Making Sense - Sam Harris - #388

00:00:00.000 Welcome to the Making Sense Podcast. This is Sam Harris. Just a note to say that if

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00:00:45.240 Well, we're about two weeks out from the presidential election in the U.S., and there's going to

00:00:52.560 be a lot of politics on the podcast. I think I have three episodes that I will drop before

00:01:00.160 the election, all more or less focused on politics, but not today, because today I'm speaking with

00:01:07.960 Sarah Walker. Sarah is an astrobiologist and theoretical physicist who focuses on the origin

00:01:15.160 of life and also the possibility of discovering alien life in other worlds. She is deputy director

00:01:22.040 of the Beyond Center for Fundamental Concepts in Science and a professor in the School of Earth

00:01:27.640 and Space Exploration at Arizona State University. She is also a fellow of the Berggruen Institute

00:01:33.880 and a member of the external faculty at the Santa Fe Institute. She is a recipient of the Stanley L.

00:01:40.280 Miller Early Career Award for her research on the origin of life, and her research team at ASU is

00:01:46.100 internationally regarded as being among the leading labs aiming to build a fundamental theory for

00:01:50.280 understanding what life is. And she has also written a very interesting book titled Life as No One Knows It,

00:01:57.420 The Physics of Life's Emergence, which is the focus of our conversation. We discuss the contributions of

00:02:04.540 physics to this topic. Erwin Schrodinger in his famous book, What is Life? The inadequacy of standard

00:02:11.640 definitions of life, the possibility of artificial life, the role of information, constructor theory,

00:02:20.360 assembly theory, the space of all possible structures, the concept of a block universe,

00:02:26.860 the existence of abstract objects like numbers, the Fermi paradox, i.e. where is everybody,

00:02:33.160 and the likelihood of life elsewhere in the universe, experiments that could decide how likely life

00:02:39.240 is to emerge, Robin Hanson's concept of a great filter, how common earth-like worlds might be,

00:02:47.180 and other topics. In all candor, this is a pretty dense conversation, but I loved it,

00:02:53.780 and some of you will too. And now I bring you Sarah Amari Walker.

00:03:06.000 I am here with Sarah Walker. Sarah, thanks for joining me.

00:03:09.740 Happy to be here.

00:03:10.940 My wife, Annika, is the person who put you on my radar, and she says hello, by the way.

00:03:16.440 Hi. Tell her I said hi. It's always nice to hear from her.

00:03:19.120 Yeah, I know you've already spoken with her. She's got an audio documentary that is soon to

00:03:25.080 be born, and I know she spoke to you for it. But let's talk about your book, which is really

00:03:33.720 fascinating. Perhaps you have other topics you want to touch here, but your book, Life as No One

00:03:38.920 Knows It, is where you address this question of, you know, what is life, and how could we recognize

00:03:45.100 it, and how could we come to understand it in the context of physics and chemistry and

00:03:52.120 a concrete conception of the universe? Before we jump in, how do you summarize your background

00:03:59.020 academically and intellectually?

00:04:02.080 I am trained in physics, and I try really hard to not let that training bias my thinking,

00:04:08.540 but I really love the rigor of theoretical physics and thinking very deeply about the nature of reality.

00:04:13.380 So I guess my training is very traditional in terms of a basic undergraduate physics education,

00:04:21.060 and then going into grad school. I was still in a cosmology group at that time, but I have

00:04:26.200 decided to work on problems that are probably very non-traditional from the perspective of physics,

00:04:30.820 but I think they're very much problems for the way we think very deeply and abstractly about the

00:04:37.380 nature of physical reality. So I guess I've just gotten really excited about that stuff. But I didn't have

00:04:41.360 any science or anything like that in my background before starting college.

00:04:46.280 Do you call yourself an astrobiologist or

00:04:49.040 a physicist, or what's the one-word descriptor of your specialization?

00:04:53.860 If I used one word, which is actually two, is theoretical physicist. But I work in problems

00:04:58.180 in astrobiology, but I think at the core I feel much more of a theoretical physicist than anything else.

00:05:04.580 As I said, you are focused on the question of life. Obviously, physicists have done this before.

00:05:12.320 You discuss Erwin Schrodinger's book, What is Life, in your book. What was Schrodinger's

00:05:18.640 contribution to this topic?

00:05:21.420 Yeah, I think the way that he asked the question was really structured, and I think it was the first

00:05:26.700 time that really anyone had laid down with the discoveries at that time, thinking in a very

00:05:32.120 logical manner about how we might reason about the fundamental nature of life based on what we

00:05:38.220 understood from physics. And so his major contribution in that book was actually to think

00:05:41.940 about the nature of genetic heredity. And what he had talked about was the fact that in order to

00:05:49.460 specify all the information in a cell, you really require a lot of information. And the most robust

00:05:54.880 kind of information storage we know in physical materials are crystals. But crystals tend to be

00:06:02.000 periodic. So this is where he came up with this idea of an aperiodic crystal, something that had

00:06:05.780 a non-repeating structure so it could encode a lot of information. And this is heralded as sort of

00:06:12.340 like a prediction of DNA as the genetic material, which is in some sense a quote-unquote non-periodic

00:06:17.640 crystal. So that was one thing that he said in that book that was really interesting and is the one

00:06:22.320 that usually people cite as being kind of, you know, like a major sort of insight. But I think

00:06:27.880 what really caught my eye about that book was how much he really went back and forth about what

00:06:33.940 physics could and could not say about the nature of life. And one of my favorite quotes toward the

00:06:39.160 end of it is about this idea that other laws of physics that we haven't established yet might

00:06:44.220 actually be necessary to explain life. And that was really kind of a big motivator for me when I read

00:06:48.460 it because I really like deeply felt that. And I didn't really see a lot of people talking about

00:06:52.840 it that way.

