so there have been two papers published
this week about one of our favorite
subjects and that's black holes in
particular what's it like falling into
black holes now i've gone and spoken to
tony all about it and he's given a bit
of an overview of what's in the papers a
bit of a taste and also in this video
i've done a few illustrations and
graphics just to dress things up a
little bit so don't take them too
seriously
if you do want to read the papers
themselves and see the slightly more
dry diagrams in the papers go and have a
look at them i've put the links in the
video description under this video
they're free to go and have a look at
but i should warn you they're a little
bit on the complicated side
every morning we read our academic
papers that have come out
and there was been something really
interesting happening today and then and
this week basically we're rethinking
what we think happens when you fall into
a black hole so the pictures change
completely there's some real big shots
over in there california that are sort
of they've there's one paper out this
one out a couple of days ago there's
this one by lenny suskind which was out
today
uh which is sort of completely
rethinking what happens when you fall
into a black hole so it's pretty big
stuff so the way that we traditionally
thought of what happens in as you fall
into a black hole but it kind of depends
on on whose perspective you're taking it
from so if you know imagine me and you
brady we went to a black hole uh you
stayed on the outside of the black hole
and
annoyingly he decided to chuck me in so
i'm falling towards it the black hole's
over there somewhere imagine right so
i'm i'm drifting towards it accelerating
towards it now what do you see you see
my clock slowing down so you see sort of
essentially time slowing down for me so
as if i'm suspended
imagine the event horizon of the black
hole is here as i approach it your time
is slowing down from your what you see
and like literally i'll as i approach
the event horizon i would then stop you
would see my you know time stop for me
now where is all this happening this is
all happening on this
what's called a stretched horizon which
surrounds this is a little region that
surrounds the event horizon it's a
thermal reason it's hot so you would see
me suspended
okay
then you would see me thermalize
well literally i would sort of burn up
okay and then i will come back to you as
a stream of hawking radiation this is
the type of radiation which comes out of
a black hole which bears the name of
stephen hawking now that's what you saw
watching me fall into the black hole
okay now what would i see as i fell into
the black hole what would i actually see
myself well this is the old picture this
is what's changed now there's nothing
special is we thought there was nothing
really special about a black hole event
horizon really it's not a region of
strong curvature strong strong
gravitational fields it's just
a place in space where something weird
does happen but there's nothing strong
about the gravitational fields there and
because of that by einstein's principle
of equivalence nothing really weird
should happen as you cross it
and indeed that's what we thought was
the case that as you cross a black hole
event horizon as i'm falling through the
black hole around bent horizon from my
perspective what do i see i don't see
anything much so you just see me
pointing the camera at you the whole
time yeah and then eventually you know
okay there'll be some effect time
effects of you know our relative um
due to our relative velocities but
generally within the re i wouldn't see
anything remarkable happen as i cross
the horizon
eventually after a long time if it was a
big enough black hole i would approach
the uh the black hole singularity which
is miles back over there now right okay
but i would approach the black hole
singularity there there are strong
gravitational fields there i would turn
to spaghetti and uh you know it would be
good night me um i don't know i have to
be mean at fans of black hole by the way
so these papers
which one which came out earlier this
week the other which came out today are
discussing that process not from your
point of view brady watching me fall in
i think everyone still agrees on that
but from the point of view of the guy
falling in
so this statement that nothing much
happens as you cross the black hole
horizon
people aren't saying that's the case now
these are real big guns right these are
guys in santa barbara and at stanford
who you know have made their names in
for various other
topics in physics
and including black hole physics and you
know these these are big guys what do i
see now as i fall into the black hole
while it's a lot more unpleasant
essentially as you approach the black
hole horizon
you just get across it and then splat
you hit what's called a firewall they're
calling it a fireball you can see so
essentially the black hole horizon
if the black hole is old enough it has
to be an old enough black hole and the
horizon will turn into a firewall and
you were literally space and time would
just end for you essentially like a
black hole singularity but now it's
jumped right out towards the event
horizon it's tricky i don't think i
fully understand it myself here as i
said i only read this paper this morning
so yeah i'm still absorbing it um okay
so let's give it a go
they're applying quantum mechanics in
