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Language: en
the whole point of Relativity is to say
there's no such thing as right now when
you're far away and that is doubly true
for what's inside a black hole and you
might think well the Galaxy is very big
it's really not it's some tens of
thousands of light years
across and billions of years old so you
don't need to move at a high fraction of
the speed of light to fill the Galaxy
the number of Worlds is Big very very
very big what do those worlds fit where
do they go the short answer
is the worlds don't exist in
space space exists separately in each
world the following is a conversation
with Shan Carol his third time in this
podcast he is a theoretical physicist at
John Hopkins host of the mindscape
podcast that I personally love and
highly recommend and author of many
books including the most recent book
series called the biggest ideas in the
universe the first book of which is
titled space time and motion and it's on
the topic of general relativity and the
second coming out on May 14th so you
should definitely pre-order it is titled
quanta and Fields and that one is on the
topic of quantum mechanics Shawn is a
legit active theoretical physicist and
at the same time is is one of the
greatest communicators of physics ever I
highly encourage you listen to his
podcast read his books and pre-order the
new book to support his work this was as
always a big honor and a pleasure for me
this is Alex Freedman podcast to support
it please check out our sponsors in the
description and now dear friends here's
Sean
Carol in book one of the series the
biggest ideas in the universe called
space-time motion you take on classical
mechanics general relativity uh by
taking on the main equation of general
relativity and making it uh accessible
easy to understand so um maybe at the
high level what is general relativity
what's a good way to start to try to
explain it probably the best way to
start to try to explain it is special
relativity which came first 1905 uh it
was the culmination right of many
decades of people putting things
together but it was Einstein in 1905 in
fact it wasn't even Einstein I should
give more credit to manowski in 1907 so
Einstein in 1905 figured out that you
could get rid of The Ether the idea of a
rest frame for the universe and all the
equations of physics would make sense
with the speed of light being a maximum
but then it was manowski who used to be
Einstein's professor in 1907 who
realized the most elegant way of
thinking about this idea of Einstein's
was to blend space and time together
into space time to really imagine that
there is no hard and fast division of
the four-dimensional world in which we
live into space and time separately
Einstein was at first dismissive of this
he thought it was just like oh the
mathematicians are over formalizing
again but then he later realized that if
SpaceTime is a thing it can have
properties and in particular it can have
a geometry it can be curved from place
to place and that was what let him solve
the problem of gravity he was always
been he had previously been trying to
fit in what we knew about gravity from
Newtonian uh mechanics the inverse
Square law of gravity to his new
relativistic Theory it didn't work so
the final leap was to say gravity is the
curvature of SpaceTime and that
statement is basically general
relativity and uh the tension with
Makowski was he was a mathematician yes
so it's the tension between physics and
and Mathematics in fact
in uh your lecture about this equation
one of
them you uh say that Einstein is a
better physicist than he gets credit for
yep I know that's hard that's a little
bit of a joke there right because we all
give Einstein a lot of credit but then
we
also partly based on fact but partly to
make ourselves feel better tell
ourselves a story about how later in
life Einstein couldn't keep up uh there
were younger people doing quantum
mechanics and quantum field Theory and
particle physics and he was just sort of
uh unable to really philosophically get
over his objections to that and I think
that that story about the latter part is
completely wrong like almost 180 degrees
wrong I think that Einstein understood
quantum mechanics as well as anyone at
least up through the 1930s I think that
his philosophical objections to it are
correct so he should actually have been
taken much more seriously about that and
what he did what he achieved in trying
to think these problems through is to
really basically understand the idea of
quantum entanglement which is kind of
important these days when it comes to
understanding quantum mechanics now it's
true that in the 40s and 50s uh he
placed his efforts in hopes for unifying
electricity and magnetism with gravity
that didn't really work out very well
all of us you know try things that don't
work out I don't hold that against him
but in terms of IQ points in terms of
trump to be a clear thinking physicist
he was really really great what does
greatness look like for a physicist so
how difficult is it to take the leap
from special relativity to general
relativity how difficult is it to
imagine
that to consider SpaceTime together and
to imagine that uh there's a curvature
to this whole
thing yeah that's a great question um I
think that if you want to make the case
for Einstein's greatness which is not
hard to do there's two things you point
at one is in 1905 his famous miracle
year he writes three different papers on
three wildly different subjects all of
which are would make you famous just for
writing that one paper um special
relativity is one of them Brownie and
motion is another one which is just you
know the little vibrations of tiny
little dust specs in the air but who
cares about that what matters is it
proves the existence of atoms he
explains browni and motion by imagining
there molecules in the air and deriving
their properties brilliant and then he
basically starts the world on the road
to Quantum Mechanics with his paper on
which again is given a boring label of
the photoelectric effect what it really
was is he invented photons he showed
that light should be thought of as
particles as well as waves and he did
all three of those very different things
