recently there's been a lot of
excitement because stephen hawking has
claimed he's solved the information
paradox so the black hole information
paradox it's sort of a puzzle that's
been around for some time now and it
kind of boils down to the fact that
information is lost in a black hole and
that violates everything that you you
think you you believe from from quantum
mechanics quantum mechanics tells you
that essentially what goes in must come
out that physics is what we call unitary
so information shouldn't be lost in a
system so for example why would
information be lost in a black hole why
do why do people believe this well take
a take a black hole
and throw something in it so what could
you throw in it well you could throw
gary neville in it now what happens well
all that happens when eventually the
black hole will its mass will increase a
little bit as a result of gary being
thrown in its charge might change if
gary carried charge if he carried
angular momentum it's angular momentum
make change but nothing else that's it
so where did all the information go
about his his you know his face is his
tattoos and all this where did all that
go
okay it's just the same black hole so i
could also throw in donald trump
okay instead right and i might end up
with exactly a very different
information for example you know he's
got
funny hair he's obnoxious personality or
all these things i could throw him in
but i end up with the same black hole
well what i'm saying is so you can say
okay can i extract the information back
so black hole will eventually radiate
eventually give off a type of radiation
called hawking radiation right now this
is what you get when you apply quantum
mechanics to the event horizon of a
black hole it gives off this hawking
radiation so you might ask can i extract
that information about whether it was
gary i threw into the black hole or
donald trump from the hawking radiation
and naively you think well absolutely no
because
the object that it came from only knows
about mass charge and angular momentum
it's
there's a saying that black holes have
no hair
okay that they all only information they
contain this mass charge and angular
momentum and so the information seems to
have become lost
tony why is this exclusive to black
holes though i mean if those two people
died and decomposed in the ground
wouldn't that information also be lost
no no no no that information wouldn't be
lost there so they're decomposing the
ground but you could extract that
information you could you know from the
whatever remains were left you could
work back and work out you know
might be difficult but you could do it
in prince but with a black hole all the
calculations that hawking did pointed to
the suggestion that this couldn't happen
okay that the information is truly lost
and that completely seems to violate
one of the things you hold sacred about
quantum mechanics
so it's a huge it was a huge issue so
for years people have been trying to
resolve this and there's various ideas
put forward maybe the black hole doesn't
completely evaporate away okay and it's
left with some remnant which contains a
lot of information contains all that
information about how the black hole was
formed right but that doesn't quite work
either because if you throw one of these
black hole remnants into a black hole
into another black hole
you violate things like the second law
of thermodynamics it just doesn't quite
work there's there's other proposals
that that people have put forward for
example baby universes that there's one
thing that hawking liked that on the
other side of the black hole there's a
baby universe and somehow the
information leaks into there again
that's based on ideas which didn't just
quite pan out in the mid 90s people
realized that actually
black holes probably don't
lose information
because something came along called the
holographic principle but still people
haven't understood how it really is
where the information where is it still
it's still not fully understood until
these recent claims by hawking and his
his collaborators there isn't a paper
out yet about this so
i'm kind of winging it a little bit and
there have been some seminars and
particularly by hawking in stockholm but
that was only a short nine minute
seminar here one of his collaborators
malcolm perry's been giving talks about
this also andy stromiger who's the other
collaborator you sometimes give talks
about stuff that's not quite completed
yet i've done it before as well
but
maybe not something as spectacular as
this but anyway but
so what the what they're saying is is
really quite interesting at least what
i'm trying to glean from what they're
saying as i said all the details aren't
out yet but it turns out you know we've
seen black holes have no hair but
actually it turns out they seem to have
a lot more they're able to contain a lot
more information than we previously
realized they have an infinite amount of
hair
so the black hole has no hair it's just
the statement all right black holes
don't know about donald trump's donald
trump's hair right okay so they don't
know about that hair so
why did when they were coming up with
the saying why did they choose hair for
black holes
i don't know they'd have to
know i guess it's sort of because
they're very smooth objects i think
that's the idea but now they do have
hair well it seems so so i mean this is
not completely non-trivial trivial thing
and it all goes back to something that
um
guys called bondi metzner and sachs did
they they were thinking about the
effects of gravitational radiation
and things like that and they discovered
that there was actually a much larger
class of symmetries called the bms
symmetries and in particular they
included something called super
translations and there's an infinite
amount of these things okay now what
hawking realized was that these things
also exist on a black hole event horizon
and so that is where the information is
being stored so when something falls
into the black hole event horizon it
causes a super translation changes the
charge there and in principle you can
store an infinite there's an infinite
amount of possible charges and this
seems to be where the information is
going
so when gary neville falls into black
hole it changes the one of these let's
call it a bms charge that's what it's
called it changes one of these bms
charges on the horizon and then what due
to a calculation that's a guy called
polcinsky did
hawkins suggesting that actually this
can now affect the stuff that's getting
emitted so when when the black hole
starts to radiate this change in the bms
charge this super translation can uh can
affect the time at which the photon says
something's emitted from the from the
black hole and so
that information is now gets stored in
the sort of times and slight delays in
the times that stuff's getting emitted
now he's also saying that it's it's
completely scrambled
the information is there but it's
chaotic so for all practical purposes
you would never really be able to
extract the information but
the claim is actually it is there so
it's not lost it's just very hard to get
so it's like anything that goes into a
black hole
at the event horizon it has like a final
selfie
yeah but the selfie is really hard to
see and read back that's one way of
putting it yeah yeah bms selfie yeah
yeah that's kind of
super selfie because these are super
translations
yeah it's kind of kind of nice way of
putting it i like that
what these super translations are so if
you imagine
this this surface this this future null
infinity it's a light like surface you
have to be
traveling at speed of light to sit on it
and it's in the far future