeing this is to think of other stories which are similar to these long sleep stories where it's sort of not so clear our intuitions about them and I'm not so clear one for example of course is Futurama fry in the pea trauma series accidentally falls into a cryogenic freezing machine and gets frozen and emerges a thousand years later in the future is that time travel to the future sort of less clear I think same thing happens in Captain America and it might be argued that the same thing happens in HG Wells is the time machine in many ways the completely typical example of a time travel story here's how the first experiment with the time machine is described in the novel I drew a breath scent my teeth grips the starting lever with both hands and went off with a thud mrs. watchett who's the housekeeper came in and walked apparently without seeing me towards the garden door I suppose it took her a minute or so didn't reverse the place but to me she seemed to shoot across the room like a rocket I pressed the lever over to its extreme position the night came like the turning out of a lamp and in another moment came to tomorrow night King black and day again not again dig and faster and faster still so this certainly seems like time travel into the future from this perspective at least but now imagine the same scene but seemed from the point of view of a third party maybe a visitor to the laboratory Stephen law philosopher imagines a retelling of the story from this point of view so he says the housekeeper led me through a door into an amazing Victorian laboratory filled with experimental equipment but the most astonishing thing of all was that that was that there sat in the saddle of his gathering brass ivory and crystal machine was the time traveler himself I thought you were off travelling in time I gasped there was no reply from the time traveller in fact he remain strangely motionless he can't hear you explained mrs. watchett but you said he was travelling in time I said he is replied mrs. watchett and now it's me that has the dust him every week so seen from this perspective the case of the time traveller in the time machine seems very like the case of the people are just going to sleep for a very long time and yet I thought the time machine was the typical time travel story so should these cases should all of these cases be classified as time travel after all so to clear up these questions we really need a definition of time travel we can't really avoid the question any longer what is time travel and almost as soon as we raised this question we almost immediately encounter a problem and it's a problem that's so thorny the many philosophers have concluded on the basis of this problem that time travel itself the very idea of time travel is itself just incoherent so here's the problem we all know what travelling through space involves it involves being in different places at different times but if that's what traveling through space involves there travelling through time must involve being different times at different times but that's nonsense right that's just complete nonsense it's no better than saying that I'm in different places in different places it makes no sense at all so somehow a definition of time travel has to face up to this problem that traveling through time involves being in different times at different times how can we make sense of that so perhaps the the most popular solution to this to this problem in the philosophical literature at least is due to the philosopher David Lewis who's my favorite philosopher she has this account of how things persist through time and according to Lewis the way that things like people and tables and chairs and so on persist through time and in particular persist through change while remaining the very same thing is exactly the same way that they extend through space namely by having paths at different times so according to Lewis enduring things like tables chairs people are as he puts it streaks through the manifold of space-time there are these four dimensional objects which are composed of temporal parts or equal stages located at various places and indeed at various times and he also gives an account of what it is for something to change what it is for a thing to change is just for there to be a difference in its various stages a qualitative difference between the stages of that thing in just the sense that change in scenery from east to west is a difference between the eastern and western spatial parts of the landscape so this landscape here changes from left to right in virtue of having a part here that's different from its party in just the same way a thing changes through time in virtue of having a part at one time that's different from its part at another time that's what it is for something to change according to Lewis now he goes on to say well if you take the stages of a common person ie a non time-traveling person these stages will manifest certain regularities with respect to external time by which he just means time first come the infantile stages last come the senile one's memories accumulate food digests hair grows and so on so if you take a common person and remember a person according to Lewis is this four dimensional object which I will attempt to represent in two dimensions by means of this diagram here and we can take the various stages of the person and we can label these stages according to how much time has passed since the person first came into existence so here one year five years thirty years eighteen years and so on and what we see is that common people manifest certain regularities as time goes on or rather that's the way the differences between their stages manifests and regularities the first stages are infantile the last stages are senile and so on but of course the stages of a time traveler are not like this the stages of a time traveler do not manifest the common regularities with respect to external time so consider a following person this is a slightly weird person it's scattered in the sense that it's made up of these two discontinuous paths and if you think about this person's life from the point of view of external time it looks like this they come into existence fully formed at this time as a fully formed adult they grew older they died some time passes then I'm born they grow older and then they disappear okay so this is a kind of a weird person as before we can label the stages of this person according to how much time has passed since they first came into existence and let's call this their external age so the external age of this stage is zero 50 150 180 and so on now of course this person the stages of this person do not manifest the usual regularities