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Instead of. Members of the Oxford Physics Department lets students, staff and faculty I'd like to welcome you to this physics colloquium,

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which is held today and the Oxford University Museum of Natural History,

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which was established as you all know in 1960, to draw together scientific across the university.

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And you should know that this lecture theatre today is thought, and this event is now being streamed live just outside the store.

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It's the world's first scientifically described dinosaur, the making of science, but ran by the world famous Oxford Dodo.

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The only soft tissue remains extinct and remarkable,

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but today's lecture on gravitational waves and prospects for motion messenger astronomy is being given by the distinguished physicist Barry Barish,

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now at Oxford, where they'd be proud of the education of students.

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They go on to make new knowledge and then go on to careers that change the world.

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And so an exhibition that we established with the first of these lectures last September.

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I've asked one of our students, Ben Fernando, as a Ph.D. student and physics and assumptions here working on seismic wave propagation

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and is also a member of the science team with the Russians Insight spacecraft,

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which recently landed on Mars to actually conduct the introduction for Barry.

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And you'll notice when Ben comes up onto the stage, it is wearing a uniform because he is a member of the Navy gear that's attached

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to this university and actually you spend some of his spare time out at sea.

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I think about how should work this evening.

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He's wearing a uniform because it's going to be conducting outreach to the local secret unit about universities such as Science at Sea,

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and he's wearing the uniform and making a group of people who, generally speaking,

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I've never gone to universities in their lives or come from different backgrounds.

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And this is a very important part of what we do in Oxford is reach out to the world.

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And so please welcome Ben, who will introduce part. Thank you, everyone, for coming.

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We're delighted to welcome here today,

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a titan of modern science professor Barry Barish from the California Institute of Technology and the University of California at Riverside.

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Barry is originally from Omaha, Nebraska, and made his first forays into the fields of physics in the 1950s,

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when he gained a B.A. from the University of California at Berkeley and later a Ph.D. from the same institution.

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From then, he went on to the staff at Caltech, first as a postdoc, working on experiments including those at Fermilab.

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Later in his career, Barry was involved in the macro experiment at Grand Sasso in Italy.

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And following that, the gem detector for the superconducting super collider proposal.

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In the mid-1990s,

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Barry became involved in like the Laser Interferometer Gravitational-Wave Observatory and was instrumental in setting up and directing the project.

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In the last 20, 25 or so years, Legault came to a head.

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As many of you will know in 2015, with the first detection of merging gravitational waves, a discovery that was nearly a century in the making.

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In 2017, Barry was awarded a share of the Nobel prise in physics for his seminal contributions to the work of Lego,

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together with Kip Thorne and Rainer Wise.

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We're delighted to welcome him here today to tell us a little bit about his life as a gravitational wave physicist

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and the prospects for science moving forward in this exciting new era of gravitational wave observation.

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So thank you. Very. Thank you, Ben.

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I've come to Oxford, you know, every few years through my career, so I'm happy to be back.

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I saw new physics building today that wasn't here last time I was here.

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So hello to my friends and colleagues and the rest of you who came to hear the lecture.

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I'm going to talk today about gravitational waves themselves, what it takes to measure them and see them.

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But in particular,

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I'm going to concentrate somewhat on what I think this opens up in terms of part of what the future will be for gravitational waves.

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The part that I won't do is the deep physics.

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The gravitational waves present us with probably the best way to test general relativity.

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I won't talk much about that.

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What I want to talk about is what I think is a new direction that you'll see in the next decades opened up partially by gravitational waves,

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which is being called multi messenger astronomy.

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So I'm going to talk about gravitational waves and then try to give you a picture of where we think this is all going.

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And in a sense, we're it's a little bit presumptuous,

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but we're in the same situation that maybe the science of astronomy was in at the time of Galileo.

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Galileo was the first to take an instrument instead of our naked eyes and look at the sky.

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Famously, he looked at Jupiter and with a telescope and discovered that there were four moons of Jupiter.

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Now we know there are more, but there were four moons of Jupiter. It's been more than 400 years since then.

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And astronomy has evolved fantastically in that era.

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We're at the very beginning. I don't know if it'll be nearly as rich, of course, as astronomy, but in a sense,

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we're at the very beginning of having come to the point where we're able to see signals

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from the universe that come from astronomical events that emit gravitational waves.

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And so you'll see how we do that and then what that might lead to in terms of astronomy today, mostly or what we call multi messenger astronomy.

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First, astronomy a little bit in recent years, what we've seen happen in astronomy is an advance that's given a lot of richness to what we know about.

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The universe through astronomy by using different ways,

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using the electromagnetic spectrum to look at astronomical events, the same events, but with different probes.

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And I, for example, I show here the Crab Nebula looked at in different ways.

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So we look at different parts of the electromagnetic spectra and look at the same phenomenon,

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and then we have a lot more information that we can use to do astronomy.

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In a sense, this is one of the big advances in astronomy that occurred in the last half of the 20th century and has led to

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many of the advances that we know about in astronomy as maybe a precursor to what I'm talking about today.

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We've seen other advances which. Have happened as we've moved into the 21st century.

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We've seen a really fantastic demonstration of how to do science on a worldwide basis

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in astronomy by combining a set of devices that weren't designed to work together.

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These are infra-red telescopes in various parts of the world to make an interferometer that

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can look at the sky and work as an instrument that basically is the size of the Earth,

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instead of the size of one of these interferometers that relied on a lot of very sophisticated methods to bring the data together.

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Computer computing around the world, bringing this all together, finding a target to look at.

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And we saw just early this spring these beautiful images of a black hole taken

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through these radio telescopes from around the world and at least in the US,

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it was a big colour picture on the front page of the New York Times.

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So astronomy has been fantastic. The next frontier, I believe, is what we're calling multi messenger astronomy,

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which is really what I'm going to come back to after I talk some about gravitational waves,

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of course, an electromagnetic setting the universe through electromagnetic waves.

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We now have a whole set of sophisticated instruments that use the different wavelengths, some on the ground, some in space.

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We have a new generation of those that'll be coming along within the next decade or so.

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In companion with that in companionship, with that,

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we now have the beginning of the ability to see a phenomenon that happen in the universe with gravitational waves,

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which I'll talk about and also with particles in this case, the particle that, well been coming through, and that is neutrinos.

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So in the future, I believe that we'll see a real richness in our ability to study phenomenon with a

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combination of sophisticated electromagnetic instruments that we're all used to.

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And the next generations of those. The gravity, gravitational waves and neutrinos from the same phenomena.

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So so far, we're just in the infancy.

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Now I'm going to go back and talk about gravitational waves and then lead to what we have done and can do, I think with multi messenger astronomy.

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We all learnt. Most of us, I think our gravity when we were in elementary school and our teacher taught us that when the apple falls out of the tree,

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the Earth pulls it down and the moon goes around the Earth, all due to the equations of Newton, which I show here.

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And that equation of of of Newton's is his theory of universal gravity,

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which is g times the product of the masses of two objects over the inverse square of the distance they are apart.

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It took about a hundred years more to determine the strength G.

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But this basic description of gravity described. Everything that involved gravity and in nature for more than two hundred years,

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it described everything from the orbits of the planets, the dropping of the apple, the tides and so forth.

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And basically, by the time Einstein came along and introduced a new theory of gravity, there were no major flaws,

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no major problems with excuse me, with Einstein's theory with with Newton's theory of gravity.

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So this is the best theory we had, Einstein came along. I'm not going to talk,

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I just show it for symmetry about the detailed equations of Einstein's theory that he came along in the early 1800s and

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made a new theory of gravity based on bringing space and time together in four dimensions and the unified space time.

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First question is why? Why do we need a new theory of gravity when we had such a successful one by Newton for all this time?

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I think the answers primarily are two.

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One is, there was at least one case of a observation that didn't exactly agree with with Einstein, with Newton's theory of gravity.

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That was the orbit of Mercury around the Sun.

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That wasn't a very big flaw, but that was the one discrepancy that existed in the early nineteen hundreds.

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But there were conceptual issues and Newton's theory, too.

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One is that it had what we call instantaneous action at a distance.

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That is, when the Apple falls, you detect it immediately, and that's fine for the apple falling.

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But if the Sun were to disappear right now, we know that it takes eight minutes for the light to arrive here.

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And certainly the effect of gravity is going to take some time, we believe.

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So Newton's theory didn't have any time for the messenger or the signal to get to pass through space.

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Einstein Einstein's theory the signal from gravity travels at the same speed.

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There's just one speed and the problem we call it the speed of light, but it's also the speed of gravitation and gravitational waves.

