Lex Fridman Podcast - #232 - Brian Greene: Quantum Gravity, Big Bang, Aliens, Life, Death, and Meaning

The following is a conversation with Brian Greene,

theoretical physicist at Columbia

and author of many amazing books on physics,

including his latest, Until the End of Time,

Mind, Matter, and Our Search for Meaning

in an Evolving Universe.

This is the Lex Friedman podcast.

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And now, here’s my conversation with Brian Greene.

In your most recent book, Until the End of Time,

you quote Bertrand Russell from a debate he had

about God in 1948.

He says, quote,

“‘So far as scientific evidence goes,

“’the universe has crawled by slow stages

“’to a somewhat pitiful result on this earth,

“‘and is going to crawl by still more pitiful stages

“’to a condition of universal death.

“‘If this is to be taken as evidence of purpose,

“‘I can only say that the purpose is one

“’that does not appeal to me.

“‘I see no reason, therefore,

“’to believe in any sort of God.’”

That’s quite a depressing statement.

As you say, this is a bleak outlook on our universe

and the emergence of human consciousness.

So let me ask, what is the more hopeful perspective

to take on this story?

Well, I think the more hopeful perspective

is to more fully understand

what was driving Bertrand Russell to this perspective,

and then to see it within a broader context.

And really, that’s, in some sense,

what my book, Until the End of Time, is all about.

But in brief, I would say that there’s a lot of truth

to what Bertrand Russell was saying there.

When you look at the second law of thermodynamics,

which is the underlying scientific idea

that’s driving this notion that everything’s gonna wither,

decay, fall apart, yeah, that’s true.

Second law of thermodynamics establishes

that disorder, entropy, in aggregate,

is always on the rise.

And that is indeed interpretable

as disintegration and destruction

over sufficiently long timescales.

But my view is, when you recognize

how special that makes us,

that we are these exquisitely ordered configurations

of particles that only will last for a blink of an eye

in cosmological time like terms,

the fact that we’re here and we can do what we do,

to me, that’s just really something

that inspires gratitude and wonder

and a sense of deep purpose

by virtue of being these unique collections of entities

that happen to rise up, look around,

and try to figure out where we are

and what the heck we should do with our time.

So it’s not that I would disagree with Bertrand Russell

in terms of the basic physics and the basic unfolding,

but I think it’s really a matter of the slant

that you take on what it means for us.

So maybe we’ll skip around a bit,

but let me ask the biggest possible question then.

You said purpose.

So what’s the meaning of it all then?

Is there a meaning to life that we can take from this,

from this brief emergence of complexity

that arises from simple things

and then goes into a heat death

that is once again returns to simple things

as the march of the second law of thermodynamics goes on?

I think there is,

but I don’t think it’s a universal answer.

And so I think throughout the ages,

there has been a kind of quest for some final way

of articulating meaning and purpose,

whether it’s God, whether it’s love,

whether it’s companionship.

I mean, many people put forward different ways

of taking this question on,

and there is no one right answer

when you recognize deeply that the universe doesn’t care.

There is nothing out there that is the final answer.

It’s not as though we need a more powerful telescope

and somehow if we can look deeply into the universe,

all will become clear.

In fact, the deeper we’ve looked,

both literally and metaphorically,

into the universe and into the structure of reality,

the more it’s become clear

that we are just a momentary byproduct

of laws of physics that don’t have any emotional content.

They don’t have any intrinsic sense of meaning or purpose.

And when you recognize that,

you realize that searching for the universal

for this kind of a question is a fool’s errand.

Every individual has the capacity to make their own meaning,

to set their own purpose.

And that’s not some platitude, that is what we are.

Because there is no fundamental answer,

it’s what you make of it.

And however much that may sound like a hallmark card,

this really is the deep lesson of physics and science

more generally over the past few hundred years.

Well, there’s some level where you can objectively say

that whatever we’ve got going on here,

it’s kind of peculiar.

It’s kind of special in terms of complexity.

And maybe you can even begin to measure it

and like come up with metrics

where whatever we’ve got going on on Earth,

these like interesting hierarchical complexities

that form more and more sophisticated biological system,

that seems kind of unique

when you look at the entire universe,

the observable part that we can see with our tools.

I mean, so I have to ask,

as you describe in your book once again,

Schrodinger wrote the book,

“‘What is Life?’ based on a few lectures he gave in 1944.”

So let me ask the fundamental question here.

What is life?

This particular thing we’ve got going on here,

this pocket of complexity

that emerged from such simple things?

Yeah, it’s a tough question.

I asked that question even to Richard Dawkins once,

and I already have my preconceived notion,

which he pretty much confirmed,

which is if one could give an answer to that question

that allowed you to sort of draw a line in the sand

between the not living and the living,

then perhaps we would have the insight that we yearn for

and trying to say, what is so special about life?

But the fact of the matter is, it’s a continuum.

There’s a continuum from the things

that we would typically call nonliving and animate

to the things that we obviously call animate

and full of the currents of life.

Somewhere in there,

it is a question of the complexity of the structure,

the ability of the structure to take in raw material

from the environment and process it through a metabolism

that allows the structure to extract energy

and to release entropy to the wider environment.

Somewhere in those collections of biological processes

is the necessity or the necessary ingredients

and processes for life,

but drawing that line in the sand

is not something that we’re able to do,

but I would agree with you.

It’s deeply peculiar.

It may in fact be unique,

but it may not.

It could be that the universe is such

that under fairly typical conditions,

a star that’s a well ordered source of low entropy energy,

that’s what the sun is,

together with a planet being bathed

by that low entropy energy,

together with a surface that has enough

of the raw constituents that we recognize

are fairly commonplace result of supernova explosions

where a star spews forth the result of the nuclear furnace

that is the core of a star.

It could be that all you need

are those fairly commonplace conditions

and maybe life naturally forms.

Look, the James Webb Space Telescope, right?

It’s going up hopefully in December.

And one of the goals of that mission

is to look at atmospheres around distant planets

and perhaps come to some sense of how special

or not life or life as we know it is in the universe.

Which part of the story of life,

let’s stick to Earth for a second,

do you think is the hardest?

If you were like a betting man,

which part is the hardest to make happen?

Is it the origin of life?

Again, we haven’t drawn the line where,

as you say, the line between a rock and a rabbit.

That part, is it complex organisms,

like multicellular organisms?

Is it crawling out of the ocean

where the fish somehow figured out how to crawl around?

Is it then the us homo sapiens,

as we like to think of ourselves special and intelligent?

Or is it somewhere in between?

As you also talk about, again, very hard to know

at which point this consciousness emerge.

If you were to sort of took us a survey

and made bets about other Earth like planets in the universe,

where do you think they get stuck the most?

Well, I would certainly say if we’re gonna go all the way

to conscious beings like ourselves,

I would put it at the onset of consciousness,

which again, I think is a continuum.

I don’t think it is something that you can draw the line

in the sand, but there are obvious circumstances,

there are obvious creatures such as ourselves

where we do recognize a certain kind

of self reflective conscious awareness.

And if we think about what it would require

for a system of living beings to acquire consciousness,

I think that’s probably the hardest part because look,

take Earth and recognize that weren’t for,

some singular event 65 million years ago

where this large rock slams into planet Earth

and wipes out the dinosaurs,

maybe the dinosaurs would still rule the planet

and they may well have not developed

the kind of conscious awareness that we have.

So for billions of years on this planet,

there was life that didn’t have the kind

of conscious awareness that we have.

And it was an accidental event in astrophysical history

that allowed a mammalian species like us

to ultimately be the end product.

And so, yeah, I could imagine there’s a lot of life

out there, but perhaps none of it’s wondering

what’s the meaning of life or trying to make sense of it,

just going about its business of survival,

which of course is the dominant activity

that life on this planet has practiced.

We are a rare exception to that.

And I really appreciate that you lean into

some of these unanswerable questions from me today.

But the, so you think about consciousness,

not as like a phase shift, the binary zero one,

you think of it as a continuum that humans somehow

are maybe some of the most conscious beings on Earth.

So you’re, so.

I mean, people will dispute that.

Yes, I mean, well, and it’s a very hard argument.

People will dispute that, rocks probably

will stay quiet on the matter.

Maybe not, right?

For the moment, they’re waiting for their opportunity.

But I agree that, look, even when you and I

look at each other, I am not fully convinced

that you’re a conscious being, right?

I mean, I think that you are.

It’s not to me.

I mean, your behavior is such that

that’s the best explanation for what’s going on.

But of course, we’re all in the position

of only having direct awareness of our own conscious being.

And therefore, when it comes to other creatures in the world,

we’re in a similar state of ignorance

regarding what’s actually happening inside of their head,

if they have a head.

And so it’s hard to know how singular we are,

but I would say based on the best available data

and the best explanations that we can make,

yeah, there is something special about us.

I don’t think that there are fish walking around

and coming up with existentialism.

I don’t know that there are dogs walking around

who’ve developed an understanding

of the general theory of relativity.

I mean, maybe we’re wrong,

but that seems the best explanation.

What do you think is more special,

intelligence or consciousness?

I think consciousness.

And I think that there’s a deep connection

between these ideas.

They are distinct, but they’re deeply connected.

But look, I mean, to me and to, of course, many philosophers

who actually coined a name for this,

the hard problem of consciousness,

David Chalmers and others,

as a physicist, I look out at the world

and I see it’s particles governed by physical law.

We can name them.

We got electrons, we got quarks

that come in various flavors and so forth.