with respect to external time but there is a way of assigning coordinates Lyra says - the time traveller stages so that the regularities that hold with respect to this assignment match those that commonly hold with respect to external time so for example this stage here is the stage that's commonly had by a person when they are one year old so we'll label that one and then this stage is this sort of similar to the stage had by a common people when they're five years old and so on thirty thirty again and then eighteen and let's call that the assignment of coordinates to the stages which preserves the regularities that normally obtained with respect to external time let's call that the the personal age of the person and what we can see is that these two assignments of numbers diverge there's a discrepancy between the external age of the person at a particular time and the personal age of the person in a particular time now Lewis calls these external time and personal time I think that's a mistake as he's very clear that there's not two different kinds of time here there's just one kind of time time rather there's just two different ways of labeling the stages of a person and according to Lewis time travel just is discrepancy between these two assignments discrepancy between a person's external age in their personal age that's just what it is for someone to travel through time so that's his definition for something to travel through time it's for there to be a discrepancy between its external and it's personal age now this definition isn't perfect for one thing you'll notice it only works for people so far now you might think that we can extend this definition in a natural way to other things which manifest certain common regularities with respect to time like bananas for example and bananas usually start off unripe and then get ripe and then go black and so on so maybe we can define a kind of personal age for bananas but what about things like electrons electrons of fundamental particles they don't go and undergo any intrinsic change at all they don't manifest any kinds of regularities with external time release on the face of it so on Lewis's definition we wouldn't be able to make sense of time travel for electrons but seem like electrons should be able to travel through time so there's a bit of work to do with this definition it's still a work in progress nevertheless it's the closest thing to to the sort of commonly held position on what time travel is in the philosophical literature it's for there to be a discrepancy between the external age of a thing and the personal age of the thing understood in some way or other we can apply therefore this definition to our question that we left hanging earlier about where the long sleeps counters time travel well according to Lewis's definition a long sleep would Countians time travel to the future but only if the processes that go into defining someone's personal age possibly things like memory accumulation hair growth and so on all slowed down by the same amount by the same rate if you like and normally in sleep that's not the case there are sort of cognitive processes that are going on don't so slow down when you sleep so it would have to be something like cryogenic freezing where all other processes really do slow down by the same amount but they go maybe a long sleep really is time travel to the future on this definition okay so let's now now that we have this definition of time travel let's go back to our starting question the question we started with is it possible to change the past so on Lewis's conception of time travel it's important to distinguish two different two different questions one is whether we can cause Lee effects the past and on Lewis's view there's really no problem with this time travelers can causally affect the past if by that you mean cause things which have already happened to happen there's no paradox there I mean it's kind of weird this idea that you can cause things which have already happened to happen but it's not inherently paradoxical on Louis's view one nice example of this from from fiction is Harry Potter and the Prisoner of Azkaban early on in the story Harry is being attacked by the Dementors and he sees in the distance this bright light which resolves into the shape of a stag and this sort of ghostly stag chases away the Dementors and saves Harry's life how he assumes that this Patronus has been summoned by his father but later in the novel Harry travels back in time and witnesses this very same scene from a different point of view and he's waiting for his father to arrive to save his past self and he he's waiting he's waiting and his father doesn't turn out and then of course he realizes that the Patronus of the stag was actually summoned by himself his future self and so sure enough he then summons the Patronus which chases away the Dementors and saving his past itself so this is an example where a time traveler causes something which has already happened to happen and there's nothing paradoxical about that on Lewis's view what time travelers can't do according to Lewis at least modulo the qualification about to make what they can't do is alter the past that is make something that has happened not have happened or on the other hand make something that didn't happen have happened that you can't do given that something has happened there's nothing you can do now that would make it the case that it didn't happen because it did so so it did the past is unshakable nothing you can do now can change that and this this sort of consumption of time travel it again goes back to the very early history of time travel stories there's this lovely scene in the novel the gaspin of all the chrony nautical journey where one character says since we are bound for yesterday and will arrive in the past bearing the experience of history wouldn't we be able to change the human condition by avoiding the catastrophes that of course so much turmoil in society in other words would we be able to change the past to which another character applies not in the slightest we may be present to witness facts consummated in preceding centuries to which Lewis would add and indeed we may cause those very facts to happen but we may never undo their existence okay so so this is really as close as you get to a sort of fundamental rule of time-travel in fiction and in fact fiction is replete with examples sort of morality tales of the things that might go wrong if you consider yourself foolish enough to be able to to alter the past so one nice example of Venice is an episode of The Twilight Zone one life furnished in early poverty in which a person goes back in time to try and prevent a lot of the traumatic