Kind: captions
Language: en
a black hole is a mirror
and the way it's a mirror is if light a
photon bounces off your face
towards the black hole and it goes
straight to the black hole just Falls in
you never see it again
but if it just misses the black hole
it'll swing around the back and come
back to you
and you see yourself
from the photon that went around the
back of the black hole
but not only can that happen
the black hole the photon can swing
around twice
and come back so you actually see an
infinite number of copies
of yourself
the following is a conversation with
Andrew strominger theoretical physicist
at Harvard whose research seeks to shed
light on the unification of fundamental
laws of nature the origin of the
universe and the quantum structure of
black holes and event Horizons this is
the Lux Friedman podcast the supported
please check out our sponsors in the
description and now dear friends here's
Andrew strominger
you are part of the Harvard black hole
initiative which has theoretical
physicists experimentalists and even
philosophers so let me ask the big
question what is a black hole from a
theoretical from an experimental uh
maybe even from a philosophical
perspective so a black hole is defined
theoretically
as a region of space-time
from which light can never Escape
therefore it's black
now that's just the starting point many
weird things uh follow from that basic
definition but that is
that is the basic definition what is
light
they can't escape from a black hole well
light is uh you know the stuff that
comes out of the Sun that stuff that
goes into your eyes
light is one of the the stuff that
disappears when the lights go off this
is stuff that appears when the lights
come on
um of course that could give you a Beth
a medical definition but or physical
mathematical definition but I think it's
something that we uh will understand
very intuitively
what is light black holes on the other
hand we don't understand intuitively
they're very weird
and
one of the questions is about black
holes which I think you were alluding to
is you know why doesn't light get out or
how is it that there can be a region of
space-time
from which light can't escape
it definitely happens we've seen those
regions we have spectacular pictures
especially in the last several years of
those regions
um they're there
in fact they're up in the sky thousands
or millions of them
we don't yet know how many
um but the proper explanation
of why light doesn't escape from
uh
a black hole is still a matter of some
debate
um
and
one explanation
which perhaps Einstein might have given
is that light carries energy
you know it carries energy because
you know we have uh photo cells and we
can take the light from the sun and
collect it turn it into electricity so
there's energy in light
and anything that carries energy is
subject to a gravitational pull
gravity will pull at anything with
energy
now it turns out that the gravital
gravitational pull
exerted by an object
uh is proportional to its mass
and so if you get enough Mass
in
a small enough region
um
you you can prevent light from escaping
and let me flesh that out a little more
um if you're
on the Earth
and you're on a rocket ship leaving this
the surface of the Earth and if we
ignore the friction from the air
um if your rocket accelerates up to 11
kilometers per second
that's escape velocity
and it can if there were no friction it
could just continue forever to the next
galaxy
on the moon which has less Mass it's
only seven kilometers per second
so but going in the other direction if
you have
enough mass in one place the escape
Velocity
can become the speed of light
if you shine light straight up away from
the earth it doesn't have too much
trouble it's going way above the escape
Velocity
and
um but if you have enough Mass there
even light can't escape the escape
Velocity and according to
Einstein's theory of relativity there is
an absolute speed limit in the universe
the speed of light
and nothing
makes any sense nothing could be
self-consistent if there are objects
that could exceed
light speed
and so uh in these very very massive
regions of space-time even light cannot
escape and the interesting thing is
Einstein himself didn't think that uh
these uh objects would call the black
holes could exist but let me actually
Linger on this yeah that's incredibly
interesting there's a lot of interesting
things here first of the speed limit
how Wild is it to you if you put
yourself in the mind in the time of
Einstein before him to come up with a
speed limit of that there is a speed
limit that and that speed limit is the
speed of light how difficult of an idea
is that is it you know you said from a
mathematic
mathematical physics perspective
everything just kind of falls into place
but he wasn't
perhaps maybe initially had the luxury
to think mathematically he had to come
up with it intuitively yes so like what
how common intuitive is this notion to
you well is it still crazy no no so it's
a very funny thing in physics the best
discoveries
seem completely obvious in retrospect
yeah
even my own discoveries which of course
are far lesser than Einsteins but many
of my papers many of my collaborators
get a little confused we'll try to
understand something we said we've got
to solve this problem we'll get all
confused finally we'll solve it we'll
get it all together
and
um
then we'll
all of a sudden everything will fall
into place we'll explain it and then
we'll look back at our discussions for
the precedings of months and literally
be unable to reconstruct
how confused we were yeah and how we
could ever have thought of it any other
way
so not only can I not fathom
how confused
Einstein was
before he when you know when he started
thinking about the issues I can't even
reconstruct my own confusion from from
two weeks ago uh you know so the really
beautiful ideas that physics have this
very hard to get yourself back into the
mindset of course Einstein was confused
about many many things
doesn't matter if you're a physicist
it's not how many things you got wrong
it's not the ratio of how many you got
wrong coming you got right it's the
number that you got right
so Einstein didn't believe black holes
existed even though he predicted them
