Transcripts of our episodes are made available as soon as possible. They are not fully edited for grammar or spelling.

Dr. Walker is an astrobiologist and theoretical physicist at Arizona State University, where she explores the origins of life, artificial life, and the detection of life on other worlds. At ASU, she has built a highly interdisciplinary research program and leads one of the largest theory groups in origins of life and astrobiology internationally. Her latest book Life as No One Knows It: The Physics of Life’s Emergence proposes an answer to the age-old question, “What is life?” Sara joins the podcast to explain how the search for alien life is related to the origins of life and whether we are more likely to find microbes on other planets or intelligent life.

Tom: Sarah, welcome to the podcast.

Sara: Thanks for having me.

Tom: I want to start by asking you: Can you tell me where you grew up and what some of your greatest interests were as a kid?

Sara: I grew up in Connecticut, kind of a rural town, and I liked the city, and I also loved being out in nature. I lived near a river and there were a lot of woods, and I used to go for long walks along the river and look for salamanders, like flipping over rocks and stuff. And that was my favorite thing to do or get on an inner tube and go down the river. So, I have a lot of memories of being in the woods and adventuring with my neighborhood friends. My mom was an antique dealer, so she would take us around Connecticut a lot, go to antique shops, and look at old things. It was a kind of quiet childhood, I guess.

Tom: Yeah. How did your interest in science begin to develop and blossom? Was that kind of in your formal educational setting with certain teachers, in family settings, or on your own?

Sara: I think it was all the above. I don’t really know when I first got interested in science. I think I was always a curious person. I always wanted to know more, but most of what I was exposed to as a child was history and art because my mother was an antique dealer and interior designer. My dad’s a hairstylist, although both of my parents were really interested in science. It just wasn’t like their main passion. And it wasn’t really till I got to high school, and I started taking, more advanced math and science courses. I wasn’t a particularly good student, but there were certain classes that I really liked.

I didn’t really know what I wanted to do. And so, I ended up just going to our local community college, because it seems like the thing to do if you don’t know to do university, but you’re not sure what you want to do with your life. And I took all the science classes they had there. So, I just registered in every science and math class I could, and I like the challenge of it, the intellectual stimulation was great for me. And that’s when I fell in love with physics. I took physics my first semester in community college. I had an amazing professor, and I just absolutely loved the subject. And I couldn’t imagine myself doing anything else.

Tom: Tell me a little bit more about your physics professor that, inspired you. What specifically, grabbed your attention?

Sara: Yeah. it’s kind of interesting because a lot of people that, become scientists Always loved it since they were a child and I had an interest but it’s not like I was like Oh my god, go look at the stars. It’s like a small child or something. I think the thing that really got me when I started at community college was my professor, professor Shaw, was talking to us in this class about magnetic monopoles. And it’s very esoteric thing in physics. But the idea is, you know, every magnet has a north and south pole.

If you cut it in half, it still has a north and south pole. And so, what he was describing to us is how physicists had predicted the existence of an isolated north or south pole, what’s called a monopole rather than a dipole. And the fact that scientists were going out to look for them in the universe. And this idea like deeply perplexed me that the human mind could come up with this idea of something that might be physically real and not know if it’s real or if it’s a real description of nature and then go out and look for it. And I just fell in love with that. the idea that our minds are capable of comprehending reality, but not just comprehending it, we can test our comprehension that for me gave me such an anchor and a security in my curiosity that I couldn’t imagine doing anything else like up to that point in my life. I think I had really thought I was going to be an artist, and I just thought that the rigor of what science provides in the constrained creativity that these things must correspond to the way the world works. I just. Absolutely love that idea.

Tom: Wow. So, great undergraduate experience studying, the sciences, taking all courses they had to offer. Did you go directly from undergraduate to graduate school, or were there some other things you explored?

Sara: No, there was an intermediate step, obviously, because I had to go and get my four-year degree before graduate school. So, I ended up going to Florida Tech, and I also had a really good experience there. For the most part, I had a great professor who had me join his particle physics group. I really wanted to do theory, and he did experiments, but it was a chance to do actual research,

I think, every physics course I took just solidified my interest in doing physics professionally. And so, I really knew, from the time I was 18 and took my first physics class that I wanted to be a professor of theoretical physics and study the deepest ideas in the universe. I was just like, “this is what I’m doing.”

Tom: Tell me a little bit more about how your interest shifted from the kinds of questions that are accustomed to studying to the kinds of questions that I would normally associate with biology. How did that go?