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The second problem that existed in Newton's theory, I find personally an embarrassing one,

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because when my teacher told me that when I jump up, the Earth pulls me down and I believe that maybe later went to learn this formula.

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I never asked why. And I don't know if any of you did or many of you did, but probably not very many.

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And that question, which is a fundamental one, was not answered by Newton's theory.

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Why does your on Einstein answers that and Newton's case?

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People tried to answer it for the hundred and some years before Einstein came along,

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usually with explanations that were electromagnetic in nature and weren't really right.

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Einstein basically does it by the fact that he in this four dimensional space, there's what we call curvature of space time,

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and it's that that affects the poles when you when the apple falls out of the tree or that you fall.

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So in this picture here, I show now the generation of gravitational waves.

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Electromagnetic waves were discovered by Hertz in the late eighteen hundreds by taking charges,

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isolating them, making a dipole, going in the next room,

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detecting electromagnetic, the electromagnetic signal from electromagnetic waves,

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a signal and moving forward and backwards, and seeing that it had a wave like nature.

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Ideally, you'd like to do the same thing for gravitational waves. It's not a dipole.

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Instead, it's a quadrupole in the case, a source in the case of gravitational waves.

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But likewise, you'd like to detect them move forward and backwards, just as Hertz did, and control all the variables that is, make a sauce.

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In this case, it would be a big bar bill that you'd rotate and detect gravitational waves because it's a quadruple.

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It turns out that that's not at all possible.

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I could go through the numbers with you, but to compare all this data, no, if I tried to do that, inventing my own experiment,

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near-surface taking a barbell like shape, rotating at a high frequency, moving, putting a detector nearby.

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The key number is that the sensitivity I'd have to have in the same units I'll show you and a little bit are about 10 to the minus thirty eight.

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I'll show you that we can barely detect gravitational waves when we have a source strong

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enough to make the Signal 10 to the minus twenty one 17 orders of magnitude better.

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So we couldn't do what a physicist wants to do, which is an experimental physicist that is to control all the variables to get it right.

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So you want to control the source, control the detectors, control the data.

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We can't do that. So we're forced to find a source that can do part of the problem for us.

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And that's what drove us to look for gravitational waves from a source in space with the.

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With the negative part now, we don't control everything and may not understand that source very well.

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And of course, the positive part that we can find sources that will give a much bigger signal

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while we're fairly primitive in our ability to detect gravitational waves.

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So that's what we did. Luckily for us, the source that we found turns out to be fantastically interesting in itself.

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So the deficit or the bad point as experimentalists,

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you don't like to have anything out of your own control in this case gave us a source that in itself

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was interesting and that we talk about probably more than we talk about the detection ability itself.

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OK. Gravitational waves. Themselves were first proposed by Einstein a year after he introduced general relativity in 1915.

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He introduced general relativity, having put acceleration into the special theory of relativity that he introduced in 1945 and

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the theory of general relativity became the new theory of gravity or his theory of gravity.

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In 1916, he noticed that if he wrote the equations of general relativity in a particular way, I'm not doing it quite like he did.

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And only those who know a little bit more about general relativity can read what I did on the left side.

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He basically noticed that if he set up the equations of general relativity in a particular way,

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the equations of the letters are different looked a lot like the equations of electricity and magnetism.

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He didn't derive the fact that there were gravitational waves.

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Instead, he just made the leap in Einstein that if the equations looked like the equations of electric,

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just magnetism, then there must be waves and gravity just as there is an electricity and then magnetism.

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So he proposed that there are electromagnetic waves. The paper itself was a terrible paper.

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It had errors in it. He didn't derive anything, but he made this presumption.

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Two years later, in nineteen sixteen nineteen eighteen, he wrote a second paper put on that second paper fix.

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He fixed the errors. He never admitted they were wrong, but he fixed the errors.

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But he did something else that was important for us, anyway,

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is that he demonstrated what kind of sauce would make gravitational waves that you need a quadrupole source.

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He only wrote one more paper on gravitational waves in his lifetime, and that was 20 years later.

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So in nineteen thirty six, he had immigrated to the US and was working at that time with Rosen.

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There's famous work of Rosen and Einstein. At that point he was at Princeton University,

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and he revisited with Rosen the question of gravitational waves because he wanted

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to see if he could derive it out of the theory of general relativity itself,

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rather than just postulating it. He failed to do that.

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And he, in his calculations with Rosen, got a bunch of infinities and his calculation.

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He then decided he must be fully himself. And he had no idea that there really weren't gravitational waves.

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There was an artefact. It turns out in general relativity for those who have ever studied it,

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it's easy to get an infinite is what we call coordinated singularities, trying to work in four dimensions.

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So it's a typical kind of problem. Anyway, he wrote a paper with Rosen.

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He submitted it to Physical Review Letters, which is the same place we submitted our paper.

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And in that paper, the paper was entitled On Gravitational Waves.

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No, I'm sorry. It was entitled Do gravitational waves exist? A funny title for somebody who had proposed it 20 years before that paper.

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It's a long story. I'll just tell you the bottom line.

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That paper went through a peer review process and was sent to a reviewer named Howard Percy Robertson,

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who happened to be at Caltech my institution on sabbatical from Princeton at that moment.

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And he saw how Einstein and Rosen had gotten these mistaken singularities and actually even showed how to get rid of them by using

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what he called cylindrical coordinates without giving the whole story that eventually went back to his physical review editor,

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who sent it to Einstein. Einstein then decided he wasn't going to publish in Physical Review.

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Rather, it wrote a rather negative letter. He never published again in Physical Review.

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And instead, he published the article about six months later in the Franklin Journal,

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which is a journal that doesn't exist today, but is from the Franklin Institute in Philadelphia.

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And that paper changed the title to on gravitational waves and starts in the first sentence,

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describing gravitational waves and cylindrical coordinates.

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But that still didn't convince the theoretical community, and it's the last paper that Einstein ever wrote on gravitational waves.

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It was about the 1950s when finally the theoretical community believe that there were gravitational waves and it happened in a meeting in Chapel Hill,

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North Carolina. And in that meeting, a actually a theoretical general relativity called Peroni,

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who I think was at this institution derived gravity gravitational waves from the fundamentals of general relativity.

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And at the same meeting, my colleague Dick Feynman was there,

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and he made the presumption that if there are gravitational waves, they have to be able to transfer energy.

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And he made a little good Duncan experiment that could demonstrate how that could be.

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So I don't have time to go through that today. This is all an excuse why it's taken 100 years to realise I'm talking about theories.

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So after that, in about the 1960s, it became an experimental problem and the idea was first tuned scene.

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Did you have to try to detect something from the universe? And I show here the generation of gravitational waves.

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You notice the top equation.

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If I set up general the message, the take home is if I set up general relativity in a particular way that I describe on the left,

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I get equations that, except for the letters, look like the wave equation.

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And so it's not surprising that I get basically a plane wave out of those equations and the plane wave I show in the next.

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And that just below that and the little h that's there is the key parameter that we try to measure and gravitational waves.

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We call it the string. It's the strength of the gravitational wave, the amplitude.

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The gravitational waves travel at the speed of light, just like electromagnetic waves.

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You'll notice in this picture, though, that the two waves are not what you learnt in electricity and magnetism.

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They're at forty five degrees to each other instead of 90 degrees to each other.

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And that's a direct result of the fact that gravity is spin, too.

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So being spin to instead of spin one, there's two components, but they're at forty five degrees to each other.

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We're on the verge in what we've measured.

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I won't show that today to be able to just to touch, to change or to pull out the two different components of gravitational waves,

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which turn out to be important for testing different ideas about general relativity might be like and whether there's variance of Einstein's theory.

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But so far, we haven't really been able to do that, but I think it's very simple.

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OK. So gravitational waves themselves come out of something like the picture that I do here.

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That is two objects going around each other, you can imagine those that say black holes.

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Gravitational waves come out. They go at the speed of light.

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And what do they do first?

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They're not like electromagnetic waves going through space, electromagnetic waves have associated with them photons.

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In the classical theory, as long as we stay with Einsteins classical theory,

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then there is no propagator art equivalent of the photon or something we might call the graviton that goes along with the gravitational waves.

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Instead, it's effectively just ripples in space and time itself, a little bit like having a pool of water still full of water.

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You throw a stone in the stone, sinks to the bottom, and there's ripples that don't have any part of the stone in them that travel along.

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So you instigated ripples that travel in space and time, and we're looking at that.

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So there's a curvature of space time, the amplitude of the waves for the kind of sources that I talked about today is about one part in 10

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to the twenty one that little each that was in my formula that turns out to be a really small number.

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I'll show you how small in a minute. And that's the number that we actually measure.

