Sometimes people ask me how do I feel about black holes, whether I find them scary or intimidating
Well they, they are a bit, but they can be cuddly too
These are two cuddly toy black holes that I got as a present for my sister-in-law. I just I just like them
They're just little cuddly. There's a little baby one, and there's a slightly bigger, you know, mummy black hole, if you like
Oh, well, a black hole is a [um] region of space
in which light can't escape from
Material comes together under the force of gravity
tries to fight that [uh] if you get too much material you can't overcome that Force and
eventually mathematics and the laws of physics
take us into this strange regime.
Well, it's an object which which forms when a star dies.
It's something hat's got many masses of the sun, has collapsed, and its own gravitational pull
It's got a lot of mass packed in a very small region
Eventually it collapses to the point where not even light can escape from it and it forms something called an event Horizon
But it's like a shell, if you like
and that event Horizon is the, within that event Horizon, light can't escape, so that's why it's black
At the center it has this mathematical impossibility called a singularity
So they say black holes can't have hair well these ones do, and eyes.
Brady: Do you know what I'd like--to make the earth into a black hole you'd actually have to make it about that size
Professor: Yeah yeah, that's right
Yeah
so if that really was a black hole it would be probably more massive than the Earth contained within a
Little tiny region
Here on Earth if you throw something up in the air
It comes back down again, okay, and if you throw it a bit harder
It goes a bit higher, but it will tend to come back down again. If you throw something hard enough
It'll actually escape from the Earth entirely
[Sounds of a rocket engine firing]
A rocket [uh] doesn't have to be
I mean, you know you could fire a projectile up anything as long as it's traveling at this escape velocity, escape speed, it will escape from the
Earth's Gravity keep going and going and going off to infinity
Rather than eventually turning around and coming back. If you were to make the earth a bit more
Compressed then the gravitational pull would get a bit more intense
And so actually you'd have to throw something a bit harder to get it to escape from the Earth's pull of gravity
And if you kept squishing the Earth and squishing the Earth
eventually you'd reach a point where the gravitational pull was so strong that that escape velocity became equal to the speed of light and
Because we can't get anything to go at the speed of light apart from light, and in particular
we definitely can't get anything to go faster than light, that means if you then squish it a tiny bit more then nothing can escape
not even light can escape
nothing can escape from it because suddenly in order to escape from it you have to make something travel faster than light in order to do so
But a black hole in general relativity is actually more sinister than that
It's not just that light can't escape, a black hole in G.R., there is literally no way to get out of it
[um] If it was just a matter of the escape velocity being high you could always build a ladder
Climb out of it and with a sufficiently powerful rocket get away
[uh] But there's actually more to that space and time
Collapses in on itself as you enter the event Horizon of a black hole in G.R.
And there's literally no way to escape from it
Brady: Now, we all know black holes involve these things called singularities, but from what you're saying
it sounds like you don't have to get as far as a singularity
to get to a point where light can't escape. It could be quite a lot bigger than a singularity.
Professor: Indeed and in fact, so these the kind of the thought experiment I've just gone through predates general relativity by a long time
I think probably laplace or someone was the first people who--person--who actually thought this through and realized there was a point
where you would create a thing called a black hole and in, you know, if you, just dealing with Newtonian Gravity
you still have a black, you can create black holes
It's only when you then say okay
So what, when you try and solve the set of equations as to what's going on within general relativity
When you've gone through this process that's when you realize that you actually create a singularity by doing that.
In G.R. space and time becomes compressed such that any movement you make in time or space
Takes you towards that center of the thing and actually that's what led
[uh] Hawking and penrose(?) in the late 60s to suggest that you have to have a singularity at the center of a black hole essentially.
And then eventually your equations inevitably break down and so you have to repeat, the very existence of black holes and the
singularity that you've seemed
It looks like you have at the center of them, tells you that Einstein's theory is not a complete theory
It's at best an effective theory that needs to be replaced by something else when you start talking about, you know, high energies, strong
gravitational fields, and so on and so forth
Even when you get to photons, things with no
mass at all, they still get pulled by gravity, and we can see that directly through this phenomenon called gravitational lensing
That way you've got like light from something passing a massive body, like a galaxy or something like that,
you can actually see that the path of light has been bent
by gravity and so actually gravity is affecting even light its path is being diverted by it so it's being pulled by the
Gravitational field so even lights not immune to gravity. Now the thing that's different
is that light doesn't slow down because light always travels at the speed of light,
so if you think about, you know, you're on one of these incredibly compact objects
and you're shining a light upwards and so, what happens?
It's not like throwing a rock in the air that it comes back down again, with light
it will keep traveling away from you at the speed of light,
but it's using up energy. Might be another way to think about these things is the energy that you're turning the kinetic energy of something
If it's a rock into its potential energy as it goes away from the surface is the same with light,
but the way that light loses its energy
isn't by slowing down, the way that light loses energy is by changing color.
It would lose its energy by changing color by,
you know, starting at the the blue end of the spectrum and ending up at the red end of the spectrum
and then beyond red it would turn into infrared, radio waves, and actually, before it escaped, it
would have used up all its energy and kind of red shifted to nothingness, there'll be no energy left to escape
So little lone black holes, which we call stellar-Mass black holes, can be one of
the several end points to the evolution of a star
They're by no means the most common end point so most stars like our own sun
Will turn into a white dwarf at the end of their life, and then fade into basically nothingness. If you have a star
that's massive enough, and the mass of the star governs pretty much its entire life, it will either turn into a neutron star
through a Supernova, or if it's even more massive, it'll turn into an even denser object.
which is the black hole, and so, you know, there are a lot of stars out there
so there are a lot of black holes out there
[um] but they're by no means sort of ubiquitous and and littering the Galaxy
we know there's always gravity, so there's always something pulling things together
so the
sort of sometimes the more salient question is what stops everything collapsing into black holes and
so, for example, in something like the sun, you've got the nuclear fusion in the middle
which is producing the energy, which is heating things up, which is kind of keeping it puffed up,
but if you were to turn off that nuclear fusion, then the sun would start to collapse down and
gravity would pull it and pull it and pull it and then you say, okay, so what else could stop it?
I'm no longer got my source of fusion, and it turns out there are few things that might help out.
[um] So this thing called Electron degeneracy pressure
[um] Which is, you know, which is a very electrons basically don't like being squashed close together
And so Electron Degeneracy pressure will stop something like the sun from collapsing all the way to a black hole
It'll turn it into a thing called a white dwarf, which is held up by Electron degeneracy pressure. Turns out
if it was a bit more massive than even electron degeneracy pressure wouldn't do it, gravity would win again,
so it would say 'to hell with this' squash all the electrons together
And then it will collapse down further, to kind of nuclear density, at that point you create a thing called a neutron star
And there's a thing called neutron degeneracy pressure, which is that neutrons don't like being squashed very close together either
[um] And so that will halt the collapsing and end up with a neutron star, but then it's a little bit more massive than that,
so maybe about twice the mass of the sun, bit more than that,
then even electron, even the neutron degeneracy pressure won't stop it
and then it will keep collapsing, and then, as far as we know, there's nothing that will stop that collapse. Now
maybe we're missing some physics. Maybe there is some process
we don't know about which would actually halt that collapse, but we don't know what it is
but they're the boring ones, let me tell you about the interesting ones, the interesting ones are the ones that live in the middle of
Galaxies which are enormous they're very very massive
[the three professor's voices are overlaid and indistinguishable]
