Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 236 | Thomas Hertog on Quantum Cosmology and Hawking's Final Theory

Episode Date: May 15, 2023

Is there a multiverse, and if so, how should we think of ourselves within it? In many modern cosmological models, the universe includes more than one realm, with possibly different laws of physics, and these realms may or may not include intelligent observers. There is a longstanding puzzle about how, in such a scenario, we should calculate what we, as presumably intelligent observers ourselves, should expect to see. Today's guest, Thomas Hertog, is a physicist and longstanding collaborator of Stephen Hawking. They worked together (often with James Hartle) to address these questions, and the work is still ongoing. Support Mindscape on Patreon. Thomas Hertog received his Ph.D. in physics from the University of Cambridge. He is currently a professor of theoretical physics at KU Leuven. His new book is On the Origin of Time: Stephen Hawking's Final Theory. KU Leuven web page Wikipedia Google Scholar publications

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Visit VitalProtene's.com to get started. These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. in combination with Hey everyone, it's Cal Penn. I'm inviting you to join the best sounding book club you've ever heard with my podcast, Earsay, the Audible and IHeart Audio Book Club. Every episode, I nerd out with amazing guests

and dive into the best new audiobooks available on Audible. It's the book club for your ears. Listen to Earsay, the Audible and IHeart Audio Book Club. On the IHeart Radio app or wherever you get your podcasts. Hello, everyone. Welcome to the Mindscape podcast. I'm your host, Sean Carroll. Stephen Hawking is known for any number of revolutionary advances in theoretical physics, the singularity theorems that he did with Roger Penrose and others in the late 60s, the evaporation and radiation from black holes in the mid-70s. And in the early 80s, with Jim Hartle, he calculated the wave function of the universe, try to explain the creation of the universe from nothing. But in 1988, Hawking revolutionized not theoretical physics. but the scientific publishing industry with the appearance of a brief history of time,

his surprise runaway bestseller. I was a little bit too young to take advantage of this, but I'm told that in the late 80s, after a brief history of time came out, if you were a theoretical physicist with a book to write, you could get a million dollar advance, no problem. Not like that anymore, but those were the days. Andre Linday is a well-known cosmologist

whose name will appear again in this episode. also a mischievous guy. He likes to tell the story back in the late 80s. He would be riding an airplane, sitting next to someone who was reading a brief history of time, and Linday would inevitably say, you know, I like the book, but I didn't really understand it. And the person reading it would go, oh, yeah, it's really not that hard. You just have to really concentrate while you're reading it. But Hawking never gave up doing science.

He wrote more books, but he also wrote a lot of technical papers in the published research literature. and his views continued to evolve about how to do quantum cosmology, how to think about the nature of the quantum universe. Today's guest, Tomas Hurtog, was one of Hawking's most frequent collaborators in those years. He was a PhD student with Hawking and then continued to write papers with him and has now come out with his own book called On the Origin of Time, Stephen Hawking's Final Theory. And it's a joint theory that he's described between himself and Stephen.

So we'll talk about that theory, but we'll talk about the genesis, the evolution of what we mean by quantum cosmology, how we go about saying, okay, you have the whole universe, we're going to apply the rules of quantum mechanics to this universe. And I think you will correctly get the impression that there's a lot that we know about how to do that and a lot that we don't know. so our views on how best to do it are continually evolving. And it brings in both philosophical ideas about the role of the observer in defining what you mean by a universe and calculating the probability of the universe looking different ways, but also very modern cutting-edge physics ideas like holography and the emergence of time from the quantum wave function. So as I apologize to Thomas in the middle of the podcast, you know, this is a tough one for me, not because I don't understand it,

but because I'm too close to the issues here. I think about these issues all the time, and so it's harder for me to put myself in the seat of the audience member who is not a super expert. I hope that I didn't interject my own views or interpretations too much here. I tried to reel myself in, but I don't think I was very successful. I think that you'll find my own views all over the place. So hopefully Thomas's views shine through because he has a different point of view that is a very interesting message. I think it's worth taking very seriously, especially because we don't know the final answers. We're still working on this. We're still moving forward. So let's go. Tomo Seratog, welcome to the Mindscape Podcast.

Hey, hi, Joan. You know, normally, and I'm sure it will happen in this episode also, here at the Mindscape podcast, we focused like a laser beam on the substantive intellectual content, and we don't dig that much into the personal fun stories of people's histories and so forth. But in your case, you are Stephen Hawking's most frequent collaborator in the last years of his life, and that collaboration forms a lot of the basis of what you're going to tell us about in the podcast and in the book that you've written. How does one become Stephen Hawking's collaborator? sure that there's a story there. Yeah, but it's it's it's it's it's it's a typical science story, right? There was a folklore, there was, there was sort of a lore at a well-known sort of story at

the department of applied mathematics and theoretical physics in Cambridge, which was whoever got top scores in their famous part three course would get an invitation to go talk to Stephen. And so that's essentially what happened and what happened to many others students in different years. So that's how I first entered into his office. The real surprise, of course, was the experience of that first conversation, which was anything but normal. It was not normal because it was interspersed with various journalists walking in and

out. And the second thing which I thought was very exceptional was that Stephen went just straight didn't and started talking about how we found that whole idea of the multiverse so paradoxical and how his colleague Andre Lindy had these outrageous theories and so there I was how could I possibly have an opinion on the multiverse and Andre Lindy as a 22 year old student but that was that was really fun and you know again we're not going to spend most of time talking about this stuff but how did it work your collaboration? I mean, again, later in life,

Stephen had a tougher and tougher time banging out the sentences, right? Right, right, right, right. Yes, yes. I think I was lucky, in a sense, for two reasons. The timing, late 90s, so Stephen and I met 98, really.

