- If we don't ask, how long do they last,

but instead ask, what's the probability

that there have been any civilizations at all,

no matter how long they lasted,

I'm not asking whether they exist now or not,

I'm just asking in general about probabilities

to make a technological civilization anywhere

and at any time in the history of the universe,

and that, we were able to constrain.

And so what we found was, basically,

that there have been 10 billion trillion

habitable zone planets in the universe.

And what that means

is those are 10 billion trillion experiments

that have been run.

And the only way, the only time that this is,

you know, this whole process from, you know,

abiogenesis to a civilization has occurred

is if every one of those experiments failed, right?

So therefore, you could put a probability,

we called it the pessimism line, right?

We don't really know what nature sets for the probability

of making intelligent civilizations, right?

But we could set a limit using this.

We could say, look,

if the probability per habitable zone planet

is less than 10 to the -22,

1 in 10 billion trillion, then, yeah, we're alone.

If it's anywhere larger than that,

then there we're not the first,

it's happened somewhere else.

And to me, that was mind-blowing.

It doesn't tell me there's anybody nearby.

The galaxy could be sterile.

It just told me that, like, you know,

unless nature's really has some bias against civilizations,

we're not the first time this has happened.

This has happened elsewhere

over the course of cosmic history.

- The following is a conversation with Adam Frank,

an astrophysicist interested

in the evolution of star systems

and the search for alien civilizations in our universe.

This is the Lex Fridman podcast.

To support it,

please check out our sponsors in the description.

And now, dear friends, here's Adam Frank.

You wrote a book about aliens.

So the big question,

how many alien civilizations are out there?

- Yeah, that's the question, right?

The amazing thing is that after 2 1/2 millennia

of, you know, people yelling at each other

or setting each other on fire, occasionally,

over the answer,

we now actually have the capacity to answer that question.

So in the next 10, 20, 30 years,

we're gonna have data relevant

to the answer to that question.

We're gonna have hard data, finally, that will,

one way or the other,

you know, even if we don't find anything immediately,

we will have gone through a number of planets,

we'll be able to start putting limits on how common life is.

The one answer I can tell you,

which was an important part of the problem, is,

how many planets are there, right?

And just like people have been arguing

about the existence of life elsewhere for 2,500 years,

people have been arguing about planets

for the exact same amount of time, right?

You can see Aristotle yelling at Democrates about this.

You know, you can see they had very different opinions

about how common planets were gonna be

and how unique Earth was.

And that question got answered, right,

which is pretty remarkable that, in a lifetime,

you can have a 2,500-year old question.

The answer is they're everywhere.

There are planets everywhere.

And it was possible that planets were really rare.

We didn't really understand how planets formed.

And so if you go back to, say, the turn of the 20th century,

there was a theory

that said planets formed when two stars passed

by each other closely,

And then material was gravitationally squeezed out,

in which case, those kinds of collisions are so rare

that you would expect one

in a trillion stars to have planets.

Instead, every star in the night sky has planets.

- So one of the things you've done

is simulated the formation of stars.

How difficult do you think it is

to simulate the formation of planets,

like simulate our solar system,

the entire evolution of the solar system?

This is kind of a numerical simulation sneaking up

to the question of, how many planets are there?

- That, actually, we're able to do now.

You can run simulations of the formation

of planetary system.

So if you run the simulation,

really, where you wanna start is a cloud of gas,

these giant interstellar clouds of gas that may have,

you know, a million times the mass of the sun in them.

And so you run a simulation of that. It's turbulent.

The gas is roiling and tumbling.

And every now and then you get a place

where the gas is dense enough that gravity gets hold of it,

and it can pull it downward.

So you'll start to form a protostar.

And a protostar is basically the young star of,

you know, this ball of gas

where nuclear reactions are getting started,

but it's also a disk.

So you as material falls inward,

'cause everything's rotating,, as it falls inward,

it'll spin up, and then it'll form a disk.

Material will collect in what's called an accretion disk

or a proto-planetary disk.

And you can simulate all of that.

Once you get into the disk itself

and you wanna do planets,

things get a little bit more complicated

'cause the physics gets more complicated.

Now you gotta start worrying about dust,

'cause actually, dust,

which dust is the wrong word, it's smoke really.

These are the tiniest bits of solids.

They will coagulate in the disk to form pebbles, right?

And then the pebbles will collide to form rocks,

and then the rocks will form boulders, et cetera, et cetera.

That process is super complicated,

but we've been able to simulate enough of it

to begin to get a handle on how planets form,

how you accrete enough material

to get the first protoplanets

or planetary embryos, as we call them.

And the next step

is those things start slamming into each other

to form, you know, planetary-sized bodies.

And then the planetary body slam into each other.

The moon came about because there was a Mars-sized body

that slammed into the Earth

and basically blew off all the material

that then eventually formed the moon.

- And all of them have different chemical compositions,

different temperatures?

- Yeah, so the temperature of the material in the disk

depends on how far away you are from the star.

- Got it. - So it decreases, right?

And so there's a really interesting point.

So like, you know, close to the star,

temperatures are really high,

and the only thing that can condense,

that can kind of freeze out is gonna be stuff like metals.

So that's why you find Mercury

is this giant ball of iron, basically.

And then as you go further out,

stuff, you know, the gas gets cooler

and now you can start getting things

like water to freeze, right?

So there's something we call the snow line,

which is somewhere in our solar system

out around between Mars and Jupiter.

And that's the reason why the giant planets

in our solar system,

Jupiter, Saturn, Uranus, and Neptune,

all have huge amounts of ice in them,

or water and ice.

Actually, Jupiter and Saturn don't have so much,

but the moons do.

The moons have so much water in them

that there's oceans, right?

That we've got a number of those moons

have got more water on them than there's water on Earth.

- Do you think it's possible to do that kind of simulation

to have a stronger and stronger estimate

of how likely an Earth-like planet is?

Can we get the physics simulation done well enough

to where we can start estimating,

like, what are the possible Earth-like things

that could be generated?

- Yeah, I think we can,

I think we're learning how to do that now.

So, you know, one part is, like, trying to just figure out

how planets form themselves and doing the simulations.

Like, that cascade from dust grains

up to planetary embryos,

that's hard to simulate because it's both,

you gotta do both the gas,

and you gotta do the dust

and the dust colliding and all that physics.

Once you get up to a planet-sized body,

then, you know, you kind of have to switch over

to almost like a different kind of simulation.

Often what you're doing is you're doing,

you know, sort of, you're assuming the planet

is this sort of spherical ball,

and then you're doing what, you know,

like a 1D, a radial calculation.

And you're just asking like, all right,

how is this thing going to,

what is the structure of it gonna be?

Like, am I gonna have a solid iron core,

or am I gonna get a solid iron core

with that liquid iron core out around it,

like we have on Earth?

And then you get, you know, a silicate,

kind of, a rocky mantle and then crust,

all of those details.

Those are kind of beyond being able

to do full 3D simulations from ab initio, from scratch.

We're not there yet.

- How important are those details,

like, the crust and the atmosphere, do you think?

- Hugely important, so I'm part of a collaboration

at the University of Rochester

where we're using the giant laser.

It's literally, this is called the Laboratory

for Laser Energetics.

We got a huge grant from the NSF to use that laser

to, like, slam tiny pieces of silica

to understand what conditions are like

at, you know, the center of the Earth

or, even more importantly, the center of super-Earths.

This is what's wild.

The most common kind of planet in the universe,

we don't have in our solar system,

which is amazing, right?

So we've been able to study enough

or observe enough planets now to get a census.

You know, we kind of have an idea

of who's average, who's weird.

And our solar system's weird

because the average planet has a mass

between somewhere between a few times the mass of the Earth

to maybe, you know, 10 times the mass of the Earth.

And that's exactly where there are no planets

in our solar system.

So the smaller ones of those we call super-Earths,

the larger ones we call sub-Neptunes.

And they're anybody's guess.

Like, we don't really know what happens to material

when you're squeezed to those pressures,

which is like millions, tens of millions of times

the pressure on the surface of the Earth.

So those details really will matter

of what's going on in there

because that will determine whether or not you have,

say, for example, plate tectonics.

We think plate tectonics may have been really important

for life on Earth,

for the evolution of complex life on Earth.

So it turns out, and this is sort of the next generation

where we're going with the understanding the evolution

of planets in life.

