Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 28 | Roger Penrose on Spacetime, Consciousness, and the Universe

Episode Date: January 7, 2019

Sir Roger Penrose has had a remarkable life. He has contributed an enormous amount to our understanding of general relativity, perhaps more than anyone since Einstein himself -- Penrose diagrams, singularity theorems, the Penrose process, cosmic censorship, and the list goes on. He has made important contributions to mathematics, including such fun ideas as the Penrose triangle and aperiodic tilings. He has also made bold conjectures in the notoriously contentious areas of quantum mechanics and the study of consciousness. In his spare time he's managed to become an extremely successful author, writing such books as The Emperor's New Mind and The Road to Reality. With far too much that we could have talked about, we decided to concentrate in this discussion on spacetime, black holes, and cosmology, but we made sure to reserve some time to dig into quantum mechanics and the brain by the end.

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Find your shade of instant eraser concealer. at your local retailer. Hello, everyone, and welcome to the Minescape podcast. I'm your host, Sean Carroll. I don't want to mess around a lot at the very beginning of this one, because you've probably heard of today's guest, who is Sir Roger Penrose, one of the most important and interesting thinkers and scientists

of recent years of my lifetime, that's for sure. If you don't know Sir Roger Penrose as a physicist, you might know him as a writer of popular-level books, the Road to Reality, the Emperor's New Mind, and so forth, but he made his bones, as it were, in the field of general relativity, Einstein's theory of gravity. Along with Stephen Hawking, he helped invent the singularity theorems, the idea that if you get enough stuff in a region of space, general relativity says you can't help but collapsing to a point of infinite spacetime curvature.

But Roger Penrose has done many other things in addition to that. Just as one example early on in the podcast, he mentions doing some early work on the nature of infinity in spacetime, and what he's really referring to is the idea of a Penrose diagram, which general relativists use to make a little picture of an entire spacetime. Roughly speaking, Penrose diagrams are as important for people in general relativity as Feynman diagrams are for people doing particle physics. And not only that, Roger Penrose has come up with the Penrose triangle,

the Penrose process for extracting energy from black holes, Penrose tilings, twister theory, the cosmic censorship hypothesis, many other extraordinarily influential ideas in general relativity. And he doesn't stop there with general relativity. As you do know, if you're a fan of Penrose, he's been working in recent years with ideas in quantum mechanics. He has his own basically idea about how wave functions collapse in quantum theory, and the implications of those ideas for consciousness

and how that relates to girdle's theorem and artificial intelligence and all these big-picture ideas. Now, in my mind, the thing about Roger Penrose is, is that he understands four-dimensional curved space time better than any person alive. So that's what I chose to talk about in our conversation, mostly in this particular podcast. We do go on to other things.

We talk about consciousness and quantum mechanics for the last half an hour or so. We could have talked about those for hours more, but we both had schedules to meet. Sorry about that. One thing I got a note is that we both got stuck on the names of the three people who were the authors on the papers on Black Hole,

the laws of Black Hole Mechanics. It's Jim Bardeen, whose name we forgot. Sorry, Jim, who wrote papers with Stephen Hawking and Brandon Carter about that topic. Now, talking to Roger Penrose, the great thing about him is he's fearless. He has enormously creative, deep ideas. Many of them have been incredibly successful and influential.

Many of them are idiosyncratic. Many of them I don't agree with. You know, I didn't agree with his ideas about entropy back when I was first learning cosmology. I later realized he was completely correct in his picture of entropy in the early universe, and that had a huge impact on my career. So I'm always happy and respectful talking to Roger Penrose whether or not I agree with his ideas. You can decide whether or not you agree or disagree with any of them in particular.

What the point of the podcast is is to give you information about what the ideas are. You're the one who's got to make a decision about what to think at the end of the day. So this was an extremely fun conversation. It's a great way to start off the year 2019. Let me just do two quick podcast announcements, of course. You can always support Mindscape on either Patreon or just directly through PayPal. Go to the webpage to find information about that. And like any podcaster, we love getting good reviews on iTunes and elsewhere.

So we have a wonderful list of guests coming up for the rest of 2019, but I can't think of a better place to start it off than talking with Sir Roger Penrose about the nature of space time, black holes, and cosmology. So let's go. Roger Penrose, welcome to the Mindscape podcast. Hello, quite present. Now, you've not only done a lot of amazingly important scientific intellectual work, but on an amazing variety of topics. And so usually for the podcast, often for the podcast, I'd like to focus in on one. I have the feeling that my people will be upset if we don't hit all the various high points of, you know, cosmology, quantum mechanics, consciousness.

