What Came Before The Big Bang? Thomas Hertog - #513

Please join my mailing list here 👉 https://briankeating.com/list to win a meteorite 💥 In a sweeping conversation drawn from his collaboration with Stephen Hawking, Thomas Hertog explores the radical “no-boundary” theory—a vision of the cosmos with no singular beginning, evolving laws of physics, and a past that isn’t fixed until observed. Rejecting the untestable multiverse, Hertog and Hawking built a fully quantum cosmology that embeds the observer within the equations, predicting inflation and replacing anthropic guesswork with a falsifiable framework. Hertog explains how time could emerge from something deeper than the Big Bang, why constants of nature may be dynamic, and how holography hints the laws of physics themselves might fade away at the origin. He then turns to the experiments—from next-generation CMB polarization to gravitational wave backgrounds—that could confirm or refute this bold vision, challenging us to see the universe as a living, evolving system whose history we help to shape. — Key Takeaways: 00:00 Intro 01:03 Thomas’s first reaction to Hawking’s theory 03:38 Hawking’s model of the Big Bang 07:20 The no-boundary proposal 22:18 The role of conscious observers in cosmology 24:34 The wick rotation and its implications 29:29 The future of physics and experimental tests 37:57 The experimental minimum 40:04 The holographic principle 43:57 Work Thomas would like to share with Hawking 49:11 Outro — Additional resources: 📚 On the Origin of Time by Thomas Hertog: https://a.co/d/ftze4JC — ➡️ Follow me on your fav platforms: ✖️ Twitter:⁠ ⁠⁠https://twitter.com/DrBrianKeating⁠ 🔔 YouTube:⁠ https://www.youtube.com/DrBrianKeating?sub_confirmation=1⁠ 📝 Join my mailing list:⁠ ⁠⁠https://briankeating.com/list⁠ ✍️ Check out my blog:⁠ ⁠⁠https://briankeating.com/cosmic-musings/⁠ 🎙️ Follow my podcast:⁠ ⁠⁠https://briankeating.com/podcast⁠ — Into the Impossible with Brian Keating is a podcast dedicated to all those who want to explore the universe within and beyond the known. Make sure to follow/subscribe so you never miss an episode! Learn more about your ad choices. Visit megaphone.fm/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

  The central quantity behind quantum cosmology, after all, the field of quantum cosmology is the attempts to think about the universe as a whole from a proper quantum mechanical perspective.

• 0:11Speaker 01

  Picture this: Cambridge, 1998. A young postdoc sits across from the world's most famous living physicist, Stephen Hawking. The great physicist delivers words that would shape both of their careers for the rest of their lives. "Thomas," said Stephen, "the universe doesn't have a unique history." What followed was a complete paradigm shift, from thinking about cosmic history as a fixed timeline to understanding it as something that crystallizes only when we observe it. As Thomas puts it,

• 0:41Speaker 00

  "The past is a wave function until you're asking a question, in which case the wave function crystallizes to a history."

• 0:48Speaker 01

  That statement was revolutionary. It implied that conscious observers might retroactively select which cosmic histories become real. This isn't science fiction. It's Stephen Hawking's final theory, developed with his closest collaborator, Thomas Hertog.

• 1:03Brian Keating?

  Thomas, take me back to that very first conversation with Stephen Hawking about the universe possibly not having a unique beginning or history. What was your honest, emotional and intellectual reaction to that bold claim?

• 1:16Speaker 00

  First of all, I was just a beginning PhD student, so I was flabbergasted to finally meet Hawking and discuss with you. And it was the first time I met a scientist, a cosmologist, who I felt was very much driven by the great old philosophical questions and trying to mold these into modern cosmology, try to elaborate on these using modern scientific methods. And to me, this felt a little bit like a homecoming. I had always sort of searched for a very fundamental field in physics, and here was Stephen just doing what I had, yeah, dream of doing. Now, the context of what was on his mind in

• 1:59Speaker 00

  the late 90s was very much, I think, these multiverse ideas, right? They were very popular at the time amongst cosmologists as an explanation, as a new sort of possible explanation for why the universe is fit for life. Maybe there's not just one universe, maybe there are many, and we find ourselves, obviously, in one that harbors life. I think Stephen was one of the first Cosmologists who deeply felt that multiverse cosmology was going nowhere, that it wasn't quite scientific, that it wasn't quite verifiable, falsifiable. But of course, he also didn't have an answer. And I think he must have felt that the answer had to be sought, at least,

• 2:42Speaker 00

  in a deeper theory of cosmology and quantum cosmology, a fully, a proper, fully quantum approach to cosmological thinking was the route that he proposed. But our paradigm shift, at which we much later arrived, came forth from the puzzles and the paradoxes associated with the multiverse. Multiverse cosmology does not tell us in which one of these universes we should find ourselves and therefore what your Simons Observatory should observe. And to Stephen, this was a catastrophe. This was telling us that we were on the wrong track.

• 3:24Brian Keating?

  Stephen was so quotable and so bold and characteristically just he

• 3:32Speaker 03

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• 3:58Brian Keating?

