Why Did The Universe Begin?

Most cosmologists agree that our universe had a beginning. But the finer details about the Big Bang remain a mystery. A history of everything would explain all, or so theoretical physicists hoped. In his final years, Stephen Hawking working with Thomas Hertog proposed a striking idea: The laws of physics were not precisely determined before the Big Bang; they evolved as the universe evolved. In this episode of The Joy of Why, Hertog speaks with co-host Janna Levin about his work and partnership with Hawking. Hertog, now at KU Leuven in Belgium, explains why they rejected the popular multiverse theory and instead explored the idea that the universe’s properties are a result of cosmological natural selection. According to Hertog and Hawking, these properties must be viewed through the lens of human observers, who are also the consequence of natural selection. So, how could the universe have created the conditions needed for life to emerge? Listen to the episode below to find out.

• 0:00Speaker 00

  Hi, I'm Terry Tao. My new book, Six Math Essentials, explores six concepts that have guided mathematicians like me throughout history. With a simple introduction to numbers, algebra, geometry, probability, analysis, and dynamics, I share how these ideas helped me think mathematically and can help everyone better understand our complex world. Mathematics isn't magic. It's a way of thinking anyone can learn. Please visit quantobooks.org to learn more.

• 0:31Speaker 01

  I'm Janna Levin.

• 0:32Speaker 02

  And I'm Steve Strogatz.

• 0:33Speaker 01

  And this is The Joy of Why, a podcast from Quantum Magazine, exploring some of the biggest unanswered questions in math and science today.

• 0:44Speaker 01

  Hey, Steve.

• 0:45Speaker 02

  Hi, Jana.

• 0:45Speaker 01

  We have a small little topic today, depending on how you look at it. Little in that we're looking back when the universe was maybe very small. Not so small in terms of profundity. We're going to be talking about the origin of the universe.

• 1:00Speaker 02

  Okay, small topic, I see.

• 1:02Speaker 01

  Yes, just that little topic. And it's interesting because there's kind of a leaning in this conversation towards why. Why did the universe begin? Or why is it the way it is? And that's actually, even though our title is The Joy of Why, not usually how people approach the Big Bang, by asking why.

• 1:21Speaker 02

  I don't even understand the question yet. This sounds interesting.

• 1:24Speaker 01

  Well, I have this 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? And they worked together through a lot of different phases of Stephen's life and well-being. 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?

• 1:58Speaker 02

  Uh huh. You're not going in an anthropic principle direction, are you?

• 2:02Speaker 01

  Well, as I understand it, Stephen was horrified by the multiverse, by this sort of anthropic approach. And yet, he couldn't deny that there was something there. And so, no, they're not exactly going to be anthropic, which would kind of say, from this God's eye view, all of these universes happen.

• 2:25Speaker 01

  It's just a matter of which one we land in.

• 2:27Speaker 02

  Yeah, yeah.

• 2:28Speaker 01

  Right? In a way, Hawking was rejecting that.

• 2:31Speaker 02

  Okay.

• 2:32Speaker 01

  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.

• 2:44Speaker 02

  Okay. It almost reminds me of Descartes. Like, the one thing you can rely on is, I think, therefore I am. So in this case, we are here observing the universe. We know at least that.

• 2:57Speaker 01

  Yeah, we know we're here. You know, in the multiverse, thinking everything that can happen, happens. And you're just sort of saying, well, I guess I'm just in the one that was viable for life. This is a little different. It's 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 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,

• 3:41Speaker 01

  Stephen Hawking's 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'm wondering, I know we call this the joy of why, but what you think of framing a why question with something like the Big Bang?

• 3:58Speaker 02

  You'd think, of course, I'm in favor of it since we're on this show.

• 4:03Speaker 02

  But it really runs counter to the way we're brought up in science. These teleological questions, as they're sometimes disparagingly referred to, like, that hearkens back to Aristotle and all. You're not supposed to ask the why question, 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 why. But with matter and the universe and energy and spacetime, it's hard to even understand what why could mean.

