 Guten Abend meine Damen und Herren, ich möchte Sie im Namen der Universität Zürich und der Universität Bern und der Einstein-Gesellschaft ganz herzlich zu diesem ersten Vortrag dieser Reihe in diesem Herbst, der Einstein-Lexes, die Physik und Astronomie gewidmet ist, begrüßen. Diese Runde ist fast wie keine andere zuvor mit Arbeiten von Albert Einstein verknüpft, im Sinne, dass er im Jahr 1916 im Juni zum ersten Mal den Begriff Gravitationswellen schriftlich festgehalten hat. Und damals war er der Meinung, dass wir man sowieso nie beobachten können und auch wenn Sie es nicht glauben, ein Monat später im Juli hat er eine Arbeit geschrieben über die Quantenatur des Lichts und hat dort die Grundlagen eigentlich gelegt für Laser, ohne es zu wissen natürlich. Und Laser sind eine absolut wichtige Komponente in den Detektoren für Gravitationswellen. So Sie sehen, er hat innerhalb von einem Monat quasi die Grundlagen gelegt für Gravitationswellen und deren Beobachtung, ohne dass es natürlich damals ahnen könnte. Und es ist wirklich einfach unglaublich, was er in gewissen Perioden geleistet hat, zuerst in Bern in kurzer Zeit und dann aber auch in Berlin. Einfach damit Sie sehen, dass was wir heute hören und auch morgen und übermorgen sehr, sehr stark auf das zurückgeht, was Einstein uns hinterlassen hat. Und möchte ich aber nicht länger werden und Herrn Wiese bitten, den heutigen Referenten einzuführen. Ja, guten Abend, everybody. I'd like to welcome you on behalf of the Faculty of Science and the Albert Einstein Center for Fundamental Physics here at Bern University. And it is my tremendous pleasure to welcome Barry Berish from the California Institute of Technology as our 10th Einstein Lecturer. Barry Berish was born in Omaha, Nebraska and then studied at the University for California in Berkeley and got his PhD there in the year 1962. One year later he moved to Caltech first as a research fellow and then he became an assistant associate and finally full professor there. And this is still his affiliation besides Riverside University. Barry Berish has been involved in many very important experimental investigations. He has done experiments for example at Fermilab also at the underground laboratory at the Gran Sasso and in many other places and what was the reason to invite him today is related to the LIGO collaboration, the large interferometer gravitational wave observatory for which he became the principal investigator in the year 1994 and the director in 1997. And this is really not a tabletop experiment, this is a huge collaboration which he formed involving more than 1000 people. And so it is not just enough to have some very good scientific ideas. You also need to coordinate all this research and you also have to make sure that the funding agencies give you the money that is necessary to engage in such an enormous enterprise. And I think Barry Berish with his many talents was exactly the right person at the right time to get this LIGO project off the ground and then extend it to what is now called advanced LIGO. And it required very advanced technologies to finally detect the gravitational waves that were following from Einstein's general relativity theory already 100 years before they were finally directly discovered by LIGO in the year 2015. And Barry Berish has played a tremendous role in making all this possible. That of course was recognized by different prizes. He received the Enrico Fermi Prize, the Draper Medal, the Princess of Asturia Award. And then in 2017 he shared the Physics Nobel Prize with Reiner Weiß from MIT and Kip Thorne from Caltech. So we are very glad to have him here tonight. And he will also speak tomorrow at 5.15 and then again on Wednesday at 7.30 again. Everything happens here in this room. And I'm very glad to have you here and welcome you for your first lecture. Thank you very much, Barry. Well, let me start by thanking the organizers for inviting me. I'm very happy to be here, especially in the place where Einstein was. And I'm happy to depend on him for all this work as you'll see. These talks are a great luxury for me because I, for example, just came from Rome and talked to a similar broad audience, not just the physics experts, but had to do it all in one hour. So the fact that I actually can take a little more time to go through the material is a great advantage I think for me, hopefully for some of you that go to the three lectures. Today's lecture is going to be a bit historic. So I'm going to begin from how the ideas of gravitational waves came about to the controversies over the theory itself to the beginning of experimentation and what it took to do it up to the discovery itself. So that much I'll cover today. In tomorrow's lecture, which will be in the afternoon, it'll be a little bit more technical, not super technical, but a little bit. And there I'm going to concentrate on how we interpret the signal that I'll show you today and what technically it took to make the measurement. What are the elements that it took to make an instrument that was sensitive enough to make this measurement. And then on Wednesday, I will talk about how we interpret these results. What's some of the other gravitational wave signals we can look forward to seeing starting a new science which is looking at gravitational wave astronomy. What some of the other ways of looking at gravitational waves will come about over the coming decades and basically what the future is of this, what we think is a brand new science. So that's the plan. Today, as I say, will be quite historical but also technical and scientific. So let me start, not with Einstein, but I'll go back to Newton. So we all learned about Newton when we were in school, all of us. And Newton probably arguably, maybe Einstein equivalently, was the most famous physicist of all time. And his theory of gravity, which is what I'm talking about here, or what he called unified gravity, is perhaps the most successful theory of physics ever. It was eventually replaced by Einstein's, which I'll discuss in a few minutes. But in, going back to the year 1687, Newton published his great works called the Principia. And in that he had, for example, is where he laid out the scientific method, something we all have learned about as the way we approach science, maybe not formally, but the way we basically approach science. And in that he also had his universal theory of gravity. So Newton's theory of gravity came about in 1687. It described what happens if you have two masses, like described here. We all learned this in school, that the force that attracts them, the gravitational force, is proportional to the product of the two masses. It's inversely proportional to how far apart they are squared. And there's a constant in front, which describes the strength of gravity. Interestingly, Einstein, Newton did not address how strong G was. It was left as a letter. This worked to describe everything for more than 200 years, almost everything. Whether it's the motion of the planets or the tides that we have in the ocean or the apple falling out of the tree. Basically, this described gravity. So that was Newton's contribution. It actually took 100 years before anyone could figure out how to determine what this capital G is. And that was done by Henry Cavendish 100 years later. And when he did it, he developed what's called the torsion pendulum. A bar that hangs down. If you twist it, it wants to restore itself. So on the bottom, he put a long bar and two weights on the ends. And then he practiced by twisting it and seeing what the restoring force was. And eventually brought up two lead balls. And so how much it twisted. And with that, 100 years after Newton introduced his formula for gravity, he told us how strong it was or what capital G was. And the answer he got, I write here, in the units we use in physics, which you can ignore for here, but I put it for completeness. It was 6.75 times 10 to the minus 11. And realizing that it took 100 years to do this, it's interesting that