  Really captured the human soul and the spirit of what he did. As I used to wander the halls of Bridge Hall during my many years as a postdoc, and he would stay where Einstein stayed. And we'd see him at the Athenaeum, the famous Athenaeum at Caltech, where Einstein and many other luminaries were. And like Einstein, I felt he was always willing to concede when he was wrong. So there were these signed documents, these bets by Stephen when he would concede. And one of the bets that I always felt, you know, he should have made or I would have liked to make with him was when he said, asking what happened before the Big Bang is like asking what is South of the South Pole. Now, you may not know, but I've been to the South Pole twice for the bicep experiment,

• 4:39Brian Keating?

  which was the last major experiment that Stephen really engaged with and seemed to be so delighted with. I have a picture of him on stage at Cambridge, I believe, speaking in his wheelchair in 2014 with the bicep results behind him. It's one of my favorite iconic images that connects me to him. Thomas, was he right? Is it really as nonsensical to ask what happened before the Big Bang?

• 5:00Speaker 00

  It's a feature, it's a property of his particular model of the Big Bang. It's a rather metaphorical way of describing the crux of his particular model, which is known as Stephen's no-boundary wave function. And the crux of his model of the Big Bang is that if you go back in time, all the way to the earliest stages of the universe, then quantum uncertainty messes up the clear difference between time and space that we experience today. In fact, time and space, Stephen boldly claimed, themselves become a little fuzzy. And we can model this, he claimed, by turning the time dimension into a space dimension.

• 5:44Speaker 00

  The intuition behind it is that if time becomes fuzzy, any clear notion of dynamics and causality will eventually fade away. And so the heart of the matter of Stephen's thinking about the Big Bang was that geometries, with all space dimensions and no time dimensions, can capture some of the quantum properties that describe the origin of the universe. This did not come out of nowhere. This idea was grounded in the observation in the 70s that those Euclidean geometries, those geometries with space dimensions, capture quantum properties of black holes.

• 6:27Speaker 00

  If that's the case, maybe they also capture quantum properties of the Big Bang. So Stephen turned time into space at the Big Bang. And of course, when you do so, you can sort of round off the past, much like you round off a sphere, much like an orange, much like the South Pole. In that sense, if time disappears when we go back to the Big Bang, there's no point, there's no notion of what came before the Big Bang. So that's why he said so. It's a feature of his model. And the key question is... How do you test this model? What do you do with it?

• 7:01Speaker 01

  Hey, everybody. I'm usually the one that asks my guests to judge their books.

• 7:06Speaker 03

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• 7:32Speaker 01

  Books by their covers, but today I'm asking myself to judge my own book by its cover. My newest book, Focus Like a Nobel Prize Winner, is chock full of advice, life tips, and focus and productivity tips from nine of the world's greatest minds, Nobel laureates ranging from economics to peace to physics, of course. It launches September 9th, which is also my birthday.

• 7:55Speaker 01

  I will go check it out. And my publisher's got an Amazon to run a special just for listeners of the Into the Impossible podcast. You can get the Kindle edition for only 99 cents. That plus than a new pocket protector. So go to Amazon and get the Kindle copy today because this special only lasts for the first week after launch.

• 8:15Brian Keating?

  What is the kind of, you know, intellectual confidence that you have in it? There's a claim that I think is made by you in the book, or at least maybe after the book, that there's really not a viable alternative as appealing as the no boundary proposal. Now, that's a very strong claim. So what gives you that confidence? You kind of took over, as I see it, from Penrose and Hartele, and you were doing the day to day, you know, kind of dirty work in the trenches with him. But what gives you the confidence to say it's basically, you know, Tina, there is no alternative?

• 8:45Speaker 00

  Okay, it remains a hypothesis, right? But the two or three aspects to me that that smell right. One, I must say, is the connection, is the close connection with inflation. Inflation remains to be firmly tested or verified, as you know, but it seems to me, based on, yeah, based on the CMB fluctuations, based on the spectrum of primordial fluctuations, it seems to me on the right track. The second point I want to make is that in all those years, we really have not had

• 9:19Speaker 00

  A proper resolution of this famous multiverse paradox, known as the measure problem in cosmology, the simple problem that the theory does not seem to predict in which one of these universes we are. Quantum cosmology, a wave function, amplitudes for different possible universes, it's all in there. Quantum cosmology provides us with a measure, provides us with a tool to get a grips on the paradoxes associated with the multiverse. And then the third point is somewhat connected. I've always felt a lot of sympathy for the sort of more philosophical point, underlying cosmology.

• 10:01Speaker 03

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• 10:27Speaker 00

  The theory, which is the fact that we are within the universe and not outside. And so we're probing even the wave function of the universe from within. And once you start to implement that observer's perspective from within, you find that in fact that the predictions of the low boundary wave function turn out really in the right ballpark. But anyway, the boldest claim, which you repeated at the start of our conversation, which is that it also seems to say that we should think of the Big Bang as a sort of epistemic horizon, where even the laws of physics evaporate and disappear. That, of course, remains to be tested.

• 11:07Brian Keating?

