# 2025-10-27 - This Is World - Proof the Multiverse Is Real - Interference and AGI [YouTube](https://www.youtube.com/watch?v=0xA5uzB6jRU) Duration: 00:39:03 ## Transcript ### David Deutsch 00:00:00 - 00:00:47 That interference shows us that the other universes are not just mathematical constructs, they are really there because they have a real physical effect on things that we see. Knowledge creating processes in a brain especially become more alike across different universes. When you're inventing a new proof, the different versions of you become more and more alike because of the error correction that is going on. Reality of evolution is unfalsifiable because nobody has ever seen a dinosaur, just because the reality that we see is affected by invisible things. Consciousness is a feature of software, it's a feature of certain computer programs like the ones we have in our brains. It's not a feature of the brain as such. ### This Is World sponsor bumper 00:00:48 - 00:00:55 The This Is World channel is partnered with IQM, one of the world's largest quantum computing companies. ### Maciej Kawecki 00:00:55 - 00:01:05 Professor, it's a great honor having you here. Why propose infinite universes when you could just have one? ### David Deutsch 00:01:05 - 00:01:58 Just because the reality that we see is affected by invisible things. That's what an interference experiment is. You have a photon, a single photon, which approaches a barrier with let's say two slits in it. The path of the photon is not the same. The path of the photon coming out of the two slits is not the same as it would be if it went through the left one or the right one. It's different. It's different from either of them. That's because in some universes it goes through the left, some it goes through the right, and those are still interfering with each other at the end of the experiment. And that interference shows us that the other universes are not just mathematical constructs. They are really there because they have a real physical effect on things that we see. ### Maciej Kawecki 00:01:58 - 00:02:14 When I solve a math problem or any other problem or I do something, how does that organize all the other versions of me somewhere there? ### David Deutsch 00:02:14 - 00:04:16 So the brain is, among other things, an error correcting system. So when we think and when there's a possibility that our thought process will go one way or another way, the outcome is not random. If there is a random process that happens to occur in the brain, like some neuron isn't working properly or a cosmic ray hits the neuron or whatever, and we do something different from what we would have done, for example in your maths thing when you were adding seven and five, you would have said 12 but your neuron is hit by the cosmic ray and you're about to say 13, but your brain contains error correcting processes. And with very high probability, if you would have said 12, some of the error correction will happen and the 13 will be converted back into a 12. That's very important because it means that knowledge creating processes in a brain, especially, become more alike across different universes. And this is especially true if you're creating new knowledge. You're solving a given maths problem may be a mechanical task, for all I know. It can be done mechanically. But inventing a new piece of mathematics, inventing a new proof in mathematics, when you're inventing a new proof, the different versions of you become more and more alike because of the error correction that is going on. So it's by trying to solve the problem, the different versions of you are becoming alike. ### Maciej Kawecki 00:04:16 - 00:04:21 It's impossible to falsify that vision, right? ### David Deutsch 00:04:21 - 00:05:55 That is a mistake. So first of all, to falsify a theory means to do an experiment where the theory predicts one thing and a rival theory predicts another thing. If there is no rival theory, then there is no experiment because that isn't a test. Because the best explanation, if you only have one theory, one explanation, the best explanation for the outcome is that it was an experimental error or any number of mistakes, just like when a few years ago the researchers on neutrinos at CERN said that their neutrinos had travelled faster than light. And some people said, ah, special relativity has been refuted. But that's not so. It would only have been refuted if somebody had produced a rival theory to special relativity which predicted all the other correct predictions of special relativity, but predicted this result for neutrinos. And sure enough, nobody did come up with such a theory. And after a while it was found that it was experimental error, a rather subtle experimental error, but nevertheless an error. ### Maciej Kawecki 00:05:55 - 00:06:03 So how would we test the multiverse theory? What would be its falsification? ### David Deutsch 00:06:03 - 00:09:49 So suppose you had another theory, like suppose you said that in a particular situation with a quantum computer or something, as Roger Penrose has suggested, that if there are more branches to the superposition than a certain number, or he says if the mass of the different branches exceeds the Planck mass, then the wavefunction will not continue to evolve with the usual equation of motion, but will have a different equation of motion that makes it home in on one particular value. Now that means that there's a different result predicted by Everettian quantum theory from this new