Thomas Hertog Explains What Came Before The Big Bang

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. Picture this. Cambridge 1998. A young posttock sits across from the world's most famous living physicist, Steven Hawking. The great physicist delivers words that would shape both of their careers for the rest of their lives. Thomas said, "Steven, 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, the past is a wave function until you're asking a question, in which case the wave function crystallizes through history. 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 Hawkings final theory developed with his closest collaborator Thomas Herthog. 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? First of all, I was just a beginning PhD student. So I was flabbergasted to finally meet Hawking and discuss with 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 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 Steven just doing what I had yeah dreamt of doing. Now the context of what was on its mind in the late '9s was very much I think this multiverse ideas right they were very popular at the time amongst cosmologists as an explanation as a new sort of possible explanation for why we 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 Steven was one of the first cosmologists who deeply felt felt that multiverse cosmology was going nowhere that it wasn't quite scientific that 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 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 Simon's observatory should observe. And to Stephen this was a catastrophe. this was telling us that we were on the wrong track. Stephen was so quotable and so bold and characteristically just he 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 posttock and he would stay where Einstein stayed near we'd see him at the aanium the famous aanium 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 Steven 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, which was the last major experiment that Steven 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? 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 crooks of his particular model which is known as Steven'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, Steven boldly claimed themselves become a little fuzzy. And we can model this he claimed by turning the time dimension into a space dimension. The intuition behind it is that if time becomes fuzzy any clear notion of dynamics will and causality will eventually fade away. And so the heart of the matter of Steven's thinking about the big bang was that geometries with all space dimensions and no time dimensions can capture the some of the quantum properties of this that describe the origin of the universe. This did not come out of nowhere. This was this idea was grounded in the observation in the 70s that those ukidian geometries those geometries with space dimensions capture quantum properties of black holes. If that's the case maybe they get they also capture quantum properties of the big bang. So Steven 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 what do you do with it hey everybody I'm usually the one that asks my guests to judge their 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. I hope you'll check it out. And my publishers 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. That less 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. What is the kind of, you know, intellectual confidence that you have in it that 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 Hardle 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. Okay. It remains a hypothesis, right? But the 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 CMBB 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 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 universe. It's all in there. where quantum cosmology provides us with a measure, provides us with a tool to get a grips on 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 cosmological theory, which is the fact that we're 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 no boundary wave function turn out not really in the ball in the right ballark. But anyway, the boldest claim which you repeated in at the start of our conversation, which is that it also seems to say that that we should think of the big bang as some sort of epistemic horizon where even the laws of physics evaporate and disappear. That of course remains to be tested. Indeed. And I think you know we we'll hear shortly what your colleague Neil Turac has claimed as the reason to abandon it and approve of a different approach the mirror cosmological model. We talked with Laam Bole 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, 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? 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, the universe better has a state. That state is a wave function of the universe. But just like a wave function of a particle, 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 ask well where does this wave function live? I'm not so sure it lives in space. Yeah, there's nothing really a wave function of the universe every quality you would assign to it would be qualities of a particular subset of histories would be intrinsic qualities. I'm not sure that the Hilbert thinking about Hilbert space is the right setting 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 larger scales. But that's not quite a statement about the wave function. 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 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 the key property of the of Stevens 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, 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 singularityfree ukleian nugget at the bottom. So in potential energy in in the engine it means smoothness in the ukidian a smooth geometry in the ukidian and that smoothness was the defining feature of the boundary wave function. Why? Because of course Steven wanted quantum cosmology to get rid of the singularity. So that's how these two things inflation and row boundary wave function go together. Now the singularity as you said is you know sort of conccomatant with the inflationary universe in a sense but could there not be some semiclass or even classical I'm thinking like a bouncing model which also avoids the you know singularity obviously as Anna Aegis and some extent Neil Turo 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 onlogical 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 Steven, by the way. I mean, I'm very interested in 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 inpalatable about the semi-class or classical scenarios that also don't have singularities? Why do you reject them perhaps? 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-class 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 cosm our observations, our CNB observations. Well, 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 and quantum theory, you give up on something, but you gain also something else. 