• 4:34Speaker 01

  Yeah. Actually, it's 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, is the argument, begins to disintegrate. Space and time become harder and harder to distinguish from each other. In some sense, maybe time itself becomes a concept we can no longer really rely on. And he says there's 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 as physical laws themselves don't even apply.

• 5:16Speaker 02

  Nice. Interesting.

• 5:17Speaker 01

  And so he's sort of saying, you know what? That why question dissolves with it.

• 5:23Speaker 02

  Ooh.

• 5:24Speaker 01

  Yeah.

• 5:25Speaker 02

  Okay.

• 5:27Speaker 01

  I don't know if I'm doing him justice. So maybe we should turn this to Thomas himself. Here's professor of theoretical physics at Belgium's University of Leuven, Thomas Herthak. Welcome to the Joy of Why, Thomas. We're so pleased to have you on the show. We haven't spoken in a while.

• 5:49Speaker 03

  Thank you, Jenna.

• 5:51Speaker 01

  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?

• 5:59Speaker 03

  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.

• 6:43Speaker 01

  So over the next 20 years, you're working together really until the end of his life?

• 6:47Speaker 03

  On and off, yeah.

• 6:49Speaker 01

  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 that in the final years?

• 7:06Speaker 03

  Right, there was a big change in the 90s. He had his computer, he was speaking fluidly, typing out sentences. And that's what I think in the end kept us going. The fact that we had the opportunity in the late 90s, 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's almost impossible to work with him later in the last five, six years of his life. 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,

• 7:51Speaker 03

  and that's what then kicks in. And these are wonderful collaborations. So indeed, we developed a truly bizarre mix of verbal and non-verbal and other creative ways of communicating.

• 8:07Speaker 01

  Hmm. Now, were you coming straight from Belgium when you came to Cambridge?

• 8:11Speaker 03

  Yes.

• 8:12Speaker 01

  What made you decide to come to Cambridge, to leave home?

• 8:17Speaker 03

  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 these pioneering works of Georges Le Metteur, this Belgian priest astronomer who came up with the idea of a big bang. But that all died out by the time I grew up. And someone told me, well, if you're interested in cosmology, you should go to Cambridge. So it was known at the time as a mecca for cosmology, I would say.

• 8:54Speaker 01

  So you were 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?

• 9:05Speaker 03

  I'm 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 the 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. To me, Stephen was really someone who stuck to the scientific method, who stuck fairly close to theories and of course, this was also the late 90s.

• 9:50Speaker 03

  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. 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, Jana.

• 10:24Speaker 01

  It happens to the best of us.

• 10:26Speaker 03

  I think so, yeah.

• 10:28Speaker 01

  So I want to ask about this epigraph in your book, "The question of origin hides the origin of the question." I believe that's from a Belgian poet.

• 10:36Speaker 03

  Well, I had to have some Belgian touch to my writings. François Jacques Maire is the poet's name.

• 10:45Speaker 01

  How do you reflect on that epigraph now?

• 10:47Speaker 03

  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.

• 11:19Speaker 03

  And that's also how my work with Hawking started out, right? I mean, many times in physics, we were driven to this paradigm shift, to turning cosmology inside out by necessity, because the rest didn't work. 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. 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.

• 12:04Speaker 03

  That's causality, that's dynamics. It's all built into the framework we use. But one might wonder that with the discovery of the Big Bang, the origin of time, we might wonder whether we haven't 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. So that's the kind of provocative question I asked at the start of my book. And frankly, it's the kind of question that was with Stephen and me for many years

• 12:48Speaker 03

  on the background track.

• 12:50Speaker 01

  You describe 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're uncomfortable with the fact that it seems to be so finely tuned for our home?

• 13:13Speaker 03

  Yeah, I do not believe, and Stephen certainly did not believe, that there was an actual designer or a god 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's as if the universe was destined to bring forth life at some point. 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's the kind of thing which was the overarching question behind our collaboration.

• 13:58Speaker 01

  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're the only ones. We're sure are here. That doesn't seem very finely tuned for life.

• 14:20Speaker 03

  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. But you're 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,

• 15:04Speaker 03

  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's a serious bottleneck. Because if there isn't a bottleneck in between the laws of physics and, say, our sort of level of complexity, then your observation that we seem to be rare is scary, yes? People, like 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.

