Within Reason - #57 David Deutsch - The Multiverse is Real

Episode Date: March 5, 2024

David Deutsch is a British physicist at the University of Oxford. He is a visiting professor in the Department of Atomic and Laser Physics at the Centre for Quantum Computation in the Clarendon Laboratory of the University of Oxford. (Wikipedia.) Buy David Deutsch's book, The Beginning of Infinity Learn more about your ad choices. Visit megaphone.fm/adchoices

David Deutsch, welcome to Within Reason. Hi. I'm excited to talk to you. I was reading your book just recently on an aeroplane, actually, which is why, for the viewers watching, they may notice it's a slightly different background to usual. I'm in a hotel room, or a motel room in Miami, and you're currently balanced on top of a microwave and a suitcase,

so that accounts for the change in circumstance. I was reading your book on the plane over here, the beginning of infinity, And right at the very beginning, you talk about the nature of our universe, you talk about our planet being part of a solar system and the solar system being part of a galaxy, the galaxy part of a universe. And then rather confidently, you say that this universe is part of a multiverse, just one universe among many others. I hear people talk about this as a matter of conjecture all the time, but the confidence with which you painted a picture of this multiverse into the very beginning of your book made me want to start by asking, what gives you such confidence? that this multiverse exists? I don't know if confidence is the right word. I have as much confidence in quantum theory as I do in any of the other superb theories of physics,

which are our best knowledge of the world. The fact that all these decades after it's almost a century since the theory was proposed and half that time it has been known that it describes a multiverse and yet that proposition is still disputed by most physicists when I say most people ask me how many I don't know but but those who adopt the many universes form of quantum theory or Everettian quantum theory, as it's called, after Hugh Everett, who proposed it in 1957, are perhaps 10% of theoretical physicists, but the proportion is much higher in certain branches of physics, where you have to actually ask

what is happening to bring about the predictions that quantum theory makes, rather than just using the predictions as a sort of almanac. And philosophically, I think that, yes, so one of the excuses that's commonly made is to call this an interpretation of quantum theory rather than calling it quantum theory. rather than calling it quantum theory. That's a kind of a trick. It's a bit like creationists calling the theory of evolution an interpretation of fossils.

Because after all, no one's ever seen a dinosaur, no one's ever seen other universes. No one's ever seen... Fossils are actually stone. which is just evidence of dinosaurs, not actual dinosaurs. And similarly, interference phenomenon are just evidence of other universes. They are not themselves other universes. So the logical connection between these things is very tight.

It's the same level of excuses being made. So you mean to say that your, you said confidence is the wrong word and perhaps it is, but your confidence in the truth of something like a multiverse is similar to your confidence in the truth of something like evolution by natural selection on the basis of the available evidence. Yes. I mean, that's quite, that's quite extraordinary. And I want to jump into why that is and why quantum physics does point to a multiverse.

But just beforehand, you said you don't know how many. scientists are on one side or the other of this debate, but for those who aren't in agreement with you here, for those who don't look at this evidence and see it obviously pointing to a multiverse, why do you think that happens? If, as far as you can see, this is a fairly straightforward deduction to make from the evidence, why are so many people missing it? It's not a deduction, it's the usual thing. It's the only known explanation of the evidence, just like in the case of dinosaurs, that we know they existed. So why has this happened in physics?

Well, I don't know is the short answer. It's some kind of sociological phenomenon of 20th century physics. You have to remember that at the time when quantum theory was proposed, there were very few physicists in the world. especially fundamental theoretical physicists. They all knew each other. It was a little culture, a little subculture or little society of its own, and it was therefore unstable to small numbers of influential ones of them,

grabbing onto a false doctrine. Now, that wouldn't matter because ultimately all doctrines turn out to be false in science, but trying to enforce it dogmatically, trying to put down questions like what brings about the outcome, trying to put down those questions as being illegitimate questions, Like, if you ask that, you haven't understood quantum theory, or else nobody can understand quantum theory. You're not supposed to understand it. Or, quantum theory isn't supposed to be a theory of the world.

It's supposed to be a theory of how we see the world. And that kind of thing, all of which are well-known excuses in all sorts of other areas. But in physics, they've somehow become. entrenched and some people say it's because the original generation then trained their students to fear, to ask certain questions. It might be that. It might be something in the wider society. The same thing could have happened in relativity.

Einstein, when he was young, was very strongly influenced by, the positivism of various people, including especially Maach, Ernst Maach, who was a physicist, but also a very emphatic positivist philosopher. And early relativity was in fact inspired by this positivistic insistence on, let's just ask questions about what we can see rather than the absolute space underneath it. Well, and Einstein, in a certain sense, took that to heart by saying there is no such thing as the absolute space. There's only relative times and distances, and therefore, for example, simultaneity

doesn't exist. So, but when it came to general relativity, That didn't work. You couldn't say that the underlying space-time doesn't exist, because the underlying space-time is actually the substance of general relativity. And so Einstein had to drop positivism completely and go across exactly to the opposite extreme, which is why he rejected the growing consensus about quantum theory. This is often said to be a case of Einstein rejected quantum theory,

but that's not at all true. He rejected the view of it, the excuses that were put forward by its pioneers for not taking it seriously as a theory of reality. So talk to me about the kind of phenomena that we're observing in quantum mechanics that leads us to this. Yes. This amazing conclusion that we've been talking about so far.

Yes. It's one of the most amazing things that physics has discovered. I don't think the most amazing, but one of the most amazing. One way of looking at it is, Traditionally, the way that people have looked at it is to look at the theory and just say, well, let's apply what we've been taught as physicists, as students, how to connect the equations of the theory with predictions about reality. And you can labour through that process.

The first people who proposed the many universes version of quantum theory did, in fact, go by that route. They were, Schrodinger came up with the idea just before Everett, probably in the early 50s, but he never published it. So Everett was the one who first published it. And so their accounts are rather mathematical. They're saying, look at the theory, let's do what we normally do and not make excuses for not believing what the theory plainly says.

sort of thing. Now, if you're not a physicist, appealing to that training is not very convincing. So I'd rather go by a different route and say, well, how do we explain the experiment? So there are very many experiments now, and they are completely uncontroversial. The outcomes of these experiments are used in other experiments and in technology, and so on. So there's no doubt about what will happen in the experiment. It's just that the prevailing view is to shut up and calculate. In other words, don't ask about how the outcome comes about. And the simplest case is the famous two-suit experiment or multiple slit experiment where a single particle is aimed, should say that a single particle is, in quantum theory, is not

quite what it is in classical physics. It's spread out a bit, which normally doesn't matter because it's spread out by a very small amount, which we can only detect in these specialized experiments. But got to remember that it spread out a little bit. And you have two slits, which are separated by less than the amount that the particle is spread out. So you can do this experiment at home, I mean, it's not that small distance, that 10th of a millimeter is fine. So you make two slits in a barrier, say a tenth of a millimeter apart, and then you aim your laser at it, and the result, and then you project that picture on a screen a few meters away. and you see that instead of just getting a shadow of two slits or of one slit, if the resolution weren't enough, you get an intricate pattern called an interference pattern.

