Sara Walker: Physics of Life, Time, Complexity, and Aliens | Lex Fridman Podcast #433

1. 0:00 – 1:07

   Introduction

   1. SWSara Walker0:00

      So, you have an original life event. It evolves for four 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.

   2. LFLex Fridman0:26

      The following is a conversation with Sarah Walker, her third time on this podcast. She is an astrobiologist and theoretical physicist interested in the origin of life and in discovering alien life on other worlds. She has written an amazing new upcoming book titled Life As No One Knows It: The Physics of Life's Emergence. This book is coming out on August 6th, so please go pre-order it now. It will blow your mind. This is the Lex Fridman Podcast. To support it, please check out our sponsors in the description, and now, dear friends, here's Sarah Walker.

2. 1:07 – 21:45

   Definition of life

   1. LFLex Fridman1:07

      You open the book Life As No One Knows It: The Physics of Life's Emergence with a distinction between the materialists and the vitalists. So, what's the difference? Can you maybe define the two?

   2. SWSara Walker1:19

      I think the question there is about whether life can be described in terms of matter and, you know, 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- it's kind of 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 kind of 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.

   3. LFLex Fridman2:10

      Is there a gray area between the two? Like, maybe all there is is matter, but there's so much we don't know that it might as well be magic?

   4. SWSara Walker2:19

      (laughs)

   5. LFLex Fridman2:20

      That- that, like, whatever that magic that the vitalists see.

   6. SWSara Walker2:24

      Yeah.

   7. LFLex Fridman2:24

      Meaning, like, there's just so much mystery that it's really unfair to say that it's boring and understood and as simple as, quote-unquote, "physics."

   8. SWSara Walker2:36

      Yeah, I think the entire universe is just a giant mystery. Um, 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, you know, what is intelligence or are there souls, or whatever- whatever question that we have that we feel like we aren't even on the tip of answering yet, I think, you know, 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, um, that we learn what those things are. And they still have an air of mystery, even when we understand them. There's- there's no- there's no sort of bottom to our understanding.

   9. LFLex Fridman3:30

      So, do you think the vitalists have a point, that they're, uh, more eager and able to notice the magic of life?

   10. SWSara Walker3:39

       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, uh, for thousands of years, for thousands of generations, there must be some truth to them. You know, we've been learning about reality (laughs) um, for a really long time, um, and w- 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 are 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 and 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, uh, measure those things, or we knew how to describe them, uh, in a more rigorous way, we would realize that the material world, matter, has more properties than we thought that it did, and 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.

   11. LFLex Fridman5:13

       So, we may still expand our understanding what is incorporated in the category of "matter" that will eventually incorporate such magical things that the vitalists have noticed-

   12. SWSara Walker5:24

       Yeah.

   13. LFLex Fridman5:24

       ... like life?

   14. SWSara Walker5:25

       Yeah. So, I think about, um, I always like to use examples from physics, so I'll probably do that to, like (laughs) , like, it's just my, it's my go-to place. Um, but, you know, in- in the history of gravitational physics, for exis- example, and the history of motion, you know, like, 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, um, time in a mechanistic way and track planetary motion, uh, and we could, you know, 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. Um, so we didn't really have to worry about the fact that my cup is heavy and the air is light. Like, the same laws describe them-

   15. LFLex Fridman6:29

       Yeah.

   16. SWSara Walker6:29

       ... um, 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, um, because we haven't really invented the right material concept yet.

   17. LFLex Fridman6:54

       So, when talking about motion, the laws of physics appear to be the same everywhere out in the universe. Do you think the same is true for other kinds of matter that we might eventually include life in?

   18. SWSara Walker7:10

       I think life obeys universal principles. I think there is some deep, underlying, exploratory 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, um, because it's too different.

   19. LFLex Fridman7:28

       You write about the paradox of defining life. Why does it seem to be so easy and so complicated at the same time?

   20. SWSara Walker7:35

       You know, all the sort of classic definitions people want to use just don't work.

