What is the Physics of life?

Daniel and Kelly talk to Sara Imari Walker, author of "Life as No One Knows it" about what life is and what aliens might look like.

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Episode Transcript

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Speaker 5 (02:30):
Daniel, it's good to see you. I have a question
that I have been dying to ask you. When is
physics going to unravel the deep mysteries of the universe?

Speaker 1 (02:38):
Wow? Right into it? Huh. Unfortunately we don't have that
on our schedule exactly, and it's going to be as
long as it takes.

Speaker 3 (02:46):
You know what.

Speaker 5 (02:46):
I need to do something important, but I keep putting
it off. I put it on a calendar and give
myself a deadline. I find that that actually makes it happen.
So you should schedule it.

Speaker 1 (02:56):
All right, let's do it. I'm putting it on my calendar.
Let's have a zoom meeting to share our ultimate enlightenment
about the universe. Have about June twenty twenty nine that
worked for you?

Speaker 6 (03:05):
Uh?

Speaker 4 (03:05):
You know?

Speaker 5 (03:05):
I feel like when you come up with budgets and timelines,
you always need to multiply by at least three. So
let's do something in the twenty forties, and I think
you'll have enough time then so there it's scheduled, and
that was easy.

Speaker 1 (03:19):
Yeah. Well, you know now I'm worried that scheduling our
ultimate enlightenment is going to lead us right into the
Hitchhiker's Guide to the Universe.

Speaker 3 (03:26):
Trap, like I'm gonna forget my towel.

Speaker 1 (03:29):
That maybe, but more importantly, we might get the answer
and have no idea what the question was.

Speaker 3 (03:36):
Yep, yep, that's a tough one.

Speaker 7 (03:54):
Hi.

Speaker 1 (03:55):
I'm Daniel. I'm a particle physicist and a professor at
u C Irvine, and I never forget my towel.

Speaker 5 (04:00):
I'm Kelly Waidersmith, and my kids are always annoyed with
me because I'm the only mom at the pool who
always forgets the towels. And I'm a junct faculty at
Race University.

Speaker 1 (04:09):
How can you be forgetting towels? I mean, you have
these young kids, they're always making messes. I mean everybody's kids,
not yours a particular. You gotta just learn to bring
the towel wherever you go.

Speaker 5 (04:17):
Right, I am resistant to learning. I guess I should
know by now, but I never All the other moms
have the big bags and they've got snacks and goldfish.
My kids are always like, where are the goldfish? Everybody
else has goldfish, and I'm like, tell it to your
shrinking a couple of years kid, I'm sorry.

Speaker 1 (04:34):
At least I have something to complain about, right. My
wife has one of those big bags, but she doesn't
just have snacks and goldfish in it. She's got all
sorts of random crap in there. And my kids are
always making fun of her for it until the moment
where they're like, hey, mom, wait, do you have chapstick
or do you have a spoon And she's like, of
course I do, and she digs it out of a
bag and then she's like the savior.

Speaker 3 (04:56):
Mom to the rescue.

Speaker 1 (05:00):
Are the best, absolutely and so Welcome to the podcast
that celebrates moms but also asks the deepest questions in
the universe. We want to know how everything works, where
it all came from, how it all can possibly make
sense to our squishy little brains. Welcome to Daniel and
Jorge Explain the Universe, a production of iHeartRadio.

Speaker 5 (05:21):
So today we're going to be talking about a topic
that's near and dear to my heart. I've spent many
hours arguing over this question with friends, but I'll let
you go.

Speaker 3 (05:29):
Ahead and introduce it. I'm just just wanting to preface
that I'm excited.

Speaker 1 (05:33):
On this podcast, we talk about some of the deepest
questions in the universe. How is everything made? Where it
all come from? What are the tiny little quantum particles,
how do they weave themselves together in order to explain
this existence? And where possibly did we leave our towels.
But today in the podcast, we're going to talk about
something maybe even deeper, something much more personal, something closer

(05:54):
to our experience, and that's the question of life itself.
What is it? How, how can we talk about it,
how can we understand it? And how possibly could we
discover it on alien planets?

Speaker 3 (06:07):
Oh, there's nothing better than talking about biology on a
physics podcast. Finally get into the good stuff.

Speaker 1 (06:12):
With a couple of physicists, right, And so today on
the podcast, we're going to be digging into the question
what is the physics of life? And we're gonna be
talking to Professor Sarah Amari Walker, author of the recent
book Life As No One Knows It, The Physics of

(06:33):
Life's Emergence. How do you feel about talking about life
with two physicists.

Speaker 3 (06:37):
Kelly wouldn't be my first choice. But no, I'm kidding.

Speaker 5 (06:41):
I mean I'm very excited to hear a different perspective
on the question.

Speaker 1 (06:44):
Well, I hope you brought a towel, because this is
gonna be messy. It's always fascinating when physicists try to
walk into another field and explain things. And I feel
like often we do that because we think maybe there
is a simpler explanation, or because we think in these
like fundamental grounding in the basic principles of the universe.
How would you say, that's typically receives Kelly.

Speaker 5 (07:06):
So you say fascinating, we say maddening, because so often physicists,
you know, seem to think that physics is up here
and biology is a few levels below, and if you
know physics, you must also know biology and maybe also chemistry,
so you can just wade in and solve all the
problems that the biologists are too silly to be able
to figure out. And so often there's you know, a

(07:26):
bit of condescension in the answers. I saw Freeman Dyson
give a talk once about group selection that was absolutely infuriating,
but because he clearly didn't understand what we were actually
arguing about. But anyway, today's conversation. You know, I read
life as No One Knows It by our author who
we're talking to today, and I really appreciated that she

(07:49):
did not have that condescending biology as easy attitude. She
clearly appreciated the nuances and was trying to meld it
with physics.

Speaker 1 (07:57):
As a physicist married to a biologist, I've definitely learned
to treat biology with great respect good and not just
for the health of my marriage, but also just intellectually
because I appreciate it. And I think, you know, the
dirty secret about physicists walking into biology or chemistry is
that we became physicists not because we thought biology was

(08:17):
too easy. Your chemistry was too simple, but because it's
too hard. You know, we can think about simple objects
and put them together to try to describe the fundamental
nature of the universe, but now we need like ten
to the forty objects to explain like a drop of rain.
It's too complicated. I can't think about all of those
things at once. That's why I let to boil things down,
for like, one particle touches one other particle, and that's

(08:38):
my whole universe.

Speaker 3 (08:40):
Man.

Speaker 5 (08:40):
I appreciate that you're trying to clean up the reputation
for your community. But I don't buy it at all.
But I do think that biology is complicated. You know,
we got the human microbiome that didn't solve all the
disease related problems we thought it was going to solve.
Now we're working on the connect dome for the brain
and there's just many levels.

Speaker 3 (08:58):
Man, it's complicated. We deserve respect.

Speaker 1 (09:02):
Maybe one day we'll have an explanation for the human
brain that goes all the way down to the fundamental physics,
but not yet. And so to take us on this
journey through the physics of life, whether we can understand
the nature of life, the meaning of life, how to
look for life, and how maybe even to discover life
on alien planets. We had a fun conversation with Professor
Sarah Amari Walker about her new book Life As No

(09:23):
One Knows It. Here's the interview. So then it's my
pleasure to welcome the podcast Sarah and Mauri Walker. She's
a theoretical physicist and deputy director for the Beyond Center
for Fundamental Concepts and Science, as well as a professor
at Arizona State University, and she's tackling some really hard

(09:45):
questions about the nature of life and consciousness and free
will using physics in her new book, Sarah, thank you
very much for joining us today.

Speaker 8 (09:53):
Yeah, I'm terrilled to be here. Thanks for having me.

Speaker 3 (09:55):
We're so glad to have you here.

Speaker 1 (09:57):
So before we dig into the topic of your book,
I just wanted to get to know you a little
bit better, especially because you're writing such a fascinating book.
You're tackling really big, really hard, almost philosophical questions, but
you're using physics as a tool to do it. So
take us back to the beginning, like what got you
into physics in the first place.