And it's the same pattern that you would get on a different scale if you put a barrier in front of some water waves. So water waves also don't produce a shadow in the water, but they produce a complicated interference pattern. But now you can put a filter, a dark filter in front of the laser until only one photon is going through at a time. Now, this you can't do it at home, but you can easily do it in an optics slab with a photo multiplier to detect the light. And you find that even when the light is going through one photon at a time, and so you only see it on the screen going blip, blip, blip, blip, it still goes blip in that same pattern, identical pattern. So there are places where it sometimes arrives, and there are places where it never arrives. and if you have only one slit open, you also get a pattern characteristic of one slit,

pattern characteristic of two slits. You can make three slits, four slits. Each number of slits produces a radically different, apparently radically different pattern. Now, the thing that tells us conclusively that there are parallel universes, wait for it, is that if you look at... there are certain points on the screen where photons, let's say you have one slit open,

where photons are arriving, and if you open the second slit, they stop arriving there. So, how come? Well, obviously, they must stop arriving because they know that there are two slits open now and not just one. And if you look at the mathematics of this, what determines whether they arrive or not depends on whether how many slits are open. There are one, two, five, 17, they all give a different pattern. And the mathematics tells us when opening a new slit will stop light arriving at a given point. So to be clear, we're still talking about firing one photon through the same slit as before. It's just that because we've opened up another slit now,

suddenly that photon just isn't going through that slit in the same way as it was before. Exactly. So then we can do various kinds of experiments. You know, something must be coming through the other slit. Something which stops our photon, the one we see finally, or the one which our photo multiplier detects. So we can do various experiments. Some people, you know, but first, you know, you might think, well, maybe when it comes to the slits, the photon splits in two, and then they join up again before they reach the slit.

Well, you can do experiments to show that that isn't the case, that you can never detect half a photon. You know, you can only, wherever you put the photo multiply between the laser and the slit, if it's going through one photon at a time, then you only ever see one photon or none if you put it somewhere. where the photon isn't, only ever one or none, never two, never more than one. Right. So then you can say, well, maybe there's something else invisible going through which we can't detect. Well, what? So you can do experiments on the other slit. Whenever you detect a photon, you also look at the readings for some other apparatus you put in the wave.

and you can put, for example, a prism in the way. You can put mirrors in the way so that whatever's going through the other slit might bounce off the mirror. And you find that if you make it bounce long enough, you get the one slit pattern again. Because the thing that came through the second slit does not, arrives too late to affect the photon we see. And then because the formula for what happens depends also on the phase of the photons, you can put a glass filter to slow down the photon just a little bit. And that actually shifts the pattern because the first photon is arriving slightly before,

but not long enough before to be unaffected by the second photon. So, in short, the thing that comes through, behaves like a photon, it's deflected by mirrors, it's half deflected by half-silvered mirrors, it's affected by prisms in exactly the way a photon is, it's just that. That is to say, in the way that a particle of light would behave. Yes, yes.

So at these very small amounts of light, it always is emitted as particles. So the photomultiplier detects a particle. And if there's less than a particle there, it sometimes detects and sometimes not. But when it does, it's always a full particle. Sure. So it is light. It's just invisible light. Well, what does that mean?

Well, you can do experiments with the energy. and find out whether some of the energy is given to the other photon, and the answer is no. And so on and so on. And the conclusion is that this photon exists always in multiple copies. Even one photon exists in multiple copies of which we can at most ever see one. You might ask which one. Well, that's like asking which identical twin am I? You know, they're identical twins.

When we look at the screen, we're seeing one of those photons, but there's another instance of us seeing each of them. Yeah. Okay, so let's slow down a bit here because this is really important. So typically, when people describe this double split experiment, this double slit experiment, you're firing a photon of light, a particle through these slits, and it's acting like a particle.

Everything you do to it makes us think that this is a particle that we're dealing with. Which just means it's localized. It doesn't spread out more than... Right. Yeah. Yeah, so it's this tiny little thing, you know? And yet, we have sort of contrary information

that this particle is also acting as if it were a wave, interference patterns being sprung up the screen behind the slips. You know, it seems to be coming out, sort of going in as a particle, coming out as a wave. And depending on what we do with it and how we look at it, it either acts like a particle or as a wave. We only ever see the whole pattern in one go, like we do with water waves, if we send through lots of photons at once so that where each one comes out is kind of blurred. But when we send through one photon at a time, then what we see is blip, blip, blip, blip, um, just, um, uh, across the screen.

By the way, I say blip, blip, um, this tells you what one photon at time means. It, like, if, if the blips are one second apart, then the actual photons are 186,000 miles apart, you know, there's no way they can affect each other, like successive photons can affect each other. Right, right. But this is a single particle phenomenon. Yeah, okay. So this is confusing. Particle, wave, what's going on? At the time that this experiment is first, that this result is first discovered, what's the first reaction of the scientific community as a means of explaining what's going on here? The first way of coping with this was before this, well, actually, I was going to say before many people knew of the single photon type experiment.

It had actually been done by a guy called GI Taylor in early 1900s, but he didn't see the significance of it. So the first thing that the mainstream physicists thought was that the particle was a wave. It's just a wave going through. And the only reason it's only seen in a narrow range of places is that it's a wave packet. It's like if you, if you, if you, if you, if you, if you, If you have a skipping rope and you're kind of making a wave in it, you can either make a wave that goes along the skipping rope. But if you suddenly move your hand, then a single called a wave packet moves along the skipping rope. Yeah, yeah.

Like a little wave will shoot all the way down the skipping rope. Yes, a little wavelet, you can call it. Yes. And it carries energy. You know, the person holding the rope at the other end will feel the kick. so it was thought that the first thing that was thought was that maybe light is like that that and by the way later they did experiments to show that electrons and neutrons do exactly the same thing so light had been thought to be a wave for about a hundred years before that so

but electrons and neutrons had been thought to be particles but when these phenomena were detected, identical phenomena were detected, then people say, well, maybe all particles are actually waves. Hmm. And so a photon would be something like light as a wave, and a photon would be something like one of these little wavelets, just being sort of shot out one at a time. So what's wrong with that interpretation? Why can't we think of photons as just tiny little waves?