and I went and I read that paper which
he wrote you know Einstein wrote down
his field equations and
1915 and short Shield solved them and
discovered the black hole solution
three or four months later in very early
and um 25 years later Einstein wrote a
paper so with 25 years to think about
what this solution means yeah wrote a
paper in which he said that black holes
didn't exist
and I I'm like whoa
you know if one of my students in my
general relativity course wrote this
you know I wouldn't pass them you get I
get to see mine oh you wouldn't pass
them okay all right good to see minus
okay same thing with gravity waves you
didn't believe oh he didn't believe in
gravitational waves either he went back
and forth but he wrote a paper and I
think 34 saying that gravity waves
didn't exist because it people were very
confused about what a coordinate
transformation is and in fact this
confusion about what a coordinate
transformation
is has persisted
and we actually think
we were on the
edge of solving it
a hundred years later
well what a hundred years later what is
coordinate transformation as it was a
hundred years ago to today let's imagine
I want to draw a map with pictures of
all the states and the mountains and
then I want to draw the weather forecast
what the temperatures are going to be
all over the country
and I do that using one set of weather
stations
and I number the weather stations
and you have some other set of
weather stations
and you
you do the same thing so the coordinates
are the locations of the weather
stations yeah they're how we describe
where the things are
at the end of the day
we should draw the same
map
that is coordinate invariance
and if we're telling somebody uh we're
going to tell somebody at a real
physical operation we want you to stay
as dry as possible on your drive from
here to California
we should give them exactly the same
route
no matter which weather stations we use
or how we you know it's a very trivial
it's it's the labeling of points is an
artifact and not in the real physics
sure
so it turns out that that's
almost true
but but not quite there's some
subtleties to it
the statement that you should always
have the same give it this the same kind
of trajectory the same kind of uh
instructions no matter the weather
station yeah yeah there's some very
delicate subtleties to that which
begin to be noticed in the in the 50s
it's mostly true but when you have a
space-time with edges
it gets very tricky how you label the
edges and space-time in terms of spacer
in terms of time in terms of everything
just based on either one okay space or
time that gets very tricky and Einstein
uh didn't
didn't didn't have it right and in fact
he had an earlier version of general
relativity
in 1914 which he was very excited about
um which was
wrong
um gave it wasn't fully coordinate and
variant it was only partially
coordinated variant it was wrong
it gave the wrong answer for bending
light to the Sun
by a factor of two
there was an expedition
sent out to measure it during World War
One
they were captured
before they could measure it and that
came that came Einstein four more years
to clean it to clean his act act up by
which time he had gotten it right
so it's a very it's a very tricky
business but once it's all laid out it
saves uh
it's it's it's clear then what do you
think Einstein
didn't uh believe his own equations and
didn't think that black holes are real
well why was that such a difficult idea
for him well something very interesting
happens in
short Shield solution of the Einstein
equation
I think his reasoning was ultimately
wrong but let me
explain to you uh
what it was
um
at the center of the black hole behind
the horizon
in a region that nobody can
see and live to tell about it
as a center of the black hole there's a
singularity and if you pass the Horizon
you go into the singularity you get
crushed and that's the end of everything
now the word singularity
means that
um
it just means that Einstein's equations
break down
they become infinite you write them down
you put them on the computer when the
computer hits that Singularity it
crashes
everything becomes infinite there's two
so the questions are just no good there
now
that's actually
not a bad thing it's a really good thing
and let me explain why
um
so
it's an odd thing
that Maxwell's Theory
and Newton's Theory
never
exhibit this phenomena you write them
down you can solve them exactly they're
really Newton's theory of gravity
they're really very simple theories you
can solve them well you can't solve the
three body problem but
um
you can certainly solve a lot of things
about them
nevertheless
there was never any reason
even though Maxwell and Newton perhaps
fell for this trap there were never any
reason to think that these equations
were exact
um and
every there's no equation
well
there's some equations that we've
written down that we still think are
exact
some people still think are exact my
view is that there's no exact equation
everything is an approximation
everything is an approximation are you
trying to get as close as possible yeah
so you think are you saying objective
truth doesn't exist in this world
we could discuss that but that's a
different that's a different thing
um we wouldn't say Newton's theory was
wrong
it had very very small Corrections Inc
incomplete small Corrections it's
actually a puzzle why they're so small
so if you watch the procession of
Mercury's perihelia this was the first
indication
of something going wrong according to
Newton's Theory mercury has an
elliptical orbit
the long part of it moves around
as other planets come by and perturb it
and so on and so this was measured by
leveria in 1859 and he compared
Theory and experiment and he found out
that the perihelion process moves around
the Sun
once every
233 centuries instead of every 231
centuries
okay now this is the wonderful thing
about science
why was this guy
are we didn't get any idea how much work
this is you know
but of course he made one of the
greatest discoveries of the history of
science without you know even knowing
what it what good it was going to be
so that's how small that that was the
first sign that there was something
wrong with Newton yeah
