Sara Walker on the Physics of Life, Time, Complexity and Aliens

Sara Walker

You have an origin of life event. It evolves for 4 billion years, at least on our planet. It evolves a technosphere. The technologies themselves start having this property we call life, which is the phase we're undergoing now. It solves the origin of itself and then it figures out how that process all works, understands how to make more life, and then can copy itself onto another planet so the whole structure can reproduce itself.

Sara Walker

I think the question there is about whether life can be described in terms of matter and physical things, or whether there is some other feature that's not physical that actually animates living things. So for a long time, people maybe have called that a soul. It's been really hard to pin down what that is. So I think the vitalist idea is really that it's a dualistic interpretation that there's sort of the material properties, but there's something else that animates life that is there when you're alive and it's not there when you're dead. And materialists don't think that there's anything really special about the matter of life and the material substrates that life is made out of, so they disagree on some really fundamental points.

Sara Walker

Yeah, I think the entire universe is just a giant mystery. I guess that's what motivates me as a scientist. And so oftentimes, when I look at open problems like the nature of life or consciousness or what is intelligence or are there souls or whatever question that we have that we feel like we aren't even on the tip of answering yet, I think we have a lot more work to do to really understand the answers to these questions. So it's not magic, it's just the unknown. And I think a lot of the history of humans coming to understand the world around us has been taking ideas that we once thought were magic or supernatural and really understanding them in a much deeper way that we learn what those things are. And they still have an air of mystery even when we understand them. There's no bottom to our understanding.

Sara Walker

I think that no tradition, vitalists included, is ever fully wrong about the nature of the things that they're describing. So a lot of times when I look at different ways that people have described things across human history, across different cultures, there's always a seed of truth in them. And I think it's really important to try to look for those, because if there are narratives that humans have been telling ourselves for thousands of years, for thousands of generations, there must be some truth to them. We've been learning about reality for a really long time and we recognize the patterns that reality presents us. We don't always understand what those patterns are, and so I think it's really important to pay attention to that. So I don't think the vitalists were actually wrong.

And a lot of what I talk about in the book, but also I think about a lot just professionally, is the nature of our definitions of what's material and how science has come to invent the concept of matter. And that some of those things actually really are inventions that happened in a particular time in a particular technology that could learn about certain patterns and help us understand them, and that there are some patterns we still don't understand. And if we knew how to measure those things or we knew how to describe them in a more rigorous way, we would realize that the material world matter has more properties than we thought that it did. One of those might be associated with the thing that we call life. Life could be a material property and still have a lot of the features that the vitalists thought were mysterious.

Sara Walker

Yeah. I always like to use examples from physics, so I'll probably do that. It's my go-to place. But in the history of gravitational physics, for example, in the history of motion, when Aristotle came up with his theories of motion, he did it by the material properties he thought things had. So there was a concept of things falling to earth because they were solid-like and things raising to the heavens because they were air-like and things moving around the planet because they were celestial-like. But then we came to realize that, thousands of years later and after the invention of many technologies that allowed us to actually measure time in a mechanistic way and track planetary motion and we could roll balls down inclined planes and track that progress, we realized that if we just talked about mass and acceleration, we could unify all motion in the universe in a really simple description.

So we didn't really have to worry about the fact that my cup is heavy and the air is light. The same laws describe them if we have the right material properties to talk about what those laws are actually interacting with. And so I think the issue with life is we don't know how to think about information in a material way, and so we haven't been able to build a unified description of what life is or the kind of things that evolution builds because we haven't really invented the right material concept yet.

Sara Walker

I think life obeys universal principles. I think there is some deep underlying explanatory framework that will tell us about the nature of life in the universe and will allow us to identify life that we can't yet recognize because it's too different.

Sara Walker

All the classic definitions people want to use just don't work. They don't work in all cases. So Carl Sagan had this wonderful essay on definitions of life where I think he talks about aliens coming from another planet. If they saw earth, they might think that cars were the dominant life form because there are so many of them on our planet. Humans are inside them, and you might want to exclude machines. But any definition, classic biology textbook definitions, would also include them. He wanted to draw a boundary between these kind of things by trying to exclude them, but they were naturally included by the definitions people want to give. And in fact, what he ended up pointing out is that all of the definitions of life that we have, whether it's life is a self-reproducing system or life eats to survive or life requires compartments, whatever it is, there's always a counterexample that challenges that definition. This is why viruses are so hard or why fire is so hard. And so we've had a really hard time trying to pin down from a definitional perspective exactly what life is.

Sara Walker

Did you see this in the jungle?

Sara Walker

Yeah, but it can't erase it. It's just restructuring it. And I think the other thing that is really vivid to me about this example that you're giving is how much death is necessary for life. So I worry a bit about notions of immortality and whether immortality is a good thing or not. So I have a broad conception that life is the only thing the universe generates that actually has even the potential to be immortal, but that's as the sort of process that you're describing where life is about memory and historical contingency and construction of new possibilities. But when you look at any instance of life, especially one as dynamic as what you're describing, it's a constant birth and death process. But that birth and death process is the way that the universe can explore what possibilities can exist. And not everything, not every possible human or every possible ant or every possible zombie ant or every possible tree, will ever live. So it's an incredibly dynamic and creative place because of all that death.

