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  • 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

I guess the first thing to say is they're a consequence of general relativity that

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B1 中級

重力波の発見-60のシンボル (Gravitational Waves Discovery - Sixty Symbols)

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    林宜悉 に公開 2021 年 01 月 14 日
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