good evening everybody
is this one is this on
hi oh it is working
hi
good evening
i'll wait maybe a few more seconds while
people settle in
but as you do uh let me introduce myself
i'm james and elitis i am chair of the
physics department and it is my pleasure
to introduce uh the 23rd annual annual j
robert oppenheimer lecture
um
[Applause]
so what i'm going to do right now is
i'll give you an introduction to what
the openheimer lecture is about before i
hand over to my colleague
professor rafael busso to introduce
tonight's speaker
the oppenheimer
annual series was established in 1998
and is made possible by the generosity
of jane and robert wilson as well as
steve and eileen krieger
the series was brought has brought a
who's who of of physicists theoretical
physicists from around the world
past oppenheimer lecturers have included
cn yang freeman dyson helen quinn
charlie kane andre linde
murray galman stephen hawking kept
thorne and marvin cohen who's joining us
via zoom i believe
many prominent theorists in the fields
of particle physics condensed
metaphysics astrophysics cosmology and
amo have stood
where i am standing today
robert oppenheimer was born in 1904
growing up in an upper-class family in
manhattan he graduated from harvard
majoring in chemistry entered cambridge
the other cambridge across the atlantic
uh in 1924 as a graduate student hoping
to work with ernest rutherford who many
of you may know as the second most
famous new zealand physicist
um
he was then
leaving in 1926 and so oppenheimer
finished his phd with max born in
gottingen
he published more than a dozen papers
while with born
mostly focused on the new theory at the
time of quantum mechanics this included
his most famous born oppenheimer
approximation that simplifies molecular
physics by separating slow nuclear
motion from the faster electron motion
more than 90 years ago in 1929 after two
years of postdoctoral study mostly in
europe oppenheimer returned to the u.s
he accepted an associate professorship
right here in berkeley
where he remained for nearly 15 years
during this period he published his
famous paper with volkov establishing
the tolman oppenheimer volkov limit on
the maximum mass of a neutron star
the mass above which a star must
collapse into a black hole
he also developed the theory of that
collapse and black hole formation both
topics are of keen interest today as
many of you may know with the recent
observation of gravitational waves from
black hole and neutron star mergers
at berkeley oppenheimer's group
typically eight to ten graduate students
and half a dozen post docs met with
oppenheimer every day
with open while open height with
oppenheimer probing them about their
progress
hans baker noted that probably the most
important ingredient he brought to his
teaching was his exquisite taste he
always knew where the important problems
are
the scientific leadership of oppenheimer
demonstrated at berkeley complicated his
later life and his role in science as
many of you know
in 1942 he was selected to lead world
war ii's manhattan project's engineering
lab cited at los alamos near a ranch
oppenheimer owned
his leadership of his effort culminated
in the successful trinity test and later
the political decision to use atomic
weapons against japan a decision that
troubled oppenheimer for the rest of his
life
after world war ii oppenheimer became
the public face of science and
technology featured on the covers of
time and life magazines
this period of his life came to a close
with a controversial loss of his
security clearance in 1954. that's his
badge shown to the right there
a time when the new cold war and
mccarthyism were stoking fears
there was some resolution about a decade
later when president kennedy presented
oppenheimer with the nation's fermi
award
my notes say kennedy but the but the
picture clearly shows lyndon johnson i
think but anyway
one of them definitely gave him the
award
oppenheimer's legacy at berkeley is a
simpler one summarized by a plaque on
the fourth floor of physics south hall
with another quote from hans beta
in these corners in these corner offices
1929 to 1942 j robert oppenheimer
created the greatest school of
theoretical physics the world has ever
known
berkeley physics strives to continue
this legacy today
so with that introduction it is now my
great pleasure to introduce my colleague
from uc berkeley physics professor
rafael busso to introduce tonight's
oppenheimer lecturer
dr lenny suskind
thank you james
well it's a great pleasure and honor to
introduce
dr leonard suskind
lenny is a legend he's also a friend and
a mentor
to me personally
um
he has made many great discoveries and
don't worry i will not tell you about
all of them
he's won great prizes like the sakurai
prize
what i want to talk about is how that's
really just half of the picture
lenny more than
almost any other
great theoretical physicist that i've
had the privilege to meet
has taught us how to think
how to pick important topics
how to tackle them
and the best way that i can try to
convey that is by by telling you the
story of how i first met lenny which he
probably doesn't actually know
um this was in in 1994 and i was
an undergrad i was trying to decide
between grad school at stanford or in
cambridge england i didn't get into
berkeley
is anybody here who
no
anyway so
so i went to stanford to sort of check
the place out and
my hope was to meet with andre linde but
he had broken his leg so he couldn't
meet with me after
after all
but but this guy comes running in
wearing running shorts sweaty shirt
um
and uh
you know i tell him i'm thinking about
grad school i'm interested in quantum
cosmology and black holes and so on and
and he sits me down and he gives me this
this very clear lecture about
how stephen hawking is completely wrong
about black holes
and
this whole idea of the wave function of
the whole universe there's nobody you
can see the whole universe doesn't make
any sense
um and
this completely shocked me i'd already
spent a little bit of time in cambridge
and you know it's a place that
definitely sees itself as the center of
the universe and shall not be criticized
um
so this was this was uh remarkable um
and i i went to cambridge anyway
but but what but what lenny had
explained to me
he explained in such a way that it
stayed and it planted a seed
and even though i became steven's
student and that was great it was a
wonderful time
i i came to realize that lenny was
probably right about black holes and
stephen hawking was wrong
that information really does come out of
black holes and is not lost
and you know before you knew it stephen
hawking decided that lenny was right
about black holes
now
this is just an example of i think how
lenny operates he's good at
making people think the right way about
something
he's good at
guiding the field and for generations of
physicists coming and going he has he
has guided us in the right directions
he's championed sometimes unpopular
ideas
and topics
that people wouldn't touch
and he's got a stellar record on those
picks
now how did he do that
i don't really know how to explain it
it's some sort of irresistible clarity
in the way he thinks about things and
conveys those thoughts it's it's a
radical inevitability it's some
contradiction in
you know
you you hear it and at once it's like my
goodness this is completely crazy and
yet you realize there's no other option
and fortunately i don't have to explain
it to you because we're about to hear
lenny explain some things to you