in one year okay but the other thing
that gets him genius status is like you
said general relativity so this takes 10
years from 1905 to 1915 he wasn't only
doing general relativity he was working
on other things he wrote he invented a
refrigerator he did various interesting
things and he wasn't even the only one
working on the problem there were other
people who suggested relativistic
theories of gravity but he really
applied himself to it and I think as
your question suggests the solution was
not a matter of turning a crank it was
was something fundamentally creative you
know the in his own telling of the story
his greatest moment his happiest moment
was when he realized that if the way
that we would modern in say it in modern
terms if you were in a rocket ship
accelerating at one g at one uh
acceleration due to gravity if the
rocket ship were very quiet you wouldn't
be able to know the difference between
being in a rocket ship and being on the
surface of the Earth gravity is sort of
not detectable or at least not
distinguishable from acceleration so
number one that's a pretty clever thing
to think but number two if you or I had
that thought we would have gone huh
we're pretty clever he Reasons from
there to say okay if gravity is not
detectable then it can't be like an
ordinary Force right the electromagnetic
force is detectable we can put charged
particles around positively charged
particles and negatively charged
particles respond differently to an
electric field or to a magnetic field he
realizes that what his thought
experiment showed or at least suggested
is that gravity isn't like that
everything responds in the same way to
gravity how could that be the case and
then this other leap he makes is oh it's
because it's the curvature of SpaceTime
right it's a feature of SpaceTime it's
not a force on top of it and the feature
that it is is curvature and then finally
he says okay clearly I'm going to need
the mathematical tools necessary to
describe curvature I don't know them so
I will learn them and they didn't have
mukes or AI uh helpers back in those
days he had to sit down and read the
math papers and he taught himself
differential geometry and invented
general relativity what about the step
of including time as just another
dimension so combining space in time is
that a simple mathematical leap as
Makowski suggested it's certainly not
simple actually um it's a it's a
profound Insight that's why I said I
think we should give mow more credit
than we do you know he's the one who
really put the finishing touches on
special relativity again many people had
talked about how things change when you
move close to the speed of light uh what
Maxwell's equations of electromagnetism
predict and so forth what their
symmetries are so people like Lorent and
Fitzgerald and poner there's a story
that goes there and in in the usual
telling Einstein sort of puts the
Capstone on it he's the one who says all
of this makes much more sense there just
is no ether it is undetectable we don't
know how fast everything is relative
thus the name relativity but he didn't
take the actual final step which was to
realize that the underlying structure
that he had invented is best thought of
as unifying space and time together I
honestly don't know what was going
through makowski's mind when he thought
that that I'm not sure if he was you
know so
mathematically Adept that it was just
clear to him uh or he was really
struggling it and he did trial in err
for a while I'm not sure I mean do you
for him or for Einstein visualize the
four-dimensional space try to play with
the idea of time is just another
dimension oh yeah all the time I mean we
of course make our lives easy By
ignoring two of the dimensions of space
so instead of four dimensional SpaceTime
we just draw pictures of one dimension
of space one dimension of time the
so-called SpaceTime
diagram but you know I mean maybe this
is lurking underneath your question but
even the best physicists we'll draw you
know a hor a vertical axis and a
horizontal axis and they'll go space
time but deep down that's wrong because
you're sort of preferring One Direction
of space and One Direction of time and
it's really the whole two-dimensional
thing that is spacetime the more
legitimate thing to draw on that picture
are rays of light are light cones from
every point there is a fixed Direction
at which the speed of light would
represent and that is actually inherent
in the structure the division into space
and time is something that's easy for us
human beings what is the difference
between space and time from the
perspective of general relativity it's
the difference between X and Y when you
draw xes on a piece of paper so there
really no difference there is almost no
difference there's one difference that
is kind of important which is the
following if you have a curve in space
I'm going to draw it horizontally
because that's usually what we do in
SpaceTime diagrams if you have a curve
in space you've heard the motto before
that the shortest distance between two
points is a straight line if you have a
curve in time which is by the way
literally all of our lives right we all
evolve in time so you can start with one
event in SpaceTime and another event in
SpaceTime what manowski points out is
that the time you measure along your
trajectory in the universe is precisely
analogous to the distance you travel on
a curve through space
and by precisely I mean it is also true
that the actual distance you travel
through depends on your path right you
go a straight line shortest distance and
curvy line would be longer the time you
measure in SpaceTime the literal time
that takes off on your clock also
depends on your path but it depends on
it the other way so that the longest
time between two points is a straight
line and if you Zig back and forth in
space time you take less and less time
to go from point A to point B
how do I make sense of that the uh
difference between the observed reality
and
the objective reality underneath it or
is objective reality a silly notion
given general relativity I'm a huge
believer in objective reality I think
that objective reality objective is real
um but I do think
that people kind of are a little overly
