i like to think of black holes as the
ultimate roach motel in the universe
once you go in you don't go out
[Music]
but beginning with the work of stephen
hawking we realized that don't worry
dude you're in this episode a lot we
realize that black holes emit radiation
they actually glow just a little bit and
they eventually dissolve completely
disappearing and this opens up a nasty
paradox called the black hole
information paradox what happens to all
the information that flows into a black
hole after it disappears the full answer
relies on a complete description of
quantum gravity which which we don't
have
[Music]
let's take a step back in einstein's
theory of relativity black holes are
extraordinarily simple objects you just
need three only three numbers to
completely describe any black hole you
need to know its mass
its electric charge and its spin and
that's it and then the black holes
themselves are extraordinarily simple
there's a singularity at the center
which is a point of infinite density and
also the subject of another episode and
then it's surrounded by an event horizon
that is the boundary the entrance to the
roach motel but that event horizon is
even more boring than you think it might
be
take a look at this waterfall
where is the point of no return it's not
the waterfall itself it's somewhere up
river there's a place on the river where
the flow is too strong for you to be
able to fight against the current
that is the event horizon that is the
point of no return and this shows us how
boring event horizons really are they
don't look any different from any other
part of the river you only discover them
when you try to escape but stephen
hawking found that event horizons are
anything but boring there's a weird
interaction happening there between the
event horizon itself and the quantum
fields that surround it and this
interaction gives rise to the emission
of particles particles that have a
temperature black holes
glow
and that's really messed up so let's go
to the chalkboard to check this out
stephen hawking's result was
extraordinary and honestly
counterintuitive who knew that that
black holes the ultimate bottomless pits
of destruction would glow
it would have a temperature and this
temperature is something that we can if
we can calculate a hawking's formula for
the temperature of a black hole is
simply
temperature
is equal to
uh the reduced planck's constant uh
which is uh planck's constant divided by
two pi times the speed of light cubed
divided by
eight
pi newton's gravitational constant times
the boltzmann constant times the mass of
the black hole this is telling us that
the temperature of a black hole is
incredibly small because the mass of a
black hole uh even the smallest black
holes have a mass of a few times that of
the sun makes this a very very big
number which makes temperature of very
very small numbers so this is incredibly
incredibly small we're talking a typical
black hole emitting you know like one or
two photons a year but it's not
zero now black holes emit radiation
through this this crazy complex quantum
mechanical process any kind of radiation
has a has a spectrum has a structure but
to describe the radiation emitted by a
black hole i i need some sort of
inspiration a light bulb moment if you
thank will
all right this is an old-school
incandescent light bulb and the kind of
radiation the spectrum of radiation
emitted by this light bulb is something
we call blackbody radiation now light
bulbs aren't the only thing in the
universe to have black body spectra uh
human bodies emit radiation with a
similar spectrum of the sun emits
radiation with the same spectrum
and so do black holes let's say we have
a very cold object very cold objects
have a blackbody spectrum of a flux
versus wavelength that looks something
like this
in very hot objects have a similar kind
of spectrum but it's shaped more like
this it's it's peakier and then it goes
down like this
any object in the universe that emits as
a black body radiator
will have something looking like these
curves including black holes that's one
of the most remarkable things about
hawking's result is how simple the
radiation emitted by black holes really
is
it's this crazy complex quantum
mechanical process
that leads to a simple equation for the
temperature and then a well-known
spectrum that we've known about for over
a century but that simplicity makes
black holes even more confusing because
this is the exact place
where the paradox comes in because the
black body radiation emitted by a black
hole doesn't carry any information it's
very boring actually so let's say you
you make one black hole made out of i
don't know a potato
all right
and you make another black hole made out
of a plant
oh yeah so i've got two black holes one
is made out of potatoes and one is made
out of plants a lot of potatoes and a
lot of plants but you get the idea
there's a lot of information in this
plant and in this potato and it's locked
up in the black hole
but then the black hole starts radiating
and they have the exact same mass they
have the exact same temperature
there's exact same black body radiation
there's no information carried away
i don't know which one is the potato
black hole and which one is the plant
black hole but as they radiate they lose
mass and eventually they disappear so
where did the information go
what happened to that this is the
paradox so let's talk to an expert to
dig into this mystery a little bit more
i am dr moya mcteer i'm an
astrophysicist a folklorist and a
science communicator well let's talk
about space and we're talking about
black holes the universe makes stars and
some stars turn into black holes how
does that happen how do you go from this
giant ball of radiation and light and
warmth to a black hole black holes can
form in a few different ways astronomers
are still trying to figure out how the
very first or early big black holes
formed we call these primordial black
holes but the most common way for black
holes to form is through the
life cycle of stars so if you have a
star that's massive enough let's say it
has to be about 40 times more massive
than our sun by the time it runs out of
all of the hydrogen in its core
something called hydrostatic equilibrium
breaks so the way that stars work is
that they're constantly imbalanced
between gravity crushing in and the
pressure that you get when you have
nuclear fusion pushing out and when you
stop fusing hydrogen in your core you
lose the outward pressure so gravity
forces the star to collapse in on itself
and then there's this rebounding effect
that creates a supernova explosion so in
the explosion a lot of the material from
the star gets flung out from the system
but what's left if it's dense enough or
if it's massive enough is a very dense
black hole these are small black holes