So, you know, we can spend three hours if you want, but I'm going to try to, you know, get the main things in there. But I thought starting with space time would be a good thing to do. I mean, this is where you sort of, you know, made your bones early on. Was it, is it safe to say the first big thing you published, the first major result was the singularity theorem for black holes? I think that's true. I'm trying to remember the order of which things were done.

I had two FISRV letter articles, and one was on conformal infinity, how one represents radiation, gravitational radiation, by squashing infinity down to a place where you can see it, more or less. And this is a nice way of talking about things like gravitational radiation, electromagnetic radiation, and so on. But then I wrote this article in 1965. I've written as 64, I think, which was about black holes, except we didn't call them then at that time. This was gravitational collapse.

Going way back to work done by Chandra Seykar in the 1930s when he was on the boat coming to England, still, I think 19 or something I can't remember. And he worked out that a white dwarf star would have a certain maximum. mass, and if it was more than that, then it would collapse, basically. Right, Chandr-Sakar Limit. That Sandra-Sakar Limit, which was worked out at that time. The White Dwarf stars being very, very concentrated, so that the mass of the sun might be concentrated in something about the size of the earth.

But these stars are certainly known, the companion of Sirius and other such stars. So it was, these are things that were well established, But what Chandra Saker showed was if the mass was above a certain amount, which was a bit more than the mass of the sun, one and a half times the mass of the sun, then there was nothing would hold it apart. And it would collapse down,

and the question is what happens to it. And Chandra was very modest about this, and we're less speculating on what happens, whereas Eddington at the time was thinking, this is ridiculous, that can't happen. I think I got a lot of pushback, right, for this idea that there would be gravitational collapse. Yes, indeed. He got into a lot of trouble because, well, it's a wild idea, so I'm not surprised, actually.

Yeah, you should get a little pushback. Yes. But, of course, that didn't get down to a black hole, which he just said, well, one is left speculating. I suppose he had some sort of idea that it might. The main, well, there's a lot of early work which people say there was this, and then I forget people's names, I'm right on the spur of the moment. Anyway, there's early work where people wondered what happened to very massive objects. But until general relativity came along, there wasn't really an issue which was problematic.

And it was in 1939 when Oppenheimer of Atomb fame and a student of his, Snyder, and they worked out an exact model according to Einstein's general theory of relativity of a collapsing dust cloud, and it was the picture of a black hole that we have now. But a lot of people were a bit suspicious of whether that was really what you'd expect, because, well, there were two things. One is the dust cloud was material that had no pressure,

so it wouldn't push itself apart at all, and it would fall into the middle, and it wouldn't stop itself from pressure. The other point was the model was exactly spherical, spiritually symmetrical. It's exactly the same all the way around. And so that the matter as it collapsed inwards would be aimed right at the central point.

And so the fact that you got infinite density there was not perhaps surprising. And people thought, well, maybe if it was irregular, it would fall in and maybe swish around a bit and then come swirling out. So they thought this was an artifact or maybe some special thing, but it's not usually what you'd expect.

That's right. I think there was argument about it. Now, I know John Wheeler was very worried about these things, and the worry came particularly to a head when Martin Schmidt, a Dutch astronomer, well, Dutch-American astronomy, and he concluded that this was the first quasar. And he noticed that the signals from this object, well, first of all, they were red-shifted,

so that meant probably it was receding from us at a very great speed. and secondly it was extremely bright. And thirdly, it had variations in its brightness of the order of a week or so, which suggested that it couldn't be much bigger than the solar system. And therefore, you had this huge amount of energy coming out, which is something like more than the whole galaxy. And it was a real problem. And the picture which was presented was that you had something

which was down at the kind of level that the... Well, Oppenheimer and Schneider model would say you have a horizon at a certain point, and that this would be where this energy would be released at that sort of scale. Sorry, this point about the variability is an important one. It's sort of not obvious to people, but it's a very simple, fun argument, right? I mean, if something varies rapidly, then it's probably not any bigger than it takes light to travel across the object, right? And so weak time scales, that's the size of the solar system, which is small. by massive extra-galactic object standards.