Sara Walker

Yeah.

Sara Walker

Right.

Sara Walker

Yeah, it does. I think the thing that's perplexing for me about these kind of examples is effectively the ant's dead, but it's staying alive now because piloted by this fungus. And so that gets back to this thing that we're talking about a few minutes ago about how the boundary of life is really hard to define. So anytime that you want to draw a boundary around something and you say, "This feature is the thing that makes this alive, or this thing is alive on its own," there's not ever really a clear boundary. And these kind of examples are really good at showing that because it's like the thing that you would've thought is the living organism is now dead, except that it has another living organism that's piloting it. So the two of them together are alive in some sense, but they're now in this weird symbiotic relationship that's taking this ant to its death.

Sara Walker

I think we have to get rid of the notion of an individual as being relevant. And this is really difficult because a lot of the ways that we think about life, like the fundamental unit of life is the cell, individuals are alive, but we don't think about how gray that distinction is. So for example, you might consider self-reproduction to be the most defining feature of life. A lot of people do, actually. That's one of these standard different definitions that a lot of people in my field like to use in astrobiology is life as a self-sustaining chemical system capable of Darwinian evolution, which I was once quoted as agreeing with, and I was really offended because I hate that definition. I think it's terrible, and I think it's terrible that people use it. I think every word in that definition is actually wrong as a descriptor of life.

Sara Walker

I know. If you want to make me angry, you can pretend I said that and believed it.

Sara Walker

Yeah, they're all interesting words and together they sound really smart and they sound like they box in what life is. But you can use any of the words individually and you can come up with counterexamples that don't fulfill that property. The self-sustaining one is really interesting, thinking about humans. We're not self-sustaining dependent on societies. And so I find it paradoxical that it might be that societies, because they're self-sustaining units, are now more alive than individuals are. And that could be the case, but I still think we have some property associated with life. That's the thing that we're trying to describe, so that one's quite hard. And in general, no organism is really self-sustaining. They always require an environment, so being self-sustaining is coupled in some sense to the world around you. We don't live in a vacuum, so that part's already challenging.

And then you can go to chemical system. I don't think that's good either. I think there's a confusion because life emerges in chemistry that life is chemical. I don't think life is chemical. I think life emerges in chemistry because chemistry is the first thing the universe builds where it cannot exhaust all the possibilities, because the combinatorial space of chemistry is too large.

Sara Walker

Yes.

Sara Walker

I know. What a coincidence.

Sara Walker

Yeah, but he would agree with that statement.

Sara Walker

Oh, sure.

Sara Walker

Or maybe, I don't know.

Sara Walker

Very precisely. It's not the first thing it creates. Obviously, it has to make atoms first, but it's the first thing. If you think about the universe originated, atoms were made in Big Bang nuclear synthesis, and then later in stars. And then planets formed and planets become engines of chemistry. They start exploring what kind of chemistry is possible. And the combinatorial space of chemistry is so large that even on every planet in the entire universe, you will never express every possible molecule. I like this example actually that Lee gave me, which is to think about Taxol. It has a molecular weight of about 853. It's got a lot of atoms, but it's not astronomically large. And if you try to make one molecule with that molecular formula and every three-dimensional shape you could make with that molecular formula, it would fill 1.5 universes in volume with one unique molecule. That's just one molecule.

So chemical space is huge, and I think it's really important to recognize that because if you want to ask a question of why does life emerge in chemistry, well, life emerges in chemistry because life is the physics of how the universe selects what gets to exist. And those things get created along historically contingent pathways and memory and all the other stuff that we can talk about, but the universe has to actually make historically contingent choices in chemistry because it can't exhaust all possible molecules.

Sara Walker

Oh, if it's not chemical. So I think some of the things that have evolved on our biosphere I would call as much alive as chemistry, as a cell, but they seem much more abstract. So for example, I think language is alive, or at least life. I think memes are. I think-

Sara Walker

Yes.

Sara Walker

Life maybe. Maybe not alive, but actually I don't know where I stand exactly on that. I've been thinking about that a little bit more lately. But mathematics too, and it's interesting because people think that math has this Platonic reality that exists outside of our universe, and I think it's a feature of our biosphere and it's telling us something about the structure of ourselves. And I find that really interesting because when you would internalize all of these things that we noticed about the world, and you start asking, well, what do these look like? If I was something outside of myself observing these systems that all embedded in, what would that structure look like? And I think we look really different than the way that we talk about what we look like to each other.

Sara Walker

I think it's the fact that it's open-ended in some sense. It's another open-ended combinatorial space, and the recursive properties of it allow creativity to happen, which is what you see with the revolution in the last century with Gödel's Theorem and Turing. And there's clear places where mathematics notices holes in the universe.

Sara Walker

Yeah.

Sara Walker

Definitely not chemical. Chemistry is one substrate.