00:06:54.280 So what is our best current definition of life? And then I guess what are the edge cases where the

00:07:01.920 definition seems to fail?

00:07:04.580 There are a lot of definitions. So in my own work, I tend not to take a definitional approach,

00:07:09.200 but other people have. So I wouldn't say that I agree with these kinds of definitions. But the one

00:07:14.700 that you'll usually see in astrobiology is that life is a self-sustaining chemical system capable

00:07:20.780 of Darwinian evolution. And at first pass, that seems pretty reasonable because you're talking

00:07:26.300 about life being chemical systems that can evolve. But as I talk about through like the first chapter

00:07:31.580 of the book, you know, like if you focus on each word in this definition, they fall apart at some

00:07:36.420 level. And there are really simple things like evolution only happens to populations. So are

00:07:42.200 individuals alive or not? Is life chemical or not? You know, we can think about means as evolving

00:07:47.480 systems or is technology actually a part of life or not? Viruses are kind of a typical definition.

00:07:55.460 They're not self-sustaining or challenging. They typically challenge definitions. They're not

00:08:00.140 self-sustaining on their own. Are they alive when they're in the cell and not alive when they're out

00:08:04.600 of the cell? And even self-reproduction is a bit problematic. So there's always the examples of

00:08:10.240 things like mules. But I like like honeybees as a better example, because it's very clear that

00:08:15.760 a colony of bees is a living thing, but you have individual members of the colony that can't

00:08:22.060 reproduce. And so all of these things seem to suggest that if we want to draw this kind of

00:08:28.340 hard distinction around life, we have all of these edge cases, as you're pointing out.

00:08:32.560 You just raised the question whether technology could be a form of life. And this poses an interesting

00:08:38.320 paradox. Maybe it's only just a semantic one. But the phrase artificial life sounds like a

00:08:46.220 contradiction, right? Because we distinguish between biology and manufactured objects and

00:08:54.920 processes, right? And so we can use a phrase like artificial intelligence without any contradiction,

00:09:00.700 because clearly we've been able to instantiate the function of intelligence in a substrate-independent

00:09:07.840 way into our machines. But the idea of instantiating life into our machines or building machines that

00:09:16.280 are in fact alive, but they're non-biological, that just sounds like a contradiction in terms.

00:09:22.560 So obviously the way you're analyzing life begins to ignore that apparent semantic boundary. How would

00:09:30.060 you think about that? But how would you parse the phrase artificial life?

00:09:34.900 I view that phrase more as a challenge, like in the sense that could we understand life enough to

00:09:42.060 actually instantiate it in a machine or recognize that we had? Because I think one of the issues,

00:09:47.300 you mentioned artificial intelligence, you know, one of the issues that we have and why people are

00:09:51.780 massively debating about whether they're intelligent or whether, you know, our technologies could become

00:09:56.880 animated in some way we might view as alive is because we actually don't have a fundamental

00:10:00.640 understanding of what these things are. And so usually I see the distinction between like when

00:10:06.440 we say things are artificial, usually we mean that they're human created. And that doesn't mean that

00:10:13.020 they're not natural and that the things that we do aren't part of, you know, some underlying

00:10:18.880 fundamental description of nature. And so I guess I don't really like the word artificial because it

00:10:25.840 makes it seem kind of like it's an epiphenomena and not telling us something very deep about the nature

00:10:30.360 of what we are and what we're doing. And that, you know, the kind of physics of the universe that

00:10:35.700 created structures like us might operate even through us to create other things that are living

00:10:39.980 or intelligent. And so this gets more into an evolutionary continuity between biology and

00:10:44.660 technology where you see less of a distinction and you see more of a progression of what evolution

00:10:49.840 does at a fundamental level. I wanted to make another pass on that terrain because I think our

00:10:56.420 intuitions or at least about what other people believe about the meanings of words might be a

00:11:00.860 little different here. I think the distinction most people assume is real is between biological and

00:11:07.840 non-biological systems. So for instance, if human beings created an artificial form of life or a

00:11:15.460 synthetic form of life based on biology, right? If we, you know, through synthetic biology created

00:11:21.640 organisms that had never existed before, but they were nonetheless wet and, you know, cellular,

00:11:29.120 I don't think anyone would hesitate to believe that those organisms were alive. And I also don't think

00:11:35.000 anyone is hesitating to believe at this moment that our artificial intelligences are in fact

00:11:39.860 intelligent. I mean, there's, we haven't achieved general intelligence yet and people are wondering

00:11:45.500 whether we're going to do that and if we do that, whether we're going to kill ourselves. But there's no

00:11:51.780 question that narrow intelligence has been instantiated in our machines. So do you just think

00:11:58.340 that this distinction between wet and dry is just entirely provincial and worth ignoring and that we

00:12:06.620 might have, that a proper definition of life really should be substrate agnostic?

00:12:14.300 Yeah, I think, I think that's exactly what I think. I think it's very provincial. And I think even,

00:12:19.100 I probably don't ascribe the level of intelligence to current algorithms that other people do. So I

00:12:24.040 maybe don't agree that we actually even have narrow forms of intelligence. I think it's very easy for us

00:12:29.760 to recognize things we think of as intelligence because we see ourselves mirrored in our technology.