  Indeed, and I think we'll hear shortly what your colleague Neil Turok has claimed as the reason to abandon it and approve of a different approach, the mirror cosmological model. We talked with Lathan Boyle, as you may know. But again, these stakes, this is such a fascinating book. It's really unique in that it's extremely technically deep for people like me that Want the details. It covers everything from the tree of life, evolution, Darwinian principles in the universe, interpretations of quantum mechanics, and it does so with rigor. And the no boundary proposal, I think we need to talk about that in some detail because to the extent that people have heard about this theory, which was presented in a brief history of time,

• 11:51Brian Keating?

  what does it mean to have the wave function of the universe? Where does that live? Who could observe that? And what are the interpretations of there being a wave function? What is a wave function of a universe?

• 12:01Speaker 00

  A wave function of the universe, just like a wave function that describes the state of a particle, that wave function of a particle will describe its many possible trajectories, its many possible histories, its many possible excitations and so forth. Similarly, a wave function of the universe, arguably the most profound or deepest physical quantity we've come up with in the 20th century, probably the central quantity behind quantum cosmology. After all, the field of quantum cosmology is the attempt to think about the universe as a whole from a proper quantum mechanical perspective. If you try to do this,

• 12:46Speaker 00

  the universe better has a state. That state is a wave function of the universe. But just like a wave function of a particle,

• 12:53Speaker 03

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• 13:19Speaker 00

  It will not describe a single history, a single expansion history of the universe. Just like a particle can have many possible positions, a wave function of the universe will describe an ensemble of many possible universes, many possible histories of the universe. So it's some sort of quantum version of this classical giant mosaic bubble universe which we had before. Now technically, you're asked, "Well, where does this wave function live?" I'm not so sure it lives in Hilbert space. There is nothing really. A wave function of the universe, every quality you would assign to it

• 14:01Speaker 00

  would be qualities of a particular subset of histories, would be intrinsic qualities. I'm not sure that the thinking about Hilbert space is the right setting here. When we talk about the smoothness, what does that mean? We can mean two things. We can mean our universe appears to be smooth on the largest scales. But that's not quite a statement about a wave function. A wave function being smooth or being spread out means that the wave function will assign a certain amplitude, a certain weight, a certain probability, if you wish, for a particular set of universes. And so, as we discussed before, the no-boundary wave function has the property that it assigns,

• 14:46Speaker 00

  that it sort of, it peaks, it sort of assigns a lot of weight to universes with an early phase of inflation. That's a key feature of that wave function. And you see, I mean, I'm not sure how technical your audience... Very technical, yeah. ...is, but so you might say, "Why is this?" Right? That's sort of the key property of Stephen's no-boundary wave function. "Why is this?" Well, you see, inflation in the early universe is a phase of accelerated expansion, and that accelerated expansion is driven by the simple fact that the universe is dominated by a source of potential energy.

• 15:23Speaker 00

  Somewhat like dark energy. Now, that feature of inflation is precisely what you need when you go back in time all the way to the beginning to sort of smoothly cap off the past, to sort of have regularity, have a regular, smooth, singularity-free Euclidean nugget at the bottom. So, in potential energy in the Lorentzian, it means smoothness in the Euclidean, a smooth geometry in the Euclidean, and that smoothness was the defining feature of the no-boundary wave function. Why? Because, of course, Stephen wanted quantum cosmology to get rid of the singularity. So that's how these two things, inflation and the no-boundary wave function, go together.

• 16:04Brian Keating?

  Now, the singularity, as you said, is, you know, sort of concomitant with the inflationary universe, in a sense. But could there not be some semi-classical or even classical, I'm thinking like a bouncing model, which also avoids the, you know, singularity, obviously, as Anna Egis and, to some extent, Neil Turok and Paul Steiner have worked on. Is there flaws in that, that, you know, that doesn't require a no-boundary proposal? In other words, it doesn't come with the kind of ontological quantum mechanical overlay that we're going to get into. But was he, you know, was that unpleasing to him in some sense, or to you? We don't have to only talk about Stephen, by the way. I mean, I'm very interested in

• 16:47Brian Keating?

  the novelties that you've employed in the last, you know, seven years since he passed away, but also long before that. So what is impalatable about the semi-classical or classical scenarios that also don't have singularities? Why do you reject them, perhaps?

• 17:01Speaker 00

  I personally have not been convinced that in these bouncing cosmologies, which, as you say, if that's true, everything would remain classical or semi-classical all the way back. I haven't quite been convinced that the fluctuations generated through these bounces, big crunch, big bang transitions, come out right and come out in line with our cosmology, our observations, our CMB observations. But the second point I wanted to make is that you're suggesting that this is a lot of baggage, this no boundary quantum kind of thing. But as usual with quantum theory, you give up on something, but you gain also something else.

• 17:44Speaker 00

  In contrast to these bouncing cosmologies, the no boundary wave function, quantum cosmology in general, has the possibility, has the potential to give us a grip on this which universe and why, to provide a measure, to provide access to a deeper question, which in the case of bouncing cosmologies, you cannot even ask.

• 18:08Brian Keating?

  So yeah, so take us through how it does that. The book has a lot of kind of an overlay of anthropic reasoning and counter anthropic reasoning. It's obvious that you and Stephen both felt that was sort of, that was distasteful. Maybe if not the bouncing model being distaste. What's the problem with anthropic? I mean, again, he ends his famous book, his most famous book with, you know, knowing the mind of God. Now, maybe he put that into some more books, who knows? All these anthropic arguments and the counters to them, as laid out in the book, in your wonderful book on the origin of time. They also sort of have a very prominent role for conscious observers,

• 18:47Brian Keating?

  which to me also kind of, you know, smells like, if not is identifiable with a teleological purpose. How do you react to that? What was, what is so distasteful about the measure problem not having an answer to it or anthropic? What don't you like about anthropic reasoning? And then how do you respond to critics who say, well, there's a role for the human or the conscious observer in this?