Sara Walker

Well, specifically it's Darwinian evolution. And I think Darwinian evolution is a problem. But the reason that that definition is a problem is not because evolution is in the definition, but because the implication that most people would want to make is that an individual is alive. And the evolutionary process, at least the Darwinian evolutionary process, most evolutionary processes, they don't happen at the level of individuals. They happen at the level of population. So again, you would be saying something like what we saw with the self-sustaining definition, which is that populations are alive, but individuals aren't because populations evolve and individuals don't. And obviously maybe you are alive because your gut microbiome is evolving. But Lex is an entity right now is not evolving by canonical theories of evolution. In assembly theory, which is attempting to explain life, evolution is a much broader thing.

Sara Walker

Yes, you're constructing yourself all the time. Assembly theory is about construction and how the universe selects for things to exist.

Sara Walker

That's fine too. But this again gets back to it. We can nitpick at definitions. I don't think it's incredibly helpful to do it. But the reason for me-

Sara Walker

Yeah, it is fun. It is really fun. And actually I do think it's useful in the sense that when you see the ways that they all break down, you either have to keep forcing in your conception of life you want to have, or you have to say, "All these definitions are breaking down for a reason. Maybe I should adopt a more expansive definition that encompasses all the things that I think and are life." And so for me, I think life is the process of how information structures matter over time and space, and an example of life is what emerges on a planet and yields an open-ended cascade of generation of structure and increasing complexity. And this is the thing that life is. And any individual is just a particular instance of these lineages that are structured across time.

And so we focus so much on these individuals that are these short temporal moments in this larger causal structure that actually is the life on our planet, and I think that's why these definitions break down because they're not general enough, they're not universal enough, they're not deep enough, they're not abstract enough to actually capture that regularity.

Sara Walker

Yeah. It's like Aristotle focusing on heavy things falling because they're earth-like, and things floating because they're air-like. It's the wrong thing to focus on.

Sara Walker

I think we're missing most of what we are. One of the issues… I've been thinking about this really viscerally lately. It's weird when you do theoretical physics, because I think it literally changes the structure of your brain and you see the world differently, especially when you're trying to build new abstractions.

Sara Walker

I think you're always on the edge of it, but I think what is amazing about being a scientist and trying to do things rigorously is it keeps your sanity. So I think if I wasn't a theoretical physicist, I would be probably not sane. But what it forces you to do is you have to hold yourself to the fire of these abstractions in my mind have to really correspond to reality. And I have to really test that all the time. And so I love building new abstractions and I love going to those incredibly creative spaces that people don't see as part of the way that we understand the world now. But ultimately, I have to make sure that whatever I'm pulling from that space is something that's really usable and really relates to the world outside of me. That's what science is.

Sara Walker

Yeah, so the issue is we evolve perception to see reality a certain way. So for us, space is really important and time feels fleeting. And I had a really wonderful mentor, Paul Davies, most of my career. And Paul's amazing because he gives these little seed thought experiments all the time. Something he used to ask me all the time was when I was a postdoc, this is a random tangent, but was how much of the universe could be converted into technology if you were thinking about long-term futures and stuff like that. And it's a weird thought experiment, but there's a lot of deep things there. And I do think a lot about the fact that we're really limited in our interactions with reality by the particular architectures that we evolved, and so we're not seeing everything. And in fact, our technology tells us this all the time because it allows us to see the world in new ways by basically allowing us to perceive the world in ways that we couldn't otherwise.

And so what I'm getting at with this is I think that living objects are actually huge. They're some of the biggest structures in the universe, but they are not big in space. They're big in time. And we actually can't resolve that feature. We don't interact with it on a regular basis, so we see them as these fleeting things that have this really short temporal clock time without seeing how large they are. When I'm saying time here, really, the way that people could picture it is in terms of causal structure. So if you think about the history of the universe to get to you and you imagine that that entire history is you, that is the picture I have in my mind when I look at every living thing.

Sara Walker

Doesn't everyone?

Sara Walker

Thank you.

Sara Walker

Well, you know what it is? The reason they're hard to write is because it's compressing a very deep idea into a short amount of space, and I really like doing that intellectual exercise because I find it productive for me.

Sara Walker

Yeah, I like language. I think it's really fun to play with.

Sara Walker

I would like to try this, but I think I use language in certain ways that are non-canonical and I do it very purposefully. And it would be interesting to me how AI would interpret it.

Sara Walker

The universe is far larger in time than it is in space, and this planet is one of the biggest things in the universe.

Sara Walker

Yeah, I think the modern technosphere is the largest object in time in the universe that we know about.

Sara Walker

I mean the global integration of life and technology on this planet.

Sara Walker

But I don't think of them as separate. They're very integrated with the structure that generated them. So you can almost imagine it like time is constantly bifurcating and it's generating new structures, and these new structures are locally constructing the future. And so things like you and I are very close together in time because we didn't diverge very early in the history of universe. It's very recent. And I think this is one of the reasons that we can understand each other so well and we can communicate effectively, and I might have some sense of what it feels like to be you. But other organisms bifurcated from us in time earlier. This is just the concept of phylogeny. But if you take that deeper and you really think about that as the structure of the physics that generates life and you take that very seriously, all of that causation is still bundled up in the objects we observe today.

And so you and I are close in this temporal structure, but we're so close because we're really big and we only are very different and the most recent moments in the time that's embedded in us. It's hard to use words to visualize what's in minds. I have such a hard time with this sometimes. Actually, I was thinking on the way over here, I was like, you have pictures in your brain and then they're hard to put into words. But I realized I always say I have a visual, but it's not actually I have a visual. I have a feeling, because oftentimes I cannot actually draw a picture in my mind for the things that I say, but sometimes they go through a picture before they get to words. But I like experimenting with words because I think they help paint pictures.