00:12:34.360 But the technology has been selected to mirror us. So it's, it's very unclear to me how much the

00:12:41.480 substrates themselves are embodying things that we might view as intelligence versus being kind of a

00:12:48.320 false mimic to us. But I think that's a really interesting question. And I kind of take both sides

00:12:52.940 of it depending on the day of the week. But this, this question you're asking about the distinction

00:12:57.760 between chemistry and silicon, for example. So if we could imagine, if we ran an original life

00:13:03.980 experiment, and we evolved a life form, I say we could actually solve the original life, and it had

00:13:08.780 completely different chemistry, then any life on earth has a completely different lineage

00:13:14.180 of evolution and information processing. It, you know, and it was, it was, it happened to be cellular,

00:13:21.180 I think probably cellularity is, is probably deeply intrinsic to, to having open-ended evolution.

00:13:26.200 So I'm okay with that as kind of a, a, a feature that we put on this thought experiment. Most people,

00:13:33.160 I guess, might be, be okay calling that biological life and saying perhaps that was more similar

00:13:37.940 to what cells evolved on earth than a computer program evolving in a computer is to cellular life.

00:13:47.180 But, you know, it is an interesting case that there's a direct lineage from the last universal

00:13:52.500 common ancestor of all life on earth. So the, the first populations of cells

00:13:56.020 that were evolving early in, you know, from the early geochemistry of our planet all the way up

00:14:02.340 to modern computers. And if I wanted to write into a sequence of DNA right now, I could encode it with

00:14:10.380 some information. I could take that sequence of DNA and I could try to amplify it by PCR and read it into

00:14:17.900 a computer. And it might just happen to be the case that with that information content, once it gets into

00:14:24.280 the computer, I actually can create a computer virus and I infect somebody else's computer,

00:14:28.220 right? So there is a possibility of having a direct, this is just sort of a thought experiment

00:14:33.040 that there's a direct line of information between us and the technologies that we're creating. It's not,

00:14:38.120 they're not independent lineages. And this is also the case with large language models. Why,

00:14:42.600 why is it that they have the structure of human language? It's because we were the environment that

00:14:47.700 selected them and, um, human language had to evolve first in the substrate of human minds and in our

00:14:54.940 writing on paper and then into our computers. And so the kind of information that they have is

00:15:01.860 actually a direct lineage from biological life evolved over billions of years. And so I actually,

00:15:08.640 I think there's more of a continuity between our quote unquote biological life and our technological

00:15:14.820 life on this planet than there is potentially between biological life on this planet and a

00:15:20.560 different instantiation of alien life and a different chemistry, which would exist in a totally

00:15:25.560 different space of possibilities and a different kind of chemical makeup.

00:15:29.800 So you've, um, invoked the concept of information here and lineage and the causal continuity between

00:15:38.640 ourselves and our technology and that somehow that being significant. How do you think about the role of

00:15:43.860 information here and is, is, is the boundary between life and non-life more likely to be

00:15:50.920 informational than merely physical or chemical?

00:15:55.200 Yes. With the caveat that I think information, when we understand what it really is

00:16:00.140 foundationally is actually a very physical feature of reality. It's just, you know, like the things we call

00:16:05.760 material are often things that we can measure and things that get regularized into our theories of

00:16:11.320 physics. So, you know, in order to talk about things like mass and charge, you know, we had to invent a

00:16:17.880 lot of technology to be able to measure those properties and they became relevant because they

00:16:21.620 were the ones that we could formalize into theories. So information to me is certainly related to the

00:16:27.080 boundary between non-life and life. And early in my career, when I first started thinking about the

00:16:31.680 original life and really thinking that theoretical physics was the right approach for addressing the

00:16:36.380 transition from non-life to life. My first sort of sets of conjectures about this were that

00:16:41.540 life is where, is the sort of boundary in the physical universe where information has to take on

00:16:50.780 a causal role. You actually have to consider it as a physical feature of, of the systems under study.

00:16:57.980 And it's really hard to do that when we talk about information as such an abstract property.

00:17:04.520 So what we've tried to do, you know, really with the formal work, and this is really a foundational

00:17:09.740 feature of assembly theory that I'm working on with my colleagues, is figure out how it is that you would

00:17:16.140 make information as a feature that constructs these kinds of evolutionary objects. These things that we see in

00:17:24.760 living systems are necessarily require information and storage of information, processing information in order

00:17:31.440 for them to be selected to exist. How do we turn that into something that we can actually understand as a

00:17:37.080 physical property and then talk about this boundary where these kinds of objects can't exist unless there was

00:17:46.020 information acquired over time in order to construct them. There was some kind of selection that happened.

00:17:50.160 So you just, you mentioned assembly theory, which is your, your theory. Let's talk about that and

00:17:57.580 its relationship, if there is one, but with constructor theory, which is, you know, David Deutsch's

00:18:03.800 approach to, I guess, adjacent matters. I know you're a fan of Deutsch's, as I am. He's been on the podcast

00:18:11.560 several times. Is there a relationship between assembly theory and constructor theory? And perhaps you can,

00:18:17.760 you can give a potted description of, of each.

