Your browser does not support the audio element.<\/p>\n Transcript<\/p>\n [Music plays]<\/p>\n JANNA LEVIN: \u200aI\u2019m Janna Levin<\/p>\n STEVE STROGATZ: And I\u2019m Steve Strogatz.<\/p>\n LEVIN: And this is The Joy of Why, a podcast from Quanta Magazine exploring some of the biggest unanswered questions in math and science today.<\/p>\n Hey Steve.<\/p>\n STROGATZ: Hi Janna.<\/p>\n LEVIN: We have a small, little topic today, depending on how you look at it.<\/p>\n STROGATZ: Yeah?<\/p>\n LEVIN: Little in that we\u2019re looking back when the universe was maybe very small, not so small in terms of profundity. We\u2019re going to be talking about the origin of the universe.<\/p>\n STROGATZ: Okay, small topic. I see.<\/p>\n LEVIN: Yes. Just that little topic. And it\u2019s interesting because there\u2019s kind of a leaning in this conversation towards \u201cWhy?\u201d: Why did the universe begin? Or why is it the way it is? And that\u2019s actually, even though our title is The Joy of Why, not usually how people approach the Big Bang by asking why.<\/p>\n STROGATZ: I don\u2019t even understand the question yet. This sounds interesting.<\/p>\n LEVIN: Well, I have his friend Thomas Hertog who I first met when he was this very young scientist just starting his studies in Cambridge. And he began working with Stephen Hawking on the big questions. Can you imagine being thrown in?<\/p>\n STROGATZ: [Laughs]<\/p>\n LEVIN: And they worked together through a lot of different phases of Stephen\u2019s life and wellbeing. And they began asking questions in a slightly different way, not just how did the universe began, but was it tuned for life? And how might these physical laws account for the emergence of life?<\/p>\n STROGATZ: Uh huh. You\u2019re not going in an anthropic principle direction, are you?<\/p>\n LEVIN: Well, as I understand it, Stephen was horrified by the multiverse, by this sort of anthropic approach. And yet he couldn\u2019t deny that there was something there. And so no, they\u2019re not exactly going to be anthropic, which would kind of say from this God\u2019s eye view, all of these universes happen. It\u2019s just a matter of which one we land in. Right? In a way, Hawking was rejecting that.<\/p>\n STROGATZ: Hmm, okay.<\/p>\n LEVIN: And I think almost in another extreme to ask about the origin of the universe, specifically from our point of view with the observer nested in the question.<\/p>\n STROGATZ: Okay. It almost reminds me of Descartes, like the one thing you can rely on is, \u201cI think therefore I am.\u201d So in this case, we are here observing the universe. We know at least that.<\/p>\n LEVIN: Yeah. We know we\u2019re here. You know in the multiverse thinking, everything that can happen happens, and you are just sort of saying, well, I guess I\u2019m just in the one that was viable for life. This is a little different. It\u2019s more a kind of Darwinian approach. We are in a tree of life, and this is our specific tree. Maybe it makes more sense to follow our tree backward, and then begin to look at the origin of the universe from the series of steps, selections that led to the life that we live\u2026<\/p>\n STROGATZ: Uh huh.<\/p>\n LEVIN: \u2026 On this viable planet. But it gets pretty challenging as we go into it. And the book that he wrote on this was On the Origin of Time: Stephen Hawking\u2019s Final Theory, with a reference as though on the origin of species. So it really is something they pursued for a very long time. But I guess I\u2019m wondering, I know we call this The Joy of Why, but what you think of framing a \u2018why?\u2019 question with something like the Big Bang?<\/p>\n STROGATZ: You\u2019d think, of course, I\u2019m in favor of it since we\u2019re on this show.<\/p>\n LEVIN: Right.<\/p>\n STROGATZ: But it really runs counter to the way we\u2019re brought up in science, right? These teleological questions as they\u2019re sometimes disparagingly referred to like that hearkens back to Aristotle and all. You\u2019re not supposed to ask the \u2018why?\u2019 question, in especially about inanimate things like there may be some justification for living things that they do have, even if not a purpose, natural selection gives a feeling of \u2018why\u2019. But with matter and the universe and energy and space-time, it\u2019s hard to even understand what \u2018why\u2019 could mean.<\/p>\n LEVIN: Yeah. And actually it\u2019s interesting that you say that because when they start to run the movie back to the very beginning, they begin to see that physics itself as the argument begins to disintegrate. Space and time become harder and harder to distinguish from each other. Some sense, maybe time itself becomes a concept we can no longer really rely on. And he says there\u2019s a kind of epistemic horizon, right? Where quantum mechanics becomes so dominant in dictating the behavior of the physical world that what we typically understand is physical laws themselves don\u2019t even apply.<\/p>\n STROGATZ: Nice. Interesting. Wild!<\/p>\n LEVIN: And so he\u2019s sort of saying, you know what? That \u2018why?\u2019 question dissolves with it.<\/p>\n STROGATZ: Ooh.<\/p>\n LEVIN: Yeah.<\/p>\n STROGATZ: Okay.<\/p>\n LEVIN: I don\u2019t know if I\u2019m doing him justice. So maybe we should turn this to Thomas himself. Here\u2019s Professor of Theoretical Physics at Belgium\u2019s University of Leuven, Thomas Hertog.<\/p>\n [Music plays]<\/p>\n LEVIN: Welcome to The Joy of Why, Thomas. We\u2019re so pleased to have you on the show. We haven\u2019t spoken in a while.<\/p>\n HERTOG: Thank you, Janna.<\/p>\n LEVIN: When we first met, you were a PhD student and you were just meeting Stephen Hawking, but he was already a celebrity. What was that experience like?<\/p>\n HERTOG: Well, it took a little while to get over the encounter with such a celebrity. But my first encounter was, to my own surprise, some sort of homecoming for me. Because, as a young boy, I had been very much interested in big philosophical questions, and I happened to be good at maths. I must say, I never quite met a physicist like Stephen, who was essentially guided by huge age-old questions, and at the same time trying to understand these questions using solid scientific reasoning. So, it was a remarkable encounter, and it sort of changed my life.