casual about the relationship between
what we observe and objective reality in
the following sense of course in order
to explain the world our starting point
and our ending point is our observations
our experimental input the phenomena we
experience and see around us in the
world but in
between there's a theory there's a
mathematical formalization of our ideas
about what is going on and if a theory
fits the data and is very simple and
makes sense in its own terms then we say
that the theory is right and that means
that we should attribute some reality to
the entities that play an important role
in that theory at least provisionally
until we come up with a better Theory
down the road I think a nice way to test
the difference between objective reality
and The observed reality is what happens
at the uh at the edge of the Horizon of
a black
hole so technically as you get closer to
that Horizon time stands still yes and
no it depends on exactly how careful
we're being so
here's a a bunch of things I think are
correct if you imagine there is a black
hole SpaceTime so like the whole
solution Einstein's equation and and you
treat you and me as what we call test
particles so we don't have any
gravitational fields ourselves we just
move around in the gravitational field
that's obviously an approximation okay
but let's let's imagine that and you
stand outside the black hole and I Fall
In And as I'm falling in I'm waving to
you you know because I'm going into the
black hole you will see me move more and
more slowly and also the light from me
is red shifted so I kind of look
embarrassed because I'm falling into a
black hole and there is a limit there is
a last moment that light will be emitted
from me from your perspective forever
okay now you don't literally see it
because I'm emitting photons more and
more slowly right because from your
point of view so it's not like I'm
equally bright I basically fade from
view in that picture Okay so that's one
approximation the other approximation is
I do have a gravitational field of my
own and therefore as I approach the
black hole the black hole doesn't just
sit there and let me pass through it
kind of moves out to eat me up because
its net energy mass is going to mine
plus its but roughly speaking yes I
think so I don't like to go to the
dramatic extremes because that's where
the approximations break down but if you
see something falling into a black hole
you see its clock ticking more and more
slowly how do we know it fell in we
don't I mean how would we because it's
always possible that right at the last
minute it had a change of heart and
starts accelerating away right if you
don't see it pass in you don't know and
let's point out that as smart as
Einstein was he never figured out black
holes and he could have it's kind of
embarrassing it took decades for people
thinking about general relativity to
understand that there are such thing as
black holes because basically Einstein
comes up with general relativity
1915 two years later Schwarz Shield uh
Carl schwar Shield derives the solution
to Einstein's equation that represents a
black hole the short Shield solution no
one recognized it for what it was until
the 50s David finlin and other people
and that's just you know one of these
examples of physicists not being as
clever as they should have been well
that's the singularity that's the kind
of the edge of the theory the limit so
it's understandable that it's difficult
to imagine the limit of things it is
absolutely hard to imagine and a black
hole is very different in many ways from
what we're used to on the other hand I
mean I mean the real reason of course is
that between 1915 and 1955 there's a
bunch of other things that are really
interesting going on in physics all of
particle physics and Quantum field
Theory so many of the greatest Minds
were focused on that but still if the
universe hands you a solution to general
relativity in terms of curved SpaceTime
and it's kind of mysterious certain
features of it I would put some effort
into trying to figure it out so how does
a black hole work put yourself in the
shoes of Einstein and take general
relativity to its natural conclusion
about these massive things it's best to
think of a black hole as not an object
so much as a region of space SpaceTime
okay it's a region with the property at
least in classical general relativity
quantum mechanics makes everything
harder but let's imagine we're being
classical for the moment it's a region
of SpaceTime with the property that if
you enter you can't
leave literally the equivalent of
escaping a black hole would be moving
faster than the speed of light they're
both precisely equally difficult you
would have to move faster than speed of
light to escape from the black hole so
once you're in that's fine you know in
principle uh you don't even know notice
when you cross the Event Horizon as we
call it the Event Horizon is that point
of no return where once you're inside
you can't leave but meanwhile the
SpaceTime is sort of collapsing around
you uh to ultimately a singularity in
your future which means that the
gravitational forces are so strong they
tear your body apart um and you will die
in a finite amount of time the time it
takes if the if the black hole is about
the mass of the Sun to go from The Event
Horizon to the singular ity takes about
one millionth of a
second and what happens to you if you
fall into the black hole like if we
think of an object as uh information
that information gets destroyed well
you've raised a crucially difficult
point so that's why I keep needing to
distinguish between black holes
according to Einstein's theory of
general relativity which is book one of
SpaceTime and geometry which is
perfectly
classical and then come the 1970s we
start asking about quantum mechanics and
what happens in quantum mechanics
according to classical general
relativity the information that makes up
you when you fall into the black hole is
lost to the outside world it's there
it's inside the black hole but we can't
get it anymore in the 1970s Stephen
Hawkin comes along and points out that
black holes radiate they give off
photons and other particles to the
universe around them and as they radiate
