basically Stephen Hawking has weighed
into the air
Great Firewall debate about black holes
and he's come to the conclusion that
black holes well basically the event
horizon which essentially defines a
black hole they don't exist
he was actually one of the first to
demonstrate possible problems in terms
of what it's known as the information
paradox but there's been work recently
over the last two years by people mainly
in this West Coast of America who have
been suggesting that the black hole
itself may not be as straightforward as
was initially thought and there may be
this so-called firewall we know that
that black holes you know the day
they're they're not as black as they
seem that they give off this radiation
how does Hawking radiation this is the
kind of radiation that you get from
black holes how does this appear well it
appears when you apply quantum mechanics
to a black hole so you can imagine near
the event horizon of the black hole
which is like sort of the point of no
return if you like near this event
horizon you get two particles created
like a particle antiparticle pair one
just inside and one just outside now
these are what we call entangled with
one another now the guy outside escapes
and the guy inside falls into the black
hole singularity this is where
everything just crunches up by the time
that the black hole has evaporated
completely you've completely lost
knowledge of the guy that fell in that
information is gone so you end up with
an end state which is basically you go
in you begin at the beginning of its
life the the the star that's collapsing
to form this black hole is described by
what we call a pure state a pure quantum
state but by the end of it by the time
the black hole is evaporated you're you
have what's called a mixed state there's
information being lost it's not pure
anymore and that information is
essentially lost to the singularity it's
locked in the singularity but as an
outside observer you just see
information is gone and this violates
one of the really sort of key principles
of quantum mechanics that called
unitarity that essentially what goes in
must come out the people were thinking
about this and
it led to ideas like black hole
complementarity that actually you don't
really lose information that is
essentially get a duplicate copy of it
but no observer can actually see both
copies you know you know no observer can
actually access both copies and so
there's no violation of what's called
the no-cloning theorem these sorts of
things that you can't make copies of
quantum information like that
and then along came the firewall okay
and this is the notion the black hole
event horizon isn't just a place in
space where nothing much happens but
actually if you cross an event horizon
you will burn up and it just seems
strange it seems too strange at many
levels but one of them it seems to break
things like the equivalence principle of
relativity I mean we discussed it
another video so no point in going over
again right as always is a link there
watch the video if the black hole is old
enough it has to be an old enough black
hole the horizon will turn into a fire
wall where does this fire wall come from
and Hawking I think has just decided to
try and understand what might be going
on in specially said it's not there a
lot of people don't like fire walls and
Ana to be honest having some time passed
since our first video Brady I'm sort of
sharing this slight dislike for fire
walls I think at the end of the day
you've got a situation where you really
have a very weak gravitational field at
the vent horizon it shouldn't be a
special place and yet you having this
very dramatic thing happen that you
would just bear not without knowing
about it it doesn't really fit the claim
is that you would you would approach the
black hole event horizon not
experiencing anything much and then
suddenly bang you're burnt up and it
just doesn't sit well but the price is
paid for saying it's not there is that
he's got rid of the event horizon as
well so he's got rid of the underlying
definition of a black hole and it's led
to press releases everywhere saying
Hawking says black holes don't exist
it's fair to say that the original
authors of the fire wall paper have
sparked something and have sparked
research and that's fantastic Hawking's
weighed in and he's very anti firewall
and he gives this paper is very short it
gives a bunch of reasons why he doesn't
like the firewall and then comes to the
conclusion
his resolution of it is that event
horizons don't actually form so I mean I
can go through a couple of his arguments
again to the firewall one of them says
that that basically he looks at a black
hole in it in what's called anti-de
sitter space it's it's a bit like this a
sort of familiar space-time but it has
this this- sort of energy pervading
through what's called a negative
cosmological constant if you take
anti-de sitter space and you put some
energy in it depending on how much
energy you kick it by you can either
have some radiation or you can form a
black hole because it anti dissipates a
bit like a box so that radiation will
eventually it can in principle gather to
form a black hole so what he says is he
says what's the what's the configuration
you would expect if I kick this this
this situation what's what's the
configuration I would expect what I've
expected in essence as sort of
superposition of both and I is the
radiation and the black hole in
principle but what he also argues is
that the situation should be symmetric
under something called CPT and through
this he argues that it should be should
be the collapse of the back hole should
be the time reverse of the evaporation
of the black hole okay so if the
collapse of the black hole and the
beginning you think about the initial
stages of collapse there's no event
horizon okay it's not it's some points
of forms but originally there isn't one
and so if the evaporation is the reverse
of that in some sense then there
shouldn't be an event horizon at the end
and I think what he means is I don't
know I haven't asked him but I think
what he means is they done you've got to
redefine what you say is a is a black
hole he's not for example suggesting if
I'm if everyone now turns their major
telescopes to the center of our galaxy
we're not going to find an object which
is behaving just as if it's a
supermassive black hole it's still there
it hasn't disappeared since Hawking
Reuters paper but whether or not it's
got an event horizon is a is a matter of
debate he's he's replacing this event
horizon with this what he's called an
apparent horizon so why is an event
horizon an event horizon is defined in
the following way it's a region of space
and time from which you cannot reach
somewhere
future null infinity future not infinity
intuitively it's really just infinitely
far in the future infinitely far away a
scan of what it is maybe we should make
a distinction there between an event
horizon and what Hawking says it's
replaced by it which is something called