Well, there are several things, several weirdnesses piled up. And the first one, which is perhaps not the most important, but since you seem to want to go back to these early interpretations, the first thing was the photoelectric effect, which, by the way, Einstein discovered why it happens before real quantum theory was invented, It's kind of an early form of quantum theory. The problem there was that, you know, the photoelectric effect is just that when light hits a photoelectric cell,

like you use to, you know, open an automatic door when you approach, when the light beam is interrupted, so the electric electricity becomes less and so on. When you put brighter light on the photoelectric cell, you get more electrical. When you dimmer light, you get less until there's some threshold below which, no matter how many, no matter how much light you put on the photoelectric cell, if the energy of each photon is not enough to excite the cell, you get nothing. So it's not dependent on the total energy incident on the photoelectric. electric cell, it's a matter of the frequency or the energy of each photon. So that tells us that photons come in particles. That, you know, photon ons meant it was a particle.

And so that was Einstein's theory. And they gave him the Nobel Prize because for various nasty reasons, they didn't want to give him the prize for relativity. So he got the prize for that. But later, there were experiments with, first of all, there were experiments with silver atoms, Stern and Gerlach, where you had silver atoms not going through a slit, but being a single, sorry, a beam of individual silver atoms. So we can talk about one silver atom.

and it's got an intrinsic magnetic moment so that when it's hit by magnetic field, it's either pulled one way or the other, or you might think if it was at an angle to the field, it would go slightly to one side, but it doesn't. It only ever goes fully to one side or fully to the other side. And so the spin or magnetic moment of a silver atom is also quantized. It also can only exist in zero or one or two units and so on. And then you can put a magnet after that. So if you shine them through individual silver atoms, you'll find them going blip,

blip, just in two different places. Blip, blip, blip, blip, blip, randomly, apparently. And then what you can do is you can put another magnet with the other polarity to make the two beams. And let me stress, these are beams. One of them has got nothing in it. It's a silver atom and a silver atom in another universe, actually.

But they didn't know that. But you put a magnet in the other direction to bring the beams together. And then you find that the state of the silver atom is what it originally was. I should have said, if you want it to go randomly up and down, you've got to start it off pointing sideways. So, classically, you'd think that if it starts off pointing sideways, it won't be affected by the magnet at all. It'll just go straight through because it's like pulled equally up and down.

Pulled in each direction. In fact, it gets split always, no matter what direction you push it. You put it in sideways. It goes through this thing. You put the magnets the other way. And then it always emerges in the same direction it started out with.

So it knows. It knows that it was once a leftward pointing rather than upward or downward pointing. But if you interrupt it halfway through, you'll get a upward pointing and a downward pointing randomly. So that was the next thing that puzzled people. And at this point, they knew that something really weird was happening because a lot of them had predicted that this thing won't work with matter.

It'll only work with light. Sure, sure. And the silver atoms were the first thing that they tried that wasn't light. and it worked exactly the same and the equations were the same and the experiment was the same and then finally now this wasn't an experiment

it was a piece of theory that had to do with the description of the experiment including the the experimenter If you think of a particle as a wave, then it's a wave in three dimensions. It's moving along just like the blip in the skipping rope or the blip in a water wave or whatever.

But the mathematical description of two particles, possibly interacting with each other or not, The real description is that it's a position is this in one universe and that in another universe and this in one universe and that in another universe. And so to describe two three-dimensional spaces. Yes. I see. Okay. So we're beginning, I'm beginning to see where we're getting sort of multi-universes jumping out of this kind of stuff. And so the next thing, which was, I think the final clincher, was really invented by David Bohm, who interestingly invented an entirely new interpretation of quantum theory, which tried to get away with not having parallel universes.

But I think his interpretation is really the Everett interpretation in a state of denial. It's very, very weird and counterintuitive. and involves lots of things that we don't see and so on. But anyway, he had the idea of why do this thing with two three-dimensional particles? Why not just use spins again? Why not have two spinning particles? And the spin of one particle can only take two values up, like can only be measured to have one of two values and the other one also.

So the combined system can be in one of four states and you can make them interact and indeed you can predict that they will behave each of them will behave as though it were affected by both instances of the other one and that is

quantum computation so they didn't call it that they didn't think of it as that but that's what it is sure okay so let's rewind a bit to this double slit experiment

and can you tell me about other i mean i'm hesitant to use the word interpretation this now because you seem to think that these are better described as as copes um yes or at least other theories which are yeah uh the the copenhagen i suppose with emphasis on the cope nice i'm wondering if you can talk us through what that interpretation is and why you think it's insufficient Why I think it's not sufficient. Yeah. Yes.

So the, well, two different things are called the Copenhagen interpretation, just to be complete about this. The original one is hardly ever advocated. That was advocated by Neal's bore, and it said that quantum theory only ever applies to microscopic things. And that macroscopic things have to be described informally, not by theories of physics. So you can see why physicists didn't like that, but it also doesn't make sense because by its inherent mathematics, quantum theory applies to things on all scales. So the best way of, oh, and then there was the wave function collapsed. theory, more or less invented by von Neumann in the 1940s, which says that when you observe a photon in one place, the other copy of it disappears.

So it really doesn't solve the problem, because if the other copy is microscopic, you still have all the many universes implications, like when there are two to the end of them, There's still two to the end different things, which are all there until you observe the particle and then all disappear. Yeah, well, this is the interpretation, this is the interpretation that I'm most familiar with that I was probably taught at school, that when we look at, when we look at sort of a wave, we're seeing this wave function of all, a sort of probabilistic distribution of all of the things that a particle can be doing, and that's what we're observing. But when we observe the particle, everything else just sort of disappears and suddenly the particle is in one place. Now, I remember the first time I learned about this thinking that seems absolutely absurd, but it was what was being taught. And I thought it was just one of those things about quantum physics that's just amazing and counterintuitive that I'm just going to have to kind of, I'm just going to have to swallow. Yes.

But that's not the case. No, it's not the case. It's worse than counterintuitive. It just doesn't make sense. It's nonsense. And the classic way of seeing that it's nonsense was invented by Eugene Wigner, yet to know a physicist, who also, this was a long time ago, but he also didn't accept Everett. But nevertheless, he invented what's called the Vignor's Friend paradox, the paradox of Vignor's friend. And it's not a paradox, it's an argument.

I mean, he thought it was a paradox. But the argument of Vignor's friend is, supposing that you have a friend who's a quantum mechanical object, now we can't do this experiment in real life because we can't control the positions of all the particles in a human being to find as fine the details as we need to do an interesting. interference experiment. But suppose we did it, then the, and let's say this friend was observing the Stern-Gerlach experiment. So he looked to see whether the silver atom was in the upper,

on the upper path or on the lower path. And when he looked at it, it would jump to being in only one of the paths. One or the other. One or the other. And he also, because until the jump occurs there's going to be two of him as well on the microscopic level Right So suppose you look at him But you don't look whether it's on one path or the other

You look on you look to see whether it was left Or not just like the on the final screen You can see whether it's you can choose To detect whether it's up down or left right Just not both So on your friend You look to see whether his consciousness is registering left or up or down or what.