Sara Walker

Yeah, for sure.

Sara Walker

Yeah, I think so. And I think this is one of the reasons that we have such an ability to abstract as humans because we are so gigantic that the space that we can go back into is really large. So the more abstract you're going, the deeper you're going in that space.

Sara Walker

Yes. And this is why the definition of life cannot be the individual. It has to be these lineages because they're all connected, they're interwoven, and they're exchanging parts all the time.

Sara Walker

Yeah, I think life is very high dimensional. In fact, I think you can be alive in some dimensions and not in others. If you could project all the causation that's in you, in some features of you, very little causation is required, very little history. And in some features, a lot is. So it's quite difficult to take this really high-dimensional, very deep structure and project it into things that we really can understand and say, "This is the one thing that we're seeing," because it's not one thing.

Sara Walker

That's very cool.

Sara Walker

Billions of years, yeah.

Sara Walker

It's crazy that there's billions of years inside of all of us.

Sara Walker

Yeah.

Sara Walker

Because it's creative. But with us, obviously. Not independently of us. And also because of this lineage view of life. And I think about life often as a planetary scale phenomena because the natural boundary for all of this causation that's bundled in every object in our biosphere. And so for me, it's just the current boundary of how far life on our planet has pushed into the things that our universe can generate, and so it's the furthest thing, it's the biggest thing. And I think a lot about the nature of life across different scales. And so we have cells inside of us that are alive and we feel like we're alive, but we don't often think about the societies that we're embedded in as alive or a global- scale organization of us in our technology on the planet as alive. But I think if you have this deeper view into the nature of life, which I think is necessary also to solve the origin of life, then you have to include those things.

Sara Walker

Every scale.

Sara Walker

Yeah.

Sara Walker

Yeah. This is the hard thing about solving the problem of life, I think, is how many things you have to integrate into building a sort of unified picture of this thing that we want to call life. And a lot of our theories of physics are built on building deep regularities that explain a really broad class of phenomena, and I think we haven't really traditionally thought about life that way. But I think to get at some of these hardest questions like looking for life on other planets or the origin of life, you really have to think about it that way. And so most of my professional work is just trying to understand every single thing on this planet that might be an example of life, which is pretty much everything, and then trying to figure out what's the deeper structure underlying that.

Sara Walker

I love that quote.

Sara Walker

Yeah. Schrodinger really tried to do what physicists try to do, which is explain things. And his attempt was to try to explain life in terms of non-equilibrium physics, because he thought that was the best description that we could generate at the time. And so he did come up with something really insightful, which was to predict the structure of DNA as an aperiodic crystal. And that was for a very precise reason, that was the only kind of physical structure that could encode enough information to actually specify a cell. We knew some things about genes, but not about DNA and its actual structure when he proposed that. But in the book, he tried to explain life is kind of going against entropy. And so some people have talked about it as like Schrodinger's paradox, how can life persist when the second law of thermodynamics is there? But in open systems, that's not so problematic.

And really the question is, why can life generate so much order? And we don't have a physics to describe that. And it's interesting, generations of physicists have thought about this problem. Oftentimes, it's like when people are retiring, they're like, "Oh, now I can work on life." Or they're more senior in their career and they've worked on other more traditional problems. And there's still a lot of impetus in the physics community to think that non-equilibrium physics will explain life. But I think that's not the right approach. I don't think ultimately the solution to what life is there, and I don't really think entropy has much to do with it unless it's entirely reformulated.

Sara Walker

Yes. From randomness.

Sara Walker

No. Physics hardly even acknowledges that the universe is random at its base. We like to think we live in a deterministic universe and everything's deterministic. But I think that's probably an artifact of the way that we've written down laws of physics since Newton invented modern physics and his conception of motion and gravity, which he formulated laws that had initial conditions and fixed dynamical laws. And that's been sort of become the standard canon of how people think the universe works and how we need to describe any physical system is with an initial condition in a law of motion. And I think that's not actually the way the universe really works. I think it's a good approximation for the kind of systems that physicists have studied so far.

And I think it will radically fail in the longterm at describing reality at its more basal levels. But I'm not saying there's a base, I don't think that reality has a ground, and I don't think there's a theory of everything, but I think there are better theories, and I think there are more explanatory theories, and I think we can get to something that explains much more than the current laws of physics do.

Sara Walker

Yeah. In physics right now, it's really popular to talk about theories of everything. So string theory is supposed to be a theory of everything because it unifies quantum mechanics and gravity. And people have their different pet theories of everything. And the challenge with the theory of everything, I really love this quote from David Krakauer, which is, "A theory of everything is a theory of everything except those things that theorize."

Sara Walker

Yeah. But it's also weird because if a theory of everything explained everything, it should also explain the theory. So the theory has to be recursive and none of our theories of physics are recursive. So it's a weird concept.

Sara Walker

I don't think so. I think you can build a theory acknowledging that you're an observer inside the universe.

Sara Walker

I don't see one.

