Lex Fridman Podcast - #41 - Leonard Susskind: Quantum Mechanics, String Theory, and Black Holes

The following is a conversation with Leonard Susskind.

He’s a professor of theoretical physics

at Stanford University and founding director

of Stanford Institute of Theoretical Physics.

He is widely regarded as one of the fathers

of string theory and in general,

as one of the greatest physicists of our time,

both as a researcher and an educator.

This is the Artificial Intelligence Podcast.

Perhaps you noticed that the people I’ve been speaking with

are not just computer scientists,

but philosophers, mathematicians, writers,

psychologists, physicists, and soon other disciplines.

To me, AI is much bigger than deep learning,

bigger than computing.

It is our civilization’s journey

into understanding the human mind

and creating echoes of it in the machine.

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at Lex Friedman, spelled F R I D M A M.

And now, here’s my conversation with Leonard Susskind.

You worked and were friends with Richard Feynman.

How has he influenced you, changed you

as a physicist and thinker?

What I saw, I think what I saw was somebody

who could do physics in this deeply intuitive way.

His style was almost to close his eyes

and visualize the phenomena that he was thinking about.

And through visualization, outflank the mathematical,

the highly mathematical and very, very sophisticated

technical arguments that people would use.

I think that was also natural to me,

but I saw somebody who was actually successful at it,

who could do physics in a way that I regarded

as simpler, more direct, more intuitive.

And while I don’t think he changed my way of thinking,

I do think he validated it.

He made me look at it and say, yeah,

that’s something you can do and get away with.

Practically didn’t get away with it.

So do you find yourself, whether you’re thinking

about quantum mechanics or black holes

or string theory, using intuition as a first step

or step throughout using visualization?

Yeah, very much so, very much so.

I tend not to think about the equations.

I tend not to think about the symbols.

I tend to try to visualize the phenomena themselves.

And then when I get an insight that I think is valid,

I might try to convert it to mathematics,

but I’m not a natural mathematician.

I’m good enough at it.

I’m good enough at it, but I’m not a great mathematician.

So for me, the way of thinking about physics

is first intuitive, first visualization,

scribble a few equations maybe,

but then try to convert it to mathematics.

Experience is that other people are better

at converting it to mathematics than I am.

And yet you’ve worked with very counterintuitive ideas.

No, that’s true.

That’s true.

You can visualize something counterintuitive.

How do you dare?

By rewiring your brain in new ways.

Yeah, quantum mechanics is not intuitive.

Very little of modern physics is intuitive.

Intuitive, what does intuitive mean?

It means the ability to think about it

with basic classical physics,

the physics that we evolved with throwing stones,

or splashing water, whatever it happens to be.

Quantum physics, general relativity,

quantum field theory are deeply unintuitive in that way.

But after time and getting familiar with these things,

you develop new intuitions.

I always said you rewire.

And it’s to the point where me and many of my friends,

I and many of my friends,

can think more easily quantum mechanically

than we can classically.

We’ve gotten so used to it.

I mean, yes, our neural wiring in our brain

is such that we understand rocks and stones and water

and so on.

We sort of evolved for that.

Evolved for it.

Do you think it’s possible to create a wiring

of neuron like state devices that more naturally

understand quantum mechanics, understand wave function,

understand these weird things?

Well, I’m not sure.

I think many of us have evolved the ability

to think quantum mechanically to some extent.

But that doesn’t mean you can think like an electron.

That doesn’t mean another example.

Forget for a minute quantum mechanics.

Just visualizing four dimensional space

or five dimensional space or six dimensional space,

I think we’re fundamentally wired

to visualize three dimensions.

I can’t even visualize two dimensions or one dimension

without thinking about it as embedded in three dimensions.

If I wanna visualize a line,

I think of the line as being a line in three dimensions.

Or I think of the line as being a line on a piece of paper

with a piece of paper being in three dimensions.

I never seem to be able to, in some abstract and pure way,

visualize in my head the one dimension,

the two dimension, the four dimension, the five dimensions.

And I don’t think that’s ever gonna happen.

The reason is I think our neural wiring

is just set up for that.

On the other hand, we do learn ways

to think about five, six, seven dimensions.

We learn ways, we learn mathematical ways,

and we learn ways to visualize them, but they’re different.

And so yeah, I think we do rewire ourselves.

Whether we can ever completely rewire ourselves

to be completely comfortable with these concepts, I doubt.

So that it’s completely natural.

To where it’s completely natural.

So I’m sure there’s somewhat, you could argue,

creatures that live in a two dimensional space.

Yeah, maybe there are.

And while it’s romanticizing the notion of curse,

we’re all living, as far as we know,

in three dimensional space.

But how do those creatures imagine 3D space?

Well, probably the way we imagine 4D,

by using some mathematics and some equations

and some tricks.

Okay, so jumping back to Feynman just for a second.

He had a little bit of an ego.


Why, do you think ego is powerful or dangerous in science?