so i welcome
professor leonard tuskett
it's very nice to be here
the last two and a half years i haven't
been more than 30 miles from my house
i'm now about 35 miles from my house
wonderful
okay
good
okay so
thank you raphael
i've known the name oppenheimer since
1945 when i was five years old
sometime after after
august 6th i think it was
i asked my grandfather
grandpa what's an atom bomb
he said that he didn't know how it
worked
but he did know the one most important
thing
he said it was a giant bomb
that had been built by a jewish man
named oppenheimer
that's all i know about oppenheimer
for about 15 or maybe a little more
years
when i started to read his papers his
physics papers
in fact i did read a number of them and
one of them had a great influence on me
a paper with a young i i'm sure he was a
student or just a young physicist by the
name of heartland schneider a brilliant
young physicist
that gave rise to the modern theory of
black holes
even if it's not explicit everything in
my lecture today rep
rests on the foundation that oppenheimer
and snyder laid in 1939
which by the way was a year before i was
born
okay let's begin
i maintain that the biggest puzzle
about physics is that it exists at all
i don't mean that the laws of physics
exist
or that they are precise and
mathematical
i mean the fact that an animal whose
closest relative is the chimpanzee
was able to ask about these laws
there he is up there
but was also able to navigate
through a sea of wrong ideas
and eventually hit on relativity
quantum mechanics the standard model of
elementary particles and more
that is a absolutely remarkable fact
that is not a physics fact it's probably
a bio biology fact
but it just blows me away
what are the tools that our ancestors
used intellectual ancestors i don't mean
the monkeys
i mean newton einstein and so forth what
were the tools that they used to be able
to answer these questions
well there were theoretical tools
thought experiments
apparent conflicts of principle
paradoxes
and of course mathematics
but some people would say the most
important tool was of course experiment
today we're going to talk about a
subject quantum gravity
which is so remote the scales of
distance so infinitesimal the energy so
enormous that direct experiment is
entirely out of the question at least
for now
the phenomena they're the phenomena
which are at the intersection of quantum
mechanics and gravity
quantum gravity to give it a name
so we theorists are on our own
can we make progress well it seems that
we have made serious progress over the
last two decades maybe even
revolutionary progress
okay as i said
the phenomena are so remote
that the possibility of experiments is
out of the question and it means we're
really on our own
we have made progress and i'm going to
tell you some of the little pieces
i'm not going to give you the whole
story i couldn't possibly but i'm going
to try to give you some feel for what
some of the pieces have been
that i think are adding up to a
revolution
okay so
where are we
the whole subject of quantum gravity
probably goes back to the first day that
anybody thought about quantum mechanics
and thought about gravity
but the modern era of it i think started
in 1958 or sometime around then
when
theoretical physicists
asked the question should we quantize
gravity now what does quantize mean
quantize was a an expression which meant
a procedure
a procedure that you do on a classical
system the classical system could be a
harmonic oscillator it could be an atom
it could be electrodynamics ordinary
electrodynamics
a procedure which if you know about that
procedure you recognize the equations
if not it doesn't matter there are
equations
and the founders of the subject
called iraq finding dewitt weinberg
wheeler
hawking their attempts
were organized around trying to describe
scattering processes processes where
particles come in and particles go out
they interact under the influence of
gravity and the here and there they may
emit
some gravitational waves gravitons
they invented feynman diagrams for uh
for gravitation
well
it was a disaster
it was a disaster everything they tried
to compute came out infinite or came out
meaningless came out nonsensical
and
that led to
what i would call an era of angst
and confusion a time when it just did
not look as if quantum mechanics and
gravity were compatible now how could
they not can be compatible they have to
be compatible the world has both
and we can't live in a world where of
inconsistency
and that was then how do you fit them
together can you fit them together it
looked impossible
today the situation is different
today as far as we can tell
not only can gravity and quantum
mechanics fit together
but it's almost as if
they were the same thing or two sides of
a coin a single coin
i'm going to tell you a little bit about
some of the
things that went into that
all right i always start with
digressions i'm a great digressor i
digress all the time
i'm going to digress about
something you've all seen
holograms
what is a hologram you've probably all
seen them the first holograms are
something like this
you had a region
inside a cavity of some sort
and the cavity was not a cavity just a
round region
and the cavity the surface of the cavity
was a film a photographic film
and if you looked carefully at that film
even through a microscope all you would
see was little scratchy um meaningless
rubbish noise
it seemed to encode nothing nothing
recognizable on the other hand if you
shined light on it of the right kind was
called coherent light
all of a sudden voila
an image would form but a
three-dimensional image right in the
middle of this cavity
a three-dimensional image of whatever it
was that had been photographed
what is a hologram
from an abstract point of view a
hologram is a two-dimensional
mathematical representation
of a three-dimensional portion of the
world
now how do you manage to take a
three-dimensional thing
and map it into two dimensions
well it's possible but it is always at
the cost of the two-dimensional image
being looking like a random hash
how you put it back together again
that
may be just shining light on it or it
may be some much more complicated
mathematical procedure but that's what a
hologram is
and what you can say about a hologram is
that things are not where you think they
are
in the hologram
the information
the um that's called the information of
what is in the hologram is in the film
the image
is in the bulk what we today call the
bulk
things or the information encoding
things are not where you think they are
i want you to keep that in mind
now let's come back to quantum gravity
in the 1990s this was the period when
stephen hawking and i were having our
fun debating and
rafael was a student
thought experiments principally
initiated by stephen himself
about black holes
led to something called the holographic
principle
what was the holographic principle and
what is the holographic principle today
it's the idea that a region of space
with everything in it
it could be astronomical space it could
just be this room
or even the whole universe
that
the information encoding everything
taking place in this three-dimensional
world
is encoded on the boundary of that
region as a kind of quantum hologram
it's of course impossible for me to
describe the mathematics of the quantum
hologram here i will try
but that's the message
and again
things are not where you think they are
or at least things are not where the