We have a list of ingredients that science has revealed

and we have a list of laws that seemingly

govern those ingredients.

And nowhere in there is there even a hint

that when you put those particles together in the right way,

an inner world should turn on.

And it’s not only that there’s no hint, it’s insane.

I mean, it’s ridiculous.

How could it be that a thoughtless,

passionless, emotionless particle,

when grouped together with compatriots,

somehow can yield something so deeply foreign

to the nature of the ingredients themselves?

So answering that question,

I think is among the deepest

and most difficult questions that we face.

Do you think it is in fact a really hard problem?

Or is it possible, I think you mentioned in your book,

that it’s just like almost like a side effect.

It’s an emergent thing that’s like, oh, it’s nice.

It’s like a nice little feature.


Well, I mean, when people use the phrase hard problem,

I mean, they mean in a somewhat technical sense

that it’s trying to explain something

that seems fundamentally unavailable

to third party objective analysis, right?

I’m the only one that can get inside my head

and I can tell you a lot about what’s happening

inside my head right now, it’s reflected

in what I’m saying, and you can try to deduce things

about what’s going on inside my head,

but you don’t have access to it in the way that I do.

And so it seems like a fundamentally different

kind of problem from the ones that we have successfully

dealt with over the course of centuries in science,

where we look at the motion of the moon,

everybody can look, everybody can measure it.

We look at the properties of hydrogen

when you shine lasers on it,

everybody can look at the data and understand it.

And so it seems like a fundamentally different problem

in that sense, it seems like it is hard

relative to the others, but I do think ultimately

that the explanation will be, as you recount,

I think that a hundred years from now,

or maybe it’s a thousand, it’s hard to predict

the timescale for developments,

but I think we’ll get to a place where we’ll look back

and kind of smile at those folks in the 20th century

and before, 21st century and before,

who thought consciousness was so incredibly mysterious

when the reality of it is, eh, it’s just a thing that happens

when particles come together.

And however mysterious that feels right now,

I think for instance, when we start to build

conscious systems, things that you’re more familiar with

than I am, when we start to build these artificial systems

and those systems report to us, I’m feeling sad,

I’m feeling anxious, yeah, there’s a world going on

inside here, I think the mystery of consciousness

will just begin to evaporate.

Well, that’s, first of all, beautifully put,

and I agree with you completely,

just the way you said it, it’ll begin to evaporate.

I have built quite a few robots

and have had them do emotion, emotional type things,

and it’s immediate that exactly what you’re saying,

this kind of mystery of consciousness starts to evaporate,

that the kind of need to truly understand,

to solve the hard problem of consciousness disappears,

because, well, I don’t really care if I understand

what can solve the hard problem of consciousness.

That thing sure as heck looks conscious.

I feel like that way when I interact with a dog.

I don’t need to solve the problem of consciousness

to be able to interact and richly enjoy the experience

with this other living being.

Obviously, same thing with other humans.

I don’t need to fully understand it.

And there’s some aspect, maybe this is a little bit

too engineering focused, but there’s some aspect

in which it feels like consciousness is just a nice trick

to help us communicate with each other.

It sounds ridiculous to say, but sort of the ability

to experience the world is very useful,

in a subjective sense, is very useful to put yourself

in that world and to be able to describe the experience

to others.

It could be just a social and the merge.

Obviously, animals, the sort of more primitive animals

might experience consciousness in some more primitive way,

but this kind of rich, subjective experience

that we think about as humans, I think it’s probably

deeply coupled with language and poetry.

Yeah, that resonates with my view as well.

I mean, there’s a scientist, maybe you’ve spoken to him,

Michael Graziano from Princeton.

Yeah, he’s developed ideas of consciousness that,

look, I don’t think they solve the problem,

but I think they do illuminate it in an interesting way

where basically we are not aware

of all the underlying physiochemical processes

that make our brains and our inner worlds

tick the way they do.

And because of that dissociation between sensation

and the physics of it and the chemistry of it

and the biology of it, it feels like our minds

and our inner worlds are just untethered,

like floating somewhere in this gray matter

inside of our heads.

And the way I like to think of it is like,

look, if you were in a dark room, right,

and I had glow in the dark paint on my fingers,

so all you saw was my fingers dancing around,

there’d be something mysterious.

How could those fingers be doing that?

And then you turn on the light, you realize,

oh, there’s this arm underlying it,

and that’s the deep physical connection explains it all.

And I think that’s what we’re missing,

the deep physical connection between what’s happening

up here and what is responsible for it

in a physical, chemical, biological way.

And so to me, that at least gives me some understanding

of why consciousness feels so mysterious

because we are suppressing all of the underlying science

that ultimately is responsible for it.

And one day we will reveal that more fully,

and I think that will help us tether this experience

to something quite tangible in the world.

I wonder if the mystery is an important component

of enjoying something.

So once we know how this thing works,

maybe we will no longer enjoy this conversation.

We’ll seek other sources of enjoyment,

but this is, again, from an engineering perspective,

I wonder if the mystery is an important component.

Well, have you ever seen,

there’s this beautiful interview

that Richard Feynman did,

great Nobel laureate physicist responsible

for a lot of our understanding of quantum mechanics,

quantum field theory and so forth.

And he was in a conversation with an interviewer

where he noted that some people feel

like once the mystery is gone,

once science explains something,

the beauty goes away, the wonder of it goes away.

And he was emphasizing in his response to that,

he’s like, no, that’s not the right way of thinking about it.

He says, look, when I look at a rose,

he says, yeah, I can still deeply enjoy the aroma,

the color, the texture.

He says, but what I can do that you can’t,

if you’re not a physicist,

I can look more deeply and understand

where the red comes from, where the aroma comes from,

where the structure comes from.

He says, that only augments my wonder.

It only augments my experience.

It doesn’t flatten it or take away from it.

So I sort of take that as a bit of a motto in some sense

that there is a wonder that comes from a kind of ignorance.

And I don’t mean that in a derogatory sense,

but just from not knowing.

So there is a wonder that comes from mystery.

There’s another kind of wonder that comes from knowing

and deep knowing.

And I think that kind of wonder has its own

special character that in some ways can be more gratifying.

I hope he’s right.

I hope you’re right.

But there’s also, I remember he said something

about like science is an onion or something like that.

You can peel back, you can keep peeling back.

I mean, there is also, when you understand something,

there’s always a sense that there’s more mystery

to understand.

Like you never get to the bottom of the mystery.

But I think it’s also different than,

you know, I don’t think you can analogize say

to a magician, right?

A magician does some trick.

You learn how it sounds like, oh my God,

that’s ridiculous when you find.

But nature is perhaps the best magician

if you wanna try to make the analogy there

because when you peel things back and you understand

how it is that things have color and you have electrons,

dancing from one orbital to another,

emitting photons at very particular wavelengths

that are described by these beautiful equations

of quantum electrodynamics,

part of which that Feynman developed,

it gives you a greater sense of awe

when the curtain is pulled back

than what happens in other circumstances

where it does flatten it completely.

Yeah, it’s very possible then say in physics

that we arrive at a theory of everything

that unifies the laws of physics

and has a very strong understanding

of the fabric of reality,

even like from the big bang to today,

it’s possible that that understanding

is only going to elevate our appreciation

of this whole thing.

Yeah, I think it will.

I think it will.

I mean, I think it has so far.

But the other side of it which you emphasize

is it’s not like science somehow reaches an end, right?

There are certain categories of questions

that do reach an end.

I think we one day will close the book

on nature’s ingredients and the fundamental laws.

Now that we can’t prove that,

maybe it goes on forever, smaller and smaller,

maybe there are deeper and deeper laws,

but I don’t think so.

I think that there’s going to be a collection

of ingredients and a collection of basic laws.

That chapter will close, but it’s one chapter.

Now we take that knowledge and we try to understand

how the world builds the structures that it does,

from planets to people to black holes

to the possibility of other universes

and every step of the way,

the collection of questions

that we don’t know the answer to only blossoms.

And so there’s a deep sense of gratification

from understanding certain qualities of the world.

But I would say that if you take a ratio

of what we understand to the things that we know

that we don’t yet understand,

that ratio keeps getting smaller and smaller

because the things that we know that we don’t understand

grows larger and larger.

Do you have a hope that we solve that theory

of everything puzzle in the next few decades?

So there’s been a bunch of attempts from string theory

to all kinds of attempts at trying to solve quantum gravity

or basically come up with a theory for quantum gravity.

There’s a lot of complexities to this.

One, for experimental validation,

you have to observe effects

that are very difficult to measure.

So you have to build,

like that’s like an engineering challenge.

And then there’s the theory challenge,

which is like, it seems very difficult

to connect the laws of gravity to quantum mechanics.

Do you have a hope or are we hopelessly stuck?

Well, I have to have to have a hope.

I mean, it’s in some sense,

but I devote at least part of my professional life toward

trying to make progress on.

And I’m glad you used the phrase quantum gravity.

I’m not a great fan of the theory of everything phrase

because it does make other scientists feel like

if they’re not working on this, what are they working on?

Man’s like, there’s not much left

when you’re talking about theory of everything.

Biology is just small details we’ll figure out.

Yeah, so it is really trying to put gravity

and quantum mechanics together.

And since I was a college kid,

I was deeply fascinated with gravity.

And as I learned quantum mechanics,

the notion of physicists being stumped

and trying to blend them together,

how could one not get fired up

about maybe contributing something to that journey?