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It's proportional to something directly experimental.

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That is the distortion of space or length in a particular direction.

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So Delta L change of lengths over length, which is what we call experimentally measure, is proportional directly to this little h,

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a number that's 10 to the minus twenty one for the sources that I'm talking about today.

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So imagine that I have a circle of three masses and now a gravitational wave comes through the board.

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What does it do? It basically distorts it so that it's a little taller and that length is this Delta L over L or 10 to the minus twenty one.

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If it's a metre in size, so it's 10 to the minus 21 metres and depending on the wavelength or frequency of the gravitational wave,

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it then oscillate the other direction becomes shorter and fatter.

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So it's a little bit like going to an amusement park where you're in front of a mirror that makes you a little taller than the next one,

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makes you short and fat and you go back and forth. And that's what happens when gravitational waves come through.

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Why is the effect so small? It basically comes down to the fact that if you think about space, which is being distorted as being,

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say, a material that the Young's modulus or space itself is just very stiff.

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So luckily for us, because we don't want space to change, too much space is very stiff.

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These gravitational waves then can't do very much to it. So it's not that they're weak or don't carry any energy.

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It's if that space itself doesn't get affected very much that the effect is so small going through space.

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OK. So how do you measure a very small thing? We do it with interferometry.

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This is a picture of an interferometer. Light comes in from the left and as it comes in, it's split in two directions.

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If those two directions are equal in length, then the light goes down.

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Shown in wave form, here comes back. And if you invert one with respect to the other, they'll just cancel.

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This is showing them cancelling and now hitting our detector, which would see nothing.

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But if one of the arms gets a little bit longer or shorter than the other one, then they won't completely cancel and we see some light.

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And that's the principle that we use. So that's called interferometry.

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And we do lots and lots of tricks and have two to make this a very sensitive instrument, and I'll just give you a sense of the key ones.

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So this is a picture exaggerated of a frequency of a couple of times a second gravitational wave going through an interferometer.

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So it makes it tall and it makes it longer in the vertical direction first than the horizontal

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direction and goes back and forth the size of the interferometers l in this picture.

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The accuracy that we have to have is to measure the difference in length in order to

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get this 10 to the minus twenty one of the light that's in the interferometer itself.

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And people who have ever used an interferometer know that you basically use an interferometer in a lab and you see these fringes,

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which come from the wavelength and you can measure something to some fraction of those fringes, maybe one in 10 or one in 100 if you're really good.

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We have to do it to one part in 10 to the 12.

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So the first part of the challenge, I'm just simplifying the challenge for us and the two numbers for you.

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One is that we have to do interferometry to an accuracy of one part in 10 to the 12th.

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That's a small number of the wavelength of the light that we use in the interferometer itself.

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Interferometers that are used for very sophisticated purposes might be one part in a thousand or ten thousand or something.

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But this is orders of magnitude beyond that. And it was the principle.

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Target goal that took us years and years to develop enough tricks and means to measure interferometry at the level of one part in 10 of the 12.

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It turned out, though, that the second challenge was even harder for us, even though maybe it seems more mundane.

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So the two challenges that we have to succeed at to be able to see something at the level of this small 10 to the minus 20 one

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were first to be able to do interferometry at one part and 10 to the 12th of the wavelength of the light that we're using.

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The second is the fact that we're living on Earth and the Earth shakes like mad in the frequency band that we're in.

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And so we have to actually do interferometry in a way that's different than in the laboratories here on campus,

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where the first thing you do is go to an optical table and put your mirrors down for them as stable as you can.

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We can't have anything to do with that because the Earth shaking is much too much.

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Instead, we have to basically isolate the interferometer even though we're living on the Earth from the Earth itself to one part in 10 to the 12th.

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And that's a tremendously small number. I'll tell you how we do that.

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And finally achieving that was what was needed to to see gravitational waves.

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So if you want your elevator speech when you leave, I'll show you what was the final experimental or technical accomplishment that we made.

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It's pretty mundane. In order to make this measurement, so we thought we could do that, we didn't have all the technologies in place.

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We got funded by the National Science Foundation and approved to do a project where we would develop two interferometers,

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two so that we could make sure within the speed of light that we saw the same signal are nearly the same signal in both interferometers.

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We proposed to the National Science Foundation in the US that we build them.

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We weren't allowed to pick a site, but of course, nobody can stop us from suggesting.

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So we turned in a proposal where we made what we called sample sites, which was our choice,

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of course, one in Southern California near the Edwards Air Force Base and not too far from Caltech.

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And the other one in southern Maine, just not too far from MIT.

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It went into the political process, and you can see that it got rotated by about 45 degrees,

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and we ended up with two very friendly and supportive senators from the state of Washington and the state of Louisiana.

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These two detectors are now three thousand kilometres apart. And so they should see a gravitational wave within the speed of light in both.

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So if we have if we have a.

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If we have a gravitational wave coming straight down, it'll be at the same time in the two if we have one coming in from this side.

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You know, if this detector 10 milliseconds before this detector and this one, 10 milliseconds before that detector.

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So we expect that a gravitational wave will be within plus or minus 10 milliseconds of the same time.

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And that's a key feature of what we do.

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We then look for out of time coincidences. We're one of them is 20 milliseconds away from the other one or 10 seconds away.

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And we do that to see if there's any accidental. And that's how we determine our our ability to actually see a signal over accidental signals.

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So that's the technique generally the two devices.

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Are shown here. They're identical. They don't look like it in this picture, but they are absolutely identical.

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One, however, is and they're in different geometry geographies.

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The one on the left is in Hanford, Washington. That's on the D.O.D. reservation, where they had reactors to develop the atomic bomb years ago.

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And we're we're not we have nothing to do with the deal. We accept that we're on the land and it's high desert.

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As you can see from the picture, the one on the right is the second detector we have.

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It's in Livingston, Louisiana, and it's in commercial pine forest, where they cut down trees every 10 years.

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Let them grow, cut them down every 10 years to make paper. And we live in those, too.

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And as I'll show you, the sensitivity of them are identical, although they're very different.

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The one on the right, by the way, is swampy. So you can see water here.

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There's water if you go kind of one millimetre below the surface in Louisiana. And we built this up.

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You can't see it very well in the picture, about six metres above the surface so that it wouldn't get flooded.

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The dirt that we got to build it up here, we took from here and it immediately filled with water, fish, alligators and everything else.

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And then the locals kill the alligators, so it's interesting, OK?

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This pictures and what happens, you put all this together, what limits you and what enables you to make a measurement?

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What I'm showing you on the left is the sensitivity.

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So first, looking at the scale on the scale, on the right side is this sensitivity.

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This is the 10 to the minus 20 what, 10 to the minus, 20 to 10 to the minus 20.

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The sensor to the shaded region is where we should be sensitive. All these lines are the different ways that affect our, our limit, our sensitivity.

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So you'll see that we're limited by three different lines immediately.

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Again, as an experimentalist who was taught when he was in school that you should control all the variables.

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I'm already not doing that. I have a terrible situation experimentally where we're limited, not by one problem, but by three problems.

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So we have three different experimental problems to be at low frequency.

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We're limited by what I wrote here is seismic noise. That's the shaking of the Earth.

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So this is how will we get rid of the shaking of the Earth that affects the low frequency and that's shown here.

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It falls like frequency to the fourth power. So even if you get a little better, you don't gain a lot at high frequencies.

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This is the frequency band at high frequencies.

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We're limited by what people that work on interferometers called shock noise, or somebody they say comes from particle physics or quantum physics.

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We call for statistics. So basically, we're limited by how many photons we have, which is less and less as we go to higher frequency.

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And in the middle level, we're limited by something else, which we call thermal noise.

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And that's basically just Caity noise because we're working at room temperature, so we're limited in these three regions.

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You'll notice that the frequency band is the frequency band, the audio band.

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So we have nothing to do with audio. But if we're living here on Earth and we want to make the most sensitive instrument,

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our ears have learnt for us through evolution that the place where you're most sensitive is in the audio band.

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So we're in the same band of frequencies where the Earth, if we're going to lower frequency shakes too much at higher frequency,

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you have to sample too quickly and you don't get any photons.

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So we're basically limited to the same frequency band as the audio band that we all communicate with.

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So for that reason, people have taken our signals, for example,

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and make an audio signal out of it and transmit it, and some of you have probably heard that.

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I don't do it because I'm too much of a purist because the frequency that we saw in the very first event was very, very low frequency.

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The ones you heard on the radio actually were multiplied by four to make them better for the years.

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And but it's in the audio band. The graph on the right is kind of the experimental history.