   21. LFLex Fridman7:40

       Mm-hmm.

   22. SWSara Walker7:40

       They don't work in all cases. So, uh, 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's so many of them on our planet, and, like, humans are inside them. And you might want to exclude machines, uh, but any definition, you know, like classic biology textbook definitions would also include them. And so, you know, he wanted to draw a boundary between, uh, these kind of things by trying to, uh, 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 counter-example that challenges that definition. This is why viruses are so hard or why fire is so hard. And so, uh, we've had a really hard time trying to pin down, from a, a definitional perspective, exactly what life is.

   23. LFLex Fridman8:42

       Yeah. You actually bring up the, the zombie ant fungus, I enjoyed looking at this thing-

   24. SWSara Walker8:47

       (laughs)

   25. LFLex Fridman8:48

       ... as an example of one of the challenges. You se- mentioned viruses, but this, this is a parasite. Look at that.

   26. SWSara Walker8:54

       Did you see this in the jungle?

   27. LFLex Fridman8:55

       Infects ants. Actually, one of the interesting things about the jungle, everything is ephemeral. Like, everything eats everything really quickly. So if you, uh, if an organism dies, uh, that organism disappears. Isn't Yeah. It's a machine that doesn't have, uh, I wanted to say it doesn't have a memory or a history, which is-

   28. SWSara Walker9:17

       Right.

   29. LFLex Fridman9:17

       ... interesting given your work on history, in defining a, uh, living being. The jungle forgets very quickly. It wants to erase the fact that you existed, very quickly.

   30. SWSara Walker9:28

       Yeah, but it can't erase it. It's just restructuring it. And I think-
3. 21:45 – 32:26

   Time and space

   1. SWSara Walker21:45

   2. LFLex Fridman21:45

      So what, what exactly are we missing by focusing on such a short span of time?

   3. SWSara Walker21:50

      I think we're missing most of what we are. So one of the issues... I- I've been thinking about this all, like, really viscerally lately. It's weird when you do theoretical physics, 'cause I think it, like, literally changes the structure of your brain. (laughs)

   4. LFLex Fridman22:02

      Hmm.

   5. SWSara Walker22:02

      And you see the world differently, especially when you're trying to build new abstractions.

   6. LFLex Fridman22:05

      Do you think it's possible if you're a theoretical physicist, like, it's easy to fall off the cliff and go descend into madness?

   7. SWSara Walker22:13

      I mean, I think you're always on the edge of it, but I think what is amazing about being a scientist, um, and trying to do things rigorously is it keeps your sanity. So I think if I wasn't-

   8. LFLex Fridman22:25

      (laughs) Okay.

   9. SWSara Walker22:25

      ... a theoretical physicist, I would, I would be probably not sane.

   10. LFLex Fridman22:29

       Right.

   11. SWSara Walker22:29

       Um, but what it forces you to do is hold the fi- like, you have to hold yourself to the fire of, like, "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, like, incredibly creative, uh, you know, spaces that people don't see, um, 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, like, relates to the world outside of me. That's what science is.

   12. LFLex Fridman23:01

       So we were talking about what we're missing when we look at a small stretch of time and a small stretch of space.

   13. SWSara Walker23:09

       Yeah. So the issue is, um, y- we evolve perception to see reality a certain way, right? So for us, space is really important and time feels fleeting. And I, I, you know, I, I had a really wonderful mentor, Paul Davies, most of my career. And Paul's amazing because he gives these, like, little seed thought experiments all the time. Like, you know, something he used to ask me all the time was a- when I was a postdoc, this is kind of a random tangent, but was like, you know, "How much of the universe b- could be converted into technology?" If you were thinking about, like, you know, long-term futures and stuff like that. And it's, like, a weird thought experiment, but, like, 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, um, 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, um, 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. (laughs) Like, they're some of the biggest structures in the universe, but they are not big in space. They are 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, I really... Like, 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 i- the picture I have in my mind when I look at every living thing.