events in his youth that he thinks led to his life going badly from then on and so he goes back in time he gives himself the alias mr. Rosenthal and he finds his young self his young past self and he befriends him and he protects him from bullies and his father and so on and then he leaves ie goes back to his his time and at the moment he leaves he has his realization he realizes that he remembers mr. Rosenthal and he remembers when mr. Rosenthal left and what a traumatic experience that was that this person came into his life and then left as soon as he'd arrived and he realized that it wasn't the bullies or his father that caused his life to go so wrong it was mr. Rosenthal in other words it was him he is responsible for his own downfall so by trying to prevent events from happening he ended up tragically causing the very events he was trying to prevent to occur so this is a good sort of warning to those who think they can change the past be careful because in trying to prevent what has happened from happening a best you'll fail at worst you'll cause the very thing you're trying to prevent now once you start thinking of time travel in this way it's easy to get a bit fatalistic or to feel a bit fatalistic about the degree of control that we have over the course of history and one way of getting at this question is to think well if time travel were possible what would stop me going back in time and killing my own grandfather hence what's become known as the grandfather paradox it seems like it's a paradox because on the one hand the time traveller of course can't kill his own grandfather because if he were to kill his own grandfather his grandfather wouldn't give birth to the Time Traveler's mother or father and so the time travel of wouldn't be born and so winters sort of gone back in time in the first place but I on the other hand it seems like well what exactly is stopping the time traveler from pulling the trigger I mean if he manages to get into this situation the line of sight is clear the weapon is working perfectly no one is waiting in the wings to stop this from happening what exactly is stopping this from happening the laws of logic so in the one hand it seems like the time traveler couldn't kill his own grandfather on the other hand it seems like well why couldn't he David Lewis of course considers this paradox and he concludes the both conclusions are true the time traveler can kill his grandfather and he can't these are compatible he says because the word can is equivocal to say that something can happen means that it's happening is composable or compatible with certain facts so to illustrate this Lewis considers the sentence I can speak Norwegian now there's a sense in which I can speak Norwegian I have what it takes physically speaking to speak Norwegian I have a tongue I have a mouth vocal cords and so on I have all of the sort of machinery needed to speak Norwegian I can speak Norwegian in the sense the bird can't speak Norwegian but of course there's another sense in which I can't speak Norwegian I'm not a Norwegian speaker both of those things seem true how is that possible well can the word can is equivocal when I say that I can speak Norwegian I'm saying that my speaking Norwegian is compatible with certain sort of physical facts about me and when I say that I can't speak Norwegian I'm saying that my speaking Norwegian is incompatible with certain sort of cognitive facts about me and both of those things could be treated similarly Urist says imagine the the time traveler here who's pointing the gun at the grandfather there's one sensing which he can kill his own grandfather his doing so is compatible with certain facts about the situation but there's another sense in which he can't because his killing his grandfather is not compatible with a broader set of facts which includes perhaps among other things the fact that grandfather did not die at this time so he but if canon he can't but notice that there's really nothing specific to the idea of killing your own grandfather here in fact exactly the same considerations apply to any attempt to alter the past of course there's something particularly vivid about killing your own grandfather but consider a much more mundane example I left the bathroom light on last night the light in the bathroom was on all night suppose I were to travel back in time what would stop me turning off the light well there's one sentence in which I can turn off the light my turning off the light is compatible with certain facts about the current situation but there's another sense in which I can't because my turning off the light is not compatible with a broader set of facts which includes for example the fact that the light stayed on all night so I can turn off the lights if I travel back in time where there's no the sense in which I can't exactly the same considerations apply there's nothing specifically problematic about killing your own grandfather and notice also that exactly the same considerations apply to any attempt to alter the future this is often a detail that's missed in discussions of time travel even by people who should know better it's often said that there's nothing inherently problematic with time travel to the future nothing paradoxical about that but exactly the same considerations apply as to time travel into the past if something will happen in the future then there's a sense in which nothing I can do now will prevent that from happening I can't prevent something that will happen from happening if it will happen then it will happen so of course there might be a sense in which my preventing something that will happen in the future from happening is compatible with certain facts about me at the time but it will be compatible with a broader set of facts which includes the fact that it does in fact happen in the future so on the sort of Lewisham way of thinking about time travel as it were the entire course of history thought of from outside of time is already or of course I can't use that phrase but already written I'm just living my life through this course of history we can say that I can make things that go differently to how they actually will go but that's only if we hold certain facts fixed if we hold all of the facts fix then nothing I can do now will change either what has happened or what will happen so those are if you like the ground rules that are laid down by a Louis Ian where they're thinking about time travel now as we know not all time travel stories stick to these rules many time travel stories will have their protagonists you know do all kinds of things in the past which according to that very same story never actually happened I hate to pick on Back to the Future but