they lose mass and eventually they
evaporate they disappear so once that
happens I can no longer say the
information about you or a book that I
threw in the black hole or whatever is
still there as hidden behind the black
hole because the black hole has gone
away so either that information is
destroyed like you said or it is somehow
transferred to the radiation that is
coming out to the Hawking radiation the
large majority of people who think about
this believe that the information is
somehow transferred to the radiation and
information is conserved that is the a
feature both of general relativity by
itself and of quantum mechanics by
itself so when you put them together
that should still be a feature we don't
know that for sure there are people who
have doubted it including Steph Hawking
for a long time but that's what most
people think and so what we're trying to
do now in uh a topic which has generated
many many hundreds of papers called the
black hole information loss puzzle is
figure out how to get the information
from you or the book into the radiation
that is escaping the black hole is there
any way to observe Hawking
radiation to a degree where you can
start getting Insight or is this all
just in the space of theory right now
right now we are nowhere close to
observing Hawking radiation here's the
sad fact the larger the black hole is
the lower its temperature
is so a small black hole like a
microscopically small black hole might
be very visible it's given off light
but something like the black hole the
center of our
galaxy 3 million times the mass of the
Sun or something like that Sagittarius A
star uh that is so cold and low
temperature that its radiation will
never be observable um black holes are
hard to make we don't have any nearby
the ones we have out there in the
universe are very very faint so there's
no immediate hope for detecting Hawking
radiation allegedly we don't have any
nearby as far as we know we don't have
any nearby could tiny ones be hard to
detect somewhere at the edges the solar
system maybe so you don't want them to
be too tiny or they're exploding right
they're they're very bright and then
they would be visible but there's an
absolutely regime where black holes are
large enough not to be visible because
the larger ones are fainter right not
giving off radiation but small enough to
not been detected through their
gravitational effect yeah
psychologically just emotionally how do
you feel about black holes they scare
you I love them I love black holes but
the universe weirdly makes it hard to
make a black hole right because we
really need to squeeze an enormous
amount of matter and energy into a very
very small region of space so we know
how to make Stellar black holes a super
massive star can collapse to make a
black hole we know we also have these
super massive black holes at the center
of the galaxies were a little unclear
where they came from I mean maybe
Stellar black holes that got together uh
and combined but that's you know one of
the exciting things about new data from
the jamesb Space Telescope is that quite
large black holes seem to exist
relatively early in the history of the
universe so it was already difficult to
figure out where they came from now it's
an even tougher
puzzle so these super massive black
holes are formed somewhere early on in
the
universe I mean that's a feature not a
bug right that we don't have too many of
them otherwise we wouldn't have a uh the
time or the space to form the the little
pockets of complexity that we call
humans I think that's
yeah it's always interesting when
something is difficult but happens
anyway right I mean the probability
making a black hole could have been zero
it could have been one but it's this
interesting number in between which is
kind of fun are there more intelligent
alien civilization than there are super
massive black holes yeah I have no idea
but I think your intuition is right that
it would have been easy for there to be
lots of civilizations and then we would
have noticed them already and we haven't
so absolutely the simplest explanation
for why we haven't is that they're not
there yeah I just think it's so easy to
make them though so there must be I
understand that's the simplest
explanation but also how easy is it to
make life or UK carotic life or
multicellular life it seems like life
finds a way intelligent alien
civilizations sure maybe there is
somewhere along that chain a really
really hard leap but once once you start
life once you get the origin of life it
seems like life just finds a way
everywhere in every condition it just
figures it out I mean I get it I get
exactly what you're thinking I think
it's a perfectly
reasonable attitude to have before you
confront the data I would not have
expected Earth to be special in any way
I would have expected there to be plenty
of very
noticeable extraterrestrial
civilizations out there um but even if
life finds a way even if we buy
everything you say how long does it take
for life to find a way what if it
typically takes 100 billion years then
we'd be alone so it's a time thing so to
you really there's most likely there's
no alien civilizations out there I just
I can't see it I believe there's a ton
of them and there's another explanation
why we can't see them I don't believe
that very strongly look I'm not going to
uh place a lot of bets here I would not
I'm both pretty up in the air about
whether or not life itself is all over
the place it's possible we know when we
visit other worlds other solar systems
there's very tiny microscopic life
ubiquitous but none of it has reached
some complex form it's also possible
there's just there isn't any it's also
possible that there are intelligent
civilizations that have better things to
do than knock on our doors so I think
you know we should be very humbled about
these things we know so little about and
it's also possible there's a great
filter where there's something
fundamental about
uh once the civilization develops
complex enough technology that
technology is more statistically likely
to to destroy everybody versus to
continue being creative that is
absolutely possible I'm actually putting
less Credence on that one just because
you need to happen every single time