Sorry, no, you can't do that. Registering left or right, sorry. And whichever of those you see, it will spoil the up-downness. So I'm not explaining it's very well, but later I proposed an experiment which would in principle be possible. But instead of Vignor's friend, you have a quantum computer. Let's say a quantum computer running an AGI program. So this quantum computer is a physicist, and it's your friend, and it decides that it's going to do this experiment.

And halfway through, when it's looking at the up-down thing, it will write, it will sign an affidavit, signed and sealed by a notary public. or whatever, and it will say, I, Mr. Quantum Computer, certify that at this moment, I am detecting either up or down. I'm not detecting left to right or both or anything like that. I'm seeing only one of up or down. I'm not going to say which, because if I said which, that would propagate to whoever reads this.

but since I'm not saying which, anyone can read it and it won't spoil the experiment. So then, again, instead of the experiment where you put the magnets up the other way, he simply reverses the dynamics of the memory element in his brain in which he remembers whether it was up or down. but not the one way he remembers that he saw up or down. Yep. So then out it comes out of the quantum computer into the detection magnet at the end, and it detects that it's sideways. So in other words, what he did did not collapse the wave function.

And yet... Right. So the computer has observed the... whether it's, has observed the actual position. Yes. Which you might want to think would collapse this wave function. Yes. But then what the computer forgets which it's, what position it saw the particle in,

but doesn't forget that it saw it in one or the other. Yes. So something has observed where this particle is, whether it's up or down. Yes. It's observed it. And yet when it comes out the other side, it's still, to us, observing the whole system, is not up or down. Yes, in fact, it's certifiably what it was before it went in.

Right, which means that something, this computer has observed the particle without collapsing the wave function. Yes, exactly. Which tells us? Well, if that happened, that would tell us that the wave function doesn't collapse. If it didn't happen, it would tell us that quantum theory is wrong, and therefore that Everett is wrong. Gotcha. So it's a test.

It's a crucial test of Everett against all the other, all the other interpretations except boom. Interesting. That's fascinating. So, okay. So people are going to be listening to this. And it's a lot. And I'm certainly no physicist.

So I might not be picking up on the right bits here. But hopefully people are following. But they're going to be listening and thinking, okay, we were talking. a moment ago about the multiverse. We were talking about this image in my head that, you know, every decision I make, there's a version of me somewhere else that's doing the opposite and ever expanding into these, these uncountable trillions of universes. And now we're kind of talking about firing silver atoms and seeing whether they're up or whether they're down and getting a quantum computer to have a look. And how does this all sort of dovetail together? Where do we

Where do we go to get from wave collapse, collapse of the wave function view of what's going on here is false to multiverse? Well, collapse of the wave function is only one of the possible copes. So what we're really testing is Everettian quantum theory against any theory that has only one universe. any theory that has only one state of the silver atom, the observer, the rest of the world, the scientific paper he writes, all the things that could be affected by whether the observation went up or down, those are all affected or not in this experiment. And in that sense, they are universes. I mean, the other way you can see, the more usual way of seeing it is just that the same equations apply no matter how many particles there are there. But to the layman, that may not be as convincing. So it's that it's going to affect things.

The differential position is going to affect things differentially that are affected by it. Note that, therefore, in one sense, the term parallel usual, The term universes is rather misleading, or it can be misleading, because the only things that can be affected are the ones that are actually hit by a photon or a silver atom or whatever, and those travel at most at the speed of light, and therefore this splitting or striation of reality into multiple realities doesn't go outside a sphere, an expanding sphere. And in fact, most of the things in that sphere are not affected anyway. So it's only certain things in the sphere, but nothing outside the sphere is affected. So it's never a universe that splits in two. It's only ever a piece of the universe that has some traveling information in it that split in two. So what's the best way of picturing this in our heads?

Because when people think of a multiverse, they might be thinking of lots of sort of orbs. and each orb is its own universe and they all exist in some kind of void or something that's not quite right here like how should we be trying to picture this in our head? Is there a good way of doing that? There are many reasonably good ways

perhaps none perfectly satisfactory but so first of all if you think of the multiplicity of the multiverse it's best to think of things not splitting in two but of two things which were originally identical becoming different.

So things in the Stern-Garly experiment, things which were originally universes or silver atoms, there were lots of them, and they were all originally pointing in one direction. And then when the magnet happens, they differentiate themselves into two groups. One of them being the upward group and the other one being the upper path group, path group and the other one being the lower path group. And then later they, usually, they don't

rejoin. But in this specialized experiment, in the Sturangalak experiment or in Vignac experiment, they can rejoin and the up and down universes interact with each other to produce a left or a right. Well, let's say it started off left. They will reliably produce a left universe again. And so the universe has become identical again until something splits them again. So it's sort of like one big universe, one big space, in which multiple kind of, I don't know what the word is here. Like if universe, the word universe is misleading, like multiple branches of events are sort of all intertangling and interacting with each other. So these aren't sort of completely mutually exclusive universe is all existing side by side, but these, you know, probably not actually infinite, but, but, you know, infinite in the sense of, in the rhetorical sense of being sort of uncountably huge numbers of branches that, that interact with each other. And this multiverse is an interactive multiverse that we're thinking about here, right?

Yes, yes. This rejoining, as I said, is very difficult to arrange, and it only happens in very specialized circumstances. Yeah, such as the one we've just talked about. Yes, but on the microscopic level, it happens all the time. Like in a chemical molecule molecule, in a benzene molecule, I think, is one of them. You have electrons can be either above the plane of the six atoms or below the plane, or both at once in different universes. So in this both at once state is stable.

So can we go back and basically run through, I mean a moment ago we talked about the double slit experiment is the one that most people are familiar with and we explained what happens there. Can we go back and can you explain it again but this time in explicit terms with reference to this many world's interpretation of quantum mechanics? what's actually going on in that experiment?

What's happening in your view? So we release one photon per universe, per Everett world. Actually, that one universe, that one photon exists in a swath of universes, the swath of all universes in which we did in fact release it. There'll be other universes in which we didn't release it, and so that will not contribute. but there'll be a swath of universes in which we release the photon. And in all of them, it will go towards the single slit or double slit or whatever we have put there in front of it.