Yeah, I think it was really a coincidence why it worked so well. First of all, on your point, in terms of communication, so Stephen was already using his computer voice at the time, but the whole system worked really well. He was used to using a mouse to select words, and he sort of, my impression was that by then, he sort of instinctively knew when to click to select certain words,

and so the whole system was working very well in the late 90s. So, of course, I had a notion of time. And so we would sit hours and hours in that department shoulder to shoulder and he would type out sentence and sentence and by, okay, if you spend so much time by, at some point, you begin to understand what he's talking about and you get going. So that was important because those years really were a foundation for when it became very difficult later on to communicate.

I think at that point these first few years we developed some sort of intuition common language. The second point I think which was equally relevant is that the late 90s were a great time in cosmology.

Stephen's famous book, A Brief History of Time, had been out for a decade. So the frenzy around that book had sort of died down. He was back to research. And he was back to research because cosmology was, it was a golden era.

You had these mystifying observations about the acceleration of the universe, the C&B fluctuations, which were pointing to an early phase of acceleration, which we now call inflation. And then you had these paradoxes to do with the multiverse, which were essentially going to the core of cosmopol. logical theory. So this was a good time. Stephen was grounded in research again and still being able to communicate. And that's what we built on, I would say. And in particular, the research that you did together, I think it's fair to say, always, as always, correct me if I'm wrong

here, is sort of downstream from the wave function of the universe work that he did with Jim Hardle in the early 1980s. So quantum cosmology in some sense. So why don't you explain to us what that is? What's about that? What is the wave function of the universe, Thomas? Okay, good. Well, so in a way, the whole wave function thinking,

the whole sort of idea of let's think about the universe in a quantum mechanical way as a quantum system, must have been sort of a moral lesson that Stephen took out of his PhD work, his own PhD work in the 1960s, when he essentially showed, using Penrose's techniques, that the Big Bang, classically, the origin of the universe, the Big Bang in Einstein's theory is a singularity where Einstein's theory breaks down. It's the origin of time.

Is this going to be an Einstein's theory, if you would take it at face value, you'd almost be driven to the statement, okay, this is not sign. This lies outside science. But of course, there's an other lesson, the one Stephen and most of our colleagues took. Well, wait a minute, it's just quantum, the quantum nature of gravity becomes important. But then how do you go about doing something about that? That's when, I think, when Jim and Stephen's pioneering work came about,

well, if the universe is a quantum system, then it must have a quantum, state somehow, a very abstract, super abstract description of reality. And the ingenuity of Stephen's work, which featured so much in a brief history of Thainim was that he came up with the first fairly explicit model of how you would go about giving a quantum description of the big bang of the creation of the universe. And their trick was really to sort of in a way bent the time dimension of Einstein's theory into a space dimension and if your reality is pure space dimensions you know what to do to close it you can just round it off like a sphere and so stephen's famous line of course was what is the big bang it's a bit

like the south pole and what was there before well it's like asking what's out of the south Paul, it's a meaningless question. So that was, of course, the typical oracular hawkinean kind of phrase, right? But by the late 90s, Stephen and many others had realized that

the creation theory, so to speak, of brief history of time, had a fundamental problem, which is that taken at face value, you'd be led to the conclusion that the universe should be empty.

that the universe should be, yeah, that there should be no stars, no galaxies, no life. And so while their original theory was beautiful in a way from a theoretical perspective, it's almost like you, and I think he felt like that, that he sort of had cracked the enigma of creation, so to speak, by giving a mathematical description of how you can make a universe, it was very much it was not the kind of universe we inhabit. So there was something missing. Right.

Actually, that's going to be a heart and soul, I think, of this conversation because it's really what your book builds up to. But I want to linger in the 80s for a little while to get the set up so that everyone comes in on the same page here. So when we say the quantum state of a system, if it's an electron or something like that, something that we are very used to treating as a quantum mechanical object. It's a wave function for every position that we could measure it in. It tells us the probability, etc. So it's a function

of every possible location we could measure it. What do you mean when you say the wave function of the universe? Is it supposed to be, it sounds hard to write down a possible quantum amplitude for every particle in the universe? Right. And it is worse than that. If you treat, I mean, what we really mean and certainly what Stephen meant in the 80s by a wave function of the universe is very much

a wave function, not just of a particle describing various positions of a particle like an electron, but really a sort of abstract description that describes a superposition of various

possible universes, including all the matter and the space and time. So it's almost like you go from one universe to a zoo of possible universes. And so you really go up a level in abstraction and a level and in confusion, right? Yeah. And frankly, I think the question, what we mean by a way function may well be at the heart of these more recent developments with Stephen and what we were.

Because of course, if the wave function predicts an empty universe, if the empty universe is the by far the dominant wave crest, so to speak, yeah, then you know something's missing, right? Yeah, good. This was Linday's complaint, right? Stephen would be saying, yeah, with your multiverse, you have infinitely many observers and you don't know where we are. and then Lindy would say, by your wave function has no observers, that's equally bad. Maybe that is even worse, honestly. Yes, yes, I didn't want to say it. But, okay, I mean, I want to give the listeners a feeling for how we operationally go about this.

I mean, clearly you're going to have to make some simplifications if you're going to think about the wave function of the universe. Yes, yes. What is the goal here? It is really, just like we do in ordinary physics problems. We try thought experiments, we try to simplify the situation. But of course in such a way that you think or that you hope to capture the essence of the problem. And my impression is that this has one. worked pretty well in this quantum cosmology program. Of course, it is not an exact, it is not

an exact way function, it is not a precise formulation, but somehow and a little bit, a little bit miraculously, the general framework of quantum cosmology, it seems to me, has been able to capture a few key foundational features of how we go about thinking. about the quantum universe, which have been very difficult to discover by other means. Do you ever feel like you're drinking from a firehouse? Paycor's intelligent HR solution empowers leaders to turn down the pressure. Their unified platform includes payroll, talent management, compliance software, and a lot more, connecting you to the people, data, and expertise you need to drive long-term business results.