information which is encoding them would
lead you to believe they are
okay let's come back now to this idea
that gravity and quantum mechanics may
be so closely related
that they really are just two sides of
the same coin
the evidence for that
is a whole bunch of parallels
between gravitational phenomena
we'll talk about what that means in a
moment and quantum phenomena things that
we had no idea were connected
things from two radically different
fields of physics
are turning out to be parallel to each
other and perhaps even
not just related to each other but the
same thing
the connection is through this
holographic principle
the gravitational phenomena are the
phenomena which are like the image the
three-dimensional image
the encoding
of that three-dimensional world is in
the form of the quantum mechanical
hologram
the correspondences are correspondences
between the two of these so i'll give
you some examples
let's go to the most primitive or basic
of gravitational phenomena
you all know what that that is if you've
fallen out of bed i've fallen out of bed
a number of times regularly
you know what gravitation is
it's falling
falling in a gravitational field
and if i wanted to if i wanted to
express it abstractly i would say that
the gravitational force just like any
other force
is a tendency for things to accelerate
in this picture here the apple is
accelerating
it's accelerating downward
caused by the gravitational field of the
earth
you can rewrite the you know the
equation f equals m a of course you can
rewrite that as f equals the rate of
change of momentum
the momentum of the apple is increasing
as it falls
so you can say that falling is the
tendency for momentum to increase
in the presence of a gravitational field
that's one side of the coin the falling
side
the other side of the coin the quantum
side is something so radically different
that it's hard to imagine that has
anything to do with it
you take some quantum system now this
quantum system i'm imagining is the
quantum system
encoding the hologram
this bunch of squiggles and uh and
random looking bits of information
out at the boundary of the region of
interest
and you come along and you tap the
system you perturb the system you might
you might hit it with an extra electron
or you might do whatever it is that you
do to it
at some spot
you perturb the system
that perturbation this is a quantum
mechanical fact will start to spread
throughout the system its influence will
spread throughout the system like an
epidemic
you touch one qubit if you know what a
qubit is that qubit will touch a few
more qubits a few more qubits will touch
a few more qubits
and the effect of perturbing the system
will spread
there's a notion of size
it's like the size of an epidemic the
size of an epidemic simply means the
number of sick people
and the number of sick people has a
tendency to grow
here's an example for the experts on
quantum computation if there are anybody
know about quantum computers
nobody good okay then this picture
doesn't mean much to you this is a
quantum circuit
and uh it proceeds from left to right
if you perturb it with a with a green
qubit
that perturbation will spread throughout
the quantum computer and that's the
phenomena of scrambling of information
scrambling
now what on earth does this have to do
with falling
i'll give you an example it comes from a
setup called adscft that may not mean
anything to you it's fine it doesn't
matter
uh let's let me just
oh i just realized i have a um a laser
pointer built into here it's not really
a laser pointer
yeah good
all right
what is this this is the boundary far
away
the boundary encoding the hologram
the bulk of space the interior is in
here
imagine coming and perturbing
the hologram perturbing means just
hitting it or something
that information that you've done so
starts to spread and starts to spread
throughout the hologram
and there is this notion of the size of
the perturbation
you can calculate these things and the
calculation as i said is purely quantum
mechanical
maybe a bit of quantum field theory
but no gravity
and what do you find you find that the
rate of change of size in this
setup
is exactly equal to the mass
of an object which was created at this
point
times the gravitational acceleration
these quantities here you get from
elsewhere but they're well defined in
the context
and what does this say
well what would gravity say
gravity would say that the time
derivative of the momentum of the
particle is falling is equal to mg
and so we see these two different fields
of physics entirely different coming
together
and giving rise to an equation
i don't know if galileo would have
recognized it in quite this form but it
was galileo's equation
and in order to make sense of it you
have to believe that momentum
of the object which was created
is simply the size of the perturbation
now you may not understand that you may
say he's talking gobbly
and the main thing that i want you to
get from this is again
this correspondence between
gravitational things this unexpected
correspondence between gravitational
things
and
quantum things here's another example
instead of a flat plane being the
hologram we can imagine the hologram as
a sphere surrounding some place
at the center of the
diagram the center of the picture
there might be a black hole or planet or
other mass
if you do the same calculation in this
context of calculating how the size of a
perturbation grows again purely quantum
mechanically
what do you find you find the marvelous
formula
that the rate of change of the size
the mass times the acceleration of the
inflowing object
is just equal to the product of the two
masses
newton's constant
and the distance between them squared in
other words newton's law of gravity
quantum mechanics
the growth of size
gravitation
gravitational attraction
to me
that is very stunning uh correspondence
okay let's come out to black holes in
paradoxes
it was a famous paradox of stephen
hawkings
uh i'm going to oversimplify it
not just stephen hawking's paradox but a
later paradox called the um the firewall
paradox
please if you're a theoretical physicist
don't shoot me for the way i explain
this because it's going to be
oversimplified
the black hole paradox we have a black
hole
the horizon of the black hole
is simply this
circle here
and we throw something into the black
hole that has some information
an encyclopedia and i'm going to call
that encyclopedia a
there's no escape from a black hole or
at least as far as we knew
uh in the 1990s there is no way that
anything can escape from a black hole
but the black hole can evaporate
that was something that stephen hawking
discovered that black holes can
evaporate and they can shrink
at some point they shrink enough
that something strange happens
namely there is not enough information
enough area enough whatever it is in
that remaining black hole
to encode
the encyclopedia that you threw in it's
called the page point
and all of a sudden the encyclopedia
simply cannot be there anymore
where is it or where are its bits of
information they're in the hawking
radiation
so the encyclopedia gets transferred
or the information of the encyclopedia
gets transferred
to the um the radiation that's a quantum
mechanical principle
that
that we've known about for a long time
and if you take that radiation imagine
somebody takes that radiation grabs all
the photons
puts them in a box and squeezes that box
down into some small box somewheres
then what this is telling us
the quantum mechanics