the logic of quantum mechanics to to a
black hole to come to this conclusion
and
in particular
i said here said at one point that this
black hole had to be an old enough black
hole now what what do i mean by old an
old black hole in this context is one
that is
emitted you know at least half of
the information it contained when it was
formed okay so you know black holes
forms and then it gives off this
radiation it's emitted more than half of
that radiation
if that's the case then you have
something important that happens
so i want you to imagine
basically a little bit of radiation
which came out early and a little bit of
radiation is just coming out now okay or
it's come out late and it's sort of
sitting on the edge of the stretch
horizon now they are entangled if the
black hole is old enough those two bits
of radiation
are entangled those two points are
entangled as in the exterior region
where all the radiation is and the edge
of the stretched horizon basically what
does it mean by them being entangled i
mean well they essentially know about
each other if i know about the the
radiation out here i can know about the
radiation sort of on the edge of the
horizon here's the other statement
there's supposed to be nothing special
about the black hole horizon
okay
that's what we previously thought
so
now you've got this little bit of
radiation just outside the stretched
horizon
there's nothing special about the event
horizon
so so imagine something just inside the
event horizon but still inside this
stretched horizon
well those two are also going to be
entangled because they're so close to
one another there's nothing special
about the horizon so they're entangled
as well
okay so now you have a situation
where
the thing
on the just outside the horizon
is completely determined by the thing
very far away and also the thing just
inside the horizon
but the thing very far away and the
thing just inside the horizon are
completely independent
systems
and you can't have something controlled
by two completely independent systems
okay so what the conclusion is there
must be actually something wrong with
our assumptions there and the conclusion
is that the thing that's wrong is that
the horizon is a special place
that the two points on either side of
the event horizon but still living in
this stretched horizon
are actually you know they aren't
entangled they are you know they don't
know about each other and the
implication then is is that you know
literally
you have a you have a singular point you
have a singularity you have a large
region of space-time curvature at the
horizon
okay and that's essentially
what happens another way of looking at
it is that you will inevitably you can
use these similar arguments to conclude
that you will inevitably encounter
quanta as you approach the black hole
horizon you will never do
sorry hi can i bring you back i'm just
doing a video brady okay no i'll call
you back in i'm nearly done i'm just in
the middle of it
all right hey
so where was i um oh yeah so another way
of looking at this and applying similar
arguments is you can argue that there's
um
as you approach the black hole you will
event horizon you will inevitably
encounter
quanta it's a little sort of fluctuation
little particles little bits of
radiation
that um you know have high energies you
know who's you know arbitrarily high in
principle and that that's what the
subject of the first paper was really
about the one by joe polczynski in
france
and um that's why it's firewall so
that's where they came up with this idea
of firewall yeah but of course it's
slightly it gets slightly complicated as
well because
you know the minute you fall in
yourself you alter the position of the
would-be horizon anyway
so actually it gets a little bit subtle
so what lenny suskin's arguing for is
that actually
so
you fall in so you don't actually
encounter anything immediately
but you do pretty damn quickly okay
basically it's proportional to your size
that you know it's into how big you are
so you know so the sort of the smaller
you are the quicker you you will
encounter something it's sort of the
other debate that they're still arguing
about is how old does the black hole
have to be so i i discussed you know it
should be more than half
the uh it should have emitted more than
half of its um
radiation
well there's some arguments about that
you know you know is it this page time
or is it something called the fast
scrambling time you know and that makes
quite a bit of difference for example if
you apply
this discussion to a solar mass black
hole so a black hole with the same mass
as the sun to be old enough for this to
happen well
if you it could be that you you know
such a black hole would have to be older
than the age of the universe for this to
be important but also it could be that
it might only have to be one tenth of a
second old so they're still
still arguing the numbers so this old
notion of spaghettification
is is it being replaced with like this
kind of incineration
yeah so so
termination is how uh okay so this is
the actually argue about this in the
papers what do you call it so joe
poltinsky and that call it burn up okay
so
actually um lenny suskin complains about
that he says it's not bad not that sort
of um indicates some sort of gradual
process where you burn up almost and he
actually says it's termination you
literally you hit this and then gone
you know it just happens