<\/p>\n LEVIN: So, over the next 20 years, you\u2019re working together, really until the end of his life.<\/p>\n HERTOG: On and off, yeah.<\/p>\n LEVIN: Probably the day you met him, he was already struggling to communicate with interfaces and various halting methods. But in the end, it came down to you bracing yourself within his field of view and interpreting his facial expressions. Can you describe the final years?<\/p>\n HERTOG: Right. There was a big change. In the nineties, he had his computer, he was speaking fluidly, typing out sentences. And that\u2019s what I think in the end kept us going. The fact that we had the opportunity in the late nineties, early 2000s to build up a common intuition, shared calculations, shared work. And I must admit that at several occasions, I thought this is it. It\u2019s almost impossible to work with him later, in the last five, six years of his life.<\/p>\n But then you discover, to your surprise, that theoretical physics is a really social affair, and that when you have a close collaborator for many years you do develop a shared intuition, and that\u2019s what then kicks in. And these are wonderful collaborations. So, indeed, we developed a truly bizarre mix of verbal and nonverbal and other creative ways of communicating.<\/p>\n LEVIN: Now, were you coming straight from Belgium, when you came to Cambridge?<\/p>\n HERTOG: Yes.<\/p>\n LEVIN: What made you decide to come to Cambridge, to leave home?<\/p>\n HERTOG: The complete lack of any serious cosmology at the time in Belgium. Which was too bad really because, in the 1930s and 40s, there was a small but wonderful cosmology community in Belgium, led or inspired by the pioneering works of Georges Lema\u00eetre, this Belgian priest astronomer who came up with the idea of a Big Bang.<\/p>\n But that all died out by the time I grew up. And someone told me, well, if you\u2019re interested in cosmology, you should go to Cambridge. So, it was known at the time as a mecca for cosmology, I would say.<\/p>\n LEVIN: \u200aSo, you\u2019re already interested in these big questions. You come to Cambridge to immerse yourself. What drew you to ask questions that seem so outside the scope of human reach?<\/p>\n HERTOG: I\u2019m really not sure. But I did start out studying physics being already interested in those philosophical questions. You see, in Belgium, we have an extremely old-fashioned educational system and we learn ancient Greek. So in high school, we read Plato and all these guys in their original texts. I really enjoyed this. And then the combination, I guess, with my interests in playing with mathematics led me to theoretical physics. And eventually, I met Stephen who happened to have a very, similar approach to physics.<\/p>\n The question of origin hides the origin of the question.<\/p>\n To me, Stephen was really someone who stuck to the scientific method, who stuck fairly close to theories. Of course, this was also the late nineties. So this was a period in cosmology in which the multiverse was very famous. It led to lots of paradoxes. So there was something to figure out, that was clear. There was all sort of bizarre things going on, and there were lots of observations in cosmology that were beginning to support the idea of inflation, dark energy, and so forth.<\/p>\n So, it was this whole context, I think, which conspired with my sort of intuitive interests in big philosophical questions and maths. It just happened, Janna.<\/p>\n LEVIN: Happens to the best of us, Thomas.<\/p>\n HERTOG: I think so, yeah.<\/p>\n LEVIN: So, I want to ask about this epigraph in your book, \u201cThe question of origin hides the origin of the question.\u201d I believe that\u2019s from a Belgian poet?<\/p>\n HERTOG: Well, I had to have some Belgian touch to my writings. Fran\u00e7ois Jacqmin is the poet\u2019s name.<\/p>\n LEVIN: How do you reflect on that epigraph now?<\/p>\n HERTOG: So, I came across his poetry while I was in the final stages of writing my book and it summarizes in one sentence my entire story, including indeed from the naive idea that we might want to understand the question of the origin based on prior truths, on transcendental, mathematical, platonic, unchanging laws. And that\u2019s also how my work with Hawking started out, right? I mean, many times in physics, we were driven to this paradigm shift to turn cosmology inside out by necessity, because the rest didn\u2019t work.<\/p>\n So, then the second part of the quote, the question of the origin hides the origin of the question, is for me really a way of saying that our human condition as observers within this universe should be somehow integrated. There should be a place for this within our mathematical framework.<\/p>\n Traditionally, in physics and in cosmology, we try to come up with causal prior explanations of things. This state of affairs leads to the following \u2014 that\u2019s causality, that\u2019s dynamics \u2014 it\u2019s all built into the framework we use. But one might wonder that, with the discovery of the Big Bang \u2014 the origin of time \u2014 we might wonder whether we haven\u2019t discovered something that does not quite have a cause. So, whether we should be looking like we do in the rest of physics for some sort of prior explanation, or whether the Big Bang is something we can only understand retrospectively from our position as observers within an old universe.<\/p>\n So, that\u2019s the kind of provocative question I asked at the start of my book, and frankly, it\u2019s the kind of question that was with Stephen and me for many years on the background.<\/p>\n LEVIN: You described in the preface of your book that the universe emerged in some sense from a violent birth, as though it knew it was destined to be our home. And you said some of the first words that Stephen ever said to you was, the universe we observe appears designed. Are you saying you\u2019re uncomfortable with the fact that it seems to be so finely tuned for our home?<\/p>\n HERTOG: Yeah. I do not believe, and Stephen certainly did not believe, that there was an actual designer or a \u2018God\u2019 behind this whole thing. He would rather keep religion out of the physics of the Big Bang. But then on the other hand, the laws of physics as we know them, seem mysteriously fit for life. They seem fine-tuned. It\u2019s as if the universe was destined to bring forward life at some point.