   14. LFLex Fridman24:49

       So you have a tw- you have a tweet for everything.

   15. SWSara Walker24:51

       (laughs)

   16. LFLex Fridman24:52

       You tweeted-

   17. SWSara Walker24:53

       Doesn't everyone? (laughs)

   18. LFLex Fridman24:54

       You have a lot of poetic, profound tweets. Um, sometimes-

   19. SWSara Walker24:58

       Thank you.

   20. LFLex Fridman24:59

       ... they're puzzles that take a long time to figure out.

   21. SWSara Walker25:02

       Well, you know what it i- the trick 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 'cause I find it productive for me.

   22. LFLex Fridman25:14

       Yeah, it's a very interesting kind of compression algorithm though.

   23. SWSara Walker25:18

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

   24. LFLex Fridman25:20

       Yeah, I wonder if AI can, uh, decompress it. That'd be interesting as-

   25. SWSara Walker25:24

       I think-

   26. LFLex Fridman25:25

       ... as a challenge.

   27. SWSara Walker25:25

       ... I, I, 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 A- AI would interpret it.

   28. LFLex Fridman25:35

       Yeah, your tweets would be a good touring test-

   29. SWSara Walker25:37

       There you go.

   30. LFLex Fridman25:37

       ... for the s- for, for a super intelligence. Anyway, you tweeted that things only look emergent bec- because we can't see time.
4. 32:26 – 36:51

   Technosphere

   1. SWSara Walker32:26

   2. LFLex Fridman32:26

      Uh, y- you mentioned just the technosphere, and you also wrote that the most alive thing in this planet is our technosphere.

   3. SWSara Walker32:33

      Yeah.

   4. LFLex Fridman32:34

      Why is it technology we create a kind of life form? Why do you, why are you seeing it as life?

   5. SWSara Walker32:39

      (inhales deeply) Because it's creative. But with us, obviously, like not independently of us, and also because of the sort of lineage view of life, and I, I think about life often as a planetary scale phenomena, 'cause that's sort of the natural boundary for all of this causation that's bundled in every object in our biosphere. And so for me, it's just sort of 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. Um, and I think a lot about the nature of life across different scales. And so, uh, you know, 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 and our technology on the planet as alive. Um, but I think if you have this, uh, deeper view into the nature of life, uh, which I think is necessary also to solve the origin of life, then you have to include those things.

   6. LFLex Fridman33:47

      All of them. So y- so you have to simultaneously think about-

   7. SWSara Walker33:50

      Every scale.

   8. LFLex Fridman33:50

      ... life at every single scale.

   9. SWSara Walker33:52

      Yeah.

   10. LFLex Fridman33:52

       The planetaria and the bacteria level.

   11. SWSara Walker33:55

       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 a, a, a unified picture of this thing that we want to call life. And, and a lot of our theories of physics are built on, um, building deep regularities that explain a really broad class of phenomena, and I think we haven't really traditionally thought about life that way. Uh, but I think to get a, 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, like, my professional work is just trying to understand, like, every single thing on this planet that might be an example of life, which is pretty much everything, and then trying to figure out, like, what's the deeper structure underlying that.

   12. LFLex Fridman34:40

       Yeah. Schrodinger wrote that "living matter, while not eluding the laws of physics as established up to date, is likely to involve other laws of physics hitherto unknown." So to him-

   13. SWSara Walker34:54

       I love that quote.

   14. LFLex Fridman34:55

       ... there was a sense that at the bottom of this, there are new laws of physics that could explain-

   15. SWSara Walker35:02

       Yeah.

   16. LFLex Fridman35:02

       ... this thing that we call life.