I think both, both, both.

I think you have to have both arrogance and humility.

You have to have the arrogance to say, I can do this.

Nature is difficult, nature is very, very hard.

I’m smart enough, I can do it.

I can win the battle with nature.

On the other hand, I think you also have to have

the humility to know that you’re very likely

to be wrong on any given occasion.

Everything you’re thinking could suddenly change.

Young people can come along and say things

you won’t understand and you’ll be lost and flabbergasted.

So I think it’s a combination of both.

You better recognize that you’re very limited,

and you better be able to say to yourself,

I’m not so limited that I can’t win this battle with nature.

It takes a special kind of person

who can manage both of those, I would say.

And I would say there’s echoes of that in your own work,

a little bit of ego, a little bit of outside of the box,

humble thinking.

I hope so.

So was there a time where you felt,

you looked at yourself and asked,

am I completely wrong about this?

Oh yeah, about the whole thing or about specific things?

The whole thing.

What do you mean?

Wait, which whole thing?

Me and me and my ability to do this thing.

Oh, those kinds of doubts.

First of all, did you have those kinds of doubts?

No, I had different kind of doubts.

I came from a very working class background

and I was uncomfortable in academia for,

oh, for a long time.

But they weren’t doubts about my ability or my,

they were just the discomfort in being in an environment

that my family hadn’t participated in,

I knew nothing about as a young person.

I didn’t learn that there was such a thing called physics

until I was almost 20 years old.

Yeah, so I did have certain kind of doubts,

but not about my ability.

I don’t think I was too worried

about whether I would succeed or not.

I never felt this insecurity, am I ever gonna get a job?

That had never occurred to me that I wouldn’t.

Maybe you could speak a little bit to this sense

of what is academia.

Because I too feel a bit uncomfortable in it.

There’s something I can’t put quite into words

what you have that’s not, doesn’t, if we call it music,

you play a different kind of music than a lot of academia.

How have you joined this orchestra?

How do you think about it?

I don’t know that I thought about it

as much as I just felt it.

Thinking is one thing, feeling is another thing.

I felt like an outsider until a certain age

when I suddenly found myself the ultimate insider

in academic physics.

And that was a sharp transition, and I wasn’t a young man.

I was probably 50 years old.

So you were never quite, it was a phase transition,

you were never quite in the middle.

Yeah, that’s right, I wasn’t.

I always felt a little bit of an outsider.

In the beginning, a lot an outsider.

My way of thinking was different,

my approach to mathematics was different,

but also my social background

that I came from was different.

Now these days, half the young people I meet,

they’re parents or professors.

That was not my case.

But then all of a sudden, at some point,

I found myself at very much the center of,

maybe not the only one at the center,

but certainly one of the people in the center

of a certain kind of physics.

And all that went away, it went away in a flash.

So maybe a little bit with Feynman,

but in general, how do you develop ideas?

Do you work through ideas alone?

Do you brainstorm with others?

Oh, both, both, very definitely both.

The younger time, I spent more time with myself.

Now, because I’m at Stanford,

because I have a lot of ex students

and people who are interested in the same thing I am,

I spend a good deal of time, almost on a daily basis,

interacting, brainstorming, as you said.

It’s a very important part.

I spend less time probably completely self focused

than with a piece of paper

and just sitting there staring at it.

What are your hopes for quantum computers?

So machines that are based on,

that have some elements of leverage quantum mechanical ideas.

Yeah, it’s not just leveraging quantum mechanical ideas.

You can simulate quantum systems on a classical computer.

Simulate them means solve the Schrodinger equation for them

or solve the equations of quantum mechanics

on a computer, on a classical computer.

But the classical computer is not doing,

is not a quantum mechanical system itself.

Of course it is.

Everything’s made of quantum mechanics,

but it’s not functioning.

It’s not functioning as a quantum system.

It’s just solving equations.

The quantum computer is truly a quantum system

which is actually doing the things

that you’re programming it to do.

You want to program a quantum field theory.

If you do it in classical physics,

that program is not actually functioning in the computer

as a quantum field theory.

It’s just solving some equations.

Physically, it’s not doing the things

that the quantum system would do.

The quantum computer is really a quantum mechanical system

which is actually carrying out the quantum operations.

You can measure it at the end.

It intrinsically satisfies the uncertainty principle.

It is limited in the same way that quantum systems

are limited by uncertainty and so forth.

And it really is a quantum system.

That means that what you’re doing

when you program something for a quantum system

is you’re actually building a real version of the system.

The limits of a classical computer,

classical computers are enormously limited

when it comes to the quantum systems.

They’re enormously limited

because you’ve probably heard this before,

but in order to store the amount of information

that’s in a quantum state of 400 spins,

that’s not very many, 400 I can put in my pocket,

I can put 400 pennies in my pocket.

To be able to simulate the quantum state

of 400 elementary quantum systems, qubits we call them,

to do that would take more information

than can possibly be stored in the entire universe

if it were packed so tightly

that you couldn’t pack any more in.