nah so the corrections to Newton's law
are very very small but they're
definitely there
the corrections to electromagnetism
they're mostly the ones that we see are
mostly coming from Quantum effects
and so so the corrections there for uh
Maxwell's equations is when you get
super tiny and then the corrections for
the
um for Newton's uh laws gravity is when
you get super big
that though that's when you acquire
Corrections that's true but I would
phrase it as saying when it's super
accurate you know if you look at the
Bohr atom
Maxwell electromagnetism is not a very
good approximation
to the force between the proton and the
electron the quantum mechanics
if you if you if you didn't have quantum
mechanics the electron would would
spiral into the proton and the atom
would collapse it's Quantum you know so
that's a huge correction there sure
so every Theory gets corrected as we
learn more
dude just be no reason to suppose that
it should be otherwise well how is this
really to the singularity why the
singular so when you hit the singularity
you know that you need some
Improvement
to Einstein's theory of gravity
and that Improvement we understand what
kind of things that Improvement should
involve it should involve quantum
mechanics Quantum effects become
important there it's a small thing
and
um
we don't understand exactly what the
theory is but we know there's no reason
to think you know Einstein's theory was
invented to describe
weekly curves things the solar system
and so on it it's incredibly robust that
we now see that it works very well near
the horizons of around black holes and
so on so
so it's a good thing
that the theory drives itself
that it predicts its own demise
Newton's gravity had its demise
there were regimes in which it wasn't
valid
Maxwell's electromagnetism had its
demise there was uh
regimes in which Quantum effects greatly
modified
the equations
but
general relativity all on its own
found drove a system which originally
was fine
would perversely wander off into a
configuration in which Einstein's
equations no longer applied so to you
the edges of the theory are wonderful
the failures the edges are wonderful
because that keeps that keeps us in
business so that one of the things you
said I think in your Ted Talk that uh
the the the fact that quantum mechanics
and uh and relativity
don't describe everything and then they
Clash is wonderful all right I forget
the adjective you used but it was
something like this so why is that uh
why is that interesting to you in that
same way that there's contradictions
that create Discovery there's no
question in my mind of course many
people would disagree with me that now
is the most
wonderful time to be a physicist so so
people people look back at at um
it's a classical thing to say
among physicists uh I wish it were 1920.
right quantum mechanics had been just
understood
uh there was the periodic table
there was but in fact that was such a
rich thing
um that
uh
well so that what a lot of exciting
stuff happened around 1920. it took it
took the whole it took a whole century
to sort out the new insights that we got
especially adding some experimental
stuff into the into the bunch actually
making observations and adding all the
experimental things all the computers
also help with visualizations and all
that kind of stuff yeah yeah yeah it was
a whole sort of Wonderful
Century I mean the seed of
general relativity
was the incompatibility
of Maxwell's Theory of the
electromagnetic field
with Newton's laws of gravity they were
incompatible because
if you look at Maxwell's Theory
there's a contradiction if anything goes
faster than the speed of light
but Newton's theory of gravity
the uh
gravitational field the gravitational
force is instantaneously transmitted
across the entire universe
so you could you know if you had a
a a a friend on you know in another
galaxy with a very sensitive
measuring device that could measure the
gravitational field that could just take
this cup of coffee and move it up and
down and Morse code and they could get
the message instantaneously over another
galaxy that leads to all kinds of
contradictions it's not it's not
self-consistent
it was exactly in resolving those
contradictions that Einstein came up
with the general theory of of
relativity and it's fascinating how this
contradiction which seems like maybe
it's kind of technical thing
led to a whole new vision of the of the
universe
now let's not get fooled because
lots of contradictions are technical
things we haven't set up the
we run into other kinds of
contradictions that are Technical and
they they don't seem to you know they
would just we understood something wrong
we made a mistake we set up our
equations in the wrong way we didn't
translate the formalisms as opposed to
revealing some deep mystery that's yet
to be uncovered yeah yeah and so we
never we're never very sure which are
which are the really important ones
but to you the difference between
quantum mechanics and general relativity
seem the the tension the contradiction
there seems to hint at some deeper
deeper thing that's going to be
discovered in the century yes because
that one has been understood since the
50s poly was the
uh first person to notice it and Hawking
in the early 70s gave it a really much
more visceral form
um and people have been
hurling themselves at it trying to
reduce it to some technicality but
nobody has succeeded and the efforts to
understand it have led to
uh all kinds of interesting relations
between Quantum systems and and
applications to other fields and and so
on well let's actually jump around so
we'll return to black holes I have a
million questions there but let's let's
go into this unification
uh the battle against the contradictions
and the tensions between the theories of
physics what is quantum gravity maybe
what is the standard model of physics
what is quantum mechanics what is
general relativity what's quantum
gravity
uh what are all the different
unification efforts
okay so again five questions yeah
it's a theory that describes
everything with astonishing accuracy
it's the most
accurate theory in the history of human
thought
Theory and experiment have been
successfully compared to 16 decimal
place