Speaker 6 (10:17):
I did not know I wanted to be a physicist
until I took my first physics class. So most of
my childhood I actually thought I was going to be
an artist. And I went to community college and I
knew at the time I liked science, so I just
took all the science classes i could, and I just
fell in love with physics. I just think it's absolutely
amazing to think about the universe in these really deep,

(10:39):
abstract ways and come to understand the world in a
way that is almost existentially shocking to humans in some sense,
because it's so different than the way that we thought
it was, and the set of broad regularities that physics
reveals I think are really beautiful, And I guess I
was really excited about transforming something about the way that
we think about the world and the way that physics

(11:01):
does that.

Speaker 8 (11:01):
So I decided I wanted to be a physicist.

Speaker 1 (11:04):
Awesome. So we just had an episode recently about thinking
like a physicist what that means, and had a little
disagreement with Jorge about whether physicists think differently than other scientists.
And so you say you're drawn to physics in particular
because of the way they think about the world. How
is that different? How does a physicist think about the
universe different than a biologist?

Speaker 6 (11:25):
Yeah, and I'm not even sure if it comes down
to training or who gets attracted to different fields. So
I actually asked this question myself. But for me, I
think a lot of the discussion about what physics is
is mostly about what physics studies and not what physics
is as a discipline. And so for me, when I
think about the history of physics and also what I'm
trying to do as a physicist, moving into territory that's
traditionally not thought of as physics. You know, the thread

(11:48):
I see there that I think is the commonality of
like what I think the core of physics is is
building new and abstract explanations for the world around us.
So I think physics to me is getting to like
the very root of like the most fundamental explanations, and
that process of doing that is you know, very unique
to physics, I think is a discipline relative to other sciences.

(12:11):
I think other sciences also do that, but I think
the kind of training and the way that we're taught
to think and trying to embed these sort of abstract
universal ideas in mathematics that are so broad, is pretty.

Speaker 8 (12:23):
Unique to what we call physics.

Speaker 6 (12:26):
Now, whether it will stay that way and it doesn't
become sort of more universal to the rest of science,
or even if that's the best way of doing science,
as you know, a whole other.

Speaker 8 (12:32):
Subject of debate.

Speaker 6 (12:33):
But for me, that's the essence of what physics is
and why I still identify as a physicist, even though
the problems I study are so different than what has
traditionally been studied in physics.

Speaker 5 (12:45):
When you've taken on a particularly in my mind, complicated
questions to tackle, so my training is as a biologist
and in grad school, like the thing that you stay
up late drinking and talking about is you know what
is life? And how do you define species? Those are
like the two questions where at the end we're all like,
I don't know. Nature doesn't fit into categories, and we

(13:07):
all laugh and go out dancing. And so what different
perspective does a physicist have on that question? And why
do we need an answer to this question as opposed
to the hand wavy nature doesn't fit into nice categories,
let's just move on.

Speaker 3 (13:19):
Why do we need to do better than that?

Speaker 6 (13:21):
So I love the resolution that nature doesn't fit into categories,
and so I love that that's like part of the conclusion.
I think that's actually a brilliant insight, because I think
we have a tendency to put problems in boxes, because
certain disciplines might become more adept to answering certain aspects
of questions. But there are some questions on the frontier,
like what is life that you know as a biologist,

(13:41):
Maybe you don't need to know the answer to you
because you can study biology on Earth without ever having
a resolution to that question. But this gets to your
point that you know there are some questions like the
origin of life that require an understanding of what life is,
because that is not even a precisely defined question unless
we say that there's some category of nature quote unquote

(14:02):
that is life that we're trying to explain the origin of,
and the other one is alien life. Are designing completely
radically new kinds of life, which I think are not
really decoupled problems with this idea of like what other
forms could life take besides the ones that have evolved
on Earth, besides the biology that we know it?

Speaker 8 (14:18):
And so I think.

Speaker 6 (14:19):
Those questions, surprisingly enough, don't seem to have answers that
fall just within the discipline of biology as it's developed,
because of exactly what you're pointing out, that the boundaries
of disciplines are kind of historically contingent, and there's a
lot of things that necessarily need to come from chemistry
or even computer science or physics or all these other
different disciplines. And so one thing I've really come to

(14:42):
notice in my career is that whatever discussion we need
to have about solving that problem doesn't fit in any
discipline as it's currently structured, And in some sense we
might need to build a new discipline to really think
about this problem because it's beyond the boundaries of like
the current confines of knowledge that we've defined them so far.
And that's exciting because it just shows you that disciplines
are kind of.

Speaker 8 (15:02):
Human centric categories of nature. We might have to rewrite them.

Speaker 1 (15:05):
Yeah, I mean, we might argue, like, what is a
physicist anyway?

Speaker 6 (15:09):
And people exactly about that, right, and just redefine that
term too.

Speaker 8 (15:14):
That sounds good to me.

Speaker 1 (15:16):
And you begin your really fun book with this question
and the sort of shocking suggestion that somebody made that
life doesn't exist. You know that if we can't categorize it,
if we can't define it, then maybe it doesn't exist
in the end. What is your definition of life?

Speaker 8 (15:34):
Yeah?

Speaker 6 (15:35):
So in my mind I usually keep I think a
running list of maybe three to ten different definitions for life,
because I think pinning yourself to one too early is
a bit premature when we don't understand what we're talking about.

Speaker 8 (15:47):
So usually my process is, you know, I really want to.

Speaker 6 (15:51):
Have a theory, like a theory that explains the broad
patterns and regularities that we see across living forms and
is deep enough to help us understand the original life.
That's sort of my modality of thinking about these problems,
and so I've spent most of my career trying to
figure out what the structure of such a theory would be.
And obviously it has to be guided by some concept
of what you think life is. So it's a little circular,

(16:12):
but you're constantly redefining everything. So I'm often very hesitant
to pin one answer when people.

Speaker 8 (16:18):
Ask me what I think life is.

Speaker 6 (16:21):
But with all that caveat, I can give you kind
of a conceptual framing of what I think it is,
but I wouldn't want to call it a definition. And
I think a lot of what the theory that we're
working on in the kinds of experiments are pointing to
is that life is something about how information structures mattercross
space and time is one way I say it, But
I think in the theory that we're building, it's much

(16:42):
more precise to say that life is the mechanism by
which the universe generates complexity. And we have a very
specific way of thinking about complexity, and it becomes very
historically contingent, and so I guess how the universe uses
memory that builds complex things in the future might be
the way I say it today, and if you ask
me tomorrow, I would say something else, because we're still
a work in progress about trying to get to the
bottom of what it is we're actually talking about.

Speaker 5 (17:04):
So for a biologists, the argument almost always comes down to, well,
our virus is alive or not, so using the definition
that you just gave our viruses alive or not and why.

Speaker 8 (17:14):
Yeah, So I think there's a little bit more nuanced.

Speaker 6 (17:17):
So usually the way I talk about it is like,
are we talking about the physics of life? Are particular
instances of things that are living? And it might be like,
are you talking about gravitational physics and like the curvature
or space time, or are you talking about a gravitational
body that was generated by these sort of fundamental mechanisms
that we know that our universe works in. And I
think there are slightly different categories. So when I say

(17:37):
things are life, I mean anything that is the product
of an evolutionary process and requires memory and information from
the past to be able to actually make that thing
exist in the present. And I think viruses are certainly
that they require evolution to form, but the sort of
active process of generating complexity and generating novelty and contributing

(17:58):
to the sort of open end growth of complexity that
we've observed in our biosphere. I think things that do
that are alive. And I'm not sure that I would
qualify a virus as an individual agent as part of
that creative process, but it certainly is when you look
at viruses as components of ecological systems for example. So
a lot of the things with issues about life is

(18:18):
trying to pick a scale and say this thing is life,
when really what you're talking about is a much more
abstract universal physics that even enables those things to exist
in the first place, to be selected. I think the
reason that viruses have been hard is because it's actually
a category error to try to partition a boundary and
around a virus and say is this alive or not
when you really should be looking at the continuity of
the evolutionary process and what it constructs.

