# 2011-09-28 - Sci-Fi London Fringe Event with Chris Patmore [Official](https://sci-fi-london.com/podcast/2011/09/319-fringe-event) Duration: 00:43:23 ## Transcript ### Chris Patmore 00:00:00 - 00:00:55 Welcome to Reality Check, the Sci-Fi London podcast. I'm Chris Patmore, Sci-Fi London's online editor, and I've hijacked the show to bring you two interviews that will be reality checks. As most people know, good science fiction needs good science as much as it needs good fiction. In this podcast we speak with a quantum physicist and a psychologist and discuss the science and ideas behind two different TV shows that are out now on DVD. First up we speak with David Deutsch, a fellow of the Royal Society and a quantum physicist at the University of Oxford. We speak to him about the parallel worlds portrayed in Fringe and the forthcoming indie film Another Earth. The parallel universes, are they metaphysics or quantum physics or just an imaginary thing of academics? ### David Deutsch 00:00:55 - 00:01:21 This is something that's highly controversial among quantum physicists and I belong to a minority faction that believes that the parallel universes are an unavoidable consequence of both quantum theory and the experiments that support quantum theory, and so that they are in fact real. ### Chris Patmore 00:01:21 - 00:01:26 Right, would you care to expand on that a little? ### David Deutsch 00:01:26 - 00:02:31 Yes, well there are various experiments that one can perform in the laboratory, typified by the two slit experiment, that go under the general heading of interference experiments. What happens in these experiments is that typically something or other can go in two or more different directions or something can happen in two or more different ways. Typically a photon hits a semi-silvered mirror and it can either go straight through or bounce off, just a single photon. And the conventional interpretation of quantum mechanics is that when this happens the photon has a probability of one half of going straight through and a probability of one half of bouncing off, which means that there's still only one photon at the end, there's only one instance of what happens and it's either the bouncing off instance or the going straight through instance. ### Chris Patmore 00:02:31 - 00:02:32 So either it will or it won't? ### David Deutsch 00:02:32 - 00:06:15 It either will or it won't, exactly. But in an interference experiment you can subsequently do things to the two paths, to those two possible paths, bring the two paths together and then what happens then is an interaction called an interference phenomenon where what happens at the end depends on what was on both of those two paths. So like if you put a piece of glass or another mirror or something else onto either of those two paths, the net effect depends on what happens on both of them. And that means that something has gone round both of the paths even though there was only one photon. And so, well, you know, the obvious interpretation of that is that the photon just splits into two smaller things, you know, mini photons or something, is not really a single particle and we're used to that kind of thing happening. And so it splits into two smaller things and they go around and then they rejoin themselves into a photon at the end. But the trouble is you can rule that out, you can rule that possibility out by putting photon detectors onto the two paths and you find that only one of them ever fires. If you shoot in one photon, then only one of the two paths ever detects a photon. And so, and it's a whole photon with the entire energy, angular momentum, everything that the original photon had. So that rules out that it's split into two. So there's only one of them there and yet what happens at the end depends on what happened on both paths. And these paths can be separated by great distances, you know, many trillions of times the size of the photon. So that tells you that this is a sort of, on the face of it, a contradiction. The photon is only in one place, but it somehow sniffs out what's happening at two or more distant locations, some of which it might have been, as it were. And then, well, at this point, in my opinion, and I have to stress again that this is a minority opinion, at this point the majority of physicists start talking nonsense and say things like it's both spread out and localized at the same time, or that what actually happens to the photon depends on what could possibly have happened to it. But in my opinion, real events can't be caused by possible events, they can only be caused by other real events. And that leads eventually, I've oversimplified the argument here, but that leads eventually and inexorably to the conclusion that both events happen, but they can't see each other except in these rare circumstances of interference experiments. And so the photon, there were actually two photons in two different universes that normally don't see each other at all, but in these special circumstances do affect each other. And the same phenomenon has been demonstrated not only for photons, but for material particles