when you get as far as writing down the equations of general relativity and start

trying to solve them you find that some of the solutions involve these wave solutions.

They're distortions of the spacetime that propagate out. -I mean they're important

for lots of reasons they're important because they are a prediction of general

relativity so actually if you then detect them you've found further

confirmation that general relativity is actually right. -Whenever matter

passes by through some region of spacetime it will distort the spacetime just like

you would distort water when if you put your fingers through water and waves

propagate out this is a propagation of the spacetime itself, so the spacetime is

is sort of moving in and out,

propagating out at the speed of light. -But also because in principle at least

they open up an entire new window on the universe, everything, almost everything we've done

in astronomy, barring a few cosmic rays and neutrinos, has been mediated through

light - we've used light to figure out what's going on in the universe. Having

an entirely new way of detecting what's going on out there in the universe is a

very exciting thing for astronomers because it - it opens up all sorts of new avenues for

us to pursue on research.

As the Earth goes around the Sun then the spacetime is distorting around the Earth

and that's propagating out in waves in all directions.

[Brady off Camera] Do those waves have nothing to do with for example why the moon is attracted to the Earth?

[Prof. Copefield] No - no they're different gravit- that- that's a different aspect, sometimes

we call them gravitational waves, but they're different here these are the-

the tides don't exist because of the nature of gravitational waves propagating.

You've got the Earth going around the Sun and then you've got the Moon going around the Earth.

These are very massive objects, right, and so there's a net attraction just due to

the pure mass of these objects and that's where the tides come from the

large mass of the Moon and the even larger mass of the Earth and as they go

around the water that's on the Earth gets distorted by the changing

gravitational field due to the huge mass of the Moon and the Earth.

But on top of that there's like a secondary effect going as the Moon

is going around it's causing ripples in the spacetime and it's those ripples -

these are minuscule ripples that are propagating out - and it's those that

recently were detected not from the Moon but from two massive, supermassive

- well not supermassive -

two massive black holes that were orbiting each other and so they

distorted the spacetime enough that these waves that propagated out -

they could be detected here on Earth. -I said space is expanding and contracting,

but the amount that space is expanding and contracting by is absolutely

minuscule. So this big result that came up with this thing that they detected - the amount

by which space was expanding over the many kilometers of their detector was

less than the diameter of the nucleus of an atom. So it's an absolutely tiny - infact

that's why we don't - sort of notice them going past 'cause if they were big effects, you know you'd see

kind of space doing all sorts of weird things. But because they're so tiny we just don't see them.

It's just amazing, I mean the numbers involved, the timing involved -

it's a spectacular event so - a billion light years away - four hundred and fifty megaparsecs away -

So a billion years ago, two black holes which were each of them thirty times the mass

of the Sun, okay? Which is unusual apparently in its own right to get this kind of

combination. They were orbiting, they'd been orbiting each other

for probably millions of years anyway. -They have to be bound together because they

were in orbit around one another and it's actually quite complicated because -

so the way you get massive black holes is you have some very massive star

exploding in a supernova, and unless that supernova is set up very carefully, if

you imagine you had a pair of binary stars in orbit around one another -

one of them goes supernova - if you're not very careful that's gonna

unbind the system because you've lost a whole load of mass, there's all sorts of energy

being transferred between one and the other - so somehow the two manage to stay

bound together, it would seem.

When first one went supernova and then a bit later the other one went supernova -

I say a bit later, you know -

probably tens to hundreds of thousands of years later the other one went supernova.

Alternatively, possibly, if both of these stars were in a cluster they might actually have

individually been separate stars and at some point in the subsequent

evolution they might have actually got sufficiently close together that they'd end up

capturing each other and end up in a binary system that way. So it is a little bit of

a mystery how you make these two massive black holes - fairly massive,

not supermassive black holes - in orbit around one another in the first place, but there are

at least kind of plausible mechanisms for doing it. -So they're doing this for millions

and millions of years and then in the final - I think it's .2 of a second -

first they're coming closer and closer together. They start going so rapidly

around one another that it begins to approach the speed of light in fact

something like sixty percent of the speed of light.

These are 30 solar mass black holes. As they're coming closer and closer, when they're

about, I think, 350 kilometers apart they basically start merging together.

Amazingly, from just looking at the kind of signal they detect they can learn a

great deal about the kind of black holes it actually was, how the amplitude

changes over time, how the frequency of the signal changes over time. So in

this case they're fairly confident that one of them was a 29 solar mass black hole

and the other one was a 36 solar mass black hole -

they got sort of errors of 1 or 2 solar masses on each one -

but they were both 30 to 40 solar mass black holes. So those are the

kind of black holes which are probably the end states of very massive stars,

although they are actually on the high side even for very massive stars.

Remember we've talked about this intricate link between the matter and the

spacetime. Just imagine - try to imagine - these two huge objects.

What they're doing to the matter, to the spacetime around it, as they- and the

spacetime must be going 'oh my god what's happening here' and it's flipping

up and down, up and down and they're just generating - these waves are beginning to

propagate out. 350 kilometers, we should find a distance, what, to London? Then you got two

30 solar mass black holes sort of orbiting one another in this region and so they're

going at close to the speed of light. So the the spacetime in which it's

revolving must be going - is having huge distortions associated with it.