That's right, yes. So it was a great puzzle, and I know Wheeler was very worried about this, and he talked to me about it, and he said, well, look, this means we've got something down at what's called the Schwartzchup radius. This was a very early solution of Einstein's equations called the Schwartzschild solution,

and Schwarzschild discovered it soon after General Davidine. Unfortunately, he died not long after that. but it was the first solution of a could represent a body he was thinking more in terms of a star or something where there was something inside which was matter and the equations changed

when you get into the matter and so that there was no singularity there but if you imagine squashing the matter down to this radius which is called the Schwarzschild radius then you get into a very curious situation which was described by the Oppenheimer-Snyder collapse model. And then so they took a long time, right, decades for people to really appreciate what the Short-Shield solution was trying to tell them all along, right?

It's a tricky little general relativity question. That's right. Well, usually people thought there was a body inside, and so you did explore what happened when you extended the solution inwards. And it was the first clear, I think the Oppenheimer-Snyder model, just before the Second World War, was a first clear model of what could be happening. Right. And then the quasars that you mentioned,

I mean, so there's a bright object in the sky, very far away, very small, seemingly, this could be a candidate for an ultra-compact object that might be what we now call a black hole. So is it really accurate to say that that experimental data nudged theorist towards trying to understand this better? Oh, absolutely true, yes.

It certainly nudged me into thinking about this. Yeah. I thought about the Schwarzschild horizon and so on, and didn't think much of it as a thing that might be out there. Usually one thought that if you squash the earth down or the sun down to that size, well, it would be pretty unrealistic. But if the thing is big enough and it collapse itself inwards, and I should say that the Schwarzschild horizon, the size of it,

the diameter of it is proportional to the mass of the object, so that if the object is much, much bigger, then the volume, which goes as the mass, cubed is something which you wouldn't, the density wouldn't be so, necessarily very large. And what is the short-shill radius for the Earth? I feel like it's some very small number. It's a, yeah, tiny.

I forget what it is, something like a centimeter or something. I can't remember, exactly, yes. A neutron star with the mass of the Earth would be the size of Los Angeles or something. Something like that, black holes a centimeter across. So that's a lot of mass in a very tiny area. Oh, absolutely, yes. But you see, I know Wheeler was very worried about this. He said, what happens?

Do you get the singularity, or does it swish around and come out again? And it was a big question. Should you trust the Oppenheimer model, Oppenheimer-Sinim model, or is that unrealistic because of irregularities? And at that time, I should say the quasars was around about 1962, I think. And around about that time, there was an argument by two Russians, Lifshitz and Kuletnikov. And they had produced a paper which seemed to say that singularities did not happen in general circumstances. Right. So that when the thing collapsed, if it was a general, irregular collapsing body, it would swish around.

It wouldn't reach these infinite densities, infinite curvatures. It would swish around and maybe come swirling out again or something like that. So that's the picture that a lot of people had. But you were able to prove otherwise. And what I love about it is, in some sense, maybe. this is exaggerating, because I don't know the history very well, but in some sense what you did represented a shift in technique in general relativity from finding exact solutions that were simple

to making an absolutely general statement that just relied on the sort of intuitive powers of the theory, but made very rigorously mathematical. You're absolutely right. You see, either people had found models which required a lot of ingenuity, but they usually had some special symmetry or something like that. And these solutions, very beautiful solutions often, but whether they were realistic, well, they were all very special. The only other way you could was to use high-power computers and work out. But then in those days, people didn't have the kind of computers we have now.

And it was pretty hard to get any impression as to what happened in the general case. So I started thinking about this. I'd been thinking about other problems a little bit like. this one of them which I never published anywhere was worrying about the old steady-state model okay see this was a cosmological model that was sort of in vogue when I was becoming a graduate student and becoming interested in general relativity in a serious way and in Cambridge where I was it was a popular view yeah yes that the universe although it was expanding and the mass sort of

got less and less by the expansion, the mass was replenished by hydrogen gas appearing spontaneously. And it was a very beautiful model in a way because as the expansion took place, the matter was replenished and you had a universe which was sort of stable and that remained forever in this way. It had some curious properties, but the question that I was interested in is, could you in any way make this consistent with Einstein's general relativity without having to introduce negative energies and things like that. And you could see pretty well that that couldn't be the case if it was very symmetrical. But then I started wondering, well, I suppose it's irregular in some, you know, quite serious way. Does this still cause a problem with general relativity? And I came to the