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 grips on this which universe and why to to provide a measure to provide access to a deeper question which in the case of bouncing cosmologies you cannot even ask. So yeah. So take us through how it does that. The book has a lot of kind of an overlay of anthropic reasoning and counteranthropic reasoning. It's obvious that you and Steven 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 knowing the mind of God. Now maybe he put that in to sell 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 which to me also kind of you know smells like if not is identifiable with a teological 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? I object to entropic 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're 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 entropic reasoning is brought into cosmology usually as some sort of principle that is exterior to the mathematics that is sort of yeah a non-scientific principle and philosophers frankly have thought it true by now and have quite clearly come to the conclusion that yeah this adds a layer into cosmology which is simply not falsifiable. your entropic principle may be different from mine and there will be no rational way for us to agree on what's typical, what's entropic, what's not entropic 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. That was the core of our program to get us into the equations in a way that main that that upholds verifiability, falsifiability and so forth. And there Steven 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 Steven 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 jeep 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 to for him to realize and the paradoxes of the multiverse of course where a stimulus is to realize gee we should really learn to think that we are to 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. Explain what is the relationship between top down and bottom up and is it a mathematical technical outgrowth or is it a philosophical outgrowth? It started out more as I would say a change of viewpoint and more some sort of yeah paradigm shift in terms of what in terms of really what should we think of this low boundary wave function? What is this theory trying to tell us? What is our position? How do we test it? How do we use it? What does it really mean? Steven was looking at this wave function just as if it 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 in in cosmology 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 at in the early days was very controversial of course because people like David Gross or like Neil Turk were saying you're putting in the answer here whereas Steven was claiming no we're getting something more out we're getting a prediction of a long period of inflation and the corresponding CMBB fluctuations out but later this sort of top down perspective oh yeah okay so that's what you were asking bottom up means you put yourself out of the wave function and you and you think about the uni the history of the universe from the big bang forward in time down means wait a minute the univer versus history isn't quite fixed. It's contingent on the kind of questions we're 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 observation situation that the past history, the dominant past history emerges from the ensemble of possible history. So the past is a wave function until you're asking a a question in which case the wave function crystallizes to 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 much more radical than we had anticipated. 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 Neanderthalss came out of caves? And what does it actually mean for the role that human beings as the only conscious entities that we know of 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? That's a bit of a misunderstanding and it has its roots in a typical Hawk Kingian exaggeration. So at some point, and I quote Steven 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 CMBB 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 eyes that constantly perform acts of observation in the quantum theory. 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 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 did 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. 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 uklitian space as merely a mathematical device or trick." He's talking about wick rotations, right? What is a wick rotation first of all? So technically a wick rotation is earning mathematically time into space. Why would you do this? It turns out that in quantum field theory in particle physics say a weak 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 terminal 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. 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 Laurentian expanding spaceime into a ukian spherical space. So then you can no longer say it's a trick somehow. And that's the boldness of the of Steven's theory of the big bang. When it comes to the origin of the universe, the trick we are familiar with from particle physics somehow becomes revealed and gives us the tools to describe the initial condition, the initial state, the birth of the universe. Normally, normal SU you know 2 SO3 rotations, you know, 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 you know t to it t right. So that changes the lorren metric from you know minus plus+ to plus+++ that allows you to simplify things I suppose. But how come it doesn't change space into time the space? Yeah because the space directions are left untouched. Is it a true rotation or is it a shift or some kind of mirror par you know a conjugate symmetry like a par reversal where it only affects one of the four components of the metric? Yeah, that's what it is. Yeah. More fundamentally I would say I mean you make a sharp distinction between the Laurentian expanding spacetime of Einstein's relativity theory and then the Uklesian all spaceelike 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 we sort of cut spacetime and now we're going to turn time. No, in fact, we think of the expanding spaceime when you go back to the big bang. It sort of smoothly deforms into a purely spatial nugget. It's and the whole thing is one geometrical settle point approximation to Steven's wave function. And the key question is what is a transition between the quantum purely spatial nugget which is really like an initial a law of an initial state to the Laurentian 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 layman's term, you could call Steven's model a model for the origin of inflation. That would be the sort of bottom lowest level at which you can describe 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 the 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? Yeah. Yeah. So at the very basic level, we would say that these constants are asmtotic values of stuff that evolved at least in the early universe and maybe there are traces of such evolution right now. 