And so we’ve been on this,

I’ve been on this for 30 years since I was a student.

We have made progress.

We do have ideas.

You mentioned string theory is one possible scenario.

It’s not stuck.

String theory is a vibrant field of research

that is making incredible progress,

but we’ve not made progress

on this issue of experimental verification validation,

which is, you know, it is a vital part of the story.

So I would have hoped that by now

we would have made contact with observation.

If you would have interviewed me back in the 80s

when I was, you know, a wild bright eyed kid

trying to make headway working 18 hours a day

and this sort of stuff,

I would have said, yeah, by 2021,

yeah, we’re gonna know whether it’s right or wrong.

We’ll have made contact.

I would have said, look,

there may be certain mathematical puzzles

that we’ve yet to work out,

but we’ll know enough to make contact with experiment.

That has not happened.

On the other hand,

if you would have interviewed me back then and asked me,

will we be able to talk about detailed qualities

of black holes and understand them at the level of detail

that we actually, I would have said,

no, I don’t think that we’re gonna be able to do that.

Will we have an exact formulation of string theory

in certain circumstances?

No, I don’t think we’re gonna have that, and yet we do.

So it’s just to say,

you don’t know where the progress is going to happen,

but yes, I do hold out hope

that maybe before I move on to wherever,

I don’t think there is an after,

but I would love before I leave this earth

to know the answer, but science and the universe,

it’s not about pleasing any individual, it is what it is.

And so we just press onward and we’ll see where it goes.

So in terms of string theory,

if I just look from an authoritative perspective currently

at the theoretical physics community,

string theory as a theory has been very popular

for a few decades, but has recently fallen out of favor,

or at least there’s been like, you know,

it became more popular to kind of ask the question,

is string theory really the answer?

Where do you fall on this?

Like, how do you make sense of this puzzle?

Why do you think it’s fallen out of favor?

Yeah, so I would actually challenge the statement

that it’s fallen out of favor.

I would say that any field of research when it’s new

and it’s the bright, shiny bicycle

that no one has yet seen on that block,

yeah, it’s gonna attract attention

and the news outlets are gonna cover it

and students are gonna flock to it, sure.

But as a field matures, it does shed those qualities

because it’s no longer as novel as it was

when it was first introduced 30, 40 years ago,

but you need to judge it by a different standard.

You need to judge it by is it making progress

on foundational issues deepening our understanding

of the subject and by that measure,

string theory is scoring very high.

Now, at the same time, you also need to judge

whether it makes contact with experiment

as we discussed before too

and in that measure, we’re still challenged.

So I would say that many string theorists,

myself included, are very sober about the theory.

It has the tremendous progress that it had 30, 40 years ago

that hasn’t gone away, but we become better equipped

at assessing the long journey ahead

and that was something that we weren’t particularly good at

back, say, in the 80s.

Look, when I was just starting out in the field,

there was a sense of physics is about to end.

String theory is about to be the be all and end all

final unified theory and that will bring this chapter

to a close.

Now, I have to say, I think it was more the younger

physicists who were saying that.

Some of them were seasoned,

even if they were pro string theory at the time.

I don’t know if they were rolling their eyes,

but they knew that it was gonna be a long, long journey.

I think people like John Schwartz,

one of the founders of string theory,

Michael Green, no relation to me,

founders of the theory, Edward Witten,

one of the main people driving the theory

back then and today.

I think they knew that we were in for a long haul

and that’s the nature of science,

quick hits that resolve everything few and far between.

And so if you were in for the quick solution

to the big questions of the world,

then you would have been disappointed

and I think there were people who were disappointed

and moved on and work on other subjects.

If you’re in in the way that Einstein was in

for a lifetime of investigation to try to see

what the answers to the deep questions would be,

then I think string theory has been a rich source

of material that has kept so many people deeply engaged

in moving the frontier forward.

There’s a few qualities about string theory,

which are weird.

I mean, a lot of physics is just weird and beautiful.

So let me ask the question,

what do you use most beautiful about string theory?

Well, what attracted me to the theory at the outset

beyond it’s putting gravity and quantum mechanics together,

which I think is its true claim to fame,

at least on paper, it’s able to do that.

What attracted me to the theory was the fact

that it requires extra dimensions of space.

And this was an idea that intrigued me in a very deep way,

even before I really understood what it meant.

I somehow had, I mean, talk about sort of

the emotional part of consciousness and the cognitive part

in some, perhaps you call it strange,

in some strange emotional way,

I was enamored with Einstein’s general relativity,

the idea of curved space and time

before I really knew what it meant.

It just spoke to me, I don’t know how else to say it.

And then when I subsequently learned

that people had thought about more dimensions of space

than we can see and how those extra dimensions

would be vital to a deep understanding

of the things that we do see in this world,

four, five, six dimensions might explain

why there are certain forces and particles

and how they behave.

To me, this was like amazing, utterly amazing.

And then when I learned that string theory

embraced all these ideas,

embraced the general theory of relativity,

embraced quantum mechanics,

embraced the possibility of extra dimensions,

then I was hooked.

And so when I was a graduate student,

we would just spend hours,

we, I mean, a couple of other graduate students and myself

who had sort of worked really well together,

it was at Oxford in England,

we would work these enormous numbers of hours a day

trying to understand the shapes of these extra dimensions,

the geometry of them, what those geometrical shapes

for the extra dimensions would imply

for things that we see in the world around us.

And it was a heady, heady time.

And that kind of excitement has sort of filtered through

over the decades.

But I’d say that’s really the part of the theory

that I think really hooked me most strongly.

How are we supposed to think about those extra dimensions?

I was supposed to imagine actual physical reality

or is this more in the space of mathematics

that allows you to sort of come up with tricks

to describe the four dimensional reality

that we more directly perceive?

No one really knows the answer, of course,

but if I take the most straightforward approach

to string theory,

you really are imagining that these dimensions are there,

they’re real.

I mean, just as you would say

that the three space dimensions around us,

left, right, back, forth, up, down,

yeah, they’re real, they’re here.

We are immersed within those dimensions.

These other dimensions are as real as these

with the one difference being their shape and their size

differs from the shape and size of the dimensions

that we have direct access to through human experience.

And one approach imagines that these extra dimensions

are tightly coiled up, curled up,

crushed together, if you will,

into a beautiful geometrical form

that’s all around us,

but just too small for us to detect with our eyes,

too small for us to detect

even with the most powerful equipment that we have.

Nevertheless, according to the mathematics,

the size and the shape of those extra dimensions

leaves an imprint in the world that we do have access to.

So one of the ways that we have hoped yet to achieve

to make contact with experimental physics

is to see a signature of those extra dimensions

in places like the Large Hadron Collider

in Geneva, Switzerland.

And it hasn’t happened yet, doesn’t mean it won’t happen,

but that would be a stunning moment

in the history of the species

if data that we acquired in these dimensions

gives us kind of incontrovertible evidence

that these dimensions are not the only dimensions.

I mean, how mind blowing would that be?

So with the Large Hadron Collider,

it would be something in the movement of the particles

or also the gravitational waves potentially be a place

where you can detect signs of multiple dimensions,

like with something like LIGO, but much more accurate.

In principle, all of these can work.

So one of the experiments that we had high hopes for,

but by high hopes, I’m actually exaggerating.

One of the experiments that we imagined

might in the best of all circumstances,

yield some insight.

We weren’t with bated breath waiting for the result.

We knew it was a long shot.

When you slam protons together at very high speed

of the Large Hadron Collider,

if there are these extra dimensions

and if they have the right form,

and that’s a hypothesis that may not be correct,

but when the protons collide,

they can create debris, energetic debris

that can in some sense leave our dimensions

and insert itself into the other dimensions.

And the way you’d recognize that is,

there’d be more energy before the collision

and after the collision because the debris

would have taken energy away from the place

where our detectors can detect it.

So that’s one real concrete way

that you could find evidence for extra dimensions.

But yeah, since extra dimensions are of space

and gravity is something that exists within,

in fact is associated with the shape of space,

gravitational waves in principle

can provide a kind of cat scan of the extra dimensions

if you had sufficient control over those processes.

We don’t yet, but perhaps one day we will.

Does it make you sad a little bit?

Maybe looking out into the future,

you mentioned Ed Witten that no Nobel prizes

have been given yet related to string theory.

Do you think they will be?

Do you think you have to have experimental validation

or can a Nobel prize be given?

Which I don’t think has been given for quite a long time

for purely sort of theoretical contribution.

Yeah, it’s certainly as a matter of historical precedent

has been the case that those who win the prize

have established, investigated, illuminated

a demonstrably real quality of the world.

So gravitational waves, the prize was awarded

after they were detected, not the mathematics of it,

but the actual detection of it.

The Higgs particle, it was an idea that came

from the 1960s, Peter Higgs and others in fact.

And it wasn’t until 2012 on July 4th

when the announcement came that this particle

had been detected at the Large Hadron Collider

that people viewed it as eligible for the Nobel prize.

The idea was there, the math was there,

but you needed to confirm it.

Indeed, the prize ultimately was awarded.

So I’m not surprised.

In fact, I would have been surprised

if a Nobel prize had been awarded

in the arena of string theory

because it’s far too speculative right now.

It’s far too hypothetical.

In fact, I am sympathetic to the view

that it really shouldn’t be called string theory.

It degrades the word theory

because theory in science, of course,

means the best available explanation

for the things that we observe in the world,

the things that we measure in experiments about the world.

And string theory does not do that, at least not yet.

