263 - Sara Imari Walker: Astrobiology, Aliens, and the Origin of Life

28:55So this is like a whole side tangent. But the idea, like in physics, the way that we write all of the equations in physics, all of the laws of physics are written as an initial condition in a fixed dynamical law. But I mentioned this problem, this open-ended evolution in biology. And also there's a lot of interest in the fact that biological systems seem self-referential, which in some sense implies that they can change the rules of how they behave as they go. The classic example is like epigenetic modifications of the genome, which can be inherited and change the dynamics of how the genomic information is read out.

29:30But there's lots of examples of this kind of feedback where the dynamics of biological systems somehow depend on the history. And so Paul and I were playing around with this idea in cellular automata models and these kind of things with asking questions about if the laws of physics weren't fixed, but they changed as a function of the state of the system, like what kind of systems would those look like? and so I was really thinking about how life basically constructs itself and how would you build a constructive process as a trajectory in a dynamical system it turns out like I don't think that's exactly the right way of the way I just said it I think is still the right way of franking about state-dependent laws I don't think work for lots of reasons one of them being you can always just write it the dynamics as a fixed law at like sort of a another sort of coarse grain scale but anyway that was what I was thinking about when I met Lee and Lee was thinking about this idea of non-trivial trajectories, which is kind of a similar sort of constructive process, but really thinking about it in an embodiment in chemical systems, like these molecules are producing things that you wouldn't expect.

30:38And he has experiments that are actually not reproducible. So one of the things that he always says is he's looking for reproducible irreproducibility. So you might get to a similar level of complexity, but you won't see the same molecules again, because the trajectories are so, there's so many of them of the way the molecules could actually react and form more complex structures that you're building into the space of more complex molecules and seeing the same sequence of events twice is almost impossible because the space of the commonatorics is so large. But you might get to like the same level of complexity or something.

31:17So that's what he was thinking about. And then I got really sold on collaborating with him and like thinking more deeply because of the way that he was thinking about embedding it in measurement, which was the initial development of assembly theory and this idea that you could measure the complexity of chemical systems by the minimum number of steps to produce a molecule, recursive number of steps. So we can get into that if you want. But the basic idea being there's some kind of shortest possible way of getting to a molecule. And if you can measure that, then you can kind of bound how complex the system has gotten.

31:52And there's some conjecture that we have that the original life is the transition above a certain threshold and complexity. There's like an abiotic upper bound of how complex a non-living system can be. And life is the mechanism that crosses that via selection and evolution. Information processing are all the words that people like to use as buzzwords to describe life. So to circle back, so that's why I started working with Lee. and I think we both have kind of a similar spirit about this nature of paradigms to get back to your original question that both of us were much more interested in actually solving the original life than working within the existing paradigms so a lot of times what you find is like when you're trained as a scientist you're trained a certain way to think and you want to kind of contribute within that way of thinking and we're not really taught to just like kind of forget the things we learned and just look at the phenomena that we're trying to study and ask from first principles, how would I solve it?

32:51And I had a really deep, probably because of my training in theoretical physics, but also my personality, I had a really deep conviction that none of the current ways that we were talking about it, we're going to solve it. And we just need to start from scratch. And Lee had a similar conviction. And so in some sense, when you're doing that kind of science, you know, you're not working within a paradigm. And then the question becomes like, what paradigm time would emerge if this problem was solved. And I think about this a lot because I think, you know, if in 20 or 30 years people are saying the origin of life is solved, what kind of new explanation does humanity have for the way that reality works?

33:26And I use this to guide the way that I think about the problem. Like what things should the original life solve? What else should explain? What other areas of science should it impact? What kind of questions can we ask? So that gets back to your point about Mike Levin. I very much agree with this perspective that science, like in some ways, when you're asking a question like what is life, people have definitions for life and then they'll work on that question within that definition. But if you have an entirely new way of framing it and it becomes a quantitative tool and maybe a new theoretical framework, that opens up all kinds of ways of asking questions that we didn't have before.

34:04And I would, I still believe this, but like I used to say this a lot more than I have been lately, but I think one of the things that I've always pushed, try to get new ideas into my field is exactly for this reason. Because even if I fail, like, you know, maybe if the program that we're working on doesn't end up being the ultimate explanation for the origin of life. Like, I think it's the most promising approach we have right now. I'm really excited about it, but it could fail. And if it does fail, you know, I at least have the hope that like our effort and all of the work that we're doing would really inspire a lot of other people to think differently.

34:33Because otherwise, if we don't do that, the field is was totally stagnated and we're not going to solve the problem. When you need to build up your team to handle the growing chaos at work, use Indeed Sponsored Jobs. It gives your job post the boost it needs to be seen and helps reach people with the right skills, certifications, and more. Spend less time searching and more time actually interviewing candidates who check all your boxes. Listeners of this show will get a$75 sponsored job credit at indeed.com slash podcast. That's indeed.com slash podcast. Terms and conditions apply. Need a hiring hero?

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35:38That was wonderful. But I do imagine that some of the more technical terminology surrounding the work that Lee does and that you and Lee did together might leave some of our audience a bit curious. so I'll have more questions for Lee when hopefully we speak one day but yeah he's also a big fan of yours so we talk about your your interviews a lot it gives us a lot of inspiration so it's really cool lovely that's awesome well first I just wanted to make sure that I can like accurately paraphrase some of what you were saying so one of the the broader questions in Lee's work are is I mean so what constraints within chemistry which I suppose would be determined by the laws of physics your area maybe limit the development of molecules that could be biologically functional is that a decent paraphrase of some of it yeah a bit I think um I mean there's a few things that he's, you know, really thinking very deeply about, I think one thing is, you know, even like, what is the definition of a molecule?

36:56I think people take this for granted. Like we think what molecule, we know what molecules are, but Lee's always going back to first principles and thinking about like, what do we actually experimentally interact with and what can we actually do in the lab? And so his concept of a molecule, I think is really different than most chemists. And it's really, you know, he has this conception, it's, you know, like a mole of something that you can hold identical copies of a mole of objects. And so a mole is just a very large number for people that don't know, it's like 10 to the 23. So it's a lot of molecules, and they have to be identical.

37:31And the reason that this becomes significant, because it excludes things like proteins, which people think are molecules from being defined as a molecule, because proteins have a lot of variation in their structure, because there's these huge macro molecules. And so when When you actually have proteins in a cell as objects, they're actually like kind of a statistical ensemble of very closely related objects, but they're not identical. And there's some kind of information blurring in some sense where like you can't actually make the exact identical structure once you get to like these much more complex chemical systems.

38:03And so that also gets into like ideas about chemical selectivity and how error correcting could a biological system be to make exact copies of a structure in these spaces that get so large that, you know, just the sequence space alone is astronomically large for proteins, right? So like, you know, people estimate even for small proteins, you know, like there aren't enough atoms in the universe to make every protein that's length 200. So, you know, like the space of possible molecules is huge, but then you get this sequence and it folds in on itself and that structural space is also large and there's a lot of variability.