Speaker 1 (18:40):
Yeah. Fascinating. I don't know that I agree that we
need a definition of life, but I do think it's
valuable to try, Like, I think we need to have
the discussion in order to try to figure out what
this is. Like if we agree or discuss, you know
what life is that shapes the way we think about
how to look for alien life and all these questions.

(19:01):
So I think it's very valuable to dig into the
question even if we don't find an answer. And from
that perspective, I was really interested in the discussion you
had at the beginning of your book about, you know,
the sort of philosophy of what life might be. And
you were going through this discussion of like panpsychism, and
there's this one particular line that jumped out at me said, quote,

(19:22):
if life is not a property of matter, and material
things are what exists, then life does not exist. I
was wondering if you could expand on that thought. You know,
it doesn't it sort of ignore the possibility that life
could just be an emergent phenomenon, as you said, an arrangement,
a complexity of basic bits that are not alive.

Speaker 8 (19:42):
Yeah.

Speaker 6 (19:43):
So part of the reason that I structured things in
specific ways early on was to kind of set up
some things that come later in the book. And one
thing I've noticed being a physicist trying to build new
physics is that we take for granted things as being
apps asolutely true about the universe that were actually invented
by human minds trying to correlate the way that their

(20:05):
theories behave with things they could actually measure. The example
I like to give is to think about mass. It
seems so obvious to us that mass is a physical
property of objects, and it is a physical property we
talk about in our theories of physics because we can
measure it, and because Galileo rolling balls down in climbplanes
and other people of his generation realized that that was
the right variable to construct theories of motion. I think,

(20:28):
of course, you can talk about life as just being
an emergent property, but you're never going to get to
an objective understanding of it as a physical phenomena unless
you can tie it to a measurement. And therefore whatever
that thing is has to become what we would call
a material property in our theories of physics, because now
it's something we measure that becomes a variable in our theory.

(20:48):
Therefore it's physical. And I think this has been the
thing that's really hard about thinking about the nature of
life is it has forever been an epiphenomena and something
we couldn't regular rise as part of a law of nature.

Speaker 8 (21:02):
Because we want to adopt this view that surely it's
just these.

Speaker 6 (21:05):
Emergent informational patterns, but we can't measure them objectively. And
so my point saying that if you want to reduce
life that way, then life doesn't exist because you don't
have an objective measurement. You don't have a way of
grounding whatever you're talking about and testing it against reality
and saying this is the thing that emerges when life emerges.

Speaker 1 (21:25):
If I could just stop you there, because I'm not
sure I'm following, Like, let's take a simpler example. You
know a thunderstorm. Thunderstorm is an emergent phenomenon bubbling up
from the you know, the properties of particles and molecules
and all this kind of stuff. Clearly thunderstorms exist, right,
we agree. We can make measurements of them, their intensity,
all sorts of stuff. We can ground them, we can

(21:46):
write even some you know, mathematical expressions that describe them
in some cases, How can we say thunderstorms exist and
we can make physical measurements of thunderstorms, but we can't
say the same thing about life.

Speaker 8 (21:58):
Well, what would be the measurement that you're taking about life?

Speaker 1 (22:00):
Though?

Speaker 6 (22:00):
So the question with thunderstorms is it's a good example
because most of the measurements you're taking are things like
wind velocity, you know, the temperature. Like these are like
physical measurements you can go in and take, and then
when you put those in some model of a thunderstorm,
you can actually model the thunderstorm's behavior. And that same
physics works if you go on another planetary body and
account for differences in pressure and differences in the particular climate,

(22:24):
you know, in that gravitational field, and things like that.
The issue with life is, you know, most people want
to say that, like when you're talking about chemistry, and like,
life is an organizational property of chemistry. When we look
at its parts, we don't see the things that we
associate to life, and so therefore life is an emergent
property of those parts. It's some organizational feature. But the

(22:45):
problem is we don't have the rules to go from
the atoms and the parts to that organizational feature in
a universal way that would allow us to go on
another planet and measure that property. It's very subjective the
way that we talk about it, right, and so it's
okay if it's subjective and not a material property. It
might be that there is no law of physics that

(23:08):
allows us to understand what life is. And it might
be that life is not a universal category of nature.
It might just be some weird emergent pattern that emerged
on Earth and there's no universality to it. But the
argument that I'm interested in, and the one I think
that will actually help us find alien life that's out there,
because life is a universal thing if it is, is
that we need to figure out what are the objective properties.

(23:30):
And when you do that, in some sense, you're making
that property material in a way that it becomes measurable.
And so that's sort of the interplay. Like life has
not been a material property. We've had this sort of
vitalism and the idea of a soul. But the question
is is that actually something that we can quantify, put
into a theory, and also measure so that we can

(23:51):
actually say this is a real universal law like feature
of our universe.

Speaker 5 (23:56):
But so then why go to life does not exist?
As opposed to we don't understand it well enough yet.

Speaker 6 (24:03):
Because the logical conclusion of the way that much of
the discussion has been is that we should assume life
does not exist. So in some sense, what I'm trying
to do is actually provoke, you know, like, if we
run the way we talk about things to their logical conclusion,
what are we really saying And is this actually what
we intend to say?

Speaker 8 (24:21):
Or do we want to say?

Speaker 6 (24:22):
There's something missing from our descriptions of nature, and maybe
we need to rethink from first principles what it is
that we're doing. So I actually think life does exist.
I was always perplexed that a lot of my colleagues,
if you really listen to the words they were saying,
were implying that it does not exist, and some of
them even saying it outright, that like, we should just
not think about the problem of defining life at all.
Life is not a category of nature. It's all reducible

(24:45):
to chemistry and physics. And I think that there's something
deeply missing in what life is and why we keep
asking this question? So why is it a question we
ponder in bars at night. It's probably because we feel
intrinsically we're missing something about, you know, the descriptions of
the world around us that's pretty fundamental. So that was
always my perspective. Now that might not be right, but

(25:06):
I think the way to find out if that's.

Speaker 8 (25:08):
Right is try to go through the process.

Speaker 6 (25:10):
Of building a theory you can test against something you
can measure, and trying to see if empirically you're actually
getting the right results, and if you're building a theory
that is all explanatory of the data that we see
in the observations we can make.

Speaker 1 (25:24):
Right, So you're not saying I don't exist, or you
don't exist, or my cat doesn't exist. You're saying that
we don't have a clear and crisp definition of life
that lets us do things that we typically do in
physical theories, like we do with thunderstorms and we do
with cats, because we can make measurements of them. We
can other than just like vaguely talking about sort of

(25:44):
fuzzy conceptual categories.

Speaker 6 (25:47):
Yeah, I mean, in part, you know, the book is
an exercise of trying to turn a philosophical question into
a scientific one.

Speaker 1 (25:52):
Yeah.

Speaker 6 (25:53):
Right, So this has traditionally been a philosophical debate because
we don't have the things that we can pin the
debate into measurements to actually go test what it is
that we're talking about. And I think that's really the
critical transition that you know, most interesting science starts as
philosophical questions. And the period where the philosophy transitions to
science is always deeply interesting because we're asking exactly these

(26:15):
like really basic questions. It forces you to even ask
what science is, what is a measurement?

Speaker 8 (26:19):
What is a theory? How do you build a new theory?

Speaker 6 (26:22):
Like what are we even talking about ever when we
talk about things in science? So this is why it's exciting,
But it is really challenging because you're confronting things that
you know, we haven't known how to think about, and
you're saying, let's pin ourselves down and be precise and
think about it and ask, how would we actually quantify
and measure this thing? And how will we build a

(26:43):
theory that describes this thing? And if you look at
the history of science, what excites me about it and
why I think any of the ideas that we're doing
might be on the right track is you know, every
time that that process has happened in the history of physics,
we come out the other side with a radically different
conception of what's going on than we had before. My
favorite example is like to think about you know, saying

(27:03):
terrestrial and celestial motion are the same thing. It's like,
you know, humans had been around for hundreds of thousands
of years seeing the stars and the night sky and
planets moving across the sky, and we never thought like
that would be you know.