So it really should be the string hypothesis, right?

We’re at an earlier stage of development

and that’s not the kind of thing

that Nobel prizes should be awarded for.

What do you think about the critics out there, Peter White,

he’s from Columbia too, I think Sabine Hafenstatter.

Is that a healthy thing or should we sort of focus

on sort of the optimism of these hypotheses?

Yeah, it’s actually a good way that you frame it

because I’m always somewhat repelled

by views of the world that start from the negative.

Try to cut down an idea, try to say that’s the wrong way

of thinking about things and so on.

I’m much more drawn, maybe because I’m an optimist,

I don’t know, I’m much more drawn to those

who go out into the world with new ideas.

And don’t try to cut down one idea,

but rather present another one that might be better.

And so you make the first idea, maybe string theory irrelevant

because you’ve come up with the better approach

to the world.

So do I think it’s healthy?

Look, I think having a wide range of views

and perspectives is generally a healthy thing.

I think it’s good to have arguments within a subject

in order that you stay fresh and you stay focused

on the things that matter.

But in the end of the day,

I think it’s a more vital contribution

to give us something new

rather than to criticize something that’s there.

Yeah, I’m totally with you.

But it could be just the nature of being an optimist

and also just a love of engineering.

It helps nobody by criticizing the rocket

that somebody else built,

just build a bigger, cheaper, better rocket.

Right, exactly.

And that seems to be how human civilization

can progress effectively.

We’ve mentioned the second law of thermodynamics.

I gotta ask you about time.


And do you think of time as emergent

or fundamental to our universe?

I like to think of it as emergent.

I don’t have a solid reason for that perspective.

I have a lot of hints of reasons

that some of which come out of string theory

and quantum gravity that perhaps would be worth talking about.

But what I would say is,

time is the most familiar quality of experience

because there’s nothing that takes place

that doesn’t take place within an interval of time.

And yet at the same time,

it is perhaps the most mysterious quality of the world.

So it’s a wonderful confluence

of the familiar and the deeply mysterious

all in one little package.

If you were to ask me, what is time?

I don’t really know.

I don’t think anybody does.

I can say what time gives us,

it allows us the language for talking about change.

It allows us to envision the events of the universe

being spread out in this temporal timeline.

And in that way, allows us to see the patterns

that unfold within time.

I mean, time allows us the structure and the organization

to think about things in that kind of a progression.

But what actually is it?

I don’t really know.

And that’s so strange because we can measure it, right?

I mean, there are laboratories in the world

that measure this thing called time

to spectacular precision.

But if you go up to the folks and say,

what is it that you’re actually measuring?

I don’t know that they can really articulate

the kind of answer that you would expect

from those who are engineering a device

that can measure something called time

to that level of precision.

So it’s a very curious combination.

What do you make of the one way feeling of causality?

Like is causality a thing or is that too just a human story

that we put on top of this emergent phenomenon of time?

I don’t know.

I can give you my guess and my intuition about it.

I do think that at the macroscopic level,

if we’re talking about sort of the human experience of time,

I do think at the macroscopic level,

there is a fundamental notion of causality

that does emerge from a starting point

that may not have causality built in.

So I certainly would allow that at the deepest description

of reality when we finally have that on the table,

we may not see causality directly at that fundamental level.

But I do believe that we will understand

how to go from that fundamental level

to a world where at the macroscopic level,

there is this notion of A causes B.

A notion that Einstein deeply embraced

in his special theory of relativity

where he showed that time has qualities

that we wouldn’t expect based on experience.

You and I, if we move relative to each other,

our clocks tick off time at different rate.

And our clocks is just a means of measuring

this thing called time.

So this is really time that we’re talking about.

Time for you and time for me are different

if we’re in relative motion.

He then shows in the general theory of relativity

that if we’re experiencing different gravity,

different gravitational fields

or actually more precisely

different gravitational potentials,

time will elapse for us at different rates.

These are things that are astoundingly strange

that give rise to a scientific notion of time travel.

Okay, so this is how far Einstein took us

in wiping away the old understanding of time

and injecting a new understanding of its qualities.

So there’s so much about time that’s counterintuitive,

but I do not think that we’re ever going

to wipe away causality at the macroscopic level.

At the macroscopic, I mean, there’s so many interesting

things at the macroscopic level

that may only exist at the macroscopic level.

Like we already talked about consciousness

that very well could be one of the things.

You mentioned time travel.

So, I mean, according to Einstein and in general,

what types of travel do you think

our physical universe allows?

Well, it certainly allows time travel to the future.

And I’m not talking about the silly thing

that you and I are now going into the future

second by second by second.

I’m talking about really the diversion

that you see in Hollywood, at least in terms

of its net effect, whereby an individual

can follow an Einsteinian strategy

and propel themselves into the future

in some sense more quickly.

So if I wanted to see what’s happening on planet Earth

one million years from now, Einstein tells me

how to get one million years from now.

Build a ship.

I got to turn to guys who know how to build stuff.

I can’t do it like you.

Build a ship that can go out into the universe

near the speed of light, turn around and come back.

Let’s say it’s a six month journey out

and a six month journey back.

And Einstein tells me how fast I need to travel,

how close to the speed of light I need to go

so that when I step out of my ship,

it will now be one million years into the future

on planet Earth.

And this is not a controversial statement, right?

This is not something where there’s differences

of opinion in the scientific community.

Any scientist who knows anything

about what Einstein taught us agrees with what I just said.

It’s commonplace, it’s bread and butter physics.

And so that kind of travel to the future

is absolutely allowed by the laws of physics.

There are engineering challenges,

there are technological challenges.

They’re close to the speed of light part, yeah.

Yeah, and there are even biological challenges, right?

They’re G forces that you’re gonna experience.

So there’s all sorts of stuff embedded in this,

but those I will call the details.

And those details, notwithstanding,

the universe allows this kind of travel to the future.

And if I could pause real quick,

you could also, at the macro level,

with biology extend the human lifespan

to do a kind of travel forward in time.

If you expand how long we live,

that’s a way to, from a perspective of an observer,

a conscious observer that is a human being,

you’re essentially traveling forward in time

by allowing yourself to live long enough to see the thing.

So that’s in the space of biology.

What about traveling back in time?

Yeah, that is a natural next question,

especially if you’re going on one of these journeys.

Is it a one way journey or can you come back?

And the physics community doesn’t speak

with a unified voice on this as yet,

but I would say that the dominant perspective

is that you cannot get back.

Now, having said that, there are proposals

that serious people have written papers on

regarding hypothetical ways

in which you could travel to the past.

And we’ve seen some of these.

Again, Hollywood loves to take the most sexy ideas

of physics and build narratives around them.

This idea of a wormhole,

like Jodie Foster in Contact went through a wormhole,

Deep Space Nine Star,

I’m sure there are many other examples

for these ideas that I’ve probably never even seen.

But with wormholes, there’s at least a proposal

of how you could take a wormhole tunnel through space time,

manipulate the openings of the wormhole

in such a way that the openings are no longer synchronous.

They are out of sync relative to each other,

which would mean one’s ahead and one’s behind,

which means if you go through one direction,

you travel to the future.

If you go back, you travel to the past.

Now, we don’t know if there are wormholes in the world.

But they’re possible according to Einstein, correct?

They are possible according to Einstein.

But even Einstein was very quick to say,

just because my math allows for something,

doesn’t mean it’s real.

I mean, he famously didn’t even believe in black holes.

Didn’t believe in the Big Bang, right?

And yet the black hole issue has really been settled now.

We have radio telescopic photographs

of the black hole in M87.

It was in newspapers around the world

just a couple of years ago.

So it’s just to say that just because it’s in Einstein’s math,

it doesn’t mean it’s real.

But yes, it is the case that wormholes

are allowed by Einstein’s equations.

And in principle, you can imagine, you know,

putting electric charges on the openings of the wormhole,

allowing you to tow them around

in a manner that could yield

this temporal asymmetry between them.

Maybe you tow one of the mouths to the edge of a black hole.

In principle, you can do this,

slowing down the passage of time near that black hole.

And then when you bring it back,

it will be well out of sync with the other opening

and therefore could be a significant temporal gap

between one and the other.

But people who study this in more detail question,

could you ever keep a wormhole open,

assuming it does exist?

Could you ever travel through a wormhole

or would there be a requirement

to some kind of exotic matter to prop it open

that perhaps doesn’t exist?

So there are many, many issues that people have raised.

And I would say that the general sentiment

is that it’s unlikely that this kind of scenario

is going to survive our deeper understanding of physics

when we finally have it.

But that doesn’t mean that the door is closed.

So maybe there’s a small possibility

that this could one day be real.

That’s such an interesting way to put it.

It will not, this kind of scenario

will not survive deep understanding of physics.

It’s an interesting way to put it

because it makes you wonder what kind of scenarios

will be created by our deeper understanding of physics.

Maybe, sorry to go crazy for a second,

but if you have like the pan psychism idea

that consciousness permeates all matter,

maybe traveling in that, whatever laws of physics,

the consciousness operates under something like that.

In that view of the university,

if we somehow are able to understand that part,

maybe traveling is super easy.

Yeah, it does not follow the constraints

of the speed of light, something like this.

Yeah, so look, I have a definite degree of sympathy

with the possibility that consciousness might be more

than what we described earlier

as just the byproduct of mindless particles.

You just made the rock happy.

Exactly, so it isn’t the approach

that feels to me the most likely, but I see the logic.