Sara: It’s really kind of my PhD advisors. So, I did my, graduate degree with, Marcelo Geiser as my PhD advisor. You know, most of his career, he’s been an early universe cosmologist, but around the time I was working with him, he got really excited about astrobiology and in particular, the origin of life, and he was looking for a student who to work on a particular problem related to the origin of life, and so he was trying to get me excited about that problem and also about astrobiology generally. So very vividly remember him, trying to recruit me to work on this problem.

I was very adamant that like, my serious stuff was the cosmology stuff, because that’s what real physicists do. And I was doing this other stuff to learn skills and think about a fun problem. But the more I read of the origin of life literature, the more I was like, no one in this field really knows what the origin of life is. There’s no consensus, there’s no conceptual framework, there’s no paradigm, there’s no theory. As somebody who has come into theoretical physics thinking, isn’t it amazing that the human mind can come up with these kinds of very deep descriptions of nature? I just thought, wow, all my heroes of previous generations confronted these unknown problems. That’s the kind of problem I want to work on. And so that’s why I pivoted during my PhD to being serious about this problem that initially, you know, I was doing because Marcelo asked me to.

And I’m grateful for mentorship for getting me excited about these kinds of problems. So, by the time I was finishing my PhD, I really knew I wanted to work on the original life and really understand what the transition from non-life to life was. And I had some ideas about what kinds of things I thought might be relevant. but that’s pretty much cascaded into my whole career and the problem that I’m really obsessed with and think about all the time.

Tom: How would you answer the question? What does physics have to do with life? Isn’t it just math?

Sara: Uh, yeah. I think math is the final stages of physics. So usually, people think all physics is math because you enter an undergrad course, you’re taught what physicists have invented in previous centuries. So, you’re taught the mathematical formalisms they came up with, but you’re not really taught the process of coming up with those kinds of theories.

And to me, that’s what physics is. Physics is the process of looking at patterns in the world and trying to come up with Universal, very deep, abstract explanations for them. And they must be deep and abstract to be universal, because if you want to describe a huge number of things in the world, with a single description, in some sense, that necessitates that it’s an abstract property that you’re looking at, and you can see this across, all the theories of physics we have so far.

So, I like to give examples like, mass seems very physical to us now, but it’s a concept that was invented over many generations of humans trying to figure out how to measure motion and what would be the relevant variables. And, by coming up with mass and being able to invent clocks that had precision to measure things on seconds, time scales, we were able to come up with laws of motion and laws of gravitation.

And then we realized that motions in the heavens were the same as motions on the earth. Right. And so that’s a very abstract idea to think that gravity is controlling, why we’re both sitting in our chair right now, having this conversation. So, think what is underappreciated about the art of doing physics, and I really do think this part is an art, so I don’t think I’ve really lost that sort of creative, artistic part of myself, is trying to look at the world and trying to understand something we don’t about it that’s rather deep and abstract. And once you understand the concepts and you know how to ask the question, then you can formalize it into the mathematical framework. That is something that you can share with other people and becomes the standard way we talk about that phenomenon.

And so, short punchline version of it is, I don’t think is, what’s studied in physics departments. I think physics is a process of coming up with these kinds of abstract laws of nature.

Tom: That’s great. That’s very helpful So when I reflect on the achievements and discoveries of physics the last a century or two, I mean, it is really amazing that scientists have been able to chart and describe and explain that distribution of matter and energy and maybe what the shape of the universe is in terms of what it contains, And yet it seems like none of those kinds of discoveries are translating to the biology outside of planet earth. What’s the discrepancy going on there?

Sara: Well, I think there are a couple of things going on with what you described. One is, we haven’t done what we’ve done in physics where, exactly as you’re describing, because we understand something about laws on Earth, we can extrapolate to, uh, other places in the universe and have high confidence that we’re talking about these phenomena accurately because they match observations. So right now, as it stands, we can talk reasonably well about life on earth. We don’t understand the origin of life. But once you have cellular life in a cellular architecture, we know a lot about the kind of processes that have happened. Obviously, there are open places where it’s still really challenging, like in, embryo development or when you get a diversity of body plans emerging all at once, like these kind of rapid complexification events are still very perplexing. And I think some of the mechanisms of those are related to what happens at the origin of life. because my view right now is the origin of life is a continuous process on earth. It’s not just, it just happened in the past. There’s the origin that started the cascade of it all, but there’s origins happening all the time.