• 19:08Speaker 00

  I object to anthropic reasoning, not because I don't sympathize. There's a role for our observer's perspective, be it conscious or not. I, in fact, I very much sympathize with the profound and at the same time evident observation that we are within the universe, looking up and looking out and not somehow outside, which I think has dominated a lot of earlier work on cosmology. So. But I object to the way that an anthropic reasoning is brought into cosmology usually as some sort of principle that is exterior to the mathematics.

• 19:51Speaker 00

  That is a source of, yeah, a non-scientific principle and philosophers frankly have thought it through by now and have quite clearly come to the conclusion that, yeah, this adds a layer into cosmology which is simply not falsifiable. Your anthropic principle may be different from mine and there will be no rational way for us to agree on what's typical, what's anthropic, what's not anthropic and so forth. So in a sense, we tried to get the observer's perspective or the observer into cosmology, in cosmological theory, without compromising the scientific method.

• 20:29Speaker 00

  That was the core of our program, to get us into the equations in a way that upholds verifiability, falsifiability and so forth. And there Stephen made a mistake in his book, In Brief History of Time, that, so you're saying, "And thus we will know the mind of God," he writes. Stephen was thinking in the 80s, he was looking at his wave function as if you look at the wave function of a particle, like you're outside and you look at it from a distance and you see, well, gee, what's the most probable position of the particle, what's the most probable universe? He was playing God with his wave function. And it took many years for him to realize, and the paradoxes of the multiverse, of

• 21:12Speaker 00

  course, where a stimulus in this, to realize, gee, we should really learn to think that we are, to probe this wave function of the universe from the inside. Quantum cosmology provides, in my view, a framework in which we can get the observer in our equations without compromising proper theory.

• 21:34Brian Keating?

  Explain what is the relationship between top-down and bottom-up? And is it a mathematical, technical outgrowth or is it a philosophical outgrowth?

• 21:43Speaker 00

  It started out more, as I would say, a change of viewpoint, a more some sort of, yeah, paradigm shift in terms of what, in terms of really, what should we think of this no-boundary wave function? What is this theory trying to tell us? What is our position? Well, how do we test it? How do we use it? What does it really mean? Stephen was looking at this wave function just as if he was outside this wave function. And the problem is, if you do that... The dominant histories will be empty universes, empty universes without galaxies, without observers, whatever. Is the theory wrong? Or are we taking the wrong viewpoint? That's a very tricky question in cosmology,

• 22:26Speaker 00

  where you're clearly taking the wrong viewpoint by putting yourself out. We started to bring in our observational situation and we realized that if you take our observer's perspective into account, in fact, the theory predicts that our past has a significant burst of inflation, a long period of inflation in line with our observations. But later, I'm not sure we have to talk a lot about this, but later this change of viewpoint, which in the early days was very controversial, of course, because people like David Gross or like Neil Hurok were saying, "You're putting in the answer here." Whereas Stephen was claiming, "No, we're getting something more out. We're getting a prediction of a long period of inflation

• 23:10Speaker 00

  and the corresponding CMB fluctuations out." But later, this sort of top-down perspective. Ah, yeah, okay, so that's what you were asking. Bottom up means you put yourself out of the wave function and you think about the history of the universe from the Big Bang forward in time. Top down means, wait a minute, the universe's history isn't quite fixed. It's contingent on the kind of questions we are asking of the wave function, on the kind of observational situation we are interested in, on the kind of correlations, if you wish, between this and that, two features of the universe that we want to probe. And that it's only when you specify the question, the correlation, the observational situation, that the past history,

• 23:55Speaker 00

  the dominant past history emerges from the ensemble of possible histories. So the past is a wave function until you're asking a question, in which case the wave function crystallizes to a history. How do you react to that? It is the realistic situation. We are within this universe. I think it's the last great unification between the observer and the observed. Between us and the universe. I think this whole unification between quantum theory and gravity is underrated. If we pursue that unification in a cosmological context, it also includes this observer's perspective and this last bit may indeed be

• 24:39Speaker 00

  much more radical than we had anticipated.

• 24:44Brian Keating?

  Does that mean that, you know, the universe didn't exist because no one was there to observe it until 200,000 years ago when Neanderthals came out of caves? And what does it actually mean for the role that human beings as the only conscious entities that we know for sure that can do experiments in quantum mechanics? What does that mean for our importance? Are we more important than people have given us credit for in the past?

• 25:04Speaker 00

  That's a bit of a misunderstanding and it has its roots in a typical Hawkingian exaggeration. So at some point, and I quote Stephen on this in my book, at some point Hawking tells me, "All right, with this top down cosmology, we've put humankind back in the center of cosmology." Of course, this isn't quite what he meant. The way that we incorporate, include, take into account the observational situation has nothing to do with consciousness. It could be conscious observers and of course we make observations, but in our framework, this could equally well be the photons of the CMB

• 25:47Speaker 00

  or a single particle or in the early universe when we are talking about these phase transitions having various outcomes, some sort of branching, possible branchings of possible histories. It's the early universe itself. And the interactions between the fields that are around here that constantly classicalize, that constantly perform acts of observation in the quantum theory.