It turns out that you actually have to think hard

about the planetary context for life.

You can't just be like, oh, there's a warm pond, you know,

and then some interesting, you know,

chemistry happens in the warm pond.

You actually have to think about the planet as a whole

and what it's gone through

in order to really understand whether a planet

is a good place for life or not.

- Why do you think plate tectonics might be useful

for the formation of complex life?

- There's a bunch of different things.

One is that, you know, the Earth went through a couple

of phases of being a snowball planet.

Like, you know, we went into a period of glaciation

where the pretty much the entire planet was under ice.

The oceans were froZen.

You know, early on in Earth history,

there was barely any land.

We were actually a water world,

you know, with just a couple of Australia-sized cratons,

they called them, protocontinents.

We went through these snowball Earth phases.

And if it wasn't for the fact

that we had kind of an active plate tectonics,

which had a lot of volcanism on it,

we could have been locked in that forever.

Like, once you get into a snowball state,

a planet can be trapped there forever, which is,

you know, maybe you already had life form,

but then because it's so cold,

you may never get anything more than just microbes, right?

So what plate tectonics does is,

because it fosters more volcanism,

is that you're gonna get carbon dioxide pumped

into the atmosphere, which warms the planet up

and gets you out of the snowball Earth phase.

But even more, there's even more really important things.

I just finished a paper where we were looking

at something called the hard-steps model,

which is this model that's been out there

for a long time that purports

to say intelligent life in the universe will be really rare.

And it made all these assumptions about the Earth's history,

particularly that the history of life

and the history of the planet

have nothing to do with each other.

And it turns out, as I was doing the reading for this,

that Earth, probably early on,

had a more mild form of plate tectonics.

And then somewhere about a billion years ago, it ramped up,

and that ramping up changed everything on the planet,

'cause here's a funny thing:

The Earth used to be flat.

What I mean by that, right,

so all the flat-Earthers out there can get excited

for one sec.

- Clip it.

(Adam laughing)

Still is. - What I meant by that,

what I mean by that

is that there really weren't many mountain ranges, right?

The beginning of,

I think the term is orogenesis, mountain-building,

the true, Himalayan-style, giant mountains didn't happen

until this more robust form of plate tectonics

where the plates are really being driven around the planet.

And that is when you get the crusts hitting each other,

and they start pushing,

you know, into these Himalayan-style mountains.

The weathering of that,

the erosion of that puts huge amounts of nutrients,

you know, things that microbes wanna use, into the oceans,

and then what we call the net primary productivity,

the, you know, the bottom of the food chain,

how much sugars they're producing,

how much photosynthesis they're doing,

shot up by a factor of almost 1,000, right?

So the fact that you had plate tectonics

supercharged evolution in some sense, you know.

Like, we're not exactly sure how it happened,

but it's clear that the amount of life,

the amount of living activity that was happening

really got a boost from the fact

that suddenly there this new vigorous form

of plate tectonics.

- So it's nice to have turmoil in terms of temperature,

in terms of surface geometries,

in terms of the chemistry of the planet, turmoil.

- Yeah, that's actually really true

because what happens is, if you look at the history of life,

you know, it's a excellent point you're bringing up,

if you look at the history of life on Earth,

we get, you know, abiogenesis somewhere

around at least 3.8 billion years ago,

and that's the first microbes.

They kind of take over enough that they really do,

you get a biosphere.

You get a biosphere that is actively changing the planet,

but then you go through this period

they call the boring billion,

where, like, it's a billion years,

and it's just microbes, nothing's happening.

It's just microbes.

I mean, microbes are doing amazing things.

They're inventing fermentation.

Thank you very much. We appreciate that.

But it's not until, sort of,

you get probably these continents slamming into each other,

you really get the beginning of continents forming

and driving changes

that evolution has to respond to,

that on a planetary scale, this turmoil,

this chaos is creating new niches

as well as closing other ones.

And biology, evolution has to respond to that.

And somewhere around there

is when you get the Cambrian explosion,

is when suddenly every body plan,

you know, evolution goes on an orgy, essentially.

So yeah, it does look like that chaos

or that turmoil was actually very helpful to evolution.

- I wonder if there's some extremely elevated levels

of chaos, almost like catastrophes,

behind every leap of evolution.

Like, you're not gonna have leaps.

Like, in human societies,

we have, like, an Einstein that comes up with a good idea.

But it feels like on an evolutionary timescale,

you need some real big drama going on

for the evolutionary system

to have to come up to a solution to that drama,

like an extra complex solution to that drama.

- Well, I'm not sure if that's true.

I don't know if it needs to be, like,

an almost-extinction event, right?

Because it's certainly true that we have gone

through almost-extinction events, right?

We've had, you know, five mass extinctions,

but you don't necessarily see that, like,

there was this giant evolutionary leap happening

right after those.

So, you know, with the comet impact, the K-T boundary,

certainly, you know, lots of niches opened up,

and that's why we're here, right?

Because, you know, our ancestors

were just little basically rodents,

rats living under the footsteps of the dinosaurs.

And it was that comet impact that opened the route for us.

I mean, that still took another, you know, 65 million years.

It wasn't like this thing immediately happened.

But what we found with this hard-steps paper,

'cause the whole idea of the hard-steps paper was,

it was one of these anthropic reasoning kinds of things,

where Brandon Carter said, oh, look,

the intelligence doesn't show up on Earth until about,

you know, almost close to when the end

of the sun's lifetime.

And so he's like, well, there should be no reason

why the sun's lifetime

and the time for evolution

to produce intelligence should be the same.

And so therefore, and he goes through all this reasoning,

anthropic reasoning, and he ends up with the idea that like,

oh, it must be that the odds

of getting intelligence are super low,

and so that's the hard steps, right?

So there was a series of steps in evolution

that were, you know, very, very hard.

And because of that,

you can calculate some probability distributions,

and everybody loves a good probability distribution,

and they went a long way with this.

But it turns out that the whole thing is flawed

because, you know, when you look at it,

of course, the timescale for the sun's evolution

and the timescale for evolution on life are coupled

because life and the timescale for evolution of the Earth

is coupled, is about the same timescale

as the evolution as the sun.

It's billions of years.

The Earth evolves over billions of years.

And life and the Earth co-evolve.

That's what Brandon Carter didn't see,

is that, actually, the fate of the Earth

and the fate of life are inextricably combined.

And this is really important for astrobiology too.

Life doesn't happen on a planet. It happens to a planet.

So this is something that David Grinspoon

and Sara Walker both say,

and, you know, I agree with this.

It's a really nice way of putting it.

So, you know, plate tectonics,

the evolution of oxygen, of an oxygen atmosphere,

which only happened because of life,

these things, you know, these are things

that are happening where life

and the planet are sort of sloshing back and forth.

And so rather than, to your point about,

do you need giant catastrophes?

Maybe not giant catastrophes.

But what happens is,

as the Earth and life are evolving together,

windows are opening up, evolutionary windows.

Like, for example, life put oxygen into the atmosphere.

When life invented this new form of photosynthesis

about 2 1/2 billion years ago, that broke water apart to,

you know, work to do, its chemical shenanigans.

It broke water apart and pushed oxygen into the atmosphere.

That's why there's oxygen in the atmosphere.

It's only 'cause of life.

That opened up huge possibilities,

new spaces for evolution to happen.

But it also changed the chemistry of the planet forever.

So introduction of oxygen photosynthesis

changed the planet forever,

and it opened up a bunch of windows for evolution

that wouldn't have happened otherwise.

Like, for example, you and I, we need that amount of oxygen.

Big-brained creatures need an oxygen-rich atmosphere

'cause oxygen is so potent for metabolism.

So you couldn't get intelligent creatures 100 million years

after the planet formed.

- So really, on a scale of a planet,

when there's billions, trillions of organisms on a planet,

they can actually have planetary-scale impact.

- Yeah. - So the chemical shenanigans

of an individual organism, once scaled out to trillions,

can actually change a planet.

- Yeah, and we know this for a fact now.

So there was this thing, Gaia theory,

that, you know, with James Lovelock introduced in the '70s,

and then Lynn Margulis, the biologist,

Lin Margulis together.

So this Gaia theory was the idea

that planets pretty much take,

or sorry, life takes over a planet,

life hijacks a planet

in a way that that sum total of life creates these feedbacks

between the planet and the life,

such that it keeps the planet habitable.

