I guess the first thing to say is they're a consequence of general relativity that
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
hey, we may have got that wrong, it may be four thousand or something, in which case you have a
bit of a different issue - you have like an LHC issue, where you've got so many
collisions. How are you gonna extract out the interesting physics here, you know,
where's the Higgs coming from - here you might just have so much radiation coming -
gravitational waves coming in from all of these binary systems that we think we
understand the binary systems and we're now interested in finding the
the weird and wonderful early universe features.
That might be a - Well that'll be a nice problem to have, I think.
You've got two very massive objects and they're in orbit around each other in a binary black hole system
when something is moving around in a circular orbit it's actually accelerating
So they weren't looking. They were in their shut down mode when this gravitational wave
when this gravitational wave passed through it. Travels for a billion years, those detectors were up and running
maybe a few months earlier, but they had just shut down and it passed through.
[Brady off Camera] Blink and you'll miss it [Professor] blink and you miss it