• 26:12Speaker 00

  Of course, eventually, these observations trickle down to our conscious observations. But we, as you point out, we are very late in the chain. And that's, and so that's not what we meant.

• 26:24Brian Keating?

  And I want to take back because, you know, I am a experimental physicist, but I'm, you know, quite conversant with some of the, at least the technical, mathematical aspects, not as technical or mathematical as you are. But I remember reading it, first of all, I bought it back in, you know, whenever it came out, I was in high school. I didn't read it for 20 years, probably. And I remember at the Royal Society meeting where I met Stephen, someone asked him, why did you write this book that allegedly no one can really comprehend and has finished? And he said, I needed to pay for my daughter's college tuition. And, you know, he was hilarious, you know that better than anybody. But I remember, you know, people kind of maybe pushing back or saying, you know, that there were some devices or concessions that, that he used.

• 27:07Brian Keating?

  And I didn't appreciate that until after I took quantum field theory as a grad student. In the middle, you know, portion where he's talking about the flowing of time, he says the following, he says, in any case, as far as everyday quantum mechanics is concerned, we may regard our use as imaginary time and Euclidean space as merely a mathematical device or trick. He's talking about Wic rotations, right? What is a Wic rotation, first of all?

• 27:28Speaker 00

  So technically, a Wic rotation is turning mathematically time into space. Why would you do this? It turns out that in quantum field theory, in particle physics, say, a Wic rotation is a handy way of describing a typical thermal state of your fields, of your system. In that context, it's a trick, as you say. It's a handy way of describing a thermal state, and then you get on with your calculations in that thermal state and you calculate whatever you want. It's a trick in that case, because of course, these fields, that system, those particles, they live in a background, they live in a universe, and that universe has a certain flow of time.

• 28:12Speaker 00

  When you do the big rotation, when you turn time into space for the universe itself, it's no longer a trick. It's a very bold and controversial statement. You're saying that, okay, we can do a similar procedure. We can turn time into space at the level of the universe itself. So there is no background notion of time left. You've just turned the geometry of the universe from a Lorentzian expanding spacetime into a Euclidean spherical space. So then you can no longer say it's a trick. Somehow, and that's the boldness of Stephen's theory of the Big Bang, when it comes to

• 28:54Speaker 00

  the origin of the universe, the trick we are familiar with from particle physics somehow becomes real and gives us the tools to describe the initial condition, the initial state, the birth of the universe.

• 29:08Brian Keating?

  Normally, normal SU2, SO3 rotations, they turn X into Y. They don't turn X into time. So how is it possible, but by applying a simple amount, it just changes the coordinate t to it, right? So that changes the Lorentz metric from minus plus plus to plus plus plus plus plus. That allows you to simplify things, I suppose. But how come it doesn't change space into time?

• 29:36Speaker 00

  The space, yeah, because the space directions are left untouched.

• 29:41Brian Keating?

  Is it a true rotation or is it a shift or some kind of mirror, you know, a conjugate symmetry like a parity reversal where it only affects one of the four components of the metric?

• 29:51Speaker 00

  Yeah, that's what it is. Yeah. More fundamentally, I would say, I mean, you make a sharp distinction between the Lorentzian expanding spacetime of Einstein's relativity theory and then the Euclidean all-space-like nugget, so to speak. Stephen and I did not make so much of a distinction of that. One goes smoothly over into the other. It's not that you sort of cut spacetime and now we're gonna turn time. No. In fact, we think of the expanding spacetime when you go back to the Big Bang, it sort of smoothly deforms into a purely spatial nugget.

• 30:30Speaker 00

  And the whole thing is one geometrical saddle point approximation to Stephen's wave function. And the key question is... What is a transition between the quantum purely spatial nugget, which is really like a law of an initial state, to the Lorentzian expanding space down? And there, there is a key point. The transition between the quantum regime and the classical expanding regime always goes through a brief burst of inflation. So that is a key single element that I would say is a universal prediction. In Lehman's term, you could call Stephen's model a model for the origin of inflation.

• 31:15Speaker 00

  That would be the sort of bottom lowest level at which you can describe.

• 31:19Brian Keating?

  In the top-down versus bottom-up predictions, I just wrote down a few of them as I was listening to the book. And one of them that struck my mind in the very truest sense, that is falsifiable potentially, is that, you know, the bottom-up model and all of physics, as I, you know, as most people interpret physics, that they're constants of nature, the speed of light, the strength of gravity. Those are constant. Whereas in top-down, your approach, these constants might be variables. Is that correct? Am I correct there?

• 31:52Speaker 00

  Yeah, yeah. So at the very basic level, we would say that these constants are asymptotic values of stuff that evolved, at least in the early universe. And maybe there are traces of such evolution right now.

• 32:05Brian Keating?