It's kind of a homeostasis, right?

I can go out, like, right now, outside,

it's 100 degrees, right?

And I go outside,

but my internal temperature's gonna be the same.

And I can go back to, you know,

Rochester, New York, in the winter,

and it's gonna be, you know, zero degrees,

but my internal temperature's gonna be the same.

That's homeostasis.

The idea of Gaia theory was that the biosphere

exerts this pressure on the planet

or these feedbacks on the planet,

that even as other things are changing,

the planet will always stay

in the right kinds of conditions for life.

Now, when this theory came out, it was very controversial.

People were like, oh my God,

you know, what, are you smoking weed?

You know, and like there were all these Gaian festivals

with Gaian dances.

And so, you know, it became very popular

in the New Age community.

But Lovelock, actually, they were able to show that, no,

this has nothing to do

with, like, the planet being conscious or anything.

It was about these feedbacks that biology,

the biosphere can exert these feedbacks.

We're still unclear whether there are true Gaian feedbacks,

in the sense that the planet

can really exert complete control.

But it is absolutely true

that the biosphere is a major player in Earth's history.

- So the biosphere fights for homeostasis on Earth.

- So, okay, what I would say right now is,

I don't know if I can say that scientifically.

I can certainly say that the biosphere does a huge amount

of the regulation of the planetary state

and, over billions of years,

has strongly modified the evolution of the planet.

So whether or not,

a true Gaian feedback would be exactly what you said, right?

The biosphere is this somehow,

and Sara Walker and David Grinspoon

and I actually did a paper on this

about the idea of planetary intelligence

or cognition across a planetary scale.

And I think that actually is possible.

It's not conscious,

but there is a kind of cognitive activity going on.

The biosphere, in some sense, knows what is happening

because of these feedbacks.

So it's still unclear

whether we have these full Gaian feedbacks,

but we certainly have semi-Gaian feedbacks.

If there's a perturbation on the planetary scale,

temperature, you know, insulation,

how much sunlight's coming in,

the biosphere will start to have feedbacks

that will damp that perturbation.

Temperature goes up, the biosphere starts doing something,

temperature comes down.

- Now, I wonder if the technosphere

also has a Gaian feedback

or elements of a Gaian feedback such that the technosphere

will also fight to some degree for homeostasis.

Open question, I guess.

- Well, I'm glad you asked that question

because that paper that David and Sara and I wrote,

what we were arguing was,

is that, over the history of a planet, right,

when life first forms, you know, 3.8 billion years ago,

it's kind of thin on the ground, right?

You've got the first species, you know,

these are all microbes, and they have not yet been,

they're not going to enough of them

to exert any kind of these Gaian feedback.

So we call that an immature biosphere.

But then as time goes on, as life becomes more robust,

and it begins to exert these feedbacks,

keeping the planet

in the place where it needs to be for life,

we call that a mature biosphere, right?

And the important thing,

and I'm sure later on,

we're gonna talk about definitions of life and such.

There's this great term called autopoiesis

that Francisco Varela, the neurobiologist,

Francisco Varela came up with.

And he said, you know, one of the defining things about life

is this property of autopoiesis,

which means self-creating and self-maintaining.

Life does not create the conditions

which will destroy itself, right?

It's always trying to keep itself in a place

where it can stay alive.

So the biosphere from this Gaian perspective

has been autopoietic for, you know, billions of years.

Now, we just invented this technosphere

in the last, you know, couple of hundred years.

And what we were arguing in that paper

is that it's an immature technosphere, right?

Because right now, with climate change

and all the other things we're doing,

we know the technosphere right now

is sort of destroying the conditions

under which it needs to maintain itself.

So the real job for us, if we're gonna last over, you know,

geologic timescales, if we want a technosphere

that's gonna last tens of thousands, hundreds of thousands,

millions of years, then we've gotta become mature,

which means to not undermine the conditions,

to not subvert the conditions that you need to stay alive.

So as of right now, I'd say we're not autopoietic.

- Wow, I wonder if we look across thousands,

tens of thousands, hundreds of thousands of years,

that perturbations, the technosphere

should create perturbations

as a way for developing greater

and greater defenses against perturbations,

which sounds like a ridiculous statement,

but basically, go out and play in the yard

and hurt yourself to strengthen,

or, like, drink water from the pond.

- [Adam] From the pond. Yeah, right.

Get sick a few times.

- To strengthen the immune system.

- Yeah, well, you know what's interesting

with the technosphere, we could talk about this more,

but, like, you know, we're just emerging as a technosphere

in terms of as a interplanetary technosphere, right?

That's really the next step for us is to,

David Grinspoon talks about,

I love this idea of anti-accretion,

like, this amazing thing that for the first time,

you know, over the entire history of the planet,

stuff is coming off the planet, right?

Used to be everything just fell down,

all the meteorites fell down.

But now we're starting to push stuff out.

And you know, like the idea of planetary defense or such,

you know, we are actually gonna start exerting perturbations

on the solar system as a whole.

We're gonna start engineering, if we make it, right?

I always like to say

that if we can get through climate change,

the prize at the end is the solar system, right?

So we'll be literally engineering the solar system.

But what you can think of right now

with what's happening with the Anthropocene,

the Great Acceleration that is the technosphere,

you know, is the creation,

that is a giant perturbation on the biosphere, right?

And what you can't do is,

you know, the technosphere sits on top of the biosphere,

and if the technosphere undermines the biosphere

for its own conditions of habitability,

then you're in trouble, right?

I mean, the biosphere is not going away.

There's nothing we could do.

Like, the idea that we have to save the Earth

is a little ridiculous.

Like, the Earth is not a furry little bunny

that we need to protect,

but it's the conditions for us, right?

Humanity emerged out of the Holocene,

the last 10,000-years interglacial period.

We can't tolerate very different kinds of Earths.

So that's what I mean about a perturbation.

- Before we forget,

I gotta ask you about this paper, pretty interesting.

There's an interesting table here about hard steps,

abiogenesis, glucose fermentation to pyruvic acid,

all kinds of steps, all the way to homosapiens,

animal intelligence, land ecosystems,

endoskeletons, eye precursor,

so formation of the eye, complex multicellularity.

- [Adam] That's definitely one of the big ones.

- Yeah, so interesting.

I mean, what can you say about this chart?

There are all kinds of papers talking about,

what the difficulty of these steps?

- Right, and so this was the idea.

So what Carter said was,

you know, using philanthropic reasoning,

he said, there must be a few very hard steps

for the evolution to get through

to make it to intelligence, right?

So some steps are gonna be easy.

So every generation, you know, you roll the dice,

and yeah, it won't take long for you to get that step,

but there must be a few of them.

And he said, you could even calculate

how many there were, five, six,

in order to get to intelligence.

And so this paper here,

this plot is all these different people

who've written all these papers.

And this is the point, actually,

you can see all these papers

that were written on the hard steps,

each one proposing a different set

of what those steps should be.

And there's this other idea from biology

of the major transitions in evolution, MTEs,

that those were the hard steps.

But what we actually found was

that none of those are actually hard.

The whole idea of hard steps,

that there are hard steps, is actually suspect.

So you know, what's amazing about this model

is it shows how important it is

to actually work with people who are in the field, right?

So, you know, Brandon Carter

was a, you know, brilliant physicist,

the guy who came up with this.

And then lots of physicists

and astrophysicists like me have used this,

but the people who actually study evolution

and the planet were never involved, right?

And if you went and talked to an evolutionary biologist

or a biogeophysicist,

they'd look at you, when you explain this to them,

they'd be like, what?

Like, what are you guys doing?

Turns out, none of the details

or none of the conceptual structure of this matches

with what the people actually study the planet

and its evolution.

- Is it mostly about the fact

that there's not really discrete big steps,

is it's a gradual, continual kind of process?

- Well, there's two things.

The first most important one was that the planet

and the biosphere have evolved together.

That's something that every,

you know, most bio geophysicists completely accept.

And it was the first thing that Carter kind of rejected.

He said like, no, that's probably not possible.

And yet, you know, like, if he'd only, sort of,

had more discussions with this other community,

would've seemed like no,

you know, there are actually windows that open up.

And then the next thing is this idea

of whether a step is hard or not,

'cause what we mean by a hard step

is that, like I said, every time there's a generation,

every time there's the next generation born,

you're rolling the dice

on whether this mutation will happen.