I put on here the best number as of today. And he almost had exactly the right answer as the first person to ever measure it after people couldn't do it for 100 years. So that was Cavendish. That was the theory. And basically for almost everything until Einstein's time, I'm trying to set the stage for what we knew and what we didn't know at the time Einstein came along. So basically that was the, even though it's a rather simple formula, that was what was known. And it was successful for everything, as I say, which involved gravity. The next person to come along that played an important role was a French mathematician who ended up being dubbed the father of celestial mechanics, Urban Leveurier. And Urban Leveurier used the formulas of Kepler for planetary motion and Newton's laws for gravity and together more or less explained everything about the orbits of planets. But he noticed that the planet Uranus had an orbit that didn't quite work. It didn't satisfy the equations of Newton and Kepler. That could have been thought to be maybe a breakdown of the rules of Newton. But instead he took the leap or hypothesized that there was a missing planet and that was why he couldn't fit exactly the orbit of Uranus. So he did the calculation to determine what this planet would be and found that there could be a planet, where it would be, how big it would be, and he even named it Neptune. And he sent a letter to an astronomer in Berlin. He was in Paris. He sent a letter to an astronomer in Berlin. He gave it five days to get from, at that time, to get from Paris to Berlin. He told the astronomer where to look in the sky. And he looked in the sky into something like one degree he found the planet Neptune. So this made Leveurier very famous and he dubbed, as I said, the father of celestial mechanics. The scientists like to use the same trick more than once often if you're successful. And this happened to Leveurier. So he basically realized 25 years later, he did all what I told you so far in maybe 1820s and 30s, and in the 1850s he noticed that the planet Mercury, our planet that's closest to the sun, also didn't quite follow the laws of Newton. And of course, since he was successful before, he hypothesized that there must be another planet or bodies between Mercury and the sun. Mercury has a peculiar orbit in that it's very, very, I'll show it in a second, it's very, very long and elliptical. Anyway, Urban Leveurier, as he had for Neptune, named what hadn't been seen, he called a Vulcan, and hypothesized that there was something in 1859, he wrote the paper, that there was a planet between Mercury and the sun. There were a lot of searches, and in fact, as happens often in physics, people who claimed a discovery, which was then disproven by better searches later, so people looked for Vulcan, but it was not found by the time Einstein came along. It also wasn't disproven, that there might be something between Mercury and the sun, it was still an active piece of research. So laying the groundwork for Einstein when he came, there was a discrepancy, maybe significant, maybe not, for Mercury around the sun, that was there considering Newton's theory, but nothing else after more than 200 years. So it makes you wonder, why Einstein, who in 1905 published several papers one month at a time on different outstanding problems in physics, and basically became the hero of all physicists since that time, with all the problems he solved so quickly, since spent 10 years developing the theory of general relativity. It's hard to believe that he did it because of this small discrepancy of Mercury around the sun, although some people have hypothesized that that's the reason. There are two kind of clear flaws. Our discrepancies are incomplete aspects of Newton's theory of gravity. The first one is that when we were all kids and we were in school and our teacher told us that the apple is pulled by the earth when it falls off the tree and told us that if we jump up, the earth pulls us down, most of us probably just believed it at that level, but maybe some of you asked your teacher why the earth pulls you down. Newton didn't explain why. Newton's theory, although he gave us a nice formula and it worked very well, didn't describe what is it that pulls two bodies together for gravity, and in fact most attempts in the next 100 years or so to describe it, try to do it by some form of electricity and magnetism. So nobody developed a new form. Einstein's theory, which I'll come to, ist, by having the reason the apple falls to the earth or the reason the earth goes around the sun has to do with the distortion or warpage of space-time around any massive object. So the massive object being the earth distorts space-time around it and other objects will be pulled to it. In any way, after Einstein came along and even came out with the theory of general relativity, NASA kept looking for whether there was any object between Mercury and the Sun. So sophisticated modern technology and satellites were used. Nothing was found. And the last time we heard about Vulcan was actually a television program. So, what about Mercury around the sun? Mercury around the sun, the discrepancy that I'm talking about is that Mercury itself, going around the sun, has a very elliptical orbit as shown here. And when it goes around, because of the positions of all the gravitational pulls of the planets, moons and so forth, it comes back to a different place. So you can calculate how far it moves each time it goes around. And that's in some units shown here, 575 arc seconds per century. And Newton's formula is not quite right, so it gives this discrepancy, which is about 10% wrong. And as I said, at the time Einstein came, that was the only discrepancy that existed in terms of measurement or anything else. So, the first way that Einstein's, that Newton's theory conceptually wasn't complete is that it didn't explain to us as school kids or later why the earth would pull us down, so why these things would work. The second failing, which Newton actually realized in his theory, is that it has in the words that we would use as physicists instantaneous action at a distance. That means when the apple falls, you know it immediately. That's okay for the apple. But if the sun were to burn out while you're here, it takes seven seconds for the light to get here or for us to know that the sun had burned out. And it's not reasonable that the gravitational signal would get here immediately. So Newton's theory didn't have any time or finite speed of information travel. Instead it's basically infinite or not addressed. So those were the two flaws that existed in Newton's theory. So even though it described everything, once we add Einstein's theory, it fixes both those problems and gets the right answer for Mercury around the sun. Einstein's theory, which I'm not going to go into any mathematical detail here, is a little more complicated looking than Newton's formula, but this is the equivalent formula from Einstein. And it basically has the feature that space and time are put together in one four-dimensional system. That's space-time. So it becomes one word in a sense. Space-time is one word. Bill Gates doesn't know that yet, so when I put it in Microsoft Word, it puts a little wavy line underneath. But it's all integrated together and it's the complexity that makes it difficult for students of physics to learn general relativity, difficult for scientists to understand it, difficult to visualize it because it's four-dimensional with space and time, and difficult to get the right answers when you calculate things, as I'll talk about. So Einstein introduced this theory. It fixed these two problems. Much of physics moved into quantum theory, so it had some people that worked on it in physics, but not all, of course. And Einstein wrote this, as I say, in 1915, 10 years after he had written this set of incredible papers. Okay. In general, we don't accept a new theory in physics, certainly after 200 and some years, like we had