38:40So I guess my point on taking this tangent is to say that I think like he's thinking about this sort of hierarchy of the complexity in chemistry and the way to engineer synthesis pathways into this more and more complex space in a way that I think is very deep and very profound and also gets into some of the hardware that he's developing in his lab because he's got this whole thing on the computer, which is a universal Turing machine for chemistry, which allows an operating language over like the standard equipment that chemists use in the lab to try to regularize all of the known synthesis and be able to build pathways that actually go further and further into chemical space and build more and more complex molecules.

39:23And I think people really underappreciate how hard this is. There's kind of a sense that like, if you dream a molecule, you can make it. And that's just not physical reality. It's like, you can dream a perpetual motion machine, but it doesn't mean that you can build one, right? Like there are constraints from the laws of physics. And so I think he is, yeah, and I guess that's where your point, your question was, like, he is really deeply thinking about, like, what are the actual constraints in the space of possible molecules? And how do you figure out how to navigate within those physical spaces, those physical constraints to actually move through this space and synthesize things that have never been synthesizable, potentially in the history of our entire universe?

40:01because I think people also don't appreciate that, you know, most of the molecular diversity that we know in the entire universe exists on Earth because we have human chemists and we have robot chemists. And so there's just like this astronomically large space of possible molecules that could be built. Biology is building them and human chemists and our technology are building them, but we don't know they exist anywhere else and they may never exist anywhere else. Just like a lot of the structure in our physical environment, same principle applies. Like, I don't know there's microphones anywhere else in the universe or red stanford hats yeah um well that's that's all really helpful and toward where i was going with my paraphrases we spoke a bit about how experiment is so vital to science and how it can show that this narrative that really sounds great might have some holes in it.

40:55And I was just wondering if there's maybe a simple experiment that Lee has done or that you and Lee have done that demonstrates one of these constraints and has been fruitful for your research. Yeah, I mean, I think the one that is most well-known and it's the easiest to explain in sight is the one where we had this conjecture that there's a threshold in this property we call the assembly index, which is this minimum recursive constructive path to make a molecule. So thinking, you know, you take the parts of a molecule, you put them together, and then you take another part and add it, and you try to build up to the molecule, there's the shortest path.

41:38We had this idea that if the abiotic universe is this combinatorial explosion, where you have no constraints, and any molecule can be produced, there should be an upper bound to how far in number of these steps you can get into chemical space and actually observe a molecule being produced outside of life. And so when you do the experiment, the expectation there is if I take totally non-living samples, things that are maybe they're dead or they're abiotic and they were never alive, and you subject them to this measurement, which we figured out actually how to measure this property using a mass spectrometer, you would expect there to be a threshold.

42:23And then above that threshold, you would expect only to see molecules if they were products of life. And that was indeed actually what we found with this experiment. It was published in Nature Communications a few years ago, where at about assembly index 15, we see like a really dramatic steep drop off because of this exponentially growing space of combinatorics is our explanation for it, where we don't observe molecules in the abiotic samples above that. And we do observe molecules above that in living things. For example, ATP, I mentioned, is about 21 for its assembly index, but something like methane is like one.

43:02So you have this huge spread. This number, this indexing of each unit of the assembly index is actually a unit in an exponentially large space. So it's kind of deceptive to think like, oh, it's only 15 steps, but it's 15 steps where each step, there's an exponentially larger number of possible molecules with that level of complexity. And so the amount of selection and evolution to get to 15 is actually, we think, quite high. And to get past that is exceptionally high. And so the conjecture of the theory is there is a firm divide there between what we would constitute being life and not life. And this gets into some interesting philosophy aside, like that's not the same distinction potentially between alive and not alive, because being alive in my mind is much more about the active construction process, the process of evolving and building novelty and constructing new possibilities.

44:00And that's sort of the dynamics of moving through this space, but this is just a feature of the space, right? It's just saying like we have this thing we call the assembly space, which is like the actual causal structure underlying the space of possible molecules, ordering them hierarchically by this recursive depth, the assembly index, and then saying there's a firm divide there of the amount of selection that you need. And then if you're past that, it's life. And then there's all kinds of living architectures that might exist in that space. but that's sort of like the starting point of it. I think it like the analogy I like to think is like sort of like trying to define your space-time background but we still need to understand like the dynamics of like how masses move in that space and those kind of things so there's still like a lot of theory development to do but I think of the assembly space is like this the physical space the combinatorial causal space that life exists in and and life is formally defined in that space which gives you new philosophical ideas about what life is but that was the experiment experiment.

45:01And the experiment is super cool. Just an aside, because I started reading Inventing Temperatures, it's like where you get in the philosophy, right? And I forgot the author's name, but I love this book, you know, about the history of temperature and like how hard it is to measure temperature. And you don't even think about these things, right? Like, it's so obvious, you know, things are hot or they're cold. And we feel like nowadays, I think like when I say it's 100 degrees outside in Phoenix, like that feels like an objective measurement about the physical world, but it's not really, it's like, it's, it's a very, you know, it's very contingent on our technology and the way that we decided to define temperature and temperature itself is, you know, notoriously hard to find because like, how do you define the boiling point of water if you don't have a temperature scale with a fixed scale to actually measure it against?

45:49So, and you know, like this one thing that's super interesting there is like why the invention of absolute zero was important even though it's not a temperature we can measure this theoretical abstraction that's the limit of having no heat becomes really important for grounding our scales and allowing to talk about an objective temperature scale that we then reference things like water relative to and so there was like a whole process of all these experiments at different temperatures and pressures and you know like you know is the water aerated or not um but anyway long story short like complexity is a concept that's been around for decades and people think we can measure complexity because we can maybe you know take a network and we can actually you know label some states of the world in the network and then quantify some feature of it but actually taking like a physical device and saying this is an intrinsic property of the object that i'm trying to extract and like ground in some theoretical abstraction like that's the process of measurement and that's incredibly hard to do.

46:49And, you know, every time that we end up doing that process, we invent entire new fields of physics. And I think this is what's the most profound thing about what Lee did with his team, and which is really dramatically underappreciated, is they basically invented the metrology of complexity, which is, you know, pretty wild. But like they, you know, the conjecture is there's an absolute scale, the assembly index, it's a causal scale, it's about the amount of causation that's like physically present in a material. And you can measure it by actually looking at the mass spectrometer and counting the peaks or actually looking at the vibration modes in the molecules.

47:26So they have several different ways. There's a paper that was published in ACS Central Science a year or two ago, looking at NMR, infrared, and mass spec and actually showing that they could measure this property of molecules. And so I think that is actually the most revolutionary part of assembly theory like the rest of it follows like once you have a measurement it's like most of the theory you're building is just based on the foundations of like what the measurements of the physical world are telling you and like how you want to build a philosophy around the measurement but the measurement itself is actually what the physical test is and like becomes the way that your theoretical ideas interface with the physical world and so so to me this is exceptionally interesting is just to think about measurement and what it gives us in terms of new theoretical possibilities.

48:17As always, I have so many thoughts about what you just said. The first one is very simple. I have never heard the word metrology before. I'm guessing it's the study of or the field of measurement. Yeah, yeah. I know. And then another thing that I'm finding very fascinating is you point out it's extremely difficult to measure temperature. And what's so funny to me is that I just had a recent episode with Tim where we were talking about measuring time. And it's in many ways, like the exact opposite, I think. Because it's very easy to measure time. I mean, You could have a sundial. I mean, it's just one second per second.

