Sara Imari Walker: Using physics to rethink the definition of life

Sara Imari Walker is a professor of physics at Arizona State University and the author of a new book Life as No One Knows It: The Physics of Life’s Emergence. As I wrote in my review of the book, I’m a big fan of Walker’s work (full disclosure, we have collaborated before on a paper and a proposal). 

The subject of her work and the new book is what might be called the “Physics of Life.” This is different from biophysics, which tries to account for specific physics aspects of biological processes. Instead, the Physics of Life has a more ambitious goal: to understand what separates living from non-living systems.

Along with her collaborators, Walker developed Assembly Theory, which focuses on “selection” and is a fundamental physics account for the difference between life and non-life. Assembly Theory quantifies complexity by measuring how many unique steps are needed to build a molecular structure. By identifying complex patterns that signify biological processes, this framework could help scientists detect life forms on other worlds — even those that may not look like anything we’re familiar with on Earth.

Assembly Theory has generated both great interest and controversy. Meanwhile, her book has generated a lot of interest and praise. I recently spoke with Walker about physics, life, information, and other topics over Zoom. This is the first of a series of pieces to come from that interview.

AF: Sara, it is great to see you again. I always enjoy our conversations and look forward to where we might end up with this one. So, I wanted to begin with the book itself. This is your first “popular” book, and I was wondering about your experience writing about your ideas in this way. Why did you decide to take this step?

SW: It was a deeply personal process for me, like a deep conversation with the reader. I really always wanted to write a popular science book but that in and of itself is not a motivator for me. It was actually about changing the nature of the discourse around this problem of the physics of life. I was really thinking about the cultural impact of that. 

I wanted more people thinking about physics and life because it is not a problem that’s seen as part of the frontier of physics. But it should be! Because it’s not, that means physicists just don’t start earlier in their careers on this problem. How can you expect to solve a big problem [like this] unless people start young? It’s that hard.

AF: OK, let’s start with the problem of looking at the phenomena of life as a physics problem. You were the one who pointed me to Schrodinger’s 1944 book What is Life? as a kind of foundation for the modern era. What do you think was important about that little book?

SW: I think my favorite thing about What is Life? is just the recognition [that] life might have something to do with physics — that physics might offer something [about] the question. When I was going through my PhD work with Marcelo Gleiser, he pointed me to the origin of life as an interesting problem to work on. But as I did my reading, I did not find a lot of deep physics. So, to see Schrodinger, this pioneer of the foundations of physics, pointing to life as an important problem and seeing him trying to think through the logic of it was really interesting to me. And then, also, he did make predictions, like life would require an aperiodic crystal to function that turned out to be right [DNA]. That was pretty profound. But obviously, that book left open way more questions than it answered.

One thing I thought was really funny is that What is Life? was published almost 80 years ago. But people are still asking, “What is life?” I just thought it was crazy that somebody as brilliant [as] Schrodinger couldn’t answer the question. Still, the good thing about the book was it got people to at least ask the question — to see that it was a physics problem.

AF: What do you think the difficulty answering the question has been?

SW: I think it may have been that the time wasn’t right to answer the question. I think a lot about history, and when Schrodinger wrote What is Life? it was the right time to ask the question, but we weren’t really adequately prepared to understand it. We didn’t yet know about, say, the molecular mechanisms of the cell. So, we had to go through a whole period of understanding biochemistry. When that was happening, everybody thought that approach was it. We had molecular biochemistry, so we understand how life works. But, of course, that perspective didn’t come to fruition either. 

AF: It’s interesting to think that we had to both learn new things and then learn that those new things were also not the “what is life?” answer we were hoping for. 

SW: Yeah. That was also true prebiotic chemistry, right? 

AF: You mean things like the Miller-Urey experiment where people could begin with “early Earth” conditions in a lab and then make building blocks for life like amino acids?