right if even one I mean this goes back
to F noyman pointing John F noyman
pointed out that you don't need to send
the aliens around the Galaxy you can
build self-reproducing probe and send
them around the Galaxy and you might
think well the Galaxy is very big it's
really not it's some tens of thousands
of light years
across and billions of years old so you
don't need to move at a high fraction of
the speed of light to fill the Galaxy so
if you were an Al uh intelligent alien
civilization the dictator of one you
would just send out a lot of probes
self-replicating probes 100% And just
spread out yes and what you should do
this is so if you want the optimistic
spin here's the optimistic
spin people looking for intelligent life
elsewhere often tune in with their radio
telescopes right at least we did before
aroso was
decommissioned that's not a very
promising way to find intelligent life
elsewhere because why in the world would
a super intelligent alien civilization
waste all of its energy by beaming it in
random directions into the sky for one
thing it just passes You by right so if
if we're here on Earth we've only been
listening to radio waves for aund or a
couple hundred years okay so if a
intelligent alien civilization exists
for a billion years they have to
pinpoint exactly the right time to send
us this signal it is much much more
efficient to send probes and to park to
go to the other solar systems just sit
there and wait for an intelligent
civilization to arise in that solar
system this is kind of the 2001 monolith
hypothesis right I would I would be less
surprised to find uh sort of quient
alien artifact in our solar system than
I would to catch a radio signal from an
intelligent civilization so you're a
sucker for in-person conversations
versus remote I just want to integrate
over time uh a probe can just sit there
and wait whereas a radio wave goes right
by
you how hard is it for for an alien
civilization again you're the dictator
of one to figure out a probe that is
most likely to find a Common Language
with whatever it finds couldn't I be
like the elected leader ofed leader
Democratic leader uh de elected leader
of a democratic alien civilization yes I
think we would figure out that language
thing pretty quickly I mean maybe not as
quickly as we do when different human
tribes find each other because obviously
there's a lot of commonalities in
Humanity but there is logic and math and
there is the physical world you can
point to a rock and go
rock right I don't think it would take
that long um I know that arrival uh the
movie uh based on a Ted Chang story uh
suggested that the way that aliens
communicate is going to be fundamentally
different uh but also they had
recognition in other things I don't
believe in so I think that if we
actually find aliens uh that will not be
our long-term problem so there's a folks
one one of the places you're affiliated
with is Santa Fe and they approach the
question of complexity in many different
ways and ask the question in many
different ways of what is life think
thinking broadly so do would be able to
find it you'll think you show up a probe
shows up to a planet we'll see a thing
and be like yeah that's a that's that's
a living thing well again if it's
intelligent and technologically advanced
the the more shortterm term question of
if we get you know some spectroscopic
data from an exoplanet so we know a
little bit about what is in its
atmosphere how can we judge whether or
not that atmosphere is giving us a
signature of Life existing that's a very
hard question that people are debating
about I mean one very simple-minded but
perhaps um interesting approach is to
say small
molecules don't tell you anything
because even if life could make them
something else could also make them but
long molecules that's the kind of thing
that life would produce so signs of
complexity
mhm I don't know I just have this
nervous feeling that we won't be able to
detect we'll show up to a planet there a
bunch of liquid on it we like dip we
take a swim in the liquid and we won't
be able to see the intelligence in it
whe
whether whether that intelligence looks
like something like you know ants or
we'll see movement perhap
strange movement but we won't be able
to um see the intelligence in it or
communicate with it I guess if we have
nearly infinite amount of time to play
with different ideas we might be able to
you know I think I mean I'm I'm in favor
of this kind of humility this
intellectual humility that we won't know
because we should be prepared for
surprises but I do always keep coming
back to the idea that we all live in the
same physical universe and
if well let's put it this way the
development of our intelligence has
certainly been connected to our ability
to manipulate the physical world around
us and so I would guess without 100%
Credence by any means but my guess would
be that any advanced kind of life would
also have that capability you know both
dolphins and octopuses are potential
counter examples to that but uh I I
think in the details there would be
enough similarities that we would
recognize it I don't know how we got on
this topic but I think it was from super
massive black holes so if we return to
black holes uh and talk about the the
holographic principle more broadly you
have a recent paper on the topic been
thinking about the topic in terms of
rigorous research perspective and just
the as a popular book writer so what is
the holographic principle well it goes
back to this question that we were
talking about with the information and
how it gets out in quantum mechanics
certainly arguably even before quantum
mechanics comes along in classical
statistical mechanics there's a
relationship between information and
entropy entropy is my favorite thing to
talk about that I've written books about
and we'll continue to write books about
so Hawking tells us that black holes
have entropy and it's a finite amount of
entropy it's not an infinite amount but
the belief is is and now we're already
getting quite speculative the belief is
that the entropy of a black hole is the
largest amount of entropy that you can
have in a region of SpaceTime it's sort
of the most densely packed that entropy