And the idea of something being a hard step,

there's two ways in which something might even appear

as a hard step and not be,

or actually not be a hard step at all.

One is that you see something that has occurred in evolution

that has only happened once, right?

So let's take the opposite.

You see something that's happened multiple times,

like wings, lots of examples of wings

over lots of different evolutionary lineages.

So that's clearly not, making wings is not a hard step.

There are certain other things

that people say, no, that's a hard step,

you know, the oxygen photosynthesis.

But they tend to be so long ago

that we've lost all the information.

There could be other things in the fossil record that,

you know, made this innovation,

but they're just gone now, so you can't tell,

so there's information loss.

The other thing is the idea of pulling up the ladder,

that somebody, you know, some species makes the innovation,

but then it fills the niche,

and nobody else can do it again.

So yeah, it only happened once, but it happened once

because, basically, the creature was so successful,

it took over, and there was no space

for anybody else to evolve it.

So, yeah, so the interesting thing about this

was seeing how much,

once you look at the details of life's history on Earth,

how it really shifts you away from this hard-steps model.

And it shows you that those details,

as we were talking about, like,

do you have to know about the planet,

do you have to know about plate tectonics?

Yeah, you're going to have to.

- I mean, to be fair to Carter on the first point,

it makes it much more complicated

if life and the planet are co-evolving,

'cause it would be nice to consider the planet

as a static thing that sets the initial conditions.

And then we can sort of, from an outside perspective,

analyze planets based on the initial conditions they create.

And there's a binary yes or no, will it create life?

But if they co-evolve,

it's a really complex dynamical system where everything

becomes much more difficult from the perspective of SETI,

of looking out there

and trying to figure out which ones

are actually producing life.

- But I think we're at the point now.

So now there may be other kinds of principles that actually,

'cause you know, coevolution actually has its own,

not deterministic, you're done with determinism, right?

But complex systems have patterns.

Complex systems have constraints,

and that's actually what we're gonna be looking for

are constraints on them.

And so, you know, and again, nothing against Carter,

was a brilliant idea, but it just goes to show,

you know, there's this great,

you know, I'm a theoretical physicist, right?

And so I love simplified,

give me a simplified model

with, you know, a dynamical equation,

some initial conditions, I'm very happy.

But there's this great XTC comic where like,

you know, somebody's working something out on the board,

and this physicist is looking over

and saying, "Oh, oh, I just wrote down an equation for that.

I solved your problem.

Do you guys even have a journal for this?"

And, you know, subtitle is,

"Why everybody hates physicists."

So sometimes that approach totally works.

Sometimes physicists, you know,

can be very good at like zooming in on what is important

and casting the details aside

so you can get to the heart of an issue.

And that's very useful sometimes.

Other times, it obfuscates, right?

Other times, it clouds over, actually,

what you needed to focus on,

especially when it comes to complexity.

- Speaking of simplifying everything down to an equation,

let's return back to the question

of how many alien civilizations are out there

and talk about the Drake equation.

Can you explain the Drake equation?

- You know, people have various feelings

about the Drake equation.

You know, it can be abused, but basically,

the story actually is really interesting.

So Frank Drake, in 1960,

does the first ever astrobiological experiment.

He gets a radio telescope, points it at a couple of stars,

and listens for signals.

That was the first time anybody done any experiment

about any kinda life in the history of humanity.

And he does it,

and he's kind of waiting for everybody to make fun of him.

And still, he gets a phone call from the government,

says, hey, we want you to do a meeting

on interstellar communications, right?

So he's like, okay.

So they organize a meeting with like just eight people,

a young Carl Sagan is gonna be there as well.

And like, the night before,

Drake has to come up with an agenda.

How do you come up with an agenda

for a meeting on a topic

that no one's ever talked about before, right?

What he does, what's so brilliant about the Drake equation

is he breaks the problem

of how many civilizations are there out there

into a bunch of sub-problems, right?

And he breaks it into seven sub-problems.

Each one of them is a factor in an equation,

that when you multiply them all together,

you get the number of civilizations out there

that we could communicate with.

So the first term is the rate at which stars form.

The second term is the fraction of those stars

that have planets, F sub-p.

The next term is the number of planets

in the habitable zone,

the place where we think life could form.

The next term after that is the fraction of those planets

where, actually, an abiogenesis event, life forms, occurs.

The next one is the fraction of planets

on which you start to get intelligence.

After that, it's the fraction of planets

where that intelligence goes on to create a civilization.

And then finally, the last term,

which is the one that we really care about, is the lifetime.

You have a civilization. Now how long does it last?

- [Lex] Why do you say we humans?

- We humans, right?

'Cause we're standing, we're staring at the,

you know, multiple guns pointing at us.

You know, nuclear war, climate change, AI.

So, you know, how long, in general,

does civilizations last?

Now, each one of these terms,

what was brilliant about what he did was,

what he was doing

was he was quantifying our ignorance, right?

By breaking the problem up into these seven sub-problems,

he gave astronomers something to do, right?

And so, you know, this is always,

with a new research field,

you need a research program,

or else you just have a bunch of vague questions,

You don't even know really what you're trying to do.

So, you know, the star people could figure out

how many stars were forming per year.

The people who were interested in planets

could go out and find techniques to discover planets,

et cetera, et cetera.

- I mean, these are their own fields.

Essentially by creating this equation,

he's launching new fields.

- Yeah, he gave astrobiology,

which wasn't even a term then, a roadmap.

Like, okay, you guys go do this,

you go do that, you go do that.

And it had such far-reaching effect on astrobiology

because it did break the problem up

in a way that gave useful, you know, sort of marching orders

for all these different groups.

Like, for example, it's because of the Drake equation.

in some sense, that people

who were involved in SETI pushed NASA

to develop the technologies for planet hunting.

There was this amazing meeting in 1978 and,

two meetings, 1978 and 1979

that were driven, in some part,

by the people who were involved in SETI,

getting NASA together to say, look, okay, look,

you know, what's the roadmap

for us to develop technologies to find planets?

So yeah, so, you know, the Drake equation

is absolutely foundational for astrobiology,

but we should remember that it's not a law of nature, right?

It's not something that's, it's not E = mc squared.

And so you can see it being abused in some sense.

People, you know, it's generated a trillion papers.

Some of those papers are good.

I've written some of those,

and some of those papers are bad.

You know, I'm not sure where my paper fits in on those.

I'm saying, you know,

one should be careful about what you're using it for.

But in terms of understanding the problem

that astrobiology faces,

this really broke it up in a useful way.

- We could talk about each one of these,

but let's just look at exoplanets.

So that's a really interesting one.

I think when you look back,

you know, hundreds of years from now,

was it in the '90s when they first detected the first-

- Yeah, '92 and '95.

'95 to me was really, that was the discovery

of the first planet orbiting a sun-like star.

To me, that was the water, the dam being broken.

- I think that's, like, one of the greatest discoveries

in the history of science.

- I agree. I agree.

- Right now, I guess nobody's celebrating it too much

because you don't know what it really means.

But I think,

once we almost certainly will find life out there,

it will obviously allow us to generalize

across the entire galaxy, the entire universe.

So if you can find life on a planet,

even in the solar system,

you can now start generalizing across the entire universe.

- You can. All you need is one.

Like, right now, it's an any,

you know, our understanding of life.

We have one example. We have N = 1 example of life.

So that means we could be an accident, right?

It could be that we're the only place in the entire universe

where this weird thing called life has occurred.

Get one more example, and now you're done.

Because if you have one more example,

you know, you don't have to find all the other examples.

You just know that it's happened more than once.

And now you are, you know, from a Bayesian perspective,

you can start thinking like, yeah, yeah,

life is not something that's hard to make.

- Well, let me get your sense of estimates

for the Drake equation.

You also written a paper expanding on the Drake equation,

but what do you think is the answer?

- So the paper, there was this paper we wrote,

Woody Sullivan and I in 2016, where we said, look,

we have all this exoplanet data now, right?

So the thing that exoplanet science

and the exoplanet census I was talking about before

have nailed is F sub-p,

the fraction of stars that have planets, it's one.

Every fricking star that you see

in the sky hosts a family of worlds.

I mean, it's mind-boggling,

'cause those are all places, right?

They're either, you know, gas giants, probably with moons,

so the moons are places you can stand and look out,

or they're like terrestrial worlds,

where even if there's not life, there's still snow falling,

and there's oceans washing up on, you know, on shorelines.