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So we started in the early 2000s and we had the same plot as this, you know, notice it has the same characteristic shape.

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That's good. But the sensitivity is up here somewhere.

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This was already the most sensitive instrument in the world, so we were able to try it run for maybe six months,

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write some papers that we didn't see anything more sensitive than ever before.

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Lick our wounds. Put in some new devices and try again, and then we come down to here and come down to here.

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Subsequently, we came down to what's shown down here and the dotted line is basically when we came to the design limit,

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or we knew we would be limited by truly by photo statistics or the remaining shaking of the Earth or thermal noise.

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So at this point, we hadn't seen gravitational waves, but we were ready and we had already proposed to the NSF how to go to better.

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Fortunately for us, the National Science Foundation stayed with us,

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even though we've been doing this for quite a few years and we started a major upgrade.

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We called the upgrade LEGO to natural thing that happens in experiments.

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They rejected that. They said If you're proposing Lego to your, you're inferring there's going to be a Lego three.

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I go for it. Therefore, we changed it to what's called advanced LEGO, which you've probably seen in the in our papers and newspapers and so forth.

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Now we have a new version that we're working on, and we call it Advanced Sligo Plus for this kind of twisting it around.

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And so it's called A-plus often, which is advanced Lego plus, and that'll be the next few years anyway.

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The improvements were many. We had started getting experience with an interferometer.

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But the NSF was supportive enough that they let us develop the technologies to make it better while we were doing the graphs that I showed,

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the curves that I show you here.

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And so we were ready to do a major upgrade with the R&D programme and engineering programme that we had carried out that.

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Involved lots of changes, we had done one smart thing,

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and that is that we built the infrastructure for Lego from the beginning so that it was bigger than it needed to be,

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had more ports and flexibility so that we didn't have to go back to the NSSF to rebuild a lot of very expensive infrastructure,

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but rather for instruments and techniques. So that was the smart decision that we made early.

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Anyway, we essentially rebuilt all the parts of Mica, which to make Advanced Sligo,

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which I show here on a chart to show you there's more to it than what I'll show you in a minute.

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That enabled us to have a more powerful laser to have a quieter thermal test, mass and so forth and so on.

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But the key one that enabled that, I promise you that it enabled the discovery is shown here.

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So let me describe the problem. I already said that we have to isolate ourselves from the Earth by one part in 10 to the 12.

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So how do we do that? We didn't invent anything magnificently clever.

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We just did a good job of something that seems pretty mundane, which once you understand it.

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The first is that we work in this frequency band from 10 to ten thousand hertz, the audio pan, and we want to get rid of the shaking of the Earth.

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So what's the technique that we all know pretty well? Many of you used it today in your car shock absorbers.

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Shock absorber doesn't get rid of the energy. It just takes the energy and moves it from the frequency band.

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That gives you a big bump when you go over it to low frequencies where you smoothly feel your car move along.

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And so we did the same thing. We made the world's fanciest shock absorbers for different layers of the shock absorbers.

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The shock absorbers themselves have just the right squishiness. We did a lot of engineering and we put that all in,

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and that was what was in for the measurements that I already showed you and we didn't detect gravitational waves.

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So we had to do something more than that and we took another.

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We knew this in the 1990s, not 2000, but we didn't hadn't developed the technology yet.

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So we asked the NSF during that period, we were making those measurements to develop the technology, and we had it developed by about two or five.

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And that is again, a technology that many of you are familiar with, and that is, you get on an aeroplane and you don't like the roar of the engines.

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And so you get one of these Bose or somebodys earphones that cancel the noise of the engines.

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What that's doing is measuring the ambient roar of the engines and basically cancelling the ambient noise.

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The stewardess comes up and asking you if you want a drink and you hear perfectly well because that's not ambient,

382
00:43:38,390 --> 00:43:45,770
so you're able to take out that noise. Again, we're not doing anything audio, but it's the same problem.

383
00:43:45,770 --> 00:43:51,230
So we basically decided we had to do something that wasn't totally passive equivalent

384
00:43:51,230 --> 00:43:56,690
of what's done in the earphones that is cancel actively the shaking of the Earth.

385
00:43:56,690 --> 00:44:03,260
So what we did is embed in these shock the system with shock absorbers,

386
00:44:03,260 --> 00:44:12,470
active size seismometers that measure the amplitude and the direction of any remaining shaking of the Earth,

387
00:44:12,470 --> 00:44:17,630
and we cancel it by pushing a force against it that took a long time to develop.

388
00:44:17,630 --> 00:44:26,480
We've developed, I think the most stable tables in the world could be very good for microelectronics or something, but they're incredibly expensive.

389
00:44:26,480 --> 00:44:30,290
So somebody else has to make them cheap anyway.

390
00:44:30,290 --> 00:44:36,920
With that, we got this factor of 10 to the 12. And one of the thing that happens is we use a pendulum and a pendulum.

391
00:44:36,920 --> 00:44:41,630
You all know when you're holding a pendulum and you move the top, it doesn't move the bottom very well.

392
00:44:41,630 --> 00:44:49,760
So we get rid of most of this kind of motion with shock absorbers and this motion by having a pendulum.

393
00:44:49,760 --> 00:44:55,520
The pendulum itself is four levels of pendulum so that all the controls for the bottom,

394
00:44:55,520 --> 00:45:00,380
which is the mirror, are controlled from above, and it's rather fancy to be able to do that.

395
00:45:00,380 --> 00:45:06,170
But that's what we do so that we have a pure, very, very fancy mirror on the bottom.

396
00:45:06,170 --> 00:45:09,770
I don't have time to show you our other technologies today,

397
00:45:09,770 --> 00:45:16,790
so I'll move on because I want to show you a little bit about the potentials of multi messenger astronomy.

398
00:45:16,790 --> 00:45:27,800
OK. So this is where we were on the left with two the two interferometers at the time I brought you to where before we installed the improvements.

399
00:45:27,800 --> 00:45:32,450
And and it has this characteristic noise curve.

400
00:45:32,450 --> 00:45:38,750
By the way, these little lines here are the natural resonances that are in any system, whether they're electronic.

401
00:45:38,750 --> 00:45:45,170
In our case, at multiples of 60 hertz or whether they're mechanical because of the mounts and so forth,

402
00:45:45,170 --> 00:45:51,880
those represent stable little lines that we notch out, and it represents less than one percent of the area.

403
00:45:51,880 --> 00:45:54,640
So don't concentrate on those, they don't matter.

404
00:45:54,640 --> 00:46:03,940
These are the two best sensitivity curves we had once optimised for low frequency, one for higher frequency.

405
00:46:03,940 --> 00:46:09,400
So they're slightly different, but that's where we were. No gravitational waves and been detected.

406
00:46:09,400 --> 00:46:16,390
Then we decided to do this improvement programme and our goal was to improve by a factor of 10 everywhere.

407
00:46:16,390 --> 00:46:24,250
At least a factor of 10 means that we measure this little edge, which is an amplitude, not a power.

408
00:46:24,250 --> 00:46:31,930
It means we are sensitive a factor of 10 more. If we're a factor of 10 better, we look for a factor of 10 further out into the universe,

409
00:46:31,930 --> 00:46:39,070
which means there's a thousand times our ten cubed as many galaxies stars potential sources.

410
00:46:39,070 --> 00:46:45,900
And so our goal was to get in a factor of 10. At high frequency, we do that by making a higher power laser,

411
00:46:45,900 --> 00:47:00,090
which we've done at at the middle frequencies where we're limited by thermal noise, by making a better, quieter test mass suspension system.

412
00:47:00,090 --> 00:47:08,310
For example, we moved all the electronics off the final test mass made it bigger and heavier and better engineered test mass and so forth.

413
00:47:08,310 --> 00:47:14,160
And at the lowest frequencies, we basically are limited, as I told you, by the shaking of the Earth.

414
00:47:14,160 --> 00:47:25,350
Seismic noise. We were approved. We did this upgrade programme between twenty eleven and twenty fourteen or fifteen.

415
00:47:25,350 --> 00:47:28,440
And we turned back on turning back on.

416
00:47:28,440 --> 00:47:40,200
We didn't immediately try to get all the improvements in, but went partway partway by not turning the laser up to full power,

417
00:47:40,200 --> 00:47:45,720
not completely in all coordinates, being able to cancel in the active seismic everything.

418
00:47:45,720 --> 00:47:49,440
You have to do a lot of measurements to do that. But we had gained everywhere.

419
00:47:49,440 --> 00:47:57,660
At least a factor of three and a factor of three for us is a factor of twenty seven and rate are possible sources because of the cube.