If you’ve got the puzzle,

how to mindless particles build mind,

one resolution might be the particles are not mindless.

The particles have some kind of proto conscious quality.

So there’s something appealing

about that straightforward solution to the puzzle.

And if that’s the case, if we do live in a pan psychist world

where there is a degree of consciousness residing

in everything in the world around us, then yes,

I do think some interesting possibilities might emerge

where maybe there’s a way of communing

with physical reality in a deeper way than we have so far.

I mean, we as human beings,

a vital part of our existence

is human to human communication, contact.

We live in social groups and that’s what it’s allowed us

to get to the place where we’ve gotten.

Imagine that we have long missed

that there’s other consciousness out there

and some kind of relationship or communion

with that larger conscious possibility

would take us to a different place.

Now, do I buy into this yet?

I don’t, I don’t see any evidence for it,

but do I have an open mind and allow for the possibility

in the future?

Yeah, I do.

So if that’s not the case

and you have these simple particles

that at the macro level emerges some interesting stuff

like consciousness, another thing you write about

in the Until the End of Time book

is the thing that it seems to emerge at the macro level

is the feeling like there’s a free will,

like we decide to do stuff.

And you have a really interesting take here,

which is, no, there’s not a free will.

I’m just gonna speak for you and then you can correct me.

No, there’s not a free will,

but there is an experience of freedom.


Which I really love.

So where does the experience,

where does freedom come from

if we don’t have any kind of physics based free will?

Yeah, and so the idea follows naturally

from all that we’ve been talking about.

Let’s make the assumption that all there is

in the physical universe is stuff governed by laws.

We may not have those laws,

may not know what the fundamental stuff is yet,

but everything we know in science points in the direction

that it’s physical stuff governed by universal laws.

And that being the case, or that being the assumption,

then you come to a particular collection

of those ingredients called a human being.

And that human being has particles

that are fully governed by physical law.

And when you then recognize it,

every thought that we have,

every action that we undertake

is just the motion of particles.

When I’m thinking thoughts right now,

of course, at this level of description,

it is the motion of particles cascading

down various neurons inside of my head and so on.

And every single one of those motions,

collectively and individually,

is fully governed by these laws

that we perhaps don’t have yet,

but we imagine one day we will.

That leaves no opportunity for any kind of freedom

to break free from the constraint of physical law.

And that is the end of the story.

So the traditional intuitive notion of free will,

that we’re the ultimate authors of our actions,

that we were the buck stops,

that there is no antecedent,

that is the cause for our decided to go left or right,

choose vanilla or chocolate, live or die,

that intuitive sensation does not have a basis

in our understanding of the physical world.

So that’s the end of the free will of the traditional sort.

But then your question is,

what about this other kind of freedom I talk about?

And the other kind of freedom,

if you focus on it intently,

I think is actually the true version of freedom

that we feel.

And that freedom is this.

You look at inanimate objects in the world,

rocks, bottles of water, whatever,

they have a very limited behavioral repertoire.


Their internal organization is too coarse

for them to do very much, right?

You try to have a conversation with a glass of water,

you send sound waves, it doesn’t do much.

It may vibrate a little bit,

but the repertoire of responses are incredibly limited.

The difference between us and a rock or a bottle of water

is that our inner organization,

by virtue of eons of evolution by natural selection,

is so refined, so spectacularly ordered,

that we have a huge repertoire of behaviors

that are finely attuned to stimuli from the external world.

You ask me a question, that’s a stimulus,

and all of a sudden,

these particle processes go into action,

and this is the result, this answer that I’m giving you.

So the freedom that we have is not from

the control of physical law.

The freedom that we have is from the constrained behavior

that has long since governed inanimate objects.

We are liberated from the limited behavioral repertoire

of rocks and bottles of water

to have this broad spectrum of responses.

Do we pick them?

We do not.

Do we freely choose them?

We do not, but yet we have them,

and we can marvel at those behaviors,

and that’s the freedom that we have.

The complexity and the breadth of that repertoire

is where the freedom emerges.

Is there something to be said about emergence?

I don’t know if you know,

I’ve looked at much about objects

that I seem to love way more than anyone else,

which is Sally or Tom,

like game of life type of stuff.

From simple things emerges beautiful complexities,

and so that’s that repertoire.

It’s like, it seems if you have enough stuff,

just beautiful complexity emerges

that sure as heck to our human eyes looks

like there’s consciousness there, there’s free will,

there’s little objects moving about and making decisions.

I mean, all of that,

you can say it’s anthropomorphization,

but it sure as heck feels

like they’re organisms making decisions.

What is that emergence thing?

Is that within the realm of physics to understand?

Is it within the realm of poetry?

What is that, like complex systems, emergence?

Will that ever be understood by science?

So here’s the way that I think about it.

So there are clearly qualities of the world

that emerge on macroscopic scales,

our sense of beauty, wonder, consciousness,

all of these kinds of qualities.

Do I feel that they ultimately are explainable

from the laws of physics?

I do.

There is nothing that’s not ultimately explainable

with the laws of physics from this physicalist perspective,

which is what I take.

So you got the particles, you got the laws,

and you have things that emerge

from the choreographed motions of those particles.

But is that the best language

for talking about these emergent qualities?

Usually not.

If I was to take something even more mundane,

like a baseball flying through the air,

if I was to describe it in terms of the quarks

and the electrons,

I’d give you this mountain of data

with 10 to the 28 particles

and all of their coordinates and spaces

a function of time.

I hand you this mountain of data,

you’d be like, I don’t know what this is.

And then if you really were clever and you’re looking,

oh, it’s a baseball,

just described in the least economical way possible.

It is much more useful and insightful

to talk about the baseball flying through the air.

Similarly, there are things at the macroscopic level

like human experience and human emotion and human action

and the sensation of free will

that we undeniably all have,

even if it itself doesn’t have a basis

in our understanding of the physical world.

It’s useful to talk about things in this very human language.

And so, yes, it’s vital to talk about things

in the poetic language of human experience,

but do not lose sight of the fact, and some people do.

They say, oh, it’s just an emergent phenomenon.

Don’t lose sight of the fact that emergent phenomena

are emerging from this deeper understanding

that comes from the reductionist account of physical law.

And there’s a lot of insight to come from that,

such as the freedom that you thought that you had,

the freedom of will that you thought you had.

It doesn’t have a basis in that reductionist account,

so it’s not real.

So speaking of the poetry of human experience,

you mentioned the images of the black holes.

How did it make you feel a few years ago

when that first image came out?

It’s truly amazing.

A sense of, well, I guess the feeling was both amazing

and there was a little sense of,

jealousy is not quite the right word,

but a sense of longing.

Yeah, I think that’s a better word,

because here’s a subject that started with Einstein

back in 1915, writes down the equations

of the general theory of relativity,

and then there are scores of individuals over the decades,

starting with people like Karl Schwarzschild

who analyze the equations,

see the possibility of black holes.

People develop these ideas.

John Wheeler, all these greats of physics.

It’s still a hypothetical subject.

It gets closer to reality through observations

of the center of our galaxy,

stars whipping around in a manner

that could only really be explained

by there being a black hole in the center of our galaxy,

but it was still indirect.

To actually have a direct image that you can look at,

what a beautiful arc, narrative arc

from the theoretical to the absolutely established.

And that’s what we hope will happen with other areas,

for instance, string theory, right?

I mean, wholly mathematical subject at the outset

and still pretty much a wholly mathematical subject today.

Yeah, do we long for that image

where we can look at it and say, string, it’s real.

I mean, how thrilling, how thrilling to be part

of that journey, to be part of that step

that moves things from the abstract to the concrete.

Yeah, so like the image of the DNA, the early images

of the DNA, for example, but there is something special.

So the problem with strings is they’re tiny.

So it’s harder to take a picture in the following sense.

When you think of a black hole, I mean, you have a swirl

of, I guess, what is, I don’t even know it’s dust,

whatever light.

A careening onto the event horizon.

And then there’s darkness in the center.

And you just imagine, so that picture in particular,

I guess, is of a gigantic black hole.

So you just, I mean, it’s terrifying.

Billions of times the mass of the sun.

Yeah, so it’s both exciting and terrifying.

I mean, I don’t know where you fall in the spectrum.

I think it’s exciting at first.

Like the longer I think about it, every time I think

about it, the more terrifying it becomes.

So it always starts exciting and then it goes to terrifying.

And both are feelings, very human feelings

that I appreciate.

It’s like terrified awe.

Somehow it’s still beautiful.

It’s a good way of saying it.

And I think I kind of share that reaction

because there is a way in which when you work on this

subject, like all the time, I teach it, I teach about

black holes, write the equations on the blackboard.

The ideas reside in a very cognitive,

I don’t know, mathematical portion of the brain,

or at least for me.

And it’s only when you like sit down and it’s quiet

and you start to contemplate, wait, wait, wait, wait,

this isn’t just like a mathematical game.

There are these monsters out there.

Now I don’t, not in a sense of I fear for my life,

but it’s a sense of how extraordinary is this universe.

And so it is breathtaking.

How powerful nature is.

Yeah, how stupendously powerful nature is.

And so there is a deep sense of humility

that I think this instills if you really allow

the ideas to sink in.

Well, I have to ask about the most stupendously

powerful thing to have ever happened in our universe,

which is the Big Bang.

What’s up with the Big Bang?

So we can, I mean, with gravitational waves,

the hope is you have more and more accurate measurements

of the gravitational waves.