  So we've heard claims in the past of, say, the fine structure constant evolving. And we can test things like that on Earth. So before we get to tests of cosmic significance that are related to what I'm involved with the Simons Observatory and other projects, what other types of tests would provide perhaps falsification, as you point out, that is the mark of a good theory, is that it can be falsified, not proven. So what could we do in the laboratory, for example, to test whether or not there's increased credulity in top-down cosmology approach?

• 32:37Speaker 00

  Well, I only know of a very futuristic lab experiment. But when it comes to testing our hypothesis, I mostly think about advanced cosmological observations. The equation of state of dark energy, is this still evolving? I think that would be a hint that there's something dynamic about the laws. Gravity wave contributions to CMB fluctuations. I think a strengthening of the inflationary paradigm would be a step in that direction. More further unification, I think, between, yeah, or more insight into how dark matter fits in the picture. Because that's presumably also a bunch of forces with their own frozen accidents

• 33:21Speaker 00

  and their own role in the emergence of life. And so those are the kind of things I'm thinking of. In terms of lab experiment, if you really want to hear it, you see there's an advanced version of top-down cosmology, which is based on that recent theory-driven revolution to do with holography. To be very brief, holography is the way we think ultimately gravity and quantum theory can talk to one another, work together. It's not relevant in most of cosmology, but it becomes very important near black holes and near the Big Bang. And to be very brief, holographic models of cosmology build in a top-down perspective,

• 34:06Speaker 00

  build in that retrospective element that you mentioned. There is a far futuristic dream amongst theorists these days that we might be able to construct certain holographic toy models in the lab and in that way begin to probe, test certain properties of quantum gravity theories. So that would, I think, be another very different avenue. It would be sort of quantum cosmology in the lab. I think this will take a few decades.

• 34:36Brian Keating?

  Yeah, well, I'll work on that after the Simons Observatory, you know. Nobel Prize. I want to ask you next about, well, a way we could test it in the lab, so to speak, which is I've had on Sean Carroll, you know, is probably one of the most prominent supporters of the Everettian many worlds interpretation. What if he's right? And I've asked him, what are the experimental tests that could be used to prove that the branching rate of quantum, you know, decisions and choices? And he said that's on the cusp of being testable at the, you know, 10 to the minus 28th of a second level. What if it's proven that the many worlds interpretation is right and Copenhagen is wrong? Would you abandon this theory? Is that a way to falsify your theory?

• 35:19Speaker 00

  No, on the contrary, it would be, it would support the theory.

• 35:23Brian Keating?

  Okay, explain how.

• 35:23Speaker 00

  The Copenhagen interpretation is just not adequate for cosmology. It's, it would be too limited. If the Copenhagen interpretation were right, which I strongly doubt, quantum mechanics essentially would be confined to a theory useful in the lab. And it would not really make sense to apply it in a cosmological context. You really need some sort of, as you were saying earlier, you need some sort of holistic wave function kind of view, which you're probing from within and which has all these histories floating around. I really don't see how you could do quantum cosmology without at least buying into most

• 36:01Speaker 00

  of the Everettian view, except perhaps proper integration of that observer's retrospective element. But in fact, this is pretty much in line with Everett. So that's something I hadn't thought about. The Everettian, Everettian view of quant, interpretation of quantum mechanics is somehow being experimentally differentiated from Copenhagen. That would, in my view, be a hint that it's not crazy to apply quantum mechanics to cosmology, to the universe.

• 36:34Brian Keating?

  And what about these tests people like Latham and others have looked at or thought about? I'm forgetting some of the other supporters of it, but what if we did see evidence of the multiverse? You know, let's say we see some patterns in the microwave sky that are anomalous relating to intersecting light cones between different universes within the multiverse. With that. Prove or falsify the top-down Hawking model?

• 37:00Speaker 00

  That's a very good question. I'd be surprised to see these things. But that's certainly not something we would predict in any sort of normal top-down reasoning, right? It would, except, I guess, in certain models, it would be unexpected. Yeah, it would put, I think, the measure problem in multiverse cosmology once again center stage in this whole business.

• 37:25Brian Keating?

  One thing I find very fascinating about this, and I'm very close friend with Paul Steinhardt and Neil Turok and Anna Heijis and many others. I've interviewed Andre Linde on the other side. But there's so much controversy around the multiverse. And it obviously was troubling to Stephen as well as you. And but it's very curious to me that, you know, there's an old probably Italian saying that the enemy of my enemy is my friend. And yet, when I interview Neil or Latham or Paul, Neil is called imaginary time beautiful, but not necessary and possibly misleading. And that's a friend and a collaborator of yours and basically claiming, you know, you both agree that inflation, you know, there are certain troubling aspects about the current instantiation of inflation.

• 38:09Brian Keating?

  He doesn't believe in inflation at all, obviously. But how do you respond to Neil in particular with his notion of the mirror universe? That that kind of eliminates, has a smooth transition and eliminates the need for WIC rotations in imaginary time? How do you react to his criticism, but also his model? I'm curious to think, what do you think of his models?

• 38:28Speaker 00

  What Neil means, I believe, is that he has more confidence that bitter ride ingredients and fields, he can get the primordial CMB fluctuations to come out naturally and right in these bouncing models. If he can show this, then I think this would be a true alternative to inflation.