Speaker 8 (27:14):
Like the theory of gravitation.

Speaker 6 (27:15):
It took you know, centuries of measurement and coming to
you know, track the regularities of heavenly motions and be
able to get clocks that were precise enough timing to
actually build into an understanding of a theory of motion
and gravitation that allowed us to look up at the
sky and be like, oh, those things are actually moving
in the same way that things on Earth move. I mean,

(27:37):
these are radical conceptual leaps that our species has taken,
and I think to think that we're done doing that
is really premature. So I get excited about life and
when people say, well, isn't it just a pattern of motions,
and I'm like, well, of course it is. But we've
explained lots of those before in really deep and fundamental ways,
so maybe we should try again.

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(31:41):
back and we're trying to sort through the slimy questions
of the physics of life with Professor Sarah Amari Walker,
author of the book Life As No One Knows It
Well in terms of taking like a radical conceptual philosophical leaps,
I was very excited to see you open the door
to getting rid of reductionism. I mean especially as of physicists,
you know, and as you described earlier, physics is so

(32:04):
ingrained in the idea of like, let's find the microscopic
explanation for everything. Start with what we hope is the
fundamental and everything will bubble up from there somehow dot
dot dot right. Leave that as an exercise for the engineers.
But in your book you actually consider the other approach,
the idea that, like, maybe not everything is determined from
the smallest bits. Maybe what we see as like emergent

(32:27):
phenomena different scales actually have their own different fundamental description that, like,
you know, the universe isn't controlled by the tiniest Can
you talk a little bit about that. I think that's
probably a very unfamiliar concept for our listeners who are
used to, especially on this podcast, hearing us talk about
how tiny particles weave together to make the universe.

Speaker 6 (32:46):
Yeah, and I also love that view of reality. I
think it's like incredibly romantic, and I was trained in
that tradition. But thinking about life and really trying to
ask the question whether there were universal laws of life
like that speaks to me that looking for something fundamental,
because it has to be universal and broadly explanatory of
any life in the universe, you start to ask questions

(33:08):
about the nature of what's fundamental. And as I already
alluded to before, a lot of the ways that we
think about what's fundamental are what things we can go
in the lab and test that build the structure of
our theories, and so some of the most precise theories
we have are these ones about microscopical physical motions, but
it doesn't mean that they explain everything. And one of

(33:29):
the points that I make in the book, which is
related to this sort of evolutionary progress of knowledge that
we've been talking about about, like disciplines changing over time,
is the notion of what's fundamental has actually changed with
our theories and with our measuring devices, and so I think,
you know, like the canonical example I like to give
is to think about atoms, which we thought for a
long time or indivisible units of matter until we build

(33:51):
better technology and we realized they had parts. And so
I'm not sure that there's really a bottom to that process.
If string theory is right, obviously all the elementary particles
and things we think are you know, the full description
and reality are not right. But we can't validate theories
that go smaller because we don't have the technology yet.
So you know, that becomes a horizon of our current

(34:13):
understanding of reality, not something that's intrinsic to reality itself.
And if you flip that around and you look at
what you're trying to do building a theory of life,
life is built out of these what we think are
fundamental building blocks. But the kinds of things that we
want to understand about life are much more about how
the universe builds complexity out of small parts. And it

(34:36):
is the case that if we can think about things
that we can measure there and build theories there, there's
no reason to think they're not just as fundamental as
these other descriptions of nature.

Speaker 8 (34:47):
So that's sort of like the.

Speaker 6 (34:47):
Baseline philosophy I'm operating from. But the sort of core
of your question, I think is more about the emergence
reductionism debate, not the philosophy of measurement and what's fundamental.
But I think like those things are coupled, so I
wanted to preface it. But the emergence reductionism, you know,
Phil Anderson had this great essay which a lot of
people have read and love about this idea of more

(35:09):
is different. You know, even if you had that fundamental description,
it doesn't mean that as you go to these other
layers like biology and mind and technology, you're going to
actually be able to recover all the features of them.
And so this has been deeply perplexing. And what I
suggest in the book is one of the reasons you
can't go between scales, is there actually are separate laws

(35:31):
that describe the idea of a hierarchy in the first place,
that the universe is actually constructing complexity, and that time
needs to be a component of that process. So if
you remove you know, evolutionary time, you know, you can
talk about me being an emergent property of my atoms.
But I've missed almost all of the physics that's in
my body. I am like the product of four billion

(35:53):
years of evolution, and so if I want to talk
about me as a fundamental structure, I need to talk
about that feature, not the atoms in my body. And
so it's really trying to invert the nature of like
what is the thing that actually explains this. You can
say I'm an emergent property, but it doesn't explain from.

Speaker 8 (36:11):
Those laws, those lower level laws, how I got here
and why I exist.

Speaker 1 (36:15):
But does that mean that there is no explanation that
you have to have fundamental laws at every scale or
does it just mean those explanations escape us currently or
our current ability to do those calculations. And in some
future where we have an amazing supercomputer, we can model
everything from the tiniest bits.

Speaker 6 (36:31):
I think it's both of the last things you said.
So I think there you cannot produce things like us
from the current structure the laws of physics. I don't
even think if you had a super massive supercomputer you
could do it. And a simple reason to cite that
is it would require more computation than is available in

(36:51):
all of the resources of the entire universe. And yet
the universe builds things like us. And so I think
that right there suggests that something fun mental is missing,
because you could say, in principle, you could simulate it,
and you could generate our conversation that we're having right now,
but actually, in principle is not in principle because it's
not even physically possible. So those things, to me suggest

(37:13):
that there's a huge gap in understanding. But I also
think fundamentally that the physics of what describes living things
is not encapsulated in the physics that we have of
gravitational fields or elementary particles or any sort of state
of the art in modern physics departments. I think it's

(37:34):
just not in those equations. And the reason I think
it's not in those equations is because life is very
much about the causation and time that's built into physical
objects that are complex. And if you think about what
the laws of physics revealed, they are often designed for
specific problems, and they tell you what's universal about a
set of questions like motion, for example, like all moving

(37:57):
objects you know, can be described by mass acceleration and
you don't care about the color of them or how
those objects feel or other things. But that doesn't mean
that those laws explain everything about every object. And so
you know, the question is how many kinds of laws
of physics do we need? What's the sort of universal
set of things that they cover? And I think life

(38:19):
is presenting some real challenges for the sort of current
paradigm that we have in really fundamental ways.

Speaker 5 (38:26):
Again, as the biologist in the conversation, so I feel
like what I'm hearing is that we don't really understand
the physics stuff, but we know that life is something
that comes about through the process of evolution. So I
feel like we're back in an evolutionary biology lab. Yes,
how how does a physicist study it differently? Like is
the question what process results in something that natural selection

(38:51):
can then act on? Like when we you know, put
the pedal to the metal or whatever, like what do
we do in a lab or on a board for
with equations that's different.

Speaker 8 (39:01):
So this is a.

Speaker 6 (39:02):
Great question because ultimately, like Daniel's question, can you know
what will ultimately resolve it is? Does providing a new
explanation provide any new experimental tests and things we couldn't do.

Speaker 8 (39:13):
Otherwise because it doesn't.

Speaker 6 (39:15):
I could tell you, you know, I could sit here
all day arguing that there's other laws of physics in life,
and you know you could nod your head, Okay, Sarah,
but show me the proof, right, And I've been doing
this most of my career because you know, I have
like some conviction it's there, but we have to do
the hard work.

Speaker 8 (39:29):
As scientists and show that it's useful for something.

Speaker 6 (39:32):
And I think, I think to your question, the challenge is,
you know, for the most part, we think about evolution
and selection as being things that life does. But that
doesn't allow us to explain the origin of life because
it has to be some spontaneous fluctuation with no mechanism, right,

(39:53):
and so or it's just a rare fluke, right, And
then if it's a rare fluke, then you know, none
of this discussion is necessary because it's just some thing
that happened and there's no explanation for it.

