 And so Dr. Brian O'Reilly is the lead scientist base at the LIGO Livingston Observatory in Louisiana. He received his BSC from Dublin City University in Dublin, Ireland in 1987, and his PhD from Northwestern University in Evanston, Illinois in 1995. His background is in high energy physics, but in 2002 he changed fields and joined the LIGO laboratory. He was the installation lead for the advanced LIGO upgrade in his current responsibilities include run management for their observation runs, which basically means that he knows everything that they're doing. So thank you very much, Dr. O'Reilly, for joining us and welcome. Thank you for the nice introduction. I'm going to share my screen. Can everybody see what I'm showing? Yeah, we can see the first observation of a binary neutron star merger. Oh, no, I give it away. Oh, gosh. All right, let me close this Q&A window. Okay, so thank you all for coming. It's nice to see so many people interested in what we're doing here. And so I'm going to talk to you about LIGO and about gravitational waves and what recent discoveries we've made. And hopefully it'll make sense and there'll be time for questions at the end. So people, I think everybody knows about Newton's gravity. It explains apples falling from the trees. It explains the orbit of the moon, planetary orbits. But one of the big problems with Newton's gravity and this is a picture of a tree, I think it observed, you know, at Cambridge, grown from the original Newton apple tree or the legendary one. One of the problems with Newton's theory is that it says that gravity is instantaneous, that if you change a mass somewhere, the effect is felt everywhere at once. Of course, relativity demands that it should be, the information should travel at the speed of light. There was also this procession of the perihelion of mercury that showed a behavior that Newton's theory could not explain. And I'll show that in a moment. The equation, which many of you probably know, the force of gravity, it depends on this universal gravitational constant and then just the product of the two masses in question divided by the square of the distance between them. Nice, simple theory, but today's standards, very successful, explained almost everything we cared about for many hundreds of years. So in 1915, Einstein generalized his special theory of relativity to include gravity. And Einstein's huge insight is that gravity is a little bit different from electromagnetic or other attractive and repulsive forces. It warps space and time. And so just like this black hole in a bucket, a demo that Brian talked about at the beginning, you can see that you think now of space as being warped by the presence of mass. So that means that you too, in your own small way, warped space in an undetectable way. This is a simplified picture. Three-dimensionally, it would look quite a bit different. But all the three-dimensional attempts to represent this look quite messy. Einstein's theory predicted several things. Gravity propagates at the speed of light in his theory, which was nice. And it also predicted gravitational waves. Let me say a little bit about this procession of the perihelion for Mercury. So this was first observed in 1859 by, and I'm probably mispronouncing this, but Arban la Verrier. And he's famous for actually discovering Neptune. And he did very precise mathematical calculations over many, many years to evaluate the effect of Newton's forces from all the planets and all the other planets, which is why he found an anomaly in the orbit of Uranus, which led him to predict where Neptune was. But it also showed that the change in the closest point of approach of Mercury to the sun was precessing or advancing at about 574 arc seconds per century. And Newton's theory could not explain this. It could explain it all up to about 47 or 45 arc seconds. Einstein's general theory of relativity predicts it almost exactly. And I haven't shown the errors here, but within errors it predicts exactly what's observed. So this was one of the big victories. The other famous tests proposed by Einstein himself took longer to observe cleanly the bending of starlight as it passed the sun and the gravitational redshift. So what are gravitational waves? So if you see here, I've drawn a ring of red dots and these represent test masses. So masses that are free falling in space. And as a gravitational wave passes through, it causes the space, space time itself to stretch and squeeze. And we measure that by calling, by measuring something called strain. We measure the change in length divided by the length. So the longer you make the measurement over, the larger distance, the more strain you will be able to observe. LIGO is designed to measure the strain. Gravitational waves are very weak. And I'll, yes. And so here's an example of the weakness of gravitational waves. So here on the right, I've written Einstein's famous general relativity equation. This tensor on, it's a, for people who don't know what a tensor is, it's like a higher dimensional vector, represents space. And this tensor here, this energy momentum tensor represents basically the energy or in some way you can think of it as the tension in space time. And they're related by this constant. And this constant, which again includes this universal constant of gravitational and the speed of light is very, very small. It's about 10 to the minus 43 meters per joule. It's a very, very tiny number. That's like a decimal point, 43 zeros and a one. If you want to, based on this equation, make gravitational waves in the lab, you can design an experiment like a thought experiment where you have two one ton masses. You join them by some super strong bar that's two meters long. You rotate it at one kilohertz. So this thing would be spinning it 1,000 times per second. So you have