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.

Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.

Transcript

[Music plays]

JANNA LEVIN:  I’m Janna Levin

STEVE STROGATZ: And I’m Steve Strogatz.

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.

Hey Steve.

STROGATZ: Hi Janna.

LEVIN: We have a small, little topic today, depending on how you look at it.

STROGATZ: Yeah?

LEVIN: 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.

STROGATZ: Okay, small topic. I see.

LEVIN: 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.

STROGATZ: I don’t even understand the question yet. This sounds interesting.

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?

STROGATZ: [Laughs]

LEVIN: And they worked together through a lot of different phases of Stephen’s 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?

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

LEVIN: 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. It’s just a matter of which one we land in. Right? In a way, Hawking was rejecting that.

STROGATZ: Hmm, okay.

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.

STROGATZ: 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.

LEVIN: Yeah. We know we’re here. You know in the multiverse thinking, everything that can happen happens, and you are 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…

STROGATZ: Uh huh.

LEVIN: … 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’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?

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

LEVIN: Right.

STROGATZ: But it really runs counter to the way we’re brought up in science, right? 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, 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 ‘why’. But with matter and the universe and energy and space-time, it’s hard to even understand what ‘why’ could mean.

LEVIN: Yeah. And 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 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’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 is physical laws themselves don’t even apply.

STROGATZ: Nice. Interesting. Wild!

LEVIN: And so he’s sort of saying, you know what? That ‘why?’ question dissolves with it.

STROGATZ: Ooh.

LEVIN: Yeah.

STROGATZ: Okay.

LEVIN: 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 Hertog.

[Music plays]

LEVIN: Welcome to The Joy of Why, Thomas. We’re so pleased to have you on the show. We haven’t spoken in a while.

HERTOG: Thank you, Janna.

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?

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.

LEVIN: So, over the next 20 years, you’re working together, really until the end of his life.

HERTOG: On and off, yeah.

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?

HERTOG: Right. There was a big change. In the nineties, 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 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’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, and that’s 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.

LEVIN: Now, were you coming straight from Belgium, when you came to Cambridge?

HERTOG: Yes.

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

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 Lemaître, 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.

LEVIN:  So, you’re 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?

HERTOG: 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 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.

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.

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.

LEVIN: Happens to the best of us, Thomas.

HERTOG: I think so, yeah.

LEVIN: 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?

HERTOG: Well, I had to have some Belgian touch to my writings. François Jacqmin is the poet’s name.

LEVIN: How do you reflect on that epigraph now?

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’s 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’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 — 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 on the background.

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

HERTOG: 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 forward 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.

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

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.

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, 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 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.

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’t have found a way?

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’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 are in that realm, I am pretty skeptical some high-level complexity could flourish and emerge. That’s my feeling.

LEVIN: 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?

HERTOG: Well, that’s the advantage of theoretical physics, right?

LEVIN: You do it on pen and paper.

HERTOG: 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 science.

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

HERTOG: 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.

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

HERTOG: He did not. We claim this is a Belgian discovery, like beer and French fries. But yeah, curiously, George 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 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 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 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.

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’t 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’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.

LEVIN: 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 Lemaître’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, 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?

HERTOG: 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 are quickly led to the idea, 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.

LEVIN: So, in the multiverse our Big Bang isn’t exactly a singularity. It’s like a plume off of a larger space-time?

HERTOG: Right, right. Like an explosion inside a bigger space. I tend to be skeptical about any pre-Big Bang era. I’ve been inclined to take the origin of time seriously, in part of course influenced by Stephen’s intuition. The multiverse is a different thing.

LEVIN: Yeah, didn’t he say the multiverse was outrageous?

HERTOG: Yes.

LEVIN: And didn’t you?

HERTOG: Yes.

LEVIN: 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.

HERTOG: The multiverse is some sort of the supreme version of taking a God’s 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.

LEVIN: So, you’re citing two successful versions of taking the God’s eye view.

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 — 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.

LEVIN: And what’s wrong with that?

HERTOG: 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 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.

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’re suggesting?

HERTOG: “Reality”… mmm-hmmm.

LEVIN: Well, okay, if that’s too strong a word — 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.

HERTOG: So, if I don’t 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’t 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.

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’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.

LEVIN: So, in some sense, instead of suggesting there’s 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’s some kind of process of selection to guide the universe that we find ourselves in thereafter?

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’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 totally 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, 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’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 — the origin of time — would also become the origin of law. The laws themselves 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.

[Music plays]

LEVIN: Steve, you are leaning back contemplating.

STROGATZ: Oh man, yeah, this is really some mind-blowing stuff.

LEVIN: Yeah. I don’t mean to wake you from your reverie over there.

STROGATZ: Well, I’m just transfixed by trying to understand, honestly, not so much contemplating as listening.

LEVIN: Mmm-hmmm.

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 — different laws, different universes — some went 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?

LEVIN: 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.

STROGATZ: OK.

LEVIN: One of the things we learn when we’re studying the universe is never pretend you can step outside of space-time and look down on it. That’s so problematic. If you’re looking down on it, you’re in space-time.