in the case of... I'm going to cool down if I can get this off. Like we had in the case of Newton. So to accept the new theory, in general what we expect is, if somebody has a new theory of physics, it's going to do more than fix some problem in an old one and this didn't seem like a huge problem, really. It fixed also two conceptual problems, as I said, which are probably more important and probably more the motivation for Einstein, but it needs to predict something new. So in this case, Einstein recognized that immediately and he made a prediction. He predicted exactly how much and that light would bend if it came near any sort of massive object. That's obvious in this theory kind of pictorially because if I have an object, this is the sun and the earth, this is a coordinate system. I can't do it in four dimensions. You have to look at every point on here and realize it has x, y, z and time and it distorts the ones near it. So each point here is distorted also in time, x, y, z and time. Here's the earth and if this wasn't the earth, clearly it was just the apple, it falls toward the lower point and would just fall like a trampoline and a bowling ball in the middle of it would roll toward it. So that's obvious and in the case of the earth, if it's going at some speed around then it tries to go out by centripetal force and it goes around on orbit. What Einstein realized in addition to this is that this picture here did not depend on the object being affected having any mass. In other words, the earth creates all this and the potential here, which was caused by the earth, it caused all this. So it doesn't matter when we talk about the fact that the earth has mass in terms of what happens to it, it gets sucked toward the fall and potential here into this well. So if I have light and it comes near this, it will also bend, it doesn't matter whether anything has any mass. So Einstein picked up that issue and he predicted that if we had light going past the sun from some star behind the sun, the light would bend as it went around the sun and he calculated exactly how much it would bend and proposed that the light be followed from a full eclipse of the sun and he proposed that it be done I think two years after 1915 and 1917 in an expedition was becoming the Soviet Union at that time, that expedition failed for political and military reasons but two years later the British scientist, Arthur Eddington, took an expedition to the southern hemisphere and measured a full eclipse of the sun when a star cluster went behind it and measured the bending as it went by and it was exactly as Einstein had predicted and that was the clinching experimental fact that made people believe general relativity and I believe made Einstein a household name because his name was in papers all over the world for this dramatic thing that light had bent and he had predicted how much in going around the sun and Arthur Eddington became Sir Arthur Eddington. Okay, so now we have a new theory it fixed the small problem that existed after 200 and some years with Newton it predicted something new, as I said and basically had some grounding. Einstein recognized over the next year that yet there was another effect or he thought there might be another effect and that was what the rest of these lectures is about and that's gravitational waves. So in 1916, only months after he had come out with the theory of general relativity, he wrote a paper which I show the front page here which predicts that there will be gravitational waves. That paper isn't a very good paper from the standpoint that it has at least one mistake of a factor of two and it doesn't develop gravitational waves from fundamental principles of general relativity. Instead, it was pure Einstein. He looked at the equations in setting it up a certain way and recognized that the equations had a similarity with different letters of course to the equations of electricity and magnetism that we all know, Maxwell's equations of electricity and magnetism. And then he made the leap. If the equations look the same, then there must, as there is an electricity and magnetism or we have electricity, we all know that from the lights and everything, magnetism, which we all know and we have electromagnetic waves, all kinds radio waves, micro waves and so forth. He made the leap that if the equations have the same form there must be also gravitational waves. So he didn't develop it from first principles. He realized that. He wrote a second version of the paper two years later in 1918 and that version of the paper fixed the factor of two. Of course, being Einstein, he never recognized that there was a mistake in the first one. He just went on and did it right in the second one. But it also set a guidance for somebody like me, an experimentalist, because what he did was talk about what would be the source of making gravitational waves. In electromagnetic waves, we all know that electromagnetic waves were discovered in the late 1800s after we had equivalently a theory of electricity and magnetism written down by Maxwell and those equations basically were, have electromagnetic waves and they were seen for the first time by a physicist named Hertz who took what you have in electricity and magnetism that is a dipole, two charges, that oscillate and make waves which are electromagnetic waves. So he made a source. He went in the next room and made a receiver, a radio receiver and detected the waves and then he moved it forward and backward and saw that it was wave-like and that was the discovery of electromagnetic waves. So likewise, we have now the same kind of form of equations and Einstein hypothesized there must be gravitational waves just like there are electromagnetic waves. I'm going backwards somehow. Maybe I just have exercise. Okay. So, before I get into gravitational waves and before I leave general relativity itself, I want to point out that we all may think of that as a very abstract subject, but almost all of us use it almost every day. And without it, we wouldn't be able to do what we do and that is the GPS system. So the GPS system, which is in your car, on your iPhone and so forth and so on basically is dependent on 24 satellites that circle the earth and the 24 satellites are going about 14,000 km an hour. That's the speed of the satellites. There's 24 of them circling the earth and if they're going at 14,000 km an hour, that's fast enough to require Einstein's special theory of relativity equals mc squared, the special theory of relativity and there's a famous expression that we all learn from relativity that moving clocks tick more slowly. That's how we remember which way it happens and in this case that's what happens. In the satellites, they tick more slowly and you have to make a correction. That correction is actually seven microseconds per day. So in the satellites themselves that correction is made, seven microseconds per day. But it's not the end of the story. The satellites are also well above the earth. They're high enough so that the gravitational field at the satellites themselves is about a quarter of what it is on the earth. Therefore the warpage of space time all what we've been talking about is less. And in this case the clocks actually tick quicker, faster. And that correction is 45 microseconds per day. So the total correction is the sum of the two und das correction is 38 microseconds per day. That correction is actually made in the satellites themselves. It's kind of amazing but the satellites that were developed by the US military the military of course didn't understand general relativity but they got convinced to make the correction when they put up the GPS system which was originally done for those purposes. Anyway, so I've just done numbers here. How much does it really matter this little correction? It does matter. It turns out that if I put the numbers in and I ask for the accuracy to stay on the road that is the accuracy to stay on the road if it drifts more than 30 nanoseconds I would go