Speaker 8 (40:02):
It was just, you know, a rare fluch event.

Speaker 6 (40:04):
And I think this is one of the challenges because
that allows you know, intelligent design and other things to
be equivalent explanations to the scientific one, because we don't
have an explanation. So if you really want to explain it,
you kind of have to assume. The original life is
itself a process of selection and evolution. So it's the
product of those kind of mechanisms, which means you need
an understanding of evolution that doesn't require.

Speaker 8 (40:27):
A cellular architecture. And this is the main challenge, right so.

Speaker 6 (40:30):
Before replication, before natural selection as we understand it now
in genes or the way it operates in biological populations,
out of chemistry, how does an evolutionary system get constructed?
And so this is really the question that we're trying
to ask is how do you get complexity when there
is none? And so that's where we need a new paradigm,

(40:54):
because we need a mechanism of selection that operates starting
from simple molecules building into the complexity of life. And
that's really the theory that we're working on. This assembly
theory is really an approach to talking about selection in
the absence of like when selection emerges, how do selection
actually start? And how to selection build evolutionary systems that

(41:15):
we would recognize as evolving things. And so it's suggesting
a much more universal structure to the evolutionary process then
the particular things that were selected and evolved on Earth.
To understanding what are the general principles of the kinds
of systems that chemistry could generate that could lead to
this process of evolution that we recognize. That's the question

(41:35):
we're trying to answer, and I think that one really
does demand new physical principles because you're asking out of
I mean, people really don't understand how big chemical space is.
It is like astronomically large. We also do a lot
of stuff with drug design with the same theories that
we're trying to sell the original life, which is kind
of crazy. So I deal with like you know, pharmaceutical

(41:56):
drugs and this kind of stuff. But like when keminformatics
go in and they want to estimate the size of
the chemical space they have to search to look for
new drug like properties. It is insane it's not small,
it's insanely large, you know. I think a really good
example is like taxol is a molecule I use a
lot of times when I give lectures that I picked
up from my colleged Lee Kronin. That molecule has an
molecrooid of about eight hundred and fifty three. You know,

(42:19):
I think it has like forty seven carbon atoms or
fifty three or something oxygen. Anyway, Like, if you wanted
to do all the molecules with that chemical formula alone,
it would fill a volume of one point five universes.
So if you think about a planet generating chemical chemistry
in an unconstrained way, no selection, the space of molecules

(42:41):
that could be you know, just randomly synthesized is so
astronomically large you would never expect anything complex to recur twice.
And so this is kind of a key argument of
the way that we constructed the theory. When you start
to see things that involve a lot of constraints in
the space of possibilities to get these really complex, really
probable objects with a high abundance that they're happening a lot,

(43:05):
that's evidence of selection happening and that selection is actually
the physics generating those objects. But it's got to be
much deeper than what we just see and involved architectures
that we have, like with the cell, because that comes
fairly late in new original Aye process.

Speaker 1 (43:17):
So I understand that the space is very large, right,
Obviously there's lots of different combinations. Chemistry is complicated. But
how do we know what's improbable or what's unusual because
the space is so large and we haven't explored it, right,
And you say, for example, like the existence of rockets
in the universe is a sign of like conscious thought
or whatever. But you know, how do we know them
that that's the case? How do we know that other

(43:39):
arrangements that our particular planet didn't explore don't lead to
greater complexity or more interesting things existing in the universe.

Speaker 6 (43:46):
So your question is a good one, and I think
a lot of us have sort of embedded in the
way that we think about the world. And this actually
comes from the way that we think about physics. It's
sort of a heritage legacy actually from the eighteen hundreds
and the development of statistical mechanics and the idea of
spontaneous fluctuation. We have an idea in our minds that
anything of arbitrarily complexity can spontaneously fluctuate into existence. And

(44:12):
this is the whole impetus of the Boltzmann brain argument,
which actually was proposed by Eddington originally as kind of
a mocking of Boltzman's ideas, but then became something that
physicists took very seriously and tried to put in our
cosmological models. And so the challenge there is, of course,
we can say, you know, a planet can make an
arbitrarily complex molecule, right, I can make that statement, and

(44:34):
it seems intuitively logical. But if I said, you know,
Mars is going to generate a cell phone tomorrow, you
would kind of laugh at me, right, Like that's.

Speaker 8 (44:42):
A silly statement.

Speaker 6 (44:43):
Is there really a chance that we could just find
the planet Mars generating a cell phone, and like, not
a cell phone, but just something equivalent amount of organization
and evolutionary processes, Like a cell phone takes four billion
years to make on a planet. It takes a lot
of selection to get to that very specific object. It's
really designed for our biology. The technology is built on

(45:04):
our planetary resources, Like everything about that object is a
direct consequence of the fact that we've had four billion
years of evolution along a specific trajectory on our planet,
and so to assume an object like that can just
form random chance somewhere in the universe, I think is
a major it's a major misstep in our reasoning.

Speaker 8 (45:25):
About how reality works.

Speaker 6 (45:27):
We have no evidence that it can actually happen outside
of evolution, and so a lot of people will try
to challenge the way that we're building the formalism on
this idea that something complex can just be made on
a planet in the absence of evolution and selection. And
I can just say back to that, like, give me
the evidence the universe builds a me that requires like

(45:48):
information to specify it, because to me, what that argument
would entail is actually the equivalent of intelligent designs, as
the universe has the design of every object at every
point in the universe. What I'm suggesting is that selection
and evolution are required to build information specific to objects
before they can exist.

Speaker 1 (46:05):
Right, And I agree with that. I'm not arguing that
cell phones should appear on Mars, so that would be probable,
and I appreciate that. You know, complexity comes as like
the tip of the pyramid built on the base of
like a huge amount of things that came before it. Absolutely,
But what I'm wondering is whether we can argue that
life on Earth is unusually complex. We don't know with

(46:28):
the space of possibilities is maybe if we go out
in the universe we discover, wow, we're actually quite primitive.
Complexity here on Earth is where in the backwater, I
mean compared to Mars. Sure, but like you were saying that,
you know, what we've achieved here on Earth or what's
happened here on Earth is unusually complex. But how do
we know that?

Speaker 8 (46:46):
Oh, where's the prior?

Speaker 6 (46:47):
I see, so I was referencing that with respect to
an abiotic prior. Imagine you don't have an evolutionary system, right,
So I think we're talking cross purposes about the question.
So I just want to kind of frame it precisely.
So if you just look at you know, so a
system that doesn't have evolutionary selection, the sort of framing
that we have the key conjectures, there's a maximum complexity

(47:08):
a non living system could build, and it can't build
any more than that until selection and sort of feedback
on the structure and self reproducing systems emerge. That actually
constrain themselves to be able to perpetuate.

Speaker 8 (47:18):
Right.

Speaker 6 (47:19):
What you're asking is, if that process starts, are we
more or less complex than that typical process? And I
think we don't know the answer to that obviously, but
I think what I can say is from the sort
of structure of the physical laws in the theory that
we're building and testing, every evolutionary system will have to

(47:43):
climb the ladder of complexity, so there is no major
jump where you just get to be like the most
complex thing in the universe by skipping steps. So that
becomes a question of did life start in the universe
on any planets much before us or have an accelerated
process moving through the sort of complexity cascade. And I
don't think anyone knows the answer to that question. That's

(48:06):
one of the reasons that we want to understand this
physics so we can actually make predictions about what that
process looks like on other planets. And I think we're
at this stage where I'm hopeful, you know, in the
next I don't know, ten years, maybe we might be
able to say something meaningful about the first steps from
geochemistry using this kind of theoretical construction like these kind
of steps about like what are the first mechanisms of

(48:26):
selection that emerge in chemistry and what do they look
like in different planetary contexts. But saying something about that
whole cascade is basically simulating the entire evolutionary process, you know,
from the geochemical original life into technology on another planet.
And I don't think that's actually computationally feasible. I think
we have to discover other life to answer that question,
which is why we need a theory to know what

(48:47):
we're looking for.