<\/p>\n So, what should we think of this? Is this a mathematical fluke? Are there many Big Bangs? Is this a deep mathematical truth? Did the universe have to be like this? And so that\u2019s the kind of thing which was the overarching question behind our collaboration.<\/p>\n LEVIN: Well, let me ask you this. When you say the universe seems so finely tuned for life, to most people who are hearing about the universe, it does not seem that way. For instance, it seems to be just us. So here we are. There are what, hundreds of billions of planets in our Milky Way galaxy alone. We\u2019re the only ones we\u2019re sure are here. That doesn\u2019t seem very finely tuned for life.<\/p>\n HERTOG: Yeah, very good point. Down at the level of fundamental physics, down at the level of the particle forces and the composition of the universe, the fact that we have three dimensions of space and all that, it seems to me fine-tuned to bring forth life. Change any of these properties of the laws, and quickly you end up with a lifeless universe.<\/p>\n It\u2019s as if the universe was destined to bring forward life at some point.<\/p>\n But you\u2019re absolutely right that it does not mean that the universe is full of advanced life forms. Indeed, it does look like we are pretty rare. The explanation is that in between the ground zero level of physics and the advanced sort of life level, there are many layers of evolution. Many improbabilities, many giant steps to take to get there. So on the one hand, these laws seem to be fit for life. On the other hand, we seem to be alone. My perhaps naive reading of that situation is that there\u2019s a serious bottleneck. Because if there isn\u2019t a bottleneck in-between the laws of physics and say our sort of level of complexity, then your observation \u2014 that we seem to be rare \u2014 is scary as people back to Fermi pointed out. Because then it would mean that there could be a bottleneck ahead of us, which would almost certainly mean that advanced life forms like us tend to be short-lived.<\/p>\n LEVIN: So this idea that if I change the laws of physics, I make the electron a little heavier, or I weaken magnetism relative to gravity, we can extrapolate that those would be lifeless universes. Do you feel very confident about that extrapolation? Are we sure nature wouldn\u2019t have found a way?<\/p>\n HERTOG: I am pretty open to the idea there could be other pockets of possible laws of physics that are fit for life. But some of these changes, and they don\u2019t need to be even very large, have such drastic implications that you often don\u2019t end up with stable atoms or stable solar systems. And once you are in that realm, I am pretty skeptical some high-level complexity could flourish and emerge. That\u2019s my feeling.<\/p>\n LEVIN: So, how do we go back and understand the beginning? We\u2019re looking back 13.8 billion years. How do you do such a thing\u2026 roll back time?<\/p>\n HERTOG: Well, that\u2019s the advantage of theoretical physics, right?<\/p>\n LEVIN: You do it on pen and paper.<\/p>\n HERTOG: Exactly. You roll back time using pen and paper. So you start with the laws of physics as we\u2019ve discovered them here on Earth, and you extrapolate them. And you use Einstein\u2019s theory of gravity, which gives us a framework to study the universe as a whole. And you come to the conclusion that if you do extrapolate Einstein\u2019s theory of gravity, you run into serious problems early on. In fact, blindly following Einstein\u2019s theory of space, time, and gravity \u2014 his theory of relativity \u2014 would lead you to conclude that the actual beginning \u2014 the Big Bang \u2014 is a singularity that lies outside science.<\/p>\n LEVIN: And by a singularity, you mean infinite energy, infinite curvatures, infinite unpredictability?<\/p>\n HERTOG: Right. So, Einstein\u2019s theory would lead you to think that the Big Bang is somehow the origin of time. And, therefore, that even your basic notions of causality may not apply in such extreme conditions. But of course, Einstein\u2019s theory doesn\u2019t tell you then what\u2019s going on. That\u2019s the whole subject of early universe cosmology, which attempts to combine Einstein\u2019s theory with ideas from quantum theory.<\/p>\n LEVIN: Even though it\u2019s derived from his theory, Einstein did not predict the Big Bang, did he?<\/p>\n HERTOG: He did not. We claim this is a Belgian discovery, like beer and French fries. But yeah, curiously, George Lema\u00eetre was younger than Einstein and became a real expert working with Einstein\u2019s theory. But he was also a priest, mathematician, astronomer. He was aware, also, of the early indications in American observatories that most galaxies were moving away from us. So, he was motivated by trying to find an explanation for that. And so, he discovered these time-dependent expanding universes as solutions of Einstein\u2019s theory, which seemed to him a better match to the observations at the time than Einstein\u2019s eternal static universe.<\/p>\n But then, of course, comes the real difficulty. Lema\u00eetre did exactly what you were alluding to earlier. He used Einstein equation, turned around time, went backwards and saw that the whole thing crunched in the beginning. Lema\u00eetre referred to the Big Bang as a primeval atom. Which was the most vague and bizarre thing, of course. I mean the atom did not really exist in space and time because it was the origin of space and time. And he thought of it as some sort of epistemic limit.<\/p>\n But his intuition was that it should not be a place for a creator or a designer. But on the other hand, he was saying we shouldn\u2019t really expect this primeval atom to hold in it the entire evolution or the nature of the physical laws. So, he was sort of trying to have it both ways. And I think it\u2019s a little bit how, in the end, Stephen\u2019s theory played out. In a way, Stephen\u2019s model, I view a little bit as some sort of geometric realization of Lema\u00eetre\u2019s idea of a primeval atom.