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Language: en
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's 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 have 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
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friedman spelled fri d
ma a.m. and now here's my conversation
with leonard susskind
you work two more friends with richard
fineman house 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 a
closed his eyes and visualize the
phenomena that he was thinking about and
through visualization
I'll flank the mathematical 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 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 even 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 math and then
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 into mathematics and I am
and yet you've worked very
counterintuitive ideas so how that's
true that's naive is something Connor
into every their rewiring your brain in
new ways yeah
quantum mechanics is not intuitive very
little of modern physics is intuitive
intuitive or what does intuitive mean it
means the ability to think about it with
basic classical physics the physics that
that we evolved with throwing stones
splashing water or 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
find many 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 I'm sort of
evolved evolved for you 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 an 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 dimension in space
if I want to visualize a line I think of
the line as being aligned in three
dimensions right well I think of the
line as being aligned 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 dimensions
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 and we learn ways we learn
mathematical ways and we learn ways to
visualize them but they're different and
so yeah I think 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 there
was a tour it's completely natural so
I'm sure there's some what you could
argue creatures that live in it
two-dimensional space yeah and there are
and um well it's romanticizing the
notion of course we're all living as far
as we know in three-dimensional space
but how do you how do those creatures
imagine 3d space well probably the way
we imagined 4d by using some mathematics
and some equations and some some tricks
okay so jumping back to a fireman just
for a second he had a little bit of an
ego yes
what 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 it was their time where you complete
you felt you looked at yourself and
asked am i completely wrong about this
oh yeah but the whole thing about
specific things the whole thing that way
which cold thing me and me and my
ability to do this thing
oh those kinds of doubts those 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 of my they were just
the discomfort and being in an
environment that my family hadn't
participated in I know 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 I never felt this insecurity am I
ever gonna get a job that veteran never
occurred to me that I wouldn't maybe you
could speak a little bit to this sense
of what is academia for because I do
feel a bit uncomfortable in it mm-hm
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 what 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 yeah 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 in the
world I wasn't the young man I was
probably 50 years old you were never
quite it was a phase transition you were
never quite free milk in the middle yeah
that's right I wasn't I always felt a
little bit of an outsider the beginning
a lot and outside Earth
my way of thinking was different
my approach to mathematics was different
but also this my social background that
I came from was different now these days
half the young people I meet their
parents were professors my that was not
my case so yeah but then all of a sudden
at some point I found myself at they're
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 put away I
mean I went away in a flash so maybe
maybe a little bit with Fineman but in
general how do you develop ideas do you
work their ideas alone do you brainstorm
with others oh both both very definitely
both the earth the younger time I spent
more time with myself now because I'm at
Stanford because I'm because I have a
lot of ex students and I you know people
who 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 it's a
very important part I spend less time
probably completely self focused and
the 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 that
everything is made of quantum mechanics
but it's not some functioning it's not a
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 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 what you're doing when you
program something for quantum system is
they're actually building a real version
of the system the limits of a classical
computer classical computers are
enormous ly limited when it comes to the
quantum systems enormously limited
because you 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 path
with 400 pennies in my pocket so we'll
be able to simulate the quantum state of
400 elementary quantum systems qubits we
call him 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 right 400 cubits on the other
hand if your quantum computer is
composed of four hundred qubits it can
do everything four hundred 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 PE so on what
kind of algorithm is just intuitively do
you think it's you know mocks for us
okay so we know that there are a handful
of algorithms that can seriously be
quantum of classical computers and which
can have exponentially more power and
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 others
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 the quantum
system you make measurements on it
quantum measurements on it the
advantages you can run it much slower
you could say why bother why not just
use the real system and why not just do
experiments on the real system well real
systems are kind of limited you can't
change them you can't
like them you can't slow them down so
that you can poke into them you can't
modify them an arbitrary kinds of ways
to see what would happen if I if I
change the system a little bit so I
think that quantum computers will be
extremely valuable in
in understanding quantum systems at the
lowest of the fundamental laws they're
actually satisfying the same laws as the
systems that they're simulating that's
right okay so in the one hand you have
things like factoring in factoring is
the great thing of quantum computers
factoring large numbers that doesn't
seem that much to do with quantum
mechanics right it seems to be almost a
fluke that a quantum computer can solve
the factoring problem in a short time so
though 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 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 you
know solve equations by the standard
methods of pencil and paper and 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 or we can change them
manipulate them make measurements on
them and find out all the things we want
to know
so in finding out the things we want to
know about very small systems right now
the is there something we can also find
out about the macro level about
something about it the function and
forgive me of our brain biological
systems the the stuff that's about one
meter in size versus much much smaller
well what the only excitement is about
among the people that I interact with is
understanding black holes that falls
black holes are big things there 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
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 I bought stuff that's in the middle
of the - it's different so but black
holes are there's something fundamental
about black holes it feels to be very
different in the brain yes and they also
function in a very quantum mechanical
way right okay it is first of all
unclear to me but of course it's unclear
to me I another I'm not a 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 there is not intrinsically a
quantum mechanical system or doesn't