00:18:21.400 Sure. I think there's definitely a relationship and Lee Cronin, who's my collaborator that developed

00:18:29.120 assembly theory and I'm working closely with, and I have both talked with David and I've had a,

00:18:34.020 you know, and I also have worked with Chiara Marletto. So I'm, I'm, you know, over many conversations,

00:18:39.640 over many years, you know, we're still trying to dig into the relationship between

00:18:43.440 their view of reality and ours. But I think, I think there are a lot of interesting parallels,

00:18:49.080 and I think they're scratching at the same kinds of fundamental understanding. So the sort of short

00:18:55.460 description of constructor theory is this idea that the laws of physics should not be cast as,

00:19:02.300 in terms of initial conditions and laws of motion, which many of us think, because that's sort of an

00:19:07.720 inadequate description for life for many reasons, which might be a tangential conversation. But

00:19:11.800 David's very adamant about this not being a final, final description of nature. And instead, what,

00:19:17.560 what he advocates, and Chiara wrote this nice book about also called The Science of Can and Can't,

00:19:23.400 is this idea that we should be talking about things that are possible to do and things that are

00:19:28.640 impossible to do and why they're possible or impossible. And so this reframing is really intended

00:19:36.420 to focus on this idea of constructors as causes for things to happen. But the theory of constructor

00:19:43.400 theory actually doesn't deal with constructors directly. What it does, it talks about tasks,

00:19:48.220 which are things that can be caused to happen. So a classic example is a chemical reaction that might

00:19:53.960 not happen unless you had a chemical catalyst, and the catalyst would be a constructor for the reaction.

00:19:59.420 So the reaction, impossible, unless you have a constructor. And so actually, it's a possible reaction

00:20:06.320 to happen. So it's not actually physically impossible, which is very different than something like a

00:20:11.900 perpetual motion machine, which we have laws of physics that say that literally cannot exist. So they want

00:20:17.220 to classify things that cannot exist as entirely separate from things that could be caused to exist. And

00:20:23.140 this idea of can be caused to exist, I think is fundamentally important, because I think when

00:20:28.120 you get into biology, if you get into evolution, and you deal with things that have knowledge, which

00:20:34.100 was, my understanding is, is David's main interest in developing constructor theory was in part to

00:20:39.960 account for knowledge as a constructor that allows things to be possible that wouldn't be possible unless

00:20:45.880 you had entities like us that understood really basic features of how reality works. So there's this whole

00:20:52.500 space of things that could be caused to exist, but they require a constructor to exist for them to

00:20:56.800 happen. And so that's the whole premise of constructor theory, they abstract away constructors, and then

00:21:01.000 they just end up talking about possible and impossible tasks, but they have a lot of ways of using that

00:21:05.940 to be able to describe really interesting features of physics that are not possible to describe

00:21:14.160 in standard approaches to physics. So that's constructor theory. Assembly theory is really

00:21:21.220 specifically developed to tackle the problem of the original life. That's, that's our main interest.

00:21:27.060 But I think throughout my career, I felt that that problem was very conceptually deep, and so deep

00:21:33.580 that whatever would explain that would reveal fundamentally new physics. And I don't know why I

00:21:39.260 had such conviction on that. But I just remember being a PhD student, studying theoretical physics,

00:21:44.840 cosmology, particle physics, quantum field theory, and thinking that, and at the same time,

00:21:50.400 starting to dig in the original life literature, and just fundamentally thinking that there was

00:21:53.900 something about these explanations of nature that didn't fit this problem. And so what assembly theory

00:21:59.240 says is that anything that requires a lot of complexity, like any object that's very complex,

00:22:08.640 has many independent parts, doesn't happen in the universe for free. There's no such thing as

00:22:15.760 spontaneously fluctuating into existence, a object like a cell phone, or DNA. These things are things

00:22:26.320 that need to be constructed over time, they need information to exist to construct them, or more

00:22:31.500 specifically, they need other objects that set the constraints to enable their existence. So if we think

00:22:36.740 about this idea of constructors, and this reaction can't happen without this catalyst existing,

00:22:41.600 if you build a structure of physical systems that relies on that property, you see that there are

00:22:48.720 certain things that can and cannot happen in the history of an evolutionary process. And so assembly

00:22:54.560 theory is really trying to get at this idea of how would we say that this particular object, if we just

00:23:00.740 looked at it in the universe, was necessarily a product of evolution, evolution being the process of

00:23:09.640 objects, constructing other objects over time, and basically getting a hierarchical stack of all of

00:23:16.360 these kind of constructors that are can only exist because the other things lower in the stack exist.

00:23:22.320 And so it's kind of a way of physically embodying information by actually asking, could this thing

00:23:28.960 exist without a very specific history for its existence?

00:23:31.980 Hmm. There's one counterintuitive claim you make in your book about at this point, which is that

00:23:38.840 you can't talk about a single object in isolation here. So like, you know, like a single screwdriver,

00:23:45.860 you need more than one to get this theory rolling. Is that the case?

00:23:50.040 It's not that you need one to get the theory rolling. I think it's an observational fact that single

00:23:56.160 screwdrivers don't exist. And what we're trying to get at...

00:24:00.540 Okay, but what about like the ceiling of the Sistine Chapel?

00:24:03.520 So this is interesting because it's not, there are some objects that are very refined that like

00:24:09.680 might exist in only one structure, but all of the parts of the Sistine Chapel exist in other objects

00:24:17.580 in our biosphere. So the way they come together as a Sistine Chapel is probably a very unique structure,

00:24:24.160 or even if you think like I would use Gaudi's church, it's like, like that one's totally crazy.