Visit paycourt.com slash leaders and go from work flood to workflow. That's paycourt.com slash leaders. Hey, everyone. It's Cal Penn. I'm the host of Earsay, the Audible and Iheart audiobook club. This week on the podcast, I'm sitting down with Ray Porter, the narrator of Andy Weir's audiobook Project Hail Mary, massive sci-fi adventure about survival and science. And what happens when you wake up alone very far from Earth? I really had to make a decision because I caught myself getting that frog in my throat and starting to get teary as I'm narrating some of these sections. And it's like, okay, yo, yeah, yo, is this indulgent?

And I really thought about it. I was like, no, at this point, it would kind of be betraying the trust the author and the listener have in telling this story if I don't go through it. But there's places in this book that deeply emotionally affected me. and I left it on the mic. That's great. Because it served the story. People will say like, oh my God, I cried at the end. It's like, yeah, dude, me too.

Listen to EIRSA, the Audible and IHeart Audio Club on the IHart Radio app or wherever you get your podcasts. It obviously runs into the question that, you know, the person on the street has been told every day of their life that we don't understand quantum gravity. So it sounds like you're doing quantum gravity even though we don't understand it. How do you get away with that? Yeah, so somehow we get away with that. I think we understand some, I think we understand more than we sometimes admit. I do think we understand we have learned a lot about sort of the conceptual framework.

Maybe we don't have a precise mathematical picture, but, and you can see where this goes, right? These toy models do capture certain essential features. The universe, the fact that the universe inflates at early times. And also, yeah, this idea that, well, as we all know from quantum mechanics, act of observation plays a crucial role. Right? An electron doesn't really have a position as long as we don't ask for it.

But that is a fundamental different thing from a classical system, which, of course, has a position and a momentum. imagine now thinking about the universe as a wave function, as a description of all possible universes, maybe it isn't quite real until we bring in the observer. And so that has been a whole fruitful area, I think, to study the kind of questions, to study ultimately the relation between our existence and the nature of the universe in any quantum mechanical setting. So, something which classically you cannot begin to ask really. So I'm a bit more optimistic.

We keep saying we don't understand quantum gravity, but I think we're still on. And that's then, and then we haven't even talked about holography. Right, we will, don't you worry. But I do want to, again, give a flavor of some of the issues that one faces here. You already mentioned. turning time into something that looks like space. I mean, this was infamously the part in a brief history of time where most people are like,

okay, I give up because he started talking about imaginary time. Yeah. So you're welcome to say no, but could you explain what imaginary time is and why it mattered? Why do you had to do that? Like, time is real to you and me. Why do you have to make it imaginary? Ah, well, well, yeah, okay, time is real to you and me here, that's all fine. But as we discussed already, when we go back in the history of the universe to the earliest stages,

the Einsteinian way of thinking about the expanding universe, we run it backwards, and time stops. So you could already have said in the 60s or even earlier, in fact, because this idea that time had an origin and that that, was the Big Bang who has been around for 90 years. So the discovery of the Big Bang to me, the fact that the Big Bang is the origin of time already shows that there must be something emergent about time. If you're going to understand the Big Bang, we better don't put in time as a prior assumption

because it's all about how the dimension or perception of time as we know it and as we experience it comes about. So I would say that the dimension of time has been a problem all along in modern relativistic cosmology. And in that sense, Stephen's trick to sort of turn time into space is in a way exactly what the doctor ordered. Of course, it's a bit radical, but then the Big Bang is a very radical phenomenon, right? Yeah. And in fact, that was later, much later, we shouldn't. probably go too deep into this but now

almost 40 years on really from Stevens time into time goes into space business now we understand that this is much this is yeah now we understand how that trick so to speak is less random as it looks but in fact emerges from our new holographic way of thinking about the universe, much more as an effective description. So, of course, this was Stephen's bold sort of characteristic way of doing physics back in the days.

He had an intuition that he could do all of physics without time, essentially. Everything could be just spatial Euclidean geometries. And I must say that since he died, that kind of physics, that kind of approach to quantum gravity, both in terms of black holes and in terms of the Big Bang has gained, regained importance. Right. We did have a podcast episode with Netta Englehart, who was one of the people working on getting information out of black holes. And the idea of Euclidean quantum wormholes loomed large. Yeah.

And so how do you would have liked that, I think. That's right. Okay, good. But I'm still stuck in the 80s because, you know, look, I'm older than you. my formative years were back in the 80s. And the big thing at that time, you already mentioned Andre Linday and Hawking had a little bit of a disagreement

about the wave function of the universe. It grew into this disagreement about the multiverse, et cetera. But back in the day, it was just about inflation and can we get inflation out of our theory of quantum cosmology. So why don't you explain to the listeners what inflation is and why it matters to us?

Okay, yeah. So inflation indeed came along in the early 80s, somewhat independent, I think, of Stephen's quantum creation model. As a way of more of inflation, what is inflation, inflation is a very rapid phase of expansion in the earliest stages of the universe's evolution. which creates a big universe in a fraction of a second. And so it sort of interconnects our entire observable universe.