conclusion using sort of topological types of arguments that you couldn't save it. Is it even possible to give us, I know this is in very intricate differential topology question. I mean, but is it possible give a flavor of the argument? It's something like gravity is always attractive, right? I mean, that's the basic principle. That's certainly, but it was mainly how it behaved on light rays. That

was the real argument. You look at how light rays behave. And it's a bit like a lens, you see. You have a parallel beam of light rays and the way that mass acts on that is to give a it focuses like a positive lens. So a convex lens

would cause this parallel ray. to focus. Now, if you have an astigmatic lens, suppose you imagine a lens which in both one direction it's positive and the other direction is negative. I mean, you don't normally find these lenses, but that's the way gravitation behaves. So if you just go through empty space, then it focuses one way in one direction and expands out in the other direction. And this is what leads to spaghettification if you fall into the black hole. Yes, it squashes you one and stretches you in the other direction. Exactly, that's right. So that's what empty space curvature does.

But the thing is, if you have combinations of lenses, it's quite interesting, you can do this just with ordinary optics. You have lenses, and you put them on an optical bench, and if you had these astigmatic lenses, what you find is that if you, suppose you had one and another, sitting right next to it, an astigmatic lens, which is at right angles. I mean, they're both, the planes are parallel,

but the direction of it. the extinguited is at right angles. Then one cancels up. The other one is so it's just like a flat piece of glass. But now suppose you pull them apart, then you find there is a residual effect, which is a focusing effect. And this is connected with the energy in gravitational waves.

So the gravity acts like an astigmatic lens. But when you have a lot of them one after the other, there's a net effect which is focusing, as though there was matter. And this is the effect that gravitational energy has as focusing. So I knew about these things by thinking about these focusing effects and so on. And it was using this kind of argument, together with topological arguments, that you could see if you reached a point of no return, and that was a critical point as knowing what that meant, then you would find that this focusing property would be irreversible.

And you could show that there had to be some singular place inside the object that was collapsing. Really, it gets to a point where you don't know what happens. It sort of stops. Space time gives up. But singular doesn't just mean special. It means the curvature is getting infinitely big in some sense. That's the expectation. Although the argument that I had didn't directly show that.

It just showed that there's something stopping. The space time comes to an end and it stops. Right. And the normal expectation is that it gets these focusing effects become infinite. And so things just crumble up and you can't get any further. And you don't necessarily, it doesn't immediately. it doesn't immediately follow logically that it's a black hole, right? But you had ideas about that.

Yes, that's true. But the argument that I hadn't prove it's what we call a black hole. It just showed that you had the singularity. You had to make a further assumption, which is what I call cosmic censorship, which is if the singularities in general are of the kinds that you, roughly speaking, can't see them, then you do get a thing that we now call a black hole.

But it did depend on this cosmic censorship hypothesis. Right. Which is still not completely proven? Is that correct? I think that's correct, yes. There's a lot of evidence that it's true, I think. Well, the argument is that it has to be. You look at general situations.

You can cook up special examples which disagree with it, but they're not realistic. So the astronomers with their data prodded you to think about this problem. Absolutely. You proved something. Did your proof prod the... astronomers to take black holes more seriously? It did eventually. It was quite interesting because

you could see somehow a huge amount of skepticism about black holes. I used to be asked to give lectures at these Texas symposia, which happened every I think every year at first. And usually I was asked to give a talk something about black holes.

And you could see gradually people beginning to take it seriously And more of the morming, but there's a whoop, it sort of went over to the other side. It sort of happened very rapidly once enough people got used to the idea. And then, I mean, on the theoretical side, Blackholes became a whole area of study, and Stephen Hawking and Brandon Carter and other people. Yes, indeed, yes, that's right. And Hawking's remarkable result about the fact that they do a very tiny amount of radiation.

