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