is that the encyclopedia
again i emphasize that by that i mean
the bits that comprise the information
in the encyclopedia is transferred from
the black hole
to the radiation
or to the black hole that the radiation
might have been compressed into
in other words as the black hole shrinks
it cannot hold any information and if it
can't hold any information nothing can
fall into it anymore
and one says that there is a firewall at
the horizon
firewall doesn't mean in the sense of
burning up
it means in the sense of an information
firewall that no information
can fall into the black hole anymore
every time you try to do so it pops out
and it appears far away
in the radiation
that's the idea now this idea of a
firewall was very very badly at odds
with what we knew about general
relativity
general relativity always said that you
can always put things into the black
hole and they will simply stay there
so
this led to a this paradox the so-called
firewall paradox
you could say it this way
either there is a firewall
or
or sometimes called
raphael were you the inventor of a
equals rb
i think you might have been
yes i think he was as a matter of fact
the idea is a generalization of the
holographic idea that things are not
where you think they are
and that in fact
the encyclopedia which is in fact behind
the horizon of the black hole but its
bits its bits of information
are found
far away in the hawking radiation
don't worry about what a and r b stand
for what it says is that the information
comprising the black the the thing
inside the black hole
is far away far away on alpha centauri
in some other system
in other words it's an extreme version
of this holographic idea that things are
just not where you think they are
well that seemed too crazy i think even
rafael thought it was too crazy
did seem too crazy
but one thing
it seemed to suggest
that if somebody far away manipulated
the radiation in this box
it would immediately have an effect on
the interior of the black hole
and an effect
which if somebody jumped into the black
hole
would detect
what was done far away
and that seemed totally inconsistent
with the idea that close things can
affect close things but they can't
affect bar things
well it needed a new idea
resolving this puzzle required a new
idea
and the new idea is called e r equals
epr
einstein er stands well let's first do
epr
how many people here know the who what
epr stands
for a good to a good fraction of you
good
it stands of course for einstein
podolski and rosen
but it also is the phenomena of
entanglement quantum entanglement
now i'm not going to tell you
exactly what the quantum entanglement is
it's what einstein called spooky action
at a distance
i'm just going to tell you a very very
simple version of it
two things they could just be two
electrons or they could be two nuclei or
they could be two macroscopic objects
are entangled
if by measuring one of them
you find out certain kinds of quantum
information about the other one
no matter how far away that other one is
let's just call that entanglement now
all my physicist friends know that i'm
being oversimplified but there is this
phenomena
of sharing
information
between two different distant systems
that's called epr entanglement and it's
a very mysterious
phenomenon
and i'm not going to explain it now
we'll just say it exists
that was the year
1935 when einstein podelski and rosen
discovered or at least
let's let's make it simple
discovered entanglement
it was incidentally a very good year for
einstein einstein i think had three
really good years
1905 when he discovered especially
discovered all of modern physics
uh except for gravity
except for the rules of gravity
1915 or so when he completed the general
theory of relativity and understood
gravity
and 1935 which is much less famous
in which he discovered this phenomena of
entanglement but the same exact year he
discovered something else
called einstein rosen bridges
einstein or sometimes called wormholes
wormholes are a solution of einstein's
equations
in which you have two black holes far
away from each other with a kind of
tunnel of space between them
you can't see that tunnel of space it's
in some interior space that can't be
seen
but
the two very distant objects are
connected
by a
i call it a tunnel i call it a bridge i
call it a wormhole
all the same idea
you've seen these things in science
fiction people jumping into wormholes
and so forth i always thought it was
nonsense
but not completely
what's the idea
if you could go as fast if
what it sounds like is you can jump into
one black hole
over here
and pop out over here or we'll see that
you can't do that but nevertheless
that's what a wormhole resembles
the science fiction idea
of a bridge between very distant places
now what does er
that's the bridge einstein-rosen bridge
have to do with epr other than they have
two letters in common
nothing
before
2013
nobody and i'm absolutely convinced that
einstein was among that nobody had
any idea that entanglement
and wormholes or einstein rosen bridges
had anything to do with each other
and after 2013 they had everything to do
with each other
in fact
the idea goes with the acronym
er equals epr
you could call it p equals one but
nobody does
er
the idea of a bridge between distant
regions of space
and the idea of entanglement
of the same idea so i'm going to show
you a little bit about how that works
the rectangle here is supposed to be
space a big region of space
over here
on earth
we have a bunch of particles
far away on alpha centauri
we have another bunch of particles two
clouds of particles those particles have
never been in contact with each other
they don't know about each other they're
completely separate
with no prior
interaction between them
i'm going to take this sheet of space
and fold it over
not because it's folded over but just
why i want to draw it that way to make a
point
but it is still true that this cloud
over here is far from this cloud because
you have to go around this long way to
get there
what happens if you let those those
clouds of particles colors
shrink
they form black holes
that's what a black hole is it's the
shrinkage and collapse of a star for
example these could form a star
eventually and after a star they could
form black holes
and those black holes will be completely
separate from each other with no
connection between them
on the other hand let's do something
else now
let's take a bunch of entangled
electrons or a bunch of entangled
particles
half of them are over here and half of
them are over here
now how do you create well the the green
line here just indicates that this
particle is entangled with this one
this particle is entangled with this one
no not that one this one
how do you create such a situation
to create it you have to create the
entangled particles near each other you
can't create entangled particles far
from each other you have to bring the
particles together you've got to let
them interact with each other and they
will become entangled
but once they're entangled you can take
the half of them
that's down here
separate it well let's say we take this
half of them and bring them all around
here so that we wind up with two clouds
of entangled particles
now we let gravity do its work
and when gravity does its work again it
creates two black holes
but the black holes are now connected by
an einstein rosen bridge
in other words
entanglement
and wormholes are in some sense the same
thing
two black holes which are entangled will