off a road that's 10 meters wide so I can put the numbers in which I didn't bother to do for you here of how much happens in a day how long does it take if the GPS system would keep you on the road for about 2 minutes if we didn't use general relativity so if you don't come home with anything else today realize that general relativity does matter for all of us. Okay Einstein and essentially nobody else visited the possibility of gravitational waves for 20 years, almost 20 years, 18 years. Einstein Most physicists were working in quantum theory. Einstein was trying to do more on unification He didn't come back to this problem until the 1930s after he had immigrated from Berlin to Princeton University and when he was at Princeton University his associate or assistant was Nathan Rosen and the two of them re-looked at the problem of gravitational waves starting in 1934, 1935 to try to develop it out of the equations of general relativity rather than the hand-waving that the equations are similar so it was an unsolved problem to do that they worked on that for some time and basically got a result and wrote a paper which is entitled do gravitational waves exist funny title for a paper written by somebody who said they did exist 18 years before so you can imagine that the content of this paper maybe wasn't what he originally intended to do which is to develop gravitational waves from first principles and the problem was that we often have in general relativity as I kind of alluded to earlier that because of having space dimensions and time all together it's difficult to do the calculations you have to be very careful how you set up general relativity problems if you set them up wrong you get often infinities and so forth which we call coordinate singularities and that's what happened in this case to Einstein and Rosen but they didn't quite recognize it they got more afraid that maybe gravitational waves don't exist they wrote a paper they submitted it to FISREV I don't know what it was called FISREV or FISREV letters at that time in the US which was the most prestigious American journal and the editor of FISREV at that time was named John Tate a rather young man you can see he's kind of young in this picture a problem of how to deal with this paper you have to realize going back to the 1930s that now when we submit papers for publication it's just part of the system that we have what we call anonymous peer review papers are sent out to anonymous reviewers depending on what those reviews say the editor decides to publish it in the 1930s I don't know whether Einstein experienced any Referenzystem in Germany before he came to the US but I do know that in the US probably not, I assume but I don't know but in the US they were just initiating for the first time peer review into the physical review and John Tate had that responsibility clearly it's anonymous peer review you generally don't have the ability then to look at what happened but now many years have passed it turns out that I served as president of the American Physical Society that runs the physical review journals and you can actually go now and look at the old records so I'll show you the old records so this was sent out so this was sent out for review this is the man that reviewed it his name was Howard Percy Robertson who also happened to be at Princeton University at the time at that time but wasn't really a close associate of Einstein he was on sabbatical at my university Caltech in Pasadena California so he got the paper to review while he was at Caltech in Pasadena California and down at the bottom here in handwriting because then we didn't have computers and all that kind of stuff is the logbook for physical review letters showing what happened to the papers that came in so the first paper on this page this is 1936 you can see he was rejected just by the editor himself so he had the right to just reject a paper or accept a paper as one down here was just accepted no referee and in between there's this paper which was sent out to review to this man Robertson and it came back about a month later so they sent it to review to Robertson and as I say was a general relativist actually looked at the problem and understood why they were getting the infinities and reset up the problem in the way they did but in what we call cylindrical coordinates and so he wrote back a letter to Tate the editor from FISREV saying what was wrong with the paper and gave Tate the problem in Einstein Tate wrote a letter to Einstein which basically said I quote from it that he would be glad to have Einstein's reaction to the referee comments and criticisms pretty mild I would say most of us had turned in a paper that had that we would get a rejection or something worse anyway so that's basically where it stood Tate wrote a very mild letter to Einstein I didn't bother to copy that but basically telling him this and Einstein got the review and basically did this and this is a copy of his letter back to physical review so he wrote a letter back to physical review and he said we, Mr. Rosen and I have sent you our manuscript for publication and had not authorized you to show it to specialists before it is printed I see no reason to address the in any case erroneous comments of the anonymous expert on the basis of this incident I prefer to publish the paper elsewhere it's true that Einstein never published in physical review again for the rest of his life he sent this paper to the Franklin Institute people have probably heard the Franklin Institute it's in Philadelphia and at that time it had a journal it has no journal now they must have been delighted they took the paper, they accepted it they didn't have peer review apparently and they were going to publish it in the meantime in the meantime Robertson went back to Princeton his sabbatical at Caltech was over he went back to Princeton and he met up with Einstein's new assistant had gone back to Europe his assistant was another well known man Infeld was well known and he didn't see Einstein didn't see Einstein himself and in fact what I'll tell you next is what people conjecture everything I've told you till now is actually documented and that is that he discussed it with Tate he showed Tate how, if you set it up in cylindrical coordinates these infinities go away and Tate then Infeld then went to Einstein this part is conjecture and Einstein said he already knew he had found the problem they then resubmitted the paper it took a long time to publish in those days luckily for Einstein and Rosen because you had to set up all the type you had to get a copy editor and then they redid it so it took months before a paper would come out in that interim is when all this happened and by the time the paper came out it actually took care of this problem and had a new title the title is rather vague it says on gravitational waves and the first sentence I'll read to you if you're too far back to see the rigorous solution for cylindrical gravitational waves is given so you remember the cylindrical coordinates and it goes on so it's not a real proof from first principles but it's the closest we had the rest of the story on gravitational waves nothing happened for another almost 20 years in general I think there was skepticism in the theoretical community outside of the general relativists nobody paid any attention and it finally came to a point in the 1950s which where a meeting was called in Chapel Hill, North Carolina this meeting was called by two well known general relativists Bryson, Cecil DeWitt and 44 of the theorists in general relativity that were somewhere close to this problem and they came together for several days in North Carolina as well as one experimentalist who I'll show you later started on the first experiments so at that meeting if you wanted a day to time I'll tell you what happened at that meeting the problem of whether there are gravitational waves in terms of the theoretical community was resolved was resolved because of two at least because of