we have that stenciled on the door where
where I work you know it's a it's an
amazing it's an amazing feat of the
human mind
it describes
um the electromagnetic interaction
unifies the electromagnetic interaction
with the so-called weak interaction
which
you need some good tools to even view
the weak interaction and then there's
the strong interaction
which binds the quarks into protons
and the forces between them are mediated
by something called Yang Mills Theory
which is a a beautiful mathematical
generalization of electromagnetism
in which the analogs of the photons
and themselves carry charge
and
um so this uh the final piece of this
of the standard model everything in the
standard model has been observed and its
properties have been measured the final
particle to be observed was the Higgs
particle
served like a over a decade ago the
Higgs is already a decade ago I I think
it is yeah wow time flies but you better
check me on that yeah it's it's true but
so much fun has been happening it's so
much fun it's been happening and so
that's all
um
that's that's all pretty well understood
there's some things that miter might not
around the edges of that you know Dark
Matter neutrino masses
some sort of
fine points or things we
haven't quite measured perfectly and so
on but it's largely a very complete
uh complete Theory and we don't
expect
anything very new
conceptually
in the completion of that anything
contradictory but I'm new because can't
you think contradictory yeah I'll have
some wild questions uh for you on that
front but yeah anything that yeah
because there's no gaps it's so accurate
so precise it's predictions it's hard to
imagine something yeah yeah yeah and
it was all based on something called let
me not explain what it is let me just
throw out the buzzword
renormalizable quantum field Theory they
all fall in the category of
renormalizable quantum field Theory I'm
going to throw that at a bar later to
impress
impress the girls
good luck thank you
all right so uh they all they all fall
under that rubric gravity will not will
not will not put that suit on
so the force of gravity
Cannot Be Tamed by the same
renormalizable Quantum field Theory to
which the all the other forces
so eagerly submitted what is the effort
of quantum gravity what are the
different efforts to um
to have these two dance together
effectively to try to unify the standard
model and um
and general relativity any kind of model
of gravity
sort of the one fully
uh consistent model that we have
that reconciles
that it it would that sort of tames
gravity and reconciles it with quantum
mechanics
uh is string theory and its cousins and
we don't know what or if in any sense
String Theory describes the world the
physical world
but we do know that it
um
is a consistent
reconciliation of quantum mechanics and
general relativity and moreover one
which
um
which is able to incorporate
particles and forces like the ones we
see around us so
it hasn't been ruled out as an actual
sort of unified theory of nature
but there also isn't a
in my view some people would disagree
with me
but there isn't a reasonable uh
possibility that we would be able to do
an experiment in the foreseeable future
which would be sort of a yes or no
to to string theory okay so you've been
there from the early days of string
theory you've seen his developments what
are some interesting developments uh
what do you see as the also the future
of string theory
and what is string theory
well the basic idea which emerged in
the early 70s
was that if you uh
you take
uh the notion of a particle and you
literally
replace it by a little Loop of string
that strings are sort of softer
than than particles what do you mean by
softer
well you know if you hit a particle
if there were particle on this table a
big one and you hit it you might bruise
yourself sure
but if there was a string on the table
you would probably just push it around
and and this the source of the
infinities
in Quantum field theories that would
particles hit each other it's a little
bit of a it's a little bit of a
a jarring effect and and uh
I've never described it this way before
but it's actually scientifically
accurate but if you throw strings at
each other it's a little more friendly
one thing I can't explain is how
wonderfully precise will the mathematics
is that goes into describing String
Theory we don't just wave our hands and
throw strings around and you know
there's some very
um
compelling mathematical equations that
describe it now what was realized in the
early 70s is that if you replace
particles by strings these Infinities go
away
and you get a uh consistent
theory of gravity without
the infinities
and um
that may sound a little trivial but at
that point it already been
15 years that people had been searching
around for any kind of theory that could
do this and it was actually found kind
of
uh by accident
and there are a lot of accidental
discoveries uh in this subject
now at the same time it was believed
then that string theory was an
interesting sort of toy model
for putting quantum mechanics and
general relativity together
on paper
but
um
but that it couldn't describe some of
the very idiosyncratic phenomena
that pertain to our own Universe in
particular the form of so-called parity
violation
our another term for the bar later
tonight yeah yeah parity violation so so
if you go to the bar and I already got
the renormalizable quantity and you look
in the mirror across the bar yes the
universe that you see in the mirror is
not identical
you would be able to tell if you show
your your your your your
the lady in the bar the photograph that
shows both the mirror and you there's a
difference if she's smart enough she'll
be able to to tell which one is the real
world and which one is you now she would
have to do some very precise
measurements
and if the photograph was too grainy it
might not be possible but as Principle
as possible why is this interesting why
is it does this mean that there is some
um not perfect determinism or uh what
does that mean there's some uncertainty
no it's a very interesting feature of
the real world
that it isn't parody of invariant and
string theory it was thought could not
tolerate that
and um then it was learned in the mid