You can crawl back further and further back in time

towards the Big Bang.

Do you have a hope that we’ll be able to understand

the early spark that created our universe?

Yeah, that and the deep interior of a black hole

I think are the biggest mysteries that we hope

the melding of quantum mechanics and gravity will reveal,

will illuminate.

And what question could be more captivating

than why is there something rather than nothing, right?

Why is there a universe at all?

And will the theories that we’re developing

take us to an answer to that?

I don’t know.

Even if we truly knew what the Big Bang is,

and that’s a big question in its own right,

one would still be left with the question,

well, okay, so you’ve explained the process

by which a tiny nugget of a universe,

a tiny nugget of space time can undergo some kind of growth

to yield the world around us.

But presumably in that explanation,

you’re gonna involve mathematics and some ingredients

like quantum fields or matter or energy or something.

Where did that stuff come from?

Can we get to that level of explanation?

I don’t know, but it is remarkable

that if you ask what happened a millionth of a second

after the Big Bang,

it’s not really that controversial any longer, right?

Even though there’s a lot of argument in the field

and it’s very heated right now I should say

regarding what is the right theory of the Big Bang?

What is the right theory of early universe cosmology

where I mean early, much earlier

than a millionth of a second,

a lot of dissent, a lot of heated arguments about that.

No pun intended.

Yeah, right, exactly.

But you go like a millionth of a second after that

and we’re on pretty firm ground.

Isn’t that amazing, right?

To understand what happened from that point forward.

But to go back is controversial.

So there is this theory called inflationary cosmology,

which I would say has been the dominant paradigm

since early 1980s.

So what does that mean?

Roughly 40 years now,

it’s been the dominant cosmological paradigm.

And it makes use of a curious feature

of Einstein’s general theory of relativity,

his theory of gravity,

where Einstein shows us mathematically

that gravity can not only be attractive,

the kind of gravity that we’re used to,

things pulled together, but it can also be repulsive.

And that fact is then leveraged by people like Alan Guth

and Andre Linde, and at the time Paul Steinhardt

and Andreas Albrecht and others to say,

okay, if we had a little nugget in the earlier universe,

which was filled with the stuff

that yields this repulsive gravity,

well, that would have blown everything apart.

It would cause everything to swell.

Beautiful explanation for what the bang

in the big bang was.

And then people mathematically analyze the consequences

of this idea and they make predictions

for tiny temperature differences across the night sky

that in principle could be measured.

You send up balloons, you send up satellites

with very refined thermometers,

and they measured the temperature of the night sky

and the statistical distribution

of the temperature differences agrees

with the mathematical predictions.

I mean, you just sort of have to stand in awe

of this insight.

So you think, aha, the theory has been established,

but scientists are an incredibly skeptical bunch.

And some scientists, including one of the people

who helped develop the theory at the outset,

Paul Steinhardt comes along and says,

well, yeah, this theory has done pretty well so far,

but there are aspects of this theory

that are making me lose confidence.

For instance, this theory seems to suggest

that there might be other universes.

Like, how do you make sense of a theory

that suggests there are other universes?

Or there are others who come along and say,

this theory seems to talk about length scales

that are minuscule even by the so called Planck length,

the sort of shortest length that we can imagine

making sense of in a theory of quantum gravity.

How do you make sense of that?

And so on and so forth, they develop a list of things

that they consider to be chinks

in the inflationary cosmological theory’s armor.

And they develop other ideas,

which they claim yield the same predictions

as inflationary cosmology

for those temperature differences across space,

but don’t suffer from these problems.

And then the inflationary cosmology folks say,

no, no, no, hang on.

Your theory suffers from different problems.

And so the arguments goes, it’s a healthy debate.

Talk about real debates in science.

So when you ask what’s up with the Big Bang,

I don’t know right now.

If you would have asked me five years ago,

maybe even less than that, three or four years ago,

I’ve said, look, inflationary cosmology has some issues,

but the package of explanations it provides is so potent

and the issues that beset it are seemingly solvable to me

that I would imagine it’s going to in the end, win out.

I would still say that today,

but I wouldn’t say it as loudly.

I wouldn’t say it as confidently.

I think it’s worth thinking about alternate ideas

and it could be the case that the paradigm

at some point shifts.

Does dark matter and dark energy fit into the shifting

of the explanations for those?

Yeah, certainly.

So dark energy has in the inflationary theory

is kind of a big mystery.

So dark energy is the observational realization

in the last 20 years

that not only is the universe expanding,

it’s expanding ever more quickly.

Something is still pushing things outward.

And the explanation is that there’s like a residual version

of the repulsive gravity from the early universe,

but it’s such a strange number.

When you write that amount of dark energy

using the relevant units in a theory of quantum gravity,

it’s a decimal point followed by like 120 zeros

and then a one.

We’re not used to those kinds of numbers in physics.

We’re used to a half, one, pi, e squared to two.

Those are the kinds of fundamental numbers

that emerge in our explanations of the world.

And we look at this bizarre number,

decimal point, all these zeros and a one,

we say something’s wrong there.

Like where would that number have come from?

And now there are people who suggest resolution to it.

So it’s not like we’re totally in the dark on it,

but those people like Paul Steinhard

who have alternate cosmological theories,

cyclic cosmologies as they call it,

claim that they have a more natural explanation

of the dark energy,

that it naturally feeds into a cyclical process

that is their cosmological paradigm.

So yeah, if the cosmology should change,

it’s conceivable our view of dark energy

may change from deeply mysterious

to deeply integrated into a different paradigm.

That is possible.

I think it’s Roger Penrose that think

that information can bleed through

from before the Big Bang to the after the Big Bang.

Is the Big Bang like a full erasure of the hard drive

or is there some information that could bleed through?

Yeah, I mean, so Roger is among the most creative thinkers

of the last 100 years,

rightly won the Nobel Prize for his insights

into singularities in space time

that we know to afflict our mathematical solutions

of black holes in the Big Bang and so forth.

And he has an enormously fertile imagination.

And I mean that in the most positive sense.

And so he has put forward this idea,

this conformal cyclic cosmology,

I think is the official title,

although I could be getting that wrong.

I can’t say that I’ve studied it.

I have seen lectures on it.

I don’t find it convincing as yet.

It feels like it’s being built to find a solution

as opposed to sort of more naturally emerging.

Maybe Roger would say otherwise.

And I don’t mean to in any way

cast aspersions on the work.

It’s vital and interesting and people are thinking about it.

I don’t consider it as close a competitor

to say the inflationary theory as for instance,

the stuff that Paul Steinhardt has put forward.

But again, you’ve got to keep an open mind

in this business when there’s so much

that we don’t yet understand.

I mean, it is wild to think

that information could survive something like that.

Just like it is wild to imagine

that information could escape a black hole, for example.

It just seems like by construction,

these things are supposed to not bleed out anything.

But one of the challenges in all of these theories

is when we talk about a singularity,

has this real sexy term, the singularity.

But a singularity is in more ordinary language,

a physical system where the mathematics breaks down.

It’s nonsensical.

It’s like taking one divided by zero.

You put that into a calculator and it says E error, right?

It does not make sense, doesn’t compute.

And so it’s very hard to make definitive statements

about things like the Big Bang or about black holes

until we cure the mathematical singularities.

And there are some who claim that in certain regimes,

the singularities have been cured.

I don’t by any means think that there’s consensus

on these ideas.

So when one talks about information sort of bleeding

through the Big Bang, you’ve really got to make sure

that the equations have no singularity.

You talk about cyclic cosmology,

you’ve got to make sure that the equations

don’t have any singularities as you go from, say,

one cycle to the next.

Now, some of the proponents of these theories claim

that they have resolved these issues.

I don’t think that there’s a general sense

that that is the case as yet, but it could be that,

look, life is so short that I haven’t had the time

to deeply delve into all the mathematical intricacies

of all the ideas that have been put forward.

If I did that, I’d never do anything else.

But that’s what the issue is.

And of course, it’s just math.

There may be holes.

There may be gaps in our understanding

in the way we’re modeling physical reality.

Well, that’s the point.

In fact, when you said, I was about to jump in

and say modeling, but you got there first,

and it’s exactly the right point.

We’re talking about the universe here, right?

And how do you talk about the universe

with a straight face, mathematically?

And the way you do it is you simplify,

you throw away those characteristics of the universe

that you don’t think are vital to a full understanding.

And so we’re gonna get to a point people are starting to

where we’ve got to go beyond those simplifications.

And so cosmology has for a long time modeled the universe

in the most simplest terms, homogeneous, isotropic.

It has just a few parameters that describe it,

the average density of mass and energy and so forth.

We have to go beyond those simplifications,

and that will require putting these things on computers.

We’re not gonna be able to do calculations there.

So much as astrophysics has gone beyond many simplifications

to now give really detailed simulations of star systems

and galaxies and so forth,

we’re gonna have to do that with cosmology,

and people are starting to do that today.

Yeah, I’ve seen some interesting work on simulation,

most simulation cosmology, by the way, is just awesome.

But just like simulation of the early formation

of our solar system to understand how the like the Oort cloud

and just, I don’t know, the whole of it,

how Earth came to be, like how Jupiter just protects us.

And then there’s like weird like moons and volcanoes

and like modeling all of that,

the formation of all of that is fascinating.

Because that naturally is the question

of how does life emerge on these kinds of rocks?

How does a rock become a rabbit?

But speaking of models,

there’s an equation called the Drake equation.

We were talking about life.