two key presentations there one by a theorist named Pirani a British theorist who in a very elegant way developed gravitational waves out of the theory of general relativity so he did what Einstein hadn't done 20 years before 40 years before and in addition at that meeting was Dick Feynman and Dick Feynman always looks a little differently at the problems and in this case he said there's gravitational waves and they're real they have to somehow be able to transfer energy otherwise it's just a fictional thing and so how might they transfer energy so he made a kind of a gedanken experiment or a fake experiment which is shown here he called it the sticky beat argument Feynman did and the argument is the following that if I have a bar like this and put a couple rings around it and the wave comes through it has, as I'll show you later it has the feature that it expands and contracts the bar which puts some frictional pressure on these these will move back and forth and due to the friction and the friction is heat and that's energy so that picture was both convincing in terms of a physical argument that gravitational waves weren't just mathematical as Pirani had showed and this method is essentially the first method that was tried as I'll show you next to detect gravitational waves so this was the argument and everything I've said to get to this was me being an experimentalist was to make the excuse that it really didn't take us 100 years experimentally to detect gravitational waves the theorists forced around for 40 years first so it only took us 60 to measure gravitational waves the first sentence of our paper says that it took us 100 years it doesn't quite say that it says that Einstein predicted it in 1916 and 100 years later and that Schwarzschild had also predicted black holes, which I'll talk about in a while the same year and 100 years later we saw them but 40 years of that I blame on the theorist then we took a long time ok, so next let's look at gravitational waves and what their effect is and how you can go about detecting them and remember what I talked about in the sticky bead argument because it's the first technique that was tried so the first statement is that gravitational waves are not like electromagnetic waves electromagnetic waves when they go through space are carried by photons are attributed to photons through quantum mechanics so we have a carrier in the case of gravitational waves at least classical gravitational waves from general relativity they have there's no carrier they're just basically distortions or ripples in space time itself a little bit like throwing a pebble in a clear pond and it goes to the bottom and the little ripples that go off are just part of the water itself taking a different form so there isn't some object carrying it so that's the first qualitative feature it's not like electromagnetic waves in that sense secondly the amplitude that is measurable by the kind of technique that I'll talk about and I'll come back to that is a little term H which is the term out of general relativity it's the strain or the amount of distortion of space time that you get and it's a very tiny number 10 to the minus 21 I'll show you that on the data there is a number that is kind of conceivable and that you need to be able to do to be able to see it is a very tiny number 10 to the minus 21 what does it do it basically distorts space and time of course but I'll work with space for the purpose of this and that is that if I have a bar 21 times its length and that's this H is delta L the change in length is 10 to the minus 21 this number times L and a way to think about it I show here if I have a ring and on the ring I put free masses these are free to move and the radius is L then and the distance is just I didn't write that well but it's 10 to the minus delta L the distance here is 10 to the minus 21 times if this was a meter wide a meter so I'd have to measure 10 to the minus 21 meters it's a small meters this big I have to measure 10 to the minus 21 meters we make that easier in the case of the experiment by making it not a meter long but kilometers long so this number 10 to the minus 21 becomes more like 10 to the minus 18 and that's the size we have to measure so Einstein's theory of gravity basically says that if there's sources that have this quadruple moment that I described and two stars going around each other would have that would have the quadruple moment then it basically will emit gravitational waves that will propagate away at the speed of light and basically that's what we'll try to measure so how should we do it as experimentalists clearly in experimental physics the most reliable thing you can do if you're trying to do an experiment is to make sure you control all the variables if you don't control all the variables then it's much harder so what you'd like to do is just what Hertz did remember Hertz made a little source went in the next room and detected it he could fool around and change the source if he wanted he could change the detector and so forth we don't have that luxury as I'll show you imagine that I made an experiment to do this to make the quadruple moment you make something like a barbell like this and rotating around and if you do that you get basically a an effect that if I put in these numbers here that is this is a one meter across these are say a kilogram that I rotated a thousand times a second and then I better get out of the room because it's pretty wild so I've exaggerated them all and I go into the next room which is only 300 meters away I'm trying to exaggerate the effect to make it as big as I can that's not far enough to see the wave like motion you have to be ten times further then how much do you get here whoops you get 10 to the minus 35 for this effect and what I'll show you by what we do from black holes is we're capable of measuring an effect so even though the effect that we're measuring is incredibly small if we actually tried to do what Hertz did it would have to be 14 orders of magnitude smaller yet which clearly we can't do so we have to rely on nature to make the source instead of make our own source fortunately nature made a source that's strong enough and fortunately that source is also very interesting so we were lucky in that sense and this is the source which I'll come to that is it looks like the barbell it's two black holes that are going around each other as they go around each other they by Einstein's equations are accelerating by going around in the circle they radiate away energy that's radiation is in the form of gravitational waves which we'll detect in a while as I'll show you and we basically can detect that okay before we get to our detection let me start with the first experimental attempt this is the 1960s and this man Joe Weber was the first to try he was the one experimentalist who was at this meeting in Chapel Hill, North Carolina and he made it so he made the first detector and the first detector was the German sticky bead argument this is the bar he made a bar that was one or two meters long a big bar a very very pure aluminum and around the waist of it here he put very sensitive detectors these are PZT detectors they're called that can measure any strain if the bar changes its shape a little bit so if a gravitational wave comes through this changes because the space gets warped space a little bit exerts a little bit of pressure on this picks up the signal and that's the gravitational waves that was done in the 1960s as I said with a very pure a very pure bar of aluminum those first searches were really very brilliant that he developed the technology and so forth unfortunately he thought he made a detection when he didn't so in 1969 he was the first to claim that he saw gravitational waves that is the picture from physical review letters that was published in 1969 where he saw what he thought was a signal from gravitational waves near Chicago and one at University of Maryland where he was a professor and he published this in 1969 