80s that not only could it tolerate that
but if you did things in the right way
you could
construct a world uh
involving strings that reconciled
quantum mechanics and general relativity
which looked more or less like the world
that
we live in
and now that isn't to say that strings
Theory predicted our world
it just meant that it was consistent
that the the hypothesis that string
theory describes our world can't be
ruled out from the get-go
and it is also
the only
proposal for a complete theory that
would describe
our world
still
nobody will believe it
until there's some kind of
direct experiment and I don't even
believe it myself sure which is a good
place to be mentally as a physicist
right always I mean Einstein didn't
believe his own uh equations right with
the black hole okay well that money was
wrong about that but
but you might be wrong too right uh so
do you think string theory is dead if
you were to bet all your money on
um no the future of strength I think
it's a
a logical error
to think that string theory is either
right or wrong or dead or alive
what it is is a stepping stone
and
uh an analogy I like to draw
is yangmil's theory
which I mentioned a few minutes ago in
the context of standard model
yangmail's theory was discovered by
Yangon Mills in the 50s
and they thought that the symmetry of
Yang and Mills
Theory
described the relationship between the
proton and the neutron that's why they
invented it
that turned out to be completely wrong
it does however
describe everything else in the standard
model
and it had a kind of inevitability you
know they had some of the right pieces
but not the other ones sure they didn't
have it quite in the right context
and it had an inevitability to it and it
eventually sort of found its place
and it's also true of Einstein's
theory of general relativity you know he
had the wrong version of it in 1914 and
he was missing some pieces and you
wouldn't say that that his early version
was right or wrong he'd understood the
equivalence principle it understood
space-time curvature he just didn't have
everything I mean technically you would
have to say it was wrong
and technically you would have to say
Yang and Mills were wrong and I guess in
that sense
I would believe just
odds are
we always keep finding new wrinkles odds
are we're going to find new wrinkles and
strings Theory and technically what we
call String Theory Now isn't quite right
but we're always going to be wrong but
hopefully a little bit less wrong every
time except exactly and I I would you
know bet the farm as they say do you
have a farm
you know I say that much more seriously
because not only do I have a farm but we
just renovated it
simply before I read it so before I
renovated better get the far my wife I
spent five years renovating it before I
you were much much looser with that
statement but now I really means
something no no it really means
something and and I would bet the farm
on the um
on the uh guess that 100 years from now
String Theory will be viewed as a
stepping stone towards a greater
understanding of of nature
and and it would I mean another thing
that I didn't mention about strings
theory is of course we knew that it
solved the Infinities problem and then
we later learned that it also solved
Hawking's puzzle about
what's inside of a black hole
and you put in one assumption you get
five things out you somehow you're doing
something right you know probably not
everything but you're you're you know
there's some good signposts and there
have been a lot of good signposts like
that
it is also mathematical toolkit and you
you've used it you've used it with
comrade Waffa uh maybe we can sneak our
way back from String Theory into black
holes yeah
um what was the idea that you and
Cameron valpha developed with the
holographic principle and string theory
were we able to discover through through
this through string theory about black
holes or um
that connects us back to the reality of
black holes yeah so that is a very
interesting story I was
interested in black holes before I was
interested in String Theory I was sort
of a reluctance
strength there is in the beginning I
thought I had to learn it because people
were talking about it but you know once
I studied it I I grew to love it first I
did it in a sort of dutiful way these
people say they've claimed quantum
gravity I ought to read their papers at
least
and then the more I read them the more
interested I got and I began to see you
know they
they phrased it in a very clumsy way the
description of string theory was was
very clumsy and mathematically clumsy or
just mathematically yeah
it was all correct but
but
mathematically clumsy but it often
happens
that in all kinds of branches of physics
that
um people start working on it really
hard and they sort of dream about it and
live it and breathe it and they begin to
see inner relationships and
they see a beauty that
is really there they're not they're not
deceived they're really seeing something
that exists but if you just kind of look
at it you know you can't you can't grasp
it all in the beginning and and um
so
our understanding of string theory in uh
uh in 1985 was almost all about uh you
know
weekly coupled waves of strings
colliding and so on we didn't know how
to describe
a big thing like a black hole and so you
know in string theory of course we could
show that strings in theory in some
limit reproduced Einstein's theory of
general relativity
and corrected it but we couldn't do any
better with black holes
than
um
before my work with command we couldn't
do any better than Einstein and tortill
had done
now
um one of the puzzles
um you know if you look at the
Hawking's headstone and also Boltzmann's
headstone and you
put them together you get a formula
for their really Central equations in
20th century physics I don't
think there are many equations that made
it to headstones
and and they're really Central equations
and you put them together and you get a
formula for the number of gigabytes in a
black hole
now a short shelves description
the black hole is literally a hole in
space and there's no place to store the
gigabytes
and it's not too hard to and this really
was wheeler and beckenstein and
wheeler beckenstein and Hawking