to use some like unobtainium material to do this. And then you have to go 300 kilometers away so that you actually see the radiated waves and not the near field effects. And that will give you a strain of about 10 to the minus 38. And to put that in perspective, I've written here, the diameter of the Milky Way is approximately 10 to the power of 21 meters. If you take this strain of 10 to the minus 38, that means that the distance for a gravitational wave of this strain strength, the distance that the Milky Way diameter would change is 10 to the minus 17 meters. The proton diameter is 10 to the minus 15 meters. So the entire Milky Way for a gravitational wave of this strength would change by a hundred times smaller than the diameter of a proton in length. So it's a very tiny effect. So the conclusion is, it's not possible to do this on earth in any kind of an experiment we can dream up to detect, to generate detectable gravitational waves. Gravitational waves come in two polarizations. There's a plus polarization that I've shown here on the right and this cross polarization here on the left. You can see they're not like the usual waves on a string or waves on a pond that you've seen where they oscillate up and down. They do this strange quadrupole oscillation. Detectable gravitational waves come from huge masses. And by huge masses, I mean black holes which are many times the mass of the sun. Neutron stars which are one and a half times the mass of the sun and perhaps from supernovae and pulsars. And so for example, two 30 solar mass black holes at a distance of around one billion light years. When those two black holes merge so they orbit each other when they merge that final merger will produce a strain around 10 to the minus 21. So this is about 17 orders of magnitude better than what we get in the lab. So even though we've gone to astrophysics, we now have something that we may have some chance of detecting. The earth's sun distance for this strain changes by about the size of an atom. So this we call this easy compared to any other option. It's not really easy. In 1969, Einstein himself wrote in his paper. So this A in this equation here which I'm not going to explain is basically the strain we have talked about. And Einstein said, if one were to measure the time in seconds, the energy in earth, the numerical factor one over C4 would enter this expression. And this chi here is about 10 to the minus 27. And so Einstein himself said that in all practical cases that A or the strain must be practically have a vanishing value. Now, of course, Einstein didn't know about black holes. Einstein didn't know that there would be lasers. He couldn't envision LIGO. And even I think until 40 years ago when Raywise first had the idea, people did not envision LIGO. I can't actually see anybody. So let me check that people can hear me okay. And see the slides. Yeah, we're going, we're right along with you. All right, great. So Raywise, following some experiments by Joseph Weber who used a different technique to try to detect gravitational waves, a technique that did not work. He tried to make them cause a resonance in a very finely polished aluminum bar that weighed several tons. Raywise started thinking about this and he came up with this wonderful unpublished paper. It's a study he wrote for the National Science Foundation. And it contains so many things that we dealt with in the last two decades to make LIGO work. And even here on this side, and you probably can't read it that well, but he talks about black holes of certain masses and neutron stars and where you expect these gravitational waves to be in the frequency spectrum. Here, if you can see my mouse, is about a hundred hertz. And here is where you will see neutron stars merging and some black holes. So this is a great document. It's very foundational. Not only does he talk about the technique of detecting gravitational waves, he talks about the noise that will prevent you from doing so. So to the stars, but not really the stars. We want the dead stars, but they're kind of dead-ish stars because neutron stars, as many of you probably know, are pulsars and emit radio waves and they're quite dynamic objects. Black holes, for the most part, we expect to be matter-free, having clear their orbits of all matter, especially binary black hole systems. There are black holes that exist in X-ray binaries where the companion is a star and they creep mass from that star and that's why you have this X-ray emission. I'll mention something about them later on. This is a video of a neutron, it's a supernova explosion. So when the supernova explodes, left at the center is this neutron star. It's about 15 kilometers in diameter. This is it compared to the San Francisco Bay Area. It's basically a giant atomic nucleus. Very little is known about the exact, what we call the equation of state, what the matter of it is in this object. We've observed them as pulsars, we've observed them in binary systems and those binary systems, we've never observed directly gravitational waves from neutron stars merging. So what we care about is, we care about binary systems and here is a reference to an article in Nature, which is in News and Views, so it's quite accessible, that talks about the implications of what LIGO measures and how some of these binary systems form. So not only do you need a binary system to form of black holes or neutron stars, in this case, we're talking about black holes, but you need them to form close enough so that they merge within what we call a Hubble time, basically within several million or billion years so that some will be merging now 13 billion years after the Big Bang. It's conceivable to form a binary that would merge, you know, never perhaps or very far from now in terms of time from the Big Bang. So there's this case