   17. SWSara Walker35:04

       Yeah. Schrodinger really tried to do what physicists try to do, uh, which is explain things. Um, and he, 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, um, and that was for a very precise reg- reason. I, y- you know, 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 as 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? Um, 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, you know, 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." Uh, (laughs) or they're like more senior in their career and they've worked on other more traditional problems, and there's still a lot of impetus, um, in the physics community to think that non-equilibrium physics will explain life. But I, I think that's not the right approach. Uh, I don't think ultimately the solution to what life is, is there, and I don't really think entropy has much to do with it, unless it's entirely reformulated.

   18. LFLex Fridman36:42

       Well, th- 'cause you have to explain how interesting order, how complexity emerges from-... the soup.

   19. SWSara Walker36:47

       Yes, from randomness.

   20. LFLex Fridman36:49

       From randomness. Physics currently can't do

5. 36:51 – 45:32

   Theory of everything

   1. LFLex Fridman36:51

      that.

   2. SWSara Walker36:51

      No. Physics hardly even acknowledges that the universe is random at its base. (laughs) We like to think we live in a deterministic universe and everything's deterministic, but I think that's probably, uh, you know, an artifact of the way that we've written down laws of physics since Newton invented modern physics, uh, in his conception of motion and gravity. Which, you know, he, he formulated laws that had initial conditions and, um, 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 and a law of motion. And I think that's not actually the way the universe really works. I think s- it's a good approximation for the kind of systems that physicists have studied so far, and I think it will radically fail, um, in the long term at describing reality at its more basal levels. But not, 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.

   3. LFLex Fridman38:02

      When you say Theory of Everything, you mean, like, everything everything?

   4. SWSara Walker38:05

      Yeah, you know, like, in f- 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, um, and, you know, people have their different pet Theories of Everything, and, and the challenge with a Theory of Everything, I really love this quote, quote from David Krakower, which is, "A Theory of Everything is a Theory of Everything Except Those Things That Theorize."

   5. LFLex Fridman38:27

      Oh, you mean, removing the observer from the thing?

   6. SWSara Walker38:30

      Yeah, but it's also, 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 just a, it's a, it's a weird concept.

   7. LFLex Fridman38:44

      Yeah, well, it's very difficult to integrate the observer into a theory.

   8. SWSara Walker38:47

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

   9. LFLex Fridman38:52

      But it, doesn't it become recursive in that way? I mean that's-

   10. SWSara Walker38:56

       You-

   11. LFLex Fridman38:56

       ... you're saying it's possible to make a theory that's okay with that?

   12. SWSara Walker39:00

       I think so. I mean, I don't think y- there's always gonna be, um, the paradox of another meta-level you could build on the m- the meta-level, right? So, like, if you assume this is your universe and you're the observer outside of it, you have some meta-description of that universe, but then you need a meta-description of you describing that universe, right? So, uh, you know, this is one of the biggest challenges that we face, um, being observers inside our universe, and also, you know, why the paradox is in the foundations of mathematics and any place that we try to have observers in the system, or a system describing itself, uh, show up. Um, but I think it is possible to build a physics that builds in those things intrinsically without having them be paradoxical or have holes in the descriptions. Um, and so one, one place I think about this quite a lot, which I think can give you sort of a more concrete example is, is the nature of, like, what we call fundamental. So, uh, we typically define fundamental right now in terms of the smallest indivisible units of matter. So again, you have to have a definition of what you think material is and matter is, but right now, the, you know, what's fundamental are elementary particles, um, and we think they're fundamental because we can't break them apart further. And obviously we have theories like string theory that, if they're right, would replace the current description of what's the most fundamental thing in our universe by replacing it with something smaller, um, but we can't get to those theories because we're technologically limited. And so if you, if you look at this from a historical perspective and you think about explanations changing as physical systems like us learn more about the reality in which they live, we once considered atoms to be the most fundamental thing, um, and, you know, it literally comes from the word "indivisible." And then we realized atoms had substructure because we built better technology which allowed us to, quote unquote, "see the world better" and resolve smaller features of it. And then we built even better technology which allowed us to see even smaller structure and get down to the standard model particles, and we think that there's m- might be structure below that, but we can't get there yet with our technology. So, what's fundamental the way we talk about it in, um, current physics is not actually fundamental. It's the boundaries of what we can observe in our universe, what we can see with our technology. And so if you want to build a theory that's about us and about what, what's inside the universe that we can observe, not what's at the boundary of it, um, you need to talk about objects that are in the universe that you can actually break apart to smaller things. So, I think the things that are fundamental are actually the constructed objects. They're the ones that really exist, and you really understand their properties because you know how the universe constructed them, because you can actually take them apart. You can understand the intrinsic laws that built them. But the things at the boundary are just at the boundary. They're e- evolving with us, and we'll learn more about that structure as we go along. But really, if we wanna talk about what's fundamental inside our universe, we have to talk about all these things that are traditionally considered emergent, but are really just structures in time that have causal histories that constructed them and, um, you know, are really actually what our universe is about.