Sara Walker

I think so, yeah. I think for me, the frontier in modern physics, where the new physics lies is not in high energy particle physics, it's not in quantum gravity, it's not in any of these sort of traditionally sold, "This is going to be the newest deepest insight we have into the nature of reality." It is going to be in studying the problems of life and intelligence and the things that are sort of also our current existential crises as a civilization or a culture that's going through an existential trauma of inventing technologies that we don't understand right now.

Sara Walker

Well, humans have always been afraid of our technologies though. So it's kind of a fascinating thing that every time we invent something we don't understand, it takes us a little while to catch up with it.

Sara Walker

Yeah, we love being traumatized.

Sara Walker

We want to learn more, and then when we learn more, it traumatizes us. I never thought about this before, but I think this is one of the reasons I love what I do, is because it traumatizes me all the time. That sounds really bad. But what I mean is I love the shock of realizing that coming to understand something in a way that you never understood it before. I think it seems to me when I see a lot of the ways other people react to new ideas that they don't feel that way intrinsically. But for me, that's why I do what I do. I love that feeling.

Sara Walker

Yeah. I'm trying to embody that. Because I think you have to live the physics to understand it. But there's a great quote about Einstein. I don't know if this is true or not, that he once said that he could feel like beam in his belly. But I think you got to think about it though, right? If you're really deep thinker and you're really thinking about reality that deeply and you are part of the reality that you're trying to describe, you feel it, you really feel it.

Sara Walker

Yes. It's a constant descent into madness.

Sara Walker

Yeah, you don't don't know.

Sara Walker

I rely on other people to tell me. Actually, this is very funny. Because I have these conversations with my students often, they're worried about going crazy. I have to reassure them that one of the reasons they'll stay sane is by trying to work on concrete problems.

Sara Walker

Yeah.

Sara Walker

The origin of life is like this boundary that the universe can only cross if a structure that emerges can reinforce its own existence, which is self-reproduction, autocatalysis, things people traditionally talk about. But it has to be able to maintain its own existence against this sort of randomness that happens in chemistry, and this randomness that happens in the quantum world. And it's in some sense the emergence of a deterministic structure that says, "I'm going to exist and I'm going to keep going." But pinning that down is really hard. We have ways of thinking about it in assembly theory that I think are pretty rigorous. And one of the things I'm really excited about is trying to actually quantify in an assembly theoretic way when the origin of life happens. But the basic process I have in mind is a system that has no causal contingency, no constraints of objects, basically constraining the existence of other objects or forming or allowing the existence of other objects.

And so that sounds very abstract, but you can just think of a chemical reaction can't happen if there's not a catalyst, for example. Or a baby can't be born if there wasn't a parent. So there's a lot of causal contingency that's necessary for certain things to happen. So you think about this sort of unconstrained random system, there's nothing that reinforces the existence of other things. So those sort of resources just get washed out in all of these different structures and none of them exist again, or they're not very complicated if they're in high abundance.

And some random events allow some things to start reinforcing the existence of a small subset of objects. And if they can do that, just molecules basically recognizing each other and being able to catalyze certain reactions. There's this kind of transition point that happens where, unless you get a self-reinforcing structure, something that can maintain its own existence, it actually can't cross this boundary to make any objects in high abundance without having this sort of past history that it's carrying with us and maintaining the existence of that past history. And that boundary point where objects can't exist unless they have the selection and history in them, is what we call the origin of life.

And pretty much everything beyond that boundary is holding on for dear life to all of the causation and causal structure that's basically put it there, and it's carving its way through this possibility space into generating more and more structure. And that's when you get the open-ended cascade of evolution. But that boundary point is really hard to cross. And then what happens when you cross that boundary point and the way objects come into existence is also really fascinating dynamics, because as things become more complex, the assembly index increases. I can explain all these things. Sorry. You can tell me what you want to explain or what people will want to hear. This… Sorry, I have a very vivid visual in my brain and it's really hard to articulate it.

Sara Walker

I know. It's so hard. It's like it's going from a feeling to a visual to language is so stifling sometimes.

Sara Walker

I hope so.

Sara Walker

Yes. So self-reproduction has this property. If the system can make itself, then it can persist in time because all objects decay, they all have a finite lifetime. So if you're able to make a copy of your self before you die, before the second law eats you or whatever people think happens, then that structure can persist in time.

Sara Walker

Right. But things that can copy themselves are very rare.

Sara Walker

And so what ends up happening is that you get structures that enable the existence of other things, and then somehow only for some sets of objects, you get closed structures that are self-reinforcing and allow that entire structure to persist.

Sara Walker

Right. So this is the classic-

Sara Walker

There is a chance.

Sara Walker

And that's what we're trying to figure out, is what are the causal constraints that close the loop? So there is this idea that's been in the literature for a really long time that was originally proposed by Stuart Kauffman as really critical to the origin life called, autocatalytic sets. So autocatalytic set is exactly this property we have A makes B, B makes C, C makes A, and you get a closed system. But the problem with the theory of autocatalytic sets is incredibly brittle as a theory and it requires a lot of ad hoc assumptions. You have to assume function, you have to say this thing makes B. It's not an emergent property, the association between A and B. And so the way I think about it is much more general. If you think about these histories that make objects, it's kind of like the structure of the histories becomes, collapses in such a way that these things are all in the same sort of causal structure, and that causal structure actually loops back on itself to be able to generate some of the things that make the higher level structures.

