How are you there. Sir Richard speaking to you again, lecturer on the final week, week seven, of this Introduction to Philosophy course, the Time Travel Lecturer. And I've been taking part in a quite a lot of online thread discussion with some of you, and I've enjoyed it very much. Thank you for all your comments. Keep your comments, keep your threads coming. I've tried to address as many questions as I can. My apologies if I haven't got right to your particular problem or your particular thread. I'm doing my best, but there's probably slightly more than I can manage. Hence in large part, this short recorded follow-up video. I want to try and revisit some of the points I've made and to try to address some of the questions that have come up. Now bear in mind that all I've been doing is to defend David Lewis's argument that backward time travel is logically possible. Neither Lewis, nor I, is saying that time travel is physically possible or physically realistic or that time travelers walk among us. Why this emphasis on logical possibility? Well, if something is logically impossible, it's contradictory. And it's a very plausible assumption that the contradictory cannot possibly be real. So it's a very powerful tool to establish that your opponent's position commits the opponent to contradiction. To make a factual error is bad, but to contradict yourself is presumably game over. So one serious implication of Lewis' argument is if he's right, then time travel can't be dismissed on logical grounds alone. And consequently, physical models that seem to permit travel, like Godel's universe. Gott's cosmic strings that, or a sewing machine, the Tipler cylinder, et cetera, cannot be ruled out logically. You need physics, you need the appeal to observation. They're a crucial part of Lewis' argument is to say that time travel is logically possible, granted a distinction between personal time and external time. Personal time is not some other temporal dimension, some meta time. But rather the way that time is registered by the traveling object. And for most of us, for non time travelers among us, I'm certainly not a time traveler by the way. For the non time travelers among us, personal time and external time march in lock step. Five minutes of increasing personal time for me will be five minutes of increasing personal time for you. But for a time traveler, personal time and external time diverge. Personal time can be registered by the accumulation of the traveler's memories, the progress of the traveler's watch, the progress of the traveler's digestion. And I stress that this distinction is a well-supported one with negativistic and quantum physics. Are very good reasons for thinking that time actually passes at different rates in different frames of reference. Time may even have different directions in different frames of reference. So it may well be that everything that's requisite for Lewisian time travel is out there. Something that has caused quite a bit of debate is the distinction between what I call replacement change and counterfactual change, or counterfactual impact. Now I stress these are two ways of registering the impact we can have. It's not our philosopher's play thing. The replacement change counterfactual change distinction is not a mere jargonese what cup that Lewis has invented to deal with a particular problem of time travel. It's actually a very well-supported tool for assessing different kinds of impact. Here again, Lewis thinks that you can make replacement change to concrete objects, but you can't make replacement change to times. You can make a concrete object go away. And another concrete object take its place. But you can't make one and the same type be replaced at that time. An example, as I speak to you now, it's approximately 12:52. Imagine that at 12:53, I bring this presumably fragile and brittle glass down sharply on the edge of this table, and the glass shatters. So at 12:53, the series of events constitutive of the intact glass phase of history ceases. And the intact glass is replaced with a series of glass fragments. That's a replacement change. And you can also see, counterfactually, if I hadn't smashed the glass off the desk, it wouldn't have generated all the fragments. So one in the same event can be assessed in replacement or in counterfactual impact terms. It's not either or. But here's the thing. What I can't do is to take a version of history where the glass is shattered at 12:53, and then replace it with another version of history where the glass is intact at 12:53. I can replace an intact glass with a shattered glass, but I cannot replace an intact glass time. That's the point. And counterfactual impact is something that we, that we use all the time. It's not something made up, eye strains. Something else that's caused a few problems is Lewis' tu quoque, or you also, argument about causal loops. Now, a causal loop occurs when an event proves to be among its own causes. It seems to present some remarkable problems because it seems to suggest that information can be generated from nothing. And various people have said, entropy explains where information is generated. Well, I would submit that entrop, that entropy explains where information is transmitted. How information gets from one place to another. Lewis's point is this, explaining the ultimate origins of a causal loop is no more and no less difficult than explaining the ultimate origins of any other causal chain, be it infinite linear or finite linear. There's no earlier event that can be appealed to in explaining where the big bang came from. If you want to explain the big bang, you appeal to the laws that govern quantum vacuum fluctuations. Where do the laws come from? Who knows? If it turns out that the big bang has an infinite chain of causes behind it,. The question where does that infinite chain come from is still equally pressing. So Lewis is not saying that causal loops only occur when the laws of nature fail or that information can be generated from nothing, and we shouldn't be worried about it. His point is that we don't have natural laws for where information ultimately comes from. One interesting possibility that I mentioned on a couple of threads is the possibility that the universe itself could be the origin of, could have its origins in a, in a causal loop. Many astrophysicists take seriously the prospect that the universe can bud or branch. And it's a view seriously considered that this universe may put forth a later branch that loops back and proves to be the branch whence the earlier universe itself comes from. A truly self-parenting universe. In which case the question of the origins of feasibility of causal loops may turn out to be the questions of the origin and feasibility of the entire physical universe. Food for thought. Various of you have objected to Lewis' analysis on the grounds that it conflicts with our sense of freedom. And frankly to that I say, so what? Who says that our sense of freedom is something that we must adhere to logically. It's not illogical truth and submit that we enjoy contra-causal freedom, freedom from all causal determination. There's an influential school of thought advanced by philosophers like David Hume and Thomas Hobbes, to name but two, that holds that it's not the case that free will and determinism are incompatible. According to this compatibilist's school, in order to have a free will, you have to be the cause of your own actions. And provided your decisions are caused in the right way, caused by thoughts about you, caused by thoughts about your preferences, about your decision making, you can be both free and caused. So a compatibilist, I have no problem in accepting that there are logical restrictions on a traveler's actions, but yet a traveler can nonetheless be free. Well, that has summarized some of the questions that I've heard. I'll try to address more questions as they come along. I put some threads out there about the physics of time travel. I put some threads out there about David Hume and testimony to time travel. I put a link to a podcast that I did recently with other academics from Edinbury University about the Scottish enlightenment and ghosts and time travel. So do keep your posts coming. Do keep your quotes coming. Do keep your observations coming. And I shall try to answer them as best I can. So thank you very much. And it's nearly time for me, my heart, and my goggles, to wish you a very good day, and to say that I hope you enjoy whatever remains of this course. Thank you very much, indeed.