But, when you try to take that process and you say, what would that look like on another planet, we have no rules, no laws that would allow us to start from planetary geochemistry and say anything meaningful about what alien biochemistry would look like or what alien technology would look like. And so, we are still in early stages of being able to take. Things that we’ve observed on earth and extrapolate them in the way that you’re describing that we’ve done for cosmology or astrophysics or planetary science.

Tom: Okay. Do the equations of physics, distinguish between life and non-life in any way?

Sara: Not themselves, because it takes thinking beings like us to write equations down. But I don’t think that there’s anything in the laws of physics that says anything about the existence of life.

Tom: Does that indicate that something is missing or is it like, hey, that’s just a matter for these other scientific disciplines?

Sara: Well, my stance, you know, and I’ll, I’ll get on my soapbox, but you know, I, I also understand I could be wrong is, that I really think there’s something missing. Where did the laws of physics come from? Why is the initial state of the universe as it is? This is why we get things like the multiverse and solutions like that that are trying to fix these kinds of problems. But I think, when I look at the phenomena of life, what I ideally want is something that tells me the mechanism about how is it that you get evolution before you have an evolutionary architecture? And you know, the laws of physics don’t have really a sense of evolution.

They’re not incompatible with the theories of evolution we have with Darwin’s theory, like can sit on top of each other, but if you try to directly connect them to take the laws of physics and build into like Newtonian physics predicts the way the universe behaves for all time, and, Evolutionary theory says you have endless forms, being created as a function of time and constant novelty and selection on that novelty. I mean, just describing what those, two paradigms talk about, like it’s very clear that there’s some dichotomy there. A lot of people have argued that it’s just an emergent property and that they should stick together. But if you try to stick them together naturally to solve the origin of life, they just, don’t fit together. There’s some gap, and that gap, I think, is the laws of physics that we don’t understand that are underlying life and allow you to connect the way that we think the non-living universe behaves and the way the living universe behaves, because obviously they occupy the same universe, and they should obey the same laws. So, in some sense, I guess part of the paradigm that I’m, trying to work on is like, how do you unify the inanimate and animate to talk about the transition between the two.

Tom: Gotcha. That’s what we’re missing. As I was reading your new book, one of the questions that you posed I found quite fascinating. You said that to answer the question, “What is life?” we might need to pivot to the question, “Why does life exist at all?” Can you explain a little to me about the difference between the way those questions are formulated and why you might need to, uh, ask, like, the why rather than the what first?

Sara: So, a question I raised in the book, I’ll start here because I think it’s a little bit of an easier entry point, is if you found a screwdriver on Mars, would it be a biosignature? Right? And so, I think most of us would think that that was evidence of life.

And certainly, if you found two of them, it’s more convincing that some intelligent being put that screwdriver there. We don’t really think it’s a valid hypothesis that if we found a screwdriver on Mars or even one of our, Robots, right? Like a more realistic scenario. There’s, a robot on Mars right now.

Um, curiosity, is it likely like if you just visited Mars and you were an alien being and you looked at curiosity, would you think that Mars just spontaneously fluctuated this one robotic entity into existence on its surface? Or is it more likely that there was something that evolved now, an intelligent being that built the robot and put it there?

So, in some sense, the robot is evidence of a designer. It’s evidence of some intelligence behind it. And this is tricky when you get into the foundations of physics, because obviously we have no, notion of what it means for an object to require design or evolution to produce it. The assumption is every object in the universe, no matter how complex can spontaneously fluctuate into existence. And so, I started thinking That’s an assumption that everyone makes. We think this is true. We have no evidence for it. We’ve never seen a complex object spontaneously form right? They’re always constructed by another system. And this is the problem.

We see life being created all the time, but we don’t see the origin of life happening because it always requires life to make more life. So, if you just start from that observation, you say, well, what if it’s the case that our universe can’t make those objects spontaneously? How would it make them? And so, this goes from the what is life to the why life question, because what I started thinking about, probably around 2015, 2016, I wrote this essay called the descent of math, which was really trying to understand mathematics as part of the evolution of the biosphere and not thinking about it as like a perfect Plutonian system. The why life question becomes.

If the universe cannot create things, unless it evolves a capacity to do so, or has the information to do so, which I think are the same thing, then life exists to expand the possibility space of what can be generated in our universe, and that’s a why life question, and that gives me A reason to think that life is actually very deep in the structure of what the physics of our universe is because it now becomes necessary to explain the existence of some things.