and atoms and molecules. And since we are made of molecules and obey all the same equations as the photons, we can infer that the same is true of us and that the universe that we see around us, you know, stars, galaxies, everything, is just one such thing that exists in reality. ### David Deutsch 00:06:15 - 00:06:41 It's a tiny sliver of a much larger entity that includes many universes, that's sometimes called the many universes interpretation, sometimes called the parallel universes interpretation, because the universes almost don't affect each other like parallel lines. But unlike parallel lines, they do slightly affect each other. Otherwise, we wouldn't have any evidence of their existence at all. ### Chris Patmore 00:06:42 - 00:06:50 So take the example of fringe, where the two universes have developed along different lines. ### David Deutsch 00:06:50 - 00:07:47 Yes. So the universes will certainly develop along different lines, sometimes radically different lines, because there are situations which are sometimes explored in the series Fringe and in other science fiction. There are situations where a small effect of a single particle such as a cosmic ray striking can, and according to the multiverse theory, when a cosmic ray strikes the earth, it strikes it at different places in different universes. And therefore, sometimes someone is struck and gets cancer as a result and then dies and doesn't contribute to human history. And in another universe, they survive. And so you can see how the knock on effects of a single subatomic particle effect can make the two universes very different from each other. ### Chris Patmore 00:07:47 - 00:07:51 Well, basically what's known as the butterfly effect, I guess. ### David Deutsch 00:07:51 - 00:09:05 Yes. Although it's funny, you should mention the butterfly effect. This knock on effect is much greater in classical physics than it is in quantum physics. So in classical physics, it's happening all the time. You know, butterflies flap their wings and it affects a storm on the other side of the world and so on. Turns out that in quantum physics, the butterfly effect is much, much smaller. And really, the only ways in which universes that are slightly different become very different is through large amplification effects that are deliberate, not sort of accidental ones like in classical physics. Quantum universes remain close to each other as much as they can. And it's only when something like a deliberate action or something like a person being dead and therefore not doing the things that he would have done, that kind of thing can make the universes diverge. But most of the time, random events do not make universes become very different. If they're slightly different, they remain slightly different. And that's that. ### Chris Patmore 00:09:06 - 00:09:22 I guess a lot of it's also to do with microcosm and macrocosm and yes, well, so where if I miss the bus or not, it's not going to have a huge effect on the rest of the world. But it may have a small effect on what goes on in my life. ### David Deutsch 00:09:22 - 00:09:50 That's right. Most of the time, a small effect will result in a small effect in real life and in quantum theory. Classical physics kind of misrepresented the nature of knock on effects. So the classical picture is wrong. And in real life, these large knock on effects are rare, but they do happen. ### Chris Patmore 00:09:50 - 00:10:09 If you look at the world of the mystics, I mean, they talk about almost parallel universes where there's sort of the reality we see, plus there's the reality with a bigger, I mean, is this a similar line to what quantum physics is going along? ### David Deutsch 00:10:09 - 00:12:12 Mysticism and science differ in their method. Of course, sometimes fiction happens to produce a true answer. You know, Sherlock Holmes had a friend called Watson, and it could be that somewhere in the world, some people who've never read the Sherlock Holmes story name their child Sherlock Holmes, and another one names their child Watson, and they become friends and so on. Now, that can happen accidentally, just because even a stopped clock is right twice a day sort of thing. It's not a case of Arthur Conan Doyle having correctly foretold these events, these namings that might have happened a century later. It's just a coincidence. And it's the similar thing with mysticism. Occasionally, mystical theories happen to hit some of the right answers. But that doesn't mean that they had the same knowledge as science does later, because those right answers are rare among their answers, among the mystical answers, because basically because science is an error correcting process, and science also says a lot of false things, but then corrects them. And then the corrected theory also has some deficiencies, which are then corrected and so on. Myths don't change, except by very gradual evolution over many centuries. And so they don't have error correction. And so they're basically stuck with their original mistakes forever. And so that's why mystical ways of thinking are full of errors and don't improve. Whereas scientific