<\/p>\n LEVIN: Let\u2019s come to that. We\u2019re now post-Einstein. It\u2019s a shock to Einstein \u2014 who wanted an eternal universe \u2014 that in fact the universe is expanding, not permanent, not static. Even though these are predictions from his own theory.<\/p>\n And as you said, you look back in time, you wind the movie backwards, and there was a hot early Big Bang. The universe is expanding out of that early period, but that initial moment all the way back is problematic. It does not sit well in our hearts. It didn\u2019t sit well in Lema\u00eetre\u2019s, Einstein\u2019s, Hawking\u2019s, yours.<\/p>\n And so, you\u2019ve returned to this question of going back further and further, past the stuff we believe to be true \u2014 the hot primordial soup, the expansion in an energetic event \u2014 to that initial moment. Tell me what were some of the other possibilities that were floated to deal with this unsettling initial singularity?<\/p>\n HERTOG: It\u2019s unsettling because it doesn\u2019t fit in any kind of framework for physics we know and love, right?<\/p>\n The unsettling thing\u2026 is that the Big Bang, the origin of time, would also become the origin of law.<\/p>\n In physics, we come up with laws of dynamics, laws that describe the evolution. That\u2019s of course problematic when your notion of time breaks down. The prediction of a Big Bang suggests we need a whole different kind of law of physics, which is a law of an initial condition. And of course that\u2019s strange. And so many attempts rightfully, I think, have been made to try to fit the idea of a Big Bang into our standard way of doing physics. And therefore, then you are quickly led to the idea, well, maybe there was some sort of evolution prior to the Big Bang. Maybe Einstein\u2019s singularity isn\u2019t the end of the world, and maybe you can evolve through it backwards into some pre-Big Bang era. Or maybe the universe is much larger than we believe, maybe there is some sort of giant space with many universes in it. That\u2019s the idea of a multiverse.<\/p>\n LEVIN: So, in the multiverse our Big Bang isn\u2019t exactly a singularity. It\u2019s like a plume off of a larger space-time?<\/p>\n HERTOG: Right, right. Like an explosion inside a bigger space. I tend to be skeptical about any pre-Big Bang era. I\u2019ve been inclined to take the origin of time seriously, in part of course influenced by Stephen\u2019s intuition. The multiverse is a different thing.<\/p>\n LEVIN: Yeah, didn\u2019t he say the multiverse was outrageous?<\/p>\n HERTOG: Yes.<\/p>\n LEVIN: And didn\u2019t you?<\/p>\n HERTOG: Yes.<\/p>\n LEVIN: And you\u2019ve said in that spirit that it\u2019s important to consider cosmology from a human perspective, rather than relying on this overarching God\u2019s eye view.<\/p>\n HERTOG: The multiverse is some sort of the supreme version of taking a God\u2019s eye view on reality, right? It started with Copernicus on the solar system, or Galileo. It is the big discovery of the scientific revolution four centuries ago.<\/p>\n LEVIN: So, you\u2019re citing two successful versions of taking the God\u2019s eye view.<\/p>\n HERTOG: Exactly. And I do think it is successful. The idea that science and mathematics can allow us to take some sort of objective viewpoint onto reality \u2014 it\u2019s clearly the success story of science. But the question is, are there limits to that view? Because the multiverse is a rather extreme version of this, right? You behave as if you\u2019re looking down onto all possible universes from some sort of abstract, mathematical, absolute viewpoint.<\/p>\n LEVIN: And what\u2019s wrong with that?<\/p>\n HERTOG: One thing that goes wrong with that is, from such a distant viewpoint \u2014 imagine you have a theory of the multiverse \u2014 it does not seem to tell you in which one of these universes you should actually be. Suppose these different universes have different laws of physics. Suppose two of them admit life. Which one should be ours? So, it seems to be you\u2019ve taken yourself so far out that you\u2019ve created some sort of distance between the universe and your viewpoint. And in many cases, for many questions, this is not important. But the question we were asking about what makes our universe fit for life, what makes these laws of physics what they are? Well, those are questions that probe the relation between, on the one hand, the laws of physics and the way the universe works, and on the other hand, our perspective.<\/p>\n LEVIN: Now, you also quote Stephen as saying, we are not angels who view the universe from the outside. What is this interplay between reality and observation that you\u2019re suggesting?<\/p>\n HERTOG: \u201cReality\u201d\u2026 mmm-hmmm.<\/p>\n LEVIN: Well, okay, if that\u2019s too strong a word \u2014 the universe we find ourselves in and the role of the observer who is in fact a product of the specific cosmos we find ourselves in.<\/p>\n HERTOG: So, if I don\u2019t want to start with the reality with all sorts of universes in it, and then figure out in which one I am, I want to turn it inside out and distill a definite concrete history based on our observation situation. So, in a way what we were trying to do is to get perspective, so to speak, because, as you say, it doesn\u2019t need to be a human observer. It could be a photon, right, without ruining the scientific methods, ruining the falsifiability, which we know and love in physics.<\/p>\n The time dimension has been the problematic one since Lema\u00eetre\u2019s discovery of the Big Bang.<\/p>\n And so, I think the framework of quantum cosmology provides a way to get this perspective in there. To not start with some sort of prior reality in which all stuff exists, but rather reconstruct the history of the universe from within, so to speak. Quantum cosmology allows you to do that very much like biology allows you to reconstruct a tree of life, starting from our observations of all sorts of species and fossils. I\u2019m advocating that the laws of physics as we know them, in our universe, are themselves the result of an early evolution \u2014 a sort of a tree of law \u2014 which in the very first stages of the universe took form. And I\u2019m advocating against the idea that there is some sort of grand equation describing all possible universes with all possible physical laws written in stone.