Lee has a beautiful example of this actually in molybdenum. It's like the first non-organic autocatalytic set. It's a self-reproducing molybdenum ring. But it's like molybdenum. And basically if you look at the molybdenum, it makes a huge molybdenum ring. I don't remember exactly how big it is. It might be like 150 molybdenum atoms or something. But if you think about the configuration space of that object, it's exponentially large how many possible molecules. So why does the entire system collapse on just making that one structure? If you start from molybdenum atoms that are maybe just a couple of them stuck together. And so what they see in this system is there's a few intermediate stages. So there's some random events where the chemistry comes together and makes these structures. And then once you get to this very large one, it becomes a template for the smaller ones. And then the whole system just reinforces its own production.

Sara Walker

If I knew how Lee's brain work, I think I would understand a more about the universe. But I-

Sara Walker

No, but I think it goes to the deepest roots of when he started thinking about origins of life. So I mean, I don't know all his history, but what he's told me is he started out in crystallography. And there's some things that he would just… People would just take for granted about chemical structures that he was deeply perplexed about. Just like why are these really intricate, really complex structures forming so easily under these conditions? And he was really interested in life, but he started in that field. So he's just carried with him these sort of deep insights from these systems that seem like they're totally not alive and just like these metallic chemistries into actually thinking about the deep principles of life. So I think he already knew a lot about that chemistry. And he also, assembly theory came from him thinking about how these systems work. So he had some intuition about what was going on with this molybdenum ring.

Sara Walker

They knew about them for a long time, but they didn't know that the mechanism of why that particular structure form was all catalytic feedback. And so that's what they figured out in this paper. And I actually think that paper is revealing some of the mechanism of the origin life transition. Because really what you see the origin of life is basically like you should have a combinatorial explosion of the space of possible structures that are too large to exhaust. And yet you see it collapse on this really small space of possibilities that's mutually reinforcing itself to keep existing. That is the origin of life.

Sara Walker

Yeah.

Sara Walker

I think it's a small space, but chemistry is very large. So there might be a lot of them out there, but we don't know.

Sara Walker

That's right.

Sara Walker

Yes. Yeah. I mean, there's also all kinds of other weird properties that happen around this kind of phase boundary. So this other project that I have in my lab is focused on the origin of chirality, which is thinking about… So chirality is this property molecules that they can come in mirror image forms. So just like chirality means hand. So your left and right hand are what's called non-superimposable, because if you try to lay one on the other, you can't actually lay them directly on top of each other. And that's the property being a mirror image. So there's this sort of perplexing property of the chemistry of life that no one's been able to really adequately explain, that all of the amino acids in proteins are left-handed and all of the bases in RNA and DNA are right-handed. And yet the chemistry of these building block units, amino acids and nucleobases is the same for left.

And so you have to have some kind of symmetry breaking where you go from these chemistries that seem entirely equivalent, to only having one chemistry takeover is the dominant form. And for a long time, I had been really… I actually did my PhD on the origin of chirality. I was working on it as a symmetry breaking problem in physics. This is how I got started in the origin of life. And then I left it for a long time because I thought it was one of the most boring problems in the origin of life, but I've come back to it. I think there's something really deep going on here related to this combinatorial explosion of the space of possibilities. But just to get to that point, this feature of this handedness has been the main focus. But people take for granted the existence of chiral molecules at all, that this property of having a handedness, and they just assume that it's just a generic feature of chemistry.

But if you actually look at molecules, if you look at chemical space, which is the space of all possible molecules that people can generate, and you look at small molecules, things that have less than about seven to 11 heavy atoms. So things that are not hydrogen, almost every single molecule in that space is achiral, like doesn't have a chiral center. So it would be like a spoon. A spoon doesn't have, it's the same as its mirror image. It's not like a hand that's different than its mirror image. But if you get to this threshold boundary, above that boundary, almost every single molecule is chiral.

So you go from a universe where almost nothing has a mirror image form, there's no mirror image universe of possibilities to this one where every single structure has pretty much a mirror image version. And what we've been looking at in my lab is that, it seems to be the case that the origin of life transition happens around the time when you start accumulating, you push your molecules to a large enough complexity that chiral molecules become very likely to form. And then there's a cascade of molecular recognition where chiral molecules can recognize each other. And then you get this sort of autocatalytic feedback and things self-reinforcing.

Sara Walker

No, it's a super interesting feature. I think chirality breaks symmetry in time, not space. So we think of it as a spatial property, like a left and right hand. But if I choose the left hand, I'm basically choosing the future of that system for all time, because I've basically made a choice between the ways that that molecule can now react with every other object in its chemical universe.

Sara Walker

And so you're actually, when you have the splitting of making a molecule that now has another form it could have had by the same exact atomic composition, but now it's just a mirror image isometry, you're basically splitting the universe of possibilities every time.