thinking also has errors, but makes them fewer and fewer by improving. ### Chris Patmore 00:12:13 - 00:12:21 Well, a lot of mystics talk about absolute truth, where there is no error, there is just the absolute reality. ### David Deutsch 00:12:22 - 00:14:28 Yes, this is a philosophical mistake that science struggled to escape from during the early years of science, about 300 years ago. When you see, if you believe in absolute truth, and then you're going to believe that you have some. And if you have some, then that leads you to judge people who disagree with you in rather harsh ways. You know, they are people who don't have the absolute truth. And if they do something that you think is absolutely wrong, you're going to feel entitled to suppress their idea. And the practice of science is the practice of not doing that. The practice of being open to criticism because one expects one's own ideas to be mistaken in some unknown ways, even if they're largely right. One expects that there are unknown errors in our ways of looking at things and that in fact, all our ignorance of the world, you know, the things that we don't yet know, whether the Higgs boson exists and whether the universe will tear itself apart at the end of time, all those things are due to misconceptions that we currently have about what the laws of physics are and how the universe is set up. So the progress of science depends on certain values of science, such as tolerance and truth seeking and fallibilism, the belief that one can be mistaken. Whereas the values that stem from a belief in absolute truth are, sorry, of absolute knowledge. Absolute truth is perfectly harmless and in fact, good concept. But the idea that one can know it, that there's a way of knowing absolute truth is a path to intolerance and to error. ### Chris Patmore 00:14:29 - 00:14:57 Because, you know, I have an interest in Vedic philosophy and a lot of the stuff they talk about there is, I mean, the really in-depth stuff is coming around to things that are also being exposed in quantum physics. Yeah, a lot of the time they're talking about the same thing, but in different languages, so to speak. ### David Deutsch 00:14:57 - 00:17:15 I fear not. The thing is, when you look at some scientific idea and then compare it with mystical text with the benefit of hindsight, then you can always see the parallels, just like in these experiments where they hand out horoscopes to people and ask them whether the horoscope matches their life, matches the description of them. And they will, with much greater probability than chance itself would allow, they will say the horoscope does match them and does describe them correctly, even if the horoscopes were handed out randomly to the sample of people. And this is due to the human mind's desire to impose order on the world, to see patterns in the world. This is what makes us human, this is the thing we do better than any other animal, but it is error prone. And if you apply that ability without also applying the hard-won methods of science to weed out the inherent tendency to error in that very ability, then you will just see patterns where there aren't any. And with the benefit of hindsight, a lot of things look like quantum mechanics. But with the benefit of foresight, none of those world views that claim now with the benefit of hindsight to be like quantum mechanics predicted a single one of the amazing experimental outcomes of quantum mechanics. And that has got to be the test, because without that test, we already tried not having science, and for hundreds of thousands of years our species discovered nothing. And has only discovered new things since we applied this error correcting process of not just looking for patterns, but looking for patterns and applying methods that can get rid of errors that look true. ### Chris Patmore 00:17:15 - 00:17:45 So would you say that in a way, this is going more into philosophy than physics, that we all actually live in parallel worlds because we have the reality that we perceive and the reality of the world around us, which is sort of what you were just saying now. You look back on things with hindsight and you can analyse them in that way. ### David Deutsch 00:17:45 - 00:17:53 Yes, I think I agree. I'm not entirely sure I understand what you just said. ### Chris Patmore 00:17:53 - 00:18:05 Well, we have an opinion of ourselves and our vision of the world, and yet someone else has another vision of the world, but it's still the same world. ### David Deutsch 00:18:05 - 00:18:07 Ah, right. I see. Okay. ### Chris Patmore 00:18:08 - 00:18:15 So we live in our world, but there's another world, but they could be perceived as parallel universes. ### David Deutsch 00:18:15 - 00:19:51 I understand. Yes, so because we're fallible, the world as we see it is not the same as the actual world. There is such a thing as the actual world, but what we see is only an interpretation of it that is bound to have lots of discrepancies from the real one. Now, there is also another world, a parallel world, which looks much more like the one that we perceive. So in a way, our mistake