SW: Yeah, that whole phase of synthesizing things under “prebiotic conditions” hasn’t worked in terms of showing us how life formed. We’re still kind of holding onto it because people think it’s the best approach. But it’s not. If something doesn’t work for 50 or 60 years, I usually assume you need a new idea. 

AF: Okay. So how about the question Schrodinger asked about needing new physics to answer the question “What is life?” One direction people take to understand life (or at least consciousness) is to propose new physics “things,” new objects. This is what panpsychism or David Chalmers propose as a solution to the hard problem. What’s your feeling about this question?

SW: I’ve always been of the stance that there was something fundamentally missing about the problem of physics and life. I started to think that when I was in grad school because I was reading so much of the literature where everybody seemed [to be] kind of grasping, but no one knew what to hold onto. 

What’s been really surprising to me, though, is the resistance to the idea that there could be anything new. I often get counterarguments to our work, which goes like, “Well, new physics should be the last resort if you can’t explain it by any known things.” I just keep thinking about the history of physics. That literally is not how it’s worked every time we’ve needed a new branch of physics.

I actually think it’s more parsimonious, more practical in some sense, to just approach the problem with fresh eyes as if nobody’s been able to answer it. Just start from first principles and then ask how would you answer it. Then it might be a case that you come up with something that actually recovers standard physics. Then there’d be no need for a new explanation. But that exercise should be done in [and] of itself. And chances are you’re going uncover something new.

AF: I like that. Especially if you’ve been working on the problem for 70 fricking years. At that point, yeah, maybe it is time to like to try something different.

SW: I think the other issue is we have blind spots, like the things you guys talk about in your book. One of those is that scientists tend to think history is complete. There is resistance to the idea that there might be radical transformations in our knowledge or that the kind of ways science needs to transform now might be really different than it was in the past. We scientists generally just don’t think about the actual history. We don’t think about ourselves living in a moment in history and history is moving through us. 

AF: Yeah, I really agree with that. I’m reading a lot of history of quantum mechanics now and it’s clear how much the founders were willing to just go wherever they needed to solve the problems at hand. Like the famous story of Heisenberg going off to the island of Heligoland for a few days where he invented the matrix mechanics, which served as a foundation of the quantum formalism. He was clearly willing to take the position of, “Oh, screw it. I’m just going to find something that works.” He wasn’t asking if it fit into existing preconceptions about what physics should look like. History was moving through him, like you say. 

SW: I’ve also been reading a lot about Boltzmann and his probabilistic derivations of entropy. Something similar happened there as well. And it’s the same thing with the assembly equation where we have this weird equation that’s exponential in the Assembly Index and linear in Copy Number. 

AF: Just for clarification, the assembly equation is a key piece of your Assembly Theory about selection and life, and the Assembly Index and Copy Number are the two key measured quantiles in that theory.

SW: Yes. Everyone is asking us, “How did you derive the assembly equation? Where does it come from?” But the equation has the properties that you would want from such an equation. That’s it. What’s really interesting about this is people forget these equations are invented in human minds for a purpose. And then they criticize you for proposing a new idea without understanding how the history of science works.

AF: I find something similar in some people’s critiques of emergence. They want to argue that everything in physics can be derived from lower levels, like the Lagrangian for the Standard Model. But as a number of condensed matter theorists have shown, that bottom-up derivation only works in certain kinds of cases. Just as important, you already have to know about the existence of the phenomena you want to derive. Then you can find the path from the lower to the higher level. You have to already know about the thing you want to explain. You don’t begin with quarks and somehow blindly derive everything in existence.

SW: Another point that I think is super important in these kinds of discussions is how much bias we put into the way that we understand the world. People get to an answer using the same perspectives they always use and therefore they think they solved the problem, but they forget about their own agency. They have chosen to keep that view and not another. They don’t ask [whether] “that perspective is representative of the whole landscape?” That’s been the problem with the lack of new thinking about the physics of life.

Next time we’ll take up the question of the role of information in life and the prospects for Assembly Theory.