Have to ask, at the highest level first,

when you look out there,

how many alien civilizations do you think are out there?

Well, zero, one, or many?

So if you say civilization,

I would bring my number way down.

It could be zero.

If you talk about life, I think it could be many.

As we were saying before,

I think the move from life to consciousness,

the kinds of beings that would build

what we would recognize as a civilization,

that may be extraordinarily rare.

I hope it’s not.

You know, as a kid, I loved Star Trek.

I just loved the idea that we would be part

of some universal community where,

look, experience on planet Earth

suggests it doesn’t always go so well

when groups who are separated try to come together

and live in some larger collective.

But again, as an optimist,

how amazing would it be to converse

with an alien civilization and learn

what they’ve figured out about physics and cosmology

and compare notes and learn from each other

in some wonderful way?

I love that idea.

But if you ask me the likelihood of it,

I would err on saying it may be so improbable

that the conditions conspire to allow life

to move to this place of consciousness

that it might be rare.

It might be oversimplifying things,

but just observing the power of the evolutionary process,

I tend to believe,

and like you read different theories of how we went,

how Homo sapiens evolved,

it seems like the evolutionary process

naturally leads to Homo sapiens

or creatures like that or much better than that.

So to me, there’s several scary scenarios.

So, okay, the positive scenario

is life itself is really difficult.

So that origin of life is difficult.

That’s exciting for many reasons

because we might be able to prove that wrong easily

in the near term by finding life elsewhere.


The scary thing to me is if life is easy

and there’s plenty of conscious intelligent civilizations

out there and we have not obviously made contact,

which means with intelligence and consciousness

comes responsibility and ultimately destruction.

So with power comes great responsibility

and then we end up destroying ourselves.

That’s the scariest.

The positive, I guess, version is that

maybe we’re being watched,

sort of like there’s a transition

to where you don’t wanna ruin the primitive villages

out there and so there’s a protective layer around us.

They’re watching.

So where do you in these possible explanations

to the Fermi paradox,

why haven’t we contacted aliens?

Do you land on?

Well, I think the most straightforward explanation

is that there aren’t any.

Now, there are many other explanations too.

So you can’t be dogmatic about things

that are just sort of gut feel,

but one of my favorite Twilight Zone episodes,

I don’t know if you ever saw this one

where this alien civilization finally comes

to planet Earth and gives us this book

that they really want us to have and to hold

and it’s in this foreign language,

you don’t understand it.

The cryptographers, they desperately try to decipher it

as humans are gonna visit this other alien planet

and they’re all sending back postcards,

how wonderful it is and so forth

and they finally decipher the title.

It’s To Serve Man and everyone’s so thrilled,

oh, they’re here to serve us, it all makes sense

and then just as one of the final cryptographers

is going on to the alien ship,

his helper runs and says,

I’ve deciphered the rest of the book.

To Serve Man, it’s a cookbook.

So yeah, is that a possibility?


And so could they be watching us

and just sort of waiting for us to get

to a mature enough level?

I don’t know, it strikes me.

Well, I think it’d be better to have this conversation

after the James Webb Telescope.

I mean, I do think that if we look

at the atmospheres of many planets,

I mean, there’s now an estimate now

that there’s on order of one planet per star on average.

So we’ve long known that the galaxy,

hundreds of billions of stars,

numbers of galaxies, hundreds of billions of galaxies.

So we’re talking about hundreds of billions

of hundreds of billions of planets, oh my.

And if we start to survey some of these planets

and one after the other after the other,

we just sort of find no evidence

for any of the biological markers.

It could be, of course,

maybe life takes a radically different form.

It’d be hard to know that.

But I think that would at least give us some insight

on the life question.

But I just don’t see how we get insight

on the civilization or consciousness question

without the direct connection.

And it strikes me that if consciousness is ubiquitous,

let’s say life is, I’m willing to grant that.

If consciousness is also ubiquitous,

then I don’t understand why they haven’t been here

or why there hasn’t been separation

because presumably they should be much further ahead of us.

How unlike would it be that we’re like,

of all consciousness in the universe,

we’re the most advanced.

That’d be such a special place for human beings

that it’s hard for me to grant that as a likely possibility.

Rather, I think we’re kind of run of the mill.

And there are many who are far more advanced than us.

And I don’t think that they would expend the energy

to hide themselves.

I don’t think they care enough.

And so see, that’s actually what I believe

that there’s a very large number of civilizations

that are far more advanced than us.

But my sense is that humans are exceptionally limited

both in our direct sensory capabilities and our physics,

our tools of sensing that just like with the string theory

and the multiple dimensions, we’re just not like,

it’s like, I honestly believe there could be stuff

in front of our nose that we’re just not seeing

because we’re too dumb, too much hubris

and I mean, there’s a bunch of stuff

and too ignorant to the fabric of reality,

all of those things.

We’re young in terms of intelligence.

But I guess what I’d say is like, I’m on board

with all of that as a real possibility,

but then it does strike me that we are sufficiently

able to observe the unit.

Look, we can look back to a fraction of the duration

from here to the, just a fraction is left

that we are unable to see.

So however young we are, we have been able

to sort of pierce the universe and it just strikes me

that there would be some signature,

but maybe that’s coming.

But look, having said that I do, look,

I certainly note the fact that it’s rare

that I stoop down while walking in Manhattan

and sort of dig up some ants in the bushes

on the side of the street and talk to the ants, right?

Because it’s just not interesting to me.

So if we’re like the ants on the cosmological landscape,

then yeah, I can imagine that the super advanced aliens

would be like, like whoever, you know,

but I feel like we’re sufficiently advanced

that there should be some signal signature of that,

but maybe it’s coming.

I think the deeper fundamental problem between us

and the ants is that we don’t have a common language.

It’s not the interest.

It’s that we don’t even have a common language.

And so the alien civilizations don’t even know how to,

like we humans have convinced ourselves we’re special

because we developed the language.

And you talked about the importance of language

to the intelligence, but it makes you wonder

like how very niche is that like club that we’ve,

like tribe we’ve created of language

and linguistic type of systems that are very specific

to our particular kinds of brains

and we share ideas together are all super excited

that we can understand the universe

because we came up with some notation and math.

I wonder if there’s some totally other kinds of language

that communicates on a different timescale

with different, very different mechanisms

in the space of information that just is not,

it’s everything, everything is lost in translation.

Yeah, and it could well be as a look.

I mean, I think part of the reason I go

toward the possibility of the soul intelligence

is there’s a certain kind of romantic appeal

to looking out in the cosmos and it’s just quiet

and it’s just eternal silence.

There’s something that appeals to me

at an emotional level that way.

But yeah, I mean, nobody knows.

And it’s certainly conceivable

that there’s just a radical mismatch

between the kinds of things

that we are able to observe and sensitive to

versus the kinds of structures that permeate the universe

in a manner that simply we’re unable to detect.

Well, if we are alone, that is exciting.

And one of the ways it’s exciting

is that it’s up to us to become,

to expand out into the universe,

to permeate consciousness out into the universe.

So that’s where space exploration comes in.

Let me ask you as somebody who’s a screen theorist,

a physicist, do you think space exploration,

a colonizing space is a physics or an engineering problem?

What would you say?

Yeah, I think it’s fundamentally an engineering problem

if we’re not trying to do things like build wormholes

the way they did, say an interstellar

to get to a different place

or trying to travel near the speed of light

so that we would actually be able

to traverse interstellar distances.

I mean, without that,

our colonization will happen in a very, very slow rate.

But one of the beauties of relativity

is if you do travel near the speed of light,

you can actually go arbitrarily far in a human lifetime.

People say, how’s that possible?

You can’t go billions of light years.

Billions of light years, well, you can actually,

because as you can do the speed of light,

the way in which space and time change

allows you to go in principle arbitrarily far.

That’s very exciting.

But if we put that physics side of the issue

and the manipulation of space and time to the side,

yeah, I think it’s a deep engineering problem.

How do you terraform other planets?

I mean, how do you go beyond our local neighborhood,

say without using the ideas of relativity?

So I think it’s all quite exciting.

And I think the idea is using solar sails

that people have developed

and trying to take that first step to Mars,

I think that’s a vital and valuable step to take.

But yeah, I think these are

fundamentally engineering challenges.

Or extending the human lifespan through biology research

or maybe reducing what it means to be a human being

into information and uploading certain parts of it.

Maybe not all of the full resolution of a human life,

but maybe the essential things like the DNA

and be able to reconstruct that human being.

But I have to ask about Mars.

Do you find the dream of humans stepping on Mars,

stepping foot first, but also colonizing Mars,

one that’s worth us fighting for?

Yeah, usually so.

I mean, I think what we have long been

not always in the best way is a species of explorers

in the literal sense of traveling

from one part of the world to another,

or in the more metaphorical sense

of trying to travel through our minds to the quantum realm

or back to the Big Bang or to the center of black holes.

So I think that’s fundamentally part of the human spirit.

So I do think that’s a vital part of our heritage

brought forward into its next incarnation.

That’s who we are.

Do you think there’ll be a day in the future

where a human being is born on Mars

and has to learn about his or her human origins on Earth?

Like, they’ll have to read in a book.

Yeah, I don’t think it’ll be a book at that stage.

It’ll probably just be uploaded into the head or something

or imprinted into the DNA,

and then they just sort of sense it.

But yeah, I think there’s, well, look,

the issue you raised before is the vital one.

Is it the case that any sufficiently advanced civilization

destroys itself?