   13. LFLex Fridman42:17

       So, we should focus on the construction methodology as the fundamental thing. But y- do you think there's a bottom to the, the smallest possible thing that makes up the universe?

   14. SWSara Walker42:27

       I don't see one.

   15. LFLex Fridman42:29

       And you, it'll take way too long. It'll take longer to find that than it will to understand the mechanism that created life.

   16. SWSara Walker42:36

       I think so, yeah. I, 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, as civilization, or a culture that's going through, uh, you know, an existential trauma of inventing technologies that we don't understand right now.

   17. LFLex Fridman43:09

       (laughs) The existential trauma, and the terror we feel that that technology might somehow destroy us, "us" meaning living, intelligent living organisms-

   18. SWSara Walker43:18

       Right.

   19. LFLex Fridman43:18

       ... and yet we don't understand what the even means.

   20. SWSara Walker43:20

       Well, humans have always been afraid of our technologies though, right? 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.

   21. LFLex Fridman43:29

       I think, also in part, humans kind of love b- being afraid.

   22. SWSara Walker43:33

       Yeah, we love being traumatized.

   23. LFLex Fridman43:34

       It's weird. The drama?

   24. SWSara Walker43:36

       We want to learn more, and then when we learn more, it traumatizes us (laughs) .

   25. LFLex Fridman43:41

       (laughs)

   26. SWSara Walker43:41

       You know, I never thought about it this- before, but I think this is one of the reasons I love what I do, is because it traumatizes me all the time (laughs) . That sounds really bad. But what I mean is, like, I love the shock of, like, realizing that, like, coming to understand something in a way that you never understood it before. Uh, I think, uh, uh, it seems to me, when, 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 like, that's why I do what I do. I l- I love, I love that feeling.

   27. LFLex Fridman44:08

       But you're also working on a topic where it's fundamentally ego-destroying.

   28. SWSara Walker44:13

       (laughs)

   29. LFLex Fridman44:14

       'Cause you're talking about, like, life. It's humbling to think that we're not, the individual human is not special.

   30. SWSara Walker44:23

       Yeah.
6. 45:32 – 1:07:10

   Origin of life

   1. LFLex Fridman45:32

      it is.

   2. SWSara Walker45:32

      I guess so. Yeah.

   3. LFLex Fridman45:33

      Uh, so what do you think is the origin of life on Earth? And how can we talk about it in a productive way?