And to me, the big bonus of inflation is that because it's such a rapid phase of expansion, it kind of, it sort of generates with it a pattern of fluctuations, a pattern of variations in the universe purely from quantum uncertainty. essentially. There are particles that are being sort of teared out of the vacuum and set you up with a big universe that is not exactly the same everywhere. It comes with some sort of roughness and that roughness is exactly what you need to over millions and millions of years generate stars and galaxies and so forth. So inflation on its own and that's, I mean, it has been, I would say there's significant observation support for such an early phase of rabbit expansion

because the roughness that you generate during inflation is reflected in the famous cosmic microwave background images which showed that the temperature wasn't equally distributed but nearly equally distributed right so inflation stands on its own really but the big question of course which which must have been the question, I think, in the early 80s, but okay, how does inflation start? And that's where all the disagreements came around. Yeah, I mean, I think that amazingly, that was not a question that most inflationary cosmologists cared about. I mean, they just said, well, as long as it starts, it gets us what we want. But Hawking and Linday and a few others like Alex Valen were a plucky minority who really tried to understand.

why it would start and that was part of what the wave function the universe was supposed to be about. Right. Okay. Okay. But of course also since then I think we've learned that it is an important question, how inflation started. Yeah. Because the pattern of variations in, say, the cosmic microwave background radiation,

that the afterglow of the Big Bang is going to depend on. precisely how inflation unfolded. So it's not that it's not an empty question. I mean, the specific mechanism that drives inflation in the early universe leaves its observational traces. So if we want to predict the details, the fine details of those fossils, so to speak, we better understand how it starts. Yeah, I agree with you.

But, you know, again, plucky minority. I think you're right that it's more. common these days. But there is, you know, a slight, I don't want to say downside, but implication of this that you already mentioned, which is that there are these quantum fluctuations that mean that inflation is a little bit rough, it doesn't end the same everywhere, and on very large scales, those fluctuations can be very big and give rise to a multiverse and, you know, different things going on in different places. And someone like Andre Linday embraced that multiverse and said, there it is. That explains why

our own universe is so unusual looking because it's a tiny, tiny part of some gigantic ensemble. My impression is that Stephen and you did not embrace that picture quite as lovingly. That is correct. Yes, yes.

And right, right. I think this is exactly the moment where I entered Stephen's office at the heart of that disagreement. somehow I think André So the problem of It's appealing in one sense

The multiverse Because I'd say Suppose you need to generate A huge expanding space Where different regions Behave like different universes Even with different effective laws of physics Yeah then you generate

Some sort of gigantic reality in which the apparent biophilic design of our universe would be just a natural fluke and that's it. Yeah. So I think it appealed to some cosmologists that this would get us around a lot of the perceived fine-tuning issues.

The idea for the observation that our universe is at a level of physics, remarkably fit for life. Of course, if there are a zillion universes out there, then once in a while you're going to have such a universe. But there was one problem, from, and which was clear in the 90s already, which is, okay, suppose you have a multiverse.

Then if you want to turn this into a fully, a full-fledged scientific hypothesis, you better tell me in which of these universes we should be. therefore what we should observe, what kind of roughness in the C&B we should observe, or what kind of value for this or that parameter we should expect to observe. And so that's something between cosmologists called a measure issue, the measure problem. And the measure problem is really, how should we, in a gigantic multiverse, associate what weight should be associated to different kinds of universes.

How important are different kinds of universes in this gigantic reality? And so I think that was a crucial point. Somehow I think Stephen thought to get a proper scientific, falsifiable hypothesis out of the multiverse, would require a radical quantum thing. game. Whereas I think other people like Linda thought, okay, the measure issue eventually it's going to go away

by some sort of anthropic principle or by another piece. And that's of course a very interesting debate because this goes to the heart of what cosmological theory is about. Yeah. How do we fit into the grand scheme?

Are we, is there a giant inflating space in which the anthropic principle is going to select our universe or is this giant inflating space not quite there without bringing in that observer's perspective in a more fundamental way interwoven with physical theory itself, with quantum thinking. And you're going to be on the latter half.

You bet. Well, let's linger lovingly over this. distinction because I think it's an important one, but it's also a difficult one. Cosmologists who do think about the universe, or for that matter, people who do black hole information or whatever, anyone who talks about quantum gravity, it seems to me, is very tempted by still drawing a classical picture of spacetime, even though they know they're talking about quantum gravity and saying, well, there are fluctuations of some sort. But I take it the point you're making, it seems from reading the book. I cheated by reading the book. That's not really fair. A truly quantum universe

isn't just a big fluctuating classical universe. Is that fair? Right. I think that is indeed the key distinction that you either assume that there is some sort of background out there, which can be wildly fluctuating in different regions, but the sort of So big, big background in which all this is happening, in which all this is happening, acts as, yeah, some sort of foundation. But this took many years, ever. Now I'm jumping around.

I mean, Stephen in the late 90s didn't have the solution. But eventually, as you suggest, we came to see that this is still too classical. This is still too much of, yeah, it's not enough quantum. I can't say differently. and so we started to try to go, try to take a fully quantum view, even though we of course didn't have a precise theory to do so and you're led to a different picture

in which we have rather a classical space around us, of course, which can be much bigger than the observable universe, but which sort of dissolves in uncertainty on the largest scale. So it's much like what we were saying earlier about the electron. The electron doesn't have quite a position in quantum theory before we ask for its position.

