Now the last few videos were made about the year before gravity waves were actually detected
but over that year the LIGO detectors in the U.S. were upgraded as David talked about,
by a team involving people from many places around the world
and when they were switched on with the new sensitivity
the discovery came rather quickly, in fact rather quicker than I think most people were expecting.
So on the 14th September, 2015, gravity waves from space were first directly discovered.
in this video I'd like to talk a little bit about what we can deduce from this very first signal. This is all very early days yet
I was going to try and interview David about it but he's off at conferences with other experts talking about these things,
so this will just be my take on what has been learned to far
based on this paper here which you can check out for yourself
here are the actual signals picked up, what we're measuring here is the strain, the fractional shift in length against time
the whole thing takes
I don't know, about point one five of a second
and here's the signal from one of the two LIGO detectors and here's the one one from the other one
what you can see is just noise and thn a signal starts to emerge from the noise and up and down signal
and the same thing here and the two signals math up
allowing for the very slight shift due to the time it takes for gravity waves to get from one as opposed to the other
and here we are comparing the
data (grey) with the theoretical model from General Relativity and you can see they match up very precisely
down the bottom we can see frequency in Hertz against time and you can see it starts off at about40 or 50 have cycles per second
and goes up to over 200 in both cases before fading out
so what's it going on here? Well, this is the classic chirp signal
this what you expect from the inspiral of merging objects. So the idea is we have two heavy compact things and they are close to each other
as they orbit round each other that means they are accelerating, therefore gravity waves are being radiated which is bleeding energy away
as it bleeds energy away they come closer. As they come closer they spin faster,
 therefore gravity waves are emitted more strongly so they come closer still,
even more emission, closer still, even more  emission until they merge and then settle down to something spherical.
So what you expect is a frequency that's increasing, intensity that increases,
peaks at  about the time when they collide and then tails away
and that's exactly what you see
what can we deduce from this?
Well the simplest deduction come from the early stages, when you're in the late stages you'd need a full
supercomputer numerical relativity simulation but in the early stages as an approximation
the chirp mass, now this is the equation for the chirp mass, its the mass of the two objects multiplied together divided by the sum of the two
masses basically and that you can work out from the data here by looking at the frequency
to the -11/3 power and f dot which is the rate of change in the frequency, so in the early
stages here you can see what's the frequency how fast it's speeding up
and that tells you the chirp mass, and the chirp mass comes out as about 30 solar masses.
Now does that mean the objects are 30 solar masses?
Well not really.
It could be that,
because of this rather complicated equation, it  could be that you've got two things of maybe 30-35 solar masses
or it could be one thing that's very low mass, say one solar mass, and another thing that's say a thousand solar masses
So if it's two objects about the same mass, they're to be about 30 solar masses each
if it's one that's less massive, the other one has to be very much more massive to make up for it
so what can we deduce from this?
Well, basically these things have to be black holes.
Why's that?
Well, we don't believe that neutron stars can exist
 much above the Chandrasekhar limit of about 1.6 solar masses so that's telling us that at least one of these
two things is much heavier than neutron star and so will have to be a black hole.
Could it be a black hole merging with a neutron star?
Well not really because if that was the case to get this chirp mass equation one object would have to be
like a 1000 solar masses merging with a one solar mass object. You could get that up here, but in that case the event horizon of the
thousand solar black hole would be so big you'd never get to these high frequencies hre, they'd merge before they got to that stage
so the fact that it kept going all the way up to frequencies of over 200 Hz tells you they really have to be two black holes.
Now to get more information we can do a full fit to the entire curve that's what this
red line is here, compared to grey which is a smoothed version of the data
and from that you learn that one of the object must be about 36 solar masses, the other one about 29 solar masses
they can't be spinning too fast, they have to be black holes
when they got to this point here, they're only 350 km apar, which is when they started merging
and what you can see is the black hole model fits very nicely.
It predicts that they should merge about here,
it gets, this is what you'd expect from the event horizons of the things,
and you can see this ring down when it keeps vibrating but less and less less,
and that's actually what the numerical general relativity calculations say should happen to a black hole:
there should be a period when you rather distorted event horizon before it settles down to a circular one
and that's indeed a very good fit to the data over here.
So, this is very interesting. We have two black holes that are not quasar-type black holes, not billions of solar masses
but still pretty hefty,
tens of solar masses, so about 30 solar masses
and that in itself is a bit surprising
because you expect in the last stages of star death huge winds that blow mass away.
These winds obviously can't be too massive otherwise you'd never get things this big
unless these black holes themselves form from the mergers of something else beforehand
so maybe we're talking about 50-60 solar mass Black Holes each, two
very massive stars, 50-60 solar masses which burnt away
blew away maybe 10-20 solar masses
leaving 20 to 30 behind
in a binary pair which then merged. So that's interesting in itself.
The fact that we saw one of these signals so early is
consistent with what we know about black hole populations,  you can try estimate how many
black holes there should be out there.
It's a very uncertain calculation
but certainly having a gravity wave signal of this  strength seen within the first few weeks of turning it on is
actually quite consistent with what we think we know about black hole populations.
Really the crucial fact is this is telling us, I think, almost beyond doubt that black holes really exist.
All we've known before is there are very dark very compact objects.
We believe that neutron stars can't get to the sorts of masses and the sorts of compactnesses needed
and we believed from a study of general relativity that if you made anything much denser than a neutron
star who would be inside its own event horizon would have to collapse.
So that kind-of indirectly said that black holes had to exist.
But if general relativity wasn't quite right under these incredible conditions and maybe you could shrink a neutron star further without it sitting
its event horizon then maybe you could get some sort of quark star or there might be some other more exotic thing
we didn't really know for sure
but here we can actually trace
who and when this thing merged and therefore measure what the event horizon is and that general relativity
explains the entire shape of the curve with exquisite precision.
So this has tested to general relativity far beyond where it's ever been tested before, at incredibly strong gravity regimes
and it passed with flying colours.
Damn Einstein - too smart for his own good.
And so it really looks like, as general relativity works that well, that black holes have to exist.
So I'm always fairly skeptical about these things but here the data are now so good there's rather little wiggle room left.
So, a very exciting result, and the dawn of gravity wave astronomy.