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 credul in top- down cosmology approach? Well, I only know of a very sort of futuristic lab experiment, but when it comes to testing or or 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 CNB fluctuations. I think a strengthening of the inflationary paradigm would be a step in the in that direction. More further unification I think between yeah or more insights into how dark matter fits in the picture because that's presumably also a bunch of forces with their own accident frozen accidents and their own role in 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 theorydriven 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 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, build in that retrospective element that you mentioned. There is a far sort of 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. Yeah, we'll I'll work on that after the Simons Observatory, you know, the Nobel Prize. I want to ask you next about well a way we could test it in the labs, so to speak, which is I've had on Sean Carol, you know, is probably one of the most prominent supporters of the ever ready and 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 and 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? No. On the contrary it would be it would support the theory. Okay. explain how 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 of the action view except perhaps proper integration of that observer's retrospective element. But in fact, this is pretty much in line with everything. So that's something I hadn't thought about. The ain vetan view of quantum 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. And what about these tests people like Laam 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 ly cones between different universes within the multiverse. Would that prove or falsify the top down Hawking model? That's a very good question. I'd be surprised we see these things, but that's certainly not something we would predict in any sort of normal top- down reasoning, right? It would except 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. One thing I find very fascinating about this and I'm very close friends with Paul Steinhard and Neil Turo and Anna Agis 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 Laam 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 you know basically claiming you know, you they you both agree that inflation, you know, there there are certain troubling aspects about the current instantiation of inflation. 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 wick rotations and imaginary time? H how do you react to his criticism, but also his model? I'm curious to think what do you think of his models? What Neil means, I believe, is that he has more confidence that with the right ingredients and fields, he can get the primordial CMBB fluctuations to come out naturally and right in these bouncy models. If he can show this, then I think this would be a true alternative to inflation 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 Steven 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 four 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. We do agree, as you say, that inflation without a completion or a 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 roots. Steven's longtime, you know, kind of 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? 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? It's very different from when you started out or when I started out. these peaks in the CMBB 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 CMBB And I go even a little further. I now teach my master students here also about gravitational wave 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 101 15 years gravitational waves will provide a third way of doing this in completely independently. So we're living through some sort of Galileo moment the birth of a new kind of astronomy. And of course, it's not so clear what it's going to bring, but it is a Galileo moment. I think it's extremely important for theorists and observers alike to at least appreciate this. 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, Maldisena 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 audio book reader pronounces the ads as ads every time. And it's 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-deitter space. We don't even live in desitter space. But these correspondencies, the holography, they are criticized often as is string theory, you know, for predicting things that are potentially unobservable, extra dimensions, which we don't have evidence 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 the holographic principle is it merely magnificent mathematics, but may not have an instantiation in reality that my colleagues and I can test with telescopes? It's certainly magnificent mathematics, but I think you're right that testing it observationally with telescopes or in the lab is 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 sort of 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 a it's such an elaborate and well theoretically tested edifice that to me there's got to be some truth in it. What exactly it means for a 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 the string of string theorists, it's clearly the holography. It's a duality 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 ADSCFT this is a space dimension. To me it's extremely unclear that in in when we apply holography to our universe it's going to be the space dimension that's the holographic projected one. I think it's going to 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 TV are going to be yeah the whole framework is going to be very different from ads CFT that said 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 hole holes are the end of time. At a very very intuitive level, you would think that if you're going to truly understand black holes and the big bang, you're going to need to develop some sort of physics in which time is a truly emergent quality of the universe so that it can disappear and not an a priority 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. Thomas, it's been seven years since the passing of Stephven 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 Steven, would you most like to tell him about and be proud about and accomplishing since his passing? Yeah. So since Steven 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 the 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. 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 trans 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 hammer this down. 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's it feels very much like your unique contributions not something that Steven 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 no nobody could have anticipated? How did this come to you and what does it mean? 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 heditaries 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 kind of the kinds of words that that we typically use. Since 2018, we've had literally hundreds of LIGO detections. We've had the event horizon telescope images. We've had the James Web deep field observations. We've had even more knowledge from the Baron 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 testing, challenging, and improving your theoretical models and predictions? I'm very excited, of course, about the next generation of CNB experiments. I'm truly impressed how far down you guys can get into measuring that gravitational wave contribution through the CNB and 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 CNB background. We all 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 early universe cosmology. Thomas, this has been great. You aren't my first guest from Belgium. I had Francis Hallsen on. He actually knew Lmetra, 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 and the greatest of thanks and, you know, happy uh happy 50th birthday, right? You just had your 50th birthday. You're half century old. Yeah. Yeah. Yeah. Almost as old as me. Wow. Amazing. Well, here's Yeah. Here's to three more. You got to, you know, you got to 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 Steven was want to do. So, thank you so much, Don. Sounds good. Thank you. Thank you for having me. Time, causality, the laws of physics themselves. Thomas Herthog and Steven Hawkins 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 Turac. Click here. And don't forget to comment, like, and subscribe.