necessarily have a bridge between them
two black holes which are unentangled
will not
this is what's called er equals epr
and it was a major discovery
it seemed ludicrous at first but it very
quickly caught on
is now part of the standard
lore
what can you do with it
okay so let's imagine now that we do
have such a wormhole connecting two very
distant black holes
one of them
franklin has control over
control means he can do things to it he
can jump into it if he wants
the other one linus
has control over and they're very far
away one is on alpha centauri the other
is on earth but they have this einstein
rosen bridge connecting them
well with enough
care
and enough fine-tuning
they can arrange these black holes so
that they can each jump into their own
black hole and in a very short period of
time can meet at the center and shake
hands
what they cannot do at least without
some
further considerations which i'll come
through or if i have time i may not have
time
linus cannot jump into one
pass through and come out the other one
that can't happen
now the fact that it can't happen is
both known from the quantum mechanical
point of view it's called the no
signaling theorem for entanglement
and for wormholes it's called the
non-traversability of wormholes the
impossibility of traversing through them
and it turns out those are the same
phenomena one quantum mechanical the
other gravitational
now let's come back to a equals rb
the encyclopedia a in black hole number
one or left-hand black hole here
is encoded
in the radiation in region 2 over here
our problem before was that sounded
crazy because somebody manipulating the
radiation over here
could perturb what's inside the black
hole
in just such a way that somebody who
jumped into this black hole over here
would detect
that a very very distant observer
had done something
to this group of photons over here that
sounded outlandish
but now we know that if these photons
over here are entangled with the black
hole which they will be
that an einstein-rosen bridge and or
from outside you can't see that
einstein-rosen bridge but the
einstein-rosen bridge will open up
and so anything anybody does over here
will affect what's behind the horizon
of the original black hole in other
words a equals rb makes perfect sense
again
it has to do with this basic idea that
information is not where you think it is
this is a radical example of it
what about the wormhole side of it
how can it be why should it be
that somebody who goes into one black
hole in alpha centauri can't get through
the wormhole
and come out
well one worm one end of the wormhole is
in new york the other end is in
california think of it as a tunnel
between the two places
why can't you ja drive through that
tunnel
and the reason is that einstein rows and
bridges and this is a gravitational
phenomena
tend to stretch and expand with time
that's the solution of einstein's
equations gravitational equations
wormholes grow
so
if who was a franklin i can't remember
franklin or linus if linus tries to
drive into the new york side
he will encounter the fact that the
wormhole is growing
and in fact it will grow so fast
that he cannot outrun
the growth of the wormhole and will
simply never get through to the other
side and come out the other side
that's the non-traversability of
wormholes you can't even send the light
signal through because even a light
signal will not go fast enough to outrun
the growth of the wormhole
what that's the that's the gravitational
side of it is there a quantum side of it
yes there is and i i don't have time to
tell you what it is i will tell you what
it is i don't have time to explain it
but i'll tell you that it's a computer
science concept
it's called the growth of complexity
the black hole quantum state of the
system is becoming more and more complex
it's very much like this information
scrambling that we talked about having
to do with falling
the quantum state of the wormhole
evolves it becomes more and more complex
and that complexity translates into the
growth of the wormhole
so these are all these very very
remarkable correspondences
which tend to make us think that they're
not just that there are deep connections
between quantum mechanics and gravity
but it's in some level as i said i'll
say it again there are two sides of the
same coin
okay so
we have the idea
of a quantum hologram encoding
information which could be far from
where the object that it's encoding is
and what is the other side of the coin
the other side of the coin is
gravity
i like this picture it's my favorite one
of all
okay
whoops
what happened here oh my
is a totally different subject let's see
if we can get it back
how can i let you
good here we are
all right now let me address let's see
how much time do i have raphael am i
running out of time
15 seconds
oh 15 minutes i don't need 15 minutes
lots of time for questions okay
there are
criticisms oh incidentally one might
point out that most of these ideas
grew not just out of a combination of
quantum mechanics and gravity
but string theory
how string theory got into it i haven't
really said very much about but let me
tell you that all of the precise
examples all the mathematically precise
examples of this correspondence
tend to come from systems
which were invented or discovered in
string theory
a string theory quantum gravity has been
the victim of an enormous amount of
criticism
the criticism
which first of all thing is unjustified
but
what does it have to do with the
criticism i would say
stems from the fact and i think it is a
fact that good science almost always
spreads its influence far and wide
into many fields of not just physics but
even outside of physics and in
particular
into engineering into technology and
that's a pattern that we've seen over
and over and over again
special relativity led to nuclear energy
general relativity we use it for
navigation by satellite believe it or
not
quantum mechanics the list of
technological advantage advances
and quantum mechanics was not invented
by people trying to do um
technology it was invented by people who
are curious about the atom
uh quantum mechanics among other things
it led to the mri machine
but so many things that i had that the
list would go on and on quantum
electrodynamics trying to understand the
quantum mechanics of electrons and
photons in particular photons led to the
laser
or at least it's closely connected with
the laser and so forth and so on
what about
quantum gravity
general relativity and its connection to
quantum mechanics it seems so infinitely
remote with no connections or
applications to the rest of science
it could be that that's true it could be
we're just stuck with that
but that has not been what is happening
first of all
this connections between quantum
mechanics and gravity have led to new
insights into strictly
uh phenomena which seem to have nothing
to do
with
gravitation
for example the surface of a black hole
the horizon of a black hole behaves as
if it were made of a fluid
that's something that general
relativistic general relativists
discovered a long time but not just a
fluid but a quantum fluid
whatever that means
one can use the fact
by knowing enough about black hole
physics and knowing enough about general
relativity you can compute properties of
fluids that were too hard to compute
otherwise
here's one example
of something that was inspired by the
connection between fluid mechanics black
holes and quantum mechanics
it's a bound on the viscosity of fluids
now that doesn't seem to have anything