400 qubits.

On the other hand, if your quantum computer

is composed of 400 qubits,

it can do everything 400 qubits can do.

What kind of space, if you just intuitively think

about the space of algorithms that that unlocks for us,

so there’s a whole complexity theory

around classical computers,

measuring the running time of things,

and P, so on, what kind of algorithms

just intuitively do you think it unlocks for us?

Okay, so we know that there are a handful of algorithms

that can seriously beat classical computers

and which can have exponentially more power.

This is a mathematical statement.

Nobody’s exhibited this in the laboratory.

It’s a mathematical statement.

We know that’s true, but it also seems more and more

that the number of such things is very limited.

Only very, very special problems

exhibit that much advantage for a quantum computer,

of standard problems.

To my mind, as far as I can tell,

the great power of quantum computers

will actually be to simulate quantum systems.

If you’re interested in a certain quantum system

and it’s too hard to simulate classically,

you simply build a version of the same system.

You build a version of it.

You build a model of it

that’s actually functioning as the system.

You run it, and then you do the same thing

you would do to the quantum system.

You make measurements on it, quantum measurements on it.

The advantage is you can run it much slower.

You could say, why bother?

Why not just use the real system?

Why not just do experiments on the real system?

Well, real systems are kind of limited.

You can’t change them.

You can’t manipulate them.

You can’t slow them down so that you can poke into them.

You can’t modify them in arbitrary kinds of ways

to see what would happen if I change the system a little bit.

I think that quantum computers will be extremely valuable

in understanding quantum systems.

At the lowest level of the fundamental laws.

They’re actually satisfying the same laws

as the systems that they’re simulating.

Okay, so on the one hand, you have things like factoring.

Factoring is the great thing of quantum computers.

Factoring large numbers, that doesn’t seem that much

to do with quantum mechanics.

It seems to be almost a fluke that a quantum computer

can solve the factoring problem in a short time.

And those problems seem to be extremely special, rare,

and it’s not clear to me

that there’s gonna be a lot of them.

On the other hand, there are a lot of quantum systems.

Chemistry, there’s solid state physics,

there’s material science, there’s quantum gravity,

there’s all kinds of quantum field theory.

And some of these are actually turning out

to be applied sciences,

as well as very fundamental sciences.

So we probably will run out of the ability

to solve equations for these things.

Solve equations by the standard methods of pencil and paper.

Solve the equations by the method of classical computers.

And so what we’ll do is we’ll build versions

of these systems, run them,

and run them under controlled circumstances

where we can change them, manipulate them,

make measurements on them,

and find out all the things we wanna know.

So in finding out the things we wanna know

about very small systems, is there something

that we can also find out about the macro level,

about something about the function, forgive me,

of our brain, biological systems,

the stuff that’s about one meter in size

versus much, much smaller?

Well, what all the excitement is about

among the people that I interact with

is understanding black holes.

Black holes.

Black holes are big things.

They are many, many degrees of freedom.

There is another kind of quantum system that is big.

It’s a large quantum computer.

And one of the things we’ve learned

is that the physics of large quantum computers

is in some ways similar to the physics

of large quantum black holes.

And we’re using that relationship.

Now you asked, you didn’t ask about quantum computers

or systems, you didn’t ask about black holes,

you asked about brains.

Yeah, about stuff that’s in the middle of the two.

It’s different.

So black holes are,

there’s something fundamental about black holes

that feels to be very different than a brain.


And they also function in a very quantum mechanical way.



It is, first of all, unclear to me,

but of course it’s unclear to me.

I’m not a neuroscientist.

I have, I don’t even have very many friends

who are neuroscientists.

I would like to have more friends who are neuroscientists.

I just don’t run into them very often.

Among the few neuroscientists

I’ve ever talked about about this,

they are pretty convinced

that the brain functions classically,

that it is not intrinsically a quantum mechanical system

or it doesn’t make use of the special features,

entanglement, coherence, superposition.

Are they right?

I don’t know.

I sort of hope they’re wrong

just because I like the romantic idea

that the brain is a quantum system.

But I think probably not.

The other thing,

big systems can be composed of lots of little systems.

Materials, the materials that we work with and so forth

are, can be large systems, a large piece of material,

but they’re made out of quantum systems.

Now, one of the things that’s been happening

over the last good number of years

is we’re discovering materials and quantum systems,

which function much more quantum mechanically

than we imagined.

Topological insulators, this kind of thing,

that kind of thing.

Those are macroscopic systems,

but they’re just superconductors.

Superconductors have a lot of quantum mechanics in them.

You can have a large chunk of superconductor.

So it’s a big piece of material.

On the other hand, it’s functioning and its properties

depend very, very strongly on quantum mechanics.

And to analyze them, you need the tools of quantum mechanics.

If we can go on to black holes

and looking at the universe

as a information processing system,

as a computer, as a giant computer.