to come to the conclusion that
if there isn't a sense in which a black
hole can store
some large number of gigabytes
that quantum mechanics and gravity can't
be consistent
we got we got to go there a little bit
so uh so how is it possible I won't say
gigabyte say there's some information so
black holes can store information how is
this thing that sucks up all light and
it's supposed to basically be you know
be super homogeneous and boring how is
that actually able to store information
where does the store information on the
inside on the surface
uh where where's yeah and what's
information
I'm liking this ask five questions to
see which one you actually answer oh
okay I'm going to ask you about that I
should try to memorize them and answer
each one in order just to answer them I
don't know I don't know what I'm doing
I'm desperately desperately uh trying to
figure it out as I go along here so
um Einstein's Black Culture short sort
of black hole they can't store
information
this stuff stuff goes in there and it
just keeps flying and it goes to the
singularity and it's gone
however
Einstein's theory is not exact
it has Corrections
and string theory tells you what those
Corrections are
and so you should be able to find some
way of some alternate way of describing
the black hole that enables you
to understand
where the gigabytes are stored
so what Hawking and beckenstein really
did was they showed that physics is
inconsistent
unless
a black hole can store an a number of
gigabytes proportional to its area
divided by four times
Newton's constant times Planck's
Constant and that's another wild idea
you said area not volume
exactly
and that's the holographic principle the
universe is so weird and that's the
holographic principle that's called the
holographic principle that is it's the
area we're just jumping around what is
the holographic principle what does that
mean well is this some kind of weird
projection going on what what the heck
uh well I was just before I came here
writing an introduction to a paper and
the first sentence was
the
as yet imprecisely defined holographic
principle
blah blah blah so nobody knows exactly
what it is
but roughly speaking it says just what
we were alluding to that
um
really all the information
that is in some volume of space-time can
be stored on the boundary of that region
so this is not just about black holes
it's about any any areas based on any
area space however we've made sense of
the holographic principle for black
holes
we've made sense of the holographic
principle for something which could be
called anti-decider space which
could be thought of as a giant as the
black hole turned into a whole universe
and
um we don't really understand how to
talk about the holographic principle for
either flat space which we appear to
live in
or
asymptotically the sitter space which
astronomers tell us we actually live in
as the universe continues to expand
so it's one of the one of the huge
problems in uh physics is to
you know apply or even formulate the
holographic principle for
more realistic
well black holes are realistic we see
them but um
yeah in in more General context so from
a general statement of the holographic
principle what's the difference in flat
space and uh asymptotic decider space so
flat space is just an approximation of
like the world we live in so like uh uh
the sitter space at some time I wonder
what that even means meaning like
uh asymptotic over what okay so for
thousands of years
you know until the last half of the 20th
well sorry until the 20th century
um we thought space time was flat
can you elaborate on flat
or what do we mean by flat
well like the surface of this table
is is flat let me just give an intuitive
explanation surface of the table is flat
but the surface of a basketball
is curved
so the universe itself
could be flat like the surface of a
table or it could be curved like a
basketball which actually has a positive
curvature
and then there's another kind of
curvature called the negative curvature
and curvature can be even weirder
because
that kind of curvature I've just
described is the curvature of space
but Einstein taught us that we really
live in a space-time continuum
so we can have curvature in a way that
mixes up space and time
and that's kind of hard to visualize
because you have to step what a couple
of Dimensions up so it's hard to you
have to step a couple but even a
if you have flat space and it's
expanding in time
you know we could imagine we're sitting
here this room good approximation it's
flat but imagine we suddenly start
getting further and further apart then
space is flat
but it's expanding which means it's
space time is curved
automates about space time okay so
what's the what's the sitter in
anti-disitter space
the three simplest space times
are flat space time which we call
minkowski's base time
and negatively curved space-time
anti-decider space
and positively curved space-time the
sitter space
and so astronomers
um think that
on large scales even though for
thousands of years we hadn't noticed it
beginning with Hubble
we started to notice that space time was
curved space is expanding in time means
that space time is curved
and the nature of this curvature is
affected by the matter in it
because matter itself
causes the curvature of space-time
but as it expands the matter gets more
and more diluted
and one might ask when it's all diluted
away
is space-time still curved
and astronomers believe they've done
precise enough measurements to determine
this
and they believe that the answer is yes
the universe is now expanding eventually
all the unit matter in it will be
expanded away but it will continue to
expand
because uh well they would call it the
dark energy Einstein would call it a
cosmological constant in any case that
the in the far future
matter will be expanded away and we'll
be left with empty decider space okay so
there's this cosmological Einstein's
cosmological constant that now hides
this thing that we don't understand
called Dark Energy what's dark energy
what's your best guess at what this
thing is
why do we think it's there
it's because of this it comes from the
astronomers
dark energy is synonymous with positive
cosmological constant