So if we think in the same way about the universe globally, we should be saying that the universe is a definite space, time and configuration around us, but on the largest scales, it's rather uncertainty which dominates, instead of a definite classical structure extending to infinity, as some people would say. So it's a picture which I came to see that builds in a certain finitude. So quantum theory

is interesting in that respect. It has always been interesting in that respect, in the sense that it's a theory of what we can know, but also a theory that sort of tells us what we can't know. And here in our model, this is sort of playing out at the level of the largest scales. By the way, this will be of interest to our listeners. Are you assuming in the background something like the many worlds interpretation of quantum mechanics? I'm certainly assuming an interpretation that is like many worlds in the sense that I'm trying to work with an interpretation. that is like many worlds in the sense that I'm trying to work

with an interpretation of quantum theory that doesn't require anything external. Yeah. Right? Or any hidden variables for that matter. Yeah, yeah, yeah, yeah, yeah. The funny thing in cosmology, though, we often think about quantum mechanics

and the many world interpretation when we think about future branchings. So we prepare an experiment, the wave function splits, the observer gets correlated with one outcome, and so forth. But the thing which I found striking in cosmology is that the current state of the universe around us is already the result of a giant question asked of the wave function. And so it's sort of the many-world interpretation in cosmology

also acts a lot or I think is important when it comes to the past. Sure. In selecting this or that. subset of histories and like anything like any branching in quantum cosmology saying it comes with limitations so in a sense you could say the multiverse it's almost we're behaving as if we have access to an infinite amount of information whereas of course from an observer's perspective within the universe there's the extent to which our observations distill one or another branch of the wave function

is finite and Stephen's tricky to close the universe to turn time into space helps in that respect owning a home is full of surprises some wonderful some not so much and when something breaks it can feel like the whole day unravels that's why homeserv exists

for as little as $499 a month you'll always have someone to call a trusted professional ready to help, bringing peace of mind to four and a half million homeowners nationwide. For plans starting at just $4.99 a month, go to homeserve.com. That's homeserve.com. Not available everywhere. Most plans range between $4.99 to $11.99 a month your first year. Terms apply on covered repairs. I think there'd be a great opportunity to clarify something about, you know, again, the person on the street who's not playing with equations, hears words and tries their best to figure out what's going on. So we hear about Feynman and his sum over.

histories, right? Like Feynman said, consider all possible histories of the particle or the universe, and there's a certain way of adding their contributions together to get the quantum wave function. And Stephen and Jim Hardle used that idea intimately when they wrote down their wave function. That's different than Everett's view of many branches of the wave function, because his individual branches are supposed to be real. They're not mathematical fictions that we add up. And in some sense, they're kind of classical, right? So is that a fair distinction the way you're thinking about it? Yes, I think so.

So I think it's more the Feynman kind of description that is perhaps at the heart of this theory. Well, let's get down to brass tax. Do you believe that there really exist other universes where things are very different, other branches? That does not quite enter in our theory. Okay. And that is because in the end,

certainly inspired by these holographic constructions, they work very much what we call top down, so backwards in time. So they're very much anchored, so to speak. The histories that play a role in, the wave function are anchored on, yeah, I would say, our observational situation around us. And so in a sense, my feeling is that the new holographic wave flooding cosmology

is going to, at the very least, trim the wave function of the universe down to, yeah, I would say, a more manageable thing. Okay, but if I observe a spin that's in a superposition of spin up and spin down and I see that it's spin up. Do you think there's another version of me that's all spin down? Well, as an operational meaning, sure. You say sure.

A lot of people think that's a radical thought to think that there's a version of me that's all spin down. Sure, but that branching, once you observe it, sure, sure. I would view that as an operational way of describing your setup, your experiment, your observation.

But once the observation has unfolded, what happens to the other view? it's lost in uncertainty again, I would say. And every branch that grows out of the other you will no longer be contributing to this universe. Okay. So let me try out the following analogy that struck me as I was thinking about your book. So if we do, let's say we do Schrodinger's cat, right? So Schrodinger puts the cat in a quantum superposition of alive and dead.

And famously, if we open the box and look at the cat, we don't. see the quantum superposition, we see the cat alive or the cat dead. I think that what you're saying is kind of like the following, that if I had an infinite series of cats spread out in space, I could look at one of them and it would either be alive or dead. But very, very far away, the cats could still be in a superposition. And it would be a mistake for me to think of this ensemble as just a random collection of cats alive and dead. It becomes more and more quantum as you go further away. Uncertain.

Indeed. Yeah. Just like the electron position. Yeah. And so you're saying that we should think of cosmology like that. We can talk about the classical world that we see, but let's not extend this classical picture too far away. Let's leave it uncertain.

That seems to me to be the lesson. And that's also at the heart of how this quantum way of thinking about it resolves the measure problem. because it is anchored on what you just said, what we see, rather than try to get us into the picture, into the cosmos, a posteriori, so to speak. Yeah. Like, for instance, someone with an anthropic principle would do.

And is this what do you mean by the top-down approach? Yes. So say, pretend that we didn't just say that. Tell us what the top-down approach is. Right, right. So what we mean by a top-down approach is indeed that we regard the universe as we observe it around us as a kind of starting point for which of the many possible histories of the universe contribute to what we see and what we observe. And that is important because it provides you selecting those histories

or selecting those subset of branches in the wave function then allows you to make predictions for future observations. Because that's kind of the problem with the multiverse, right? If you have many different universes and you want to predict something for a future observation for the next satellite, yeah, you need some sort of criteria. And that's good. That was very much at a sort of, that was sort of the guideline also for Stephen and me. So to get, to get me sort of had this intuition that a proper quantum way of thinking about the universe should somehow resolve this.

Should sort of give us a measure and give us, give us an unambiguous criterion for future predictions. But it comes with a radical different perspective, of course, because we used to be. able, we used to think that we would one day be able to predict from first principles how the universe should be, how the universe should turn out.

That was the kind of attitude that Hawking took in brief history of time, like a sort of transcendental theory that tells us why and how the universe is the way it is. That's how he phrased it. And he totally turned, he totally turned

turn 180 degrees on this point, which I think, well, was a very interesting, a very interesting evolution to witness in his thinking. So I want to make sure that the listeners know what we mean when we say the measure problem. In a multiverse, in a very big multiverse that we do, as you and I agree, it would be sloppy and careless to think of it as a big classical ensemble of things, but let's think of it that way anyway. There's a lot of observers, they see a lot of different things, you know, different cosmological constants, different masses of the electron or whatever. And a traditional multiverse cosmology thing to do would be to say, what is the chance that you observe the electron mass to be a certain number?