happens to match up with the real thing. Just like in the imaginary story I told, a fictional story might accidentally match up with a real event. And even though the author thought he was writing fiction, in fact, it was a true account of a real event elsewhere on Earth or at a different time. And similarly, it will always be true that a fictional event that makes sense will be a real event in some other universe. So that is true. However, again, just as with the fiction, that does not mean that when we perceive that happening, we are accurately perceiving the other world, because as I just said, it's bound to be true. If what we see makes sense at all, it's bound to be true that it exists in another world. That doesn't mean we've perceived the other world, it just means we've imagined it. ### Chris Patmore 00:19:54 - 00:20:08 I'm going back to the mystics again. I mean, the mystics talk about seeing the world as nothing more than vibrations. Yes. Which does sort of correspond to what quantum physics is talking about in a way, doesn't it? ### David Deutsch 00:20:08 - 00:20:13 Well, again, only with hindsight, I'm afraid. For example... ### Chris Patmore 00:20:13 - 00:20:17 Also, we've got no proof of what they actually saw. ### David Deutsch 00:20:18 - 00:21:44 Well, yes, quite. And if they had seen something, they could have made some startling predictions, which would, you know, for example, if we look at early physics of the ancient Greeks or the Arabs in the golden age, we can sometimes find that they surprisingly did have some scientific knowledge that we thought had only been invented much later. But in those cases, this knowledge led them to make a prediction which was surprising, and that prediction is borne out. And if anybody had known the truth of quantum theory before the 1920s, they would have easily made predictions that are absolutely startling. So, and they didn't. And so when they speak of, for example, vibrations is a very good example. Vibrations is a phrase that could mean... It isn't used in fundamental physics. It's only used for engineering type applications of physics, and regarded as denoting something in fundamental physics, it could be a lot of things and it could be waves. And there are interpretations of quantum mechanics in which particles are waves and... ### Chris Patmore 00:21:44 - 00:21:45 Waves of energy. ### David Deutsch 00:21:45 - 00:23:20 Yeah, particle wave duality. Unfortunately, from my perspective, particle wave duality is part of the equivocation and nonsense that was talked by the early pioneers of quantum theory in an attempt to avoid the parallel universe's implications. And in fact, there is no particle wave duality. What the wave is, is not a vibration in anything. It's the extended... it's the extended nature of a wave that they were talking about. Not its changing, going up and down like a vibration. It's just the fact that it's in more than one place at a time. And the more than one place at a time in reality translates into being in different places in different universes. Not to moving from one to the other in a vibratory way. So if somebody had understood the parallel universe's implication of quantum mechanics somehow before the 1920s, vibration would have been the last word they would have used. They would have denied that quantum theory has anything to do with vibrations. They do use a word nowadays, fluctuations, which is likewise very misleading. Fluctuations is another of these double-talk words that pretends that objects kind of go from one to the other very rapidly. But there's no such process described by the equations. What the equations describe is existing in both places at once. ### Chris Patmore 00:23:22 - 00:23:59 Well, I guess one of the things that hampers all this is actual language. I mean, you asked someone to describe God, which lots of scientists have issues with anyway. But how can you describe something as immense as that? It's you trying to bring something down just into words that cannot be explained almost. Well, I guess this is an issue that also scientists face, is you're trying to explain something. Whenever you give it a name, it is, but it isn't at the same time. ### David Deutsch 00:24:01 - 00:26:40 The thing is, science is hard to understand, hard to explain, and hard to test. But tests are possible. And when you have a weird theory that says that the world is very different from what our intuition and our apparent experience seems to say, what science does is in the process of sharpening that idea, correcting the ambiguities in the idea, and correcting the errors in the idea, it eventually comes to a prediction. And that prediction is the opposite of all those things. It's the opposite of ambiguous. It is clear that everybody involved knows that if the spot of light falls on one place on the photographic plate or on the sensitive receiver, then the theory is true. And if it falls on the other place, then it's false. And the exciting thing that happens in science is that in some cases, these theories can make predictions that are absolutely amazing. The idea that the place where