Sara Walker

In two, but molecules can have more than one chiral center, and that's not the only symmetry that they can have. So this is one of the reasons that Taxol fills 1.5 universes of space. It's all of these spatial permutations that you do on these objects that actually makes the space so huge. So the point of this sort of chiral transition that I am pointing out is, chirality is actually signature of being in a complex chemical space. And the fact that we think it's a really generic feature of chemistry and it's really prevalent is because most of the chemistry we study on earth is a product already of life.

And it also has to do with this transition in assembly, this transition in possibility spaces, because I think there's something really fundamental going on at this boundary, that you don't really need to go that far into chemical space to actually see life in terms of this depth in time, this depth in symmetries of objects, in terms of chiral symmetries or this assembly structure. But getting past this boundary that's not very deep in that space requires life. It's a really weird property, and it's really weird that so many abrupt things happen in chemistry at that same scale.

Sara Walker

I like that movement's in there. That's cool.

Sara Walker

Well, it's efficient for things to eat other things that are already alive because they don't have to go all the way back to the base chemistry.

Sara Walker

You got to live.

Sara Walker

Oh, I see.

Sara Walker

I actually think it's a gift that we don't have much time.

Sara Walker

Yeah. I like invention as a word for them. I think that's good.

Sara Walker

No, I think the origin of life is a continual process, that's why. I'm interested in the first transition and solving that problem, because I think it's the hardest, but I think it's happening all the time.

Sara Walker

I like sight as an invention, because I think having sensory perception and trying to comprehend the world, to use anthropocentric terms, is a really critical feature of life. And I also, it's interesting the way that site has complexified over time. So if you think at the origin of life, nothing on the planet could see. So for a long time, life had no sight, and then photon receptors were invented. And then when multicellular evolved, those cells eventually grew into eyes and we had the multicellular eye.

And then it's interesting when you get to societies like human societies, that we invent even better technologies of seeing, like telescopes and microscopes, which allow us to see deeper into the universe or at smaller scales. So I think that's pretty profound, the way that site has transformed the ability of life to literally see the reality in which it's existing in. I think consciousness is also obviously deeply interesting. I've gotten kind of obsessed with octopus. They're just so weird. And the fact that they evolved complex nervous systems kind of independently seems very alien.

Sara Walker

Do you have any examples?

Sara Walker

Oh, I've seen videos of that. It's so gross when the babies come out. Did you see that in person? The baby's coming out?

Sara Walker

Have you seen videos of that? It's so gross. It's one of the grossest things I've ever seen.

Sara Walker

I guess, if I was one of those frogs, I would think that was the most beautiful event I'd ever seen. Although, human childbirth is not that beautiful either.

Sara Walker

Well, we come into the world so violently, it's just like, it's amazing.

Sara Walker

You know what? This actually makes me think of one that's not up there, which I do find really incredibly amazing, is the process of the germline cell in organisms. Basically, every living thing on this planet at some point in its life has to go through a single cell. And this whole issue of development, the developmental program is kind of crazy. How do you build you out of a single cell? How does a single cell know how to do that? Pattern formation of a multicellular organism, obviously evolves with DNA, but there's a lot of stuff happening there about when cells take on certain morphologies and things that people don't understand, like the actual shape formation mechanism. A lot of people study that, and there's a lot of advances being made now in that field. I think it's pretty shocking though that how little we know about that process. And often it's left off of people's lists, it's just kind of interesting. Embryogenesis is fascinating.

Sara Walker

Yeah. And the genes and all the cells are the same. So the differentiation has to be something that's much more about the actual expression of genes over time and how they get switched on and off, and also the physical environment of the cell interacting with other cells. And there's just a lot of stuff going on.

Sara Walker

Yes.

Sara Walker

Right.

Sara Walker

Yeah.

Sara Walker

Probably, yes. I think most of the things about understanding anything about what we are embedded in time.

Sara Walker

I know. So predictable. It's turning into a deterministic universe.

Sara Walker

Yeah, always was, but it won't be in the future.

Sara Walker

Yes.

Sara Walker

Yeah. I think the first thing is the observation that life seems to be the only thing in the universe that builds complexity in the way that we see it here. And complexity is obviously a loaded term, so I'll just use assembly instead because I think assembly is more precise. But the idea that all the things on your desk here from your computer, to the pen, to us sitting here don't exist anywhere else in the universe as far as we know, they only exist on this planet and it took a long evolutionary history to get to us, is a real feature that we should take seriously as one that's deeply embedded in the laws of physics and the structure of the universe that we live in.

Standard physics would say that all of that complexity traces back to the infinitesimal deviations and the initial state of the universe that there was some order there. I find that deeply unsatisfactory. And what assembly theory says that's very different is that, the universe is basically constructing itself, and when you get to these combinatorial spaces like chemistry, where the space of possibilities is too large to exhaust them all, you can only construct things along historically contingent paths, like you basically have causal chains of events that happen to allow other things to come into existence.

And that this is the way that complex objects get formed, is basically on scaffolding on the past history of objects, making more complex objects, making more complex objects. That idea in itself is easy to state and simple, but it has some really radical implications as far as what you think is the nature of the physics that would describe life. And so what assembly theory does formally is try to measure the boundary in the space of all things that chemically could exist. For example, like all possible molecules, where's the boundary above which we should say these things are too complex to happen outside of an evolutionary chain of events, outside of selection. And we formalize that with two observables. One of them is the copy number of the object. How many of the object did you observe? And the second one is what's the minimal number of recursive steps to make it? If you start from elementary building blocks, like bonds for molecules, and you put them together, and then you take things you've made already and build up to the object, what's the shortest number of steps you had to take?