   4. SWSara Walker45:40

      The origin of life is, like, this boundary, um, 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, like, it's in some sense the emergence of, like, a deterministic structure that says, you know, "I'm gonna exist, and I'm gonna keep going." (laughs) Um, but, uh, you know, 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, uh, in an assembly theoretic way when the origin of life happens. But the basic process I have in mind is, like, a system that has no causal contingency, no constraints of objects basically constraining the existence of other objects or forming, or, or allowing the existence of other objects. Um, and so that sounds very abstract, but, like, you can just think of, like, 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. Um, so there's a lot of causal contingency that's necessary for certain things to happen. So, um, you think about this sort of unconstrained random system, there's nothing that reinforces the existence of other things. So, so, the sort of resources just get washed out in all of these different structures, and none of them exist again, um, or they just, you know, they're n- 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, um, you know, like, just molecules basically recognizing each other and being able to catalyze certain reactions, uh, there's this kind of, uh, 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 o- 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 this selection and history in them is what we call the origin of life. And pretty much everything beyond that boundary, um, is holding on for dear life to all of the causation and causal structure that's basically put it there, um, and it's carving its way through this possibility space, um, 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, like, really fascinating dynamics, because, you know, like, 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, uh, me to explain (laughs) or, or what people want, w- will want to hear. Um, this, uh... Sorry, I have, like, a very vivid visual in my brain-

   5. LFLex Fridman48:53

      Mm-hmm.

   6. SWSara Walker48:53

      ... and it's really hard to articulate it.

   7. LFLex Fridman48:55

      Got to convert it to language.

   8. SWSara Walker48:56

      I know. (laughs) It's so hard. It's not, it's like, it's going from, like, a feeling to a visual to language is so stifling sometimes.

   9. LFLex Fridman49:03

      And then I have to convert it-

   10. SWSara Walker49:05

       Yeah.

   11. LFLex Fridman49:05

       ... from language to, to a visual-

   12. SWSara Walker49:07

       Yeah.

   13. LFLex Fridman49:07

       ... to a feeling.

   14. SWSara Walker49:09

       Yeah.

   15. LFLex Fridman49:09

       I think it's working.

   16. SWSara Walker49:11

       I hope so.

   17. LFLex Fridman49:12

       I really like the self-reinforcing objects. I mean, just so-

   18. SWSara Walker49:16

       Yeah.

   19. LFLex Fridman49:16

       ... just so I understand, one way to create a lot of the same kind of object is make them self-reinforcing.

   20. SWSara Walker49:24

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

   21. LFLex Fridman49:47

       So, that's a way to sort of emerge out of a random soup.

   22. SWSara Walker49:51

       Yes.

   23. LFLex Fridman49:51

       Out of the randomness of soup.

   24. SWSara Walker49:52

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

   25. LFLex Fridman49:55

       Yeah. Rare.

   26. SWSara Walker49:55

       Um, and so what ends up happening is that you get structures that enable the existence of other things.

   27. LFLex Fridman50:05

       Mm-hmm.

   28. SWSara Walker50:06

       And then somehow, only for some sets of objects, you get closed structures that are self-reinforcing and allow that entire structure to persist.

   29. LFLex Fridman50:16

       Right. So, the one, object A reinforces the existence of object B, but, you know, object A can die.

   30. SWSara Walker50:24

       Yeah.
7. 1:07:10 – 1:23:24

   Assembly theory

   1. LFLex Fridman1:07:10

      about the past, the assembly theory.

   2. SWSara Walker1:07:11

      Yeah.

   3. LFLex Fridman1:07:12

      Can you, uh, explain assembly theory to me? I listened to Lee talk about it for many hours and I understood nothing. No, I'm just kidding. Uh...

   4. SWSara Walker1:07:19

      (laughs)

   5. LFLex Fridman1:07:20

      I just want to take another per- uh, well, you've been already talking about it, but just...

   6. SWSara Walker1:07:23

      Yeah.

   7. LFLex Fridman1:07:24

      ... just, just ano- j- j- just what, from a big-picture view, is the assembly theory way of thinking about our world, about our universe?