00:24:29.900 I don't even like know how you would possibly reproduce that. But if I can take a step back

00:24:34.840 for a second to explain a little bit more of the structure of the theory, it might become a little

00:24:37.780 bit more evident what this, this copy number feature comes in, which you're talking about,

00:24:42.580 because this is actually the hardest and most conceptually deep feature of the theory that I

00:24:47.660 think is really hard to wrap our heads around. Because it seems like when we talk about complexity,

00:24:52.180 that seems obvious to us when we start getting into this issue of copy number reproducibility of structure

00:24:57.300 as being necessary to understanding that physics, that gets a little bit more difficult.

00:25:03.000 So the sort of key conjecture of assembly theory is if you imagine, again, this sort of idea of all

00:25:07.480 possible things that could be caused to be constructed or could possibly exist, assembly theory's

00:25:13.820 conjecture is that objects can only be made from objects that already exist.

00:25:19.220 And the way that, so if you wanted to try to build into a space of new objects, you have

00:25:24.980 to do it recursively. You have to use structures that are already selected to exist.

00:25:29.140 So we're talking atoms, molecules.

00:25:31.860 Sure. Yeah. So if we think about in chemistry, if you, if you want to think about, yeah, you

00:25:37.780 want to think about making a molecule, you have to use, you know, like you have to use atoms and

00:25:42.220 the atoms have to come together to make bonds. And the conjecture is if you're just thinking

00:25:46.760 about fragments of molecules, if you want to make the next structure, you have to have

00:25:50.900 pieces already in place to make the next structure. So it's sort of a very abstract view of reality.

00:25:54.940 It's a little easier to think about Lego. You put two pieces of Lego together and then

00:25:58.680 you can use that to build the next structure and in a hierarchical way to get to a more complex

00:26:03.360 object. Now, this is important because if you imagine you're randomly searching this

00:26:07.520 space, every time you combine two objects, the space is super exponentially growing because

00:26:12.160 you have this, a lot of diversity of components that you could stick together. And it, it ends

00:26:17.220 up being such a large space, even for very small molecules that the universe couldn't exhaustively

00:26:22.360 search the entire space to instantiate even one copy of every object.

00:26:26.920 So you're talking that you're talking about the space of all possible objects that could

00:26:29.700 be built from those parts.

00:26:31.100 Yep.

00:26:31.580 Yeah.

00:26:32.200 And so I think this is a very underappreciated fact of our reality that there are far more

00:26:38.220 structures that could exist that don't than will ever exist. And we can

00:26:42.140 imagine a lot of that space, which I also think is an underappreciated feature of our

00:26:46.360 reality and is telling something that's quite deep about how much information is encoded

00:26:51.360 in us as physical structures. But just to go back to this idea of this possibility space

00:26:55.720 of, you know, like thinking about all possible molecules, for example, what assembly theory

00:27:00.120 says is that there's actually a hard boundary that the universe cannot cross a threshold

00:27:06.340 complexity. What we call it, we call it assembly index is the measure of this. If you

00:27:12.060 take an object and you take it apart to basic building blocks, and you try to do this process

00:27:17.060 of building it from those atoms by making bonds, or if you're sticking your Lego together, there's

00:27:22.940 a minimal number of steps that you have to take to get to that object. And we call that

00:27:26.560 the assembly index is the size of the minimum space necessary to construct that object from

00:27:32.680 elementary parts. And our conjecture is that if the assembly index is too high, the universe can't

00:27:39.480 make that object spontaneously. Because if you imagine every step you could have an error, and the

00:27:44.320 space is exponentially growing, it's becoming exponentially less likely that you would hit that

00:27:49.000 specific object even once. And if you want to make it twice, that's, you know, a double exponentially

00:27:54.380 less likely by a random process with no selection. And if you want to make it three times, it's even less

00:27:58.560 likely. And so yet, on our planet, at least, we see complex objects, things that take many steps for

00:28:05.580 their construction, their assembly index, the minimum path, the shortest path for producing them is quite

00:28:10.240 high. And we see them in high abundance. And so our conjecture is that there's actually a boundary in the

00:28:16.940 space of all possible objects, all possible molecules, if you want to use that as an example, above which you

00:28:22.440 need to have physical systems that can constrain the space of possibilities to construct specific

00:28:28.260 objects, you need to have constructors themselves that persist in time, the things that can cause this

00:28:33.580 thing to exist themselves need to be persisting in time. So you end up getting these structures that

00:28:39.460 are their their coexistence necessitates that they need to be able to produce each other. And actually,

00:28:47.060 their existence at all, they need something else to exist in order to exist, they need to be caused to

00:28:52.660 exist by another physical object. So that boundary, we think is the origin of life. And it's in a very

00:28:59.400 abstract space, it's in a very large, causal space, which we call the assembly space. And it gives us a

00:29:06.540 way of formalizing this boundary. And in fact, when we look at this property for molecules, you can measure

00:29:11.400 the assembly index for molecules using mass spectrometry, NMR and infrared. And Lee's lab did an

00:29:16.820 experiment where they went in and they found the assembly index for a whole bunch of molecules from

00:29:22.600 abiotic and biological samples. And they were able to show that there is indeed a threshold above which

00:29:27.860 they only find molecules in life of a given assembly index. So that's sort of the basic structure of it.