• 38:51Speaker 00

  And probably eventually, observationally distinguishable. What I guess I'm a bit surprised about is that he's certainly very open and very keen on asking these deeper questions that Stephen was driving at which universe and why and how do we fit into this game? And I don't see much of that coming to the fore in these bouncing models. Maybe that reflects their state of development. I mean, after all, inflation has been around for so many decades and so we've had plenty of opportunity to sort of think this all through. Now, I'm really talking about a matter of taste, almost.

• 39:31Speaker 00

  We agree, you say, that inflation without completion or alternative, inflation as it stands, has serious paradoxes and should be completed into a proper theory. That's, we certainly both agree with that. We've just taken different routes.

• 39:48Brian Keating?

  Stephen's longtime, you know, kind of the foil, Lenny Suskin, was on the podcast not too long ago, and we talked for a long time about the black hole war. We haven't really talked much about the black hole war or the implications for black holes, although you do discuss that in your book. But more, before I get to that, I do want to ask you the question I asked Lenny, which was, he's written books like The Theoretical Minimum. Now, you're a theorist, and I want to know, what is the experimental minimum? What should a bright, young, theoretical PhD student in your group know about experimental physics, cosmology, astrophysics?

• 40:30Brian Keating?

  What do you teach him or her to be conversant at the level that they need to be a success in theoretical physics, as you have become?

• 40:38Speaker 00

  It's very different from when you started out or when I started out. The peaks in the CMB were still being searched for, right? During my PhD, the acceleration was discovered, so we knew so little, we knew so. It's truly amazing. I think every theorist these days should have a profound appreciation of this precision era in cosmology. Which is truly very special. We know this history and its composition so well, so precise. And then there's this wealth of information in the CMB. And I go even a little further. I now teach my master's students here also about gravitational wave

• 41:18Speaker 00

  observations, their future. Because in 10, 15 years, we now have the Hubble tension, the discrepancy between the rate of expansion measured from early universe data and now in 10, 15 years gravitational waves will provide, whoops, a third way of doing this completely independently. So we're living through some sort of Galileo moment, the birth of a new kind of astronomy. And of course, not so clear what it's gonna bring, but it is a Galileo moment. And I think it's extremely important for theorists and observers alike to at least appreciate this.

• 41:54Brian Keating?

  I do want to talk about black holes in the context of the holographic principle, which obviously you discuss in the book. What is the status of it? I remember that, you know, Mal D'Saena moment in 1997, you know, in grad school, everyone was so excited about ADS-CFT. And you probably do the best job of explaining what ADS-CFT is, although the audiobook reader pronounces the ADS as ads every time. And it was a little bit jarring to me, but that's the only error in the whole book. But, you know, we don't live in anti-disenter space. We don't even live in disenter space. But these correspondences, the holography, they are criticized often as a string theory, you know, for predicting things that are potentially unobservable, extra dimensions, which we don't have evidence

• 42:39Brian Keating?

  for. Yes, they could be there, but so could a purple unicorn on Neptune's south pole. But answer this, Thomas, what is the status of it? You know, it's a remarkably beautiful piece of mathematics, as is string theory. But some have called it, like Roger Penrose, fantasy, and even fantasy squared, in that, you know, it involves string theory and then superposing string theory on top of inflation, which he also doesn't believe in. So how do you react to those criticisms? Is of a holographic principle, is it merely magnificent mathematics, but may not have an instantiation in reality that my colleagues and I can test with telescopes?

• 43:13Speaker 00

  It's certainly magnificent mathematics, but I think you're right that testing it, observationally with telescopes or in the lab, is a long way away. But is this the only way of testing things? There exists, at some point I think we should admit that there exists something like theoretical data. Theoretical consistency, mathematical consistency, is also a certain driver, a certain guideline, a certain type of knowledge which is there, which can't be ignored. That's how I feel holography. We should see holography by now. It's such an elaborate and well-theoretically-tested edifice

• 43:58Speaker 00

  that, to me, there's got to be some truth in it. What exactly it means for our real world and our expanding universe is profoundly unclear. To give you an example, in ADS-CFT, so in this mathematical toy-model universe of string theorists, it's clearly the holography. It's a duality, a connection between gravity theory and quantum theory in which the gravity theory lives in one dimension more, hence the name holography. One of the dimensions in our familiar world is a holographic projection. Now, in ADS-CFT, this is a space dimension. To me, it's extremely unclear that when we apply holography to our universe, it's gonna be the space dimension that's the holographic projected one.

• 44:43Speaker 00

  I think it's gonna be the time dimension. So, this just to illustrate you that there are profound uncertainties in how holography applies to our real world. Because, as you can imagine, if it's the time dimension that holographically pops out, the predictions of the theory are gonna be, yeah, the whole framework is gonna be very different from ADS-CFT. That's it. Since 90 years, 95 years, it's the time dimension which is the problematic one in cosmology. The Big Bang is the origin of time. Black holes are the end of time. At a very, very intuitive level, you would think that if you're gonna truly understand black holes and the Big Bang, you're gonna need to develop some sort of physics

• 45:28Speaker 00

  in which time is a truly emerged quality of the universe, so that it can disappear and not an a priori thing. Well, holography does seem to deliver exactly that. How, precisely how and how we will test it is completely unclear. But once again, to me, it smells right.