but unfortunately it wasn't correct and several experiments were done that were at least 10 or 100 times better than his that didn't see such an effect so he was incredibly important for experimentalists in starting this field he even did several things that are basically built into what we do in LIGO which I'll list here just to give him credit he did sensitivity calculations for what bothers you in the end the various sources of noise we do the same thing he did a coincidence which he did between two different detectors distant from each other in order to try to reduce the problems that you might have we do the same he finally did a way to evaluate what the backgrounds would be by looking at these when they're not in time instead of in time to see how many times you get deviations that look like they're in time but they can't be and we do the same thing so he started the field he was very clever experimentalist unfortunately he used some ideas that he did as part of what we do before I get to our experiment I want to go through one other thing and that is that another very important experiment was done by Hulse and Taylor this is Hulse and this is Taylor in that they saw indirectly very strong evidence in an experiment over a period of over 20 years looking at pulsar signals with a radio telescope they looked at a particular pulsar system called PSR 1913 plus 16 with a radio telescope so what is a pulsar and what was the experiment we have in our own galaxy neutron stars which are products of the death of a star if the star is not so big that it might make a black hole it makes a neutron star we don't know in detail what a neutron star detail looks like but the best models we have of what they look like I described here they are about one and a half times as massive as our sun they have a solid crust on the outside that's about a kilometer or two kilometers thick the whole thing isn't much bigger than the size of a medium sized city about 20 kilometers across and the inside is made up of a lot of neutrons and other particles that are supposed to be heavy and very dense liquid so that's a neutron star it then has made and been detected as signals in radio pulsars because they tend to rotate they have currents in them the currents make signals that come out of the end so if it's rotating around radio signals come out of the ends of a radio pulsar and you see periodic signals it works a little bit like a lighthouse that when it points at you you see it when it isn't it's dark and that gives a signal and so pulsars are watched by seeing this in order studying a particular pulsar system the one that I said and they were going this is Halson Taylor they were going to study it in great precision they weren't looking for gravitational waves and in order to see how well it all fit together they were studying a system called PSR 1913 plus 16 and it consisted an around 17x2 so they have a signal that's going like a lighthouse 17x2 they're detecting it with a radio telescope and with that radio telescope they were longitudinally measuring it over some period of time and they noticed they weren't searching for it they noticed that there was an 8-hour modulation on the signal that it changed kind of every 8 hours after a while they realized that meant that there was a second object that this wasn't a single pulsar but like the earth and the moon there was another pulsar that was going around the first one and they inferred that the period of that was about 8 hours so the picture they had was this picture here they measured then the period of this 8-hour cycle by inferring over a period of about 20 years they first were measuring everything very accurately so they measured that one object was 1.4 times the mass of the sun the other one was 1.36 that the orbit was elliptical with an ellipticity of 0.617 and they were separated from each other by about a million miles the predictions from general relativity once you have the two straight forward to calculate because we have the two going around each other and there's a certain amount of energy then given away because of the accelerations to general relativity and calculate it and after it goes around each orbit it loses a little bit of energy and solely spirals in so that this loses basically 3 mm per orbit and the rate that changes the rate or the time it takes this to go around just down just a little bit and they measured it here this is after they were awarded for this but so this is the beginning the time, this is seconds and the years from 1975 to 2000 and you can see the line the dots along here the dots are their measurements which they'd make how long it took to go around what the period was very accurately small enough actually the line you'll see goes right through them and in contrast to typical physics experiments that line is not a fit to the data but a calculation using the parameters of the system from general relativity and it goes right through the data so this was indirect proof that there's a system that's losing energy and that energy presumably in die Form of gravitational waves okay so now comes the new scheme and what I'll talk about the rest of the time and that is using interferometry so the idea here is that light comes you divide it in two perpendicular directions it goes down a path which we've made kilometers long hits a mirror and comes back they'll cancel each other and nothing will be detected in a detector but if one of the arms gets a little bit longer because of distortion of space-time then they will add to each other instead of subtract and you measure a signal here in simple minded terms that's how interferometry works and it's what we use in LIGO our detector basically is that simple in general in experimental physics it's really difficult to measure something absolutely like how big is a meter stick or how much is a kilogram and we have big institutions that work to make in the US it's a national bureau of standards and there's a big institution similarly in France and so having the standards for everything we have is a big deal and it's tough to do it's easier often to measure something that should be the same or barely is different from each other so this is almost ideal because we measure a difference of something that should be the same the two arms are the same the light comes back and cancels if one of them gets a little longer then we can detect it so that's the scheme of interferometry that I'm showing a couple of times a second here what happens one gets longer in one direction then shorter in the other direction then goes the other way and that okay I'll tell you in words so there's a piece of that slide missing and I'll tell you in words basically there are two challenges experimentally most of which I'll talk about tomorrow but I was just going to tell you the numbers today in order to be able to measure accurately enough to see the kind of effect that we see this little 10 to the minus 21 that I talked about in order to be able to do two things one is do the interferometry where I showed wavelengths you have to be able to do it to one part the wavelength in one part in 10 to the 12th this is called splitting a fringe if anybody ever used an interferometer people who use an interferometer and a freshman or southmore laboratory maybe see fringes and can split a fringe to one part in 10 or 100 very good interferometry might do it a factor of a thousand better than that but we have to do it to one part in 10 to the 12th so much of what we've done over 20 years is learn how to do interferometry itself with unprecedented accuracy and there's an incredible number of very clever experimental techniques and tricks that are used to do that the second problem is that we live here on earth and we're trying to detect gravitational waves but the earth shakes a lot and so we have to isolate ourselves from the earth itself and again the number is similar we have to isolate ourselves from the earth we don't want the ice we have to suspend this thing