00:29:34.560 But I think this helps build a little bit of the intuition about copy number, because what happens in this

00:29:41.280 kind of physics is in order to even get to high assembly index structures, you have to have reuse

00:29:47.020 of parts and you have to have objects that persist in time long enough to make the same structures

00:29:52.560 again and again. And this feature means that in order to even get to that space, you have to have a

00:29:59.360 reliability of all of those construction processes all the way down the causal chain, which means that

00:30:04.400 most of these objects will never come into existence once they'll come into existence multiple times,

00:30:08.620 because the constructors that are necessary to make them must themselves persist in time.

00:30:13.780 So of course, we'll get some novelty on the edges. And one way I think about it is we have these sort

00:30:17.940 of diagrams we draw of the assembly space where you're just adding parts by taking parts of the

00:30:23.020 history and stacking them on top of each other to build more and more complex objects. And so most of

00:30:28.420 the things that structure creates, when you actually look at the causal depth in time, they have the same

00:30:34.320 exact history in their construction. It's only the things that the tips that start to have a lot of

00:30:38.240 novelty and variation. And so the copy number feature is actually all the way down that causal

00:30:43.640 history. And when we get things at the tips, you know, that they'll be genuinely novel objects, but they

00:30:50.440 won't persist in time into the future unless they can actually be entirely reproduced all the information

00:30:54.880 in them. So the things that we care about in assembly theory are also very similar to the ones in

00:30:59.120 constructor theory in the sense that you want the transformations that are reliable, you want the

00:31:03.100 causation that the universe has selected to continue to exist to actually be the core of the theory.

00:31:07.740 Because that's the thing that actually ends up being the content of evolutionary lineages over

00:31:12.540 billions of years.

00:31:14.600 Right. Yes, that final sentence is worth reiterating. Because if you're going to imagine

00:31:22.500 that Darwinian principles are at the bottom of this process, you're by definition talking about

00:31:30.100 many copies of things, right? You're not talking about single novel objects.

00:31:34.540 Yes. And I think this is also where this physics really demonstrates that having spontaneous design

00:31:43.640 or spontaneous fluctuation of objects is not explanatory. And I think the copy number is actually

00:31:49.760 at the crux of that. The fact that when we see complex objects, we see them in abundance.

00:31:54.580 And we don't just see spontaneous formation of things that are very deep in time that have a

00:31:59.420 very large assembly index just forming instantaneously. But they actually require

00:32:03.840 evolutionary lineages. They require physical time, time instantiated as a physical material

00:32:08.860 and the objects to construct them. It means that there's not really a possibility for the

00:32:12.800 information content for those objects to exist at every point in time. The information is embodied

00:32:18.480 in the constructors that also must be physical that construct those objects.

00:32:22.480 What would an infinite universe or an infinite many worlds multiverse do to this picture? I mean,

00:32:31.280 given how big infinity is, doesn't that give us the possibility of high assembly index objects like

00:32:38.360 cell phones spontaneously appearing out of nothing?

00:32:41.700 I think that interpretation of reality is not correct. So I think this is one place where

00:32:46.980 maybe the deep foundations of assembly theory and constructor theory diverge. Because I know that David

00:32:51.720 really, you know, firmly believes in many worlds is the most explanatory interpretation of quantum

00:32:57.240 mechanics. And in assembly theory, you know, I think we intuitively feel that there is one universe

00:33:03.760 and it is constructed over time. But the possibility space that's folded up in physical structures that

00:33:09.600 that's actually embedded in time as a physical dimension is so large that when we interact with

00:33:15.560 physical objects, we're only interacting with the tips of their actual, that structure.

00:33:20.980 The tips of their lineage that embodies all the information required to create them?

00:33:26.640 Yeah. Yeah. So I, you know, like if I think about, you know, what I am as an evolutionary structure,

00:33:32.920 you know, like it's easy to think about me as a three-dimensional object that's, you know,

00:33:37.540 about five, three in height and, you know, like I have a, you know, a spatial extent, but we don't

00:33:42.200 really think about the fact that I'm, you know, partially or like parts of me are literally 3.8

00:33:47.480 billion years old because they've been constructed on this planet that long. And I think that's

00:33:52.080 actually a real physical feature of what I am. And so some of the things that we see in quantum

00:33:57.580 mechanics that I think lead to the many worlds interpretation, I think are looking at the

00:34:02.680 the, the physical, the structure of physical reality from the wrong end. So when we do quantum

00:34:07.860 experiments, we look at objects that are very small, they're instantaneous in time and have no

00:34:12.960 memory. And so like elementary particles and the universe has, you know, it doesn't require an

00:34:18.400 evolutionary history for the universe to create elementary particles. But when we think about things

00:34:22.560 like quantum uncertainty or entanglement, it seems to be the case that when you start to build

00:34:28.080 causation and contingency in the system, you have these, these elementary particles that are kind of

00:34:35.160 existing in this non-deterministic underlying reality that doesn't have a lot of structure to it.