In some situations, okay, in the case of one slit, there will still be interference because photons that, touch the left side of the slit will interfere with photons that touch the right side of the slit because that will make them different from each other. And they will then, and that's why there's an interference pattern, even with one slit, but it's smaller and less pronounced. Yeah, because some of these photons are sort of bumping up against the side of the slit. Yes. So physicists of old called those different kinds. kinds of diffraction but we can just call it interference that it's the kind of interference when there are two they differ by by more their phases differ by more and so they they have a

more pronounced effect on each other when they when they are rejoined so then so they come out of in the two universes or two branches because each branch consists of a whole sway of universes itself. But in the two branches, in the first branch it comes out in a, in a sort of little cone, and in the other one it comes out in, also in a little cone, but displaced sideways from the first one. So these, these ones are displaced by, say, a tenth of a millimeter from each other. I mean, it's big enough to see. It's not that microscopic. And then they travel at the speed of light towards the screen and when they when they're reaching the screen the ones which the ones which head away from each other don't do anything special but the ones which are heading towards

a given point where they can where light can reach that point from both slits right not all points are like that you go far enough sideways um but if you have a point where light can reach from both slits, the photons, the photons in one, the photon in one universe will interfere with the photon, the other universe, in such a way that it either stops it getting there, deflects it to somewhere else, or it doesn't. And if it doesn't, how, how are photons from different universes interacting with each other in this way? How are they sort of bumping off each other? Sort of. So the universes, as you said just now, the universes are in constant interaction with each other.

It's just that most of the time things we see, it all cancelled each other out and we don't see interference phenomena. But if we look at things in theory, we see that there are interference phenomena all the time. Solid matter is an interference phenomenon. When we try to display an interference phenomenon in the laboratory, we're trying to explain something that can only be explained by interference. So we know that solid matter can only exist because of interference. What do you mean by that? Solid matter can only exist because of interference.

How do you mean? Yes. So the individual atom, in a, say in a crystal, are in this lattice. And if they were interacting with each other by classical electrodynamic fields, as was thought before quantum theory, then the whole thing would collapse into a sort of puddle. There isn't enough of a, there isn't enough of

potential well at each point. In fact, there's no potential well at each point to keep the atom there. What keeps it there is that it is in more than one place at a time and so are all the other atoms.

They're all in more than one place at a time. And this state of being in more than one place at a time, just like the silver atom in the Stoangalloch experiment, being being in the up path and the lower path at the same time is equivalent to pointing sideways, which is the thing that can't be expressed in just being up or being down.

And in a crystal, it could be that this other way of being, a way of being in this place and that place at the same time, has a lower energy than either of them separately. And therefore, the crystal settles into this lower energy state where the particle, are no longer in individual positions. Because that's a lower energy, it requires energy to change the shape, and that's why there are solids. So we require a multiverse in order to get solid matter. Yes.

This seems to me like a revolutionary claim, and yet you were talking about it earlier as if it's obvious and you can't understand why scientists are unwilling to accept this. But you understand that on the face of it, that seems like an extraordinary claim to make. Yes, I don't think it's as extraordinary as relativity, frankly. For some reason, it's resisted more. And there are different kinds of codes. I mean, there used to be copes in regard to relativity as well, but they were of a different kind.

Maybe I should say, one thing that is very convincing to people, to physicists, is that when you actually use the theory, you have to write down the equations, and you can either not ask what the equations are saying happens, in between the initial state and the final state, or you can ask. If you ask, you have to talk about other universes. For example, when you write down the equation that takes light from a laser to a 15 slit barrier, you have to write down 15 equations. one for each of the slits, sorry, an equation with 15 terms, sorry, a single equation with 15 terms in it, and nothing less will do. And this is, again, why quantum computers are important, because that means that quantum

computers can do computations, which involve 15 or 2 to the power of 15, separate computations, which are then brought together to give an answer. So, if you're a quantum computer, computing person and are inventing algorithms for quantum computers to do difficult tasks, you have to work out what each one of those two to the 15 branches will do. There's no way of getting around. In fact, for the interesting cases, there is provably no way of getting around it. So you know that the whole reason why you want the quantum computer to do this is that no way of following

through individual paths or probabilistic individual paths or individual paths with collapse or you know whatever there is simply no way of getting the answer in less than whatever it is two to the 15 pages or two to the 500 pages whereas the quantum computer can be run and can do the motion referred to by those two to the 500 terms in the equation all at once all at once. Right, because it can sort of, so those, and that's because those different terms all represent different, say, branches rather than, to not use the word universe, they all represent different branches, which a quantum computer is able to actually just sort of have coexist

and interacting all at once, whereas any human being looking at any of these terms is suddenly going to be in one of those branches and thus no longer able to look at the whole system of branches interacting altogether. And that's what quantum computing will do for us. Yes. And how far off are we from this technology really becoming something that's usable? I don't know. I haven't been working on quantum computation for about 10 years because I've been working on constructor theory with my other hobby horse and so I'm not I'm not familiar with the state of the art every so often there's a new idea and I think oh now now it's going to work but it's it's extremely hard the reason is extremely

hard is that any kind of perturbation of the quantum computer while it's doing its multiple computation will spoil the interference. It will make only some of the universes different, and the others will be the same in all the universes in which the perturbation happened. So you've got to either prevent the perturbations happening, or you can do this magical thing called quantum error correction, which is a kind of quantum computation that, a specialized kind of quantum computation that removes the errors.

It's like an error correcting code except that in classical computers an error correcting code always involves redundancy where let's say a majority vote is taken. You have

five bits doing the same computation and if one of them is different you reset it to what the other four are same. In quantum computation, you can't do that because to detect that one of them is different would be a classical computation and that would spoil the interference. But fortunately, there are quantum algorithms that do the same thing that can internally detect without ever telling you, they internally detect which cubit was different from the others, and they change it

accordingly. And you never find out which one was wrong. Right. I've got to say, this all still sounds, I mean, this idea that we've got multiple universes interacting, multiple branches interacting. Yes. And I understand the idea that when I observe one of these branches, it's kind of just like finding out which branch I'm in. It's not like a wave function collapse. It's just that I find out which all of the options do actually happen. I'm just finding out which one I'm in. It still seems strange to me, though, that if I don't make that observation,

then at the end, I can look at the whole system as if all of the branches sort of all happened in this one universe. You know what I mean? Well, because they're all the same by then. So you're still only seeing one of them, but they're all the same. So like the silver atoms in the Stoengalek experiment, for a while in the experiment, they're doing different things. But by the time they come to the end, all the universes, or both the branches, or however you want to put it, are identical. They are both pointing to the left.

So no matter who looks at it in what universe, they will see. something pointing to the left. So why does our observation of an individual particle in the double slit experiment change from, change the end result from an interference pattern into two singular slits, two singular lines on the screen? Yeah. So in the many worlds interpretation. Yeah.