And so if you're imagined in the very remote future of the universe, And these huge black holes, well, you see now we see huge ones. Initially, there were just the odd double star system where you'd see one star and the other one. It seemed to be going around something else that you couldn't see. But maybe there were some plane where material was coming out of it. You couldn't see the object itself. And so people speculated that that was a black hole. The evidence was pretty indirect for a while.

but now the evidence is well indirect in a sense that you don't actually see directly into the hole but you see things like in our galaxy we have a what we call a supermassive black hole which is about four million times the mass of the sun and it's really remarkable you can see these pictures over you you have to speed up a little bit

but not that much and you can see stars going around in these sort of elliptical orbits and there's nothing in the middle. Yeah, they're going around a big heavy thing that you can't see. Yes, that's right. Except it's not that big. It's a small, compact, heavy thing you can't see. That's right.

It's pretty small by the, yeah, that's right. A four million solar mass. I'm not sure quite how big that is, but it's not huge. It's less than a light year, right? Yeah, it's very tiny. But so when Hawking, circa 1974, argued that black holes are not completely black, if you take quantum mechanics into account, they radiate,

how did that strike you? I mean, was that controversial at the time? I can tell you exactly. what happened. You see, I think I was away from Cambridge. So I was working in Cambridge at the time. No. Yes, it wasn't. When was this? That was 70s. 74, early 70s. I must have been in Oxford by then. But you and Hawking had become close and worked collaborators. Oh, yes. No, I knew him well. But the point was I'd come back from somewhere exactly where I can't remember that I haven't worked that out. But Dennis Sharma, you see,

who is a great friend of mine and I learned a lot of cosmology from. and he was Stephen Hawking, a supervisor. And Dennis was a great person for getting the right people to meet each other and so on, people who might benefit from encounters with other people. And he knew all the physics that was going around. So I learned an awful lot of physics from Dennis. But anyway, I come back and Dennis told me, oh, if you heard the latest, Stephen Hawking showed the Blackhomes radiate, you see.

And I said, what? So I got hold. I phoned him up. see, I phoned up Stephen, and he, I said, what's this all about, you see? And he said, well, it's tied in with the ideas about thermodynamics. And I said, ah, I see, that makes a lot of sense. So, no, I was, it didn't take me long to come to the conclusion. It was probably right, what he said. Well, to be fair to the audience, you had previously shown that you could extract some energy from a rotating black hole, a limited amount, and you had to do it sort of

intentionally. It wouldn't be automatic. But that kind of, you know, pushed people on the road to think about these questions. Yes, that's right, that you could get stuff out in a sense. You could get energy out of it. But this was going a little further. Well, it was curious because now I must have been in Oxford because I remember how this came about. We used to have these meetings every Friday in my office in Oxford with my graduate students, postdocs and people interested. And if somebody was visiting, we'd have them to give a nice little talk. And Stephen was visiting at the time. And he was telling us about He was imagining little black holes which might be created in the Big Bang.

And the idea was, if they were rotating, would they lose their energy in the rotation? And he showed some calculation about this. And it was actually not very long after that, you see. I must have been visiting. I can't remember exactly how it was. I saw Dennis at that point. But I phoned Stephen up, and it was really following up on this calculation he'd done previously about the rotation. about the rotating ones.

I think he found that they didn't need to be rotating, which rather surprised. Black hole does it, right. Yes, that's right. And I think that in the famous textbook by Kip Thorne and also Mizzner and Wheeler, there's this picture of an advanced civilization that has built an energy extraction device

around the supermassive black hole, the center of their galaxy. There's a lot of energy we could, in principle, get out. Absolutely. We'd be free from our dependence on foreign oil sources if we could really harness black holes this way. Well, you see, this was the argument. I wrote a paper.

in the Italian Journal. There was a conference in Italy somewhere. I forget where now. And there were people talking about very massive stars and so on. I was asked to give a talk on black holes, which I did. But I'd come to this conclusion

that you could extract energy. And I imagined that you could do this by having particles which would split up into two and so on. But when it came to the talk, I can see which way around was it. I think I did give it that way around. But I thought also about the civilizations

rotating around the hole and extracting energy out. And there's a simple argument. You can see they could do this from quite general principles. They just lower, you know, fill buckets full of rubbish and lower them into the hole and tip the rubbish in and they would extract more energy

than the mass of the rubbish. And when Hawking did explain that black holes give off radiation, this all came as part of this, as you mentioned, thermodynamics. Yes, that's right. This package that black holes have entropy. Yes.