and um
uh we think it's there
because the astronomers have told us
it's there
and
um they they know what they're doing and
we don't know really really hard
measurement but they know they really
know what they're doing
and
we have no freaking idea why it's there
another big mystery another another
reason it's fun to be a physicist and if
it is there why should it be so small
why should there be so little why should
it have hit itself from us
why shouldn't there enough be enough of
it to substantially cons curve the space
between us and the moon why did there
have to be such a
small amount that only the crazy best
astronomers in the world could find it
well again the same thing be said about
all all the constants
all of the content be said about gravity
can't they be said about the speed of
light
like why is the speed of light so slow
so fast so slow
relative to the size of the universe
can't it be faster
or no well the speed of Lights is a
funny one because you could always
choose units
in which the speed of light is one you
know we measure it in kilometers per
second and it's 100
86 000 or miles per second is 186 000
miles per second
and but if we had used different units
yeah then we could make it one but you
can make dimensionless ratios
so
um you know you could say why is the
time scale set by the expansion of the
universe so large compared to the time
scale of a human life or so large
compared to the time scale for a neutron
to Decay you know yeah I mean ultimately
you know the reference the temporal
reference frame here is a human life
maybe isn't that the important thing for
us uh descendants of Apes isn't that a
really important aspect of physics
like uh because we kind of experienced
the world we Intuit the World Through
The Eyes of
the these biological organisms I guess
mathematics helps you escape that for a
time but ultimately isn't that
how you wonder about the world
absolutely that like a human life yeah
time is only 100 years because if you
think of everything
um if you're able to think in I don't
know in billions of years
uh then maybe everything looks way
different
maybe universes are born and die and
maybe all these physical phenomena
become much more intuitive that we see
at the Grand scale of general relativity
well that is one of the a little off the
track here but that certainly is one of
the nice things about being a physicist
it's
you spend a lot of time thinking about
you know insides of black holes and
billions of years in the future and and
it's sort of uh
gets you away from the day-to-day uh
into into another fantastic
realm
um
but I was answering your question about
how there could be information in a
black hole yes
so
Einstein only gave us
an approximate description and we now
have a theory that corrects it string
theory
and now sort of was the Moment of Truth
well when we first discovered String
Theory we knew we knew from the get-go
that string theory would correct what
Einstein said
just like Einstein corrected what Newton
said
um but we didn't understand it well
enough
to actually compute the correction to
compute how many gigabytes there were
and sometime in the early 90s
we began to understand the mathematics
of string theory better and better
and it came to the point where it was
clear that this was something we might
be able to compute
and it was a kind of Moment of Truth for
string theory because
if it hadn't given the answer
that beckenstein and Hawking said it had
to give for consistency
String Theory itself would have
been inconsistent and we wouldn't be
doing this interview well
that's a very dramatic statement but yes
uh that's not the most that's not the
most dramatic thing
I mean but like okay that's very life
and death you mean like that that uh
because string theory was Central to
your work at that time is that what you
mean well String Theory would have been
inconsistent yeah okay so that would be
a string theory would have been
inconsistent but those inconsistencies
can give birth to other theories like
you said the inconsistency right
something else could have happened yes
yeah it would have been a major a major
uh change in the way we think about
string theory if it and it was a good
thing that you know one supposition that
the world is made of strings solves two
problems not not one solves the infinity
problem and it solved the Hawking's
problem
and also the way that it did it
was very uh was very beautiful
um it it gave an alternate description
so
alternate description thing of things
are are uh
are very common I mean we could to take
a simple example this bottle of water
here is
90 percent full I could say it's 90 full
I could also say it's ten percent empty
those are obviously the same statement
and they're it's trivial to see that
they're the same but there are many
statements that can be made in
mathematics and mathematical physics
that are equivalent
but might take years to understand that
they're equivalent
and might take the invention or
discovery of whole new fields of
mathematics to prove their equivalent
and this was one of those
we found an alternate description
of the certain black holes and string
theory
which we could prove was equivalent and
it was a description of the black hole
as a hologram
that can be thought of a holographic
plate
that could be thought of as sitting on
the surface of the black hole
and the interior of the black hole
itself sort of arises as a projection
uh or the near Horizon region of the
black hole arises as a projection
of that holographic plate so the two
descriptions were the hologram
the three-dimensional image and the
holographic plate
and the whole gram is what Einstein
discovered in the holographic plate
is what we discovered
and this idea that you could describe
things very very concretely in string
theory in these two different languages
of course took off and was applied to
many uh many different
many different contexts within string
string theory so you mentioned the
infinity problem in the Hawking problem
uh witch-hawking problem the the that
the black hole destroys information or
that the what which Hawking problem are
we talking about well there's really two
Hawking problems they're very closely
related