can be and what that means is there's
sort of a maximum amount of information
that you can fit into that region of
space and you call it a black hole and
interestingly you might expect if I have
a box and I'm going to put information
in it and I don't tell you how I'm going
to put the information in but I ask how
does the information I can put in scale
with the size of the Box you might think
well it goes as the volume of the Box
because the information takes up some
volume and I can only fit in a certain
amount and that is what you might guess
for the black hole but it's not what the
answer is the answer is that the maximum
information as reflected in the black
hole entropy scales as the area of the
black holes Event Horizon not the volume
inside
so people thought about that in both
deep and superficial ways for a long
time and they proposed what we now call
the holographic principle that the way
that SpaceTime and quantum gravity
convey information or hold information
is
not different bits or cuits for Quantum
information at every point in SpaceTime
it is something holographic which means
it's sort of embedded in or located in
or can be thought of as as pertaining to
one dimension less of the three
dimensions of space that we live in so
in the case of the black hole The Event
Horizon is two dimensional embedded in a
three-dimensional universe and the
holographic principle would say all of
the information contained in the black
hole can be thought of as living on the
Event Horizon rather than in the
interior of the black hole I need to say
one more thing about that which is that
this was an idea what the idea I just
told you was the original holographic
principle put forward by people like
Gerard
and Leonard huskin Super Famous um
physicist Leonard huskin was on my
podcast and gave a great uh talk he's
very very good at explaining these
things mindscape podcast every listen
that's right yes and you don't just have
physicist on I I don't I love mscape oh
thank you very much curiosity driven
yeah ideas exporation of ideas from
smart people yeah but anyway what I was
trying to get at was susin and also a
Ted were a little vague they were a
little handwavy about holography and
what it meant where H graphy the idea
that information is sort of encoded on a
boundary uh really came into its own was
with Juan
Mala in the
1990s uh and the ads cftd correspondence
which we don't have to get into that
into any detail but it's a whole
full-blown theory of it's two different
theories one theory in N dimensions of
SpaceTime without gravity and another
theory in N plus1 dimensions of
SpaceTime with gravity and the the idea
is that this n dimensional theory is you
know casting a hologram into the N plus
one dimensional Universe to make it look
like it has gravity and that's
holography with a Vengeance and that's
the that's an enormous source of
interest for theoretical physicist these
days how should we picture what impact
that
has uh the fact that you could store all
the information you could think of as
all the information that goes into a
black hole can be stored at the Event
Horizon yeah I mean it's a good question
um one of the things that Quantum field
Theory indirectly
suggests is that there's not that much
information in you and me compared to
the volume of space time we take up as
far as Quantum field theories concerned
you and I are mostly empty
space and so we are not information
dense right the density of information
in us or in a book or a CD or whatever
computer RAM is in speed uh encoded by
volume like there's different bits
located different points in space but
that density of information is super
duper low so we are just like the speed
of light or just like the big bang for
the information in a black hole we are
far away in our everyday experience from
the regime where these questions become
relevant so it's very far away from our
intuition we don't really know how to
think about these things we can do the
math but we don't feel it in our bones
so you can just write off that weird
stuff happens in a black to do better
but we're trying I mean that's why we
have a information loss puzzle because
we haven't completely solved it so here
just one thing to keep in mind once
SpaceTime becomes flexible which it does
according to general relativity and you
have quantum mechanics which has
fluctuations and virtual particles and
things like that the very idea of a
location in SpaceTime becomes a little
bit fuzzy right because it's flexible
and quantum mechanics says you can't
even Pin It Down
so information can propagate in ways
that you might not have expected and
that's easy to say and it's true but we
haven't yet come up with the right way
to talk about it that is perfectly
rigorous it's crazy how dense with
information of black hole is and then
plus like quantum mechanics starts to
come into play so you know you almost
want to romanticize the kind of an
interesting computation type things that
are going on inside the black hole you
do you do but I will I'll point out one
other thing um it's
information dense but it's also very
very high entropy so a black hole is
kind of like a very very very specific
random
number right it takes a lot of digits to
specify it but the digits don't tell you
anything they don't give you anything
useful to work on so it takes a lot of
information but it's not of a form that
we can uh learn a lot from but
hypothetically I guess as you mentioned
the information might be preserved the
information that goes into a black hole
it doesn't get destroyed so what what
does that mean when the entropy is
really high well the black hole I said
that the black hole is the highest
density of information but it's not the
highest amount of information because
the black hole can evaporate and when it
evaporates and people have done the
equations for this when it evaporates
the entropy that it turns into is
actually higher than the entropy of the
black hole was which is good because
entropy is supposed to go up but it's
much more dilute right it's spread
across a huge volume of SpaceTime so in
principle all that you made the black
hole out of the information that it took
is still there we think in that
information but it scattered to the Four