make use of the of the special features
entanglement coherent superposition are
they right
I don't know I sort of hope that wrong
with just because I like the romantic
idea that the brain is a quantum system
and but I think that I think probably
not
the other thing big systems can be
composed of lots of little systems
materials the materials are that we work
with and so forth are three large
systems and a large piece of material
but they're Bagan 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 with discovering
materials and quantum systems which
function much more quantum mechanically
then than we imagine topological
insulators this kind of thing that kind
of thing those are macroscopic systems
but they just superconductors
superconductors I have a lot of quantum
mechanics in them you can have a large
chunk of superconductor so it's a big
decent 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 mm-hmm and looking at the
universe as a information processing
system as a computer as 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 or information
processes there's no extra magic to us
humans it certainly feels consciousness
intelligence feels like magic
sure though where does it emerge from if
we look at information processing what
are the emergent phenomena that come
from viewing the world is an information
processing system here is what I think
my thoughts are not worth much of this
if you ask me about physics my thoughts
may be worth something
yes 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 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
the 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
the brain yeah doesn't seem to we where
when my brain works my brain seems to
work 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 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 but how will we get it right I
think maybe computer scientists will get
it right eventually I don't think that
any ways near it I don't even think
they're thinking about it but by
computer 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 will
evolve systems
and we may start to discover mechanisms
that that have implications for how we
think and for what what does
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 by
introspection so that's a really
interesting point you've in many cases
if you look even at string theory when
you first think about a system it seems
really complicated like the human brain
and through some basic reasoning then
trying to discover a fundamental
low-level behavior of the system you
find out that it's actually much simpler
you one have you you know is that
generally the process and to 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 an
analyzer yeah I mean there are lots of
examples where physics has taken very
complicated systems analyze them and
found simplicity in them for sure I said
superconductors before it's an obvious
one a superconductor seems like
monstrously complicated thing with all
sorts of crazy electrical properties
magnetic properties and so forth and
when it finally is boiled down through
its simplest elements it's a very simple
quantum mechanical phenomenon called
spontaneous symmetry breaking and which
we in other context 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
right 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
that it doesn't seem to be any
theoretical reason why we can't
artificially create a 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
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
ah not for Google for Google X I am the
senior academic advisor third to a group
of machine learning physicists at 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 myth there is a real machine
learning problem mainly 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 have any right to expect
the machine learning systems are systems
that the system is not too different
than the kind of systems if this is a
study there's not all that much
difference between quantum construction
of mathematics physically yes but in the
structure the mathematics between a
tension network designed to describe a
quantum system on the one hand and the
kind of networks that are used in
machine learning so they're 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 having lunch on having
lunch 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 those physics way of thinking I
think will add will I had great value to
UM
will bring value to the machine learning
I believe it will and I think it already
has and what time scale do you think
predicting the future becomes useless in
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 for 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 we're 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 for me 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
problems are 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 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 it's
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 theorists 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 has had a number
of main uses originally string theory
was invented then 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 hey groans
hey drones are sub-nuclear particles
protons neutrons mesons and
at that time the late 60s early
seventies it was clear from experiment
that these particles call hydrants 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 hydrants 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 nineteen 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 to find 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 that nineteen orders of
magnitude too small or things that were
interested in but we can test them out
mathematically and analyze their
internal consistency by now forty years
ago thirty five years ago 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 um that certainty
that quantum mechanics and gravity can
coexist that's not a small thing that's
a very huge thing it's a huge thing
Einstein be proud
Einstein he might be appalled I don't
know I'm like a very much yeah he would
certainly be struck by it yeah I think
that maybe 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 theme of quantum
mechanics is there something exactly
yeah yeah I think string theory is just
an example of a quantum mechanical
system that contains both gravitation
and in quantum mechanics so is there
something underlying quantum mechanics
perhaps something deterministic so have
something deterministic my friend far
out it wolf whose name you may know he's
a very famous physicist Dutch not as
famous as he should be but the heart
dispels names it's hard to say his name
you know it's easy to spelling ' he's
only person I know his name begins with
an apostrophe and he's one of my heroes
in physics and it's a little younger
than me but it's nonetheless one of my
heroes
the Tufte believes that there was some
sub structure to the world which is
classical in character the deterministic
in character which somehow by some
mechanism that he has a hard time
spelling out emerges as quantum
mechanics I don't the wavefunction is
somehow emergent the wavefunction and
not just the wavefunction but the whole
making the whole thing that goes with
quantum mechanics uncertainty and pango
meant all these things are emergent do
you think quantum mechanics is the
bottom of the well as is the right here
I think is 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 yes I think we 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 a tuft is a Sperry
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 unobserved
is 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 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 and I am as puzzled
as anybody there's echoes of it in the
observer effect yeah 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'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 measure
now does that satisfy me personally as
an observer hmm 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
yeah the conscious level right is 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 of
and it was just well we wouldn't be
observing it but on the other hand you