It’s a giant computer.

What’s the power of thinking of the universe

as an information processing system?

Or what is perhaps its use

besides the mathematical use of discussing black holes

and your famous debates and ideas around that

to human beings,

or life in general as information processing systems?

Well, all systems are information processing systems.

You poke them, they change a little bit, they evolve.

All systems are information processing systems.

So there’s no extra magic to us humans?

It certainly feels, consciousness intelligence

feels like magic.

It sure does.

Where does it emerge from?

If we look at information processing,

what are the emergent phenomena

that come from viewing the world

as an information processing system?

Here is what I think.

My thoughts are not worth much in this.

If you ask me about physics,

my thoughts may be worth something.

If you ask me about this,

I’m not sure my thoughts are worth anything.

But as I said earlier,

I think when we do introspection,

when we imagine doing introspection

and try to figure out what it is

when we do when we’re thinking,

I think we get it wrong.

I’m pretty sure we get it wrong.

Everything I’ve heard about the way the brain functions

is so counterintuitive.

For example, you have neurons which detect vertical lines.

You have different neurons

which detect lines at 45 degrees.

You have different neurons.

I never imagined that there were whole circuits

which were devoted to vertical lines in my brain.

Doesn’t seem to be the way my brain works.

My brain seems to work if I put my finger up vertically

or if I put it horizontally

or if I put it this way or that way.

It seems to me it’s the same circuits.

It’s not the way it works.

The way the brain is compartmentalized

seems to be very, very different

than what I would have imagined

if I were just doing psychological introspection

about how things work.

My conclusion is that we won’t get it right that way,

that how will we get it right?

I think maybe computer scientists will get it right eventually.

I don’t think there are any ways near it.

I don’t even think they’re thinking about it,

but eventually we will build machines perhaps

which are complicated enough

and partly engineered, partly evolved,

maybe evolved by machine learning and so forth.

This machine learning is very interesting.

By machine learning, we will evolve systems

and we may start to discover mechanisms

that have implications for how we think

and for what this consciousness thing is all about

and we’ll be able to do experiments on them

and perhaps answer questions

that we can’t possibly answer by introspection.

So that’s a really interesting point.

In many cases, if you look at even a string theory,

when you first think about a system,

it seems really complicated, like the human brain,

and through some basic reasoning

and trying to discover fundamental low level behavior

of the system, you find out that it’s actually much simpler.

Do you, one, have you, is that generally the process

and two, do you have that also hope

for biological systems as well,

for all the kinds of stuff we’re studying at the human level?

Of course, physics always begins

by trying to find the simplest version of something

and analyze it.

Yeah, I mean, there are lots of examples

where physics has taken very complicated systems,

analyzed them and found simplicity in them for sure.

I said superconductors before, it’s an obvious one.

A superconductor seems like a monstrously complicated thing

with all sorts of crazy electrical properties,

magnetic properties and so forth.

And when it finally is boiled down

to its simplest elements,

it’s a very simple quantum mechanical phenomenon

called spontaneous symmetry breaking,

and which we, in other contexts, we learned about

and we’re very familiar with.

So yeah, I mean, yes, we do take complicated things,

make them simple, but what we don’t want to do

is take things which are intrinsically complicated

and fool ourselves into thinking

that we can make them simple.

We don’t want to make, I don’t know who said this,

but we don’t want to make them simpler

than they really are, okay?

Is the brain a thing which ultimately functions

by some simple rules or is it just complicated?

In terms of artificial intelligence,

nobody really knows what are the limits

of our current approaches, you mentioned machine learning.

How do we create human level intelligence?

It seems that there’s a lot of very smart physicists

who perhaps oversimplify the nature of intelligence

and think of it as information processing,

and therefore there doesn’t seem to be

any theoretical reason why we can’t artificially create

human level or superhuman level intelligence.

In fact, the reasoning goes,

if you create human level intelligence,

the same approach you just used

to create human level intelligence

should allow you to create superhuman level intelligence

very easily, exponentially.

So what do you think that way of thinking

that comes from physicists is all about?

I wish I knew, but there’s a particular reason

why I wish I knew.

I have a second job.

I consult for Google, not for Google, for Google X.

I am the senior academic advisor

to a group of machine learning physicists.

Now that sounds crazy because I know nothing

about the subject.

I know very little about the subject.

On the other hand, I’m good at giving advice,

so I give them advice on things.

Anyway, I see these young physicists

who are approaching the machine learning problem.

There is a real machine learning problem.

Namely, why does it work as well as it does?

Nobody really seems to understand

why it is capable of doing the kind of generalizations

that it does and so forth.

And there are three groups of people

who have thought about this.

There are the engineers.

The engineers are incredibly smart,

but they tend not to think as hard

about why the thing is working

as much as they do how to use it.

Obviously, they provided a lot of data,

and it is they who demonstrated

that machine learning can work much better

than you have any right to expect.