Winds we just talked about the event
horizon of black hole what's on the
inside what's at the center of it no
one's been there so it came back to this
is a theoretical prediction but you know
I I'll say one super crucial feature of
the black holes that we know in love
that you know the kind that short shield
first invented there's a singularity but
it's not at the middle of the black hole
remember space and time were parts of
two different two uh parts of one
unified SpaceTime the location of the
singularity in the black hole is not the
middle of space but our future it is a
moment of time it is like a big crunch
you know the Big Bang was an expansion
from a singularity in the past big
crunch probably doesn't exist but if it
did it would be a collapse to a
singularity in the future that's what
the Interiors of black holes are like
you can be fine in the interior but
things are becoming more and more
crowded SpaceTime is becoming more and
more warped and eventually you hit a
limit and that's the singularity in your
future I wonder what time is like on the
inside of a black hole time always Ticks
by at 1 second per second that's all it
can ever do time can tick by differently
for different people and so you have
things like the twin paradox where two
people initially are the same age one
goes off near the speed of light and
comes back now they're not you can even
work out that the one who goes out and
comes back will be younger because they
did not take the shortest distance path
but locally as far as you and your
wristwatch are concerned time is not
funny time the your your neuro
neurological signals in your brain and
your heartbeat and your wristatch
whatever is happening to them is
happening to all of them at the same
time so time always seems to be scking
along at the same rate but if you fall
into a black hole and then I'm an
observer just watching
it and then you come
out once it evaporates a million years
later I guess you be exactly the same
age have you aged at all you would be
converted into
photons you would not be you anymore
right so it's it's not at all possible
that information is preserved exactly as
it went in it depends on what you might
preserved um it's there in the
microscopic configuration of the
universe it's exactly as if I took a
regular book made of paper and I burned
it the laws of physics say that all the
information in the book is still there
in the heat and light and Ashes you're
never going to get it yeah it's a matter
of practice but in principle it's still
there but what about the age of things
from The Observer perspective from
outside the black hole from outside the
black hole doesn't matter CU they're
inside the black hole no so but is there
okay there's no way to escape the black
hole except to let it evaporate to let
it evaporate but also you know by the
way just in relativity special
relativity forget about general
relativity it's enormously tempting to
say okay here's what's happening to me
right now I want to know what's
happening far away right now the whole
point of Relativity is to say there's no
such thing as right now when you're far
away and that is doubly true for what's
inside a black hole
so you're tempted to say well how fast
is their clock ticking or how old are
they now not allowed to say that
according to
relativity cuz space and time is treated
the same and so it doesn't even make
sense what what happens to time and the
holographic Principle as far as we know
nothing dramatic happens um we're not
anywhere close to being confident that
we know what's going on here yet so
there are good unanswered questions
about whether time is
fundamental whether time is emergent
whether it has something to do with
quantum
entanglement whether time really exists
at all uh different theories different
proponents of different things um but
there's nothing specifically about
holography that would make us change our
opinions about time whatever they happen
to be but holography is fundamentally
about it's it's a question of space it
really is yeah okay so time is just a
like an time just goes along for the
ride as far as we know yeah so all the
questions about time is just almost like
separate questions whether it's emergent
and all that kind of yeah I mean that
might be a reflection of our ignorance
right now but yes if we figure out a lot
you know millions of years from the
about black holes how surprised would
you be if they travel back in time and
tell told you everything you want to
know about black holes how
much do you think there is still to know
and how mindblowing would it
be it does depend on what they would say
you know I think
that there are colleagues of mine who
think that we're pretty close to
figuring out how information gets out of
black holes how to quantize gravity
things like that I'm more skeptical that
we are pretty close I think that there's
room for a bunch of surprises to come so
in that sense I suspect I would be
surprised the biggest and most
interesting surprise to me would be if
quantum mechanics itself Were Somehow
superseded by something better as far as
I know
there's no empirical evidence-based
reason to think that quantum mechanics
is not 100% correct but it might not be
that's always possible so and there are
again respectable friends of mine who
speculate about it so that's something I
would that's the first thing I would
want to know oh so like the black hole
would be the most clear illustration
yeah that's where it if there's
something it would it would show up
there I mean maybe the point is that
black holes are mysterious for various
reasons so yeah if our best theory of
the universe is wrong that might help
explain why but do you think it's
possible we'll find something
interesting like black holes sometimes
create new universes or black holes are
a kind of portal through SpaceTime to
another place or something like this
like and then our whole conception of
what is the fa fabric of SpaceTime
changes completely because black holes
it's like swiss cheese type of situation
yeah you know
um that would be less surprising to me
cuz I've already written papers about
that we don't have again strong reason
to think that the interior of black hole
leads to another universe but it is