Speaker 5 (48:48):
So would geologists come across molecules that look complicated or
in chemistry? Was the class I did not get?

Speaker 3 (48:53):
As in, yeah, so how do you know that you've got.

Speaker 5 (48:56):
Like a complicated molecule that's life or do you get
similarly complicated molecules in a geology lab that.

Speaker 3 (49:02):
Are not alive.

Speaker 6 (49:02):
It's a non trivial question, highly non trivial, and I
think part of it comes from the different uses of
the word complexity. So I think a lot of my
planetary science colleagues and geologist would say, of course we
find complexity in chemical systems.

Speaker 8 (49:16):
We see it all the time.

Speaker 6 (49:18):
The kind of complexity that they're talking about is often
what we see in prevotic chemistry experiments also, which is
oftentimes like the most problematic thing you see in prevatic chemistries.
If you just run an unconstrained reaction, you get what
we call a tar, which is a non differentiated mess
of molecules. So these kind of systems can generate a
lot of molecular diversity, but it tends to be things

(49:41):
that would be low complexity as individual molecules, and for
things that are higher complexity, you don't see a lot
of them. And that's very different than what we see
in life, which is we see, you know, there's a
lot of diversity and molecules in life, but compared to
the size of the chemical space that that could be
you know, equivalent structure like molecule or way, or you know,

(50:02):
number of chiral centers or you know, number of elements
in the molecule, like any of these features you might
say is equivalent molecules. You see very very very very
low diversity in biology. And so what biology does is
it builds molecules that are deep in the space of chemistry,
like they require a lot of parts to build the
specific molecule, so that molecule is complex in an evolutionary sense,

(50:25):
it's found in high abundance, but there's not a huge
diversity of exploring in an unconstrained way the space of
all molecules. So I think the kind of complexity we
talk about in a planetary science sense doesn't discover the
kind of complexity that biology generates.

Speaker 1 (50:41):
So let's dig into that a little bit, because I
think that's really at the core of the argument in
your book, right, this concept of complexity and how we
discover it and what it means for life to be complex.
So you're saying that life is something that uses a
relatively small number of building blocks but puts them together
in a way that's very complex. Right. The complexity comes

(51:04):
from the arrangements of a few bits, not from the
inherent complexity of the pieces, and that we of the
chemical space that exists out there. As you're saying earlier,
it uses a tiny, little, very tiny, yeah, very tiny,
little aspect of that. And so again I wonder, like,
how do we know how unusual that is, or how
special that is, how distinct that is? How do we

(51:26):
know that it's not possible using other building blocks to
create complexity? How do we know that complexity doesn't always
arise if you start from you know, basic bits.

Speaker 6 (51:36):
Yeah, this is the more exciting question, and it's actually,
you know, the one experimentally, I'm really interested in. If
you designed an original life experiment that was modeling a
planetary environment and it was unconstrained and selection and evolutionary
processes emerged in the system just by the dynamics of
the chemistry, would it discover the same biochemistry?

Speaker 8 (51:58):
And I'm not convinced it would.

Speaker 6 (51:59):
I think space is so large that there are a
lot of potential chemical architectures for life that could be
quite different, but we haven't discovered them yet. Because life
on Earth, you know, had one solution that allowed it
to perpetuate itself and had to use that architecture for
billions of years. And you know, the question is if

(52:20):
the original life happened again, would it discover the same
architecture or not?

Speaker 8 (52:23):
And nobody knows the answer to that.

Speaker 6 (52:25):
I think that's an experimental question, you know, and that's
one that we want to ask because you know, part
of the thing I you know, try to really highlight
in the discussion in the book, but I'm also really
trying to just pitch to people as an experimental program
that we need to do because it requires a lot
of investment of technology and resources, is to think through
how we would build an experimental program to discover alien life.

Speaker 8 (52:46):
In the lab and the idea of Eric exactly in
the lab.

Speaker 6 (52:51):
Yes, it's exactly what you're saying though, like we don't
know if life could exist in other chemistries. If you
do a proper original life experiment and you really want
to know the spontaneous probability for a universe to generate life,
you can't just cherry pick and use them like our
structures that were selected on Earth and say I'm going
to make these because you're inducing selection based on what
was produced on Earth right prior knowledge of it. You

(53:12):
really want to see from chemistry what would it generate
without any intervention from us, as you know, designers of
the chemistry, knowing what we know of Earth life. So
I think the original life problem is actually that's exactly
what it is, is an experimental problem to try to
discover what is the mechanism chemistry generates living forms. And

(53:36):
if you know that mechanism, is it universally going to
converge on what life on Earth did? Or are there
many solutions because this space is so huge.

Speaker 3 (53:44):
I'm sure you could get a lot of evolutionary biologies
very excited if you could repeatedly generate them.

Speaker 8 (53:49):
You need their talent, so that's good. Yeah, I think
you know.

Speaker 6 (53:54):
Part of my motivation with right, it's hard to write
a book, right, And for first and foremost, I'm about
this science, like I just really want to solve the
original life. So part of my motivation is to get
people excited about like what are the actual critical challenges here?
And I think this one is like the main one
is like what does the experimental program look like that
solves the original life? And how related is it to

(54:14):
all these other questions that we're talking about. And for me,
it is going to require you know, really intense technological infrastructure,
building digital chemistry and AI driven robotic chemistry to actually
search chemical space like a search engine, and then looking
for these kind of chemistries and when they undergo selection
and they start emerging things that we might call life.
And the reason for having to build a theory in

(54:37):
part besides all the reasons I have as a physicist,
is if the chemistry is radically different, how do you
even know it's a living thing, and so we have
to be able to measure evolution in molecules agnostic to
what molecules evolution creates, and know that they have a
comparable level of selection in them to say that, you know,
this is past the threshold that we expect it to

(54:59):
be a live form versus you know what, we expect
a big systems that aren't evolving to be able to generate.

Speaker 1 (55:06):
And so this is sort of the core argument of
your book. You describe assembly theory, where you measure the
complexity of something as the number of steps it takes
essentially to build some of the shortest path to go
from building blocks to this thing, right, and then you
do this thing where you say, well, anything above fifteen
steps is life, and anything below fifteen steps is not life.

(55:28):
And I was with you until I got to that point.
I found myself asking, as you actually did in the book,
like well, why fifteen doesn't it feel sort of arbitrary?
How do we know it's fifteen?

Speaker 8 (55:38):
It's like it should be forty two, right, forty two
would have been better?

Speaker 1 (55:42):
Yeah, gives a description of that thought process.

Speaker 8 (55:45):
No, it's a good question.

Speaker 6 (55:46):
So fifteen is maybe an approximate bound, right, But where
that comes from is actually the experimental data. So it's
not like anointed, you know, like the assembly theory cult
said it's going to be fifteen.

Speaker 8 (55:56):
It's just like, you know, we build it.

Speaker 1 (55:57):
How do I do any cult?

Speaker 4 (55:59):
I know?

Speaker 1 (55:59):
Right?

Speaker 6 (56:00):
And there is no cult actually, Like it's really funny
because you know, there's a lot of challenges we're getting
developing this theory from people picking it apart different ways,
which we love, but I don't think anyone's harder on
it than the people we have actually working on it.

Speaker 8 (56:12):
So it's like, which is the best kind of science?

Speaker 6 (56:14):
Right, Like you pick apart every idea and you re
evaluate everything you're doing at every step.

Speaker 1 (56:18):
Well, if you're denying the existence of the cult, that
tells me that the cult definitely exists.

Speaker 8 (56:22):
Oh sure, yeah, I know right, you're just I'm going
to wink link wink.

Speaker 1 (56:27):
Tell us about the data that supports the choices.

Speaker 8 (56:29):
Yeah, so that comes from the data.

Speaker 6 (56:31):
So this idea of this minimal past, so it involves
you recursions. So the idea is like it's easier with
lego than chemistry, you know, because most of us aren't chemists.
So it's like you think you're building a particular lego structure.
I usually use Howkwarts castle with people know, but like
the taj Ma hall or whatever you know, building you
might prefer, you know, you wouldn't expect to form it
by randomly shaking the building blocks in the box.