• 15:43Speaker 01

  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't have found a way?

• 16:04Speaker 03

  I'm 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't need to be even very large, have such drastic implications that you often don't end up with stable atoms or stable solar systems. And once you're in that realm, I'm pretty sceptical some high-level complexity could flourish and emerge. That's my feeling.

• 16:38Speaker 01

  So how do we go back and understand the beginning? We're looking back 13.8 billion years. How do you do such a thing? Roll back time?

• 16:46Speaker 03

  Well, that's the advantage of theoretical physics, right?

• 16:49Speaker 01

  You do it on pen and paper.

• 16:50Speaker 03

  Exactly. You roll back time using pen and paper. So you start with the laws of physics as we've discovered them here on Earth, and you extrapolate them, and you use Einstein's 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's theory of gravity, you run into serious problems early on. In fact, blindly following Einstein's theory of space, time, and gravity, his theory of relativity, would lead you to conclude that the actual beginning, the Big Bang, is a singularity that lies outside time.

• 17:27Speaker 01

  And by a singularity, you mean infinite energy, infinite curvatures, infinite unpredictability?

• 17:34Speaker 03

  Right. So Einstein's 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's theory doesn't tell you then what's going on. That's the whole subject of early universe cosmology, which attempts to combine Einstein's theory with ideas from quantum theory.

• 18:02Speaker 01

  Even though it's derived from his theory, Einstein did not predict the Big Bang, did he?

• 18:07Speaker 03

  He did not. We claim this is a Belgian discovery, like beer and french fries. But yeah, curiously, Georges Lemaître was younger than Einstein and became a real expert working with Einstein's 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 sort of trying to find an explanation for that. And so he discovered these time-dependent, expanding universes as solutions of Einstein's theory, which seemed to him a better match to the observations at the time

• 18:53Speaker 03

  than Einstein's eternal, static universe. But then, of course, comes the real difficulty. Lemaître did exactly what you were alluding to earlier. He used Einstein's equation, turned around time, went backwards, and saw that the whole thing crunched in the beginning. Lemaître 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. 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't really expect this primeval atom

• 19:37Speaker 03

  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's a little bit how, in the end, Stephen's theory played out. In a way, Stephen's model, I view a little bit as some sort of geometric realization of Lemaître's idea of a primeval atom.

• 20:00Speaker 01

  Let's come to that. We're now post-Einstein. It's a shock to Einstein, who wanted an eternal universe, that in fact, the universe is expanding, not permanent, not static, even though these are predictions from his own theory. 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't sit well in Le Matre's, Einstein's, Hawking's, yours. And so you've returned to this question of going back further and further, past the stuff we believe to be true, the hot primordial soup,

• 20:45Speaker 01

  the expansion in an energetic event, to that initial moment. Tell me, what were some of the other possibilities that were floated to deal with this unsettling initial singularity?

• 20:57Speaker 03

  It's unsettling because it doesn't fit in any kind of framework for physics we know and love, right? In physics, we come up with laws of dynamics, laws that describe the evolution. That's, 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's 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're quickly led to the idea,

• 21:42Speaker 03

  well, maybe there was some sort of evolution prior to the Big Bang. Maybe Einstein's singularity isn't 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's the idea of a multiverse.

• 22:05Speaker 01

  So in the multiverse, our Big Bang isn't exactly a singularity. It's like a plume off of a larger spacetime?

• 22:13Speaker 03

  Right, right, like an explosion inside a bigger space. I tend to be skeptical about any pre-Big Bang era. I've been...

• 22:26Speaker 03

  Inclined to take the origin of time seriously, in part, of course, influenced by Stephen's intuition. The multiverse is a different thing.

• 22:35Speaker 01

  Yeah, didn't he say the multiverse was outrageous?

• 22:38Speaker 03

  Yes.

• 22:39Speaker 01

  And didn't you?

• 22:40Speaker 03

  Yes.

• 22:40Speaker 01

  And you've said in that spirit that it's important to consider cosmology from a human perspective, rather than relying on this overarching God's eye view.