49:08But you have no idea what it is that you're actually measuring. I mean, there's so many questions about what time is. Right, right. Whereas temperature is kind of like a stipulated definition. Yeah, it is more definitional. But I guess this is probably that's actually a really interesting point. It's kind of weird because time is intrinsic to like almost every object, right? Like you can make almost anything into a clock. and and that's kind of an odd property of it which also you know like if we get into the philosophy of time like I think time is pretty fundamental to the structure of a universe and I think assembly theory like we take time as the fundamental substrate but it's much more of a sense of causation but but this idea that you know measuring time it doesn't have an objective scale is actually you know that's the whole premise of relativity and there I think you know the thing that you do take as objective is the measurement of the speed of light.

50:03And if you assume that that's fixed in every reference frame, an objective feature of the physical world, you get all these other properties about like the relative properties of clocks. So the scale of time becomes rooted in this other, again, physical limit of the universe, like absolute zero or speed of light. Like you can't actually, I mean, light obviously travels the speed of light, but we can't accelerate anything massive to it. So it's like, it's interesting to me that these kind of objective features of the universe are also kind of intangible, like they really exist in the sort of space of abstractions of our theories that allows us to then anchor our theories and measurements in these ways that we feel are intrinsically objective, and they end up being much broader and much universal explanations.

50:48Since we've been talking about assembly indices, and you've referenced assembly theory, maybe now would be a good time to just ask, I mean, what is the crux of assembly theory? Yeah, I think, you know, we're still developing the theory. So it's like an incredibly exciting time. And I'm like, kind of changing my descriptions of it all the time, which I think is really intellectually healthy. But the way I think about it is really to say that like causation itself is actually a physical substrate or time is a physical substrate. And so we always think like, you know, like the ticking of clocks, we're just passing through time.

51:28We don't think about time as being a property of objects. And I think what assembly theory is saying at its core is like sort of a foundational feature of physical reality is that the objects that evolution produces, actually, they physically embody the causation, the assembly index as we measure it, of this recursive history of their construction. And so objects can be deeper and deeper in time, depending on how much evolution was necessary to produce them. And so that's sort of the key conceptual, I think, and philosophical innovation of the theory is really to start treating time as a material property.

52:07I haven't seen that in any other theory of physics. And it's really counter to the way that we talk about time and most theories of physics. But I think it's actually really necessary to explain living complexity and like these things that we have sort of had hand wavy words for, like what is an emergent property? You know, like, like, like life is an emergent property, right? Like it's not reducible to its atoms. It's something else. And if it's like you can't reduce it to the physical structures like of like, you know, atoms bumping around in space, like what what is it about me that's different than just another random pile of atoms?

52:44Well, the difference between me and just all of my atoms randomly assembled is that I'm the product of 4 billion years of evolution that have constructed every single molecular component of me, every single organ, every single idea in my head over the process of billions of years up into the last few thousand years and all the way into the modern age of cultural evolution and the things that have led to the way I think and interact with the world. So I am a constructed object and all of that structure, I feel like, and this is what I think assembly theory is telling us is actually a physical feature of me.

53:20If I look at myself as an extended causal structure in time and I can like unroll and measure this recursive history as a physical attribute. But I wouldn't see that feature unless I was actually like, you know, trying to look at that kind of measurement seriously. um and so you know like we don't you know it's it's sort of like like the abstractions in our theories are not like you know the reality of our everyday experience all the time but they they somehow become deep you intuitive so like you know the curvature of space-time you know that's keeping me in my chair right now is something i have awareness of but our ancestors had no awareness of because it's it's a learned attribute of the world that comes from our theories and it becomes a way that humans actually can communicate the regularities in the world that we see around us and it becomes societal knowledge and technological knowledge.

54:09And what I see for assembly theory is if we had some kind of idea of the physical space that living things exist in, this kind of causal space that we call the assembly space, you know, it would have similar explanatory power and would give us some intuition for why living things seem so odd. So yeah, that's that's the short answer I guess uh yeah one thing that you said earlier that I really liked was you drew this distinction between life and alive and it makes sense it's not not something I've ever really thought much about but you would still want to point to a dead thing and say well this is an example of life even though yes it's not alive and this is a very basic question but for something to qualify as life because it'd have to have been like gone through this assembling process yes okay I would like to ask you a very tricky question that just comes to mind and maybe you you just won't want to answer it um but you you mentioned Dan Dennett in in your book and And when I, one of the times I spoke to him, I asked him about, I don't remember what the question was, but it involved a very, it's like canonical philosophical example or thought experiment called like a swamp man.

55:40where I think what happens in, I don't remember the original thought experiment, but basically a tree is struck by lightning and it just suddenly is an atom for atom replica of somebody without his causal history. And I'm just wondering if lightning strikes my mug and it suddenly is just an Adam for Adam replica of me that remembers everything I've ever done and is walking around and talking but it was kind of just assembled by magic maybe we wouldn't want to say it was assembled at all would your definition still allow for this like mug version of me to be living and alive, even if it wasn't produced by evolution?

56:31So I think it's an interesting feature of thought experiments that we can basically make stuff up. And so like when you're saying this, you're talking about it as if it's a physical possibility. And I'm sitting from the perspective that that's actually not physically possible. Because in order to make that mug, it requires a certain set of like causal constructive circumstances. And that is actually the physical mechanism how that mug is made. It isn't possible for lightning to spontaneously make a mug. You know, like it's so it's it's an interesting thing to me because I you know, this is also where like the philosophy comes in.

57:14You know, I was trained in traditional physics where we have this sense that anything in the universe can spontaneously fluctuate into existence, because if it's permitted by the laws of physics, it would just be very low probability. Right. I was going to ask you about Boltzmann brains next. Yeah, yeah, exactly. I mean, you're basically asking the Boltzmann brain question, right? You're saying, except your mechanism here is lightning. And I'm saying, no, they're not actually physically possible. The only mechanism our universe has for making brains is by the process of evolution of a billion years on a planet.

57:48And they don't fluctuate into existence in isolation. They come, you know, with a population of other organisms that have similar architectures because that's actually how our universe generates these structures. And I think it's so weird because it's so obvious that that's actually how it happens, right? Like that's what we observe. yet we have these other theories and ideas in our head that these things can just happen magically or spontaneously. And because we've invented the concept of probability, we can just hide it under the concept of probability and say it's a low probability event and therefore we don't observe it.

58:24And I think by doing that, we're actually missing the entire mechanism of how the universe even actually does these things that it does. And in some sense, what you're advocating for when you say lightning can do that is that intelligent design is possible, which I don't subscribe to. I think all of the design is self-constructed by the universe. So the universe can make lots of things permitted by the laws of physics, but it actually has to do the evolutionary work of designing those objects and building them. And there's no design for free. And it's super funny. I was actually right. because we and I are working on this piece on the philosophy of assembly theory.

59:02And I was writing about this. And I was just thinking about the fact that like, spontaneous generation was ruled out as a possible explanation for biology in like the 1800s, right? This is a very famous set of experiments that were done about, you know, like, could you get mold to grow if you like, you know, had a vacuum tube and, you know, like, you didn't let anything get in and stuff. Because before that, like, you know, things would just appear on bread and stuff. So people thought the original life might be happening everywhere all the time. People thought mice would appear in haystacks. Yeah, exactly.