can ask though 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 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
this time so the arrow of time are you
comfortable with that arrow do you think
time is an emergent phenomena or is it's
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 my friends yeah my friends
no we all ask the same question that you
just asked in space we have a pretty
good idea is emergent and it emerges out
of tan tangle mint and other other
things
time always seems to be built into our
equations as just what Newton pretty
much were for Newton modified a little
bit by Einstein would have called time
and and mostly in our equations it is
not emergent time in physics is
completely symmetric forward and
bathymetric so you don't really need to
think about the area of time for most
physical phenomena the most microscopic
phenomena no it's only when the
phenomena involves systems which are big
enough for thermal
Amyx to become important the entropy to
become important for small subsets a
small system entropy is not a good
concept an entropy is something which
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 sub structures 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 I think we
understand better than most people think
that most people say they think we
understand it yeah I think we understand
it it's just a statistical idea the you
mean like second law thermodynamics
entropy and so on yeah the pack of cards
and you're flinging it in the air and
you look what happens to it yeah but
what's random we understand 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 Humpty Dumpty who fell on the
floor yeah 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
how do we do that how do we reverse it a
clue you're saying 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 and you don't like it go back
you can make it reverse it steps you can
make it reverse its trajectory yeah how
do we do 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 uh no sense of
the 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 say you can
break the law because it's not you can
break German isn't the break it but it's
hard it requires great care the bigger
the system is the more to 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 a with some effort 400 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 so note no time
travel for engineering reasons oh no no
no what is time travel time travel time
travel to the future that's easy
yes you just close your eyes go to sleep
and you wake up in the future yeah yeah
good nap gets you there yeah good map
gets you there right
but in reversing the second law of
thermal is a very difficult engineering
effort I wouldn't call that time travel
because it gets to me too mixed up with
what the science fiction calls
time-travel right 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
input you can do it with one molecule if
you find a particle moving in a certain
direction let's not say a mama 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 baseball's colliding
well you can do it but you have to be
very very careful to get it just right
now ten baseball's really really tore
better yet tend ten billiard balls on an
idealized frictionless billiard table
mm-hmm
okay so you start the balls all in a
triangle right and you're whack them yep
depending on the game you're playing you
the wacom where you're really careful
but the you're welcome
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 dated
sometime at some point and reversing its
motion so 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
billion 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 it just made me
think if we think the unrolling of state
that's happening as a program if we look
at the world so the 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 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 well I think
we could simulate it in a computer and 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 or
big enough one robust enough one we
could probably simulate a universe a
small version of an anti-de sitter
universe and that the 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 decent or geometry and
we don't really understand the 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 you know we could ask oh we
could we build in the laboratory a small
version a quantum mechanical version the
collection of quantum computers
entangled and the couple together which
would reproduce the the phenomena that
go on in the universe even on a small
scale yes if you were anti de sitter
space know if it's the sitter space can
you a slightly describe the sitter space
and anti-de sitter space yeah what are
the geometric properties of big
different they differ by a this is the
sine of a single constant called the
cosmological constant one of them is
negatively curved the other is
positively curved the 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 the sitter space is
the one we really live in and that's the
one that's exponentially expanding
exponential expansion dark energy
whatever you 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 their an understanding of the
birth the origin no the bing bang so
knows what normally 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 my real opinion
okay
infinity on both sides oh boy yeah yeah
that's why is that your favorite because
it's the the most just mind-blowing no
because we want a beginning no why do we
want a beginning I practiced it was the
beginning of course and practice it was
a beginning but could it have been a
random fluctuation in an otherwise
infinite time maybe in any case the 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 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 essential good lines all
right yeah so there's no there's really
no way to visualize some of these things
like ya know this is a very good
question those physics have any is does
infinity have any place in physics right
right and well I can say is very good
question
so what do you think of the recent first
image of a black hole visualized from
the event horizon telescope it
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 I
I we might but the kind of things we
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 gonna happen you know 30 40
years ago and I think it's a magnificent
the 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 so
you're just astonishing though we zoom
all this just happy for us to this right
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
know lies that no it simply confirmed
you know it's a magnificent triumph to
have confirmed confirmed a direct
observation yeah that Einstein's theory
of gravity at the level of black hole
collisions actually works is awesome and
it's really awesome you know I know some
of the people who were involved in that
they just thought married people yeah
and the idea that they could carry this
out I just don't I'm shocked yeah just
these little Homo sapiens
yeah just these little monkeys got
together right and took a picture of
slightly advanced lemurs I think what
kind of questions can science not
currently answer but you hope might be
able to soon well you you've already
addressed them what is consciousness for
example do 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 I'm not a physicist
perhaps with the helper haps 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
volve 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 a 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 itself against each other this is
a form of evolution these machines
evolved they evolved and intelligence
they evolved in intelligence without
anybody telling them how to do it and
we're 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
there goes on your life and intelligence
is last question what kind of questions
can science not currently answer and may
never be able to answer yeah
is there an intelligence out there
that's underlies the whole thing you can
call them with the 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 this it seems
to me a real question
it's an answerable question well 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
monitor thank you so much for talking
about okay
you