the single particle lands depends on what the whole apparatus was doing, apparatus several meters across, so that something about that particle was probing that whole apparatus, is something that you can't see, is absolutely astounding, in my opinion, more astounding than any of the things imagined by mysticism or religion. And we can understand them. And then somebody else can go away and say, well, if that's true, then this other amazing thing should happen. And lo and behold, it does happen. So, for example, well, we haven't actually verified this yet, but one of the predictions of quantum theory is that you can build computers using quantum theory that are enormously faster: for certain types of problem, a quantum computer could solve it faster than any classical computer, including one that was built of all the matter in the universe. So here's the experiment. You take all the matter in the universe, you build a computer out of it, you run that computer on this problem, and it takes a certain time to solve the problem. A simple quantum computer made of a few atoms could solve that problem in a microsecond. ### Chris Patmore 00:26:41 - 00:26:42 We all know the answer is 42. ### David Deutsch 00:26:43 - 00:28:48 Yeah, exactly. Yeah, yeah. And no, but you can make it so that you can choose a problem such that the answer is easy to verify once you have it, but very hard to find in the first place. So even a normal computer can verify that it's the right answer, unlike in the case of Deep Thought and 42. So what you would do is run this program, then verify it gave the right answer, and then that shows that some process produced that answer that was more complex than the entire visible universe, which means that there is more there than the entire visible universe, but it means something much more exciting than that. It means that this other stuff that is there in addition to the entire visible universe obeys equations that we know. We have knowledge that is so powerful and sophisticated that it can describe and predict and understand the motion of this entity that is trillions of times more complex than the entire universe, and we can harness it to do a little mathematical calculation for us. Now, that's something that was never guessed by any mysticism, but it has been predicted by quantum theory, and we're hoping to verify this in the next few decades. And also, if we fail to verify it, and if the answer comes out different, we will know that we were wrong, that the theory was wrong. Whereas theories about mysticism are never exposed to this kind of danger of being proved wrong, and therefore when they are wrong, they just remain in their erroneous state. Attractive though they might be, but lots of attractive things are false. ### Chris Patmore 00:28:49 - 00:29:31 There's a new film coming out soon called Another Earth, an identical planet Earth, complete with exactly the same people living on it appears in the sky. Complete with a moon. The story seems to lack any scientific grounding whatsoever. I mean, the fact that if an identical planet Earth appeared visibly in our skyline, just the physical attraction of the two planets would create absolute chaos. Even the idea that these two planets were exactly the same and all the people on it lived exactly the same lives seems a little improbable to me. To my mind. ### David Deutsch 00:29:31 - 00:30:06 Yes, but that couldn't happen in our universe. So they must be assuming that this planet came from another universe, which is contrary to the laws of quantum physics as we know them. But you know, in science fiction, you could imagine, and that's what Fringe does, it imagines that the laws of physics are slightly different from what we know them to be, from what we believe them to be, which after all is perfectly allowed by science. You know, we expect our opinion about what the laws of physics are to be slightly wrong. So that's a legitimate piece of science fiction there. ### Chris Patmore 00:30:09 - 00:30:13 Even if you make up the science, you've got to make the science sound plausible. ### David Deutsch 00:30:13 - 00:32:14 Yes. Now that's a very important point. In my book, The Beginning of Infinity, when I discuss parallel universes, I also discuss the issue of what is the difference between fiction and fictional science and other universes, because they all overlap conceptually, those things. One of the things is that the fictional science has to make sense in its own terms. So that's, and I try to explain the multiverse theory in my book in terms of a fictional story first. I start with a fictional story about parallel universes, and then I point out that this doesn't make sense in certain ways. So we fix it, and we get a better theory of it, and so on, and we eventually end up with the actual theory of quantum parallel universes. I'm afraid I don't know about the film you spoke about, but there was a film a few years ago called Sliding Doors, also about parallel universes, and that one, rather interestingly, it was a sort of tour de force, I thought, because it managed to have an interesting story about parallel