Is that sort of a commonplace quality?

I mean, that’s the other potential answer

to the Fermi paradox.

Why aren’t they here?

Because by the time they got to the technological development

where they could travel here, they blew themselves up.

They destroyed themselves.

And that’s an unfortunate,

but not a hard to imagine possibility

based on things that have happened here on planet Earth.

But putting that to the side,

I think that’s the big obstacle,

but putting that to the side,

we will resolve the engineering challenges.

And I should probably modify my answer

from before when you said, is it engineering or physics?

It’s really both, right?

So we will surmount the engineering challenges

and that will then make the physics challenges relevant.

It’ll make it relevant to figure out

how to travel near the speed of light.

It’ll make it relevant to learn

how to manipulate the shape of space time and so forth.

So I think it’s a multi stage process

where it is engineering and ultimately physics.

And if we stick around long enough,

those are the kinds of challenges

I think that we’re ultimately gonna surmount.

And then the physics side is figuring out

how to harness energy enough

to travel outside the solar system,

which seems like a heck of a difficult journey.

But even Mars itself,

I don’t know, maybe because I was born in the Soviet Union

and was born with the,

looking up at the stars and that dream

of like the highest of human achievement

is the ability to fly out there,

to join the stars.

I really liked the idea of going to Mars

and not just stepping foot on Mars.

And it wasn’t until maybe misinformed,

but for me personally,

it wasn’t until Elon Musk started talking

about the colonization of Mars,

did I realize like we humans can actually do that.

And first of all, the importance of somebody saying

that we can do these seemingly impossible things

is immeasurable because the fact that he placed that

into my mind and into the minds of millions of others,

maybe hundreds of millions, maybe billions of others,

young kids today, I mean, that’s gonna make it a reality.

I, for some reason, am deeply excited,

even though my work isn’t AI that echoes all of this.

I’m excited by the idea that somebody would be born,

as we were saying, on Mars and sort of look up

and be able to see with a telescope Earth

and say, that’s where I came from.

I don’t know, that idea scale to other planets,

to other solar systems, that’s really exciting.

And hugely exciting.

I think you’re absolutely right.

I mean, the vital thing is to dream, right?

I mean, and it sounds hackneyed,

but it is so important for young kids,

for the next generation,

to think about the things that are seemingly impossible.

I mean, that’s what makes them possible.

And this is one which is concrete enough.

I mean, this is something that’s gonna happen soon

in terms of actually going to Mars.

And then the next step of establishing some presence,

some semi permanent or permanent presence.

This is not something that’s gonna wait

to the 25th century.

I mean, this is something that’s gonna happen

relatively soon.

So, I mean, it could well be in your lifetime,

unlikely mine, but possibly in your lifetime

that that kid will be born

and have the experience that you described.

So yeah, it’s spectacularly exciting.

And I actually, I would love to go on Mars

on one of the early.

You would? Yeah.

It would if it’s one way.

I’m happy to do it one way. Really?

Wow. Yeah.

I’m single if there’s ladies out there

that wanna start that family.

Let’s go out to Mars.

No, I think.

See, I have to tell you something.

You spoke about terror, thinking about like black holes.

If I actually think about going to Mars

and being on Mars and put myself in there fully,

that’s terror inducing.

The idea of to be in this foreign world

where you can’t come back,

where you’ve made this choice that can’t be reversed.

Well, at some point it may be,

but in that guise, that to me carries a deep sense of terror.

I feel that sense of terror every time Kerouac,

Jack Kerouac talked about this on the road

is when you leave a place, if you’re honest about it,

like life is short.

And when you leave a place, you move to a new place

and you think of all the friends, maybe family

you’re leaving behind as you drive over the hill,

that really is goodbye.

Like we sometimes don’t think of it that way

when we’re moving, but that really is goodbye to that life,

to the person you were, to all the people.

Maybe if it’s close friends, you’ll see them maybe 10, 15

more times in your life and that’s it.

And you’re saying goodbye to all of that.

And so in the same way, I see it as way more dramatic

when you’re flying away from earth and it’s like,

it’s goodbye to Dunkin Donuts and Starbucks.

And it’s goodbye to whatever, I don’t know why I picked

those, but some, all the things that are special to earth,

it’s goodbye, but that’s life.

I suppose more, what excites me about that kind of journey

is it’s a distinct contemplation of your mortality,

acceptance of your mortality.

You’re saying, just like when you take on any difficult

journey, it’s accepting that you’re going to die one day

and might as well do something truly exciting.

Yes, I mean, I will, you know, I’m with you on that.

I’m a strong believer that deep underneath human motivation

is this terror of our own mortality.

Yeah, there’s this a wonderful book that had a great

influence in me called The Denial of Death by Ernest Becker.

And when you are aware of the ways in which our mortality

influences our behaviors, it really does add a different

slant, a different kind of color to the interpretation

of human behavior.

Yeah, it’s funny that that book had a big influence

in me as well.

Oh, is that right?


And the terror management theory and I, again,

from an engineering perspective, I don’t know how many

people that book influenced because I talk to people

about the fear of death and it doesn’t seem like

about the fear of death and it doesn’t seem to be

that fundamental to their experience.

And I don’t think on the surface it’s fundamental

to my experience, but it seems like an awfully,

in terms of we talk about models and strength theory

and theories, in terms of theories of this macro experience

of human life, it seems like a heck of a good theory

that the fear of death is at the kind of is the warm

at the core.

Yeah, well, I mean, and the terror management theories

that you make reference to, I mean, this is a group

of psychologists, social psychologists who devise

these very clever experiments, real world experiments

with real people, where you can directly measure

the hidden influence of the recognition

of our own mortality.

I mean, they’ve done these experiments where they have

group of people A, group of people B,

and the only difference between the two groups

is that group B, they somehow reminded them

in some subtle way of their own mortality.

Sometimes it’s nothing more than interviewing them

with a funeral home across the street.

And influence is there, but it’s subtle,

you don’t even think you’d take note of.

And they can find measurable effects that differentiate

the two groups to a high degree of statistical significance

and how they respond to certain challenges

or certain kinds of questions that shows a direct influence

of the reminder of their own mortality.

And I’ve read a number of these studies

and they are really convincing.

And so yeah, I would say that the reason why

so many people would say that, yeah, fear of mortality,

it’s not front and center in my worldview.

Yeah, I don’t really think about it much,

it doesn’t really matter too much.

The reason why they’re able to say that

is because this thing called culture has emerged

over the course of the last 10,000 years.

And part of the role of culture is to give us a means

of not thinking about our mortality all the time,

of not living in terror of the inevitable end,

which faces us all.

So it’s completely understandable that that’s the response

because that’s what culture is at least in part for.

Is it at least possible that the fear of death,

the terror of your mortality is the creative force

that created all of the things around us

at this human civilization?

And I think about from an engineering perspective,

this is where I lose all of my robotics colleagues

is I feel like if you want to create intelligence,

you have to also engineer in some kind of echoes

of this kind of fear.

Fear is such a complicated word,

but it’s kind of like a scarcity,

a scarcity of time, a scarcity of resources

that creates a kind of anxiety,

like deadlines get you to do stuff.

And there’s something almost fundamental to that

in terms of human experience.

Yeah, well, that’s an interesting thought.

So you’re basically in order to create a kind of structure

that mirrors what we call consciousness.

You’d better have that structure confront the same kinds

of issues and terrors that we do.

Consciousness and suffering only makes sense

in the context of death.

If you want to, I feel like,

if you want to fit into human society,

if you’re a robot and if you want to fit into human society,

you better have the same kind of existential dread,

the same kind of fear of mortality,

otherwise you’re not gonna fit in.


That’s good.

It might be wild, but it’s at least,

like we’re talking about all the theories

that are at least worth consideration.

I think that’s a really powerful one.

And definitely one has resonated with me

and definitely seems to capture something

beautifully like real about the human condition.

And I wonder, it’s of course,

it sucks to think that we need death to appreciate life,

but that’s just maybe the way it is.

Well, it’s interesting if this robotic

or artificially intelligent system understands the world

and understands the second law of thermodynamics

and entropy, even an artificial intelligence will realize

that even if its parts are really robust,

ultimately it will disintegrate.


The timescales may be different,

but in a way, when you think about it, it doesn’t matter.

Once you know that you are mortal

in the sense that you are not eternal,

the timescale hardly matters

because it’s either the whole thing or not.

Because on the scales of eternity,

any finite duration, however large is effectively zero

on the scales of eternity.

And so maybe it won’t be so hard for an artificial system

to feel that sense of mortality

because it will recognize the underlying physical laws

and recognize its own finitude.

And then it’ll be us and robots drinking beers,

looking up at the stars and just,

having a good laugh in awe of the whole thing.


I think that’s a pretty good way to end it,

talking about the fear of death.

We started talking about the meaning of life

and ended on the fear of death.

Brian, this is an incredible conversation.

My pleasure, thank you.

I enjoyed it enormously.

I really, really enjoyed it.

It’s been a long time coming.

I’m a huge fan of your work, a huge fan of your writing.

Thanks for talking to me, Brian.

Thank you.

Thanks for listening to this conversation

with Brian Greene.

To support this podcast,

please check out our sponsors in the description.

And now, let me leave you with some words from Bill Bryson.

“‘Physics is really nothing more

“’than a search for ultimate simplicity.

“‘But so far, all we have is a kind of elegant messiness.’”

Thank you for listening and hope to see you next time.

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