And there was this sort of analogy between what black holes do in the laws of thermodynamics. And this really said it's not an analogy. It's true, right? Black holes have entropy. Well, it was curious because, you see, the entropy of the black hole,

it was really, Beckenstein had a fairly rough argument. But I was quite happy with that because it seemed to be very consistent that the area of the black hole should have an entropy. And that Begenstein's argument looked pretty convincing to me. But Stephen then had a much more exact argument, which was more impressive mathematically.

But this came after a lot of discussions between Stephen and Brandon Carter and... Who is the other one? I know the paper you're talking about because the laws of like-hole mechanics, but I'll tell the audience afterwards. Yes, tell the right-right people. I think there were only three people who were before.

Yeah. But they were exploring the analogy between thermodynamics and black holes. But Stephen definitely just thought it was an analogy. I remember talking to him about this. And I was sort of thinking it was real, you see. But you see, in a way, he'd been looking more deeply into it than I had because he realized if it was really thermodynamic, then the black holes had to have a temperature.

And that didn't seem to be the case. Yeah, of course, they just swallow things. there's nothing nowhere it can have a temperature, you see? Right. But then he found that they did. I mean, I think that in a brief history of time, he mentioned that he was literally annoyed at Beckenstein for suggesting the entropy was real and that motivated him to do this.

That's interesting. I hadn't quite realized it was that argument, but that's right. Did that help get you interested in entropy? Because it was just a few years after that that you started talking about the entropy of the early universe. It must have done. I'd have to try and put it together. But to be able to assign a value,

to this entropy. That was crucial for that, you see. I can't quite remember the order of my thinking here, but a big, certainly something which I played around with a lot was thinking about, well, I thought about it in the steady state model, you see, at the issue of entropy balance troubled me, you see. But there, you see, you had this creation of the hydrogen, which the fact that it could condense, you see, in form objects. could be a source of the entropy, and so a source of low entropy, I should say, a reservoir of low entropy. So that's how it sort of squared itself in my mind with the second law issue. I've been worrying about it then already. But when that had to be given up,

and one was looking at models which expanded and didn't have creation of matter, at some point, I was worrying about this question, but it didn't really have substance to it until the Hawking entropy, well, Begenstein Hawking, entropy value for black holes and how huge it was. Yeah, so just, I need to be very fair to all of the audience listeners,

entropy, they've heard the word before, but it's roughly speaking a measure of how random disorganized a certain system is. Sure. And in some sense, intuitively, you might have thought that a black hole has zero entropy. It's not a lot of different ways a black hole can be, right? It's not like a cup of coffee where there's a lot of, of arrangements of the atoms inside. But Hawking and Beckenstein show that is actually a huge number.

And that inspired you in some sense to start writing about... The second law of the thermodynamics says that entropy tends to increase in closed systems, and therefore it's not surprising that entropy used to be lower in the past. But cosmologists have a long history of being confused about this issue. And I think that let's just be honest that this is the most profound, that it's been quite a profound effect that you have had on my career, because it was your papers emphasizing this problem of why the early universe had such a low entropy, which I thought were completely convincing and puzzled by why my fellow cosmologists don't take this seriously.

So why don't you explain how you think about it. No, you were one of the few people that really latched onto it immediately. But I was just as puzzled why people didn't take it seriously. But the basic argument is this, as you say, I mean the entropy or the randomness, if you like, increases with time, and that's the second law. Another way of saying exactly the same thing is if you go back in time, it decreases. So as you go back and back and back in time,

you should find the entropy very small. Now, what's the earliest evidence that we directly see of the universe's state? That's the cosmic microwave background. So this is radiation coming from all directions, electromagnetic radiation. And this radiation has a lot of entropy in it, but the main point that I'm going to sort of,

concentrate on here is if you look at the curve which represents the intensity for different frequencies you have this thing the curve goes up and then comes down again and it has it's what's called a plank curve the amount of light at different wavelengths that's right so that so there's a certain temperature where it's maximum and then it the radiation at higher at higher frequencies it goes it goes down and this plank curve is observed the Kobe satellite when it went up. You'll have to tell me the dates I don't remember.

1991, 92, yeah. Okay. The Kobe satellite measured this curve, the intensity for different frequencies of this radiation, to find an extremely good fit, an almost perfect fit to the Plank Spectrum. What does the Plank Spectrum tell us? It tells us what we're looking at is maximum entropy.