The machine learning systems are systems.

The system’s not too different

than the kind of systems that physicists study.

There’s not all that much difference

between quantum, in the structure of mathematics,

physically, yes, but in the structure of mathematics,

between a tensor network designed

to describe a quantum system on the one hand

and the kind of networks that are used in machine learning.

So there are more and more, I think,

young physicists are being drawn

to this field of machine learning,

some very, very good ones.

I work with a number of very good ones,

not on machine learning, but on having lunch.

On having lunch?



And I can tell you they are super smart.

They don’t seem to be so arrogant

about their physics backgrounds

that they think they can do things that nobody else can do.

But the physics way of thinking, I think,

will add great value to,

or will bring value to the machine learning.

I believe it will.

And I think it already has.

At what time scale do you think

predicting the future becomes useless

in your long experience

and being surprised at new discoveries?

Well, sometimes a day, sometimes 20 years.

There are things which I thought

we were very far from understanding,

which practically in a snap of the fingers

or a blink of the eye suddenly became understood,

completely surprising to me.

There are other things which I looked at and I said,

we’re not gonna understand these things for 500 years,

in particular quantum gravity.

The scale for that was 20 years, 25 years.

And we understand a lot

and we don’t understand it completely now by any means,

but I thought it was 500 years to make any progress.

It turned out to be very, very far from that.

It turned out to be more like 20 or 25 years

from the time when I thought it was 500 years.

So if we may, can we jump around quantum gravity,

some basic ideas in physics?

What is the dream of string theory mathematically?

What is the hope?

Where does it come from?

What problem is it trying to solve?

I don’t think the dream of string theory

is any different than the dream

of fundamental theoretical physics altogether.

Understanding a unified theory of everything.

I don’t like thinking of string theory

as a subject unto itself

with people called string theorists

who are the practitioners

of this thing called string theory.

I much prefer to think of them as theoretical physicists

trying to answer deep fundamental questions about nature,

in particular gravity,

in particular gravity and its connection

with quantum mechanics,

and who at the present time find string theory

a useful tool rather than saying

there’s a subject called string theorists.

I don’t like being referred to as a string theorist.

Yes, but as a tool, is it useful to think about our nature

in multiple dimensions, the strings vibrating?

I believe it is useful.

I’ll tell you what the main use of it has been up till now.

Well, it has had a number of main uses.

Originally, string theory was invented,

and I know that I was there.

I was right at the spot

where it was being invented literally,

and it was being invented to understand hadrons.

Hadrons are subnuclear particles,

protons, neutrons, mesons,

and at that time, the late 60s, early 70s,

it was clear from experiment

that these particles called hadrons could vibrate,

could rotate, could do all the things

that a little closed string can do,

and it was and is a valid and correct theory of these hadrons.

It’s been experimentally tested, and that is a done deal.

It had a second life as a theory of gravity,

the same basic mathematics,

except on a very, very much smaller distance scale.

The objects of gravitation are 19 orders of magnitude

or orders of magnitude smaller than a proton,

but the same mathematics turned up.

The same mathematics turned up.

What has been its value?

Its value is that it’s mathematically rigorous in many ways

and enabled us to find mathematical structures

which have both quantum mechanics and gravity.

With rigor, we can test out ideas.

We can test out ideas.

We can’t test them in the laboratory.

They’re 19 orders of magnitude too small

are things that we’re interested in,

but we can test them out mathematically

and analyze their internal consistency.

By now, 40 years ago, 35 years ago, and so forth,

people very, very much questioned the consistency

between gravity and quantum mechanics.

Stephen Hawking was very famous for it, rightly so.

Now, nobody questions that consistency anymore.

They don’t because we have mathematically precise

string theories which contain both gravity

and quantum mechanics in a consistent way.

So it’s provided that certainty that quantum mechanics

and gravity can coexist.

That’s not a small thing.

It’s a very big thing.

It’s a huge thing.

Einstein would be proud.

Einstein, he might be appalled.

I don’t know.

He didn’t like it.

He might not be appalled, I don’t know.

He didn’t like quantum mechanics very much,

but he would certainly be struck by it.

I think that may be, at this time,

its biggest contribution to physics

in illustrating almost definitively

that quantum mechanics and gravity

are very closely related

and not inconsistent with each other.

Is there a possibility of something deeper,

more profound that still is consistent with string theory

but is deeper, that is to be found?

Well, you could ask the same thing about quantum mechanics.

Is there something?


Yeah, yeah.

I think string theory is just an example

of a quantum mechanical system

that contains both gravitation and quantum mechanics.

So is there something underlying quantum mechanics?

Perhaps something deterministic.

My friend, Ferad Etouf, whose name you may know,

he’s a very famous physicist.

Dutch, not as famous as he should be, but…

Hard to spell his name.

It’s hard to say his name.

No, it’s easy to spell his name.

Apostrophe, he’s the only person I know

whose name begins with an apostrophe.