59:32It's wild, right? So then like when you do the careful experimentation and you control for all the causation that could possibly go in the experiment, you don't see it happening. Like it just doesn't happen. And you see this also with origin life experiments. This is my point about like people make ATP in these experiments, but they're putting the causation in. Like if they just let the system sit there and they didn't actually try to design ATP into the experiment, they wouldn't get it. And of course, this is like why the intelligent design is a debate, you know, like why people that subscribe to that view feel like they have a foothold in original life because we're not being explicit about the fact that we're sneaking in our own design.

1:00:13And so that leaves room for these alternative hypotheses. And I'm just saying, actually, no, like if we actually just said, you know, design is not a spontaneous process. It's something that has to happen over time and it requires selection. Like and then that that's actually the physics you want to understand. It just really changes your perspective on things and it changes the questions you ask. And it's completely consistent with all the observational evidence, but it's completely counter to standard physics. which is also super interesting and probably underappreciated. Quite interestingly, Dan didn't answer the question the way that you did exactly, but he said, well, I just don't trust those crazy thought experiments that are so far from the edges of reality that we just can't trust our intuitions about them at all.

1:01:13Yeah, I feel the same about philosophical zombies and things. There's like all kinds of things that people use to reason their way into a box or even like many worlds I feel like is in the space. It's just like it's like these. Yeah. And I just I guess I'm a very strict physicalist and I really want to work from what we actually see and actually observe to understand things, because that's the best bet to have testable theories. and sometimes thought experiments are useful. Like I think the way that Einstein did thought experiments was absolutely brilliant, but he was always trying to anchor his thought experiment in a physical way.

1:01:59And I'm saying, no, actually, you have no mechanism for that thing. happening in your thought experiment. So it's not a good physical thought experiment because there's no maximistic explanation. And I bet if that actually happened, there would have been some human-like intelligence behind the lightning designing the actual cup. It's not like the causation has to be there. That's just the way the universe works. And this is a bit orthogonal to the conversation, but you don't think that Boltzmann brains are physically possible? No, I don't. Maybe I don't totally grasp the problem of Boltzmann brains for physicists because clearly they offend our intuitions that these are possible and that we're more likely to be Boltzmann brains.

1:03:01But my understanding of the problem was that our best theories of physics do hold Boltzmann brains as possible physically. So is what you're saying that you just think our theories of physics are in some way wrong if they allow for the possibility of Boltzmann brains, even though they're what we use right now? Yes. Do you know, can you point to where it is that you think the problem lies? Well, I think it goes back to this issue of where does complexity come from? So I'm just using complexity as kind of like a not very well defined word right now, just like colloquially. But, you know, like a brain is a very complex object.

1:03:51And the argument, you know, I guess from the physics side is, you know, there's a small probability that a particle could fluctuate into existence. So there's a smaller probability that two could and there's a smaller probability that an atom could. And then, you know, like you can just do the whole cascade all the way up to a brain. and I think it's not, I think that is physically not possible and I think it's not accounting for a few things. Like one, it's not accounting for the persistence time of each object along the chain. So the probability of like getting to a brain requires all the pieces to stay stable long enough to actually come into the configuration of a brain.

1:04:29It's not like a poof moment when it comes into existence. And I think that a brain physically is evidence of the constraints that actually built it. And this kind of hypothetical experiment, there are no constraints. So like why in the common, like the combinatorial space, I mean, the size of a brain in terms of the number of combinations and configurations of those atoms is not even astronomically large. It's like uncountably large. Like I don't, you know, when I was talking about the size of chemical space, It's like chemical space is so large, no one knows how to estimate its size. This is why I laugh when people want to say that they're going to do machine learning and explore all of chemical space, or they're going to solve the drug design problem indefinitely.

1:05:13And I'm like, I don't think that you understand how big these spaces are. They are so big that the universe cannot generate every object. How are you going to compute this space? It's like, it's just, it's huge. And so it's really, so that's a probabilistic argument that it's just so low probability, but I actually think it's not physically possible. I think there's something about the mechanism for generating structures that's not spontaneous. I think there's a non, like my sort of, my model of reality right now, which changes pretty often, but is that the underlying structure is non-deterministic.

1:05:56And it's not really predictable in a sense. So like this comes from like, you know, if you look at quantum foundations, it's like very clear that there's some indeterminism or non-deterministic behavior at the base level. And I think the deterministic structure that we see is all the product of selection. So and co-constructive constraints in the physical world, because there's all of these structures that have emerged that are interacting and basically allowing certain structures to persist in time and then construct new structures, which can also persist in time. And so I think if you take time seriously and you think objects that exist require a mechanism for their existence, and that mechanism often is the physical embodiment of other objects that actually can be the cause for their existence.

1:06:45That's not like a totally radical thing to say, but it's a very obvious feature of our environment. I think if you take that as your base level of reality, it's way more explanatory, way more consistent with what we see, and to the point that we had earlier, opens new questions. So, you know, one of the things Lee and I are working on is actually, like, how would you, what experiment would you do to, say, rule out the possibility of Boltzmann's brains by saying complex objects actually require a minimum constructive history? And so I think there is a way to actually say, you could do an experimental test, is spontaneous generation possible?

1:07:23Or is it the case that this object has to be constructed over time? And if it's the latter, then that would be consistent with what we think in assembly theory and the foundations of physics that we're building. If it's the former, it would be consistent with current physics. But I don't see any empirical evidence to suggest that Boltzmann brains are an actual real physical possibility, even though they might be a logical possibility based on current theories. Okay, that's very, very helpful. I'd like to take a step back for a moment and think about how assembly theory helps us make sense of and perhaps define life.

1:08:03And you write about the paradox of defining life, and there are lots of major difficulties with it. A few that come to mind are, you mentioned, like, universality is very important, especially as an astrobiologist. You want to divide a definition that's not parochial that we can apply elsewhere. You also want one that's testable so you can definitively say and back up your claim that something is life. And then, I mean, a very difficult part of this is going to be avoiding arbitrariness and satisfying our intuitions about the divide between the inanimate and the animate and this sort of like magical leap between the two.

1:08:56Are there any other sort of major concerns about defining life or do you want to say anything else about the ones I've mentioned? yeah i think the other one that i have personally as a criteria but i don't think that most people would hold is this idea of explanatory power like like whatever whatever way we come to understand the original life or the nature of life i think that that theory will be really deep like very explanatory deep and so and this probably is a fundamental physicist to me i'm obviously approaching this from, you know, a certain paradigm of training, but also just like, what do I find intellectually interesting?

1:09:41The things I find most interesting and powerful are like the theories of physics that we've developed over the last few centuries. And I always thought like long before even working on assembly theory, I always thought since I started working on this problem, that whatever theory would explain the original life would be as deep as our theories of quantum physics or gravity and would radically change some of our perceptions of other fields of science because when you build knowledge that deep it really should transform problems in other areas and so one of the hallmarks i look for for the science that we're doing is that it has implications in totally surprising ways for other areas of research that are that seem completely disconnected um and those could be everything from like It has implications right now for drug design, thinking a lot about how would you detect life in all kinds of different substrates, including technological ones.