an apparent horizon to all intents and
purposes look pretty close to an event
horizon there are still regions within
which light sort of goes into what you
would call the black hole but the slight
difference is it like can escape from
this over a long long period of time
it's a kind of a metastable
configuration and so it's different from
the event horizon in that sense now the
key difference is the following
basically an apparent horizon can
disappear okay so you would when you
look at a parent horizon a the thing
about an apparent horizon is it's it's
it's dependent on an observer on a set
of coordinates okay and so you take her
slicing a snapshot in time and you look
at yours your space and you realize that
there's a place where all the light rays
are bunching up so this kind of a place
where light can't get across okay that's
your apparent horizon okay that's not
the same as an event horizon an event
horizon says that I look at the
space-time globally I look at it overall
time and identify those regions of space
and time from which I cannot reach this
fusional infinity place okay and that's
not the same thing as the apparent
horizon funny thing about an event
horizon is that it requires you to know
about the whole space-time it's kind of
weird it's sort of you need to know
about the future it's it that way it's
you know more might worry about
causality but actually there's no issue
really but it's it's a kind of strange
property it's defined by some boundary
condition in the future we're taking a
snapshot in time I'm seeing this what
looks like an event horizon but it's not
really it's an apparent horizon where
the light rays are bunching up I see you
cross it I think you know what you're
coming back but I hang on and I hang on
and I hang on and hang on and eventually
as a partner eyes it disappears and you
escape that's it for me the most
exciting thing about this current debate
that's going on of which as you can tell
I'm not an expert and is that people are
really using these environments of
strong gravity so
actually test out theories of gravity
and quantum mechanics and so that's the
crucial element here that has led to all
of this that they're trying to properly
incorporate quantum theory quantum
mechanics and and and they're suggesting
things get modified either through the
creation of these firewalls which seem
to be emerging or if you don't want the
firewalls you you have you getting rid
of the event horizon so the thing the
true classical solution isn't
necessarily the solution in a in a
theory including quantum theory well
actually that's probably no massive
surprise problem with the paper this is
no calculations okay so it's all very
nice but to say oh everything's chaotic
inside the apparent horizon and this
prevents me from predicting what's going
to come out blardy blardy blar all very
nice all very I mean he has great
physical insight is no denying that
there's no calculation in here though
and it's sort of like what's he really
saying I mean you need you need to see
the mass before you can you can see that
whether or not Stephens managed to
really determine what the true outcome
is of this situation it remains to be
seen it's a very short paper with idea
with the bulk of the paper is just
reasons why you shouldn't leave the
firewall ideas and then right at the end
he has a little section on where he says
he has a resolution of his own in which
the ends up getting rid of the event
horizon replacing it with this apparent
horizon so this thing still can look
like a compact object which is very
massive in other words like a black hole
if all Caine was right and we don't have
event horizons we have apparent horizons
yeah what does this mean for the way we
think of black holes as as beasts well I
think you would even call them black
holes anymore I think that's the bottom
line
if he's right then yeah there's no such
thing as a black hole I would define a
black hole by the by the existence of an
event horizon okay as formally as in
light cannot not even like nothing could
escape from us from a particular region
of space-time to this future null
infinity place there is no such place
now there is no such a region of
space-time which which can't reach
future
finiti now this just doesn't exist
anymore so therefore there's no black
hole and there is a lot light could get
out one day yeah it just has to wait
okay but I mean I don't I think this is
a pretty radical notion he's saying here
but there are no equations in this paper
it's full of ideas and I think until
there are equations there that we can
that we can look at and solve and and
try and understand how we're
incorporating the quantum mechanics with
the gravity then it still remains a bit
open Stephen Hawking made his reputation
on black holes he did he did but now
he's saying that I'm not even there well
yeah yeah yeah he is but you know I mean
Canada is this not part of the story
right that that he's developed such a
great understanding of black hole
physics that he is that it you know he
can make this seat make these insights
be refers to a number of papers to
justify why not to believe the firewall
and those do have calculations in but
this particular idea where he gets rid
of the event horizon and it replaces it
was an apparent horizon I I presume in
the back of his head he's got some
equations that he's thinking will do it
for him but he probably just hasn't
managed to you know get them down a heap
you know as you know he it's difficult
for him to write down anything and then
so he's he's probably thought through it
in his own way I think Stephens a very
intuitive kind of physicist he's he
probably knows the answer before he's
even bother solving the equations that's
the way he probably thinks his way
through it firewalls have sort of thrown
off a puzzle and people are calling
radical really radical suggestions that
these these entangled states are
connected by air by wormholes to
stretching right through through through
air space it's yes it but this is what
this is the beauty the file it's really
got people imagining imaginative and
then Hawking has gone off and said no
black holes would it be best to say that
black hole temporary that they're
temporary
well Hawking of course showed they're
temporary he showed that they evaporate
which is one of them for me that's his
magic major contribution that is just
such a staggering result and has led to
all of this that the we always thought
you know
cause suck right they just everything
collapses in and he then demonstrated
when you include quantum theory actually
know the evaporate they lose everything
so they're that we've always known
they're temporary but we've they're
temporary on long timescales he's saying
effectively again this is another way in
which they're temporary there they've
got so parent horizon and light can
escape eventually through that
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 imagine something just inside
the event horizon but still inside this
stretched arising