I will so Back to the Future is one particularly hilarious example of this so in Back to the Future for those who don't know Marty travels back in time to 1955 in a souped-up DeLorean car he then immediately goes about messing about with the past by pushing what turns out to be his father out of the way of an oncoming car driven by what turns out to be his mother now we learn later in the film that Marty's mother was supposed to run into Marty's father and then nurse him back to health and that's how they get to know each other and then there starts a long love story which inevitably ends with Marty's being born we know this because Marty meets the young doc Brannon in the past and tells him what he's done and this makes doc very worried that all these changes in the space-time continuum are being affected by Oh Marty's having arrived in the past and messing things around now the film makes some concessions to the server inherent paradoxically of all of this by making Marty you start to disappear which sort of sparks a hilarious narrative rebel he has to get his parents back together to avoid him disappearing completely something written out of history of course from a Louison points of view all of this is complete nonsense right if you if you try and change the past you won't disappear you'll just fail because what you're trying to do is impossible so you already know that you will fail now of course you know we can forgive Back to the Future these these kinds of messing about with the ground rules in fact you know some time-travel stories are just perfectly explicit about how little they care about sticking into these rules one sort of example is the film looper in which sort of weird Joseph gordon-levitt Bruce Willis hybrid travels back in time to meet himself in a diner and there's a scene where young weird Joseph gordon-levitt Bruce Willis hybrid asks what seems like a perfectly reasonable question do you know what's gonna happen you done all this already it's me great question young weird Joseph gordon-levitt Bruce Willis hybrid effectively what he's asking is is this past alteration the old version by going back in time is he changing the past or is he just reliving what has already happened from a different perspective here's the reply I'm gonna talk about time trappers if we start talking about don't be here all day talking about and making diagrams and strong extraordinary so this is the the writers of this film basically telling us look you don't care if this doesn't make sense you're not to know you know you're not here for a lecture on time-travel you're just here to be entertained so just sit there quietly and don't ask too many questions but unfortunately for you guys you are an electron time travel also no such luck for you so we really knew do need to address this question head-on are these stories where people are apparently changing the past in other words preventing things which did in fact happen from having happened are these stories simply logical contradictions do they simply make no sense at all or maybe just maybe is there a way to make sense of past alteration and indeed future alteration on which it's not a contradiction after all whether it's physically possible as a different question but it's at least not the logical contradiction so one idea that's been floated and which is very popular in science fiction circles much less so actually in philosophical circles for reasons that might become apparent is this idea that paradox can be avoided if time is allowed to split or branch so the idea here is nicely expressed by Martin Gardner in his book on time-travel he says look the basic idea is as simple as it is fantastic the moment a time traveller enters the past the universe splits into two parallel worlds each with its own time track along one track rolls the world as if no leaping had occurred along the other track spins the newly created universe its history permanently altered so I guess we're supposed to imagine something like this in the grandfather case so we've got this one timeline here where the grandfather stays live if I was born a man's born and then the man travels back in time thereby creating this new branch where the grandfather is killed by the time traveler and therefore the time travelers never bore and although they do in this branch come into existence by time-traveling from the future now one problem with this attempt to make sense of past alteration it's sort of obvious when you think about it namely that the time travel is not really altering the past at all so much is just creating a new past so it's not as if anything on this branch is being changed rather a new branch has been created where things go differently and this again was realized by the very early time-travel fiction authors including in the first ever explicit discount discussion of branching timelines in David R Daniels the branches of time the time traveller sort of recollect you know reminiscing about his his intentions comes to a sort of depressing realization he says I did have an idea to go back to make past ages more livable but it isn't any use think of all they suffered I could go back and save them those wrongs and yet all the time they would still have known their own happiness and their agony because in this world line the one who's just left I guess those things happened at the end it's all unchangeable so when we have branching timelines we're not really having past alteration as such and yet many time-travel stories do present themselves as stories where the past is being altered not just new pasts being created but there's a more fundamental problem with this suggestion even before we get to to this problem so let's go back to this quote from from Gardner here he says the moment a time traveler enters the past the universe splits but on reflection this doesn't really make any sense because what we were being asked to imagine is that a new timeline is created at a particular time right so for a new time and to be created we need to suppose that it didn't exist at one time and then does exist after that time but it's time itself that's being created so we're back to the very first paradox we started with where we're starting where it seems like we're talking about things being time itself being different at different times the only way to really make sense of this is to imagine that there's a second dimension of time relative to which we can make sense of time itself the structure of time itself changing but if we don't have a second dimension of time then this really doesn't make any sense so presumably what we're being asked to imagine here is not new timelines being created but rather just two timelines existing a