And so it begins to send out gravitational waves - they've been happening

all the time but at a much lower amplitude because they've not been

feeling this effect, like this. And then these two black holes keep coming in together and they merge.

And what happens is when you've got two black holes they'll merge into a bigger black hole.

So one of them is 29 solar masses, the other one is 36 solar masses.

If you add those two together you get 65 solar masses, so you would think by merging

these two together you make a 65 solar mass black hole. Turns out they can also tell

you what the mass of the black hole ended up with was. Again, just by looking at the

kind of signal, and it's not 65 solar masses, it's about 62 solar masses.

And the reason why is because three solar masses has disappeared, and via Einstein's

famous formula E=mc^2 those three solar masses of energy

have all been turned into the energy of the gravitational waves. So three solar masses

by E=mc^2 has been turned into a huge amount of energy liberated

in this gravitational wave explosion. And in fact if you work out what the luminosity

of the thing was, how bright it was in gravitational waves, in that fraction of a

second as all this happened, it was brighter, it was liberating more energy,

more power than all the stars in the entire observable universe -

for that fraction of a second - all in gravitational waves.

But there was no light? -There may well have been some light as well, but that

was just what was coming out in gravitational waves, mostly the energy

of this merger was coming out in these sudden bursts of gravitational waves.

They're traveling now,

they've got a billion years, they're traveling in all directions - they propagate, and then it - as it

happens, there's a detector - two detectors in America, been recently updated, and

they've just been turned on - they were doing I think they call it the engineering run - they

haven't even started doing the proper science run.

They'd been turned on for a few weeks, and a few billion years later these waves are

coming through - now they've lost a lot of their energy, right? Just as light loses

its energy and becomes dimmer and dimmer. The huge amplitude associated

with the waves early on is now dimmed down, down, down, down. They pass through

this detector, and the detector consists of two arms - four kilometer long arms.

An interferometer, classic interferometer has two arms to it and you basically

shine a light down each arm - usually a laser 'cause you want it to be nice coherent light - and

in essence you shine the light backwards and forwards along each of these arms - by recombining the light you

can essentially tune the thing so that the two arms are exactly the same

length as each other. And if you set up your interferometer right, then the light

that's gone down this arm, and the light that goes down this arm exactly cancel

each other out so you end up with no signal at all. And so that's a thing called

a nulling interferometer it's set to - you get zero signal when the two arms are actually

tuned in that way. Now, of course, when one of these waves goes past, in one

direction it actually causes a contraction and in the other direction it actually causes an expansion.

"This back and forth stretching and squeezing happens over and over

until the wave has passed."

As the wave goes past, by this tiny, tiny amount

the arms will no longer be exactly the same length - and the effect of that is

then that exact cancellation ceases to work, and suddenly some of the light gets

through your interferometer. So the way they actually detect it is that they actually

start seeing light in the interferometer because the arms have changed in length by

that tiny amount. -This huge amount of energy required

this desperately accurate detector in order to be able to find the gravitational

waves. And then you might ask: "How do you know you've found gravitational waves,

surely everything distorts?"

[Brady off Camera] Seems like an instrument that a mosquito sneezing would effect them.

[Proffessor] And they get huge numbers of false positive detections, so any kind of

earth tremor, a truck driving by, all those kinds of things produce signals that they end up

detecting in these interferometers. There's two things that save them: one is that

actually it has - the things that you're looking for - so things like these black

hole signatures - have a very characteristic shape to them that the

way that the oscillations increase and decrease in amplitude with time - has this very

classic signature to it that tells you the kind of thing you're looking for -

so they know what sort of thing to look for, and then the second thing that saved them

is that there isn't just one interferometer, there's two working at the

same time a large distance apart from one another -

and so the chances of the same pathological truck going past both of them

at the same time producing something that looks exactly like a

black hole merger signature is at that point astronomically small -

so they can, by doing this kind of coincidence

thing of detecting it in both detectors almost simultaneously - tells them that

actually they have detected a real astrophysical result. One of the upsides

to actually having two detectors; if the gravitational wave is coming from over

here somewhere - it'll hit one detector first and then a bit later it'll hit the other detector

So the wave came through, hit Louisiana first, and then the

light travel time - because they're going at the speed of light - it then passed

through the Washington detector - exactly the same profile -

7 milliseconds later which corresponds to the light travel time - and that

enabled them to sort of give an estimate of where in the sky this original thing had started from.

So for example this thing that they've detected, they know it's

somewhere in the southern hemisphere. They can't say much more than that,

it's somewhere in the southern sky, is about as close as they can get - but they do

at least get some directional information. When they start getting third and fourth

detectors up obviously that will give them more information, so they'll actually be able to

triangulate much more exactly where these sources are. -Potentially an issue

for the gravitational wave community: it could be that we're on the

verge of being inundated now with

black hole by neutron star.. black hole binaries..

So all of a sudden they're everywhere and we just hadn't had the sensitivity

to detect them and now *poof*. No one really knows how many there are out there

because all that we have to work on are theories where you estimate

how many you expect there to be -

so that, I was reading that, you know, they're expecting an order of 40 per year, but