one is how does the black hole store the
information
and
um
that is the one that we
solved in some cases so it's sort of
like you know your your smartphone
how does it store at 64 gigabytes well
you rip the cover off and you count the
chips and there's 64 of them each with a
gigabyte and you know they're 64
gigabytes
but that does not solve the problem of
how you get information in and out of
your smartphone
you have to understand a lot more about
the Wi-Fi and the internet and the
cellular and and that's where Hawking
radiation this prediction it starts
that's where Hawking radiation comes in
and that problem of how the information
gets in and out you can't you couldn't
have explained how information gets in
and out of an iPhone without first
explaining
how it's stored in the first place so
just to clarify the storage is on the
plate
on the flight on the holographic plate
and then it projects somehow inside the
the bulk the the space time is the
Hologram the Hologram but man I mean did
you have any intuitive when you sit late
at night and you stare at the stars do
you have any intuitive understanding
what a holographic plate is
um like that there's two Dimension no
projections that store information
how a black hole
could store information on a holographic
plate
I think we do understand in in great
mathematical detail and also intuitively
and it's very much like an ordinary
hologram or you hold up a holographic
plate and you sh it contains all the
information you shine a light through it
and you get an image which looks
three-dimensional yeah but why should
there be a holographic plate
why should there be yeah why
that is the Great thing about being a
theoretical physicist is
anybody can very quickly stump you if
they going to the next level of wise
yeah so if I can just keep asking yeah
you could just keep asking and it won't
take you very long to
so the trick in being a theory a
theoretical physics is finding the
questions that
you can answer sure so so the questions
that we think we might be able to answer
now and we've partially answered
is that
um there is a holographic explanation
for certain
kinds of things and string theory Sure
we've answered that
now we'd like to take what we've learned
and that's what I've mostly been doing
for the last 15
20 years I haven't really been working
so much on string theory proper I've
been sort of taking the lessons that you
we learned in string theory
and trying to apply them to the real
world
using only
assuming only what we know for sure
about the real world so on this uh topic
you you co-wrote co-author the paper
with Stephen Hawking called soft hair on
black holes yes that makes the argument
against Hawking's original prediction
that black holes destroy information can
you explain this paper
yeah and the title yeah
okay so um first of all
um
the hair on black holes
is a word that was coined by the
greatest phrase master in the history of
physics John Wheeler invented the word
black hole
and he also said that uh he made the
statement that black holes have no hair
that is every black hole in the universe
is described just by its mass and spin
they wrote they can also rotate as was
later shown by Kerr
and
um
and this is very much unlike a star
right every Star of the same mass is
different
in a multitude of different ways
different chemical compositions
different motions of the individual
molecules every star in the universe
even of the same mass is different in
many many different ways
black holes are all the same
and that means when you throw some in
Einstein's description of them
which we think must be corrected
and um
if you throw something into a black hole
it gets sucked in
and if you uh throw in a red book or a
Blue Book
the black hole gets a little bigger but
there's no way within Einstein's theory
of telling how they're different
and that was one of the assumptions
that Hawking made in his
1974-75 papers in which he concluded
that black holes destroy information you
can throw encyclopedias thesis defenses
the Library of Congress it doesn't
matter it's going to behave exactly the
same uniform way yeah so what what
Hawking and I showed and also Malcolm
Perry
um
is that one has to be very careful about
what happens
at the boundary of the black hole
and this gets back to something I
mentioned earlier about when two things
which are related by a coordinate
transformation are and are not
equivalent
and
um
and what we showed is that they're very
subtle imprints when you throw something
into a black hole
they're very subtle imprints left on the
horizon of the black hole which you can
read off at least partially what went in
and
um so this
invalidates uh Steven's original
uh argument that the information is
destroyed and that's a soft hair that's
the soft hair right so and soft is the
word that is used in physics for things
which have very low energy and these
things actually carry no energy
there are things in the universe which
carry no energy
you said I think to Sean Carroll
um by the way everyone should go check
out Sean Carroll's mindscape podcast
it's incredible and Sean Carroll is an
incredible person I think he said there
maybe in a paper I have a quote you said
that a soft particle is a particle
that has zero energy just like you said
now and when the energy goes to zero
because the energy is proportional to
the wave of length it's also spread over
an infinitely large distance if you like
it's spread over the whole universe
it somehow runs off to the boundary what
we learned from that is that if you add
a zero energy particle to the vacuum you
get a new state and so there are
infinitely many vacuole
plural for vacuum which can be thought
of as being different from one another
by the addition of soft photons or soft
gravitons right can you uh elaborate on
this wild idea
if you like it spreads over the whole
universe when the energy goes to zero
because the energy is proportional to
the wavelength it also spreads over an
infinitely large distance if you like
it's spread over the whole unit it's
spread over the whole universe what
um can you explain these soft gravitons
and photons
yeah so the soft gravitons and photons
um have been uh known about since the
60s
but exactly what we're supposed to do
wi