420
00:47:57,660 --> 00:48:02,160
So it didn't take very long to have more data than we had before.

421
00:48:02,160 --> 00:48:08,490
But you'll notice the low frequencies here. We actually had gained a factor of 100.

422
00:48:08,490 --> 00:48:15,930
That was because we put it in the active seismic isolation. So it's the elevator take-home speech if you want, because at that point,

423
00:48:15,930 --> 00:48:22,860
we had gained a factor of 100 cubed and our ability to see gravitational waves at low frequency.

424
00:48:22,860 --> 00:48:31,200
And that's why within a few days of turning this detector on, we could see something we hadn't seen in ten years of previous running.

425
00:48:31,200 --> 00:48:42,440
So that's the final message experimentally. We then turn this on here, even though we're not all the way down to the bottom because we're anxious.

426
00:48:42,440 --> 00:48:48,570
We are anxious to try to get experience with it and then continue to improve it.

427
00:48:48,570 --> 00:48:53,970
Then on September 14th, 2015, we saw this.

428
00:48:53,970 --> 00:48:56,820
So this is the picture that many of you have seen,

429
00:48:56,820 --> 00:49:08,250
and I've even seen it on Madison Avenue in a dress shop where they put it on fancy dresses for a woman and made ties for us that we wore in Sweden.

430
00:49:08,250 --> 00:49:14,910
What you see is first, the scale from here to here is the 10 to the minus twenty one.

431
00:49:14,910 --> 00:49:18,790
So we can see something that happens then that scale the scale.

432
00:49:18,790 --> 00:49:29,340
This direction is about two tenths of one second. And the signal is this we're going it gets narrower and bigger as it propagates along.

433
00:49:29,340 --> 00:49:34,920
That's the so-called chirp signal as they come together to objects is I'll show you the frequency

434
00:49:34,920 --> 00:49:40,500
gets higher and the signal gets larger until they merge and then they merge and come together.

435
00:49:40,500 --> 00:49:45,300
This is the signal in Hanford, Washington.

436
00:49:45,300 --> 00:49:50,460
This is the signal in Livingston, Louisiana. This one was seven milliseconds before that one.

437
00:49:50,460 --> 00:49:56,970
This is the two put on top of each other, and they look very much alike.

438
00:49:56,970 --> 00:50:07,760
So at that point, we. We're pretty sure that we had detected gravitational waves immediately looking at the data even more impressively.

439
00:50:07,760 --> 00:50:18,470
If I take that data and I calculate the expected signal for gravitational waves using Einstein's equation,

440
00:50:18,470 --> 00:50:23,750
then the signals themselves are shown in the top frame.

441
00:50:23,750 --> 00:50:36,390
Here, the Hanford, Washington and Livingston, Louisiana, and these are the calculated signals for two, essentially 30 solar mass black holes.

442
00:50:36,390 --> 00:50:45,150
Merging into each other to give a gravitational wave signal as calculated using Einstein's theory of general relativity.

443
00:50:45,150 --> 00:50:49,110
Then I subtract one from the other down here, and what you see is just wiggles.

444
00:50:49,110 --> 00:50:50,820
That's the noise in the system.

445
00:50:50,820 --> 00:50:59,400
Nothing systematic, and that's the easiest way to look at what we saw initially immediately before we did anything much with the data.

446
00:50:59,400 --> 00:51:03,240
I just show this to illustrate that it's the low frequencies.

447
00:51:03,240 --> 00:51:09,150
I've started off everything below 40 hertz or so. Uh, the signal.

448
00:51:09,150 --> 00:51:19,900
This is a time frequency plot that it was only by improving the low frequencies that enabled us to make this measurement.

449
00:51:19,900 --> 00:51:28,390
Interestingly, even though we only make one measurement that she has an incredible amount of information that we can calculate the shape,

450
00:51:28,390 --> 00:51:36,880
we call it a chirp pass, but it doesn't matter. I've just shown you the shape can be written down in an analytical form that's quite detailed.

451
00:51:36,880 --> 00:51:44,830
There's there's a second set of terms that I just don't show here that allow you to measure all the detailed parameters.

452
00:51:44,830 --> 00:51:51,640
So when people have wondered how we can take one event and tell so much, it's because there's really a form,

453
00:51:51,640 --> 00:51:57,610
a formulation that's in detail where we can tell everything from the masses how far apart they are,

454
00:51:57,610 --> 00:52:05,260
how far away they are, the chirp mass, the ratios of the spins, how redshift than it is,

455
00:52:05,260 --> 00:52:10,060
and blah blah blah, so we can measure almost everything from this shape.

456
00:52:10,060 --> 00:52:14,710
There's also orbital perception and spin misalignment and so forth.

457
00:52:14,710 --> 00:52:21,700
We don't see any evidence for that yet. That's not in this term. It's in the next terms and they're harder to measure.

458
00:52:21,700 --> 00:52:28,630
We also get the sky location from the time the signal arrives in the two detectors from the amplitude,

459
00:52:28,630 --> 00:52:37,510
we get to the distance away and we get the binary orientation information from the time delays between the two.

460
00:52:37,510 --> 00:52:42,010
So basically, we get all that information for the detectors.

461
00:52:42,010 --> 00:52:50,410
And from this first event, we were able to publish the first paper that told us that what we have seen is two black holes,

462
00:52:50,410 --> 00:52:52,300
one thirty six solar masses,

463
00:52:52,300 --> 00:53:02,230
the other twenty nine solar masses with a reasonable error on them giving a final black hole after the merger of sixty two solar masses.

464
00:53:02,230 --> 00:53:09,310
You'll notice there's some left over here. That's the energy that was transmitted away in gravitational waves, which is significant.

465
00:53:09,310 --> 00:53:16,330
And in that, two tenths of a second was the brightest object in terms of energy in the universe.

466
00:53:16,330 --> 00:53:21,430
Then we can measure something about the spins, how far away it is and so forth.

467
00:53:21,430 --> 00:53:31,950
So all of that we knew from this first measurement, which was the discovery of the gravitational wave signals.

468
00:53:31,950 --> 00:53:38,200
We generally can look for gravitational waves much more sensitively.

469
00:53:38,200 --> 00:53:46,510
Then what you saw there and we expected and use for most of our events since that time, a template bank.

470
00:53:46,510 --> 00:54:01,000
And the idea is to calculate those forms for as many from the from for the whole space of possible masses and and and mergers that we might detect.

471
00:54:01,000 --> 00:54:08,020
And then compare that with the data we use what's called a match filtering technique that is to take the shape that we calculate.

472
00:54:08,020 --> 00:54:17,830
Multiply it by the background. And when you do that, if you run into some real signal like I'm showing this illustrative one,

473
00:54:17,830 --> 00:54:23,920
you actually find you can pick it out even though it's smaller than the noise because it's there for a while.

474
00:54:23,920 --> 00:54:35,650
And by seeing that spike, we then go in and analyse the data and pick out the merger by its shape and signal much below the actual noise level itself.

475
00:54:35,650 --> 00:54:39,370
That's the technique that's used for most of the signals that we've seen.

476
00:54:39,370 --> 00:54:44,830
This is the second event that we saw, which was in December of the same year, 2015.

477
00:54:44,830 --> 00:54:49,840
And you can see that it's amplitude is lower than the noise level.

478
00:54:49,840 --> 00:54:56,020
But there are many, many oscillations, not like the very first one that you saw that had about six oscillations.

479
00:54:56,020 --> 00:55:05,120
The reason there's many is that these were not as heavy. And so they took went to higher frequency before they finally merged.

480
00:55:05,120 --> 00:55:14,960
So using that, we basically have done a lot of work to look at that detail wave form and ask how well

481
00:55:14,960 --> 00:55:21,050
does it really in detail fit or not fit Einstein's equations of general relativity?

482
00:55:21,050 --> 00:55:29,870
The fit doesn't disagree at all, and everything we've done so far and the best tests are the ones that oscillate many times like these.

483
00:55:29,870 --> 00:55:39,290
Not the first event which was bigger, but we see no evidence so far of any deviations from Einstein's theory.

484
00:55:39,290 --> 00:55:44,930
OK, now let me move on. We have two detectors, which I showed you.

485
00:55:44,930 --> 00:56:01,070
And in August of 2017, we had a third detector join us and that was a detector in Italy made by the Dutch, the French and Italians called Virgo.

486
00:56:01,070 --> 00:56:10,730
And we immediately saw an event on the 14th of August Twenty Seventeen, which is shown here.

487
00:56:10,730 --> 00:56:18,560
And what to notice is this is a picture of the Earth on the right and how well we can tell from when the signals arrived,

488
00:56:18,560 --> 00:56:21,650
where the signal source was, where it came from.