   8. SWSara Walker1:07:38

      Yeah. I think the, uh, first thing is, you know, that, like, the observation that, uh, 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, like, a loaded term, so I'll just use "assembly" instead, uh, 'cause I think "assembly" is more precise. Um, but the idea that, like, you know, all those things on your desk here, from your computer to the pen to, uh, you know, 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, um, 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. Uh, standard physics would say that, you know, all of that complexity traces back to the infinitesimal (laughs) , uh, deviations in, like, the initial state of the universe, that there was some order there. Um, I find that deeply unsatisfactory. And, uh, what assembly theory says, uh, that's very different is that the universe is basically constructing itself. And when you get to these commonest tutorial spaces, like chemistry, uh, where the space of possibilities is too large to exhaust them all, um, 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, uh, 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, um, is the nature of the physics that would describe life. And so what a ph- assembly theory does, formally, is try to measure the boundary, um, in the space of all things that, you know, chemically could exist, for example, like all possible molecules. Where is the boundary above which we should say, "These things are too complex to happen outside of an evolutionary chain of events, outside of selection"? Um, and we've formalized that, um, with two observables. One of them's the copy number of the object, so how many of the object did you observe? And the second one is, what's the minimal number of recursive steps to make it? So 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, uh, it's about 15 steps, and then you only see molecules, uh, that ... you know, the only molecules that we observe that are past that threshold are ones that are, are in life. And in fact, one of the things I'm trying to do with this idea of, like, trying to actually quantify the origin of life as a transition in, uh, like a phase transition in assembly theory, is actually be able to, um, explain why that boundary is where it is, 'cause I think that's actually the boundary that life must cross. So the idea of going back to this thing we were talking about before about these, these structures that can reinforce their own existence and move past that boundary, uh, 15 seems to be that boundary in chemical space. Uh, it's not a universal number. It will be different for different assembly spaces, um, but that's what we've experimentally validated so far. And then...

   9. LFLex Fridman1:11:19

      So literally, 15, like the assembly index is 15, and some-

   10. SWSara Walker1:11:23

       It's 15 or so for the experimental data, yeah.

   11. LFLex Fridman1:11:25

       So that's when you start getting the self-reinforcing...

   12. SWSara Walker1:11:29

       That's when you have to have that feature in order for, to observe molecules in high abundance in that space.

   13. LFLex Fridman1:11:35

       So look, copy number is the, the number of exact copies. Well, that's what you mean by high abundance, and...

   14. SWSara Walker1:11:40

       Yeah.

   15. LFLex Fridman1:11:41

       ... assembly index, or the complexity of the object, is how many steps it took to create it. Recursive?

   16. SWSara Walker1:11:47

       Recursive, yeah. So you can think of objects in assembly theory as basically re- recursive stacks of the, the construction steps to build them. So they're like, it's like, uh, you take this step and then you make this object, and you make 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.

   17. LFLex Fridman1:12:06

       What if you took the long way home, as the song goes?

   18. SWSara Walker1:12:08

       You can't take the long way.

   19. LFLex Fridman1:12:10

       Why not?

   20. SWSara Walker1:12:11

       The long way doesn't exist.

   21. LFLex Fridman1:12:12

       It's a good song, though.

   22. SWSara Walker1:12:14

       (laughs)

   23. LFLex Fridman1:12:14

       Uh, what do you mean, the long way doesn't exist?

   24. SWSara Walker1:12:16

       Um...

   25. LFLex Fridman1:12:16

       If I do, if I do a random walk from A to B, I'll eventually... If I start at A, I'll eventually end up at B. And that random walk would be much shorter than the-

   26. SWSara Walker1:12:25

       So it turns out-

   27. LFLex Fridman1:12:26

       ... longer than the short-

   28. SWSara Walker1:12:27

       No. If you look at objects and you, you... So the, we, we define something we call the "assembly universe." And the assembly universe is ordered in time. It's actually ordered in, in the causation, the number of steps to produce an object. And so all objects in the universe are, in some sense, um, exist at a layer. That's defined by their assembly index. Um, and the size of each layer is growing exponentially.So, 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 sort of 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, uh, 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 they're close by in that space, and they can actually get to it by an almost shortest path for that object.

   29. LFLex Fridman1:13:30

       Yeah, the, the almost shortest path is the most likely. And the-

   30. SWSara Walker1:13:33

       Yeah.

Episode duration: 2:51:58