to do with either of those subjects well
it's a little bit quantum mechanical it
is quantum mechanical
ada is the fl is a viscosity of a fluid
the stickiness of it
s on the right hand side is the entropy
per unit volume of the fluid
the heat per unit volume
what was discovered
in the context
not discovered by people doing fluid
dynamics people comparing properties of
black quantum mechanical black hole
horizons with fluids
is that the viscosity is always greater
than equal to some number that includes
h-bar that includes the quantum constant
times the entropy density
will that have
impact into fluid dynamics and into uh
probably
there are things called strange metals
strange metals are a form of matter that
was discovered by condensed
nanophysicists
are they important in technology i don't
really know but they were discovered
about 30 years ago
and they were met metallic systems which
behaved just differently than ordinary
metals
it's turning out that those strange
metals
are mathematically identical
to certain special kinds of black holes
called extremal black holes or near
extreme or black holes
both sides are quantum mechanical one
side is also gravitational extremal
black holes the other side is the pure
quantum mechanics of certain
materials
information scrambling the thing which i
told you accounts for the falling of the
apple as it does as it accelerates in
the gravitational field
information scrambling is an important
thing in quantum computer science
the information scrambling from black
holes led to a bound again another bound
that a certain constant called the
japanese exponent
in information scrambling is always less
than some other constant
that now is considered a reliable
fundamental bound on how fast
information can spread through a quantum
mechanical system
i told you
that linus cannot get through the
wormhole
well i was a little bit too pessimistic
with a little bit of help from something
called classical in
the exchange of classical information
these little purple dots being sent from
one side to the other
that's just ordinary
morse code for example
but has no information about it about
linus or about
anything else inside the wormhole with a
little bit of help from a little bit of
classical information
you can slow down that growth of the
wormhole
you can slow it down enough so that
indeed linus can get through it
that phenomenon which was discovered in
the context of gravity and quantum
gravity
has
led to a new protocol
and new experiments
for quantum teleportation quantum
teleportation is a real thing
you can teleport information what it
means you can't exceed the speed of
light but you can send information in a
way that is completely hidden 100 hidden
cannot be decoded by an eavesdropper
and so it's led to new protocols for
quantum teleportation experiments are
now being done
to confirm that this can happen not in
black holes but in quantum computers
i'm not allowed to tell you that the
experiments are are successful because i
promise not to
mention that they're being done and
they're working out successfully
so i won't
quantum complexity theory the growth of
wormholes
the whole idea of gravitation being
controlled by the growth of complexity
has led to many new insights
into how complexity of quantum systems
evolve
we've seen advances coming from gravity
in error correction error correction is
the big hang up in trying to build a
quantum computer it's too easy to make
errors in a quantum computer
you have to error correct for them and a
whole new
insight
into error correction
has come from thinking about
gravitational systems again
and
finally in the hands of one of my
favorite physicists juan alvasena
uh
much of what we're talking about
has had a very interesting influence in
cosmology in inflationary cosmology
so far from being a totally isolated
thing
outside the framework of any other
science
this quantum gravity is beginning to
have
an effect which let's just put it this
way condensed metaphysicists
and quantum computer physicists
and
theoretical cosmologists are being
forced to learn
what ads cft means
and they are learning it they're
learning it and using it
so this is exciting this is a very very
exciting period in the development of
physics
it is also one which is very very
difficult to explain to a general
audience
you know when you give a lecture like
this or what did lincoln say you
completes half of the people half of
them
all the time well
you can please half of the audience uh
half of the time and so forth and so on
i think the real truth is in a lecture
like this you can
you you're lucky if you can uh
if you can satisfy any piece of the
audience even a little bit
because the ideas are complicated
they're difficult and so forth
you do your best uh you do your best to
try to explain
and i've tried to explain as well as i
can i hope you have gotten something out
of it i hope there's at least one person
who has gotten something out of this
lecture
uh and i thank you for listening
[Applause]
okay we have some time for a few
questions
um
from the audience i also have some from
zoom
would you like to take a ques ask a
question
well i got a couple on zoom so
oh
[Music]
okay how are you there's an experiment
going on on using drones to detect
using drones
to detect quantum waves
drones
drones
there's stuff flying around in ukraine
drones drones drones yeah stones yes
and they're used they're
you're planning on uh there's
experiments going on and using these to
detect
quantum waves
how are they uh
okay i guess they it's an interesting
thing i have no idea i'm interested in
the drone so okay that's that's i'm
sorry tell me
well i never heard of maybe
okay
okay we have a question
hi hello just out of curiosity did you
draw the charlie brown drawings yourself
or
did you find this having a little
difficulty hearing can somebody uh
closer did you draw the cartoons
yourself can i draw the cartoons did you
draw those cartoons the peanuts cartoons
did you draw them yourself oh yeah
some of them some of them i just copied
but they were all hand drawn by me yes
yes
[Music]
my first um
my first uh ambition
had been to be a painter
not a house painter a a picture painter
the problem was i had no talent whatever
or at least my talent was
maybe being able to draw charles schultz
cartoons but i really wanted to be
picasso
didn't work out
i have a couple of questions for online
um
how can a wormhole grow faster than the
speed of light well
locally each piece of it is growing
the rule is not that one thing can't
exceed the speed of light it's one thing
cannot pass another
close by
faster than the speed of light
the universe for example in some sense
grows faster than the speed of light
the accelerated expansion of the
universe tells you that if you're far
enough away
just the just the hubble law
that things will be moving away from you
faster than the speed of light
but it does tell you that you can't get
information from uh from behind from
that far away and that's what creates a
horizon it creates a cosmic horizon
the same is true here the wormhole can
grow
but if it's growing that fast you simply
you can't get through it for one thing
and you can't receive signals uh
from far away along the wormhole
unless
this idea of classical information being
sent back and forth can come and slow
the uh the growth of the wormhole
and that is something that um
that uh there's mathematics for it
and in in some way it's being tested in
the laboratory not on real wormholes but