It's incredible to think how many places

and stories there are out there.

So right, the first term was F sub-p,

which is how many stars have planets.

The next term is how many planets

are in the habitable zone, right, on average.

And it turns out to be 1/5, right?

So, you know, you know, around 0.2.

So that means you just count five of them.

Go out at night and go one, two, three, four, five.

One of them has an Earth-like planet,

you know, in the habitable zone, like, whoa.

- So what defines a habitable zone?

- Habitable zone is an idea that was developed

in 1958 by the Chinese American astronomer, Shu Shang,

and it was a brilliant idea.

It said, look,

you know, I can do the simple calculation if I take a planet

and just stick it at some distance from a star of,

what's the temperature of the planet?

What's the temperature of the surface?

So now all you're gonna ask,

you give it a standard kind of,

you know, Earth-like atmosphere and ask,

could there be liquid water on the surface, right?

We believe that liquid water is really important for life.

There could be other things that's happening, fine,

but you know, if you were to start off trying to make life,

you'd probably choose water as your solvent for it.

So basically, the habitable zone

is the band of orbits around a star

where you can have liquid water on the surface.

You could take a, you know, glass of water,

pour it on the surface, and it would just pool up.

It wouldn't freeze immediately,

which would happen if your planet is too far out,

and it would just boil away

if your planet's too close in.

So that's the formal definition of the habitable zone.

So it's a nice strict definition.

There's probably way more going on than that,

but this is a place to start, right?

- Well, we should say it's a place to start.

I do think it's too strict of a constraint.

- [Adam] I would agree.

- We're talking about temperature

where water can be on the surface.

There's so many other ways

to get the aforementioned turmoil

where the temperature varies,

whether it's volcanic, so interaction of volcanoes and ice

and all of this on the moons of planets

that are much farther away, all this kind of stuff.

- Yeah, well, for example,

we know in our own solar system,

we have, say, Europa, the moon of Jupiter,

which has got 100-mile deep ocean

under 10 miles of ice, right?

That's not in the habitable zone.

That is outside the habitable zone.

And that may be the best place.

It's got more water than Earth does.

All of its oceans are,

you know, it's twice as much water on Europa

than there is on Earth.

So, you know, that may be a really great place

for life to form.

And it's outside the habitable zone.

So, you know, the habitable zone is a good place to start,

and it helps us,

and there's reasons why you do wanna focus

on the habitable zone, because like Europa,

I won't be able to see from across telescopic distances

across light-years.

I wouldn't be able to see life on Europa

because it's under 10 miles of ice, right?

So the important thing about planets in the habitable zone

is that we're thinking they have atmospheres.

Atmospheres are the things we can characterize

for across 10, 50 light-years,

and we can see biosignatures, as we're gonna talk about.

So there is a reason why the habitable zone

becomes important for the detection of extrasolar life.

- But for me, when I look up at the stars,

it's very likely that there's a habitable planet

or moon in each of the stars, habitable defined broadly.

- Yeah. I think that's not unreasonable to say.

I mean, especially since the formal definition,

you get one in five, right?

One in five is a lot. There's a lot of stars in the sky.

So yeah, saying that, in general, when I look at a star,

there's a pretty good chance

that there's something habitable orbiting it

is not a unreasonable scientific claim.

- To me, it seems like

there should be alien civilizations everywhere.

Why the Fermi paradox? Why haven't we seen them?

- Okay. The Fermi paradox.

Let's talk about the, I love talking about the Fermi paradox

because there is no Fermi paradox.

Dun-dun-dun-dun.

Yeah, so the Fermi paradox,

let's talk a about the Fermi paradox and the history of it.

So Enrico Fermi, it's 1950,

he's walking with his friends

at Los Alamos nuclear weapons lab to the cantina.

And there had been this cartoon in "The New Yorker."

They all read "The New Yorker."

And the cartoon was trying to explain

why there had been this rash

of garbage cans disappearing in New York.

And this cartoon said, oh, it's UFOs,

'cause this is already, you know, it's 1950.

The first big UFO craze happened in '47.

So they were laughing about this as they're walking.

And being physicists,

started talking about interstellar travel,

interstellar propulsion, blah, blah, blah.

You know, conversation goes on for a while,

conversation turns to something else.

You know, they've gone to other things.

About 40 minutes later, over lunch, Fermi blurts out,

"Well, where is everybody?" right?

Typical Fermi sort of thing.

He'd done the calculation in his head,

and he suddenly realized that, look,

you know, if intelligence is common,

that even traveling at sublight speeds,

a civilization could cross,

you know, kind of hop from one star system to the other

and spread it out across the entire galaxy

in a few hundred thousand years.

And he realized this,

and so he was like, why aren't they here now?

And that was the beginning of the Fermi paradox.

It actually got picked up as a formal thing in 1975

in a paper by Hart

where he actually kind of went through this calculation

and showed and said, well, there's nobody here now,

therefore, there's nobody anywhere that, you know.

Okay, so that is what we will call the direct Fermi paradox.

Why aren't they here now?

But something happened where after SETI began,

there's this idea of the great silence.

People got this idea in their head that like, oh,

we've been looking for decades now

for signals of extraterrestrial intelligence,

and we haven't found any,

therefore, there's nothing out there.

So we'll call that the indirect Fermi paradox.

And there absolutely is no indirect Fermi paradox,

for the most mundane of reasons, which is money.

There's never been any money to look.

SETI was always done by researchers

who were kind of like scabbing some time,

you know, some extra time from their other projects

to, you know, look a little bit,

you know, at the sky with a telescope.

Telescopes are expensive.

So Jason Wright, one of my collaborators,

he and his students did a study

where they looked at the entire search space for SETI.

You know, and imagine that's an ocean.

All the different stars you have to look at,

the radio frequencies you have to look at,

when you look, how often you look.

And they looked, then they summed up all the SETI searches

that had ever been done.

They went through the literature.

And what they found was if that search space,

if the sky is an ocean and you're looking for fish,

how much of the ocean have we looked at?

And it turns out to be a hot tub.

That's how much of the ocean that we've looked up.

We've dragged an a hot tub's worth of ocean water up,

and there was no fish in it.

And so now are we gonna say,

up, well, there's no fish in the ocean, right?

So there is absolutely,

positively no indirect Fermi paradox.

We just haven't looked.

But we're starting to look,

so that's what's, you know, finally we're starting to look,

that's what's exciting.

The direct Fermi paradox,

there are so many ways out of that, right?

There's a book called

"Seventy-Seven Solutions to the Fermi Paradox"

that it just, you know, you can pick your favorite one.

It just doesn't carry a lot of weight

because there's so many ways around it.

We did an actual simulation, my group,

Johnson Carroll, one of my collaborators,

we actually simulated the galaxy,

and we simulated probes moving at sublight speed

from one star to the other,

gathering resources, heading to the next one.

And so we could actually track the expansion wave

across the galaxy, have one abiogenesis event,

and then watch the whole galaxy get colonized or settled.

And it is absolutely true that that wave crosses,

you know, Hart was right, Fermi was right,

that wave crosses very quickly.

But civilizations don't last forever, right?

So one question is, when did they visit?

When did they come to Earth, right?

So if you give civilizations a finite lifetime,

you know, let them last 10,000, 100,000 years,

what you find is you now have a steady state.

Civilizations are dying.

You know, they're coming back,

they're traveling between the stars.

What you find then is you can have big holes opened up.

You can have regions of space where there is nobody

for, you know, millions of years.

And so if we're living

in one of those bubbles right now,

then maybe we were visited,

but we were visited 100 million years ago,

and there was a paper that Gavin Schmidt and I did

that showed that if there was a civilization,

whether it was, like, dinosaurs

or aliens that was here 100 million years ago,

there's no way to tell.

There's no record left over.

The fossil record is too sparse.

The only way maybe you could tell

is by looking at the isotopic strata

to see if there was anything reminiscent

of an industrial civilization.

But the idea that, you know, you'd be able to find,

you know, iPhones or toppled buildings

after 100 million years, there's no way.

- So if there was an alien camp here,

an alien village, a small civilization,

maybe even a large civilization.

- [Adam] Even a large civilization,

even if it was a large. - 100 million years ago.

- And it lasted 10,000 years,

fossil record's not gonna have it.

