The following is a conversation with Leonard Suskind.
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
is 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,
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And now, here's my conversation with Leonard Suskind.
You worked and were friends with Richard Feynman.
How has he influenced you and 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, I would flank 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, you can 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 or 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. Experiences that other people are better
at converting it to mathematics than I am. And yet, you've worked
very counterintuitive ideas. So, how do you... No, that's true.
So, how do you 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.
What does intuitive mean? It means the ability to think about it with
basic classical physics, the physics that we evolved with,
throwing stones, splashing water, whatever it happens to be.
Quantum physics, general relativity, quantum field theory are
deeply unintuitive in that way, but you know, 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're 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 want to 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 going to happen.
The reason is, I think, 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.
I think we do rewire ourselves, whether we can ever completely
rewire ourselves to be completely comfortable with these concepts,
I doubt. So that's completely natural.
To where it's completely natural. I'm sure there's
somewhat, you could argue, creatures that live in a two-dimensional
space. Yeah, maybe there are.
While it's romanticizing the notion, of course, 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
some tricks. Okay, so jumping back to Feynman just for a second,
he had a little bit of an ego.
Yes. Do you think ego is powerful or dangerous in science?
I think 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. Wait, which whole thing?
Me and me and my ability to do this thing. Oh, those kinds of doubts.
Well, 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, well, for a long time.
But there weren't doubts about my ability or my...
They were just the discomfort and 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.
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 going to 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.
You know, 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.
Right. That was not my case.
So yeah, but then all of a sudden at some point I found myself at the 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. I mean, 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
with a piece of paper and just sitting there staring at it.
What are your hopes for quantum computers?
Machines that have some elements of leveraged quantum mechanical
ideas. It's not just leveraging quantum mechanical ideas.
You can simulate quantum systems on a classical computer.
Simulate them means solve this 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 the quantum state
of 400 spins, that's not very many. 400 can put in my pocket.
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 anymore 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 be classical computers and which can have
exponentially more power. This is a mathematical statement.
Nobody's exhibited this in the laboratory. That'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 the simulated 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 the quantum system.
You make measurements on it, quantum measurements on it.
The advantage is you can run it much slower.
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, the fundamental
laws. They're actually satisfying the same laws
as the systems that they're simulating. That's right.
In 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.
Those problems seem to be extremely special,
rare. It's not clear to me that there's going to be a lot of them.
On the other hand, there are a lot of quantum systems. There's chemistry,
there's solid state physics, there's material science,
there's quantum gravity, there's all kinds of quantum field theory.
Some of these are actually turning out to be applied sciences as well as
very fundamental sciences. 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.
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 want
to know. In finding out the things we want to know
about very small systems, is there something 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 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.
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. We're using that relationship.
You didn't ask about quantum computers as 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
brains? Yes, 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 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 of the
special features, entanglement, coherent 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 big and they're made out of
quantum systems. Now, one of the things that's been happening over
the last a good number of years is we're discovering
materials and quantum systems which function much more quantum
mechanically than we imagine. Topological insulators, this
kind of thing, that kind of thing. Those are macroscopic systems, but they
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? But 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. 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 and 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 counter-intuitive.
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.
It doesn't seem to me 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 that are...
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.
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
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 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.
Is that generally the process and 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. 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.
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 super human 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 super human 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 there is a real
machine learning problem namely why does it work as well as it does.
It 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 had any right to expect.
The machine learning systems are systems that the system is not too
different than the kind of systems that physicists study.
There's not all that much difference between in the structure of the
mathematics physically yes but in the structure of the
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 right yeah 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
will add great value to 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
and your long experience and being surprised at new discoveries?
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 going to 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 we 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 theorist. 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 as 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 there 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 sub-nuclear particles.
Protons, neutrons, mesons and
at that time the late 60s early 70s
it was clear from experiment that these particles called Hadrons had
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 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 have 19 orders of magnitude
too small of 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.
It's provided that certainty that quantum mechanics and gravity
can coexist. That's not a small thing. It's a huge thing.
Einstein will be proud. Einstein, he might 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? Exactly.
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.