<\/p>\n LEVIN: So, in some sense, instead of suggesting there\u2019s a multiverse of possibilities where every option will be tried by nature, you are saying all of that could be happening in the very early universe, and that there\u2019s some kind of process of selection to guide the universe that we find ourselves in thereafter?<\/p>\n HERTOG: Right, but in a sense that selection is something which acts retrospectively. We find ourselves in the universe around us, and we try to reconstruct our history. I\u2019m not saying there couldn\u2019t have been any other universes. But the big difference is that in the multiverse cosmology, all these universes are somehow totally out there, and they play a role in trying to figure out what we should predict.<\/p>\n In our framework, all these universes might or might not be out there, but they don\u2019t enter into the predictive scheme that we try to develop. Just like there might be life on another planet, but it doesn\u2019t interfere with biology on this planet.<\/p>\n So, I\u2019m trying to advocate a more humble viewpoint, I guess, on cosmology and physics, which is more in line maybe with the way we think about things in biology, and which has the implication that if you go backwards in time, the laws as we know them merge. The different particle forces and particle species lose their specificity. That\u2019s this whole idea of unification. It\u2019s been tested to some extent. And I guess we\u2019ve always thought that we would arrive at some sort of rock-solid, ultimate mathematical, eternal truth. I guess the crux of the hypothesis that Stephen and I ended up developing is that this process of simplification and unification, maybe it just goes on all the way, and maybe ultimately even the distinction between space and time disappears. That\u2019s the crux of his hypothesis. And the unsettling thing, of course, is that the Big Bang \u2014 the origin of time \u2014 would also become the origin of law. The laws themselves sort of evaporate going all the way backwards.<\/p>\n So, I guess I\u2019m advocating a fundamentally evolutionary understanding of what we call the laws of physics.<\/p>\n [Music plays]<\/p>\n LEVIN: Steve, you are leaning back contemplating.<\/p>\n STROGATZ: Oh man, yeah, this is really some mind-blowing stuff.<\/p>\n LEVIN: Yeah. I don\u2019t mean to wake you from your reverie over there.<\/p>\n STROGATZ: Well, I\u2019m just transfixed by trying to understand, honestly, not so much contemplating as listening.<\/p>\n LEVIN: Mmm-hmmm.<\/p>\n STROGATZ: Because I, for example, have some points of confusion that maybe you can clear up for me. When I hear reference to an evolutionary picture, it makes me think that different species, so to speak \u2014 different laws, different universes \u2014 some went extinct, and we\u2019re left with the one that we\u2019re in. But then I also hear him saying that there are many universes that are, he doesn\u2019t use this phrase, but like causally decoupled from ours \u2014 that they don\u2019t interfere with ours. So, are they existing at the same time as ours, or not? Have they gone out of existence? What is the claim?<\/p>\n LEVIN: So let me try and I\u2019m not sure if I\u2019m going to be projecting, but I think he would say that\u2019s just not the right way to think about it.<\/p>\n STROGATZ: OK.<\/p>\n We shouldn\u2019t think of what we call fundamental laws of physics and laws of biology as ontologically completely opposite to one another.<\/p>\n LEVIN: One of the things we learn when we\u2019re studying the universe is never pretend you can step outside of space-time and look down on it. That\u2019s so problematic. If you\u2019re looking down on it, you\u2019re in space-time.<\/p>\n STROGATZ: Sure, yeah.<\/p>\n LEVIN: Right? So, so he would say just, you shouldn\u2019t be thinking that way anymore. There\u2019s no meaning to it. The only meaningful approach is to acknowledge that we are within this one universe. This is it. That\u2019s a tree. And by following that tree of life, we get to the origin of life here on Earth. What happened on another planet isn\u2019t part of that story, and we shouldn\u2019t make it part of that story. To think of every single possible combination that could have led to every single possible kind of animal, kind of plant, kind of kingdom in the living world, and then somehow play some random game, we\u2019re just a random toss of the coin in this infinite sea. That\u2019s really not how we think.<\/p>\n Now as we go back and back, okay yeah, these are the laws of physics that we have that are unified but then we might get to a point where there is no such thing as the laws of physics and where all of those possibilities are kind of existing there in the same way it might have in the primordial oceans here on Earth. He\u2019s sort-of trying to suggest that we stop talking about what\u2019s out there, quote unquote, in some spatially causally, temporally, disconnected multiverse.<\/p>\n STROGATZ: Mmm-hmmm. So, but when we go back that far to the place where things start to dissolve \u2014 \u201cplace\u201d\u2014 I guess it\u2019s hard to use the words, right? What words to use, place and time. I\u2019m not even supposed to talk about that language anymore, but I don\u2019t know what, I\u2019m getting stymied here<\/p>\n LEVIN: Well, no, but I think he would say, yes, exactly. You\u2019re getting stymied.<\/p>\n STROGATZ: Yes, grasshopper.<\/p>\n LEVIN: Now you\u2019re getting it. Because why are we asking questions that are so rooted in a kind of language and reality that ceases to exist back there in the very origin of the universe? So our questions are faulty, and we have to begin to kind-of give in to this very quantum nature, and I don\u2019t know what happens then.<\/p>\n But maybe we should take a breather and head into a break, and let Thomas catch us up again. So, more from Thomas Hertog in just a minute.<\/p>\n [Music plays]<\/p>\n LEVIN: Welcome back to The Joy of Why, we\u2019ve been speaking with theoretical physicist Thomas Hertog.