And he’s one of my heroes in physics.

He’s a little younger than me,

but he’s nevertheless one of my heroes.

Etouf believes that there is some substructure to the world

which is classical in character,

deterministic in character,

which somehow by some mechanism

that he has a hard time spelling out

emerges as quantum mechanics.

I don’t.

The wave function is somehow emergent.

The wave function, not just the wave function,

but the whole thing that goes with quantum mechanics,

uncertainty, entanglement, all these things,

are emergent. So you think quantum mechanics

is the bottom of the well?

Is the…

Here I think is where you have to be humble.

Here’s where humility comes.

I don’t think anybody should say anything

is the bottom of the well at this time.

I think we can reasonably say,

I can reasonably say when I look into the well,

I can’t see past quantum mechanics.

I don’t see any reason for there to be anything

beyond quantum mechanics.

I think Etouf has asked very interesting

and deep questions.

I don’t like his answers.

Well, again, let me ask,

if we look at the deepest nature of reality

with whether it’s deterministic

or when observed as probabilistic,

what does that mean for our human level

of ideas of free will?

Is there any connection whatsoever

from this perception, perhaps illusion of free will

that we have and the fundamental nature of reality?

The only thing I can say is I am puzzled by that

as much as you are.

The illusion of it.

The illusion of consciousness,

the illusion of free will, the illusion of self.

Does that connect to?

How can a physical system do that?

And I am as puzzled as anybody.

There’s echoes of it in the observer effect.

So do you understand what it means to be an observer?

I understand it at a technical level.

An observer is a system with enough degrees of freedom

that it can record information

and which can become entangled

with the thing that it’s measuring.

Entanglement is the key.

When a system which we call an apparatus or an observer,

same thing, interacts with the system

that it’s observing, it doesn’t just look at it.

It becomes physically entangled with it.

And it’s that entanglement which we call an observation

or a measurement.

Now, does that satisfy me personally as an observer?

Yes and no.

I find it very satisfying

that we have a mathematical representation

of what it means to observe a system.

You are observing stuff right now, the conscious level.

Do you think there’s echoes of that kind of entanglement

in our macro scale?

Yes, absolutely, for sure.

We’re entangled with,

quantum mechanically entangled with everything in this room.

If we weren’t, then it would just,

well, we wouldn’t be observing it.

But on the other hand, you can ask,

do I really, am I really comfortable with it?

And I’m uncomfortable with it in the same way

that I can never get comfortable with five dimensions.

My brain isn’t wired for it.

Are you comfortable with four dimensions?

A little bit more,

because I can always imagine the fourth dimension is time.

So the arrow of time, are you comfortable with that arrow?

Do you think time is an emergent phenomena

or is it fundamental to nature?

That is a big question in physics right now.

All the physics that we do,

or at least that the people that I am comfortable

with talking to, my friends, my friends.

No, we all ask the same question that you just asked.

Space, we have a pretty good idea is emergent

and it emerges out of entanglement and other things.

Time always seems to be built into our equations

as just what Newton pretty much would have thought.

Newton, modified a little bit by Einstein,

would have called time.

And mostly in our equations, it is not emergent.

Time in physics is completely symmetric,

forward and backward.

Right, it’s symmetric.

So you don’t really need to think about the arrow of time

for most physical phenomena.

For most microscopic phenomena, no.

It’s only when the phenomena involve systems

which are big enough for thermodynamics to become important,

for entropy to become important.

For a small system, entropy is not a good concept.

Entropy is something which emerges out of large numbers.

It’s a probabilistic idea or it’s a statistical idea

and it’s a thermodynamic idea.

Thermodynamics requires lots and lots

and lots of little substructures, okay?

So it’s not until you emerge at the thermodynamic level

that there’s an arrow of time.

Do we understand it?

Yeah, I think we understand better

than most people think they have.

Most people say they think we understand it.

Yeah, I think we understand it.

It’s a statistical idea.

You mean like second law of thermodynamics,

entropy and so on?

Yeah, take a pack of cards and you fling it in the air

and you look what happens to it, it gets random.

We understand it.

It doesn’t go from random to simple.

It goes from simple to random.

But do you think it ever breaks down?

What I think you can do is in a laboratory setting,

you can take a system which is somewhere intermediate

between being small and being large

and make it go backward.

A thing which looks like it only wants to go forward

because of statistical mechanical reasons,

because of the second law,

you can very, very carefully manipulate it

to make it run backward.

I don’t think you can take an egg, a Humpty Dumpty

who fell on the floor and reverse that.

But you can, in a very controlled situation,

you can take systems which appear to be evolving

statistically toward randomness,

stop them, reverse them, and make them go back.

What’s the intuition behind that?

How do we do that?

How do we reverse it?

You’re saying a closed system.

Yeah, pretty much closed system, yes.

Did you just say that time travel is possible?

No, I didn’t say time travel is possible.

I said you can make a system go backward.

In time.

You can make it go back.