STROGATZ: Sure, yeah.

LEVIN: Right? So, so he would say just, you shouldn’t be thinking that way anymore. 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, we’re just a random toss of 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 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.

STROGATZ: Mmm-hmmm. So, but when we go back that far to the place where things start to dissolve — “place”— I guess it’s hard to use the 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

LEVIN: Well, no, but I think he would say, yes, exactly. You’re getting stymied.

STROGATZ: Yes, grasshopper.

LEVIN: 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 head into a break, and let Thomas catch us up again. So, more from Thomas Hertog in just a minute.

[Music plays]

LEVIN: Welcome back to The Joy of Why, we’ve been speaking with theoretical physicist Thomas Hertog.

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 a singularity, but you seem to want to say that as you go back, you actually now have to imagine that you’ve lost time as a dimension entirely, that the universe isn’t three spatial dimensions and one time, you’ve gone down a dimension.

HERTOG: So, if you insist on following Einstein’s classical relativity, and you go back in time like Lemaître did in the thirties, you hit that singularity. Infinite curvature, breakdown of science. Stephen’s 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 — two very distinct, clear things in our current universe — 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’s try to model this.

LEVIN: This is Jim Hartle?

HERTOG: Right, yes, so Hartle and Hawking then said let’s 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.

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 a geometry in which a clear time dimension gradually emerges. And so that was the birth of their model.

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 view that we discussed earlier.

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.

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

LEVIN: It imparts some humility. Now, you’re 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’s based in 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?

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’s 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’s going to be. So that was a 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. 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. The interior of a black hole is an emergent quality, and it goes as far as it goes in this way of thinking.

LEVIN: So, it’s a holography, meaning I have a 2D surface that projects a 3D image.

HERTOG: Right.

LEVIN: The black hole’s 2D surface of its event horizon projects an illusion of an interior, in some sense, or of a solid.

HERTOG: Right. That’s 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.

And of course, it’s useless in most circumstances, right? Let’s be clear on that. If you want to talk to a gravitational wave astronomer, he’s going to use Einstein’s theory with three dimensions of space and one dimension of time. It’s 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’s the time dimension. And the time dimension has, of course, been the problematic one since Lemaître’s discovery of the Big Bang.

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

LEVIN: So, we shouldn’t even ask what happened before the Big Bang?

HERTOG: No, it’s 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’s a little bit like in biology, right? I mean, what laws of biology are left before the origin of life?

LEVIN: Your title actually resides exactly in that tension.

HERTOG: Right.

LEVIN: You have on one side a very specific reference, I believe, to Darwin’s 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?

HERTOG: 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.

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.

LEVIN: And of course, Stephen’s resting place is between Darwin and Newton.

HERTOG: 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 you should be able to mathemize the world.

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?

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.

LEVIN: And not necessarily hospitable to us.

HERTOG: 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 are beginning to understand, and where holography provides, in my view, much more intuition and knowledge.

LEVIN: So, it saves us from asking the wrong questions.

HERTOG: 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.

LEVIN: You’re not necessarily following where Lemaître had with religion, but rather you’re saying the science itself is removing this as a viable question?

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

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

HERTOG: Yeah, that’s 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’re right that I’m coming to the conclusion that the theory of everything may not be what we thought it was going to 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.

LEVIN: Hmm,  are you still loving what you’re doing even in the face of that really quite confounding possibility of unknowability?

HERTOG: 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. 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.

LEVIN: You’re not despairing.

HERTOG: No, I’m okay. So far, I’m okay.

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

HERTOG: Thank you, thank you so much. It was wonderful. Thank you.

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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’t know. And, in some ways it’s thematically very consistent with lessons of the 20th century, like with Göedel 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. 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, 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’m making it sound like it’s all continuous with the trend we’ve had for a hundred years. Am I missing something? Is it more radical than that?

LEVIN: 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.

STROGATZ: Yes.

LEVIN: And they led to huge discoveries.

STROGATZ: That’s right.

LEVIN: So, Heisenberg’s uncertainty principle wasn’t just like, ah, darn.

STROGATZ: That’s right.

LEVIN: We’re totally jammed up. It was actually, it led to discoveries.

STROGATZ: Yes, it did.

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.

STROGATZ: Wouldn’t that be great?

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’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. Maybe we should revisit it.

STROGATZ: Okay, good, good to hear because I’m sure a lot of people will hear like, where’s the science in this?

LEVIN: Right.

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.

LEVIN: Yeah, 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 years.

STROGATZ: Good.

LEVIN: A hundred maybe.

STROGATZ: What’s time anyway?

LEVIN: Yeah. I don’t know. I’ll see you soon.

STROGATZ: Very good. See you soon.

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STROGATZ: If you’re enjoying The Joy of Why and you’re not already subscribed, hit the subscribe or follow button where you’re 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.

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.

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.

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’s editor in chief.

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’m your host, Janna Levin. If you have any questions or comments for us, please email us at [email protected] Thanks for listening.