theory, Penrose's theory, or whatever other theory contradicted quantum theory. Then what you could do is replace the observer of the outcome with another quantum computer. And on this quantum computer you run an AGI program, that is a general intelligence which has all the cognitive attributes of a person, or of any other cognitive thing. Then the observer measures the outcome, let's say the outcome can be left or right, and this computer then is instructed to say not it was left or it was right, but I know the answer, and it is either left or right. I have seen it, I have seen the outcome, it's either left or right. Now if Penrose's theory were true, then the outcomes that the computer would have seen will be random, randomly left or right, because the coherence has been lost, the coherence which provides the one correct answer. And if Everettian quantum theory is true, then the answer would be, let's say, left, whichever has been set up as the correct answer under quantum interference. Then you reverse the process, which you can do, but you have to, if you reverse the path of the particle through the apparatus, you can do that provided that you also reverse the memory of the second computer to forget whether it was left or right. But the crucial thing is, and I'm sorry I have taken rather a long time to describe it, the crucial thing is that you do not have to erase the memory of having known it, of having known one precise value. It has just forgotten whether it was left or right. So there are two completely different outcomes, one where it will be left and right randomly, and the other where it will be always left whenever you do the experiment. And if it's left or right randomly, then Everettian quantum theory is refuted. ### Maciej Kawecki 00:09:50 - 00:09:58 That's the way how quantum computers can shed new light on the nature of the reality? ### David Deutsch 00:09:58 - 00:09:59 Correct. ### Maciej Kawecki 00:09:59 - 00:10:14 When a quantum computer breaks encryption by trying millions of solutions simultaneously, where is all that parallel processing actually happening? ### David Deutsch 00:10:14 - 00:11:37 Yes, I don't think I've ever said it proves it because I don't think one can prove scientific theories. But the thing is, that result is not explicable by any other theory. So that is, by explicable I mean literally there does not exist an explanation of how that result comes about under any other theory, because some variants of quantum theory say that you're not allowed to ask what happens between the beginning of an experiment and the measured ending. So if they say you're not allowed to ask, that means they don't have an explanation, which means that in my scheme of things, they are not science. Those aren't explanations. But yes, it is not explicable in any other way other than by unmodified quantum theory, Everettian quantum theory. And the same conclusion applies even when it's only two computations, like a photon passing through two slits and then interfering. That also gives the same conclusion. ### Maciej Kawecki 00:11:37 - 00:11:48 Don't you think that sometimes such controversial theses can expose physics to the charge of being unfalsifiable? ### David Deutsch 00:11:48 - 00:13:32 Well, the charge is often made, but it's simply not true. To give an example, creationists often say that the theory of evolution is unfalsifiable because nobody has ever seen a dinosaur and nobody ever will. And exactly as in the case of the experiment with the photons going through the slits, although we have never seen a dinosaur, we have seen fossils. And fossils are the effect of dinosaurs. They're not dead dinosaurs. They're not made of the same material as a dinosaur, but they are the effect on geological materials that the dinosaur had after it died. And the only explanation for why the fossil looks as it does is that there was, unseen by us, a dinosaur. Millions of years ago, when there were no humans to see it, no beings that could form theories of evolution or theories of biology were around. And yet, even though nobody was around, because they affected things that we do see, we can form a theory that they were there. And again, that theory that there were dinosaurs has no rival. There is no other explanation. You can form a non-explanation by saying you're not allowed to ask that question, but that's not an explanation. ### Maciej Kawecki 00:13:33 - 00:13:51 Once you said good scientific theories are hard to vary, but theories like Freudian psychology can be twisted to explain anything. What is the key difference between real science and non-science for you? ### David Deutsch 00:13:51 - 00:15:16 Well, there are many ways in which theorizing can go wrong and stop being scientific. So the one you just mentioned is an example of how it can go wrong, because if it could explain anything, then it actually explains nothing. Another way it can go wrong is what we mentioned before. So it's kind of the opposite of Freudianism. If the theory is purely predictive, then it also explains nothing and is also not a scientific theory. And my favorite example of this is a conjuring trick. Let's suppose you go to a theater for a conjuring performance and you see the conjurer put some cups on top of balls and then he swirls it around and then he removes the cup and the ball is not there. Now if you go again the next day, you see him doing the same thing, maybe you go every day for a week, and at the end you can predict perfectly when the ball is going to be there