Yeah, yeah. The fossil record is too sparse, right?

Most things don't fossilize.

And 10,000 years is a, you know, blink in the eye

of geological time.

So we call, or Gavin called this the Silurian hypothesis

after the "Doctor Who" episode

with the lizard creatures, the Silurians.

But that paper got a lot of press,

but it was a, you know, it was an important idea,

and it was really Gavin's,

I was just helping with the astrobiology,

that to recognize that like, yeah,

you know, we could have been visited a long time ago.

There just would be no record.

Yeah. It's kinda mind-blowing.

- It's really mind-blowing.

And it's also a good reminder that we've been,

intelligent species have been here

for a very short amount of time.

- Very short amount of time. Yeah.

And this is not to say that there was.

Like, so, oh, whenever I gave, you know,

like, I was on Joe Rogan for exactly this paper,

and I had to always emphasize,

we're not saying there was a Silurian, you know,

but we're just saying that if there was,

that's why I love Gavin's question.

Gavin's question was just like, how could you tell, right?

It was a very beautifully scientific question.

That's what we were really showing is that you really,

you know, unless you did a very specific kind of search,

which nobody's done so far,

that, you know, there's not an obvious way to tell

that there could have been civilizations here earlier on.

- I've actually been reading a lot

about ancient civilizations,

and it just makes me sad

how much of the wisdom of that time is lost

and how much guessing is going on,

whether it's in South America,

like what happened in the jungle.

- Yeah. Like, the Amazon, like the Amazon.

You know, the conquistors came and wiped everybody out.

And just even, like, the plague may have decimated.

So yeah, how much of that civilization.

- And there's a lot of theories,

and you know, because of archeology only looks at cities,

they don't really know the origins of humans.

And there's a lot of really interesting theories,

and they're, of course, controversial,

and there's a lot of controversial people

in every discipline,

but archeology is like, yeah, it's a fascinating one

'cause we know so little.

They're basically storytellers.

You're assembling the picture

from just very few puzzle pieces.

And it's fascinating.

It's humbling and it's sad

that there could be entire civilizations,

ancient civilizations that are either almost entirely

or entirely lost.

- Yeah, well, like the indigenous peoples of North America,

there could have been, like, millions and millions.

You know, we get this idea that like, oh, you know,

the Europeans came and it was empty, you know?

But it may have only been empty

because the plague gets swept up from the, you know,

from the what happened in Mesoamerica.

So, and yeah, and they didn't really build cities,

but they had,

I mean, they didn't build wooden, or stone cities.

They built wooden cities, you know?

- Everybody seems to be building pyramids,

and they're really damn good at it.

I don't know what- - What does that have to do

with a, like, why does that apply?

Like, what archetype in our brain is that?

- And it is also really interesting, speaking of archetypes,

is that independent civilizations formed,

and they had a lot of similar kind of dynamics.

Like human nature,

it builds up hierarchies in a certain way,

it builds up myths and religions in a certain way,

it builds pyramids in a certain way,

it goes to war, all this kind of stuff,

independently emerges, fascinating.

- Santa Fe Institute,

the stuff the Santa Fe Institute does on this

as complex systems, you know, the origin

of hierarchies and such, very cool.

- Yeah, Santa Fe folks.

Complexity, in general,

is really cool. - Really cool.

- What phenomena emerge

when a bunch of small things get together and interact.

Going back to this paper,

"A New Empirical Constraint on the Prevalence

of Technological Species in the Universe,"

this paper that expands on the Drake equation,

what are some interesting things in this paper?

- Well, so the main thing we were trying to do

with this paper is say, look,

we have all of this exoplanet data, right?

It's gotta be good for something,

especially since two of the terms

that have been nailed down empirically

are two terms in the Drake equation.

So F sub-p, that's the second term,

fraction of stars that have planets.

And then N sub-e, the average number of planets

in the habitable zone.

Those are the second and third term in the Drake equation.

So what that means

is all the astronomical terms have been nailed.

And so we said like, okay, how do we use this

to do something with the Drake equation?

And so we realized is, well, okay, we gotta get rid of time.

The lifetime thing, we can't say anything about that.

But if we don't ask, how long do they last,

but instead ask, what's the probability

that there have been any civilizations at all,

no matter how long they lasted?

I'm not asking whether they exist now or not.

I'm just asking in general about probabilities

to make a technological civilization anywhere

and at any time in the history of the universe,

and that, we were able to constrain.

And so what we found was, basically,

that there have been 10 billion trillion

habitable zone planets in the universe.

And what that means

is those are 10 billion trillion experiments

that have been run.

And the only way that we're the only time that this is,

you know, this whole process from, you know,

abiogenesis to a civilization has occurred

is if every one of those experiments failed, right?

So therefore you could put a probability,

we called it the pessimism line, right?

We don't really know what nature sets for the probability

of making intelligent civilizations, right?

But we could set a limit using this.

We could say, look, if the probability

per habitable zone planet

is less than 10 to the -22, 1 in 10 billion trillion,

then, yeah, we're alone.

If it's anywhere larger than that,

then there we're not the first.

It's happened somewhere else.

And to me, that was mind blowing.

Doesn't tell me there's anybody nearby.

The galaxy could be sterile.

It just told me that, like, you know,

unless nature's really has some bias against civilizations,

we're not the first time this has happened.

This has happened elsewhere

over the course of cosmic history.

- 10 billion trillion experiments.

- Yeah. That's a lot of experiments.

- That's a lot. - Right.

- 1,000 is a lot. - Yeah.

- 100 is a lot. - Yeah.

- If we, normal humans, saw 100 experiments

and we knew that, at least one time,

there was a successful human civilization built.

I mean, we would say for sure,

in 100, you'll get another one.

- Yeah. Yeah, so that's what,

I mean, so this, you know, these kinds of arguments,

you have to be careful with what they can do.

But I felt like what this paper showed

was that, you know, the burden of proof

is now on the pessimists.

Right, so that's why we called it the pessimism line.

You know, throughout history there's been a,

you know, alien pessimists and alien optimists,

and they've been yelling at each other.

That's all they had to go with, right?

You know, and like with Giordano Bruno in 1600,

they burned the guy

at the stake for being an alien optimist.

But nobody really knew what pessimism or optimism meant.

You know, we sort of thought this was like the plank length.

This was sort of the plank length of astrobiology,

gave you an actual number that, you know,

if you could somehow calculate

what the probability, you know,

of forming a technological civilization was,

this thing sort of shows you where the limit is.

As long as you're above 10 to the -22,

then, absolutely, it has occurred in the history,

other civilizations have occurred

in the history of the universe.

- So to me, at least, the big question is fe,

which is basically abiogenesis.

How hard is it for life to originate in a planet?

'Cause all the other ones seem very likely.

Everything seems very likely.

The only open question to me

is like, how hard is it for life to originate?

- There's lots of ways to, again, you know,

we don't know unless we look,

and, you know, you had Sara Walker

on not too long ago.

You know, she's very interested in origins of life.

So, you know, lots of people are working on this.

But I think it's hard looking at the history of the Earth,

you know, and again, you can do Bayesian arguments on this,

but yeah, forming life, I don't think is hard.

Getting, like, basic biology started,

I don't think is hard.

It's still wild.

It's an amazing process

that actually, I think, requires some deep rethinking

about how we conceptualize what life is and what life isn't.

That's one of the things I like about Sara's work.

We're pursuing on a different level

about the life as the only process,

or the only system that uses information.

But still, regardless of all those kinds of details,

life is probably easy to make.

That's my gut feeling, you know?

- Yeah. I mean, day by day, this changes for me.

But I just see that once you create bacteria,

it's off to the races.

You're gonna get complex life.

As long as you have enough time,

I mean, that boring billion,

but I just can't imagine a habitable planet

not having a couple of billion to spare,

couple years. - Yeah, a couple billion years

to spare, you know, there is a mystery there

about why did it take so long,

like, with the Cambrian explosion,

but that may be, again, about these windows

that, like, it couldn't happen until the window,

the planet and the life had evolved together enough

that they, together, kind of opened the window

for the next step.

You know, intelligent life and how long intelligent,

civil and technological civilizations,

I think there's a big question about how long those last.

And you know, I'm hopeful, you know,

but in terms of just, like,

I think life is absolutely gonna be common,

you know, pretty common in the universe.