<\/p>\n So, when you\u2019re going back to thinking about the very early universe, there\u2019s no longer this realm that you and I are familiar with, where things are behaving according to our intuitions, but rather it\u2019s been taken over by this quantum kind of realm. So you\u2019re tracing time backwards towards a singularity, but you seem to want to say that as you go back, you actually now have to imagine that you\u2019ve lost time as a dimension entirely, that the universe isn\u2019t three spatial dimensions and one time, you\u2019ve gone down a dimension.<\/p>\n HERTOG: So, if you insist on following Einstein\u2019s classical relativity, and you go back in time like Lema\u00eetre did in the thirties, you hit that singularity. Infinite curvature, breakdown of science. Stephen\u2019s idea in the eighties was indeed, but wait a minute, perhaps before you hit that singularity, quantum uncertainty becomes important even at the level of space and time. And because of this, maybe space and time \u2014 two very distinct, clear things in our current universe \u2014 maybe they get intermingled in a way. Maybe an interval in space is sometimes an interval in time. And then, I guess, the act of genius from Stephen and Jim was, ah, but let\u2019s try to model this.<\/p>\n LEVIN: This is Jim Hartle?<\/p>\n HERTOG: Right, yes, so Hartle and Hawking then said let\u2019s try to model this deep fundamental quantum mechanical thing happening in the beginning. Alright, how do we model this? Then they took inspiration from earlier work, in part by them and by others in the seventies, which had shown that geometries that are purely spatial, that have no time dimension, give us access to the underlying quantum aspects of space-time. And in the seventies, the discovery was made that black holes radiate, that quantum processes are responsible for this, and that this radiation could be captured by purely spatial geometries. And that was the inspiration to try that out on the Big Bang as well.<\/p>\n And so the idea came, let\u2019s try to model the origin of time by a purely spatial geometry, which has no time built in. And so you get these very bizarre, geometric pictures, in which the birth of our universe is very much a purely spatial affair which then gradually morphs a geometry in which a clear time dimension gradually emerges. And so that was the birth of their model.<\/p>\n Our hypothesis is the beginning of some sort of marriage between Darwin\u2019s fundamental idea of evolution and\u2026 Newton\u2019s great idea that you should be able to mathemize the world.<\/p>\n But I think the intuitive understanding of this strange geometry is the idea that when we go back far into the Big Bang, even space and time lose their meaning and get intermingled. And quantum uncertainty takes over.<\/p>\n So, you might wonder, in a way, where are all these other universes in the multiverse? I think Stephen\u2019s theory is saying they\u2019re lost in total uncertainty. They may or may not exist, but from our perspective as observers in this universe, we cannot know. And that\u2019s the crucial difference between our cosmology and the one from a God\u2019s eye view that we discussed earlier.<\/p>\n LEVIN: So in a way, this philosophical shift, this paradigm shift stops you from asking silly questions about the probability of us existing in an infinite multiverse.<\/p>\n HERTOG: Exactly. It stops you from wanting to know too much.<\/p>\n LEVIN: It imparts some humility. Now, you\u2019re describing these brilliant ideas from Jim Hartle and Hawking back in the seventies, but your final theory that you worked on with Stephen is new and it\u2019s based in newer ideas like the holographic principle. Can you explain how you\u2019ve sort-of revised these very early universe stories by using a holographic principle for time?<\/p>\n HERTOG: There are two reasons why the old theory of Jim and Stephen failed. The first reason is the one we discussed. Jim and Stephen in the eighties did not quite take this sort-of observer\u2019s viewpoint from within. I think they thought of their model really as a model of creation, as some sort of preexisting, transcendent law, and this is how it\u2019s going to be. So that was a shift.<\/p>\n A thoroughly quantum interpretation of that model puts you as observer inside, and makes you work backwards in time. But then, around 2010, the criticism was, ah, but this is a choice. Surely, I can come up with a better theory that gives me this a priori explanation. We started to develop that theory further, using new ideas from string theory and holography. The observer\u2019s viewpoint from within is built in from the start. You can\u2019t avoid it. And that left a very strong impression on Stephen.<\/p>\n Holography is a new way of thinking about quantum gravity. And as the name suggests, one of the dimensions is truly an emergent dimension. The holographic way of thinking about black holes, for instance, will resemble very much a physical picture of a black hole, in which the degrees of freedom are located on the surface. It won\u2019t directly speak of the interior of a black hole. The interior of a black hole is an emergent quality, and it goes as far as it goes in this way of thinking.<\/p>\n LEVIN: So, it\u2019s a holography, meaning I have a 2D surface that projects a 3D image.<\/p>\n HERTOG: Right.<\/p>\n LEVIN: The black hole\u2019s 2D surface of its event horizon projects an illusion of an interior, in some sense, or of a solid.<\/p>\n HERTOG: Right. That\u2019s the way holography comes about in theoretical physics today. It attempts to make quantum theory and gravity talk to one another, it has emerged that yes, they can talk to one another, they can be compatible. But strangely enough, one appears to be the hologram of the other. The quantum description of a black hole or of the universe appears to live in one dimension less.<\/p>\n And of course, it\u2019s useless in most circumstances, right? Let\u2019s be clear on that. If you want to talk to a gravitational wave astronomer, he\u2019s going to use Einstein\u2019s theory with three dimensions of space and one dimension of time. It\u2019s only if you want to know what happens to a really old black hole, or in the deep interior of a black hole, you might want to switch to that holographic way of thinking. Same in cosmology. A holographic viewpoint is extremely impractical, except of course where the usual thinking in cosmology breaks down at a Big Bang. And strikingly enough, in holographic models of the universe, one of the dimensions pops out. It\u2019s the time dimension. And the time dimension has, of course, been the problematic one since Lema\u00eetre\u2019s discovery of the Big Bang.