And the problem is there's an infinite number of observers in this universe who observe it to be a certain number and also an infinite number that observe it to be a different number. And it's very hard to take infinity divided by infinity to figure out what fraction of people will see a certain thing. That's the measure problem in my mind, yeah? Yeah, I think it's one version. There's a different aspect which I think is closely related to what you say. And so we asked the question, faced with the multiverse, we would ask the question, what's the probability that we see this or that? Yeah.

But there's this subtlety in what we mean by we. Yes. depending if you have a different definition or a different description of what the V means, what physical characteristics you associate to an observer, be it a human observer or a habitable planet or just a galaxy, depending on how you chose to define that, you're going to get a different answer. Yeah. And so there is, it's almost not setable, setable by rational arguments, because you could turn all, you could turn a negative outcome into a positive outcome by changing what you mean by we. And so that's another version of, I think, the measure problem, a version which

points very clearly to the underlying problem with the multiverse, that it is a construction, a kind of platonic construction that is out there independently of whether it is observed or not. It's out there with an independent existence from us. And that, frankly, it took many years by the time I sort of fully realized the depth of the problem. The gods I view, in other words. Yeah. It's what Stephen calls, called indeed, and many others, I think.

the gods eye view, which indeed, by 2005 or so, we were absolutely convinced that we had to, yeah, construct cosmology in a different way from what's what Stephen called a warm's eye view. Not a very good term, I think. You get the idea, right. Well, I do think, I actually really like the philosophy behind it. And I think it's kind of a shame that Stephen famously, you know, went to rhetorical war against the philosophers because I think that there's useful... I agree. I agree. Yeah. No, that's a good point. And one can wonder why that was. To sell books. You think so?

I, you know, he... Maybe people more than I do. Well, I can say that people who knew about, you know, have... he constructed his famous sentences about philosophy being dead and so forth in the grand design. It was very clearly to sell books. Oh, but right. That is probably true, but my feeling is, I'm not sure I'm not a biographer, right? But my feeling is that the whole philosophy is that thing of how he predates the grand design.

He was never a fan of philosophy. No, that's true. But that doesn't distinguish him from plenty of other physicists, right? Like plenty of physicists will... Right, but wrongly so, I think. I think so too. If you now look back on our, just on the conversations, we had. This issue, God's eye versus, let's call it, worm's eye, is foundational.

Yes. Because it is really about what is it ultimately that physical theory finds out about the world? Is it some sort of eternal, transcendental? truth or is physical theory once you get the observer fully incorporated in there a different beast from what we thought it was

contingent on our existence within the universe and that's for this I mean I think we must admire Stephen for just for the simple fact that he was able to change his mind on this. Oh yeah sure

and so at the end towards the end of my life, of his life, he said, literally, by, okay, we top down, with that new approach to cosmology, somehow we put humankind back in the center. That is a very different, Stephen, from the one we could read in a brief history of time. Very much. And I will, I do, I should apologize to you because all of what you do and talk about in the book is too close to things that I care about.

So instead of asking you questions, I keep saying, what about this? But hopefully you can deal with my question. asking style. So let me do it again. Let me say what about this? Because I think that this question of predicting what we should be like if the certain multiverse were true is exactly wrongheaded. That's the point on which I completely agree with you. Like, what do you mean what we should be like? We're us. You know, we are what we are like. I do think it's possible. And you could probably say this even classically in a big fluctuating unsolved. you could ask, what is the probability that your theory predicts the existence of anybody like you?

And if that probability is one, who cares if there's many more people not like you? You're going to be there in the multiverse, right? Or in the theory. That's right. That's right. Yeah, yeah, yeah. Certain probabilities we just don't care about. Do you ever feel like you're drinking from a firehouse?

Peacor's intelligent HR solution empowers leaders to turn down the price. pressure. Their unified platform includes payroll, talent management, compliance software, and a lot more, connecting you to the people, data, and expertise you need to drive long-term business results. Visit paycourt.com slash leaders and go from work flood to workflow. That's paycourt.com slash leaders. Hey, everyone. It's Cal Penn. I'm the host of Earsay, the Audible and Iheart audiobook club. This week on the podcast, I am sitting down with Ray Porter, the narrator of Andy Weir's audiobook Project Hail Mary, massive sci-fi adventure about survival and science. And what happens when you wake up alone very far from Earth? I really had to make a decision because I caught myself getting that frog in my throat and

starting to get teary as I'm narrating some of these sections. And it's like, okay, yo, yeah, yeah, yo, is this indulgent? And I really thought about it. I was like, no, at this point, it would kind of be betraying the trust the author and the listener have in telling this story if I don't go through it. But there's places in this book that deeply emotionally affected me, and I left it on the mic. That's great. Because it served the story. People will say like, oh my God, I cried at the end.

It's like, yeah, dude, me too. Listen to Eursay, the Audible and IHeart Audio Book Club on the IHeart Radio app or wherever you get your podcasts. And Jim Hardle and Mark Shrednicki made a point. about this with the example of Jovians. Do you know their example about the Jovians? Sure, sure. Can you tell it to us to the readers, to the listeners? Oh, I don't remember the details, but they, they, you can correct me, but their point was that should we expect in some giant ensemble of inhabitants say should we expect to be typical in in one or another sense.