somehow and make it free from the earth so that any shaking of the earth is reduced by a factor of 10 to the 12th those are the two experimental challenges which I'll talk about tomorrow but let me go back to what we detected first so I end this with a piece of physics 1.3 billion years ago there were two black holes shown in this picture here and those two black holes that are shown in the picture here are drawn by a computer rendition of our data not by an animator what's shown is the universe and the two black holes but before I show them what they do let me remind you what a black hole is at least so we can all understand in simple minded terms it's of course a very difficult concept mathematically I won't deal with that let me do it physically it's a region of space created by matter that's so dense that nothing can escape from that matter preserved by general relativity so nothing can get out of it then it's defined as a black hole a region of space like that is created when a star burns up all its fuel and collapses not our sun because it's too small it can't make enough density but we think that stars larger than about three times the mass of our sun can give this very big density that we call a black hole all of these black holes are the ones that I'll show you that we detect are about 30 times the mass of our sun this is from our data the two black holes going around each other as they go around each other they attract each other and eventually they're in the meantime affecting everything around them as you can see in the galaxies and so forth eventually they radiate enough so they merge together and then they'll settle down with some shaking a little bit and all of that has a lot of physics in it but that's the process of what happens and then you end up with one bigger black hole the bigger black hole is not the sum of the masses of the smaller black holes but the sum of those masses plus the energy that went away in the form of radiation which is gravitational waves now showing the gravitational waves so as they're coming in they're emitting gravitational waves those gravitational waves that we detected started 1.7 1.3 billion years ago they then were coming toward us on the earth I show them getting to the earth but in this picture I exaggerate a little bit how much it affects the earth but it's going to distort it's going to distort space time itself so it distorts the earth but as I said the earth itself is a little hard to deal with so we have to put an instrument instead to see that distortion that instrument's called LIGO which I'll talk about in more detail tomorrow but it's the instrument that does these two things it does interferometry at a level of 10 to the minus 12 of what the size of the wavelength of the light that's in it and is isolated from the earth 10 to the 12 okay, I lost a picture here too this picture showed a picture of the earth and the first thing that happened is that this gravitational wave that we detected that was the first ever detection of a gravitational wave was on September 14th, 2015 and in this picture you'd see that we know that it came up through the southern hemisphere but not very accurately and there's a picture that shows that I'll show it in a future slide this is the first detection so in Livingston, Louisiana on September 14th at 5 o'clock in the morning this trace was seen so what are we looking at this is time going this way this is the size of the signal this way and if you can't read it this is what we saw before 10 to the minus 21 so 10 to the minus 21 is from here and you'll see this has a shape that does this what it's doing is to you might look just like wiggles but it's a particular kind of wiggles that we expect that is they get bigger as time goes along and they get narrower as time goes along so we call it a chirp signal it goes whoop and I'll try to show that 7 milliseconds later in a second piece of apparatus that we have 2 thirds of the way across the US one in Louisiana the second one in the state of Washington state of Washington we see this and even to your eye you can see that they look a lot alike and so this is the raw data it's many times in physics computers doing all kinds of things before you ever see the data and then we show you the data after we massage it it's what we saw right away from the online data so that's the signal and in here you can hear this chirp signal so what happens is it comes what's plotted this way is frequency and this is time and as they merge together they are going faster and faster so it has a shape that goes like this which is the so called chirp shape and the detail registration of it is shown here in the two labs so that's what we detected in terms of what we're thinking is then and I'll show you how that works in a minute this is what you expect from general relativity for such a system that as you start with two objects they're going around each other like the earth around the moon these are two more or less the same size and it's doing this as it gets closer together it gets narrower and somewhat taller then they merge together that's here and eventually make a little ring down what's shown down here this is a fit to our data by the way but what's shown down and I'll show you how that works in a minute but what's shown down here is two things one is how fast these are going they're going starting at about a third the velocity of light up to about here six tenths the velocity of light so we have these two objects that are going around each other at a good fraction of the speed of light radiating away gravitational radiation the distance there apart is about the scale here is in units that you don't know but it's about a hundred kilometers apart so what I've shown here and it's where I'm going to end I think is those same two signals but maybe you can see it or maybe you can't there's a little white line that goes through them that's the fit to general relativity so we got this and immediately one can see how well without doing anything it fits the theory of general relativity four two massive black holes that are in the distant universe that are about 30 solar masses each and that's the end of what I'll show today what I'll show in the next lectures in addition to how and why it's possible to make a detector that can see this more detail about what we can learn from this shape and how well we know the shape what parameters and what we can tell about the system itself that we detected and other systems that we can detect in the future so with that I'll end for today thank you okay thank you so much Barry for a really clear and very exciting lecture I'm very curious to hear what's coming tomorrow and on Wednesday but I'm sure there many questions and so now we're ready to have you interact with the audience just raise your hand and we'll come to you microphone just to begin there was a dispute between Newton and Leibniz about gravitation and if I remember well Leibniz was skeptical about gravitation but I have forgotten his argument could you tell us the argument of Leibniz against Newton I'm not really an expert either so somebody else probably knows better than I sorry okay there's a question over there so I'll get some exercise I thought we might have a second microphone if somebody knows where it is please let me know just a very naïve question how often do appear these gravitational waves and how you can be sure that I would expect to see many waves coming from many different places in the universe how can you single out one specific phenomenon okay so this question I can answer thank you we're trying to detect something that's fairly rare and we have to be able to make a detector that can see out far enough so that some of these events can happen this particular event was about 5% of the way to the edge of the universe which is the distance at the time we did this as far as we could look so what we're limited by is the fact that we can't see very far yet and so it's rare for these to happen since that time