temporally speaking as it were to start with and it's not as if the time traveller himself is creating these new timelines they just exist full-stop and once we think of things in that way it looks like these sorts of branching time structures are going to have some pretty weird consequences that it doesn't seem like explicitly acknowledged in time travel stories so for example think of what is the case at this particular time here just before time splits into two different branches call that time T at that time it's true the grandfather will die tomorrow why is that well because there's a time one day after that time at which grandfather dies so bran father will die tomorrow true at this time but it's also true at this time that grandfather won't die tomorrow why well because there's a time one day after this time over here where grandfather doesn't die so grandfather will die tomorrow true grandfather worked out tomorrow also true at the very same time now whether you think that such contradiction depends on how you like your temper or logic about which more in a moment but at the very least it doesn't seem like what's really going on in time travel stories you know in in 1954 just before marty is about to arrive back in time it doesn't seem true both the marty will exist in a year's time and that he won't exist in a year's time at least that's not however Back to the Future film represents itself so for all those reasons I think branching timelines is is not the right way to think about genuine past alteration on the assumption that such a thing is even coherent to start with so in last sort of five minutes of this lecture I want to talk about a different approach a different way perhaps of making sense of past alteration so so far we've been assuming what philosophers called the Beave theory of time the the label comes from the philosopher JME McTaggart according to the B theory of time reality fundamentally speaking is just this static four dimensional block with time of course as one of the dimensions and what happens at different points in that block is as it were fixed it's not clear that we can even go here and you say that it's fixed because that would suggest there's an extra dimension of time relative to which we can think of what's going on in the four dimensional block as changing but of course there isn't but nevertheless fundamentally what is the cases just this four dimensional block and a description of what's going on in different places in the four dimensional block so according to the B theory when I say the package will be delivered tomorrow what I mean is that there's a bit of reality that the bit in the future one day in the future where the package is being delivered that bit of reality exists doesn't exist in my bit of reality but it exists in a bit of reality in the future but perhaps that's not what the universe is really like maybe change genuine change is more than just variation along the temporal dimension as Lewis posit maybe the universe is fundamentally dynamic in nature so maybe when I say the package will be delivered tomorrow I'm not saying there's a bit of reality where the package is being delivered on this view the package is not being delivered in any bit of reality rather what reality is like itself fundamentally speaking changes over time and that change can't be understood as merely variation along a bit of dimension of reality rather reality itself is changing over time this is what's called the a theory of time and if the a theory is correct then once we get used to the idea that there can be genuine change and how things are it's actually not that hard to start to see how they might be genuine change in how things work or indeed genuine change and how things will be so to see how this might work let's go back to back to the future here is Marty he's escaping from the Libyan terrorists who just killed Brown at the beginning of the film and according to the theory here's what the fundamental description of the world is right now so right now doc is dead and of course Marty did not exist in the 1950s because this is before time travel has occurred but here's another thing that's true according to the film Back to the Future here's another thing that's true at this time quite soon in particular when the DeLorean hits 88 miles an hour and the plutonium reactor is activated quite soon things will be quite different to how they were in particular Marty will have met his parents as teenagers in the 1950s he will have played Johnny be good at a 50s high school prom and he will have warned doc about the terrorists and saved his life right now none of those things happened but after he hits 88 miles an hour all of those things will have happened in the past that's what's true according to the Back to the Future film unfortunately it turns out that these claims are inconsistent in standard tense logic and in particular the reason for that is because they violate was called the GP axiom of tense logic which roughly speaking is the claim that if something is the case it will always at some time have been the case and once you think about it it is clear how this axiom is going to rule out past alteration scenarios like these because for example Doc Brown is dead right now and so on this according to this axiom it will always have been at some time the case that he is dead killed by the Libyan terrorists given that you know his body currently is ridden with with bullets it will always at some time have been the case that his body is written with bullets this basically encodes this axiom basically encodes the claim the just cannot be altered and so we want a tense logic that will allow for past alteration scenarios to come out as consistent we need one on which their GP axiom can be violated and that's basically what I've been spending my time trying to do in the past few months is to come up with a tense logic that allows for past alteration scenarios where you get violations of the GP axiom turns out this is fairly easy to do and the results are slightly interesting and I can talk about them more in the Q&A after this but somewhat ironically we have run out of time for the lecture so I will end things there just a few notes of thanks before I finish in particular to two people one my investment and his his wonderful book on the paradoxes of time travel which is the definitive guide to the philosophical literature on time travel and secondly Michael Maine's big list of time travel adventures which is exactly as it sounds the most comprehensive archive of time-travel stories from antiquity to the present day it's a really wonderful resource so thanks very much to both of those and to all of you for listening thank you