in entangled quantum computers
thanks for the talk
so if the information in everything is
really all scrambled up and delocalized
why does it act in any way that makes
sense to us
like why do things like why is physics
local
to us that is a really good question
now if you just take some random quantum
system it will have some features that
look somewhat gravitational in character
but
it will not have the degree of locality
that we expect in the real world it's
only very very special quantum systems
very very special candidates for the
holographic screen let's say
which show the kind of locality which
you're asking about
those special quantum systems
are generally gauge theories whatever if
you don't know what that means that's
okay and they're also very very strongly
coupled very difficult systems to
analyze
so
you're pointing at one of the most
important questions to understand what
the nature of the very very special
systems that behave with this kind of
ultra locality that the real world seems
to have
and that's that's as far as i can go
with it because i think the answers are
yet to be given
first of all fantastic lecture it was
great but i'm not going to admit i'm not
going to state that i understood no no
but i mean one of the the
a person who happens to have worked on
strange metals and thought a little bit
about quantum computing uh
how much of the connections between
uh the quantum gravity and everything
you've been saying and and these other
fields that some of us work on
is because of the mathematics discovered
in the process of of working out
theories string theory and working with
theories and how much is some deeper
connection between the phenomena okay i
think the connections are deeper
certainly certainly that's part of it
just the mathematics seems entirely
similar you probably know about the such
dev d a category theory that's an
example
yeah they're the but a lot of the
physical phenomena
uh
experienced in that system
are identical to the physical phenomena
that you would expect in a near extreme
or black hole
so i'm hesitant to say that it's just
the mathematics so don't think of it i
think there is a real physical
similarity if that's what you're asking
i think there is yeah
we'd have to sit down and talk about
that obviously but uh
i have another question from online if
i'm if i may
yeah so uh
can you say a few more words about the
growth of quantum complexity and how it
corresponds to the non-traversability
words you wouldn't believe it
and i have
but um i'm not sure this is the venue
for it
quantum complexity well i'll tell you
what quantum complexity is first of all
let me tell you what complexity is
complexity means lots of different
things to different people
you might think for example
a
beautifully designed building or
something or a beautifully designed car
is complex no it's less complex than
almost anything you can think about uh
in that sense
complexity is used in other words i have
a very complex relation with my
mother-in-law unfortunately i had well
fortunately i had a very good relation
with my mother-in-law but you could use
the term that way
in fact a very special thing is meant
complexity if you have a given problem
complexity is a measure
of the shortest number of steps to solve
that problem let me give you an example
theorem proving
you start with a bunch of axioms
and now you have some theorem that you
think might be a theorem
and so you go to try to prove it and you
prove it okay it took you a certain
number of steps
that number of steps might not be the
cheapest and least number of steps that
it takes you to prove the theorem what
am i being misstep steps are the use of
the uh of the logical um
axioms together with the rules of logic
each one being used once is a step
your proof
might involve
550 steps
the complexity of a theorem is a measure
of the least possible number of steps
that it would have taken to prove the
theorem
another example from quantum mechanics
or quantum computation is you have a
quantum computer and you're going to put
into it some simple state
and you want to run that computer and
get
to another state another state which may
have some interesting information in it
okay quantum state of a bunch of qubits
or something like that
you can think of different ways of
getting there there are different routes
different routes different series of
gates different series of processes
that could bring you to that um
to that target state
the complexity of the state
is a measure of the fewest number of
simple operations that can get you there
that's the notion of complexity now
that's exactly the notion that's used in
this wormhole situation
the wormhole grows which is simply
another way of well which means that the
quantum state
of the uh of the
two black holes evolves with time
it's a general feature
that complexity increases with time in
other words
you go to states which are harder and
harder to get to which would take few
more and more steps to get to them and
curiously interestingly
there appears to be a connection between
how complex
the quantum state of the black holes is
and how big the wormhole is
that seems to be something that's fairly
well confirmed by now
and so
it is believed that the growth of the
interior of the wormhole is equivalent
that's the gravitational side that's the
general relativity side
equal to the tendency for quantum
complexity to increase with time for a
complex
uh for a chaotic quantum system so i'll
have to leave it at that because uh
as i said i could say a lot more but it
would take
more time than
we can expand that was great hello uh so
i was wondering since you said that the
center of a black hole or at least the
holographic surface of the center of a
black hole acts like a fluid
uh
would that would solving the horizon
which is a kind of holographic surface
yes okay so if it acts like a fluid with
solving uh the navier-stokes equation
be a step forward to figuring out what
happens at the center of life i rather
think
frankly i think it will happen the other
way
that people will be able to solve the
gravitational equations for this fluid
and uh make steps
that
direct attack on the navy stokes
equations would be too hard for
so my guess is it will happen the other
way that not solving the navy stokes
equation won't teach you that much about
black holes because i think it's just
too hard to do
but solving black hole equations which
are a lot easier because they're just
einstein's equations
might very well teach us a lot about um
uh
dynamics of fluids
that would be my best guess as to which
way the information will flow there
but there are other experts on that
subject here
and um
i think you're an expert right
yeah yeah he has a nori
well i don't know who's somebody's in
there
oh i kind of question uh here i don't
know if he knows oh anyways i was
wondering if there is a relationship
between the
surface and the entropy between the what
oh it's here
hello
i was i was wondering if uh so you
mentioned there is a uh bound on the
viscosity uh of this of the black hole
uh and the entropy uh so since the
viscosity somehow seems to be on the
surface of the black hole is there a
relationship between the surface of the
black hole and the entropy and if so why
black holes
and horizons have entropy
they have a uniform entropy distributed
over the horizon
they also have viscosity
so it was
realized by
i think it was a condensed matter
physicist the sun
son
that
that no matter what he tried to do to
the black hole