• 45:48Brian Keating?

  Thomas, it's been seven years since the passing of Stephen Hawking. If you had 30 seconds, you know, to visit him in heaven or wherever he is right now, what would you want to talk to him about? What questions would you want to raise with him? And what new, about, what about your new discoveries with your research team, completely independent of Stephen? What you most like to tell him about and be proud about and accomplishing since his passing?

• 46:12Speaker 00

  Yeah, so since Stephen passed away, I've been developing holographic models of top-down cosmology, of the theory we developed together. And in those holographic models, this retrospective element which you mentioned of top-down cosmology is sort of building from the start. And you go back in time by taking sort of a fuzzier and fuzzier look at those holograms. And eventually, when you go all the way back to the Big Bang, you sort of run out of bits. It's as if, it's as if the laws themselves don't go further back. And so the Big Bang really turns into a limit of science without breaking down. And this, I would like to have his opinion on.

• 46:57Speaker 00

  It seems to me that a proper implementation of our top-down ideas seems naturally to lead to the stunning conclusion that even the laws of physics sort of evaporate away, fade away into the Big Bang. These transcendental, platonic, mind-of-god kinds of truths, which, yeah, he told in the 80s. And I think he was very close to this, to thinking, to this change of paradigm. But it was sort of when he died, it was still sort of left up in the air because we didn't have, we didn't have enough tools to sort of really... Hamard is done.

• 47:42Brian Keating?

  Yeah. Now, you've written that cosmology, in a sense, could or maybe should be like evolutionary biology, Darwinian, and in retrospect, you know, kind of reconstructing the past from the present. And that's a hallmark of your, as I see it, it feels very much like your unique contributions, not something that Stephen maybe could have been capable of doing. So how did you come to this, you know, kind of paradigm or analogy? And what does it mean that cosmology could be an aspect of Darwinian evolution? That Darwin or Hawking or nobody could have anticipated? How did this come to you? And what does it mean?

• 48:21Speaker 00

  It came to me through building these holographic models and realizing, but gee, these laws disappear. It's not that we sort of hit some sort of eternal mathematical foundation. And then, of course, I began to think, okay, vice versa, how do these laws emerge? It's much like a tree of physical laws. The tree of physical laws could have emerged differently. And there you are with your Darwinian viewpoint. But of course, it's a bit of an analogy in the sense that selection or heterozytes, this is normal, I mean, the branching of the tree of physical laws in the early universe is very much like a quantum phenomenon, right? There's variation and selection, but these are not the kinds of words that we typically use.

• 49:05Brian Keating?

  Since 2018, we've had literally hundreds of LIGO detections. We've had the Event Horizon Telescope images. We've had the James Webb Deep Field observations. We've had even more knowledge from the barren acoustic oscillations of the DESI instrument, which have revealed tensions perhaps between the late and early time measurements. So here's a question for you. How have the observations that my colleagues and I have made since 2018, how have they shaped and maybe influenced you? And maybe given you courage to see things that Stephen and maybe anyone else is unable to see? What are the future observations that you're most excited about for test?

• 49:50Brian Keating?

  Challenging and improving your theoretical models and predictions.

• 49:54Speaker 00

  I'm very excited, of course, about the next generation of CMB experiments. I'm truly impressed how far down you guys can get into measuring that gravitational wave contribution to the CMB. I'm very intrigued by the first hints of a possibly evolving dark energy density. And I guess the future gravitational wave observations and the cosmological gravitational wave backgrounds would be. So these are really, just like we are now familiar with our CMB background.

• 50:32Speaker 00

  We also have gravitational wave backgrounds and sort of beginning to probe those. I think will sort of truly open up this, unlock these earliest stages of the universe in a very tangible way. So frankly, I'm excited about all these observations and there's not a single one, I think, that sort of. I think it's really the collection, the different angles that are bringing the richness to the field for the universe cosmology.

• 51:01Brian Keating?

  Thomas, this has been great. You aren't my first guest from Belgium. I had Francis Halsen on. He actually knew Le Metre, who you talk about a great deal in the book and took a course with him, I think. That's incredible. Yeah, I want to wish you the best of luck and the greatest of thanks and, you know, happy, uh, happy 50th birthday, right? You just had your 50th birthday. You're a half century old.

• 51:24Speaker 00

  Yeah, yeah, yeah.

• 51:25Brian Keating?

  Almost as old as me. Wow. Amazing. Well, here's, yeah, here's to three more. You gotta, you know, you gotta wish someone a little bit more than what they anticipated. Thomas has been phenomenal. I've enjoyed it very much and I do hope that we'll get to meet in person someday and discuss these great ideas, you know, and maybe make some bets one another, as Stephen was wont to do. So thank you so much, Tom.

• 51:46Speaker 00

  Sounds good. Thank you. Thank you for having me.

• 51:49Speaker 01

  Time causality, the laws of physics themselves. Thomas Hertog and Stephen Hawking's final theory challenges our deepest assumptions about reality. Whether conscious observers truly shape cosmic history or this represents the most beautiful failure in physics, let me know in the comments below. And if you found this exploration of time's true nature fascinating, check out my most recent episode with Neil Turok. Click here and don't forget to comment, like and subscribe.