universes that do not in fact interact with each other. So that story was perfectly consistent with existing quantum mechanics. Such things do happen in reality, and making a story about them is a terrific challenge, and I thought they did it very well, and they also showed how the butterfly effect isn't right. The things which become different as a result of a small change in these two universes are all human-mediated things, and that is exactly what would happen, what does happen in the real multiverse all the time. I thought it was a brilliant film. ### Chris Patmore 00:32:14 - 00:32:22 It is a concept that's been used quite a lot in science fiction films. There's also a film called Primer that came out a few years ago. I don't know if you've seen that. ### David Deutsch 00:32:22 - 00:32:24 That I didn't see, no. ### Chris Patmore 00:32:24 - 00:32:31 But that was more to do with time travel, and these guys built a machine that could only travel forward 10 minutes. ### David Deutsch 00:32:32 - 00:32:33 Ah, yes. ### Chris Patmore 00:32:33 - 00:32:41 But they ended up with four or five different versions of themselves, and they had to keep watching themselves to make sure that they didn't do something else. ### David Deutsch 00:32:43 - 00:34:55 That's interesting, because the theory of time travel and the theory of, ah, if time travel is possible, that's the same kind of variant theory of quantum mechanics as the theory that travel between universes is possible. The science fiction stories that are imagining contact between different universes, like Fringe, are imagining the same kind of variant laws of physics as the stories that imagine time travel. And we can't rule that out. We can't absolutely rule out the existence of such things at the moment. What we can say, though, is that if time travel is possible, time travel into the past is possible, then it necessarily also involves traveling to other universes. You can't necessarily make yourself go to an earlier point in your own universe. If you go to an earlier point, it is in general to the past of another universe, and that's what avoids the paradoxes. So although you can end up with multiple copies of yourself, what you can't end up with is a paradox where you prevent yourself from setting out in the first place, or killing your grandfather. I don't know why the paradigm of this paradox is killing your grandfather. Nothing so violent is required. All you have to do is set up an experiment where you agree that if you appear out of the time machine, you're not going to go in it during that experiment. And that's a simple paradox in itself. If you apply quantum theory to this paradox, you find that indeed the laws of quantum physics prevent you from going to the past of your own universe, but they do not prevent you from going to the past of other universes. And what happens then is that some universes end up with two or more copies of you, and in some universes you've gone into a time machine and never came back. ### Chris Patmore 00:34:56 - 00:35:03 Yeah, I love time travel movies because they can just really mess up your head basically. ### David Deutsch 00:35:03 - 00:35:29 Yeah, most of them unfortunately do not bother with making sense, and I don't like science fiction that doesn't make sense. I don't mind if they imagine different laws of physics. In fact, I like that. That's the good thing about science fiction. And I think Fringe, for example, although sometimes it's deliberately silly, I think a lot of the episodes are kind of joke episodes. ### Chris Patmore 00:35:29 - 00:35:32 Yeah, yeah. But they did the same with X-Files as well. ### David Deutsch 00:35:32 - 00:36:42 Well, X-Files was, I think, that was a case of really not trying to make sense, I'm afraid. But the basic storyline of Fringe, if you neglect the joke episode aspects, really are trying to make sense and do quite nicely explore the issues that arise out of taking seriously the existence of multiple copies. You know, like, you know, which one is you? Which one is the real you? Does it make sense to ask which one is the real you? And if you have the same thoughts and the same feelings and fall in love with the same people, are you really any different from each other? And they slightly simplify the problem, at least in the episodes I've seen. I haven't seen all the episodes, but in the episodes I've seen there are only two parallel universes. But in real life, there are enormous numbers of them. So practically anything you can imagine that makes sense happens in some universe or other. ### Chris Patmore 00:36:42 - 00:36:50 Because going back to the time travel, I mean, one could say that actually time doesn't exist, because there is only the present. ### David Deutsch 00:36:50 - 00:39:25 Yes, well, that's a mistake. The physics, at least according to physics, physics doesn't distinguish between the present and any other time. So what the present is, is a perspective on the multiverse. And in fact, the different times are just different universes. They happen to be the ones that physics allows us to know more about by direct observation. There