he could never get the viscosity of the
surface fluid
to be less than a certain amount
which was proportional to the entropy
uh
and uh
that was a pure black hole study
and it turned out that when people
looked at experiments on the most on the
least viscous fluids
no matter how they manipulated the
fluids experimentally
nobody found the fluid who has that ex
that ex exceeded them the uh the bound
whose sun and staronets uh who
discovered this bound
uh i just took that as an example
there's nothing particularly special
there are lots and lots of examples like
this this one was easy to explain
there's a nice simple equation that goes
with it and uh
and um
it's so down to
earth that it's very easy to understand
not understand where it comes from why
it's true but what it says
um sir
okay i have two questions um the first
one is um i'm not very familiar with
this quantum gravity before this lecture
and there's one thing that seems very
well to me is that um
where is the quantum holograph
and does it physically exist and this is
a you're asking where the client yeah i
think the answer is that you take any
region
any region whatever this region here of
space and you want to link the room
and you want to ask
how the information is encoded
and the answer is
that the amount of information you need
to describe the interior of this room is
never more
than the surface area of the boundary of
the room and so you can always say you
pick your region
and then the answer will be on the
boundary of that region
so it's a mathematical statement
uh what about the universe
well
the universe does have a horizon
uh the natural place for the hologram
describing the entire universe would be
at the horizon of the uh
at the cosmic horizon of the universe
but that's something that is still being
studied
so the natural the natural thing to say
is we take the whole entire observable
universe
it's bounded by something called the
cosmic horizon
and that would be the natural place
where you might want to say the hologram
was
now i'm not sure that all my colleagues
agree with that and that's still still
something that i think is um
work in progress
okay and the second one is
rather and is related to a former
question
how do you define
on two points are nearby as we discussed
that you cannot travel um greater than
at a speed greater than the speed of
light between two nearby points and to
what extent does these two points can be
uh can be considered as nearby
as you stated send messages back and
forth in small amount of time you'll say
they're nearby
i'm not sure what else to say
i don't know raphael wendy said two
points are nearby
yeah okay
hi thank you for the lecture um
for sure oh yeah here
uh so the famous quote-unquote
historical dilemma
is that the copenhagen interpretation of
quantum mechanics is
not irreversible is not reversible while
the schrodinger equation is very much
time symmetric do you believe that these
deeper insights into the workings of
quantum mechanics and as they relate to
general relativity as you laid out in
the lecture we'll deepen our
understanding of our interpretation of
the wave function or even perhaps change
or offer a new interpretation of the
wave function i'm only going to say that
that's a wonderful question i think that
is
a really really good question is all of
i've always felt
that
the puzzles of quantum mechanics the
many worlds interpretation all the very
very confusing things
about quantum mechanics would only
eventually get solved when we understand
the connection with gravity i still
think that
but i don't if i knew how to do it
how to make those connections i would
have published them
the no no
i mean
my feeling is
that the answer to your question is
understanding these aspects of quantum
gravity will
tell you more about the inner workings
of quantum mechanics but nobody has
really um actually that's not been
a primary concern of the people doing
this kind of work
they tend to be very
focused uh pragmatic people if you can
call a theoretical physicist pragmatic
who will tend to solve problems
that can be solved this is part of the
art of being a good theoretical
physicist is to identify those problems
which are hard enough that the results
of them of a solution will matter and be
important but we're not so hard that
we'll just sit around for a hundred
years
scratching our butts over them
and so the general feeling of my friends
and so forth and i sort of share it
is that these problems of the
foundations of quantum mechanics are
real problems but boy they've been
around a long time people have struggled
with them people not been able to make
any real sense out of it it's not even
clear they're real you know what feynman
said about it he said the problems are
so
confusing that he can't even tell if
there's a real problem
and that is the way it feels
you know it's like thinking about
consciousness or uh
so
the pragmatic side of physicists tends
to make them steer away from problems
like that
will they come back to it
perhaps but at the present time i think
all of this has not impacted
the um
the most fundamental understanding of
quantum mechanics
which is disappointing in a way
okay we have time for two more questions
i think one at the front here
yes i also have a question about
we talked before about collapsing a set
of entangled particles into a wormhole
at one ear and one at alta centauri and
i wanted to ask if that in turn also
means that if we have a wormhole and it
decays that we also get back a set of
entangled particles
i am still having a little trouble
hearing can
anybody repeat it
some voices it's nothing special some
voices i simply have difficulty there's
a certain range of frequencies
yeah absolutely
yeah for example you have these two
entangled black holes which form the
wormhole
um what happens if those two black holes
evaporate
they each just get transformed into a
radiation particles that go out but
those particles will be entangled so
that's exactly right
okay one last question just yet yeah
last question because i'm beginning to
fade
try now
okay i'll say that again i'm i'll ask a
much simpler question than i was i was
planning to um you mentioned that
there's uh a definition of the leoponoff
exponent that has to do with quantum
gravity i'm i was just wondering if you
could say more about
what system that exponent is referring
to and where that definition comes from
uh
well it's a wide variety of systems but
any system
the the most well understood case i
would say is this syk model which is
also a theory of strange metals
the yampanov exponent
has to do with exponential growth
when anything grows exponentially
it it varies like e to some constant
times time that constant in this context
is called the yapon of exponent
um what is it that's growing well the
simplest way to say it is it's the
growth of the region of influence
of the perturbation
in other words this growth of what i
call size earlier
um what did you ask me
what system is it referring to any kind
pretty much any kind of quantum chaotic
system
but as i said the best understood one at
this point is this um
syk theory of strange metals which is
also a theory of
extreme or near-extremal black holes
s-y-k that's the initials
sashtev yeah and kitayev
so i feel
that's where most of it has been worked
out in the greatest detail
well lenny i think the only way that
people would be left unsatisfied is only
insofar as they're hungry for more
so
thanks again professors
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