possible and it's also very possible
that that's true for some black holes
and not others um this is stuff we don't
know it's easy to ask questions we don't
know the answer to the problem is the
questions that are easy to ask that we
don't know the answer to are super hard
to answer because these objects are very
difficult to test and to explore for us
the regimes are just very far away so
either literally far away in space but
also so in energy or mass or time or
whatever uh you've uh published a paper
on the holographic principle or that
involves the holographic princi what can
you explain the details of that yeah you
know I'm always interested in since my
first published paper taking these wild
speculative ideas and trying to test
them against data and the problem is
when you're dealing with wild
speculative ideas they're usually
not well defined enough to make a
prediction right like it's kind of a I
know it's going to happen in some cases
I don't know what's going to happen in
other cases so we did the following
thing as I've already mentioned um the
holographic principle which is meant to
reflect the information contained in
black holes seems to be telling us that
information there's less information
less stuff that can go on than you might
naively expect so let's upgrade naively
expect to predict using Quantum field
Theory Quantum field theory is our best
theory ofun Al physics right now unlike
this holographic black hole stuff
Quantum field theory is entirely local
in every point of space something can go
on and then you add up all the different
points in space okay not holographic at
all so there's a mismatch between the
expectation for what is happening even
in empty space in Quantum field Theory
versus what the holographic principle
would predict how do you reconcile these
two things so there's one way of doing
it uh that had been suggested pre
previously which is to say that in the
quantum field Theory way of talking it
implies there's a whole bunch more
States a whole bunch more ways the
system could be than they really are and
the answer and just I'll I'll do a
little bit of math just because there
might be some people in the audience who
like the math if I draw two axes on a
two-dimensional geometry like the
surface of the table right you know that
the whole point of it being two-
dimensional is I can draw two vectors
that are perpendicular to each other
other I can't draw three vectors that
are all perpendicular to each other
right they need to overlap a little bit
that's true for any numbers of
Dimensions but I can ask okay how much
do they have to overlap if I try to put
more vectors into a vector space than
the dimensionality of the vector space
can I make them almost perpendicular to
each other and the mathematical answer
is as the number of Dimensions gets very
very large you can fit a huge extra
number of vectors in that are almost
perpendicular to each other so in this
case what we're suggesting is the number
of things that can happen in a region of
space is correctly described by
holography it is somewhat
overcounted by Quantum field Theory but
that's because the quantum field theory
states are not exactly perpendicular to
each other I should have mentioned that
in quantum mechanics states are given by
vectors in some huge dimensional vector
space very very very very large
dimensional Vector space so maybe the
quantum field theory states are not
quite perpendicular to each other if
that is true that's a speculation
already but if that's true how would you
know what is the experimental deviation
and it would have been completely
respectable if we had gone through and
made some guesses and found that there
is no noticeable experimental difference
because again these things are in
regimes very very far away we stuck our
neck necks out we made some very very
specific guesses as to how this weird
overlap of States would show up in the
equations of motion for particles like
neutrinos and then we made predictions
on how the neutrinos would behave on the
basis of those wild guesses and then we
compared them with data and what we
found is we're pretty close but haven't
yet reached the detectability of the
effect that we are predicting in other
words well basically one way of saying
what we predict is if a nutrino and
there's reasons why it's neutrinos we
can go to it into if you want but it's
not that interesting if a nutrino comes
to us from Across the Universe from some
Galaxy very very far away there is a
probability as it's traveling that it
will dissolve into other nutrino because
they're not really perpendicular to each
other as vectors as they would
ordinarily be in Quantum field Theory
and that means that if you look at
neutrinos coming from far enough away
with high enough energies they should
disappear like if you see if you if you
see a whole bunch of nearby neutrinos
but then further away you should see
fewer and there is an experiment called
Ice Cube which is this amazing Testament
to the Ingenuity of human beings where
they go to
Antarctica and they drill holes and they
put photo detectors on a string a mile
deep in these holes and they basically
use all of the ice in a cube you know I
don't know whether it's a mile or not
but it's like a kilometer or something
like that some big region that much ice
is their
detector and they're looking for flashes
when a cosmic ray or nutrino or whatever
hits uh ice molecule water molecule in
the ice flashes in the ice they're
looking for they're looking for flashes
in some crazy I mean what what do the
detector of that look like it it's a
bunch of strings many many many strings
with 360° photo detectors yeah and you
will that's really cool it's extremely
cool and they've um done amazing work
and they find neutrinos they're looking
for neutrinos yeah so the whole point is
most cosmic rays are protons because why
because protons exist and they're
massive enough that you can accelerate
them to very high energies so high
energy cosmic rays tend to be protons
they also tend to hit the Earth's
atmosphere and Decay into other
particles so neutrinos on the other hand
punch right through at least usually
right to a great extent so not just
Antarctica but the whole earth