are countless other ones that are much harder to see, that we see only through their indirect effects via interference phenomena. And the past and the future are just the names that we give to parallel universes that we have much more direct evidence of and interaction with. The difference between different nows, different present times, is the same thing as the difference between different perspectives in space. So if you imagine the different copies of you of different times, they're rather like different people standing around a monument, looking at it from different angles. And in space, we can see them all at once. So we're not tempted to say that they are real and the other one isn't. But in fact, in time, they are all real in exactly the same sense that all the parallel universes of quantum theory are real. The past, the present and the future are all real. When we say that, one's mind immediately jumps to the mistake of thinking, oh, well, if they're real, they're all real at the same time. That isn't true. They're not all real at the same time, because they're all different times. So that would be rather like saying that all those observers of the monument are all real at the same angle. No, they're not. They're all real at different angles. All the different angles are real. All the different perspectives on the monument are real perspectives, but none of them is the monument. The monument contains all the perspectives. The actual three dimensional object determines all the perspectives and all the different perceptions of it. Same with time. Someone told me once that we talked about aliens and they said aliens exist, but only in the future. I'm not sure what he might have meant by that. Did he mean that we are going to become the aliens as we spread out? Well... ### Chris Patmore 00:39:25 - 00:39:41 I think it means that, like you said, with all these different parallel universes, if people see them, they're seeing something that's in a... If they see aliens, they're not seeing them in this actual present day. They're seeing something that's leaked through from a future. ### David Deutsch 00:39:42 - 00:41:19 I see. Well, of course, there's no evidence that anything can leak through from the future. As I said, if it did, that would have massive implications for parallel universes, because it would always leak through into another universe. If we saw an alien coming out of a time machine, it would have come... That alien would have come from a universe that isn't our future. Yeah. So the only reason that we might believe that there would be aliens in our future is if we also had reason to believe that the alien came from a universe that was very like ours, which we might have. We might have such evidence. But again, this is all different from the question of whether there are aliens out there, because whether there are aliens out there is a question that's open to experimental testing. A few hundred years ago, people imagined that there might be... Or less than that, even a hundred years ago, people imagined that there might be civilizations on the moon, or perhaps on the far side of the moon, which we can never see from Earth. And that was then tested, because we can now look at high-resolution photographs of the far side of the moon on Google Earth. You can actually look for yourself and see whether there are any cities there, and there aren't. So that theory was tested, and science made progress. So a hundred years ago, we did not know this, and now we do know it. ### Chris Patmore 00:41:19 - 00:41:23 Nikola Tesla was convinced that there was life on Mars. ### David Deutsch 00:41:24 - 00:43:03 Was it Tesla? Yes. I know that Lowell, the astronomer, was convinced of that. And by the way, the history of Mars astronomy is a very good example of the ability of humans to see patterns when they aren't there, because different astronomers all saw these canals on Mars. And actually, the canals are just an artifact of what happens when you stare very hard at something and expect a pattern to be there. And if it's just at the limit of what your eye can resolve, your brain inserts the pattern. It's not a matter of fraud or anything like that. It's just a matter of the ability of the brain to see patterns, which is absolutely crucial in our discovering real patterns. But far more often than real patterns, it sees things that aren't there at all. And the method of science is to find ways of systematically eliminating the majority of perceptions, which are of things that aren't there at all, and finding the ones that are there, testing that, and then going with that idea, which itself is usually flawed in some other way. But in that way, we get more and more knowledge. This is the thing that methods of proceeding with knowledge other than science don't do. They don't correct their errors. ### Chris Patmore 00:43:07 - 00:43:15 Excellent. Oh, so good. Well, that's very enjoyable. Thanks for the conversation. I appreciate it. ### David Deutsch 00:43:15 - 00:43:16 Thank you. ### Chris Patmore 00:43:16 - 00:43:22 It's pretty much blown my mind for the rest of the day.