And so, I think the why life question is important because when you’re asking what life is, it’s been hard to do that. People have been asking that question for over a century and not had success. If you ask what life does and what life does uniquely, you can ask questions about the complexity that life creates. And this also motivates. Why life might be a feature of the universe. And so, it’s sort of third order question you get to when you’re trying to really build an explanatory paradigm for what life might be.

Tom: I want to ask you a few questions about extraterrestrial life. Can you tell me, is the search for extraterrestrial life related to the search for the origin of life? Because those, I think at a first glance, seem quite distinct.

Sara: If you go to astrobiology conferences, the biosignature people looking for life in the solar system, the biosignature people separately looking for life on exoplanets, and then the origin of life people traditionally have been in a whole different, Set of rooms talking about a different set of problems and so those things even by professional scientists haven’t been considered strictly related from my perspective.

I want to understand this thing that we call life and how we might find it elsewhere, but also how we might generate it and how the universe generates it. So, when you’re going to that level of description and you really want a theory that works, it needs to be able to explain life on other planets as well as the origin of life on Earth. And so, you start to realize that these problems are very deeply connected.

And so, my thought always was, is kind of similar because we’re looking for, A universal phenomena, if life exists on other planets and life is a category that nature recognizes and not like a weird fluke that happened on our planet, it’s a universal phenomenon that happens on some planets, maybe not all planets.

And we understand the mechanisms or the foundations of what that thing is. We should be able to test those principles in the laboratory here. you know, first contact with alien life is going to be in the lab. We’re going to solve the origin of life, and the origin of life is going to yield some, different chemistry,

Tom: Yeah. there’s a great quote from your book. It said, aliens visiting the earth might assume that automobiles are the dominant life form on our planet. How might they come to a conclusion like that?

Sara: So that I got from Carl Sagan. He was being a little flippant. I think, just talking about standard definitions of life, like, it metabolizes, it reproduces, it, moves you open any standard biology textbook, you’ll be introduced in chapter one. What is life? Life is this. Description of things if it meets these seven categories, it’s life. And, he was just pointing out like, well, cars fulfill all these things. Are they life? So, he was trying to use that to poke fun at these definitions and that they were inadequate. I think his examples are brilliant and good at pointing out that if you draw a boundary around a definition, you’ll have things like cars that you meant to exclude that get included.

You also get things that you want to include that get excluded. Viruses are always in this boundary, too. So, I try to use that as a launching point to say that this hard boundary is maybe not the right way of thinking about it at all. I draw a lot of inspiration from the way he thought about it.

Tom: Do you think we’ll have a better chance of, discovering, finding, very basic forms of life on other planets? Or that it’s more likely to discover intelligent forms of life?

Sara: So, if the original life is common, then, something simple like a microbe or cellular life should be lots of places, but it’s harder potentially to detect because the distinguishability of those kinds of planets from abiotic worlds might be tough, but technology, is a later stage of evolution. It might be rarer because it requires. much more time to generate, but it might be more obvious when we see it. and so, these are sort of the trade-offs that usually are discussed: microbes might be common, technology is rare, technology is easier to see, microbes are harder to see.

So, I tend to be agnostic about a lot of the questions about the search for alien life, because I’m not sure it’s there. without a mechanism for the origin of life, we had no idea if any other planet is inhabited.

I think it could be everywhere and completely obvious, and we just haven’t seen it.

Tom: Yeah. It makes me reflect on what kind of evidence is indisputably biological in nature. And if you’re looking from a great distance, any kind of evidence you want to look at is on the scale of a planet or a planet’s atmosphere. And so much of biology is at a much smaller scale than that. And so, you’re looking at Something that life produces rather than the life itself.

Sara: There’s an interesting point here also, which is, I think that we have in our heads, and maybe this is because of science fiction and like, Hollywood depictions, we just think, when we detect alien life, we’re going to immediately recognize it. And I think the actual discovery of alien life is not any individual detection or set of scientific data. It’s the explanatory paradigm you build about what the nature of life is because answering the question of alien life is answering the question of what life is because you’ve understood it deep enough to recognize another example.

So, the example I like to give, and I did talk about this in the book, is gravitational waves. They’ve been permeating our planet for as long as we’ve been here, as long as life has been on this planet, and no organism or technology was able to recognize that they were there.

It took the brilliant insights of many generations of scientists, culminating with Albert Einstein’s theories of relativity, to predict the existence of gravitational waves. But it took us a hundred years to develop the technology and precision to measure them. And we have no theory for alien life, no explanatory paradigm.