1:10:38And then also, there's a lot of artists that I'm talking with that are very interested in fundamental concepts of life and really interesting ways. So I think there's all kinds of ways that theories interact with the world and become parts of cultural knowledge. And I'm deeply interested in that process. And I think we don't think about that enough because we don't really think, it goes back to what we were saying earlier, when we're doing science, we don't really think about ourselves as existing within a paradigm or being one of the people that's trying to build a new paradigm. And I'm always very cognizant of the fact that I'm working in the latter category.

1:11:14And the kind of knowledge you build when you're doing that is not just like, I'm my one individual person in my lab, but there's some kind of radical transformation that needs to happen in our knowledge and that will happen, you know, in all kinds of areas of human society all at once. And so I'm just always looking for like, where are the patterns where people are thinking about the same problem as me, but they don't know that they are. And so I do a lot of my theoretical work actually going into like, totally different communities. And it's like very funny, because it's like academics think they're interdisciplinary if they like, go to the next department over, but I'm like, you know, like, I don't know, I think it's like, you know, philosophy and art and humanities and technology and like everything.

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1:12:50daily routine can be remarkable. That's the science of wow. Colgate Optic White. ...is impacted by this problem potentially. And I also want people to be excited about working on Origins of Life. So I think that's very important to make those connections because you get like, you get more people like really understanding the depth of the problem and what we're trying to solve. And then that gets more people like, you know, brains on it, which maybe potentially helps solve it. I want to dig a bit more into a feeling you mentioned at the beginning of your response that our theory for the origin of life might involve new transformative physics and this is something that jumped out at me as I was reading your book because you mentioned I think being in the audience of a talk that Sean Carroll gave where Sean says something to the effect that the standard model of particle physics is sufficient to account for everything we know about life and you found this deeply troubling and I'm wondering why that is or was at the time just because I'm not a physicist I said it still is yeah no I know Sean's always going on about the core theory I you know I obviously have a huge amount of respect for Sean but I have like an allergic reaction to these statements.

1:14:14Like I find them offensive. So, yeah. Well, I think, I think there's a couple of things about it that bother me. One is, I mean, you know, like physics as a discipline or like as a mode of human inquiry is only a few hundred years old. And if I think about the billions of years of evolution and acquisition of knowledge on this planet or the thousands of years of human history, it just seems like incredible hubris to me that we would stand at this point in history and actually have an ultimate explanation for anything. I tend to think of science in an evolutionary context, that scientific ideas are part of the evolutionary structure of our biosphere, and they're evolving modes of knowledge.

1:14:58And in some sense, just like you have an abstract, you have a conscious experience, and having representations in your brain. In some sense, I think like what science contributes to human society is like the deepest abstract cultural representations of the way the physical world works, right? So when I'm developing theories, I'm really thinking about it as like the societal construct. And so if I look at things like the core theory, or like the standard model, or I look at current theories of physics, I think those are amazing explanations in their domains of validity and for the problems that they were addressing, but they will be subject to revision.

1:15:36And the kinds of ideas that overturn them are probably going to be quite radically interesting. Because if you look at the progression of human history and the ideas that we think over time, and you look at the way scientific theories have developed alongside of them, they're very culturally embedded. As far as our conception of the world is the lens through which we interpret experiments and build theories and build understanding of the world. And so, so I think, I think there's no such thing as a final theory. I think there's no such thing as a theory of everything. I think there's evolving states of human knowledge.

1:16:09So that's sort of just my baseline. And then, so I could never sit up and make a statement that like, this is the final theory and it explains things. And like these other things are details on top. I would think that no matter what, even our deepest theories will be revised over time. Like there, there's no fundamental ground to our understanding. Um, but I think that's actually critically important because my fundamental ground right now is actually this very constructive evolutionary process. And so to me, theories being a part of that process, you know, they must have, like, if you want, if you want the fundamental structure, you know, like the evolutionary structure to keep evolving, the theories themselves should be getting deeper and deeper or more explanatory, or they should change with sort of the historical context that allows us to see more of the world.

1:16:57And I really love, you know, thinking about the history of Thomas Moynihan is a historian of science. I'm a fan of his work. But he, you know, he's been thinking about like how big humans think history is over time. And it's radical to me that like, you know, a few hundred years ago, people literally thought that they would be reliving their life a few hundred years from then. Because like, they just thought the universe was so small and like the possibility space was so small that, you know, like they would just be reliving the same moments over and over again. You know, and as we study the planet more and we realize the planet is very deep in time and we study space and we realize the universe is very large, we're also seeing like, you know, in the fossil record, all of these animals that have gone extinct, we're starting to see our technologies emerge in our environment around us that are things that had never existed before and our ancestors would have no language to describe.

1:17:50And so we're just seeing this rapid expansion of like the space of possibilities around us. And I think it's just suggesting that this model that the universe has an initial condition and a fixed law and none of the structure can change. And it's not like it's fundamentally not an evolving structure doesn't fit the way that humans feel about the reality around them and the history we've actually lived. And so to me, that suggests like it is a time when we need revolutions in our theories of science because they need to actually account for these features. And so the core theory is great. It was developed in the 1900s when we felt like we had control over reality.

1:18:25Our technology was new, but, you know, like we could really just get to the fundamental structure. And now we're living in a time where nobody knows what's going on. You know, like artificial intelligence is like blowing stuff out of the water for people. And, you know, the internet and like social media and the way that we see active manipulation of social structures and understanding that the reality as we perceive it is not the reality out there and all kinds of things are changing just like the psyche of the human species, I think we really need explanations that match the reality we live.

1:18:58And so I don't think the core theory is wrong. Like I think it's a really great explanation for particle physics. But I think this idea that the structure of that theory, you know, and the sort of mathematical structure we impose on that part of physical reality as being the whole map of the entire territory of everything it's just like it's just massive overreach i could talk about the standard model and fundamental theories all day but i in the interest of making sure that that we get toward aliens uh by the end of this conversation i would we can steer away from that and just briefly before for aliens, I want to ask why assembly theory, just briefly, might meet some of these criteria that we've laid out.

1:19:47So universality, testability, satisfy intuitions, explanatory power. Yeah. So universality is easy. And so is testability, because I think those two things are kind of like very core to what the theory is about. So the universality part, like just to give some context for people who might not be so familiar with astrobiology in the audience, the sort of way that we've looked for life in the universe so far is mostly predicated on looking for structures that we found on earth. And so those structures could be specific molecules. And oftentimes they are. So we might go to Mars and try to look for amino acids or DNA.

1:20:28We go to exoplanets and we look for evidence of oxygen in the atmosphere because that's what photosynthesis on Earth has produced as an abundant signature of life on this planet. And so assembly theory is a bit different because we're not saying we're looking for particular molecular structures. Like we wouldn't go to Mars and look for DNA. In my mind, it doesn't make sense to do that unless you're looking for planetary exchange between Earth and Mars because they might have actually shared material and maybe life on Mars is the same as life on Earth. But if I'm thinking about like a planet, you know, having a genova original life event, there's no reason it should discover the same exact life that we have on Earth.