Speaker 8 (56:52):
Right, there's instructions that allow you to make it. In
assembly theory, you realize.

Speaker 6 (56:56):
If you take the instructions, there's actually lots of different
ways of getting to the end if you don't use
the instructions. Right, So in assembly theory we have to
ground it in something that's always there about that structure,
and not just because you had a particular set of
instructions in a particular environment to make that. And so
we do this based on this minimal path idea. And
the assumptions of the minimal path are you can only

(57:17):
build things if you already build them, and you want
to try to find the shortest path where you're reusing
parts you've built already to build the final structure.

Speaker 8 (57:24):
So very minimal set of assumptions.

Speaker 6 (57:26):
It turns out that feature you can actually measure in
the lab, and we measured it in the paper that
we tried to do biosignature validation in using mass spectrometry,
But there was a later paper from Leekronin's group that
came out showing that you can also measure assembly index
this shortest path measure with NMR and infrared.

Speaker 8 (57:45):
So we have a strong sort of.

Speaker 6 (57:47):
Conviction right now as a RUNN assumption with a theory
that it's an intrinsic property of a molecule, this feature,
it's one that you can measure with independent measuring apparatus.
It doesn't depend on where the environment where the molecule
was made. That gives us some of the baseline criteria
of something being an objective measure of the amount of
selection in an object. And then what you have in

(58:07):
a molecule that you can go and measure in the lab.
So you know, if I went to Enceladus and I
measured it, you know the molecule would have the same
value it would have on Earth.

Speaker 8 (58:16):
Okay, so that's important. And so the question is then
if you use.

Speaker 6 (58:20):
That, does it actually separate out things that are uniquely
produced by life from not? And so to do this,
Lee's lab took a bunch of samples, you know, abiotic
and biotic, even some Scotch whiskey, and then they had
some samples that were blinded from NASA that were both
biological and non biological, sort of an adversarial test case.

(58:43):
And what they did is they use this mass spect
method to measure assembly index on the actual samples, and
they showed that using assembly theory combined with the mass
spect data, they only saw molecules that were more than
fifteen steps in this minimal path being produced by living samples.

(59:04):
And this gets to the complicated versus complexity issue because
one of the adversarial samples that NASA cent was actually
Murchison meteorite, which oftentimes in prebatic chemistry literature you will
see as being like the most complex sample of prebatic chemistry,
and it was still below fifteen for the molecules in
the sample. So what you see Murchison is again this tar.

(59:26):
You have a lot of molecules, but none of them
are high abundance and a high assembly. And so it's
not just that the molecule is high assembly. You actually
have to have it in a high enough abundance to
say that it was selected in that particular context.

Speaker 1 (59:42):
Right, So, but does that mean that if I find
some sample on an alien planet and you measure its
assembly index and it comes out to forty two, You're
going to be like, this is life? Are you proposing
this as a definition of life? A discrow way to decide?

Speaker 6 (59:57):
I think that would be the ultimate outcome based on
the path that we're on, but I think we have
a lot more work to do to be like definitively
there is a threshold and assembly spaces above which you
would say that a sample was life or not.

Speaker 8 (01:00:10):
And that's sort of where we're going.

Speaker 6 (01:00:12):
So right now, I would be pretty high confident if
you discovered something on Enceladus that was forty two that
it could only be produced by life. But the sort
of next steps are really actually theoretically predicting why it
should be the number fifteen and what number it should
be if you have variations in your elemental distributions or
other things about the structure of the assembly space, which

(01:00:32):
is the underlying mathematical structure that we build this.

Speaker 8 (01:00:35):
Whole formalism out of.

Speaker 6 (01:00:37):
And so one of the projects in my lab right
now is actually trying to quantify why there's a transition
in assembly theory at the particular assembly index values that
we see and might see in different systems.

Speaker 1 (01:00:48):
Because as you say, the chemical space is very large
and petible, that there's structures out there that are quite
complex but that nobody would call alive.

Speaker 6 (01:00:57):
Right, So there are many such structures, right, So it's
not necessarily that the threshold must be at fifteen.

Speaker 8 (01:01:03):
That's where we.

Speaker 6 (01:01:04):
Have for the kinds of chemistry that we test in
labs so far. When we have a general theory, which
we're working on very fervently right now with our teams,
we would be able to predict, based on the sort
of elemental composition and other features of your chemistry, where
that threshold should be. And then that allows us further
tests to really validate that this works in different systems.

(01:01:26):
And you know, one place where it might be more
challenging is actually with minerals because minerals don't uphold all
of the properties of molecules. They have very different structure,
but they're also made out of elements bonding and minerals
because they have very different structure to how you build
up a mineral in this kind of way that we do,
this kind of recursive construction and what you would define
as a repeat in a mineral, they might have a

(01:01:49):
you know, a threshold that's much higher than what we
observe for aqueous molect our chemistry. And so, as with
any new theory of physics, it's a process of suggesting
an idea, testing it, iterating on the idea. And so
you know, the process of theory building doesn't happen overnight.
It happens over a decade or two. And so these
are some of the frontier questions that we're asking about, Like,
you know, is this actually you know the right approach,

(01:02:10):
then these things should work out, but we don't know
the answer yet.

Speaker 8 (01:02:14):
So that's why it's exciting.

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Speaker 1 (01:05:17):
I hope your brains are arrested and ready for some
more overwhelming conversation about incredible topics like the physics of life.
It is exciting. One question I would have in my
mind if I was working on this is like, whether
the value is fifteen or nineteen or fourteen or whatever,
how do we know that all the information is captured
by this one quantity. You back it up so far

(01:05:39):
with like, well, we have a bunch of these examples,
then it seems to have worked, and you're working on
the theoretical underpinnings.

Speaker 11 (01:05:44):
You know.

Speaker 1 (01:05:45):
I work on machine learning problems all the time where
we convince ourselves we've learned how to distinguish between A
and B because we've only seen some kind of examples,
and then we go off into the world where like, oops,
turns out that's just one dimension of a multidimensional problem.
You know, for example, if I was not intelligent about
this and I said, well, look I took a bunch
of examples of living things that all have brown hair

(01:06:07):
and non living things that don't have brown hair, and
then I developed my definition of life to be like,
does it have brown hair? I could also come up
with a way to distinguish between quote unquote living and
unliving things because my sample was limited, and here your
sample is limited because you only have examples from you know,
life we've seen and so how do I know that
there is a number that distinguishes.

Speaker 8 (01:06:29):
So there's a couple of things there.

Speaker 6 (01:06:31):
One of them is assembly theory is not a scaler theory,
so it's not just about this number. It's actually about
also the abundance of objects at different assembly indites.

Speaker 8 (01:06:39):
So it's higher dimensional than that.

Speaker 6 (01:06:41):
And the idea is you're talking about selection in this
massive combinatorial space, and you have to talk about the
reduction of the size of the space represented in the
objects you observe. And we have rigorous ways for thinking,
like based on the mathematical formalism of the theory, why
we're capturing the relevant features of that reduction of.

Speaker 8 (01:06:58):
The size of the space.

Speaker 6 (01:07:00):
People usually do do exactly what you're saying, where they
try to, you know, separate living and non living things
and then classify them. And it's becoming very popular in
natro biology right now to do machine learning on those
kind of problems. I don't think that that approach is generalizable.
The reason I have confidence in what we're doing is
because there's a whole theoretical infrastructure building in a whole
bunch of ideas about the nature of information in life,

(01:07:22):
what is causation in life?

Speaker 8 (01:07:23):
How do we measure these features in molecules? You know?