- Yeah, I think it's absolutely, like, I think, again,

if I were to bet everything,

even advanced civilizations are common.

So to me, then, the only explanation is the L.

Our galaxy is a graveyard of civilizations.

- Yeah, because, you know, you think about it,

we've only been around, I mean,

truly, you know, when we think about, in Drake's definition,

you had to have radio telescopes,

that's been 100 years.

You know, and if we got another 10,000,

100,000 years of history,

for us, it'd be pretty amazing, right?

But that's still, that wouldn't be long enough

to really pop up the number of civilizations

in the galaxy.

So you really need it to be, like,

hundreds of millions of years.

And that raises a question, which I am very interested in,

which is, how do you even talk about,

I call it the billionaire civilization, right?

How do we even begin to hypothesize

or think about, in any kind of systematic way,

what happens to a technological civilization

across hundreds of millions to a billion years?

- Yeah, like, how do you even simulate the trajectories

that civilizations can take across that kind of timescale

when all the data we have

is just for the 10,000 years or or so,

20,000 years that humans have been building civilizations?

And then just, I don't know what you put it at,

but maybe 100 years that we've been technological.

- Yeah, and we're ready to blow ourselves to bits

or, you know, drive ourselves off the planet.

Yeah. No, it's really interesting.

But there's gotta be a way.

I think that's really a frontier.

So you had David Kipping on not too long ago,

and David and I did a paper,

and Caleb Scharf, David really drove this,

where we, you know, it was a Bayesian calculation

to sort of ask the question,

if you were to find a detection,

if you were to find a signal

or, you know, a technosignature,

would that come from a civilization

that was younger, your age, or older?

And you could see,

I mean, this is not hard to do,

but it was great, the formalism was hard.

You know, it's kind of intuitive,

but the formalism was hard to show that,

yeah, they're older, you know, probably much older.

So it means you really do need to think about like, okay,

how do billion-year civilizations manifest themselves?

What signatures will they leave?

And yeah, can you even...

I mean, what's so cool about it,

it's so much fun because you gotta, like,

you have to imagine the unimaginable.

Like, you know, I mean, obviously, biological evolution

can happen, you know, on those kinds of timescales.

So you wouldn't even really be the same thing

you started out as, but social forms,

what kind of social forms can you imagine

that would be continuous over that?

Or maybe they wouldn't be continuous,

they drop out, you know,

they destroy themselves, and then they come back.

So maybe it's, you know, it's a punctuated evolution.

I mean, but we gotta sort of, this is the fun part,

we have to sort of work this out.

- Mm-hmm, well, I mean, one way to approach that question

is like, what are the different ways to achieve homeostasis

as you get greater and greater technological innovation?

So, like, if you expand out into the universe

and you have optic Kardashev scale,

what are the ways you can avoid destroying yourself?

Just achieve stability while still growing?

And I mean, that's an interesting question.

I think it's probably simulatable.

- Could be, I mean, you know, agent-based modeling,

you could do it with.

So you know, our group has used agent-based modeling

to do something like the Fermi paradox.

That was agent-based modeling.

But you can also do this,

people at Santa Fe have done this,

other groups have done this,

to use agent-based modeling

to track the formation of hierarchies,

the formation of stable hierarchies.

So I think it's actually very doable,

but understanding the kind of assumptions

and principles that are going into it

and what you can extract from those,

that is what is sort of the frontier.

- Do you think if humans colonize Mars,

the dynamic between the civilization on Earth

and Mars will be fundamentally different

than the dynamic between individual nations

on Earth right now?

Like, that's a thing to load

into the agent-based simulation we're talking about.

- If we settle it,

Mars will very quickly wanna become its own nation.

- Well, no, there's already gonna be nations on Mars.

That's guaranteed.

- Yeah, go be your own- - Moment you have

2 million people,

the moment you have 1 million people,

there's gonna be two tribes.

- [Adam] Right.

- And then they're going to start fighting.

- [Adam] Right.

- And the question is, interplanetary fighting,

how quickly does that happen?

And does it have a different nature to it,

because of the distances, you know?

- Are you a fan of "The Expanse"?

Have you watched "The Expanse"?

Great show, 'cause it's all about the,

I highly recommend to everybody.

It's based on a series of books that are excellent.

It's on Prime, six seasons,

and it's basically about the settled solar system.

It takes place about 300 years from now,

and the entire solar system is settled.

And it is the best show about interplanetary politics.

The first season, actually, the journal, what was it?

"Foreign Affairs" said the best show on TV about politics.

It's interplanetary, so yeah, I think, you know,

human beings being human beings, yes,

there will be warfare, and there will be conflict.

And I don't think it'll be necessarily all that different,

you know, because really,

I think within a few hundred years,

we will have lots of people in the solar system.

And it doesn't even have to be on Mars.

We did a paper where we look based on,

'cause I always wanted to know about whether an idea

in "The Expanse" was really possible.

In "The Expanse," the asteroid belt,

what they've done is they have colonized the asteroid belt

by hollowing out the asteroids and spinning them up

and living on the inside, right?

Because they have the Coriolis force.

And I thought like, wow, what a cool idea.

And when I ran the blog for NPR,

actually talked to the guys and said,

did you guys calculate this, see whether it's possible?

Sadly, it's not possible.

The rock is just not strong enough

that if you tried to spin it up to the speeds,

you need to get 1/3 gravity,

which is what I think the minimum you need for human beings,

the rock would just fall apart, it would break.

But we came up with another idea,

which was that if you could take small asteroids,

put a giant bag around them, a nanofiber bag,

and spin those up, it would inflate the bag.

And then even a small couple

of kilometer-wide asteroid would expand out to,

you could get like a Manhattan's worth of material inside.

So forget about even colonizing Mars. Space stations, right?

Or space habitats with millions of people in them.

So anyway, the point is that I think,

you know, within a few hundred years,

it is not unimaginable

that there will be millions, if not billions,

of people living in the solar system.

- And so you think most of them will be in space habitats

versus on Mars, on the planetary surface?

- You know, it's a lot easier,

on some level, right?

It depends on how, like, with nano fabrication and such.

But, you know, getting down to gravity well is hard, right?

So, you know, there's a certain way

in which there's a lot of, you know,

it's a lot easier to build real estate out of the asteroids,

but we'll probably do both.

I mean, I think what'll happen is, oh, you know, the next,

should we make it through climate change

and nuclear war and all the other, and AI,

the next thousand years of human history

is the solar system, right?

And so, you know, I think we'll settle every nook

and cranny we possibly can.

And it's, you know, it's a beautiful,

what I love about what's hopeful about it

is this idea you're gonna have all of these pockets,

and, you know, I'm sure there's gonna be

a Mormon space habitat, like, you know?

There's gonna be whatever you want,

a libertarian space habitat.

Everybody's gonna be able to kind of create their,

there'll be lots of experiments in human flourishing.

And those kinds of experiments will be really useful for us

to sort of figure out better ways for us to interact

and have maximum flourishing, maximum wellness,

maximum democracy, maximum freedom.

- Do you think that's a good backup solution

to go out into space,

so to avoid the possibility

of humans destroying themselves completely here on Earth?

- Well, I think, you know,

I wanna be always careful with that because, like I said,

it's centuries that we're talking about, right?

So, you know, the problem with climate change,

you know, and same with nuclear wars,

breathing down our necks now.

So it's not a, you know,

trying to establish a base on Mars is gonna be so hard

that it is not even gonna be close to being self-sufficient

for a couple of, you know, a century at least.

So it's not like a backup plan now.

You know, we have to solve the problem of climate change.

We have to deal with that.

There's still enough nuclear weapons

to really do, you know, horrific things

to the planet for human beings.

So I don't think it's like a backup plan in that way.

But I do think, like I said, it's the prize.

You know, if we get through this,

then we get the entire solar system

to sort of play around in and experiment with

and do really cool things with.

- Well, I think it could be a lot less

than a couple of centuries if there's a urgency,

like a real urgency, like a catastrophe,

like maybe a small nuclear war breaks out

where it's like, holy shit, this is for sure,

for sure a bigger one is looming.

Maybe if, geopolitically,

the war between China and the United States escalates

where there's this tension that builds and builds and builds

and it becomes more obvious that we need to really,

really, really excavate. - Yeah.

I think my only dilemma with that

is that I just think that a self-sufficient base

is so far away that, like, say, you start doing that

and then there is a full scale nuclear exchange.