1:21:04And so what we want is a way to detect life, no matter what its physical instantiation is, as far as the molecules that compose it. And we do that with this assembly index, which you can measure for any molecule and the copy number of molecules. So I didn't talk about copy number too much. It's actually very deep in the theory. but there's this idea that you have to look at the reproducibility of the structure in order to actually understand that it's a product of life and that's in part related to these probability arguments but I stand by what I said before and I don't think it's like even ruling out a improbable thing I think it's an impossible thing which is more related to deep intuition but anyway long story short if you're looking for complexity in the universe you're not looking for a particular structure and so the universality part is you know our conjecture is all life in the universe would display evidence of having a high assembly which is this sort of joint feature of assembly index and being abundant so abundant complex objects anywhere in the universe our life um and so that's just universal across any substrate it could be you know silicon life it could be carbon-based life It could be, you know, I don't know.

1:22:15I mean, the options are limited on the periodic table, but there you have it. So that's the universality part. And we, you know, like we think that the physics of assembly theory is universal across our universe. So there then the definition of life is a universal process. Our universe engages in is the process of constructing complexity and doing this along lineages of causation where, where, you know, life is this constructive process that happens over time. So that gets into the measurement process. Like it's that feature is measurable for molecules. We're working on extending it to other substrates.

1:22:49So I mentioned, you know, trying to detect life and technology as also like minerals. One thing that I'm working on in my lab right now is detecting an exoplanet atmospheres. So there you're not talking about the assembly of an individual molecule. You're talking about the assembly of an entire atmosphere, which is like the causal imprint of a biosphere on an atmosphere. Is that measurable over interstellar distances? And we think it is, and we have ways of getting at that. But the measurement there is always looking for this evidence of high assembly. That would be the universal metric for evaluating life.

1:23:24I have now forgotten your second two questions. Well, so there was universality, there was testability, there was explanatory power, And then there was the satisfaction of our intuitions about the distinction between life and non-life. I love these as criteria. This is good. I feel like everybody should have these standards for theories. I actually like I like this is one thing I'm also trying to advocate in the astrobiology community is like it's great. You like detected oxygen. But like what does it tell you about life? And like do you know anything else about life? on the like i i actually feel like if you're if you're trying to measure life it should tell you something about what life is um and so um yeah so the the measurement part like there's this idea of the threshold the threshold would be substrate like it would it would kind of vary with substrate so like we have a generalizable theory uh we think that we can universally apply it because it's just looking for complexity by this kind of causal definition that we have this metrological definition, I should say, which is embedded in causal principles.

1:24:30But yeah, so the explanatory power, part of it is the fact that this theory is generalizable. So, you know, like I would use the same theory to go in the lab with Lee's computers. And if he was, you know, running all of these robots, doing these messy chemical soups, trying to look for the emergence of life in the lab, we would use the same theory for that as we would use in a telescope looking for evidence of life in an exoplanet atmosphere. And there is no other framework in astrobiology that has that breadth of explanatory power. And this is also something I thought very early in my career.

1:25:07I wrote this paper. I think it was like the original life, a key problem for physics or something. And I was just trying to make the argument that original life and life detection were the same problem. And it's wild because people don't think they're the same problem. But fundamentally, you're just asking, like, what is the difference between non-life and life? And how do I measure where that happens? And that's exactly what assembly theory is designed to do. So that's one place of the exploratory breadth, right? So you can connect these very different areas of astrobiology. But as I mentioned, you know, the theory also, you know, potentially holds promise, like, if we do want to try to apply it to technology and look for evidence of the emergence of life in technology, or we want to look for the emergence, like these successive original life events on Earth.

1:25:54Like, when is it the case that a new substrate emerges in the evolution of biology, let's say tissues, and they start to have enough complexity in their morphological shapes that we say, you know, this is now in of itself a living structure and not just a collection of cells as a multicellular aggregate. So there are ways where you can think about these transitions and evolution also being measured in this framework. And then, you know, Lee's done a lot of stuff with drug design. So he uses assembly theory and this idea of the assembly space to imagine new molecules, and then, you know, go and actually produce them in the laboratory.

1:26:26So there's a lot of places, there's people that are applying it, the theory to music, actually, and doing musical composition. So we get like a lot of emails from people doing all kinds of weird things, which is really cool. Um, so, um, so that's, but, but for me, like the explanatory power is actually, you know, gets to your last question is like, you know, what is it telling us about life and what is it telling us about, you know, more fundamental questions we might have. And so one of the reasons I got kind of obsessed with the, the metrology and when I started really doing, I think deeper philosophy was really trying to think about like, what does the assembly index mean?

1:27:01If I'm saying that the causal history, not the actual history, I don't even know what what actual history is, right? History is reconstructed from the present, right? It's always an inference about a sequence of events. But we're saying there's some objective feature of objects that tells you something about the minimum causation for them to exist. That's what the assembly index captures. If we say that that's a physical property of an object, like what are the implications of that for thinking about life? And when I was early in my career, I was writing with Paul Davies about the original life transition from a more philosophical perspective.

1:27:36we wrote this paper called the algorithmic origins of life and the conjecture there was that the origin of life would somehow be quantified by a physical transition informational and causal structure um like and what does that mean and like and you know i like remember trying to think about how to describe it at this time it's like information gains causal efficacy over matter um which um you know it's basically the idea that like you know we see all these kind of things that look very abstract and quote unquote informational in our environment, but they seem to actually shape the physical world around us.

1:28:11So one way that that's happening right now is just through human language, right? You and I are talking, we're separated in space and we're separated in time from your audience, but there's this communication happening and information that we're saying might actually physically change something somebody does in their day. Like, right, you know, they might stop and rewind and be like, what were they saying? or, you know, like these events will happen because this information was transferred. And it feels very non-physical, right? Like what is a word as a physical object? It's kind of like, obviously they physically exist, but to us, they don't seem physical.

1:28:47And so, you know, like the big mystery in the original life is like, you know, where does the information and DNA come from? Like DNA has kind of similar properties. It's, you know, it's an inert molecule, but like if you put it in the cell, the cell has machinery that reads in quote unquote information out and like makes all the structure that has function. And so these are like very big mysteries as far as, you know, understanding biology from kind of any physicalist perspective, um, in the deep foundations, if you take this idea of these kind of self-referential informational causal properties seriously.

1:29:20And so Paul and I just said, you know, like if you could get a physics that could explain that maybe it has something to do with top-down causation was their conjecture at the time that there actually are higher level causes um that would be the original life um and actually i think assembly theory does a much better job than any of the things that we put in that paper so some of the things we conjecture really state-dependent dynamics top-down causation information being causal all of these kind of things are very hand wavy and very difficult to reconcile with physics as we understand it now or even the physical world um and assembly theory just has this really simple explanation that the objects that life produces are deep in time.

1:30:00And when you talk about things having informational properties, you're really looking at time having material attributes because all of this time in history is actually a physical feature of now. And the evidence of that physical feature of now, like this time in our environment now, is what we call information. All these abstract properties are just because these objects are causally deep. And so I found that deeply satisfying because it suggests to me that life exists in this space we hadn't really accounted for as a physical space before. And so all these sort of mysterious animated properties and things, you know, we don't have a language to describe them because we're not actually treating causation as a real physical material space.

1:30:41And if we think about it as real physical material space, life becomes understandably like it's natural to talk about the properties of life in that space. It's natural to measure the properties of life in that space. It's natural to understand the novelty and the exponential growth of like the combinatorial diversity on a planet in that space. But those are not like, but that's not a space that exists in current physics. And it's not a space that we treat as physical in the same way that we treat the coordinates of space and time. But as you pointed out, like time is also hard to measure. And we had to invent clocks in order to have a physical concept of time.