In my thought experiment, it doesn't change it. Or at least it, it doesn't change it if you perform the experiment. as specified because you'll bring the branches together again. That's the key step. Now, when we look at that in real life, we can't bring the branches together again because that would involve sort of a massive 10 to the 26 pronged magnet converting us back to the same state that we were at the beginning other than having remembered and all that stuff.

So it's not feasible to do that with an actual human, but it is feasible to do it with a quantum computer. By the way, you said just now there are the universe is operating in a quantum computer. And then I think you change your mind and said branches. Branches is much better because especially in a quantum computer, the rest of the world isn't affected. If it were, the interference would be spoiled. So the whole multiplicity happens inside the quantum computer and then gets undone before it presents its result. Yeah, so I think that's actually a really helpful way of picturing what's going on here. Like if you, I mean, when I think of a computer, I'm thinking of like a big sort of plastic box and it's all happening inside of there and there are these sort of multiple universes, so to speak, but it's everything outside of that box is completely unaffected.

Yes. So it's branches within one one universe. And the trick is bringing those branches back together again. Yes. So in the double slit experiment, the reason why we, in fact, get two different results is because by observing one particle and which slit it goes through, we have sort of found ourselves on one of the branches and that branch is never going to rejoin the others again. Yes. But if it did, whereas if it did, then if it did then we would still be able to observe. that one particle and have it come out as an interference pattern.

Yes. But because that's unfeasible, that's not something that we're really going to see happen. And the reason why we end up getting the interference pattern is because when we don't observe that particle, we don't find out which branch we're on and those branches come back together again to produce this interference pattern on the back of the screen. Yes. Yes. Exactly right.

Though again, just because people will misunderstand this, it's not a matter of finding out It's a matter of the information, it's a matter of us having differentiated, the different versions of us, which are always there, would, if the information leaks out of a quantum computer, then we would have differentiated into people who see one configuration and people who see another configuration. And then you could only get the right answer from the quantum computer if you brought that information back and erased it from every, where it had got to, or made the, the, did something to make all the different versions of the observer identical again. So everything happens inside of the quantum computer and the exterior world isn't affected unless we observe which branch the, which branch the quantum computer is on. And then, like, there are multiple versions of me created one which observes one branch

quantum computer, one which observes another. We don't actually have to do the observation. It's enough if an air atom hits the qubit in question. Right. So that if we observe, that it's already, the interference is already spoiled then. That's why they have to add. It's too late. Yeah, it's too late.

So it's not a matter of. So it's not, it's not the observation that makes the difference. It's something else that makes the difference which observation necessitates. Yes, which would permit. observation. But whether the observation actually takes place is irrelevant. And what's the best term to use to describe instead of observation there? Like what is, what is the thing that happens? It's called decoherence. Decoherence. Yes. And if you had to sort of summarize the definition of decoherent into a sentence or a couple of sentences? It's where some other

physical object, like an air molecule, let's say, or an oxygen molecule in the the air hits a cubit and becomes differentially affected by whether the cubit is up or down. After that, there are two versions of the oxygen, two branches for the oxygen molecule as well. Unless you can make it come back and undo that phenomenon, undo that collision, the phenomenon won't happen. Which is possible in principle, but far from possible in practice. Yes. Well, you can do quantum error correction, which is kind of magic. It's almost the same. Yeah. I mean, it sounds like magic, but then they say that any sufficiently advanced technology is indistinguishable from magic.

Just to close out, what are some of the philosophical implications of this in your view? And the reason that I'm asking, the reason I'm asking you, I don't know if you would consider yourself, much of a philosopher as opposed to a, as opposed to a physicist. But the reason why I think it's interesting to ask you is because you understand this far better than someone like I will. And so when I think of the philosophical interpretations of the multiverse, I'm kind of picturing this, you know, bunch of different orbs kind of multiverse, which you're telling me is not what quantum mechanics is implying. Yes. So what are some of the philosophical interpretations that someone could make of a correctly construed many worlds quantum realm?

Well, the philosophical implication, so I don't know whether you call this philosophy or meta philosophy or whatever, but the whole story of trying to understand what the equations of quantum theory tell us about the world is a story of realism versus everything else. It's positivism, instrumentalism, woo-woo, you name it, you name the bad philosophy, it has derailed understanding quantum physics. You have to be, and I think this is true of all science, but you know, you want to know what the lesson of quantum, of the history of quantum theory is, that to do proper science, you have to be a realist. And Einstein, by the way, at the early days of quantum theory, was a realist. And this picture of him as being a stick in the mud and not wanting to accept quantum theory is false. He didn't want to accept anti-realist copes, as you call them, quantum theory. He didn't know what the right version was. Unfortunately, he died. sort of just before, I mean, I think Everett was already producing his theory, but he wasn't quite ready. So, you know, if he'd lived another few years, Bryce DeWitt says he's sure that Einstein would have switched immediately to it, but we don't know.

We can't tell. So that's one. And also, I myself, I'm sure that quantum computation would have been invented at least. least, at least 30 years earlier, if not 50 years earlier, if people had not been instrument, if the leading lights of theoretical physics had not been instrumentalist, positivist, and worse. Wow. That's a hell of an indictment on, let's say, closed-minded thinking.

I think it is a scandal. But now maybe what you really wanted was not this meta-philosophical implication, but an actual one, I think the most direct one turns out to be, not very, not very earth-shaking. The one I'm referring to is, since you know that every time you cross the road, in some universes, you will be killed by a bus that you didn't see. On the other hand, in some universes, you will be saved from the meteor that hit your house just immediately after you left it.

Yeah. So what do you do about this? Should you be risk averse? Should you be happy, go lucky, and think that, you know, no matter what happens, I'll survive in some universes. There is a mistaken line of thought called quantum suicide, which says that if your life is going wrong and you're unhappy, what you should do is buy a lottery ticket and set up a machine to kill you in your sleep

if you don't win. And so if you wake up, you'll only wake up in the universes where you did win. Yeah. And that is using the frequency interpretation of probability, which is exactly wrong according to quantum theory. I'm glad to hear you say that that's wrong. Tell me why that's wrong. Just in case anybody's getting any ideas listening to this podcast.

Tell us why that's wrong. The idea that probability in the sense of what you expect to happen and probability in the sense of if there are many copies of you or many people who you identify with or whatever, that any one of them has a one over N, you have a one in N probability of being that one. Of being that one. Of being that one.

That is a mistake. And it's a known mistake in the philosophy of probability, and people didn't really know what to do about it. But quantum theory solves this. It so happens that there is a structure within the physics of the multiverse that actually is relevant to decision theory. And I was the first one to propose this. It's now been called the decision-theoretic approach to probability in quantum theory. And you have to work quite hard to extract the answer. And the answer, kind of fortunately or unfortunately, is that the effect on your decision,

knowing that you are in multiverse should be exactly the same as it is if you knew you were in a stochastic universe where things happened by chance. So in other words, it has no effect. If you win the lottery in one out of 80 universes, that's not the same thing as saying that you have a one in 80 chance of winning the lottery in this universe. Yes. In your universe, whatever that means, yes. So is that because the probability is slightly different, or is it because the probability in your universe of you doing that is, you know, either 100 or zero?