• 22:52Speaker 03

  The multiverse is some sort of the supreme version of taking a God's eye view on reality, right? It started with Copernicus, the solar system, or Galileo. It is the big discovery of the scientific revolution four centuries ago.

• 23:05Speaker 01

  So you're citing two successful versions of taking the God's eye view.

• 23:09Speaker 03

  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. It's 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're looking down onto all possible universes from some sort of abstract, mathematical, absolute viewpoint.

• 23:41Speaker 01

  And what's wrong with that?

• 23:42Speaker 03

  One thing that goes wrong with that is from such a distant viewpoint, imagine you have a theory of the multiverse, 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've taken yourself so far out that you've 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

• 24:27Speaker 03

  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.

• 24:34Speaker 01

  Now, you also quote Stephen as saying, "We are not angels who view the universe from the outside." So what is this interplay between reality and observation that you're suggesting?

• 24:45Speaker 03

  Reality.

• 24:47Speaker 01

  Well, OK, if that's too strong a word. The universe we find ourselves in.

• 24:53Speaker 03

  Yeah, yeah, yeah, yeah, yeah.

• 24:54Speaker 01

  And the role of the observer, who is, in fact, a product of the specific cosmos we find ourselves in.

• 25:00Speaker 03

  So if I don't want to start with a reality with all sorts of universes in it, and then figure out in which one I am, I want to turn this inside out and distill a definite, concrete history based on our observational situation.

• 25:22Speaker 03

  So in a way, what we were trying to do is to get perspective, so to speak, because, as you say, it doesn't need to be a human observer. It could be a photon, right? Without ruling the scientific method, ruling the falsifiability, which we know and love in physics. 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.

• 26:07Speaker 03

  I'm advocating that the laws of physics, as we know them in our universe, are themselves the result of an early evolution, a sort of a tree of law, which in the very first stages of the universe took form. And I'm advocating against the idea that there is some sort of grand equation describing all possible universes with all possible physical laws written in stone.

• 26:39Speaker 01

  So in some sense, instead of suggesting there's a multiverse of possibilities where every option will be tried by nature, you're saying all of that could be happening in the very early universe? And that there is some kind of process of selection to guide the universe that we find ourselves in thereafter?

• 27:00Speaker 03

  Right, but in a sense, that selection is something which act retrospectively.

• 27:06Speaker 03

  We find ourselves in the universe around us and we try to reconstruct our history. I'm not saying there couldn't have been any other universes, but the big difference is that in the multiverse cosmology, all these universes are somehow taught to be out there and they play a role in trying to figure out what we should predict. In our framework, all these universes might or might not be out there, but they don't enter into the predictive scheme that we try to develop. Just like there might be life on another planet, but it doesn't interfere with biology on this planet. So I'm trying to advocate a more humble viewpoint, I guess,

• 27:49Speaker 03

  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,

• 28:06Speaker 03

  the laws, as we know, then merge. The different particle forces and particle species lose their specificity. That's this whole idea of unification. It's been tested to some extent. And I guess we've 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's the crux of his hypothesis. And the unsettling thing, of course, is that the Big Bang,

• 28:47Speaker 03

  the origin of time, would also become the origin of law. The laws themselves would sort of evaporate, going all the way backwards. So I guess I'm advocating a fundamentally evolutionary understanding of what we call the laws of physics.

• 29:08Speaker 01

  Steve, you are leaning back contemplating.

• 29:11Speaker 02

  Yeah, this is really some mind blowing stuff.

• 29:14Speaker 01

  Yeah, I don't mean to wake you from your reverie over there.

• 29:18Speaker 02

  Well, I'm just transfixed by trying to understand honestly, not so much contemplating as listening. 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, different laws, different universes, someone extinct and we're left with the one that we're in. But then I also hear him saying that there are many universes that are, he doesn't use this phrase, but like causally decoupled from ours, that they don't 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?

• 30:02Speaker 01

  So let me try. And I'm not sure if I'm going to be projecting, but I think he would say that's just not the right way to think about it.

• 30:09Speaker 02

  Okay.