489
00:56:21,650 --> 00:56:30,290
And you'll notice as long as there was Leo itself, which is all these top banana looking shapes, we couldn't tell very well.

490
00:56:30,290 --> 00:56:38,360
But then when we added this third detector, not surprisingly, we get a much better localisation of where something's coming from.

491
00:56:38,360 --> 00:56:45,260
That's a key to enabling us to be able to work with astronomers and with astronomy

492
00:56:45,260 --> 00:56:49,520
to try to actually know where any signal we have comes from in the universe.

493
00:56:49,520 --> 00:56:54,140
We could do that to about a thousand square degrees when there was only like.

494
00:56:54,140 --> 00:57:03,290
And now it's a 10 or 20 square degrees with the addition of another particle.

495
00:57:03,290 --> 00:57:13,030
I'm sorry. Last thing on General Relativity, before I got into astronomy,

496
00:57:13,030 --> 00:57:18,010
we are in a situation where you'd like to test Einstein's theory of general

497
00:57:18,010 --> 00:57:24,760
relativity against any sort of other possible theories of general relativity.

498
00:57:24,760 --> 00:57:33,730
Unfortunately, most of the very variation of theory theories that are variants of Einstein's theory are not predictive enough

499
00:57:33,730 --> 00:57:40,840
so that we can use those directly to ask whether they fit what we see better or worse than Einstein's theory.

500
00:57:40,840 --> 00:57:44,530
So instead, we look for deviations from Einstein's theory.

501
00:57:44,530 --> 00:57:51,820
I'll just show you one. We haven't seen any. If we look and just ask, is there any evidence of dispersive term?

502
00:57:51,820 --> 00:57:59,050
What might give a dispersive term if we had some mass where people like to call it gravitation connected to the

503
00:57:59,050 --> 00:58:05,770
gravitational waves that would put a dispersive term in if we added dispersive term to Einstein's equations,

504
00:58:05,770 --> 00:58:11,170
that changes the waveforms? How much mass can we allow to put in before that?

505
00:58:11,170 --> 00:58:22,090
They don't fit the data very well. It turns out we're able to limit the mass of a graviton to 7.7 10 to the minus twenty three f overseas squared.

506
00:58:22,090 --> 00:58:29,680
So we don't have any evidence in that formulation of any deviation from general relativity.

507
00:58:29,680 --> 00:58:41,170
But we're just beginning to detect. We've now reported 10 examples of mergers of black holes.

508
00:58:41,170 --> 00:58:46,630
They're all shown together up here and in the mergers of the black holes you'll see.

509
00:58:46,630 --> 00:58:50,560
They look different. If you look at the wiggly lines, some are longer, some are shorter.

510
00:58:50,560 --> 00:58:54,220
Now they're starting to be different masses, different orientations.

511
00:58:54,220 --> 00:59:01,180
And we're starting to develop in the information to see, for example, the distribution of masses,

512
00:59:01,180 --> 00:59:09,580
distribution of distances and so forth that will, we hope, enable us to understand the origin of these heavy black holes.

513
00:59:09,580 --> 00:59:19,390
They weren't expected by astronomers, so what we discovered was not expected by everything we knew from electromagnetic measurements.

514
00:59:19,390 --> 00:59:22,120
They're heavier than what was expected.

515
00:59:22,120 --> 00:59:27,820
We're now trying to understand what the origin is of those by making enough measurements to constrain the problem.

516
00:59:27,820 --> 00:59:30,610
What is the distribution of the masses?

517
00:59:30,610 --> 00:59:39,640
Spins are the spins of the two or aligned or not aligned and so forth, and we're just starting to get enough data to do that.

518
00:59:39,640 --> 00:59:44,620
I want to move on from that because these events have nothing to do at this point,

519
00:59:44,620 --> 00:59:48,940
at least with multi messenger astronomy, since they don't give any electromagnetic signal.

520
00:59:48,940 --> 00:59:51,280
Ah, so far none has been seen.

521
00:59:51,280 --> 01:00:03,930
If you look at the bottom right, you see the 11th signal that we've seen since the original, and that has a timescale that goes four minutes.

522
01:00:03,930 --> 01:00:09,720
Instead of two tenths of a second, it goes for like 60 seconds.

523
01:00:09,720 --> 01:00:13,710
Very different. And it goes to very, very high frequency, it turns out.

524
01:00:13,710 --> 01:00:28,260
And you can see the signal here. This was a signal that we analysed and showed was a binary neutron star merger instead of a binary black hole merger.

525
01:00:28,260 --> 01:00:36,070
First, let me say I'm a black holes, I said this already. So.

526
01:00:36,070 --> 01:00:44,680
Once we saw that, then there's the immediate question, once we see something like a neutron star merger that it's a nuke,

527
01:00:44,680 --> 01:00:50,980
it's not a black hole, it's a nuclear physics system that combines together.

528
01:00:50,980 --> 01:00:56,410
What, if anything, can be seen by electromagnetic instruments?

529
01:00:56,410 --> 01:01:00,550
The first step in doing multi messenger astronomy?

530
01:01:00,550 --> 01:01:09,250
In order to do that, we have to be able to develop and give information very quickly to the electromagnetic community.

531
01:01:09,250 --> 01:01:20,020
And so what's shown here is that we can develop quite quickly candidates the sky location by what happened in the three detectors,

532
01:01:20,020 --> 01:01:24,460
and that's within a few minutes and we can alert astronomers.

533
01:01:24,460 --> 01:01:26,920
We then try to do event evaluation,

534
01:01:26,920 --> 01:01:35,380
which can take up to 30 minutes just to make sure that we can avoid it immediately if we if they're starting to turn their telescopes.

535
01:01:35,380 --> 01:01:42,970
And if we then try to understand more of the physics, we have to fit into Einstein's equations and so forth,

536
01:01:42,970 --> 01:01:46,990
and that can take hours or days or weeks or even a month or so.

537
01:01:46,990 --> 01:01:59,380
So this process for us in establishing it from the gravitational side has an immediate alert and as best information we have to give astronomers.

538
01:01:59,380 --> 01:02:05,770
And then if upon analysis wasn't real, we say oops and cancel it.

539
01:02:05,770 --> 01:02:10,660
But otherwise, if they are interested, they can turn their instruments.

540
01:02:10,660 --> 01:02:21,550
This first event that we saw had the feature that we were able to locate it in the sky quite well.

541
01:02:21,550 --> 01:02:28,540
So originally, as I said, the sky locations we had were typically hundreds or even more than a thousand square degrees.

542
01:02:28,540 --> 01:02:35,410
That's not exactly vocal enough for astronomers to want to turn and use their instruments.

543
01:02:35,410 --> 01:02:51,670
But once we added the third detector just in time to see this event on August 14th and on August 17th, two or three days later, we got lucky.

544
01:02:51,670 --> 01:02:57,790
All these events, by the way, with black holes have been looked at to some extent with electromagnetic devices.

545
01:02:57,790 --> 01:03:06,130
Nothing has been reported. No one's seen anything that they've reported with from any of the 10 events we've seen the 11th event,

546
01:03:06,130 --> 01:03:09,790
the one that I just showed you, we saw in all three detectors.

547
01:03:09,790 --> 01:03:16,990
Because the Virgo detector was then working, we were able to locate the position in the sky to 31 square degrees,

548
01:03:16,990 --> 01:03:24,070
and we could tell by the amplitude of the signal there was 40 plus or minus eight mega parsecs away.

549
01:03:24,070 --> 01:03:29,950
That's pretty good. So at that point, this is just what that signal sounded like.

550
01:03:29,950 --> 01:03:37,180
So it's already detectable here and the signal you can't see very well, but this this time versus frequency.

551
01:03:37,180 --> 01:03:44,050
And as we get near the end, you should be able if your ears are good to hear the final what we call chirp signal.

552
01:03:44,050 --> 01:03:58,420
But I'm not changing the frequency. You're going to hear the real frequency. So as it gets near zero over here, just in here, you'll hear the cheer.

553
01:03:58,420 --> 01:04:02,810
I don't know how well you heard it, but that was it.

554
01:04:02,810 --> 01:04:10,910
So we saw this signal, you can see that the three devices, these are the two like ones, this is frequency versus time.

555
01:04:10,910 --> 01:04:15,770
So it goes to higher frequency and time. That's his character characteristic shape.

556
01:04:15,770 --> 01:04:20,380
The signal is much weaker in the Virgo detector, but fortunately for us,

557
01:04:20,380 --> 01:04:29,420
it was located in the sky in a position that made them able to tell the position, help us tell the position very well.