And, you know, sometimes enough slow pieces come together that we get these rapid paradigm shifts. And I think that’s what we’re undergoing right now in the study of life. And if you get to the other side of the paradigm shift, you have some kind of consensus about what this phenomenon is and, that First contact.

Tom: Yeah. Sounds like your life’s work really is in, building foundations. I’m thinking really like architectural metaphor of you’re working hard on what is the basis by which this will be constructed. But at this point, it’s let’s get the foundation solid.

Sara: Yeah, I realized early in my career. It wasn’t going to just be a scientific problem, but it was also a cultural challenge, when you look at major transitions in science, the shifts in science were never isolated events. They are always coincident with things happening in other areas, like what was going on in art or what was going on in politics, or what was going on in economics; all these things are connected because we’re all living at the same time. And there are a lot of changing views about how reality works from many different perspectives. And so, I always, had a deep sense that solving the origin of life was not only going to require shifting the culture scientifically, but just shifting the broader culture and that other people might be recognizing features of the same kinds of problems. And I look for inspiration, not just in my colleagues in science, but also across everyone basically alive now and how they think about this problem which I think, is a little bit unusual, but I think it’s critically necessary. And again, it goes back to looking at history. If you look at history, everything’s connected to everything else.

Tom: Yeah. That’s fascinating. It makes me think that the world as we live in it right now has all kinds of environmental problems, political problems, and social problems, but the way you’re describing how scientific advances get made is they’re wrapped up in the cultural and historical turbulence that they’re a part of. So, I wonder if perhaps some of the problems that we’re experiencing in human society might themselves be springboards to some of the bold discoveries that you’re imagining. Yeah.

Sara: I also think, one of my motivations, I recognize that a lot of the ways that we talk about what we are, as life or things in the universe, seem self-limiting to me.

So, you know, traditional physics, you’ll have a lot of popular science communicators saying the universe is deterministic. We know everything at the level of elementary particles; therefore, you have no free will, and you don’t matter to the universe they don’t maybe say it that starkly, but that’s the implied message. And I think that observation. observational evidence in our daily existence about how rapidly humans are causing change in our world is completely contrary to that narrative. And I think it leads to a lot of dichotomies and a very deep epistemological divide between what science is telling us reality works as and what we’re actually seeing in our life.

And I think that’s a powerful argument, big problem. And so, a lot of what I think about and see is that the kind of physics that we see in life is much more descriptive of how we feel about our existence in this point in history. And that it’s important to build those narratives and not just build them as scientists, but build them collectively as a society how is it that we want to talk about ourselves?

How is it we want to talk about how the universe works? And for me, that needs to be an empowering narrative because I think life is the thing that generates the future on our planet. And if we understand that mechanism, we understand that process. And we understand that we have some agency and control in that process.

It radically changes our perception of the future and our role in it. And so, I think we need new narratives, new ways of talking about how the world works.

Tom: Uh, with the time that we have left, I want to circle back kind of where we started here. And I’m imagining you in your childhood as an aspiring artist who loves, to draw, to create, to imagine. Come back in a time machine, to interact with her, what kind of conversation would you have? What kinds of experience would you want to impart? or questions you might want to ask? What might the interaction look like from your vantage point now?

Sara: I talked about how I was pursuing science out of the curiosity, but it was also part of escape in some sense. So, I’ve always used deep intellectual thought to escape the pain of existing. and I kind of laugh about that, but, you know, like, everybody in their life has challenging things about their childhood and things that are hard to cope with.

Some things in your life don’t end up the way the way that you thought they would and a lot of our, stories of our lives are built by the people around us. And sometimes that can be perplexing if you realize one day that, the story you were sold is not actually the reality that you think is there.

And so that was. very much, my coming of age, without getting, into too many details. So, I think, my perspective on it now is I don’t think I would change anything about the trajectory I had because I, I think it shaped me so much as an individual and allowed me to really be resilient in ways that I think are important, to who I am and to really be.

Stubborn in ways I also think are important. Like the fact that I really wanted to work on these ideas, and I was insistent on them.

So, I guess the old me would tell the young me to keep doing it but try to learn from every experience. Everything about me is built by my history, and there’s no way of changing that if I want to be me. I’m happy with where I ended up.

Tom: Well, I appreciate you taking time to talk to me today. I love reading your book and I’m looking forward to, having many people, exploring these kinds of topics and conversations.

Sara: Yeah, me too. I hope we have a whole revolution.