<\/p>\n If we truly want to understand the origin of time, we want to develop the physical framework in which time is not built in a priori. Holography does exactly that. So, it\u2019s as if your physical reality really simplifies. When you go back to the Big Bang, you lose more and more information encoded in the hologram. And now you begin to see something truly amazing. The Big Bang is going back so far in time that you run out of qubits. You run out of information, and that is resonant with what we were discussing earlier. It\u2019s as if quantum uncertainty becomes so large that it washes out anything you might want to know. This observer\u2019s perspective within the universe and built into that holographic cosmology, seems to carry with it fundamental limitations of what, from this perspective, we can know.<\/p>\n LEVIN: So, we shouldn\u2019t even ask what happened before the Big Bang?<\/p>\n HERTOG: No, it\u2019s stronger than that. I would say that the question itself disappears because this holographic cosmology seems to build in limitations to the questions that are posed. It might not be satisfying, but it does resolve some of the multiverse paradoxes. It\u2019s a little bit like in biology, right? I mean, what laws of biology are left before the origin of life?<\/p>\n LEVIN: Your title actually resides exactly in that tension.<\/p>\n HERTOG: Right.<\/p>\n LEVIN: You have on one side a very specific reference, I believe, to Darwin\u2019s origin of the species. And on the other side, you are using this very abstract quantum, space-time thinking. How did biology come to you as a way to rethink these ideas, to talk about this in terms of a Darwinian-style evolution, or a pressure and selection?<\/p>\n HERTOG: Well, that\u2019s a good question. I discussed this Darwinian analogy a few times with Stephen, but as you can imagine, after dinner, right? Because in our technical work on this stuff, that sort of broader semi-philosophical discussion is present, but in the background.<\/p>\n There\u2019s stuff I believe we cannot know as observers within this universe, and yet it does not mean that science breaks down.<\/p>\n Whereas, if you try to put the big picture together when you\u2019re writing a book, you feel quickly that those broader considerations become more important. Because, of course, the mechanism of variation and selection in the early universe is completely different than what we are familiar with from biology.<\/p>\n But there\u2019s an element of chance, and there\u2019s an element of necessity, operating in these earlier stages as well. These are elements that we are familiar with from quantum dynamics. And then, of course, when it comes to the more epistemic implications of this theory \u2014 namely that the laws themselves, when you go back to the Big Bang, evaporate and disappear \u2014 then, of course, you come very close to the sort of deeper implications of Darwinism in biology. In a sense, I think, I\u2019m trying to advocate that we shouldn\u2019t think of what we call fundamental laws of physics and laws of biology as ontologically completely opposite to one another.<\/p>\n LEVIN: And of course, Stephen\u2019s resting place is between Darwin and Newton.<\/p>\n HERTOG: That\u2019s right. Well, his ashes are buried in Westminster Abbey in London. And are indeed in between the graves of Newton and of Darwin. So somehow, it seems to me, that indeed our hypothesis is the beginning of some sort of marriage between Darwin\u2019s fundamental idea of evolution, and on the other hand, Newton\u2019s great idea that you should be able to mathemize the world.<\/p>\n LEVIN: Now, if the laws of physics, when you go that far back, cease to be, does that mean that running the movie forward again, that the universe could have been a very different way?<\/p>\n HERTOG: I think so, yes. Just like the tree of life could have turned out completely different, the tree of laws of physics could have turned out to be completely different.<\/p>\n LEVIN: And not necessarily hospitable to us.<\/p>\n HERTOG: Right. I think I guess I\u2019m saying also that a law of the origin is also the origin of law. That\u2019s, I think, the thing which was confusing. A true initial condition of the origin of time, I think that\u2019s what we are beginning to understand, and where holography provides, in my view, much more intuition and knowledge.<\/p>\n LEVIN: So, it saves us from asking the wrong questions.<\/p>\n HERTOG: Well, exactly. In a way that\u2019s been the challenge for almost a century, right? What\u2019s the right question? There\u2019s stuff I believe we cannot know as observers within this universe, and yet it does not mean that science breaks down.<\/p>\n I think it\u2019s a refinement of what are the limits of science, and the beauty of quantum theory is that it sort-of tells us what can be asked and what can be known and what cannot be known. So it\u2019s a beautiful story.<\/p>\n LEVIN: You\u2019re not necessarily following where Lema\u00eetre had with religion, but rather you\u2019re saying the science itself is removing this as a viable question?<\/p>\n HERTOG: Exactly. It carries the limits of what can be known within its own self-consistent mathematical framework.<\/p>\n LEVIN: Now, is this unsettling to you that there might be, in some fundamental sense, a limit to knowledge?<\/p>\n HERTOG: Yeah, that\u2019s a very good question. As a young kid, yeah, you would roll into this field thinking, oh we are going to figure out the theory of everything. And I think you\u2019re right that I\u2019m coming to the conclusion that the theory of everything may not be what we thought it was going to be.