And what Lee's been able to show in the lab with his team is that for organic chemistry, it's about 15 steps. And then you only see molecules that the only molecules that we observe that are past that threshold are ones that are in life. And in fact, one of the things I'm trying to do with this idea of trying to actually quantify the origin of life as a transition in… A phase transition and assembly theory is actually be able to explain why that boundary is where because I think that's actually the boundary that life must cross.

The idea of going back to this thing we were talking about before about these structures that can reinforce their own existence and move past that boundary, 15 seems to be that boundary in chemical space. It's not a universal number. It will be different for different assembly spaces, but that's what we've experimentally validated so far. And then-

Sara Walker

It's 15 or so for the experimental data. Yeah.

Sara Walker

When have to have that feature in order to observe molecules in high abundance in that space.

Sara Walker

Recursive. Yeah. You can think of objects in assembly theory as basically recursive stacks of the construction steps to build them. They're like, it's like you take this step and then you make this object and you make it this object and make this object, and then you get up to the final object. But that object is all of that history rolled up into the current structure.

Sara Walker

You can't take the long way.

Sara Walker

The long way doesn't exist.

Sara Walker

It turns out, now if you look at objects… And so we define something we call the assembly universe. And assembly universe is ordered in time. It's actually ordered in the causation, the number of steps to produce an object. And so, all objects in the universe are in some sense existed, a layer that's defined by their assembly index.

And the size of each layer is growing exponentially. What you're talking about, if you want to look at the long way of getting to an object, as I'm increasing the assembly index of an object, I'm moving deeper and deeper into an exponentially growing space. And it's actually also the case that the typical path to get to that object is also exponentially growing with respect to the assembly index.

And so, if you want to try to make a more and more complex object and you want to do it by a typical path, that's actually an exponentially receding horizon. And so most objects that come into existence have to be causally very similar to the things that exist because close by in that space, and they can actually get to it by an almost shortest path for that object.

Sara Walker

By a lot.

Sara Walker

Yeah, imagine yourself-

Sara Walker

Yeah. I mean basically, the more complex we live in a space that is growing exponentially large. And the ways of getting to objects in the space are also growing exponentially large. And so, we are some of the most deterministic things in the universe. And so, we can generate very regular structure and we can generate new structure along a particular lineage. But the possibility space at the tips, the things we can generate next is really huge.

There's some stochasticity in what we actually instantiate as the next structures that get built in the biosphere. It's not completely deterministic because the space of future possibilities is always larger than the space of things that exist now.

Sara Walker

I said before, right now, I think the origin of life is a continuous process on earth. I think this idea of combinatorial spaces that our biosphere generates not just chemistry, but other spaces often cross this threshold where they then allow themselves to persist with particular regular structure over time.

Language is another one where the space of possible configurations of the 26 letters of the English alphabet is astronomically large, but we use with very high regularity, certain structures. And then we associate meaning to them because of the regularity of how much we use them. Meaning is an emergent property of the causation and the objects and how often they recur and what the relationship of the recurrence is to other objects.

Sara Walker

Well, this is why you can play with language so much actually. Words don't really carry meaning, it's just about how you lace them together.

Sara Walker

—the entire context.

Sara Walker

Yeah. And you can figure out that structures from the regularity of objects that we use, not from the actual meaning we assign to them. Which is why I think your tweet compression technique works so well because you're leaving space and you're letting people's minds fill in the gaps based on the regularity of their own experience. So your tweets are really compressed contextual meaning.

Sara Walker

Oh, there's so many ways. The assembly universe is a very high dimensional space. It's like a hypergraph or a network. In some sense, all objects are nodes and the edges between them are the relationships of causation that allow some objects to make other objects. And so it's a causal network of all possible constructions. And the universe is creating objects that are connected in this space, getting further and further away from the base objects and into this exponentially growing space. The frontier of all of what's possible is expanding, and life is at the frontier. Life is exploring what's possible.

Sara Walker

Well, not quite. It means that it has a history of selection. So whether you call that life or not is another question. But the selection that shows up in the assembly index is a property of the assembly, not necessarily the substrate or the thing itself. So a molecule can be alive in one context and not alive in another. A molecule that shows high copy number and high assembly index in a biological context would not be alive in, say, a space of mathematical proofs. So it depends on what space you're sampling from.

Sara Walker

Very much. The universe is context-dependent. Meaning emerges from context. Objects that appear to be alive in one assembly space might not be alive in another. That's what I find so fascinating about assembly theory. It's not a monolithic definition of life. It's a framework for understanding when objects can be considered alive based on their historical construction and their current distribution.

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*Note: The full transcript continues with discussions on Aliens, the Great Perceptual Filter, Fashion, Beauty, Language, Computation, Consciousness, Artificial Life, Free Will, and Why Anything Exists. Due to length limitations, only the first major sections have been included. The complete transcript would continue through the remaining chapters listed in the Table of Contents.*