I mean, that's the whole point of it. It's what they call the blackbody radiation, which meant maximum entropy. So here, this is what I call the mammoth in the room. You go back and back in time where the entropy is supposed to be going down and down and down until it, which is a maximum, which is sort of the wrong way around. And I don't know why people didn't worry about that more. But the point is the answer, I think partly because it's muddled up with the expansion

of the universe, that people sort of think, oh, well, the universe is expanding, and so maybe there's not much room for entropy down there or something like that. I think it's partly that and partly that when we're taught thermodynamics, gravity is not a part of it, right? We don't think about black holes and so forth. I mean, there were people like Tolman, one of the early mathematical physicists who studied cosmology, and he understood pretty well about the entropy issue.

So people certainly did understand it, the right people at that time. But anyway, yeah, you've got this huge amount of entropy in the radiation, which is a lot, actually. and then why is the second law hole when it starts off at the top and it's got nowhere to go if you like and you can have to convince yourself that the expanding expansion of the universe doesn't help

which is a point told me I did understand but the other issue you see you've got this radiation coming from all directions in which it has this thermal character but the other point about it is it's also very uniform so it's almost completely uniform over the whole sky if you take account of the earth motion through the radiation, it is uniform over the sky to about one part in 100,000.

So it's really pretty uniform. Now, you see, what does that indicate? You say, well, okay, suppose you have a gas in a box, then the temperature would be pretty uniform if it's at a maximum entropy stay. You just leave it in the box, and the temperature, apart from gravity and all that stuff, it would be uniform temperature. So that also finds, represents high interest. So where is it low? Well, it's low if you think of not a gas in the box, but suppose you think of a huge box of galactic scale, and it's got stars running around in it. Now those stars will tend to clump because of their gravitational attraction and eventually become black holes. And as they do this, the entropy goes shooting up, particularly with the black holes, because, as we know from Beckenstein and Hawking, the entropy is absolutely enormous. So that represents an increase in entropy. So you have these two things about the early universe. One is the Planck spectrum, which tells you that the matter and radiation.

You see the plank spectrum was telling you that the early stages of the universe, photons and matter were kind of randomized as much as they could be. And so that radiation comes to us and you see this plank spectrum. But the other feature about it is that it's uniform and that, as far as gravity is concerned, is very low entry. Because as things start to clump, and a good example of this is our sun. Our son used to be, well, a long time ago, a distribution of gas all over the place and went through various stages, but it clumped together and produced this hot body.

It would be hot even if there were no thermonuclear. at all. So it gets that heat, which is, when compared the darkness of the background sky, is a very low entropy situation. You can get energy out of it by simply... And we do. We do, absolutely. That's why we're here.

The plants do by photosynthesis, and we live off plants and animals that eat plants and so on. And so that's where it all comes from. So it comes from the fact that the sun is a hot spot in a cold background sky. And this is why we are interested in the second law of thermodynamics, because it's low and it's creeping up is what we get our structure from and all that stuff. So ultimately the cosmic microwave background, it looks like it's maximum entropy, but that's only because you forgot that there could have been all these lumpinesses, which would have made the entropy much lower. I think it's true, the entropy of the black hole at the center of our galaxy is larger than the entire

entropy of the cosmic microwave background. That could well be. Yes. And if you look at it, all the stars, all the galaxies that are around, it's absolutely enormous. The entropy in the black holes is absolutely stupendous by comparison with anything else. So this is very good news for the second law of thermodynamics. It explains that the early universe had a low entropy, actually, not a maximum entropy. That's right. It's bad news for cosmology because we're stuck with this question of why the early universe was like that. Exactly.

A lot of cosmologists like the theory of inflation proposed by Alan Booth around 9. 1980, and you were one of the first gadflies there saying that inflation is a bit of a cheat, I think. Yes. Well, you see, I thought this is a crazy idea. It won't last a week, and how wrong I was. No, that's completely wrong. It just seemed to be such an artificial theory. You had to invent a special field. Well, in the old days, they call it it a Higgs field, because they hoped that the Higgs field would be the same, but that didn't work. We should explain that the basic idea is that the universe underwent very, very early times, a period of super fast accelerated expansion,

which I think you would agree. It's true that if that happened and if the energy driving that expansion turned into matter and radiation, it would give you a universe looking like our universe. But it doesn't help explain the initial conditions. Yes, it doesn't, because you've got to have it pretty uniform already, otherwise it doesn't even work.