00:34:40.580 When they start interacting with each other, determinism emerges out of that. And the, the

00:34:46.880 causal structures actually are self-constraining. And so my view of it is there's a non-deterministic

00:34:52.920 underlying reality and determinism is an emergent property of causal constraints that emerge in,

00:34:59.440 in, in, in the observable universe. What we interact with and the parts that we actually

00:35:05.920 can interact and have structure with are the, the things that we, you know, actually have those

00:35:10.160 deterministic properties. So it's very counterintuitive, but in some, some sense, what we think in

00:35:14.960 assembly theory is living structures that are the most deterministic things in the universe. They have

00:35:19.520 the most causation in the universe because of this feature of the fact that they wouldn't exist

00:35:23.780 without all of these, this causation built into them. Whatever happened to the concept of a block

00:35:29.340 universe in physics? I mean, because one, this isn't my reason for thinking this, but it would, it's kind

00:35:35.940 of, it's an instantiation of, of one way this might be true. I'm wondering whether the very notion of

00:35:43.060 possibility is just that a notion that doesn't actually map on to reality. So then maybe what

00:35:49.480 if there is no possibility space? What if the only thing that is possible is what is in fact actual

00:35:54.760 and there is what happens and everything else is our idea about counterfactuals that aren't just that,

00:36:02.320 not factual. You know, what, what if the future is not only determined, but, you know, just as real

00:36:09.180 and some as viewed from above as the present or the past. And there, there are no such thing as

00:36:15.520 events or processes. There's just a single object. Is there something in fit in modern physics that's

00:36:22.000 discredited that idea? I think it's an interpretation. Again, it's, it's sort of like many worlds. It's one

00:36:28.260 way of building a philosophy out of current theories of physics. So I don't know anything that's refuted

00:36:33.960 that idea, but at the same time, nothing's refuted many worlds. And the reason is because these are

00:36:39.140 philosophical interpretations of theories that give sort of a broad explanatory framework that's

00:36:44.520 consistent with the data and the structure of the theories that we built that correspond to those

00:36:49.040 sets of data. So I think the block universe is maybe not as popular as it was, but I think the idea

00:36:55.200 still has some favor in the foundations of physics. And I have colleagues that, that certainly view

00:37:00.420 reality that way. Does it play well with quantum mechanics or is it just, does it somehow ignore

00:37:06.780 quantum mechanics? I think it lives in a separate space, at least for me, conceptually, there may

00:37:12.520 have been people that have thought about the two together, but I haven't really meditated on thinking

00:37:17.520 about the block universe and its correspondence to quantum events. What I tend to think about the block

00:37:24.120 universe is that it's easy to view reality that way. If you can take a God, God's eye view and think you

00:37:32.260 exist outside of the universe and you can write down laws that also exist outside of the universe and

00:37:38.320 initial states. So again, it goes kind of, kind of to this idea that, that David was rolling against about

00:37:43.860 what he calls a prevailing conception, that initial conditions and laws of motion, you know, are really not the

00:37:50.340 right framing of the physical universe. And if you take that seriously, most of modern physics

00:37:55.280 eventually needs to be thrown out, but a direct consequence of that entire chain of, of the

00:38:01.300 evolution of theoretical physics from Newton up to Einstein is the creation of the block universe idea.

00:38:07.040 But I think, you know, the reason I never felt very comfortable with that conception is that living

00:38:13.220 inside reality and not thinking that our laws of physics can exist outside of it. I tend to think that

00:38:19.340 like my conception of laws of physics is very different than most physicists, but I think

00:38:23.640 about the laws of physics as information that our biosphere has constructed by intelligent beings like

00:38:29.920 us, happens to be us in this case, that have regularized a large set of observations we've seen

00:38:36.460 in the physical world to the point that we, we can say, we feel like these are objective features of the

00:38:41.200 physical world, but the laws themselves actually are also constructive processes in our biosphere. So

00:38:46.260 an example I give in the book is the fact that we understand things like Newton's laws of

00:38:51.700 gravitation or Einstein's theories allows us to do things like build satellites and, and other

00:38:57.120 technologies that wouldn't be possible without that kind of knowledge. And to me, that's much more

00:39:01.740 interesting. The fact that we have a description like the block universe allows us to do other

00:39:06.680 things that we wouldn't be able to do without that kind of description. So I tend not to think that

00:39:11.980 any of our theories of physics are platonic ideals that really describe the way reality works. And I

00:39:16.560 think this, even with assembly theory, assembly theory is a very, um, constructive theory of

00:39:22.460 physics. I, I literally have a conception of the universe that it's constructing itself and even the

00:39:27.900 theories I build are a part of that process. So, so I would expect that theory to play a role for a

00:39:33.680 certain time in our understanding of reality, but eventually it will be replaced by something else

00:39:37.440 that's better. But I think it's, it's to me the best explanation for the nature of life right now

00:39:42.060 that I, I can find. Um, and I really do take seriously the fact that we can imagine counterfactuals

00:39:48.080 as being causal to the reality that we live in. We see that every day as humans, it's, it's like

00:39:52.120 literally a part of our human experience. And that's the part of physical reality that I want

00:39:56.880 to understand is, is the part that's us. Well, so imagining a counterfactual is obviously that

00:40:03.040 act of ideation is in our case, physically instantiated in our brains, right? So it's,

00:40:08.820 as an operation we're performing in spatio-temporal terms. And, and I'll grant you,

00:40:14.940 it has cause, it seems to have causal consequence, right? Because we can, obviously we can talk about

00:40:19.920 it as we're doing now. I guess it's a, you know, we're talking a lot about objects and information

00:40:25.400 and causation. And I mean, this, this is of necessity, a conversation that's pushing us

00:40:32.200 into metaphysics, right? So what, what does it mean for something to exist? What is an object?