and when it's not going to be there. But you still have no idea of how that outcome comes about. That is a completely different kind of knowledge. It's explanatory knowledge, not predictive. ### Maciej Kawecki 00:15:16 - 00:15:38 Why couldn't evolution have simply created minds that solve problems without any subjective experience at all? Why don't our brains just solve problems in the dark without any inner awareness? ### David Deutsch 00:15:38 - 00:18:23 Well, the process of evolution did solve many problems without any explanation. But natural problems, but not about abstract thinking, abstract math, physics revolutions. So that's what I mean. So there are fundamental limitations on the kind of creativity that evolution is capable of. One major thing it can't do is it can't jump over a conceptual gap. So if there is an organism which could do with a certain improvement, like if there's a lizard and it would really benefit the lizard if it could breathe fire like a dragon, then the only way that evolution can generate a new organism which can breathe fire would be to have a sequence of organisms in successive generations which could come closer and closer to the ability to breathe fire. And every single one of those would have to be not only a viable organism in its own right because it has to survive, to reproduce, to be mutated and so on, but also be useful because if it's not useful, then its competitors who didn't have that slight mutation would outbreed it. So it's very difficult therefore for evolution to cross a conceptual gap like that. Whereas thinking, we can say, imagine that you had a flying car. Now the flying car won't work if you just imagine it. I used to imagine all sorts of flying and swimming and underwater cars when I was a child and at first I did not try to bridge that conceptual gap. But the way that humans do bridge it when they do is not by making something, by making a sequence of things that starts with the first one and ends with the one with the new ability. They think of how to bring it about. That is the explanatory part and evolution doesn't have explanations. ### Maciej Kawecki 00:18:45 - 00:19:10 You claim all humans are universal explainers. ### David Deutsch 00:19:10 - 00:19:13 Yes. ### Maciej Kawecki 00:19:13 - 00:19:20 Is human brain capable of solving any problems? ### David Deutsch 00:19:20 - 00:22:05 Well, universal explainers refers both to the capacity to generate explanations and also the capacity to understand them, which is really the same capacity. It's the same ability. Now this does not mean that given a problem, a given human brain or any brain existing at a certain time will manage to solve it. That's a different issue. One of the things that prevents most conceivable problems from being solved is that nobody wants to solve them. Nobody wants to find an integer with a given problem. Only very few people want to find whether the Riemann conjecture is true and so on. There are far more potential conjectures. There are infinitely many potential conjectures, most of which nobody will want to solve. The issue is different. The issue is, are we prevented by the constitution of our brains from creating a piece of knowledge which could in principle exist? Not which could be produced, but which could in principle exist? And the answer is no, as I argue in my book, because if there were such a thing that could not be comprehended by humans, but which nevertheless impinged on us in such a way that we would want to solve it, then if that couldn't be solved, then this is the same thing as believing in supernatural beings. Because then we say that we can comprehend the universe this far, but no further, and there is no explanation for why it's this far and no further. And therefore, since the thing beyond the barrier affects us, we can't therefore understand the things that are within the barrier either. So it's a contradiction. So we either have unlimited capacity for explanation or we have no real capacity for explanation, because everything that we think we understand is actually an illusion according to that theory. ### Maciej Kawecki 00:22:05 - 00:22:25 Why some people struggle with basic math while others discover relativity? How can we all be universal when our abilities are so different? ### David Deutsch 00:22:25 - 00:24:30 Well, people struggle with basic math when somebody is trying to force them to do it and they don't want to. They don't want to is the key to this state of affairs. They have something better to do, or at least something that they think is better for them to do. As I said, very, very few people in the world want to solve the Riemann hypothesis, want to prove the Riemann hypothesis or disprove it or whatever. Very few people want to learn calculus. Very few people want to learn to solve simultaneous equations. Unfortunately, we have a tradition of education that is based on making people do things they don't want to do. And under such circumstances, creativity is impaired. How much it is impaired depends on sheer luck mostly. Some people are forced to do exactly the thing that they would have wanted to do anyway. Some people are forced to do things where they can do their own thing and not be noticed and not be punished and so on. So many people get away with it. But most people, in the case of maths for instance, which I think would be interesting for a lot more people than it actually is, get put off by the coercion of it. So Einstein said something like, even a starving wild beast