The singularity, well I mean that's just something that we just don't understand.
It's a singular point and that means the curvature of the spacetime at
this point is gone
infinite and all the mass is collapsed to one point. So it''s a point
literally a point, with this mass in there. So it's got infinite density, you know a density is mass per
unit volume, well if this got no volume, it's an infinite density. These black holes
that are defined, are defined in the sense of using General Relativity and this is this
wonderful theory of Einstein which explains how matter and
curvature of spacetime are intricately linked together. So you really need some
quantum theory of gravity to try to describe that, so I'm sure that our
quantum theory of gravity will resolve the question of what the singularity is at
the centre of a black hole. It's just that we don't know what that is yet. So that solution
as it stands, is there classically but when you include the world of quantum mechanics
where things get, can get smeared out, what isn't clear is whether that singularity
will remain or maybe it will get smeared out. Will this event horizon still be there?
So will that, what we think of as the black hole still be there? It probably will.
-What's that area like between the event horizon and the singularity?
Is it like a, is that a tumultuous place or do you not know you're in that like
almost like the eye of a hurricane? -If you're heading into the black hole and
if this is a big enough black hole you actually don't even know you cross this
event horizon. In fact the density of the black hole around the event
horizon for really massive black hole is of order water, that of water and so
you'd just sort of pass through. But what begins to happen is you begin
to experience the gravitational attraction. It begins, as you get closer
and closer to the singularity it gets bigger and bigger and so you will
begin to stretch and you will begin to feel the effects of the pull of the
black hole. And on the other hand for a solar mass black hole,
you know one of order the mass of the Sun
then the pull of gravity on the event horizon is much stronger and you would have
already been spaghettified by the time you cross the event horizon. You won't see
the event horizon, there's no sign post saying you know: 'event horizon no return!'
but you will pass through this, you would
already be gone in terms of living by then. But a supermassive black hole you
actually want even realize. The pull is far less, the density of
the matter is far less significant there, then it would be in the
smaller black hole.
It's a good question. Ok so if the black hole is a big, there's a big black hole like maybe the one in the
center of the Milky Way, then you know when it crosses the event horizon,
well forget firewalls for a second, but when it crosses the event horizon
generally you'd expect nothing much to happen because actually the
gravitational field's pretty weak out there. As you start to go deeper and deeper
inside this black hole, you might think that indeed does it retain its status
as a particle? I would say probably not because you know you're entering that
regime where even your theory of gravity is breaking down. Well let's think
about how election became an electron, ok? So where does the electron get its mass from?
It gets its mass from the Higgs particle, from the Higgs field actually more
appropriately and for it to get its mass from the Higgs field something has to
happen called spontaneous symmetry breaking, a symmetry has to be broken. Now if you
restore that symmetry and that symmetry does get restored at high energies,
then the electron looses its mass and you know you can't really talk about, eventually the
electron just becomes part of some greater thing. And that's what would
happen I think as you approach the black hole singularity
things would just, symmetries would be restored, you would no longer be able to describe
matter in terms of what we think of the low-energy matter dynamics you would
have to replace that by some high-energy version of it and eventually you'd have to
replace things ultimately by the full quantum theory of gravity and everything
i.e. maybe string theory. So you'd really be talking about strings so you would never even
talk about an electron, you can't talk about electron very very close to singularity
you don't have a singularity really, you probably got some sort of
stringy type structure of which that electron
has now become a part itself, some stringy excitation.
We do not know how to use gravity and quantum mechanics
in regimes where gravity is incredibly strong and quantum mechanics is playing
a big role. We can use our quantum theory is really well understood in
regimes where gravity is not that strong, gravity is really well understood in
regimes where you don't need to worry about quantum mechanics, but we know both
have to exist. We know we have to be able to understand the world of the macroscopic
and the microscopic and there are regimes where these two clash and the black hole
is a regime where you have the world of the microscopic quantum theory and the
world of high strength gravitational fields coming together and a
manifestation of this problem is this information issue because it's defined
simply in terms of its mass, its angular momentum and its charge. It's got what's
known as no hair, there's nothing coming out to tell you what's bit gone in there.
The world of quantum mechanics on the other hand should also be able to
describe this process. In the world of quantum mechanics there is what's known
as a wave function. It's integral to the world of quantum theory that we can keep
track of this wave function we solve it by the Schrodinger equation. Knowing
about this wave function tells us about the probabilities of everything we
always expect probabilities to add to one. If they don't then it's a bad,
something's wrong with our system. When you lose information it means that this wave
function no longer is deterministic in the sense that you lose that ability to
have probabilities adding up to one. Something's gone wrong with the old
world of quantum mechanics but we believe our world of quantum mechanics
so we have to be able to reconcile this classical picture of the black hole where
the information gets lost, to this quantum mechanical picture where we know
the information shouldn't get lost.
We need to be able to retrieve it and in order to do it you're having to match these two together.
The very existence of black holes and this singularity that you seemed,
it looks like you have at the center of them, tells you that Einstein's theory is
not a complete theory
It's the best and effective theory there needs to be replaced by something else
when you start talking about high-energy strong gravitational fields and so on
and so forth. You need something else which is most likely string theory I guess.
It's believed that the centers of galaxies have got these supermassive black
holes right, to billions of solar masses which actually raises an
interesting question: how did they form? How did these have time to,
I think there's might be some cosmic strings involved but no one likes that idea. I'm just hoping
they'll keep finding these supermassive black holes earlier and earlier and
earlier utntil eventually there's just not been enough time for black holes
coalesce and you need another seed, something like a super, like a massive
cosmic string but anyway that's just my own pet hope. -Is it possible that as you start
reconciling quantum mechanics and General Relativity and understanding black
holes better, that singularities could come under threat? And people might think 'oh, it's ok
there's actually a little bit of volume there.'
-Yes, absolutely and I think the majority I don't about the majority but a lot of
people that think about what will be the effect of quantum mechanics, is that,
that singularity which again is a breakdown of your, demonstrates the breakdown of your
field equations, the breakdown of your mathematics, that singularity will get
smoothed out.
Example of a star or an object, whose gravity was so great, that light couldn't escape from it.
So basically what's being hinted at here for the first time is a black hole!
Of the existence of bodies and either of these circumstances we could have no
information from site.