558
01:04:29,420 --> 01:04:32,270
We also could tell where it was, what the distance was.

559
01:04:32,270 --> 01:04:40,920
And with the help of astronomical measurements, we could tell what galaxy it actually came from, which was important in the following measurements.

560
01:04:40,920 --> 01:04:50,730
Two seconds later. A signal we're seeing by the Fermi satellite and the same region of the sky.

561
01:04:50,730 --> 01:04:55,830
They're looking at high energy photons coming from what are called gamma ray bursts.

562
01:04:55,830 --> 01:05:03,270
So what the gamma ray bursts were suspected or thought by many people to come from the merger of two neutron stars.

563
01:05:03,270 --> 01:05:08,370
So we immediately within two seconds out of 100 million light years, by the way.

564
01:05:08,370 --> 01:05:17,580
So the first thing to mention is that the speed of light gravitational waves is the same to two seconds out of 100 million light years.

565
01:05:17,580 --> 01:05:27,930
The signal from the electromagnetic signal from the merger is supposed to be lighter because it happens not from the merger itself,

566
01:05:27,930 --> 01:05:38,430
but from the nuclear physics after it emerges. So a couple of seconds later, they saw this signal that enabled us to alert the astronomical community,

567
01:05:38,430 --> 01:05:47,580
and something like 2000 out of 4000 instruments in the world pointed in the sky over the coming seconds, months, days.

568
01:05:47,580 --> 01:05:59,160
And that included gravitational waves. Of course, it included visible infra-red, radio waves, X-rays, gamma rays, even neutrinos.

569
01:05:59,160 --> 01:06:05,060
Oh, no, signal was saying with neutrinos. Then.

570
01:06:05,060 --> 01:06:06,890
This was done over some period of time.

571
01:06:06,890 --> 01:06:16,610
So first, we saw that short signal 1.7 seconds later in the family satellite, which was presumably from a short gamma ray bursts.

572
01:06:16,610 --> 01:06:25,070
Within five hours or so, we were able to get those details sky location from another electromagnetic observation and

573
01:06:25,070 --> 01:06:31,250
so on out through the days and through the different bandwidth that people do astronomy.

574
01:06:31,250 --> 01:06:32,900
I can't show all this,

575
01:06:32,900 --> 01:06:45,380
but it fits taking the measurements that happen over a period of time to a picture of a neutron star merger that happens in a phenomenological scan,

576
01:06:45,380 --> 01:06:52,490
which we call a kilonova. That kilonova predicts even how the wavelength will change with time.

577
01:06:52,490 --> 01:06:56,870
That fits pretty well with the light curves were that were detected, which I show here,

578
01:06:56,870 --> 01:07:04,730
and even with the emission is like as a function of wavelength, and all of this fits the picture quite well.

579
01:07:04,730 --> 01:07:15,820
So I'm not going to go through that in detail. I'll show you one result from this one event, and that is that we as.

580
01:07:15,820 --> 01:07:20,800
Physicists try to describe everything in nature, always, whether we can or not.

581
01:07:20,800 --> 01:07:25,720
And so I grew up learning that.

582
01:07:25,720 --> 01:07:30,710
The heavy elements were a problem, how do we how do the heavy elements get into the Earth?

583
01:07:30,710 --> 01:07:35,360
We know that most of the universe is hydrogen and helium.

584
01:07:35,360 --> 01:07:41,210
We know that stars burn using the fusion process, and that works up to about iron,

585
01:07:41,210 --> 01:07:51,410
and then it burns up the fuel and stars collapse, calling us making a supernova so that the elements up to something like iron.

586
01:07:51,410 --> 01:07:55,610
We understand where they came from and how they got into the Earth.

587
01:07:55,610 --> 01:07:59,540
But where did the heavier elements come from? That was always a puzzle.

588
01:07:59,540 --> 01:08:10,460
The explanation that we've been taught or believe is that there was an enhanced process in the burning of stars called the R process.

589
01:08:10,460 --> 01:08:18,290
I won't talk about it in detail, which enhanced the production of heavy elements in the burning of stars.

590
01:08:18,290 --> 01:08:25,280
That was never very satisfactory, actually artificially making the table.

591
01:08:25,280 --> 01:08:26,270
How was that done?

592
01:08:26,270 --> 01:08:33,590
That's done generally by going into a laboratory, taking a heavy element, bombarding it with lots of neutrons, and you make a heavier one.

593
01:08:33,590 --> 01:08:40,070
And if you're successful, you make something that somebody didn't see, and that's the end of the plot.

594
01:08:40,070 --> 01:08:50,870
So how did they get into nature? Into the Earth has been kind of a not a very well established and not a very satisfactory explanation.

595
01:08:50,870 --> 01:08:59,900
Well, the merchant way they're made artificially with neutrons bombarding some elements sounds a lot like what happens when neutron stars collide.

596
01:08:59,900 --> 01:09:05,810
So the phenomenology of what happens when neutron stars collide has been put together,

597
01:09:05,810 --> 01:09:11,210
and it's only as reliable as a single event, which is all that we saw.

598
01:09:11,210 --> 01:09:14,630
So it's not statistically reliable at that point at this point,

599
01:09:14,630 --> 01:09:26,990
but it's consistent with all the your favourite heavy elements say platinum and gold being made by the collision of neutron stars.

600
01:09:26,990 --> 01:09:33,650
So if I look at the collision of this neutron star, presumably the ones in the Earth happened a long time ago,

601
01:09:33,650 --> 01:09:39,380
it actually produced about 100 hundred earth masses of gold.

602
01:09:39,380 --> 01:09:43,930
Now, we don't know for sure all of this is this one event, as I said, but that's the take up.

603
01:09:43,930 --> 01:09:48,290
So where are we going from here?

604
01:09:48,290 --> 01:09:54,320
We basically now have three detectors working two in the US, one in Italy.

605
01:09:54,320 --> 01:09:58,610
The one in Italy, we hope, becomes as sensitive as legal instruments.

606
01:09:58,610 --> 01:10:03,470
But that'll take a while. We're also commissioning one.

607
01:10:03,470 --> 01:10:10,310
The Japanese are commissioning one in Japan, and that has some innovations in it too.

608
01:10:10,310 --> 01:10:18,470
That might eventually be the kinds of things that we do in the future, which is mostly to go from room temperature to a cold detector.

609
01:10:18,470 --> 01:10:23,810
And we're building one in India that'll be finished by twenty twenty five.

610
01:10:23,810 --> 01:10:26,930
That'll enable us to do much better in localisation.

611
01:10:26,930 --> 01:10:33,200
So the first problem and doing multi messenger astronomy is we have to be able to localise what we see.

612
01:10:33,200 --> 01:10:37,640
This is where we are now and this is where we'll be in about twenty twenty four.

613
01:10:37,640 --> 01:10:44,570
That is, we got lucky being able to see where the event we saw now was because of where it happened in the sky.

614
01:10:44,570 --> 01:10:53,960
But we'll be able essentially anywhere in the sky, be able to tell to roughly 10 to 20 or 30 square degrees where an event came from.

615
01:10:53,960 --> 01:10:59,990
So that's the first necessary ingredient to be able to compare with astronomy.

616
01:10:59,990 --> 01:11:07,280
There's lots of other sources besides the one that I've talked about today, and as we get better sensitivity, we'll have a better chance.

617
01:11:07,280 --> 01:11:15,620
Let me just list them a little bit. We can see if they're close enough and we don't know very well a supernova.

618
01:11:15,620 --> 01:11:20,720
So certainly a supernova that might happen in our own galaxy.

619
01:11:20,720 --> 01:11:29,150
We can see in the most optimistic conditions where we improve the detectors we might see out as far as the Virgo cluster,

620
01:11:29,150 --> 01:11:38,570
where there's one two year type two supernova. And within that range, there are periodic sources note spinning neutron stars in our own galaxy.

621
01:11:38,570 --> 01:11:43,670
To the extent that they're not perfectly spherical, they'll have a quadrupole moment and they'll give a signal.

622
01:11:43,670 --> 01:11:46,970
We've looked for those we've limited for, known Paul,

623
01:11:46,970 --> 01:11:57,590
the best known pulsars that they don't have mountains on them bigger than, say, a millimetre out of the 12 miles or so across.

624
01:11:57,590 --> 01:12:05,030
So that's small, but we haven't seen yet a signal. That signal will be a continuous signal, and we're not looking in the most sensitive way.

625
01:12:05,030 --> 01:12:11,840
If you expect to see a signal for gravitational waves, the neutron star should be not perfectly symmetrical.