00:40:38.580 And what is the, I mean, in what sense can a law of physics exist and be causally effective,

00:40:47.640 right? And, and, and not, and be something other than merely it's many instantiations, right? Like

00:40:53.900 is the, is the law of gravity something beyond the fact that every object that has been dropped has

00:40:59.420 fallen already? Or like, does it, how does it impose its will on the next object that I, I let go from

00:41:06.840 my hand? If it, it has to be something in addition to the instance of that object falling, or, or, or so

00:41:13.300 it would seem, I guess I'll ask you, having vomited all that philosophy on you, let me just ask you a

00:41:20.340 simple question. What, what, how do you view the existence of abstract things like numbers? How

00:41:27.280 does, how do mathematical objects fit into your ontology here? Yeah, I've thought a lot about the

00:41:33.440 physicality of math almost my entire career. And I, I have a very intuitive understanding of what I think

00:41:43.800 math is that I don't really see reflected in the way that I see most people talking about math.

00:41:48.660 And just to take a step back from that, one of the things that I, I think about a lot, and it,

00:41:56.300 it comes, I, I have a formal way of talking about it now with the structure of assembly theory, but it's

00:42:00.980 something that I've thought about a lot longer than that, is this idea that what we call information

00:42:07.260 is deeply tied, like what we think of as abstract things in our environment, human language, mathematics,

00:42:15.480 I don't know, the information content of genomes, like things that seem substrate independent because they

00:42:20.920 can be copied between different kinds of physical meat, materials. You know, that, that feature seems

00:42:26.080 very perplexing. This is one of the reasons that information has been really hard to

00:42:29.840 understand as a physical property. And I think what we're really talking about when we talk about this

00:42:34.100 property of information is we're talking about objects that have a physical size and time, because what

00:42:39.040 you're looking at when you see something that's informational is you're seeing something that was the

00:42:42.520 product of a history to generate it. And so I have this sort of philosophical interpretation of what

00:42:49.940 we're doing in assembly theory, which I think, which comes from the measurements and also just the

00:42:54.060 ontology of how the theory is set up, that evolutionary objects are deep in time and, and the, the size and

00:43:01.960 time, the assembly index is actually a physical feature of the object. And so if you have things that

00:43:07.260 have a, you know, like a smaller depth in time than you do, that you can isolate as physical structures

00:43:13.980 and look at them, you know, they look very physical to us. So, you know, we're built out of elementary

00:43:18.320 particles and cells, and it's easy for us to see those as physical objects, because, you know, we're larger

00:43:24.500 than them, we have enough capabilities within the bounded physical structure we are to acquire most of the

00:43:29.700 information about those objects. So they seem physical to us. But things like human societies or human

00:43:35.480 language or mathematics, I think are, are much larger physical structures than we are. They're

00:43:40.800 much larger in time. And so as smaller bounded objects in time, we can't process, we can't possibly

00:43:47.380 observe all of them at once, and they look very abstract to us. And so the sort of idea of a platonic

00:43:52.060 world to me is just saying that we actually are embedded in this massive causal structure.

00:43:57.300 I'm going to give you an abstract object that is going to stay abstract. And I think it's nonetheless

00:44:02.520 identifiable, at least it's going to seem so when I utter the sentence. So, you know, we have a sense

00:44:11.180 that, we have a very deep sense mathematically that, that there are an infinite number of prime numbers.

00:44:17.160 And if you just assume that we are finite, and our interaction with, with these numbers is going to

00:44:23.600 be, you know, finite in time, however many millions or billions of years we do math, we're not going to

00:44:31.600 get, we're not, there's going to be a final prime number that we interact with or instantiate on any

00:44:38.820 of our hard drives or otherwise use in our cryptography or et cetera, et cetera. So there's a

00:44:44.700 largest prime number that any member of our species, and let's just for the moments assume we're alone

00:44:51.080 in the universe and no one else is going to do math. There's a largest prime number that we're

00:44:55.100 going to talk about, write about, use in some way. But of course there are, there's a, there's a next prime

00:45:01.820 beyond that one, right? So I'm talking about that next prime beyond that one. Does that exist?

00:45:07.780 It can exist in the future.

00:45:10.040 But no, but I'm talking about that. We know that however, however long we live,

00:45:13.800 we're never going to, there's a last one.

00:45:16.080 It exists as an idea, which I think is still a physical object, but the actual embodiment of that

00:45:21.220 idea doesn't exist yet. And I think sort of, it's hard to think about numbers as physical things.

00:45:26.440 So I'll, you gave me a very abstract example. I'll give you another very abstract example.

00:45:30.720 And then after that, it might be good to talk about time getting bigger as part of this,

00:45:35.100 this process, but I'm just going to put a pin in that because I can get to that later if we come

00:45:39.700 back to it.

00:45:40.280 But hold on, but just before we go to other examples, when you say it exists as an idea,

00:45:46.060 it's a little bit more than that because one, you know, we know if we know anything about

00:45:51.220 numbers at this point, we know that it exists in that it is potentially discoverable, right?

00:45:57.660 And we know that it doesn't end in two.

00:46:00.600 Right. Yeah. No, this is a good point. I think I actually like that you're pushing on this quite

00:46:05.480 a lot. This is very fun. So I think if I was going to be more precise in my language,

00:46:11.160 I would say that the knowledge to generate that structure exists. So a constructor that can build

00:46:16.460 that next prime exists, but it hasn't actually mediated that transformation.

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Making Sense - Sam Harris - October 21, 2024

#388 — What Is Life？

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