That base is, you know, it's not gonna last

'cause it's just, you know,

the self-sufficiency requires a kind of economy,

like, literally a material economy

that we are so far from with Mars

that we are centuries from.

Like I said, you know,

three centuries, which is not that long.

Two to three centuries, you know, look at 1820,

nobody had traveled faster than 60 miles an hour

unless they were falling off a cliff, right?

And now, we routinely travel at 500 miles an hour.

But it is sort of centuries long.

So that's why I think we'd be better off trying

to solve these problems than,

you know, I just think the odds

that we're gonna be able

to create a self-sufficient colony on Mars

before that threat comes to head is small.

So we'd have to deal with the threat.

- Yeah, it's an interesting scientific

and engineering question of

how to create a self-sufficient colony on Mars

or out in space as a space habitat,

like where Earth entirely could be destroyed,

you could still survive.

- Yeah, yeah, 'cause it's really what about,

you know, thinking about complex systems, right?

A space habitat, you know, would have to be

as robust as an ecosystem, as the kind of thing,

you know, you go out and you see a pond

with all the different webs of interactions.

You know, that's why I always think that, you know,

if this process of going out into space

will help us with climate change

and with thinking about making

a long-term sustainable version of human civilization,

'cause you really have to think about these webs,

the complexity of these webs

and recognize the biosphere has been doing this forever.

The biosphere knows how to do this, right?

And so, A, how do we build a vibrant,

powerful technosphere that also doesn't,

you know, mess with the biospheres,

mess with the biosphere's capacity

to support our technosphere?

So, you know, by doing this,

by trying to build space habitats,

in some sense, you're thinking

about building a small-scale version of this.

So I think the two problems

are gonna kind of feed back on each other.

- Well, there's also the other possibility of,

like, the movie Darren Aronofsky's "Postcard From Earth"

where we can create this kind of life gun

that just shoots,

so as opposed to engineering everything,

basically seeding life on a bunch of places

and letting life do its thing,

which is really good at doing, it seems like.

So as opposed to, like, with a space habitat,

you basically have to build the entire biosphere

and technosphere, the whole thing

by yourself. - The whole thing, yeah.

You know, if you just,

hey, the aforementioned cockroach with some bacteria,

place it in Europa,

I think you'd be surprised what happens.

- Yeah. Yeah. - Right?

Like, honestly, if you put a huge amount of bacteria, like,

a giant number of organisms from Earth

on Mars, on some of these moons

of the other planets in the solar system,

like, I feel like some of them

would actually find a way to survive.

- You know, the moon is hard

'cause the moon is just, like, there's no,

you know, the moon may be really hard,

but, you know, I mean, I wonder,

somebody must have done these experiments, right?

Like, 'cause we know there are extremophiles, right?

We know that you can go down, you know,

10 miles below the Earth's surface,

and there are things where there's no sunlight.

There's, you know, the conditions are so extreme,

and there's lots of microbes having a great time,

living off the radioactivity, you know, in the rocks.

But, you know, they had lots of time

to evolve to those conditions.

So I'm not sure if you dumped a bunch of bacteria,

you know, like, somebody must have done these experiments.

Like, you know, how fast

could microbial evolution occur

in under harsh conditions

that you maybe get somebody who figures out,

okay, I can deal with this.

I think the moon's too much 'cause it's so sterile,

but you know, Mars, I don't know, maybe, I don't know.

We'd have to that, but it's an interesting idea.

- I wonder if somebody has done those experiments.

- Yeah, you think somebody would.

Like, let's take a bunch of microbes-

- Take the harshest possible condition

of all different kinds, temperature, all this kind of stuff.

- Right, pressure, salinity,

and then just, like, dump a bunch of things

that are not used to it.

And then just see, does everybody just die?

You know, that's it? There's, you know-

- The thing about life,

it flourishes in a non-sterile environment

where there's a bunch of options for resources,

even if the condition is super harsh.

In the lab, I don't know

if you can reconstruct harsh conditions

plus options for survival.

You know what I mean?

Like, you have to have the huge variety

of resources that are always available on a planet somehow,

even when it's a super harsh condition.

So that's actually not a trivial experiment.

If somebody did that experimental in the lab,

I'd be a little bit skeptical,

'cause I could see bacteria doesn't survive

in this kinds of temperature,

but then I'd be like, eh, I don't know, I don't know.

- Is there enough? Right.

You know, are there other options?

Like, you know, is the condition rich enough?

- [Lex] Rich enough, yeah.

- You know, there's an alternative view though,

which is, there's this great book

by Kim Stanley Robinson called "Aurora."

You know, so there's been a million century ship stories,

like where, you know, Earth sends out a,

you know, generation ship or century ship,

and it goes to another planet,

and they land, and they colonize.

And on this one, they get all the way there,

and they think the planet's gonna be habitable.

And it turns out that it's not habitable for Earth life.

You know, there's, like, you know,

bacteria are prions actually,

you know, super that just, like, you know,

kill people in the simplest way.

And the important thing about this book

was the idea that, like, you know, life

is actually very tied to its planet.

It may not be so easy.

I just thought this was a really interesting idea.

I'm not saying necessarily supporting it,

but that actually life reflects the planetary conditions,

not the planetary, the planet itself,

the whole lineage, the whole history of the biosphere.

And it may not be so easy just to just sort of be like,

oh, just drop it over here and it'll, you know,

'cause the bacteria,

even though they're individual examples of life,

and I kind of believe this, the true unit of life,

it's not DNA, it's not a cell,

it's the biosphere,

it's the whole community. - The whole thing.

- Yeah.

- That's actually an interesting field of study,

is how, when you arrive from one planet to another,

so we humans arrive to a planet that has a biosphere,

maybe a technosphere,

what is the way to integrate

without killing yourself or-

- [Adam] Or the the other one.

- Or the other one.

Let's stick to biology,

Like, that's an interesting question.

I don't know if we have a rigorous way

of investigating that.

- Because everything on life is,

you know, has the same lineage.

We all come from LUCA,

you know, the last universal common ancestor.

And what you see is, often, in science fiction,

people will do things like, oh, well, it's okay

because, like, that bio,

that metabolism, that biochemistry

is so different from ours that we can coexist

because they don't even know each other.

You know, right? Like, the...

You know, and then the other version is you get there,

you land and instantly, you know, the nose bleeds,

and you're dead, so it's...

- Unfortunately, I think it's the latter.

- Yeah, it sort of feels like

the more alien kind of thing. - Is the more likely.

So as we look out there,

according to the Drake equations we just discussed,

it seems impossible to me

that there's not civilizations everywhere.

So how do we look at them? This process of SETI.

- I have to put on my scientist hat

and just say my gut feeling

is that dumb life, so to speak, is common.

I am a little agnostic about,

I can see ways

in which intelligent civilizations may be sparse,

but, you know, we gotta go look.

It's all armchair astronomy.

- That's from a sort of rigorous scientific perspective.

From my bro-science perspective, it seems,

again, smoking the aforementioned weed.

- Smoking the weed, yeah, after the bong.

Yeah, it seems right. - I mean, honestly.

- Yeah.

- It's really just seems impossible to me

that there's not potentially dead,

but advanced civilizations everywhere

in our galaxy.

- Yeah, yeah, the potentially dead part, I think, right.

It could be that, like, making civilizations is easy,

they just don't last long.

So when we went out there,

we'd find a lot of extinct civilizations.

- Extinct civilizations. Yeah, apex predators don't survive.

Like, they get better, better, better, better,

and they die and kill themselves all.

Anyway, so, just, how do we find them?

- Yeah, so SETI,

search for extraterrestrial technology,

is a term that I am not fond of using anymore.

I mean, some people in my field are, so I'm sorry, folks,

but what I really like is the idea of technosignatures.

I think, you know, to me,

SETI is the, first of all, intelligence.

We're not really looking for intelligence.

We're looking for technology.

I mean, you know, and SETI,

the classic idea of SETI is the radio telescopes,

you know, in "Contact," Jodi Foster with the headphones.

That whole thing is still part, it's still active,

there's still great things going on with it,

but suddenly, this whole new window opened up.

When we discovered exoplanets,

we now found a new way to look for intelligent civilizations

or life, in general,

in a way that doesn't have any of the assumptions

that had to go into the classic radio study.

And specifically what I mean is we're not looking