Speaker 6 (01:07:26):
What is it about these emergent properties of life? Like
you know, I've spent my entire career basically working on
every kind of biological system imaginable, and you know, Lee's
lab has worked on kind of every chemical system imaginable,
and putting all of that knowledge into one theoretical structure
that we can really stand behind and say, this captures
the features we think are important, and we can measure it,
and we want to test it by actually building living

(01:07:47):
things in the lab. So it's not ever going to
be convinced by just this measure and the copy number.
What you get convinced of is an explanatory paradigm that
allows you to explain not just this one data set,
but a whole bunch of different things, And so ultimately
the proof of the theory is going to come from
the generalizability and the new questions it allows us to answer.

(01:08:07):
Not what we've done so far, so't I wouldn't hang
my hat on it the data we have so far.
But what I'm hanging my hat on is the potential
things that this is opening up and the way it's
allowing a unified explanation for a whole bunch of problems
in astrobiology that have traditionally been considered completely separate. And
also on what we talked about at the very beginning
about what is physics and what does physics do? And

(01:08:30):
for me, it's about the explanation for the nature of
life that's more important than anyone experiment.

Speaker 3 (01:08:35):
That feels like a nice note to end things on,
like a nice wrap up is did you have another question, Daniel.

Speaker 1 (01:08:41):
Yeah, I would love to hear just as a final vision,
like how you think this could, in the best case
scenario come together like you talk in the book about
building these computers, how you search for the origin of
life and when I do science, I was trying to imagine,
like what's the fantasy data I would have, Are you
hoping that we discover like alien life here on Earth

(01:09:03):
before we discover it on Enzi? Let us would actually
be your preference? Take us through your scientific fantasy, right.

Speaker 6 (01:09:09):
So my preference is that we just discover more life
and we understand what life is. So my preference is
understand what life is and have enough scientific evidence to
support that explanation. The progress of science is that's never
a single aha moment. It's like, you know, a cultural transformation,
and the way we understand a certain set of phenomena
mediated by a whole bunch of experiments and observations and

(01:09:31):
a consilience of like a whole bunch of things coming together. Right,
So I think that process is highly nonlinear. For me,
the reason I really advocated more for this experimental approach
in the lab is the idea that you could iterate
between the observation and theory very quickly. So the challenge
for looking for life on alien worlds is we don't

(01:09:51):
know the prior probability for life, and we don't know
what we're looking for, and so we don't know how
many planets we have to survey before we find something.
And nor is it the case that we can actually
rule out a planet is alive or not. And so
I think by trying to bring the paradigm of looking
for alien life into an experimental paradigm, and when we
were actually building large enough chemical search engines to look
for alien life, we actually make it tractable to understand

(01:10:14):
the mechanism of the original life, the probability it doesn't happen,
and also build the theory and experiments to really test
how it does happen at the same time. So to me,
it seems the most efficient route to get actually getting
to the answer to the question. And so and you know,
part of it might be biased by my background in
cosmology and just looking at the way that particle physicists
and cosmology have collaborated to really like, you know, why

(01:10:36):
do we understand the mechanisms of like the Big Bang
so well, it's.

Speaker 8 (01:10:38):
Because we build particle accelerators.

Speaker 6 (01:10:41):
And so I think without an experimental paradigm for these
questions that really ties the planetary and the alien search
to the original life, you have to couple those problems
through the same problem at their core. And I also
just like the radical idea that like alien life will
discover it on Earth and an experiment.

Speaker 8 (01:10:56):
Because it's fine, but then we'll know what we're looking for.

Speaker 6 (01:10:59):
So it was more obvious we can find it elsewhere, right, So,
like to me, that just seems logical, but it's also
fun because you know, like, it means we can do
the science really rapidly, and I want to see it solved,
so I want the most efficient route to an answer.

Speaker 1 (01:11:11):
And then we'll have to argue about whether it's really
alien if it was on Earth after all.

Speaker 6 (01:11:15):
Right, that's a good problem to have, right, because yeah,
a really good problem to.

Speaker 3 (01:11:20):
Have come full circle to good beer conversations.

Speaker 8 (01:11:22):
Yes, yes, there you go.

Speaker 6 (01:11:24):
See if you answer one of the beer questions, but
you have a better beer question, it's not a problem.

Speaker 3 (01:11:29):
Yep.

Speaker 1 (01:11:30):
Well, thank you very much for coming on and talk
to us about assembly theory and for giving us an
advanced peak at your book. Everyone, it's called Life As
No One Knows It, the physics of Life's emergence. Congratulations
on the book and best of luck.

Speaker 8 (01:11:43):
Thank you so much, Thank you both.

Speaker 3 (01:11:46):
Yeah, thank you.

Speaker 1 (01:11:47):
All right, and that was our conversation with Sarah and
Maria Walker, author of the recent book Life As No
One Knows it, which you can now pick up at
all fine booksellers. Kelly, what's your takeaway is physics is
going to help us understand what life is.

Speaker 5 (01:12:00):
Well, you know, my takeaway is pretty similar to my
takeaway many years ago when I was arguing with my
fellow grad students, which is, Wow, this is complicated, which
is pretty much the conclusion to every ecology paper you'll
ever read is it's complicated and it depends and anyway.
So yeah, it's complicated.

Speaker 3 (01:12:19):
What do you think.

Speaker 1 (01:12:20):
I think it's a valiant effort and it's worth doing,
and we're going to learn things along the way. I
don't know that we're going to figure it out. I
don't know that this is the right approach, but I
think thinking hard about it is the first step to
figuring it out. No matter where it goes. Often you
go down to the root of something and you discover
something completely different than solving the problem you meant to,

(01:12:42):
or you make breakthroughs in other areas. So I'm excited
to see where this goes. I'm excited that people are
thinking about these problems. I think it helps clear away
some of the cobwebs and provides a little bit of clarity,
But I guess I have to put it into like
we'll see category before I'm actually convinced that this is
the right way to think about life.

Speaker 5 (01:13:00):
I mean, it would be pretty epic if like on demand,
new life could be created in the lab and we
could look at different evolutionary trajectories and stuff like that.
I mean, the evolutionary biologists would go wild if that.

Speaker 3 (01:13:10):
Could be accomplished.

Speaker 5 (01:13:11):
I don't know where you start to do something like that,
but it would be pretty exciting if it could be accomplished.

Speaker 1 (01:13:17):
Yeah. And then, of course, the next philosophical question would
be if you've created a new kind of living goo
in your lab, is it an alien or is it
just another kind of earthling?

Speaker 3 (01:13:27):
And how long before it begins to eat human flesh?

Speaker 1 (01:13:32):
Day one? I'm hoping. I mean, if this movie's going
to be any good, right, yeah.

Speaker 3 (01:13:36):
No, it's going to happen fast.

Speaker 8 (01:13:38):
Yeah.

Speaker 1 (01:13:39):
I mean you could also flip the question around and
like say that new kind of living goo becomes intelligent,
how do we know it doesn't call us aliens?

Speaker 6 (01:13:46):
Right?

Speaker 1 (01:13:46):
And like we're at home, how can we be aliens?

Speaker 5 (01:13:48):
That's right, that's right, let's all just get along and
be Earthlakes.

Speaker 8 (01:13:52):
Am I right?

Speaker 1 (01:13:53):
That sounds good exactly. I look forward to a big,
cozy family meal, passing the bread and the corn cobs
around with our fellow Earth new.

Speaker 5 (01:14:01):
I'm going to schedule it one week after our meeting
where you tell me about how physics have solved all
the problems.

Speaker 4 (01:14:06):
Of the world.

Speaker 1 (01:14:07):
Sounds good, and I'll bring some towels.

Speaker 3 (01:14:09):
That's good because I'll forget mine.

Speaker 1 (01:14:10):
Bring extra, please, Katriina, We'll have an extra win in
her bag. No worries, all right. Thanks everyone for joining
us on this squishy discussion about the physics of life.
Hope you learned something I certainly did. And thank you
very much Kelly, my friend and co host, for joining
me on today's episode.

Speaker 3 (01:14:27):
My pleasure. Thanks Daniel.

Speaker 1 (01:14:30):
Tune in next time for more science and curiosity. Come
find us on social media where we answer questions and
post videos. We're on Twitter, Discorg, Instant, and now TikTok.
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. For more podcasts

(01:14:52):
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
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