• 30:09Speaker 01

  One of the things we learn when we're studying the universe is never pretend you can step outside of spacetime and look down on it. That's so problematic. If you're looking down on it, you're in spacetime.

• 30:21Speaker 02

  Sure.

• 30:22Speaker 01

  Right? So, so he would say, just, you shouldn't be thinking that way anymore.

• 30:26Speaker 02

  Okay.

• 30:26Speaker 01

  There's no meaning to it. The only meaningful approach is to acknowledge that we are within this one universe. This is it. That's 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't part of that story and we shouldn't 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, oh, we're just a random toss the coin in this infinite sea, that's really not how we think. Now, as we go back and back, okay, yeah, these are the laws of physics that we have that are unified.

• 31:06Speaker 01

  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's sort of trying to suggest that we stop talking about what's out there, quote unquote, in some spatially, causally, temporally disconnected multiverse.

• 31:32Speaker 02

  Mm hmm. So but when we go back that far to the place where things start to dissolve, place, I guess, it's hard to use words, right? What words to use? Place and time, I'm not even supposed to talk about that language anymore. But I don't know what I'm getting stymied here.

• 31:48Speaker 01

  Well, no, I think he would say, yes, exactly. You're getting stymied.

• 31:51Speaker 02

  Yes, grasshopper.

• 31:52Speaker 01

  Now you're 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't know what happens then, but maybe we should take a breather and let Thomas catch us up again. So more from Thomas Hertog in just a minute.

• 32:50Speaker 01

  Welcome back to the Joy of Why. We've been speaking with theoretical physicist Thomas Hertog.

• 32:57Speaker 01

  So when you're going back to thinking about the very early universe, there's no longer this realm that you and I are familiar with, where things are behaving according to our intuitions, but rather it's been taken over by this quantum kind of realm. So you're tracing time backwards towards the singularity, but you seem to want to say that as you go back, you actually now have to imagine that you've lost time.

• 33:22Speaker 03

  Yeah.

• 33:23Speaker 01

  As a dimension entirely, that the universe isn't three spatial dimensions and one time. You've gone down a dimension.

• 33:31Speaker 03

  So if you insist on following Einstein's classical relativity and you go back in time, like Le Maitre did in the 30s, you hit that singularity. Infinite curvature, breakdown of science. Stephen's idea in the 80s was indeed, but wait a minute. Perhaps before you hit that singularity, quantum uncertainty becomes important even at a level of space and time. And because of this, maybe space and time, two very distinct, clear things in our current universe, maybe they're intermingled in a way. Maybe an interval in space is sometimes an interval in time. And then I guess the act of genius of Stephen and Jim was, ah, but let's try to model this.

• 34:15Speaker 01

  This is Jim Hardo.

• 34:16Speaker 03

  Right, yes. So Hartle and Hauking then said, let's try to model this deep fundamental quantum mechanical thing happening in the beginning. All right, how do we model this? Then they took inspiration from earlier work, in part by them and by others in the 70s, which had shown that geometries that are purely spatial, that have no time dimension, give us access to the underlying quantum aspects of spacetime. And in the 70s, the discovery was made that black holes radiate, that quantum processes are responsible for this,

• 34:58Speaker 03

  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. And so the idea came, let's 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 in a geometry in which a clear time dimension gradually emerges. And so that was the birth of their model.

• 35:39Speaker 03

  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. So you might wonder, in a way, where are all these other universes in the multiverse? I think Stephen's theory is saying they're lost in total uncertainty. They may or may not exist, but from our perspective as observers, in this universe, we cannot know. And that's the crucial difference between our cosmology and the one from a God's eye

• 36:24Speaker 03

  view that we discussed earlier.

• 36:26Speaker 01

  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.

• 36:37Speaker 03

  Exactly. It stops you from wanting to know too much.

• 36:41Speaker 01

  It imparts some humility. Now, you're describing these brilliant ideas from Jim Hartle and Hawking back in the 70s, but your final theory that you worked on with Stephen is new, and it's based on newer ideas like the holographic principle. Can you explain how you've sort of revised these very early universe stories by using a holographic principle for time?