626
01:12:11,840 --> 01:12:18,620
The best ones known by radio telescopes are, by definition, the ones that have been around so long they give a very stable signal.

627
01:12:18,620 --> 01:12:27,410
They're therefore pretty Saracho. So what we need to do is to detect the ones that are young and not seen yet by radio telescopes.

628
01:12:27,410 --> 01:12:32,270
That's a tremendous computing and technical. Problem, but we're working on that.

629
01:12:32,270 --> 01:12:40,160
Lastly, and I'll mention that at the very end is to see cosmological signals.

630
01:12:40,160 --> 01:12:50,500
So. We will be improving. My gosh, we have so far over the next decade, but we're still not limited by nature.

631
01:12:50,500 --> 01:12:58,780
So when people are impressed by how well we've done, we still can go further and we know that more for LEGO itself,

632
01:12:58,780 --> 01:13:08,200
roughly a factor of 10, which is a factor of a thousand. And right before we had some problems that are more fundamental that we will limit us.

633
01:13:08,200 --> 01:13:13,270
They're technical at this point. We then can move and we're starting to design.

634
01:13:13,270 --> 01:13:19,090
What I show here is a European design of what a third generation detector might look like.

635
01:13:19,090 --> 01:13:25,030
Third generation detector, the one they talked about here, may not be what's built, but looking on the left.

636
01:13:25,030 --> 01:13:30,340
These are some of the parameters they're all likely going into a third generation detector,

637
01:13:30,340 --> 01:13:35,870
presumably in the twenty thirties, and the Einstein telescope has almost all those features.

638
01:13:35,870 --> 01:13:39,490
I show that first you might want to go underground.

639
01:13:39,490 --> 01:13:45,400
Going underground gets rid of a lot of the seismic shaking of the Earth, especially at very low frequencies.

640
01:13:45,400 --> 01:13:52,810
So going underground, if it's important enough to go to yet more frequency, that's heavier a mass black holes.

641
01:13:52,810 --> 01:13:59,770
You go underground. The arms can be longer than what we have at the ideal length is actually 100 kilometres.

642
01:13:59,770 --> 01:14:05,500
So I'm sorry, 40 kilometres. But they are talking about making 10 kilometre arms.

643
01:14:05,500 --> 01:14:11,050
The present ones are three and Italy and four kilometres in the US.

644
01:14:11,050 --> 01:14:17,410
You can have a configuration that, instead of being L-shaped, is triangular.

645
01:14:17,410 --> 01:14:22,480
That has some advantages in determining the two polarisations of gravitational waves.

646
01:14:22,480 --> 01:14:29,890
And also in being able to internally be able to crudely but be able to point somewhat.

647
01:14:29,890 --> 01:14:35,470
A big step for us and one that we're working on really the hardest is to get rid of

648
01:14:35,470 --> 01:14:40,900
the noise in the most sensitive region by cooling the detector becoming cryogenic.

649
01:14:40,900 --> 01:14:47,170
That's a big challenge, but only a technical challenge. It means that we have to be able to get the heat out,

650
01:14:47,170 --> 01:14:51,340
which is made all the time when the laser beams going through without shaking it because

651
01:14:51,340 --> 01:14:56,560
everything has to be quiet and we have to add optics that will work at cold temperatures.

652
01:14:56,560 --> 01:15:01,900
So we have to develop the right materials and the right coatings to reflect the light.

653
01:15:01,900 --> 01:15:07,120
And we're working on all those problems and the these people are as well.

654
01:15:07,120 --> 01:15:11,770
Lastly, we show up when I showed you was a broadband instrument.

655
01:15:11,770 --> 01:15:19,870
It looks at gravitational waves at all frequencies when in fact, if you want to, you can do better in different parts of the frequency band.

656
01:15:19,870 --> 01:15:28,000
So for example, in the case of the neutron star merger, we'd like to understand what happened to the nuclear physics after they merged,

657
01:15:28,000 --> 01:15:32,920
not just the general relativity that these two objects merge. And then a lot of light came in.

658
01:15:32,920 --> 01:15:40,960
But what happened in the nuclear physics that happens if we to detect that at higher frequencies than we now are very good at?

659
01:15:40,960 --> 01:15:47,080
So you want to optimise things at higher frequency. So anyway, you'll see them increasing.

660
01:15:47,080 --> 01:15:55,480
Our proposal in the US is to make a maybe longer version of, like we call it, the cosmic explorer.

661
01:15:55,480 --> 01:16:03,730
It's to go instead of four kilometres on an arm, something like 40, which is pretty ideal in terms of the parameters that you're measuring.

662
01:16:03,730 --> 01:16:13,990
And with that, and using the same technologies that we're developing over the next decade, but put into a new detector of longer length,

663
01:16:13,990 --> 01:16:22,750
we can see we can become an instrument that starts studying cosmology that looks at high redshift.

664
01:16:22,750 --> 01:16:28,480
Right now, we're not. And for example, for the black holes, we can see that the edge of the universe.

665
01:16:28,480 --> 01:16:37,090
And that's the future. Lastly, the whole game is not on the Earth, just like an astronomy.

666
01:16:37,090 --> 01:16:41,140
And I started by talking about astronomy doing done in different wavelengths.

667
01:16:41,140 --> 01:16:47,050
You'd like to do a graph and we will do eventually gravitational waves and the different frequency bands.

668
01:16:47,050 --> 01:16:53,110
So everything I've talked about so far is on the Earth's surface, and I show that on the right side, that's legal.

669
01:16:53,110 --> 01:17:08,170
And it's that is the audio band. There's a proposal and there's a approved space experiment called Lisa being built through the European

670
01:17:08,170 --> 01:17:15,190
Space Agency with small collaboration from the US that looks at not ten hertz to ten thousand hertz,

671
01:17:15,190 --> 01:17:22,660
but 10 to the minus one to 10 to the minus four hertz by having three satellites that bounce

672
01:17:22,660 --> 01:17:27,430
the beam back and forth the distance between them two and a half million kilometres.

673
01:17:27,430 --> 01:17:36,400
And that does the region in a lower frequency. There's also ideas how to do the region and between the two, but there's no approved experiment now.

674
01:17:36,400 --> 01:17:41,380
It's certainly doable. And lastly, on the left.

675
01:17:41,380 --> 01:17:45,880
You see that I have something called PTA, that's what that is,

676
01:17:45,880 --> 01:17:53,530
is fine is pulse pulsar timing a pulsar timing array, meaning that we see these pulsars.

677
01:17:53,530 --> 01:17:58,780
They're very good clocks. If you measure the whole array of them and a gravitational wave comes through,

678
01:17:58,780 --> 01:18:04,450
it changes the relative timing and you can pick out a low frequency gravitational wave.

679
01:18:04,450 --> 01:18:16,660
The ultimate goal this is the space experiment, 2.5 kilometres, 2.5 million kilometres apart on a triangle like the experiment below ground.

680
01:18:16,660 --> 01:18:22,150
The Pulsar timing array to see things. And lastly, I'd say not immediate,

681
01:18:22,150 --> 01:18:30,250
but the dream that certainly I had for the moment I got into this is how do we really learn about the early universe?

682
01:18:30,250 --> 01:18:37,240
And I think the ultimate technique is gravitational waves, everything we know and it's a tremendous amount.

683
01:18:37,240 --> 01:18:47,320
We've learnt from the cosmic microwave background experiments, which only probe what happened up to four hundred thousand years after the Big Bang.

684
01:18:47,320 --> 01:18:50,770
Before that was a fault. So I can go back before that.

685
01:18:50,770 --> 01:18:57,880
We project back, but we can't measure it back. If you want to go to earlier times and probe it, there's two choices.

686
01:18:57,880 --> 01:19:02,950
The two choices are neutrinos or gravitational waves. Neutrinos.

687
01:19:02,950 --> 01:19:12,700
They go back to a few seconds after the Big Bang. The problem is that neutrinos that were made at that point in time have thermals in between.

688
01:19:12,700 --> 01:19:18,430
They're very low energy and experimentally that makes them almost undetectable.

689
01:19:18,430 --> 01:19:27,790
Gravitational waves aren't absorbed almost at all. So they're our ultimate probe, I think, to go back to the very first instance of the Big Bang.

690
01:19:27,790 --> 01:19:33,920
We don't yet have the detector to do that. But I told you, I took 400 years to get where we are from.

691
01:19:33,920 --> 01:19:38,260
Mark Galileo was. So if you give us 400 years or maybe less,

692
01:19:38,260 --> 01:19:46,090
maybe we can measure signals from the very earliest moments of the Big Bang and understand really how it all began.

693
01:19:46,090 --> 01:19:56,662
Thank you.