we've improved the distance maybe a factor of two so we see maybe one tenth of the way out to the edge of the universe and we've seen quite a few more how many are there you can take the fact that since we've done this one somewhere between five and ten how far out we look in the universe assuming that they're uniformly distributed, which we don't know through the universe we can calculate how many there are but what limits us is really how far out we can see this little sensitivity that I used that I said we were sensitive to some level in the words of experimental physicists is an amplitude and that means that as the gravitational wave source is twice as far away it'll give half as big a signal not a quarter as big you don't have to square it so as we improve our detector by a factor of two we see a factor of two further out or eight times the volume or eight times the rate and so we have numbers in the literature now for what the rate is of these black holes is fine because we only have a handful of examples so far but it's basically that you said there's a lot of them going around we're just not that you're right but we're just not sensitive enough to see them we're barely able to see far enough to see any at this stage okay are there further questions this is the great location for our next question how much is the amplitude of the gravitational wave if we are very close to the event well as I say the amplitude of the gravitational wave itself is a number that I gave which is that 10 to the minus 21 then you have to measure what effect there is on a length to see what the effect will be it varies linearly with the distance so I showed you at basically one giga parsec how big the signal is 10 to the minus 21 if we had a signal that was a half of a mega parsec it'll be twice as big if it was a quarter of a mega parsec it'll be four times as big so it varies linearly with the distance what are the prospects of refining these instrument in such a way that one can open a new window to the universe and detect even I mean this is an extremely violent event colliding what are the chances of in the near future let's say so that we can also observe less violent gravitational events so I'm being asked what my next two lectures are which I'll do two things one is I'll project and say why how this window will crack open over the coming decade and beyond and what other kinds of physics sources will become available because of that and that I'll talk about I think what I'll say is the following today that this has been a hard long task to do this experimentally but we aren't limited by nature so we haven't run into the problem that we don't know how to make better detectors so it's easy but there's nothing fundamental in nature that's stopping us right now it's our ability experimentally to have a quiet enough environment to make a good enough isolation from the ground and so forth and so on we know how to do better and I'll just name a few things which I'll talk about a little bit in the next days one is that we're limited much better because they lower the temperature and will lower the temperature in the future that's an obvious way it's not easy because we have to make an interferometer like this lower the temperature without shaking anything and so forth and so on but basically that's one scheme we can have a more powerful laser we're not anywhere near the limit of what people can use for lasers we've concentrated mostly on the quality of the laser and not how much power it develops and there's many other ways this will improve so I think it's going to be slow as it's always slow to make big expensive things but I think we can look forward to generations of our kind of continual upgrading and improvements and hopefully we've already seen a second kind of source which I'll describe in another lecture with stars instead of black holes and I think as time progresses we'll see many more kinds of sources of gravitational waves okay I'll allow at least one more question if there's no question right now I'm sure there is one so how long did it take you to realize that you have detected the gravitational wave from your raw signal in your detector how long did it take you like minutes or hours was there a nice alarm we have the feature that because we're looking for signals from astronomical events not this one but astronomical events in general that might also be seen by optical or electromagnetic radiation some sort of telescopes basically what we call an online analysis an online trigger if something becomes a strong enough candidate we alert the astronomical community our own analysis to do it in detail might take longer we do that regularly and then we say oops a few days later when we figure out that it wasn't real so this fit that category we immediately that it looked like it could be a gravitational wave it isn't the first time and as I said it happened at 4.50 in the morning in Louisiana which was two, three o'clock in the morning in California and almost noon in Europe so the first person in LIGO that actually identified this as a possible event was not at the two laboratories that we have in Hanover where he saw the data it was an Italian postdoc and he saw the data and made a lot of emails and telephone calls and so forth by the time I woke up in California three hours later there was a long string of email and that is the first time there is an alert and there is some email but this was longer and kept looking better instead of worse as you read through them and I would say then that's the scientific story at the beginning that we knew it was different than anything we had ever seen and we had to check it some of my colleagues I believe had a Eureka moment at that point I had a closer to a panic I think and that is kind of two questions one is this was as I mentioned before we were continually improving the apparatus and it had been really redone and this was the very early running on an apparatus that we hadn't had that long we had tested it but not in detail so one question is how are we fooling ourselves meaning how is this new apparatus making us think we see something that we are not thinking that we knew exactly how to test that of course but it took one month and the second panic if you want is that data had traveled all the way from Louisiana to Caltech to Hanover how do we know that some devious graduate student or somebody else had gotten into the data and put something in there and so we had to trace these signals back to the origin and the detectors themselves because as we assemble that little curve that you saw we leave data in the apparatus itself and at the labs themselves before we bring it together and that took about a month so it was about a month later that I think all of us were convinced that we saw gravitational waves and I thought I was totally convinced we saw our second example of gravitational waves three months later on Boxing Day in December and for me personally I had a sigh of relief so you can ask when you get as a human convinced and of course we've seen many since then one very last question I wonder if you double the laser frequency could you increase the sensitivity of the laser so the question is if we increase the laser frequency could we be more sensitive actually in principle going to shorter wavelength would be a little better than where we are but we're more limited by how we can make a laser stabilize it, make it run so we work in the infrared but we debated a lot whether to work in the green but we could make a better laser and more stable laser that more than counter balanced the gain that we would have so in principle you're right in practice for a real laser we picked a neodymium YAG laser one we could commercialize and one that we understood very well and one that we could make very stable and that was more important than the shorter wavelength second lecture at 5.15 and then on Wednesday but I'm sure you'll join me in thanking Barry Berish for an absolutely superb first lecture