BRIAN: All right, Paul.
These black holes sound like a good idea theoretically, but as an observer,
I like to see things before I believe them.
Now black holes by their very name seem to conjure up a problem.
How on earth are you going to see something
which no light can get out of?
PAUL: Yes.
And astronomy is the ultimate noncontact sport.
You can look, but you can't touch for anything outside our own solar system.
And how can you see something that doesn't emit any light?
Or any radio waves?
Or any sort of waves?
Even gravity waves couldn't get out of a black hole.
Any sort of wave is going to be trapped in there so how are you
going to see these things?
BRIAN: Well, one thing when we looked at the spaghettification of myself,
I would certainly be screaming as I got ripped apart like that.
So if-- OK, you can't hear me scream in space,
but it does conjure up the idea that as I get ripped apart,
if I were a star or something I'd probably
make a pretty interesting display.
PAUL: Yeah, and remember we saw that for the dwarf novae.
The lights from the dwarf novae was coming not from the white dwarf,
but from the mass that was spinning round it going faster and faster
and radiating gravitational potential energy.
So if something falls into a black hole--
we talked about this in the quasar section of the first course--
there's a lot of energy before it goes into the event horizon.
Going from infinity to 2 or 3 Schwarzschild radii out.
You could liberate maybe 30% of the rest mass' speed.
If it bumps into something else in there, you can perhaps radiate.
BRIAN: Right.
So you have a star come by, it'll get ripped apart if it were to come by,
then all the gas would probably collide with each other
and want to radiate all that energy.
As it radiates the energy, then it wants to get closer and closer.
So you could imagine getting a big, it's probably
a disk of material form around the thing that
is going to get heated up from all the stuff colliding.
Radiate that gravitational energy away, and that should glow pretty brightly.
PAUL: Yeah.
So we can't see the black hole itself, but maybe we
could see stuff swirling down its throat.
Another possibility though if you remember in the exoplanets course
we couldn't see the exoplanets, but we worked out they
were there by their effect by making the star wobble backwards and forwards.
So maybe if there's a black hole around, you
will see stars wobbling backwards and forwards
or doing loop de loops or something around it.
And we could tell there was something dark and massive there.
BRIAN: Ah, so I'd use the motion of the stars themselves
to infer how much gravity is there.
OK.
That makes sense.
We weigh the sun, for example, by the Earth's motion.
PAUL: Yeah.
And in fact, it's a combination of these two things
that leads to probably the best case study of a black hole.
If you remember going back to x-ray astronomy,
they found all this strange x-ray resources, and some of them
turned out to be neutron star binaries.
But one of the very early discovered ones, Cygnus X-1,
turns out is not a neutron star binary or it's probably not.
The star, just like the other binaries we talked about,
we've got a star going loop de loops around something that emits x-rays.
But that it's going around rather faster.
And the star is very massive.
It probably weighs something like 10 or even more solar masses.
BRIAN: So this is a giant-- blue super giant.
So that's sort of like--
PAUL: not small red stars like the other ones we were talking about.
BRIAN: OK.
So that's a star like Rigel in the constellation of Orion.
So that's a big 10 to 20 solar mass star that is young and burning very brightly
because it has a big nuclear reactor because it has so much mass
to compress its interior so much.
PAUL: Yeah.
Can you imagine the Doppler effect of the star?
You can see it is indeed moving back and forwards at pretty high speeds.
So as we've done many times before in the series of courses
you can work out the mass of what it's going around.
And that mass comes out as 10 to 20 solar masses.
If it was going around a neutron star-- it actually
wouldn't be because it's so massive the neutron star would be going around it.
It wouldn't be wobbling very much.
So to have it wobbling as much as we see, it must be going around something.
That's pretty damn heavy.
BRIAN: All right.
So we have something that's 10 or 15 times the mass of the sun,
yet seems to be very small.
Now neutron stars we know a bit about.
We know they're made out of neutrons.
And we know that when you have a neutron star
and you start adding material to it, it starts becoming smaller.
PAUL: It's just like the white dwarfs do.
It's a rather strange situation when you make them heavier, they get smaller.
BRIAN: And so we know that a neutron star even if it's 1 and 1/2 times
the mass of the sun is already only 8 or 9 kilometers across.
And we know that's very close to a black hole.
So if I make one 15 solar masses across, I
think, given our understanding of neutrons,
that that thing would be smaller than the Schwarzschild radius.
And so it almost has to be a black hole if it's that heavy and small.
PAUL: Yeah, once it's compressed that much,
no matter what force is holding it up, it can't.
Has to fall in.
BRIAN: Yeah.
OK.
PAUL: We also know this thing is flickering in x-rays
on time scales of even milliseconds, which also implies
it's only [? light ?] milliseconds across and therefore, very small.
So we seem to be looking at something that's very small and very massive
and that sounds like a black hole.
Here's an artist's impression of it.
We don't know for sure.
It's certainly consistently.
We can't think of anything else that could have that mass given
our understanding of neutrons, but maybe our understanding of neutrons
is rather deficient.
People have been trying very hard to look
at the x-rays coming from the central region
and see if there's really conclusive proof that it is actually falling down
a hole rather than bouncing off a surface.
And it all seems to be consistent with that.
There's probably some evidence there that it's
actually falling into something, not just hitting a surface,
but it's very model dependent and not, perhaps, 100% sure.
But probably between 98% or 99%.
BRIAN: But unfortunately, we can't go out
and take a picture that looks just like this right now.
Instead, we are relegated to an artist's impression
of what we think it might look like, but I
think the bets are in that we have a pretty
good case for a black hole in this case.