No, it's because it's very unlikely, if I can misuse the term, the probability even applies in this situation. For example, suppose that this is done in a hotel with 100 rooms and they're sealed off. and they're all quantum mechanically sealed off so that no information could get in or out and and there's a machine that makes an exact copy of you um so um let's say you're going to be killed in that say there are two copies of you to make it simple and one of them's got to be killed going to be killed um then you should act as though there was a one in two chance so so the the you know the mistaken theory says. But the trouble is that if you know that you're never going to be let out of this room,

as is the case with universes, then you could, what's the cost, how much would you pay to have a copy of you made in another room, which was an exact copy and therefore it would behave exactly as you did for the rest of your life? You might say, well, that's, that's zero because that will not change any of my experiences. It's totally irrelevant to me. Totally irrelevant. Because it's not me in that room.

It's something like a copy of me. Yes. Well, yeah, now you've said it, you see, because if one of those is going to be killed, you're going to say that it's, there's a two-thirds chance now that the one which we're talking about, was going to be killed and only one chance that he isn't. And yet, you're indifferent

to whether you are duplicated or not. So... Yeah, yeah. So the versions of you that exist in different universes or different branches within the universe are not actually you. You're not identical with them.

They're just sort of another person that happens to share the same, I don't know, like the same sort of set of atoms working in roughly the same way. And so it's wrong to think of this natural interpretation of the multiverse that, oh, well, you know, if things go badly for me today, it doesn't matter because things went well for me in another universe. That's not really you. Well, so it's an interesting philosophical question, whether that's really you.

But the number of, them is not the probability of being them that's gotcha yeah I think that makes sense yeah I didn't want to ask you about one more thing that's just come to mind which is I think it comes from Nick Bostrom he wrote a paper and it was I can't remember the name of it but it was something about how the multiverse undermines consequentialism as an ethical theory You know, an ethical theory which says that the thing that matters morally is what the consequences of your actions are. So take utilitarianism, what we want to do is maximize pleasure.

If there really is this sort of infinitely branching set of universes that anytime something could happen differently, it just sort of does actually happen differently in two different branches or two different universes. It seems to suggest that if I cause suffering, in this universe, there is another universe in which I didn't cause that suffering. If I cause pleasure in this universe, there's another universe in which I didn't cause that pleasure. And so across the whole system, it seems like it doesn't matter what I do, because any effect that I have on any set of consequences, so maybe the amount of suffering in the world, is just going to be equally and opposingly matched by another branch in which the exact opposite happens. And so if our

moral commitment is just to maximize pleasure or minimize suffering overall, then as soon as we accept that there's a multiverse, suddenly we just have moral license to do whatever we want, because anything we do doesn't affect the overall balance of suffering in the universe. It just affects the amount of suffering in our particular branch. Yeah. So many things come to mind. First of all, this isn't an argument about consequentialism. It's about utilitarianism.

It's about the idea that we should make decisions according to some kind of calculus of good or bad. And as Popper has pointed out, suffering and pleasure are not the reverse of each other, because, as he puts it, suffering, other people suffering, for example, makes a moral claim on us, whereas other people's pleasure does not. We don't have any kind of, you know, you mean, you. might argue how much claim it makes, but we don't have the same kind of obligation to give somebody more pleasure as we do to give them less pain. Yes.

So that's one reason why that story of Bostroms isn't right from the get-go. Then remember that the branches exist in different amounts. It's not just that different things happen, that all the possible things happen. It's that different things happen in different branches. And in some situations, some not all situations, those like thicknesses or weights of the branches should be treated as probabilities. and in some it shouldn't. So if the utilitarian calculus applied,

then you would expect to want to maximize the expectation value of the total good that you're doing. On Popper's criterion, you'd want to probably minimize the total amount of harm you're doing. Yeah, yeah. Oh, yes, I remember what the argument is. Now, these branches in which different things happen are changing according to the decisions,

according to many decisions that we make sequentially. And in particular, our thinking, creative thinking or rational thinking, depends on error correction. Therefore, there are branches which become alike because our thinking applied to both of them says that this version is better than that version, so we change that version into this version.

Now, this isn't talking about quantum interference because we are decohering all the time. So the branches aren't coming together and interfering, they're coming together and just increasing the weight of the branch with that decision in it. It's as though you looked at the silver atom, but then you manually took the silver atoms and put them in the same slot, which, where they won't interfere, but there will be twice as many silver atoms in that slot. Similarly, when we decide that a certain thing is immoral and we switch, in the universes where we thought that was moral, we switched to calling it immoral,

and the other universe where we thought it was immoral all along will then become broader. It will have more weight, more probability in the situations where applying probability is correct. So this calculus is wrong for that reason as well, because we are, when, we think when we're making moral decisions, we're just, we're not just calculating all the possible outcomes and their probabilities. We are changing the outcomes. We're making them, we're making sure that some of the ones we think are bad are converted into ones we think are good.

By the way, the same applies to his thought experiment about the, or his allegory, about taking marbles out of a jar if you know that one. Yeah, yeah, yeah. They're all white except for a few that are black and black is doom. But the thing is,

every time you take out a white marble, you change the number of white and black marbles. Every time you take out any kind of marble, you're changing the number of black and white marbles because you are thinking. And just like I said just now about the branches coming together or not coming together, you're trying to make knowledge better or you're trying to make the outcome better. Let's suppose you're a moral person.

You're trying to make the outcome better. So you're changing the possible bad outcomes into good outcomes. In terms of the marbles, you're changing some of the black ones into white ones. So it's not like Russian roulette. In Russian roulette, nothing you can do according to the rules of the game. In real life, of course, you could. But in Russian roulette, if you obey the rules of the game, you've got a one in six chance every time you spin the chamber.

But in real life, the outcome depends virtually entirely on what we do, or actually entirely on what we do in the long run. Because even being wiped out by an asteroid depends on what we do and think now. There's no such thing as the probability will be wiped out by an asteroid unless it's already very near. That probability is affected by what we do, unlike Russian roulette, where it's always one in six. The probability of whether an asteroid will hit the Earth is affected. by what we do. And think, mostly what we think and then do, yes.

For example, set up an asteroid watch and prepare rockets which can deflect the asteroid. Oh, I see, right. I see what you mean. Yeah, okay. So it's not, when you were talking a moment ago about, you know, there's a universe in which I decide that something is immoral and I don't do it, Isn't there just then another universe or another branch in which I decide that it is moral and that I do do it? Is there always, in other words, this equal and opposite me that does the good thing and the bad thing? So there's always a you that does the bad thing, even if that might happen in only one in a trillion trillion of the universes.