• 37:02Speaker 03

  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 80s did not quite take this sort of observer's viewpoint from within. I think they thought of their model really as a model of creation. As some sort of pre-existing transcendent law and this is how it's gonna be. So that was the shift. 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."

• 37:47Speaker 03

  We started to develop that theory further using new ideas from string theory and holography. The observer's viewpoint from within is built in from the start, you can't avoid it. And that left a very strong impression on Stephen. 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't directly speak of the interior of a black hole.

• 38:32Speaker 03

  The interior of a black hole is an emergent quality and it goes as far as it goes in this way of thinking.

• 38:39Speaker 01

  So it's a holography, meaning I have a 2D surface that projects a 3D image?

• 38:44Speaker 03

  Right.

• 38:45Speaker 01

  The black hole's 2D surface of its event horizon projects an illusion of an interior in some sense, or of a solid?

• 38:53Speaker 03

  Right. That's the way holography comes about in theoretical physics today. In 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. And of course, it's useless in most circumstances, right? Let's be clear on that. If you wanna talk to a gravitational wave astronomer, he's gonna use Einstein's theory with three dimensions of space and one dimension of time. It's only if you wanna know what happens to a really old black hole or in the deep

• 39:34Speaker 03

  interior of a black hole, you might wanna 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 the Big Bang. And strikingly enough, in holographic models of the universe, one of the dimension pops out. It's the time dimension. And the time dimension has, of course, been the problematic one since Le Matre's discovery of the Big Bang. If we truly want to understand the origin of time,

• 40:16Speaker 03

  we want to develop the physical framework in which time is not built in a priori. Holography does exactly that. So it's 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's as if quantum uncertainty becomes so large that it washes out anything you might want to know. This observer's perspective

• 41:00Speaker 03

  within the universe and built into that holographic cosmology seems to carry with it fundamental limitations of what, from this perspective, we can know.

• 41:14Speaker 01

  So we shouldn't even ask what happened before the Big Bang?

• 41:18Speaker 03

  No, it's stronger than that. I would say that the question itself disappears because this holographic cosmology seems to build in limitations to

• 41:30Speaker 03

  the questions that are posed. It might not be satisfying, but it does resolve some of the multiverse paradoxes. It's a little bit like in biology, right? I mean, what laws of biology are left before the origin of life?

• 41:45Speaker 01

  Your title, actually, resides exactly in that tension.

• 41:50Speaker 03

  Right.

• 41:50Speaker 01

  You have, on one side, a very specific reference, I believe, to Darwin's Origin of the Species. And on the other side, you're using this very abstract quantum spacetime 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?

• 42:14Speaker 03

  Well, that's 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. Whereas if you try to put the big picture together when you're 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.

• 42:54Speaker 03

  But there's an element of chance and there's 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, namely that the laws themselves, when you go back to the Big Bang, evaporate and disappear, then, of course, you come very close to the sort of deeper implications of Darwinism in biology. In a sense, I think, I'm trying to advocate, that we shouldn't think of what we call fundamental laws of physics and laws of biology as ontologically completely opposite to one another.

• 43:34Speaker 01

  Then, of course, Stephen's resting place is between Darwin and Newton.

• 43:38Speaker 03

  That's 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's fundamental idea of evolution and on the other hand Newton's great idea that we should be able to metamorphose the world.

• 44:06Speaker 01

  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?

• 44:17Speaker 03

  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.

• 44:26Speaker 01

  And not necessarily hospitable to us.

• 44:29Speaker 03

  Right. I think, I guess I'm saying also that a law of the origin is also the origin of law. That's, I think, the thing which was confusing. A true initial condition of the origin of time. I think that's what we're beginning to understand. And very holography provides, in my view, much more intuition and knowledge.

• 44:49Speaker 01

  So it saves us from asking the wrong questions.

• 44:53Speaker 03

  Well, exactly. In a way, that's been the challenge for almost a century, right? What's the right question? There's stuff, I believe, we cannot know as observers within this universe. And yet it does not mean that science breaks down. I think it's 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's a beautiful story.

• 45:26Speaker 01

  You're not necessarily following where Le Maitre had with religion, but rather you're saying the science itself is removing this as a viable question.