Given that, all we know is that we exist. Yeah. And so they made a big point in explaining that the mere observation of the fact that we exist is very, very different if you don't have access to other civilizations or planets or extraterrestials from saying that we should be typical. Right. And the reason is the same as what we were discussing earlier.

typicality in the end always boils down to treating certain features of our living systems or biosphere or planet or galaxy as preferred as the most probable and and and but that is that is that is that is that is that is that is fallacious thinking really so good so I think that we're very much on the same wavelength there you I take it you would agree that there's no reason to think that you or I as individuals or human beings as a species are typical in the universe. And even, and this is the key point, and even that nature of the physical laws that we observe, is the typical outcome of some grand cosmic evolution. Exactly. So neither is the tree of life on Earth as sketched first by dark. The typical outcome of that evolution, in a sense, Stephen and I pushed that same kind of thinking further down, and we're saying, well, wait a minute, maybe the physical laws as we have them are also not a typical outcome.

But, and this is the crucial point, just like Darwin, just like Darwin didn't need a zillion other planets to do biological evolution on this planet. we claim we don't need a zillion other universes to study the evolution of this universe. But it comes crucially, as you point out, it comes crucially with the caveat that there is no assumption of typicality and there is no, it could have turned out differently. Yeah, that said.

And to me, the big surprise, and this is really when the moment that Stephen sort of told me, look, now it's time for a new book. the real surprise this is as a matter of giving some homework right was that holographic way of thinking about cosmology

builds in much of that top-down reasoning because the holography in a cosmological context really flows backwards in time from data from an observational situation in the present. The time, the past,

the time, dimension is in a sense the emergent dimension and it's contingent on the kind of questions you ask. And that for me was sort of the key transition point because previously much of that top-down reasoning we were preaching, so to speak, remained controversial because it felt like a choice. It felt to some people like David Gross would tell us, ah, but wait a minute, you were putting the answer. So that's this typicality reasoning again, right? You're putting in the answer. I'm trying to explain.

I'm trying to predict the answer. And you kind of feel like, maybe he's right. He has a Nobel Prize and all that, you see. But then holography sort of solidified the top-down reasoning precisely because it flows backward. It works

backwards in time. And I was very surprised by that. And I think Stephen was too. So, okay, that's when it all sort of began to click together our picture. Well, if you want us to believe that, which is a good thing to do, you're going to have to tell us more about holography and how it goes backward in time. I don't know where you want to start. But what do you mean when you're saying holography in this context?

Okay. So we want to talk a bit about holograph. Yeah. you wrote a book it's your job so holography has been let's face it holography has been the talk of the town

in theoretical physics for 25 years right? Yeah but of course it's true it's been mostly practiced in highly idealized abstract non-realistic mathematical situations universes that have nothing to do with ours but there's a general lesson

behind holography which is I think it's been the way which we're finding out in which quantum theory and gravity can finally work together more or less harmoniously. And the way this works is that one appears to be the hologram of the other. The clearest example perhaps is the case of a black hole. We think about a black hole. we've seen images of a black hole that's all very nice

and a black hole is something very gravitation. The space time is curved highly curved. Einstein says there is a surface, there is a horizon and inside the horizon inside the black hole

space time really crumbles. So that's the gravitational description of a black hole but then when you start thinking about a black hole from a quantum perspective, you begin to discover, going back to the work of Beckenstein and Hawking and many others, that well, maybe all there is to know about a black hole is in fact located in bits of information that are living on the horizon surface, that are living on the

surface. So if you start reasoning about a black hole that way, you might arrive at the conclusion that the inside of the black hole doesn't really exist or isn't or is in a sense yeah not quite there is some sort of emergent emergent phenomenon

which you may not need if you want to ask physical questions you could ask physical questions from a quantum perspective and just only talk about the horizon or tick end horizon so I think that's much of the more motivation or inspiration for maybe there is a fully quantum way of thinking about the universe,

about space and time, is in a sense holographic, in the sense that there is one dimension, in the case of a black hole, the interior dimension, that is emergent, that is not quite phenomenal. And now you begin to think, well, wait a minute, wait a minute, the Big Bang is another problematic thing, just like black hole, space time grumbles. What dimension is a minute? What in cosmology could be the one that is holographically projected, that is sort of encoded in a lower dimensional screen-like thing, just like a hologram. Well, as we discussed, in cosmology, it is very much the dimension of time,

which is the problematic one. It's the one that has an origin. It's the one that disappears with the Big Bang. It's the one that causes us a headache. And the development of those holographic ideas in theoretical physics, indeed suggests that it is the dimension of time in a cosmological context that can be holographically encoded in a hologram. So we start with a moment of time or some spatial description of the universe and then we kind of do

holographic tricks to understand how that could be projected into time evolution. I'm just stringing words together like chat GPT here. You can fix them. Right, right. The way I see it is that there is, okay, I want to say two things here. First of all, this holographic way of thinking about reality is completely useless in normal circumstances, right? Today, here around you, around me, around everywhere, there is time and there is space

and we can work with that. But where holography becomes important, I think, is where Einstein's theory, where the description of reality in terms of space and time that we experience, where that description doesn't hold. So inside black holes and at a big bang, my feeling is that in those extreme regions of the universe,

the more fundamental holographic, quantum nature, rises to the forefront. And so what I mean by that is that in those extreme regions of the universe, one of the familiar dimensions disappears. So in the case of the black hole, it's the interior of the black hole. In the case of the universe, if we trace the history of the universe backwards, it's all fine. But at some point, the bending of time becomes so strong.

And you think what holography is telling us is that, well, effect the dimension doesn't reach further. The holographic way of saying the same thing would be that the hologram doesn't quite uncode the information to push history further backwards. And so the Big Bang in holographic way of thinking about the universe becomes almost like, it becomes almost like an epistemic horizon. A region where you call, yeah, you run out of bits almost literally. That's kind of where it stands. Of course, this is a grand new hypothesis. It has to be developing so many ways.