would be put off its dinner if it were forced to eat it, or something like that. He put it better than I just have. So this is true in general and it's rather sad. And I think if you interview anyone who's been successful in a given field, whether it's... ### Maciej Kawecki 00:24:30 - 00:24:34 Like you. ### David Deutsch 00:24:34 - 00:25:39 Yes, but fields that aren't socially rewarded. So not someone who becomes a professor, but someone who becomes very, very good at something, let's say making model ships out of matchsticks or something. You will find, first of all, that they're very interested in doing it. You will find that nobody forced them to do it. You will find a whole constellation of things that are characteristic of creativity that happened. And usually that person will say that they were lucky to escape, they were lucky to be able to continue this thing and acquire a great expertise in it, because they very narrowly were stopped. Sometimes they can be stopped by being forced to do that very thing, like the hungry lion. ### Maciej Kawecki 00:25:39 - 00:25:49 What is the function of consciousness for you? We see intelligence without consciousness in machines and some animals, I think. ### David Deutsch 00:25:49 - 00:26:06 I wish I knew. But note that consciousness is a feature of software. So it's a feature of certain computer programs like the ones we have in our brains. It's not a feature of the brain as such. ### Maciej Kawecki 00:26:06 - 00:26:26 Amazing, because a few days ago in Pasadena, I talked to a Turing Award winner, Judea Pearl, and he said that for him, consciousness is the blueprint of its own software. ### David Deutsch 00:26:26 - 00:27:02 That's a very deep sounding statement. Judea Pearl is a very deep thinker on this very issue. He understands that, for example, the difference between prediction and explanation. Very few people do. And he is a campaigner for explanatory ideas as opposed to predictive ones, especially in the field of probabilistic prediction. So, yes, I agree. But I'm not quite sure I can decipher that statement itself right now. But I'm sure he's right. ### Maciej Kawecki 00:27:02 - 00:27:28 You said a few minutes ago that quantum theory is not the last one. Physics is largely predictive. So, if constructor theory that you work on asks what's possible instead, can you give an example of how this changes everything around? ### David Deutsch 00:27:28 - 00:27:50 Yes. I have to say, to begin with, that constructor theory at the moment is an idea for a theory. It's not yet a theory. We're working on it. Just as Penrose's theory of quantum mechanics is an idea for a theory, he hasn't actually written down a coherent theory with those properties, but he might be able to. ### Maciej Kawecki 00:27:50 - 00:27:53 A few months ago, I talked to him about it. ### David Deutsch 00:27:53 - 00:30:59 Ah, good. So, let me see. I think one nice way of explaining how constructor theory might be a better approach is with thermodynamics. Now, in thermodynamics, one of the fundamental laws of thermodynamics is that entropy increases in the forward direction in time, and entropy cannot decrease in the forward direction in time. So, this is, from the point of view of explanation, this is problematic, because we know, or at least at present, our fundamental theories of physics are reversible in time. So, if you had the ability to look at the molecules of air in this room with a suitably powerful microscope, and you'd see them bouncing off each other, if you took a movie of them bouncing off each other, you wouldn't be able to tell whether you were playing that movie forwards or backwards. In other words, the forward version is obeying the laws of motion of air particles, exactly as well as playing it backwards. And yet, the backwards version never happens in real life, only the forwards version. So, this is a sort of firewall between thermodynamics and the rest of physics, where on the face of it, thermodynamics violates the rest of physics. Now, in constructor theory, we don't ask what happens given a certain initial condition, which is what we would be doing with this movie of air molecules. We ask what can be caused to happen by a constructor. And the crucial thing is that if you have a constructor to do a particular thing, like stir a liquid with a stirrer to increase the temperature of the liquid, then there's no particular reason in the theory, expressed in constructor theory forms, that tells you that the reverse of that process can be caused by any constructor. There need not be a constructor that can do it backwards. There is no mystery, because all the fundamental laws in constructor theory are expressed in terms of what can be brought about by a constructor and what cannot be brought about. And it's perfectly possible for a process, a transformation, for it to be possible to bring about a transformation, but not possible to bring about the reverse transformation. ### Maciej Kawecki 00:31:00 - 00:31:04 How was the idea born in your brain? ### David Deutsch 00:31:04 - 00:32:50 Well, there were several things that came together when thinking about them, and I was sort of struck by the fact that I came to the same idea from many different directions. So that was one of them. That was one of the directions. When I wrote, I then eventually wrote a philosophical paper about, you know, why I think constructor theory would be a good idea if we could construct one. And I had a section called motivations, and I kept adding more