• 45:36Speaker 03

  Exactly. It carries the limits of what can be known within its own self-consistent mathematical framework.

• 45:43Speaker 01

  Now, is this unsettling to you that there might be, in some fundamental sense, a limit to knowledge?

• 45:49Speaker 03

  Yeah, that's a very good question. As a young kid, yeah, you roll into this field thinking, well, we're gonna figure out the theory of everything. And I think you're right that I'm coming to the conclusion that the theory of everything may not be what we thought it was gonna be. 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's a framework that lets us understand better and better the unity in nature. It's a bit of an epistemic shift, and I haven't 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.

• 46:34Speaker 01

  Hmm. Are you still loving what you're doing, even in the face of that really quite confounding possibility of unknowability?

• 46:42Speaker 03

  Yes, because I somehow find it an exciting shift. I guess I've always been a little worried or skeptical about truths that would be purely mathematical, independent of our perspective on the universe. 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.

• 47:24Speaker 03

  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'm not depressed over here in Belgium.

• 47:43Speaker 01

  You're not despairing?

• 47:45Speaker 03

  No, I'm okay. So far, I'm okay.

• 47:47Speaker 01

  Beautiful, Thomas. Such a pleasure to have you on the show. I so appreciate you taking the time.

• 47:53Speaker 03

  Thank you. Thank you so much. It was wonderful. Thank you.

• 47:58Speaker 02

  Well, it sounded to me like the question was being changed from "what can we know?" to a question like...

• 48:08Speaker 02

  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't know. And in some ways, it's thematically very consistent with lessons of the 20th century, like with Gödel 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. So it feels like that's been the humbling message of the 20th century, that it's an anti-hubris message.

• 48:51Speaker 02

  There are limits to what human beings can know, and not because of our frailty as people. It's intrinsic to what it is as observers, 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'm making it sound like it's all continuous with the trend we've had for 100 years. Am I missing something? Is it more radical than that?

• 49:12Speaker 01

  I don't know if it's more radical than that, but I think if you look at the examples you just gave, they all led to enormous revolutions.

• 49:20Speaker 02

  Yes.

• 49:20Speaker 01

  And they led to huge discoveries.

• 49:23Speaker 02

  That's right.

• 49:23Speaker 01

  So Heisenberg's uncertainty principle wasn't just like, ah, darn.

• 49:29Speaker 02

  That's right.

• 49:29Speaker 01

  We're totally jammed up. It was actually, it led to discoveries.

• 49:34Speaker 02

  Yes, it did.

• 49:35Speaker 01

  And now the most widely tested quantum theory is the most 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 actually has a physical consequence.

• 49:50Speaker 02

  Wouldn't that be great?

• 49:51Speaker 01

  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 spacetime. So maybe we really do lose time, and it's all holographic, and that could actually lead to... A concrete way about talking about the early universe that helps us understand. I think that's the step that is in progress.

• 50:16Speaker 02

  Okay.

• 50:17Speaker 01

  Maybe we should revisit.

• 50:18Speaker 02

  Good to hear, because I'm sure a lot of people will hear, like, where's the science in this?

• 50:22Speaker 01

  Right.

• 50:23Speaker 02

  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.

• 50:29Speaker 01

  Right, and I think that is the deeper connection that they're pursuing. So maybe we're 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's check in again in a year, a couple of years.

• 50:48Speaker 02

  Good.

• 50:49Speaker 01

  A hundred, maybe.

• 50:50Speaker 02

  What's time, anyway?

• 50:52Speaker 01

  Yeah, I don't know. I'll see you soon.

• 50:55Speaker 02

  Very good. See you soon.

• 50:58Speaker 02

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• 51:15Speaker 01

  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. The Joy of Why is produced by PRX Productions. The production team is Caitlin Foltz, Levin Brock, Genevieve Sponsler, and Merritt Jacob. The executive producer of PRX Productions is Joselyn Gonzalez. Edwin Ochoa is our project manager. From Quanta Magazine, Simon France and Samir Patel provide editorial guidance with support

• 51:57Speaker 01

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• 52:37Speaker 01

  From PRX.