they're called stellar mass black holes
only a few times more massive than our
sun but if you get enough of them
together then they can grow into a
bigger black hole how many black holes
are we talking about in a galaxy one two
maybe ten we have one super massive
black hole in the milky way galaxy but
there are
tens of thousands of these smaller
stellar mass black holes in the milky
way alone and there are hundreds of
billions of galaxies so there are lots
of black holes out there in the universe
you mentioned this super
black hole in the center of the milky
way how do you get something to be super
massive i think in a lot of
space contexts astronomers are still
trying to figure out how these really
massive black holes form
but we are pretty sure that at least
regular supermassive black holes not
these like hyper massive ones that they
form through galactic collisions because
a lot of galaxies most galaxies have
black holes at their centers and that's
just another effect of gravity this
really heavy thing is going to sink
towards the center of the galaxy and
when these galaxies collide so do their
black holes
after many millions of years of spinning
in towards each other when you get all
of these black holes colliding then
their masses add up so you get a super
massive one in the center these black
holes they just sit there minding their
own business and they don't do anything
else
oh i wish well it actually is a common
misconception that black holes suck
things into them and they do have very
strong gravitational pulls but
they aren't like vacuums actually
suctioning stuff in it's just a gravity
well it is a pit for stuff to fall into
if we look at the anatomy of a black
hole there's the black hole in the
center and the edge of the black hole is
called the event horizon
after the event horizon if the black
hole is massive enough and active enough
it has something called an accretion
disk so this is the disk of material
around the black hole that it's heating
it's material that has been
gravitationally attracted to the black
hole and is actively in the process of
spinning into this gravity pin
sometimes there's enough friction and
heat in that accretion disc that it
lights up
so we can see it that's how we study a
lot of black holes uh or we study their
gravitational effect on things around it
like the stars that are orbiting it one
of the things we're exploring in this
episode is is hawking radiation
can you just talk about hawking
radiation and how annoying it is
yes i can so we say
that black holes are so dense that
nothing can escape them unless they're
traveling faster than the speed of light
but that creates this this paradox
because black holes have energy
and they can dissipate that energy but
if something can't escape a black hole
how can it possibly dissipate this
energy stephen hawking came up with this
hypothesis for
radiation that gets trapped basically on
opposite sides of the event horizon some
get sucked in some dozen the stuff that
doesn't get sucked in can be radiated
away as energy as this hawking radiation
yeah i've been thinking a lot about the
long-term end of the universe and one of
the potential ways that the universe
could end is in the the big freeze this
is the scenario where the universe keeps
expanding forever long enough that all
of the gas gets made into stars and then
all of those stars die and they cool off
and eventually many many
trillions of years from now
the
only thing left in the universe is black
holes after all of the stars have pulled
down to dark chunks of rock so some
scientists have tried to figure out
what happens after that how do the black
holes lose energy because the only way
for the universe to really be dead is
for there to be no energy in it so one
way that black holes can lose their
energy is through this talking radiation
and it would take many many billions of
years for a single black hole to lose
all of its energy through hawking
radiation but it is
theoretically possible so the ultimate
fate of the universe
rests in understanding hawking radiation
the ultimate fate of the universe in
this one potential scenario thank you
moya yeah thank you
to achieve his result hawking combined
our understanding of gravity which is
general relativity with our
understanding of the very small which is
quantum mechanics but he only did that
in an approximate sense he assumed that
the gravity was relatively weak at the
event horizon that it didn't couple
strongly to the quantum fields that were
there it could be that in a full theory
of quantum gravity something much more
complex is happening at the event
horizon something we call a firewall now
studio regulations prevented me from
having an actual wall of fire to show
you what would happen so instead i'm
just going to smash some eggs the key
idea here is that in physics
information is preserved and everything
can be traced back to its roots if i
smash this egg
all the information is preserved if i
wanted to i could retrace all the steps
of all the molecules and reconstruct the
egg it would be extraordinarily
difficult but it wouldn't be impossible
so what happens in firewall theory is
that if this egg were to hit the event
horizon it would get obliterated and all
its information would be spread across
the event horizon and then that
information would get tangled up with
the hawking radiation leaking out into
space
that way when the black hole disappears
all the information is still there in
the universe and the paradox is resolved
a little bit messily but but still
resolved you know what this is actually
oddly satisfying anyway we should move
on is there someone here to clean it up
or
it's me it's me okay
as intriguing as firewall theory is it
does have its shortcomings most
importantly
we don't actually know how it works how
the information actually gets encoded on
the event horizon and how it gets
tangled up in the hawking radiation so
it doesn't actually solve the paradox it
just moves it somewhere else but in our
studies of black holes we found
something very intriguing and that has
to do with holograms yeah yeah
holographs but not the holograms you're
thinking of holographic theory where
multi-dimensional information is stored
on a lower dimensional of the substrate
if you will we found that when
information flows into a black hole its
surface area increases in proportion to
the amount of information not its volume
so this is telling us that somehow the
information that goes into a black hole
is getting stored or encoded on the
surface area not within the the bulk of
the black hole itself what is this
telling us what we're not exactly sure
it could mean that a full theory of
quantum gravity really only lives in two
dimensions not three or something else i
mean honestly black holes are just plain
confusing
thanks steven
you