<\/p>\n But you gain something else. You gain some sort of deeper understanding, and there are other things you can predict. It indeed might not give us final ultimate answers, but it\u2019s a framework that lets us understand better and better the unity in nature. It\u2019s a bit of an epistemic shift, and I haven\u2019t really fully thought this through. But I feel that, okay, giving up on one thing, giving up on ultimate answers, may open the door for fruitful discoveries of another kind.<\/p>\n LEVIN: Hmm, \u200aare you still loving what you\u2019re doing even in the face of that really quite confounding possibility of unknowability?<\/p>\n HERTOG: Yes. Because I somehow find it an exciting shift. I guess I\u2019ve always been a little worried or skeptical about truths that would be purely mathematical independent of our perspective on the universe.<\/p>\n Interestingly, I asked the same question that you just asked to Stephen, and he responded saying, look, it was all well to try to look for a final theory. But suppose we had a final answer. We would stagnate. Whereas now, in this evolutionary picture, we will always have stuff to figure out, in a way. It feels right to me to have deep down the human condition be somewhere in there in our cosmology, in ways yet of course to be discovered and made precise. So yes, I\u2019m not depressed over here in Belgium.<\/p>\n LEVIN: You\u2019re not despairing.<\/p>\n HERTOG: No, I\u2019m okay. So far, I\u2019m okay.<\/p>\n LEVIN: Beautiful, Thomas. Such a pleasure to have you on the show. I so appreciate you taking the time.<\/p>\n HERTOG: Thank you, thank you so much. It was wonderful. Thank you.<\/p>\n [Music plays]<\/p>\n STROGATZ: Well, it sounded to me like the question was being changed from what can we know, to a question like how can we know that there are limits to what we can know? That is a triumph in this framework is explaining why there are things we can\u2019t know. And, in some ways it\u2019s thematically very consistent with lessons of the 20th century, like with G\u00f6edel and other logicians showing us why there are limits to what logical systems can do. And Heisenberg showing us why there are limits to things we can know simultaneously. Even in chaos theory, a very classical subject in a way, it puts limits on what we can predict in certain types of systems.<\/p>\n So, it feels like that\u2019s been the humbling message of the 20th century, that it\u2019s an anti-hubris message. There are limits to what human beings can know and not because of our frailty as people, it\u2019s intrinsic to what it is as observers, uh, or maybe just deep in the structure of the universe itself. And so this seems to be a step in that direction. But maybe I\u2019m making it sound like it\u2019s all continuous with the trend we\u2019ve had for a hundred years. Am I missing something? Is it more radical than that?<\/p>\n LEVIN: I don\u2019t know if it\u2019s more radical than that, but I think if you look at the examples you just gave, they all led to enormous revolutions.<\/p>\n STROGATZ: Yes.<\/p>\n LEVIN: And they led to huge discoveries.<\/p>\n STROGATZ: That\u2019s right.<\/p>\n LEVIN: So, Heisenberg\u2019s uncertainty principle wasn\u2019t just like, ah, darn.<\/p>\n STROGATZ: That\u2019s right.<\/p>\n LEVIN: We\u2019re totally jammed up. It was actually, it led to discoveries.<\/p>\n STROGATZ: Yes, it did.<\/p>\n LEVIN: And now the most widely tested quantum theory, you know, is the mostly widely tested paradigm in the history of science. So, I think what they need is that, right? They need to say, look, the uncertainty or the unknowability has a physical consequence.<\/p>\n STROGATZ: Wouldn\u2019t that be great?<\/p>\n LEVIN: For instance, like dovetailing with the ideas of holography would be great. Because what does holography says? It says kind of we lose a dimension that everything can actually be encoded in a lower dimensional space-time. So maybe we really do lose time and it\u2019s all holographic. And that could actually lead to a concrete way about talking about the early universe that helps us understand. I think that\u2019s the step that is in progress. Maybe we should revisit it.<\/p>\n STROGATZ: Okay, good, good to hear because I\u2019m sure a lot of people will hear like, where\u2019s the science in this?<\/p>\n LEVIN: Right.<\/p>\n STROGATZ: So, it would be nice if we could get something, and of course they know that too, Thomas and Hawking would have felt that way.<\/p>\n LEVIN: Yeah, right. And I think that is the deeper connection that they\u2019re pursuing. So maybe we\u2019re in a moment like a century ago when the ideas were so radical that they really had a hard time before they could have a concrete usable theory, but they do now of quantum mechanics, for instance. So, let\u2019s check in again in a year, a couple years.<\/p>\n STROGATZ: Good.<\/p>\n LEVIN: A hundred maybe.<\/p>\n STROGATZ: What\u2019s time anyway?<\/p>\n LEVIN: Yeah. I don\u2019t know. I\u2019ll see you soon.<\/p>\n STROGATZ: Very good. See you soon.<\/p>\n [Music plays]<\/p>\n STROGATZ: If you\u2019re enjoying The Joy of Why and you\u2019re not already subscribed, hit the subscribe or follow button where you\u2019re listening. You can also leave a review for the show. It helps people find this podcast. Find articles, newsletters, videos, and more at quantamagazine.org.<\/p>\n LEVIN: The Joy of Why is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests, or other editorial decisions in this podcast or in Quanta Magazine.<\/p>\n The Joy of Why is produced by PRX productions. The production team is Caitlin Faulds, Livia Brock, Genevieve Sponsler and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzalez. Edwin Ochoa is our project manager.<\/p>\n From Quanta Magazine. Simon Frantz and Samir Patel provide editorial guidance with support from Matt Carlstrom, Samuel Velasco, Simone Barr and Michael Kanyongolo. Samir Patel is Quanta\u2019s editor in chief.<\/p>\n Our theme music is from APM Music. The episode art is by Peter Greenwood, and our logo is by Jaki King and Kristina Armitage. Special thanks to the Columbia Journalism School and the Cornell Broadcast Studios. I\u2019m your host, Janna Levin. If you have any questions or comments for us, please email us at [email\u00a0protected]<\/a> Thanks for listening.<\/p>\n","protected":false},"excerpt":{"rendered":"Most cosmologists agree that our universe had a beginning. 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