1:31:18and we had to invent, you know, the metric system and like, you know, physical measurements. And it wasn't until we had really good, sophisticated measuring rods and clocks that we could come up with laws of motion. And I'm just saying like the metrology like suggests that causal depth is a real physical feature of objects. So if we take that seriously and we build out that space and we can do that for other substrates and we start to see, you know, causal depth emerging and new substrates, that's life. And I find that explanatory. I don't know, like people have different definitions of explanatory.

1:31:51But I also find it satisfying because, you know, a lot of the discussion, we talked a little bit about emergence. You know, like life is an emergent property. It's not reducible to the atoms. But then what is emergence? It's some magical glue that animates atoms when they're in a particular configuration. That's not what assembly theory would say. Assembly theory would say is that configuration of atoms is actually has this causal architecture. and this causal architecture is associated with a living thing so it's alive so it's making life a physical property and it's also assigning to life causal power like these properties of free will that we feel or agency and things are not just epiphenomena that sit on top of the core theory they're actually real deep physical features of our universe and in some sense the process of life is the process of how the universe generates structure so like things that exist most of the things that could exist will have to be made by a living process like because the space of things above this kind of complexity threshold that the universe could create is far larger than can exist in a non-living universe it's not even exponentially large it's like infinitely large if you believe in infinities I'm not sure I believe in infinities of physical object but like intelligibly larger so almost everything the universe will ever generate as a unique object will be the product of life and i think to me uh i don't know it's like you get those happy feelings of like you know like you hear carl sagan or like somebody talking about like the mystery of the cosmos and where the way the universe gets to know itself i just feel like this kind of explanation is so deep at that level about like how deep and intrinsic to the structure physical reality we really are um i find that empowering and i find the fact that like you know thinking that the universe is not actually deterministic the future hasn't happened yet it has to be constructed no one can actually even predict what the future looks like like we actually have to make it um like those kind of narratives i think are are really interesting and they feel much more deeply intrinsically alive um than the sort of philosophy around current physics Speaking of philosophy, there's one last theoretical question I want to ask, and it has to do with what I see as a bit of a conflict, not a problem with your theory at all, but a conflict that we intuitively have where we want our definition to not be arbitrary.

1:34:28it we don't want there to be arbitrary cutoff points so for instance you mentioned being deeply offended by the by sean's comment about the standard model one thing that i found deeply offensive in high school was learning that viruses are technically not alive like that just seemed to totally offend my intuitions about what life is and my question then is what assembly theory says about this this dividing line between life and non-life and whether there's sort of an in-between period yeah like a sort of vague penumbral region where things are neither one yeah i think i think there's definitely some some ambiguity and and some like i wouldn't i wouldn't even put the sharp threshold at 15 myself, because I think it's not like a phase transition where there's some kind of singularity at 15 or something mathematically.

1:35:37I would be very convinced if we found an assembly index 40 molecule, for example. That's absolutely, for me, would be life. I think I've done some calculations on cosmological bounds on assembly index for an abiotic universe and it's like around 40 or 42 which is kind of hilarious if it's 42 but um just about like the highest like complexity object the universe could make spontaneously uh in assembly theory um so something above that would convince me but but anyway to your point like i think there's a little bit of fuzziness around the boundary and i used to think that um like oh like when i was working with paul and this origin life transition stuff like there's there's a lot that's really related very deeply to the like theories of computation and like fundamentally like is computation like a theory of physics and are living processes embodiments of computation and I have lots of reasons I think that life is not computation and I'm I and including formal arguments that are like really deeply embedded in physics but that's a whole nother thing and I'm trying to work that out now but but the point being like I actually thought the origin life transition was uncomputable like it's like like you just wouldn't know like that you were actually at the the boundary point it would be like on either side um but i i think um like to your point it goes back more to this idea of paradigms and like redefining questions and so i don't really think like even when i say life and alive i'm using that to try to guide to a better way of describing these regularities of the physical world embedded in current ways of describing things.

1:37:20And so one of the things that's very hard about doing physics in new conceptual spaces is you don't have the words to use. So oftentimes what I do is I try to subtly change the meanings of words as I use them to try to get to what I actually mean, but start from something familiar that people actually understand. And that's a very active cognitive process for me because I'm very aware of this feature that we don't know how to define life. We don't know the right language for describing it. I think that the case is going to be if we had a theory that explained the original life, then the definitions of life should derive from the theory.

1:37:56So we're starting in this place where we have some rough definitions. We're talking about this phenomena. We're trying to tunnel through using experiments and metrology to build a new theory that might allow us to actually just look at the regularities in the physical world we associate with living objects. and then we somehow have to carry people along so that they understand that that's actually still the same thing that we're talking about when we're talking about life because that's a problem we're trying to solve but then in that process eventually you know i think if the theory and the program is successful there'll be a totally different way of describing what life is in 10 or 20 years and we'll just have a different language for the way that we talk about the phenomena and so sometimes like these these gray boundaries and things they're gray because we don't have any way of talking about them.

1:38:43So, yeah, so I don't know if I'd say like, this is life, this is not life, I would say, you know, this process is very causally deep. And it's, you know, like, it's so deep, like, this could not form spontaneously. And this, you know, like, and some of these structures are much deeper than others. And like, so, you know, like evolution must have been happening on this planet for a really long time, because there's intelligent things that, you know, have as much ability for explanatory power and universal computation as humans do, or whatever the criteria is. But yeah, so yeah, I guess it just goes back to, we try to label the physical world in like discrete categories, and it just doesn't really, human labels are not reality.

1:39:31And I think we misunderstand that a lot, that our languages of description are not the physical world. Chronic migraine is 15 or more headache days a month, each lasting four hours or more. Botox, onabotulinum toxin A, prevents headaches in adults with chronic migraine before they start. It's not for those with 14 or fewer headache days a month. It prevents on average eight to nine headache days a month versus six to seven for placebo. Prescription Botox is injected by your doctor. Effects of Botox may spread hours to weeks after injection, causing serious symptoms. Alert your doctor right away as difficulty swallowing, speaking, breathing, eye problems, or muscle weakness can be signs of a life-threatening condition.

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1:40:37The Colonel's cooked up a new$10 bucket of the day just for you. Monday, 24 nuggets for$10. Tuesday, 8-piece fried chicken for$10. Wednesday, 10 wings for$10. Thursday, 8 tenders for$10. Friday, 24 nuggets for... Ooh, you guessed it, didn't you? $10. The$10 bucket of the day deal. Every weekday, only at KFC, it's finger-lickin' good. Prices and participation vary while supplies last. Not available on third-party ordering platforms. Tax extra. Right. I agree with you completely on that point. I'm okay if you don't, though, also. No, no. I mean, it leads to tons of philosophical problems where people conflate our descriptions with reality.

1:41:25Crazy to me. A description might be really useful, and then that leads people to believe that what's, I mean, our descriptive apparatus is reality itself. So I don't want to take us too far afield, but I think that that's what happens with math. I mean, mathematical objects are so useful for describing things that people end up thinking, well, they're real in some physically salient sense. I think it's also hard because the architecture of our mind, like literally like we're manipulating labels. So like people just use that as their interface with the world. I could have a whole separate conversation.