Right. So you're going to have quite general arguments to show that it can't really be the explanation of the low, not just the low entropy in the early universe, this particular form that it's low, namely in gravitational degrees of freedom. And it just struck me strange why cosmologists,

you know, you can see a list of what are the problems of cosmology, and you look down the list and say, where is this? Yeah, I'm totally on your side there. But so both of us are, you know, in the small band of people who've been trying to invent models of the universe, which naturally explain, this early entropy, but that's where we diverge. So I think that inflation,

I would honestly give it a 50% chance of being part of the final answer. That's a lot more than I'm giving it. Exactly, but a lot lower than all my friends give it, right? Oh, absolutely. Yes. And you've been working very recently on a new model of the universe on its super-largest scales. That's right. Well, recently isn't even all that recently.

I forget now, it's over 10 years anyway. Okay. But nobody paid. Well, you see, it took me. It took me a long time to... I used to lecture about... Let me explain the model first. Please.

The argument is that... Think of the two ends of the universe. We have this future which seems to be dominated by this exponential expansion. So this is observations of supernova stars and other things together, which persuaded cosmologists

enough to give the two... two groups, the Nobel Prize, which was very deserved, to see that the universe is having this, what's called, an exponential expansion. It's a sort of self-similar, the rate of expansion is proportional to the size and so on. So it's something which seems to be taking place. Now, those of us who were brought up on cosmology and read the cosmology books would have seen that these kinds of models with that exponential expansion

are perfectly well described in the books. These are models in which there is a thing called the cosmological constant, which is positive. And it's usually referred to as a lambda, capital lambda, so it's like a V upside down. Right. Basically the energy of empty space. Well, you can think of it as energy.

People call it dark energy, yes. I'm not sure I'm happy with that term, but never mind. It's certainly, well explained by this by this Einstein cosmological constant Einstein introduced this term

in 19, when was it, 17? 1917, for what was the wrong reason? He wanted a model which was static. He hoped that the universe was static. I think it's a sort of appealing idea. The steady state model was again the same sort of appeal philosophically appealing in some way. To be fair,

I think it was also the observations at the time. The universe looked static to astronomers in 1917. I think, I guess Hubble hadn't quite... Right, that was 1920s. But it was after, I can't remember, it was after the Vesto Slyfer, wasn't it? Because Vesto Sleifer had already seen the expansion. Maybe it wasn't such a convincing argument. Well, you saw that there were distant, you saw that there were objects that were moving away from us.

Yes, I guess, I guess, that's true. It probably wasn't terribly persuasive, but there was some indication. but not enough to rule out a model like Einstein, the one he produced then. But for that model, he needed this term. And it's the only thing you can really do to Einstein's equations without wrecking them, in my view. You just add this term. And he didn't like to do this at first, but then I guess he thought, well, let's put it in, and then you get a static model, which we refer to as the Einstein cosmology.

But then, not long after that, Hubble showed pretty much. convincingly that there was this expansion in proportion to the distance. So the whole universe seemed to be taking part in this expansion. And so Einstein, I guess, kicking himself for not having stuck to his original equations, which seemed to indicate he would have predicted it. He could have become famous, yeah. He could have become famous, yes. Well, he considered this to be his greatest blunder.

And I think that's true. It's on record. I think Gammoth has recorded, Einstein having said this. So, okay, this is... But it's all in the cosmology books, despite Einstein sort of retracting it. And, yeah, people studied.

I studied it, I studied, look to see what the infinity looked like in this model and so on, so I was familiar with it, although I was a bit slow on the uptake to taking it seriously. I think it was Jerry Ostriker who told me, I said, well, you know, maybe this distant supernovae look red because of dust and so on,

which people thought maybe it was the case. And he said, oh, no, there are all sorts of other things that's got you. It's not just that. You have to take it seriously. So I thought, okay, I'll take it seriously. And it wasn't, I don't know how long after that conversation, that I began to think about the remote future. Now, you see, the remote future, and I mean the very remote future,

we have the universe expands and expands and it. expands and it gets more and more rarefied and nothing much happens. You get sort of dead stars and it gets pretty boring. And the most exciting things are around these black holes. But they're pretty boring too. You sit around waiting it for it to evaporate according to Hawking's evaporation. And for the big ones, it takes about a Google year, something like that. So you're thinking about...