and more motivations until in the end there were 18 motivations. And that very fact made me think, okay, there must be something to this. If there's no constructor theory, something else must explain this way that all sorts of different problems can be addressed by changing one's view from the conventional view of initial conditions, laws of motion, final conditions, to what can be brought about and what cannot be brought about. So in constructor theory, you don't necessarily, if you know that, for example, that something can be brought about, that doesn't mean that you know how to bring it about. And if it can't be brought about, you don't necessarily know from that fact what stops you. You just know from the fundamental theory that this is working out of that a certain thing has to be possible, a certain thing has to be not possible. ### Maciej Kawecki 00:32:50 - 00:32:58 Can quantum mechanics work without wave function collapse? ### David Deutsch 00:32:58 - 00:34:10 You're asking whether a theory that doesn't exist at the moment could exist. So logically, it could exist, as it could also be that the world is only 6,000 years old and there never were any dinosaurs. But don't hold your breath waiting for such a theory. In both cases, the best existing theory is not only the only one, it's extremely detailed in its capacity to predict for the future. So I'm sure that quantum theory will not be the last theory of physics and I'm sure that the present theory of evolution will not be the last one. After all, it has been amended several times in between the time when Darwin invented it and today. I'm sure that there will be further improvements in both theories. But those improvements will not contain the irrational alternatives that are hoped for by the irrational people. ### Maciej Kawecki 00:34:10 - 00:34:17 So why did physicists think they needed to add it all the time? ### David Deutsch 00:34:17 - 00:37:04 This is a bit of a scandal in the history of physics. It was the early 20th century. The trend in philosophy since the late 19th century had been towards positivism and then logical positivism. And those two theories would have liked to recast physics entirely without explanation, only with prediction. It was because of empiricism and the history of philosophy of science and so on. They got hold of this really bad idea that science isn't about explaining the world, it's only about predicting it. And so they tried to, when the outcomes of measurements in quantum theory were bizarre and counterintuitive, they thought, well, we can predict them, why do we have to explain them? Let's not explain them. Because positivism authorizes us and indeed instructs us to do that. And then what happened, now some physicists took that view, some physicists opposed that view like Einstein, some tried to find a middle view like David Bohm. But what happened next was not this debate that that theory originally sparked, but this also happened at the time when physics became more and more important and there was more and more training of young people to become physicists. You know, there are, I don't know how many more times, 100 times more physicists now than there were in, say, 1900. I don't know the exact number, but it's an enormous increase. And those were all put through a system of education where when the students asked, yes, but how does the outcome come about? Why is the pattern of dots on the screen as it is? The professors would say, if you ask, you're not allowed to ask that question. Or if you ask that question, worse, if you ask that question, you don't really understand. And so they shut the students up and the students went on to shut their own students up. And this has become an anti-rational tradition in physics. It is very, very gradually dissipating. ### Maciej Kawecki 00:37:04 - 00:37:12 Do you remember the moment you created the first theory of quantum algorithm on Earth? ### David Deutsch 00:37:12 - 00:37:14 Yes. ### Maciej Kawecki 00:37:14 - 00:37:16 Tell me something more about it. ### David Deutsch 00:37:16 - 00:38:49 Well, I was just trying to. So I've often told the story of how I got interested in computational properties of quantum. Because a colleague was telling me that contrary to what I had thought that complexity theory is actually rooted in physics. That the limitations of complexity theory, what's P and what's NP and that kind of thing is determined by physics. So I said, well, if it's physics, you're using the wrong physics because they were using Turing's physics, basically, which was classical. So I went home and I just jotted down on a piece of paper what would be the equivalent of Turing's argument in the quantum. In the quantum world. And then I thought, well, what I better do to make this an elegant derivation is prove universality. And I couldn't prove universality. And then I realized that some of the quantum operations, some inherently quantum operations are needed to make quantum universality. And so I realized that there would be such a thing as a universal quantum computer, which would be more powerful than a universal classical computer like Turing's. ### Maciej Kawecki 00:38:49 - 00:38:53 Thank you very much for your time. It was a great pleasure. Grand honor. ### David Deutsch 00:38:53 - 00:38:55 Same here. ### Maciej Kawecki 00:38:55 - 00:38:56 Bye bye then. ### This Is World sponsor bumper 00:38:56 - 00:39:02 The This Is World channel is partnered with IQM, one of the world's largest quantum computing companies.