Perhaps something deterministic. My friend Ferrad Etouft 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 a
metastrophe. And he's one of my heroes in physics. He's a little
younger than me but he's nevertheless one of my heroes.
Etouft believes that there is some sub-structure 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 and the whole, not just the wave function but the whole
mech and 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? 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 it to be
anything beyond quantum mechanics. I think Etouft 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.
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 it,
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, 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 at the people that I am comfortable
talking to, my friends. 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 back.
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 involves
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, it's a statistical idea,
and it's a thermodynamic idea. Thermodynamics requires lots and lots
and lots of little substructures. 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 are. Most people say they think we understand it.
Yeah, I think we understand it. It's just a statistical idea.
You mean like second law, thermodynamics, entropy and so on?
Yeah, you take a pack of cards and you fling it in the air and you look what
happens to it. It gets random. 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-humpy-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.
Yeah. 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? What I'm saying is you can do it on 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 deterministic. You can break it, but it's hard.
It requires great care. The bigger the system is, the more care and 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 100
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? No, no, no, no. What is time travel?
Time travel to the future? That's easy. Just close your eyes,
go to sleep, and you wake up in the future. Yeah, yeah. The good nap gets you
there, yeah. The good nap gets you there, right. But
in 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.
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, or better yet,
10 billiard balls on an idealized frictionless billiard table.
Okay, so you start the balls all in a triangle, right? Yep.
And you whack them. Yep. 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, 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, you could, that, but I wouldn't call that time travel.
Yeah, that's something else, but if you think, think of 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-dissider 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-dissider
universe. Anti-dissider 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-dissider geometry, it's the
dissider 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 could ask, could we build in the laboratory a small version,
a quantum mechanical version, the collection of quantum computers entangled
and coupled together, which would reproduce the phenomena that go on
in the universe, even on a small scale. Yes, if it were anti-dissider space,
no if it's dissider space. Can you slightly describe
dissider space and anti-dissider space? Yeah.
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.
The anti-dissider 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. Dissider space is the one we really
live in, and that's the one that's exponentially expanding.
Exponential expansion, dark energy, whatever we want to 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? No.
The bing-bang, so there's one problem with the other.
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? Okay.
Infinity on both sides. Oh boy. Yeah, yeah, that's...
Why is that your favorite? Because it's the most just mind-blowing? No.
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?
Maybe. In any case, the eternal inflation theory, I think if correctly understood,
it would be infinite in both directions. How do you think about infinity?
Oh God. So, okay, of course you can think about mathematically.
I just finished this discussion with my friend Sergey Brin.
Yes. How do you think about infinity? I say, well, Sergey Brin is infinitely rich.
How do you test that hypothesis? Okay. That's such a good line.
Right. Yeah, so there's really no way to visualize some of these things.
Yeah. Now, this is a very good question. Does infinity have any place in physics?
Right. Right. And all I can say is a very good question.
What do you think of the recent first image of a black hole visualized from the event 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. I can't. 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 going to tell us anything new about black holes.
But it's a triumph in the sense that you go back a hundred 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 going to happen 30, 40 years ago.
40 years ago. And I think it's a magnificent structure, a magnificent thing, this evolution of
general relativity, LIGO, high precision, ability to measure things on a scale of 10 to the minus
21. So, yeah. So you're just astonishing though we've done this path took us to this picture.
Is it different? You know, you've thought a lot about black holes. Is it how did you visualize
them in your mind? And is the picture different than you can tell us that?
No, no. It's simply confirmed. You know, 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. You know, 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'm shocked.
Yeah, just these little homo sapiens. Yeah, just these little monkeys got together and took a picture
of slightly advanced limers, 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. Perhaps 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 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 an architecture, not under our control, only
partially under our control, but under the control of a machine learning. I'll tell you
another thing that I find awesome. You know, this Google Bing that they taught computers
how to play chess. Yeah, yeah, okay. They taught 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. Well, life in intelligence is last question. What kind of
questions can science not currently answer and may never be able to answer? Yeah.
Is there intelligence out there that's underlies the whole thing? You can call in 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. 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.