You can make it reverse its steps.

You can make it reverse its trajectory.


How do we do it?

What’s the intuition there?

Does it have, is it just a fluke thing

that we can do at a small scale in the lab

that doesn’t have?

Well, what I’m saying is you can do it

a little bit better than a small scale.

You can certainly do it with a simple, small system.

Small systems don’t have any sense of the arrow of time.

Atoms, atoms are no sense of an arrow of time.

They’re completely reversible.

It’s only when you have, you know,

the second law of thermodynamics

is the law of large numbers.

So you can break the law because it’s not

a deterministic law. You can break it,

you can break it, but it’s hard.

It requires great care.

The bigger the system is, the more care,

the more, the harder it is.

You have to overcome what’s called chaos.

And that’s hard.

And it requires more and more precision.

For 10 particles, you might be able to do it

with some effort.

For a hundred particles, it’s really hard.

For a thousand or a million particles, forget it,

but not for any fundamental reason,

just because it’s technologically too hard

to make the system go backward.

So, no time travel for engineering reasons.

Oh, no, no, no, no.

What is time travel?

Time travel to the future?

That’s easy.

You just close your eyes, go to sleep,

and you wake up in the future.

Yeah, yeah, a good nap gets you there, yeah.

A good nap gets you there, right.

But reversing the second law of thermodynamics,

going backward in time for anything that’s human scale

is a very difficult engineering effort.

I wouldn’t call that time travel

because it gets too mixed up

with what science fiction calls time travel.

This is just the ability to reverse a system.

You take the system and you reverse the direction

of motion of every molecule in it.

That, you can do it with one molecule.

If you find a particle moving in a certain direction,

let’s not say a particle, a baseball,

you stop it dead and then you simply reverse its motion.

In principle, that’s not too hard.

And it’ll go back along its trajectory

in the backward direction.

Just running the program backwards.

Running the program backward.

Yeah. Okay.

If you have two baseballs colliding,

well, you can do it,

but you have to be very, very careful to get it just right.

If you have 10 baseballs, really, really, better yet,

10 billiard balls on an idealized,

frictionless billiard table.

Okay, so you start the balls all on a triangle, right?

And you whack them.

Depending on the game you’re playing,

you either whack them or you’re really careful,

but you whack them.

And they go flying off in all possible directions.

Okay, try to reverse that.

Try to reverse that.

Imagine trying to take every billiard ball,

stopping it dead at some point,

and reversing its motion

so that it was going in the opposite direction.

If you did that with tremendous care,

it would reassemble itself back into the triangle.

Okay, that is a fact.

And you can probably do it with two billiard balls,

maybe with three billiard balls if you’re really lucky.

But what happens is as the system

gets more and more complicated,

you have to be more and more precise

not to make the tiniest error,

because the tiniest errors will get magnified

and you’ll simply not be able to do the reversal.

So yeah, but I wouldn’t call that time travel.

Yeah, that’s something else.

But if you think of it, it just made me think,

if you think the unrolling of state

that’s happening as a program,

if we look at the world,

silly idea of looking at the world as a simulation,

as a computer.

But it’s not a computer, it’s just a single program.

A question arises that might be useful.

How hard is it to have a computer that runs the universe?

Okay, so there are mathematical universes

that we know about.

One of them is called anti de Sitter space,

where we, and it’s quantum mechanics,

I think we could simulate it in a computer,

in a quantum computer.

Classical computer, all you can do is solve its equations.

You can’t make it work like the real system.

If we could build a quantum computer, a big enough one,

a robust enough one, we could probably simulate a universe,

a small version of an anti de Sitter universe.

Anti de Sitter is a kind of cosmology.

So I think we know how to do that.

The trouble is the universe that we live in

is not the anti de Sitter geometry,

it’s the de Sitter geometry.

And we don’t really understand its quantum mechanics at all.

So at the present time,

I would say we wouldn’t have the vaguest idea

how to simulate a universe similar to our own.

No, we can ask, could we build in the laboratory

a small version, a quantum mechanical version,

the collection of quantum computers

and tangled and coupled together,

which would reproduce the phenomena that go on in the universe,

even on a small scale.

Yes, if it were anti de Sitter space,

no, if it’s de Sitter space.

Can you slightly describe de Sitter space

and anti de Sitter space?


What are the geometric properties of?

They differ by the sign of a single constant

called the cosmological constant.

One of them is negatively curved,

the other is positively curved.

Anti de Sitter space, which is the negatively curved one,

you can think of as an isolated system

in a box with reflecting walls.

You could think of it as a system

of quantum mechanical system isolated

in an isolated environment.

De Sitter space is the one we really live in.

And that’s the one that’s exponentially expanding,

exponential expansion, dark energy,

whatever we wanna call it.

And we don’t understand that mathematically.

Do we understand?

Not everybody would agree with me,

but I don’t understand.

They would agree with me,

they definitely would agree with me

that I don’t understand it.