So that is virtually always the case. But it's meaningful to ask whether there are more of you doing one thing or less of you. Right, because the number of branches or universes isn't actually infinite. If it were actually infinite, then there would be, I mean, like there would be an infinite number in which you do the bad thing, an infinite number in which you do the good thing. Even then, like, if it were a measurable infinity, that'd be, that's why I referred to the thickness of the universes. Even if there's an infinite number in the sense of the cardinality, there was still, there would still be meaningful to say that some of them, that some branches are thicker than others. There's more universes in some universes than others. And in some, there are so few that it's mind-bogglingly few.

The English language doesn't have the words to explain how few they are. And in many situations, those thicknesses are probabilities or should be treated as probabilities. So especially situations where it is a good approximation to think of the process that is choosing one or the other as being random. I know. Sorry, I can't continue. So you may, a cosmic ray may strike you and turn you evil. In the overwhelming, in the sense that you have an impulse to do an evil thing, in the overwhelming majority of those universes, your values kicking and say, actually, no, I don't want to do X.

So that's turning most of the universes hit by the cosmic ray. which happened to cause an evil change in your in your brain to turn back to being good again that that's how you think you error correct you think I mean you don't think that's wrong so I can't do it you usually you would say wait a minute I didn't think that that's just a misthought I know that this might seem like an impossible or even a stupid question it's going to be impossible to know how many universes are implied by the many world's interpretation of quantum mechanics. But is there a way that you can give us an idea to help us conceive of just how many different universes or branches we're talking about here? Well, if you think of

collisions that happen among atoms, like the ones we were talking about in the the quantum computer. Most of them, lots of those happen all the time in the world. In all the air molecules in this room, there are many collisions. I don't know how many there are per second, but

a very large number. And each of those collisions produces a spread of trajectories following the collision. And they're not all the same proportions in the multiverse either. There'll be a majority and then there'll be minority ones and there are some in which by chance, as it were, or in very few universes, all the molecules in this room are

deflected to the other end of the room and I suffocate. Right. Yeah, yeah. But in most of the ones in which they go to the other end of the room, they immediately come back. There'll be a sonic boom or something. which might in itself kill me, but that's another, you know, the, there is, it's not error correction, it's just elasticity, um, happening where, which will, uh, and so most, most of the randomness or most of the multiplicity of universes is immaterial to the, to the gross outcome. It only is in special cases like a cosmic ray striking a neuron, and usually not even then.

So how many? Well, imagine the number of atoms in the universe and then the number of collisions that they make and then the number of possible trajectories from each collision. Then it's first number times the second number to the power of the third number. It's very, very, very large. And most of them are to all intents and purposes the same. Some of them actually interfere with each other, but there's very few of those as well. Because they would have to be a very specially fine-tuned collision to then have another collision,

come back together again. So, yeah, a lot, in other words. A lot, yes. That's amazing to think in terms of that equation. Yeah. The timesing it one times the other to the power of the other. Yes. And that's just unfathomably huge.

It's one of the problems we're talking about this kind of stuff. And a lot of physics in general is that the same thing happens in astronomy, where you hear these numbers. and your brain just can't even be amazed by them because of the fact that it can't even conceive of them it's one of the many it's one of the greatest tragedies of our ape brains I think that they were built to deal with

how to get enough food to not starve and not to conceive of stars I think they were built to understand the world and we can understand infinity as well let alone in large numbers but it's difficult to I think Descartes made a distinction between, I can't remember the terminology he used, it was something like conceiving and imagining, and I can't remember which way around he used, but he talked about like, take a 125,000-sided shape, you know, you can imagine it in the sense that you can, you can know true facts about it, you can do maths with it, you can treat it as an object and you can mess around with it just in your head. But you can't conceive. of it in the sense of really seeing it in your mind's eye. You can't imagine that many sides

all sort of in front of you. And I suppose there's one sense in which our brains can understand the universe. And that's the first sense where we can know how many stars there are in a galaxy and we can do maths. And we can work out the gravitational pull of each of those and the trajectory of the galaxy and this kind of stuff. But that's another thing from, that's a separate thing from conceiving of it in the sense of being able to really understand what it means to think of a number that large, you know? It's, I don't think this is anywhere near as bad as you think. That's good, that's true.

I can't really imagine a cube. For a start, you know, if I try to imagine the far side of the cube, then in some sense, I'm imagining it as being transparent. Yeah, yeah, sure. And I don't know if you've ever been given the puzzle. If you think, look at the long diagonal of a cube and hold it up against a bright light so that you see only the silhouette. Yeah, and it sort of switches. It can look like it's coming out.

No, that's yet another one. No, I mean a solid cube. And you're holding opposite corners of the cube. Yeah, okay. most distant corners and you're holding it up in in front of you now it's going to its silhouette is going to be a two-dimensional shape yeah what shape is that now if you ask this question to roger penrose he will instantly tell you and he can even do that in four dimensions let alone three dimensions so you know mathematicians who work on these

things can not only work on them mathematically, they can understand them. They can grok them. Now, probably even Penrose couldn't do 20-dimensional one, but actually the answer is it's a perfect hexagon. And another kind of person who knows that off the top of their head is a crystallographer because they know that a I think it's a a face-centered cube or is it a body-centered cube. One of them anyway, in the crystal structure is the same as a

hexagonal, close-packed cube. You only got to look at it from 30 degrees different angle and it's a completely different crystal. Yeah. Yeah, what you said a moment ago about not being able to imagine the far side of a cube is so fascinating. Because you're so right, like the moment you're thinking about that face, you're not, it's no longer on the back of the cube. And yet there's no trouble. I mean, this is weirdly consoling that there's no trouble in our brain not being able to really conceive of that. I still know what it means to say the other side of a cube.

And maybe there is a sense in which we can, we can grasp the majesty of the universe. in at least that sense, even if we can't really picturing in our mind's eye in the same way that we can't picture the far side of a cube. The pictures in our mind's eye are tools, just as the mathematical equations are tools, the diagrams we draw on paper are tools. They're all tools that we use, and we can actually use them to understand the world, not just predict. We can understand it in however much of its majesty we want.

well what a what a brilliant place to wrap things up after such a wide-ranging and technical conversation that's a that's a wonderfully optimistic place to leave things um i appreciate you taking the time i need to uh take you off this this fridge that i've laid on its side and put it upright again and onto the other side of the hotel room before someone notices that i've completely upended this place so i had better hop to it but uh david deutch thank you so much for coming on within reason it's been fun oh thank you for having me Thank you.