You sort of get a gist, right? Yeah, no, I do. And so maybe a motto might be classically, we would say, if we kept going backward in the history of the universe, time would end because we hit the Big Bang singularity. And what you're saying is time kind of ceases to be a thing. It's not that it ends, but it ceases to be a useful way of talking about the universe.

Yeah, I think indeed, right, right. Gradually, maybe. In a way, what we have been trying to do in cosmology ever since the discovery of the Big Bang is to let time when we go backwards disappear in a controlled fashion. Sure. That's essentially, that has essentially been the goal. And of course it's kind of interesting to look back on this

because this is what the singularity theorems in the 60s tell us to do. Find a better way to let time disappear into the Big Bang so that physics doesn't break down. And of course, these ideas about the multiverse or about pre-Big Bang cosmologies, there are strong of ideas that all go in the direction, well, maybe the Big Bang was. wasn't really the origin.

Maybe there's something, maybe we can just push through. Do physics as we normally do it. But it's kind of interesting that this hypothesis that I developed with Hawking is very different. It's taking the idea of an origin very seriously. In fact, even more seriously than the early Hawking would have done it, in the sense that if you let the time I mentioned disappear, it's as if the laws of physics disappear.

And so it's really sort of placing that notion of an origin very central in our thinking about the early universe. And in that sense, I think we can now begin to see clearer the difference between this hypothesis and other hypotheses that evoke an evolution before the Big Bang and all that. Well, let me consider two different cosmological scenarios. One is one much like we think is real. In other words, we have observers like us today, and we trace back 14 billion years, and there was a big bang.

And that big bang, by the way, was a very low entropy special condition, as Roger Penrose and others have pointed out. Another one might be there's sort of a galaxy, kind of like the Milky Way and people like us. but the whole background space time is otherwise empty. So there's no Big Bang. There's just a weird random fluctuation in which all the particles came together to make our galaxy and then they'll disperse in the future and there's no beginning to end of time. Does your theory explain why our universe looks like the former rather than the latter? Right. I'm writing a paper on that.

Good. be writing your paper Thomas. Okay. My claim is the following that if you specify in sufficient detail the local galactic

configuration that you sketched, by which I mean really the actual configuration, so you specify enough data. Yeah. Then you will see a switch I'm revealing really the latest research here. You will feel a switch in from your second scenario to your first scenario. So if you only sort of loosely say, well, I've got some sort of milky way,

I'm not very much interested in its precise description, then you might well favor an empty universe without anything else. But if you begin to describe that observation situation, the fact that actually in sufficient detail, then at some point it'll become, you will see a phase transition, you will see a shift towards the universe like we actually observe it. Okay, I will go on the record as saying,

that would be great, and I don't believe you. But we can talk about that. You should be skeptical. I am skeptical. I think that even I could, I would claim that you can specify to whatever level of detail you want. the world around us to, you know, 100,000 parsecs in every direction, surrounded by vacuum, and everything is perfectly fine. Okay.

Wait, and what is the statement then? The statement is that it doesn't matter how carefully I specify my current observations here in the Big Bang. I can always embed them in a universe. Sorry, here in the Milky Way. It's my mistake. I can always embed them very easily in the universe without a Big Bang at all. And I suspect strongly, and though I don't know, maybe I'm wrong about this, maybe this is what you can convince me of,

but I suspect strongly that in most known principled ways of comparing the likelihood of those two possibilities, the empty universe is going to come out more likely. Yeah, yeah. So indeed, what I'm going to reveal is indeed, of course, a different way to compare these likings. Good. I look forward to seeing that. But okay. I guess the last loose end here, this has been excellent.

Thank you very much for explaining a lot of this modern research to us. I just want to get straight one last time the comparison to the multiverse story. So if I understand what you're saying, which I think I mostly do, I can kind of conditionalize on here I am, here's the Big Bang, here's what I observe, and then I use your theory to reconstruct the past and the future of the universe. Could I have conditionalized on completely different kinds of people and completely different laws of physics and things like that and told a similar story all still within your framework? Oh, yes, yes. Sure, sure. You could do a thought experiment and conditionalize. In fact, we do many of those

thought experiments in theory and conditionalize or start so to speak from an entirely different configuration. Yeah. And you'd get a different past and future. Good. All of these past and futures are limited, just like we discussed as. So good. So I'm just trying to get it right, so I keep repeating.

So what you're saying is that once I say who I am, the classical world around me is finite, it's limited, because it sort of dissolves into quantum uncertainty if I go too far. But I can think of it as an ensemble of many different patchwork classical realities, all of which are there in the wave function of the universe. Good. This is his last point I am no longer convinced of. Okay, good. All these different, all these different classical worlds fit in one grand wave function. That's indeed the heart of that top-down approach taken fully.

And the evidence we have for this, and this is a crucial point, I think, yes, the evidence we have for this is that they're not all there in one grand way function when we think holographically about this. Holography really sort of, yeah, it's kind of interesting. It brings in this observational perspective with... the theory, but at the same time it then also limits the range of that wave function that we've been talking about. It limits the range of different realities that the wave function encompasses. Now, of course, we are talking really cutting-edge stuff, right? But you're absolutely right.

Is there a grand overarching wave function uncompricing all possible holographic theories? Or is there a limitation on the reach of physical theory that is contingent on, say, a boundary configuration or an observational configuration. Yeah. And we don't know yet. Still work to be done. That remains to be seen. Yes, sure, sure.

It's good that not all the questions are yet answered because that leaves something for you to say in your next book, which I predict is going to come out eventually. So, Thomas Hartog, thanks very much for being on the Minescape podcast. Thank you so much. Lovely.