What about, is there an understanding of the birth,

the origin, the big bang?

So there’s one problem with the other.

No, no, there’s theories.

There are theories.

My favorite is the one called eternal inflation.

The infinity can be on both sides,

on one of the sides and none of the sides.

So what’s eternal infinity?


Infinity on both sides.

Oh boy.

Yeah, yeah, that’s.

Why is that your favorite?

Because it’s the most just mind blowing?


Because we want a beginning.

No, why do we want a beginning?

In practice there was a beginning, of course.

In practice there was a beginning.

But could it have been a random fluctuation

in an otherwise infinite time?


In any case, the eternal inflation theory,

I think if correctly understood,

would be infinite in both directions.

How do you think about infinity?

Oh God.

So, okay, of course you can think about it mathematically.

I just finished this discussion with my friend Sergei Brin.

How do you think about infinity?

I say, well, Sergei Brin is infinitely rich.

How do you test that hypothesis?


Such a good line.


Yeah, so there’s really no way

to visualize some of these things.

Yeah, no, this is a very good question.

Does physics have any,

does infinity have any place in physics?


Right, and all I can say is very good question.

So what do you think of the recent first image

of a black hole visualized from the Horizon Telescope?

It’s an incredible triumph of science.

In itself, the fact that there are black holes

which collide is not a surprise.

And they seem to work exactly

the way they’re supposed to work.

Will we learn a great deal from it?

I don’t know, we might.

But the kind of things we’ll learn

won’t really be about black holes.

Why there are black holes in nature

of that particular mass scale and why they’re so common

may tell us something about the structure,

evolution of structure in the universe.

But I don’t think it’s gonna tell us

anything new about black holes.

But it’s a triumph in the sense

that you go back 100 years

and it was a continuous development,

general relativity, the discovery of black holes,

LIGO, the incredible technology that went into LIGO.

It is something that I never would have believed

was gonna happen 30, 40 years ago.

And I think it’s a magnificent structure,

magnificent thing, this evolution of general relativity,

LIGO, high precision, ability to measure things

on a scale of 10 to the minus 21.

So, astonishing.

So you’re just in awe that this path

took us to this picture.

Is it different?

You’ve thought a lot about black holes.

How did you visualize them in your mind?

And is the picture different than you’ve visualized it?

No, it’s simply confirmed.

It’s a magnificent triumph to have confirmed

a direct observation that Einstein’s theory of gravity

at the level of black hole collisions actually works

is awesome, it is really awesome.

I know some of the people who are involved in that.

They’re just ordinary people.

And the idea that they could carry this out,

I just, I’m shocked.

Yeah, just these little homo sapiens?

Yeah, just these little monkeys.

Yeah, got together and took a picture of…

Slightly advanced limer’s, I think.

What kind of questions can science not currently answer

but you hope might be able to soon?

Well, you’ve already addressed them.

What is consciousness, for example?

You think that’s within the reach of science?

I think it’s somewhat within the reach of science,

but I think that now I think it’s in the hands

of the computer scientists and the neuroscientists.

Not a physicist, with the help.

Perhaps at some point, but I think physicists

will try to simplify it down to something

that they can use their methods

and maybe they’re not appropriate.

Maybe we simply need to do more machine learning

on bigger scales, evolve machines.

Machines not only that learn

but evolve their own architecture.

As a process of learning, evolve in architecture.

Not under our control, only partially under our control,

but under the control of machine learning.

I’ll tell you another thing that I find awesome.

You know this Google thing that they taught

the computers how to play chess?

Yeah, yeah.

Okay, they taught the computers how to play chess,

not by teaching them how to play chess,

but just having them play against each other.

Against each other, self play.

Against each other, this is a form of evolution.

These machines evolved, they evolved in intelligence.

They evolved in intelligence

without anybody telling them how to do it.

They were not engineered, they just played

against each other and got better and better and better.

That makes me think that machines can evolve intelligence.

What exact kind of intelligence, I don’t know.

But in understanding that better and better,

maybe we’ll get better clues as to what goes on

in our own intelligence.

What life in intelligence is.

Last question, what kind of questions can science

not currently answer and may never be able to answer?


Is there an intelligence out there

that’s underlies the whole thing?

You can call them with a G word if you want.

I can say, are we a computer simulation with a purpose?

Is there an agent, an intelligent agent

that underlies or is responsible for the whole thing?

Does that intelligent agent satisfy the laws of physics?

Does it satisfy the laws of quantum mechanics?

Is it made of atoms and molecules?

Yeah, there’s a lot of questions.

And I don’t see, it seems to me a real question.

It’s an answerable question.

Well, I don’t know if it’s answerable.

The questions have to be answerable to be real.

Some philosophers would say that a question

is not a question unless it’s answerable.

This question doesn’t seem to me answerable

by any known method, but it seems to me real.

There’s no better place to end.

Leonard, thank you so much for talking today.

Okay, good.

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