1:42:02Like I'm obsessed with like human minds labeling the world and our confusion over that process. But especially in the age of artificial intelligence, I think this is why people are getting so confused about the technologies that we're creating because of this misrepresentation of labels as reality. But anyway, yeah, it's fun. I like thinking about that. Well, I think, I mean, this conversation, I mean, that could be a follow up conversation, but it's this conversation certainly demands a follow up just about alien life. But in the time that we have, I want to ask a couple of very basic questions that I would be remiss not to ask you.

1:42:40One is the universe is so large. Why do you think that we have yet to discover anything that would qualify as life, even under the assembly theory? Yeah, actually, it's very funny because this is one place where I think Lee and I really disagree. So he's very like sort of optimistic life is everywhere. And I'm much more conservative on this front. And probably I get that from from Paul Davies because he just really instilled in me like, you know, the pendulum swings both ways over history. Like in his career, it's like some people think aliens are everywhere. And then there's like a decade where people think they're nowhere.

1:43:16And, you know, and his point is like, the evidence hasn't changed. And I'm like, you're right, the evidence hasn't changed. So I actually, I feel very strongly that I need to be really agnostic about that point because until you have a theory to test and you're out there testing it, it's like impossible to make a statement. So I guess actually my answer is the reason that we haven't detected life and why maybe, you know, Lee can say that life's probably everywhere. And like, and people can, like, I think that's a valid viewpoint is until you have a theory to go and look for the phenomena, um, you're not, you're just literally not going to see it.

1:43:51And, you know, the evidence I give there is very abstract, but it's like gravitational waves. Like, you know, like they're permeating my body right now. They have been, you know, the very active process in our universe. they've been happening on this planet for billions of years that they've been passing through. And it wasn't until Einstein's theories of relativity and really conceptualizing the idea of gravity waves that we could actually go and detect them. It took us a hundred years to build the technology to do that. And if we're saying, you know, the space of the common, you know, saying physical space is large, which has been the traditional concept that people have been worried about with the search for alien life.

1:44:27And I'm saying it's not just that physical space is large is the combinatorial space is large. Like the things a planet could possibly do is astronomically large, just on a single planet. If you think about all the combinations of the chemistry, and then if the chemistry evolves into something like a cell, like how many possible structures are there in the physical universe that could be evolving entities? We don't know. And so, and we don't, we never thought to look for that process as a physical process that happens on planets. And so I think it's just such an early stage that I just don't have any confidence that it's not there.

1:45:02And we just don't see it because we don't have the theoretical framework. With aliens, it's weird because people think there's going to be some aha moment. We're just going to like have some, you know, magical moment where like the UFO descends or we go and, you know, put a rover on another planet. And there's some like little, you know, microbe like right there or some little animal skitters out and like, you know, like sniffs us or something. Right. So, but you know, like it could be, it could be like right in front of you and you wouldn't even recognize it as life. Like, I think we're doing that all the time.

1:45:32We have that in our own history, you know, like there's microbes all over my table and my body that, you know, most of human history, we didn't have any idea we were surrounded by all this life because we didn't have microscopes to see it. So I think technology as a sensor, like technologies as perceptual apparatus that allow us to see the world in new ways and theories as perceptual apparatus that allow us to see the world in new ways are critically important to actually resolving the structure of the world. And I think it's radical to me that at any point in history, we think we've seen everything and we know everything when all of human history has basically been like, you know, we have so much to learn and we have so much of the world that is right in front of us that we can't even see or don't know is there.

1:46:16So I think aliens are just in that space right now. They're in the unknowns and we need new ways of seeing the world in order to see that they're there. Assembly theory, it seems, would tell us that if we land on Mars and we do a soil sample and we find some molecules with assembly index of 43, it's highly probable, very highly probable that their their life in some way i'm pretty sure that geochemistry of mars could not do that but as for much longer range detection so where we're not like in situ there with a robot does assembly theory even though it's in this very nascent stage tell us what kind of biosignatures we might be looking for that would be indicative of life existing on another planet or solar system?

1:47:13Yeah, right now we're thinking a lot about exoplanet atmospheres, as I mentioned, and there, like, the way to think about it is, like, the entire atmosphere, like, the molecular composition is encoding something about the complexity of life on the planet. And so what we're seeing evidence of is the sort of complex ecologies of Earth's organisms with all of these kind of different gases that are being put into the atmosphere by different kinds of ecosystems are actually imprinting on the atmosphere in terms of the structure of this assembly space, the constructed steps for all the molecules in the atmosphere in a way that's very unique to Earth and we don't see on other planets that we know have atmospheres that are not inhabited.

1:47:54And so it's very much looking for this idea of like a causal fingerprint, like how do you detect constructed complexity in an atmosphere? It's very hard because atmospheric gases are very simple, but we still see signatures of it. And it's really cool, actually, because my student working on this, Estelle, totally brilliant, but she actually went and looked over the surface of the earth at different ecological and urban environments. And so if you look over urban China, the assembly is higher than over the Amazon, which is higher than over the Sahara. So you kind of see the imprint in the atmosphere of this biological activity on the surface of planet as far as like what is able to actually enable to happen in terms of atmospheric chemistry and and it's a it's it's really cool to see like in an atmosphere like the same kind of principle applies um so that's the place so and we can actually because of this metrology part of it and we've already developed methods for infrared um estelle and louis who's another person in my lab have been actually um working on taking infrared spectra as we would get from a telescope and getting the assembly index of the atmosphere directly from the spectra.

1:49:04So it will be a measurable property. And then we're just trying to like validate the basic theoretical approach. And then hopefully, you know, people will develop it from there. And it will actually become mission ready to look for entire biosphere fingerprints in the atmosphere. Well, I think the universality of the theory is really just like, I'm not saying this just to be like a cheerleader over here, but one of the most exciting things I've heard of come from astrobiology, just because often when I talk to astrobiologists, they're looking for the same sort of signature you would look for on Earth on other planets.

1:49:49But I like that this theory, it has nothing to do with what we would find on earth it's a test to tell us what other kinds of life or it's it's sensitive to other non-earth-like life systems and that's great yeah ultimately i think like for me that's exciting because i want detection of life to actually tell us something fundamental about what life is so it's like usually people just want to detect aliens because like aliens but i'm like actually aliens are a window into this fundamental physical phenomena that we don't know if there's anything else like us. So like we understand more about, you know, what life is by making a detection, but it has to be because it's already, you know, a theory-related detection.

1:50:31Like it's talent, like the theory is underlying the ability to detect and the detection validates the theory and allows us to better understand the structure of our own understanding of the world. So I, that, that's science to me. And I think this is one of the reasons that astrobiology is pre-paradigm because it hadn't, you know, before assembly theory, it hadn't stepped into this kind of mode of inquiry where you have theories that are, you know, like explanatory that you're testing against the data. It was much more life uses this. Let me go look for it. Well, Sarah, I'm genuinely looking forward to the development of assembly theory and hopefully one day hearing about all of the experiments that are going on astrobiologically.

1:51:16Yeah, so thank you so much for having this conversation with me. I really enjoyed it. It was really fun. Thanks for having me.