where you've got two stars, one blows up, becomes a supernova, it turns into a black hole. If the star is enough mass, it becomes a black hole instead of a neutron star and then its companion is now going through this gas envelope which brings them close together. The companion blows up and now you have two black holes and the spins of these black holes tell you something. You can also have B where they're rotating very fast so that they don't expand, they just go straight to the black hole stage and now the spins will look very similar to each other. And finally, you have this star cluster where they're formed, we call it dynamical formation where a star comes in and scatters off a black hole and knocks it into a binary system. It basically pairs it up with another black hole. So these are three of the many possibilities that LIGO explores. So I said that we had observed neutron stars and we have, we've seen pulsars, we've seen this radio wave clocks emitted, you know, at very high frequency, several hundred hertz and a very precise and periodic. Hulse and Taylor studied the first of these binary pulsars which they discovered at the Aracibo Observatory in Puerto Rico and they followed it. If you look at this graph, these are measurements starting in like around 1975 and continuing to 2000. This graph of course has continued, people have continued to measure this pulsar. And what they're looking for is they're looking for how the orbit of this pulsar shrinks and again, how this closest point of approach precesses because that's calculable very precisely from general relativity. And so this Hulse and Taylor and Weisberg did this measurement over many years. This line here is a general relativity fit to the data. So this is convincing proof that gravitational waves do exist as Einstein predicted. It's not the same as direct detection of gravitational waves but it was worthy of I think the 1994 Nobel Prize. Okay, so as a reminder, here again is the test masses being stretched and squeezed by a gravitational wave. And so this naturally leads us to the creation of a detector for gravitational waves and we use something called an interferometer. So in this picture, the light comes in, it's split into two arms and then when it comes back, it recombines at this beam splitter and goes to the right there to the dark port. And when the waves of the light overlaps so that the peak of one overlaps the trough of another, you get cancellation. So you get no light at the dark port. When they overlap so that their peaks align, you get a very bright spot at the dark port. And LIGO, we set up our detector so that we have very little light at the dark port. When a gravitational wave passes through our detector, it makes one arm a little longer and one arm a little shorter. And that's that oscillation that you just saw in that picture. And then this light spot here, this light that we detect is our signature of a gravitational wave. So this is basically a transducer. It turns a gravitational wave strain, induced strain into a light signal, which we then turn into a digital signal with our electronics and detectors. So this is what advanced LIGO looks like in very, very schematic form. As you might imagine, it's quite a bit more complicated than this. We have a laser, very highly stabilized. It goes to this beam splitter and then a split into two long arms. These Fabry-Perot cavities they're called. We have a test mass at each end. The test masses are hung from a pendulum suspension, but they're basically free to move above one hertz. So above any frequency, but with a period of once per second, they basically move inertially in space. This is what this test mass structure looks like. We call this a quadruple suspension. It hangs from an isolation platform in our vacuum system. There's two stages of suspension up here and then there's two more down here. They look just like a pendulum. If you've ever seen a pendulum wall clock, it's basically the same principle. And then this output detector is where we will see the gravitational wave signal. So the light comes in, splits into two, reflects back, recombines at the beam splitter, and then at the output, you will either get darkness or bright light depending on the gravitational wave. So just to give you an idea of the size we're talking about, this is a person working in chamber on one of these suspensions. There's actually two people in here working on the alignment and installation of the suspension. You can see it's resting on a table. It hasn't been bolted to its platform above yet. And here in the lower right corner, we have some of the optics on the inputs. These actually sit on a single stage isolation table. You can see a person here working on those. So these chambers are smaller, but the scale is still quite large. In fact, it's one of the largest high vacuum systems in the world. I think NASA may have a system that's slightly bigger. It's about 10,000 cubic meters of vacuum at 10 to the minus nine torr. Atmospheric pressure is about 10 to the three torr. So it's about a trillion times, a trillion orders of magnitude below atmospheric pressure. The reason we have to keep it in vacuum is that the molecules in residual gas in our vacuum system would contribute noise to our measurement. They would cause the light to scatter and then the light would take, some of the light would scatter and disappear, which is okay. We have lots of light usually. So losing a small fraction is no big deal. But some of the light will hit a gas molecule and stay in the main beam. But now it's picked up the small extra path length and that's going to look like noise because we can't distinguish that extra path length from a path length that would have been caused by a gravitational wave passing through. The LIGO laboratory consists of two observatories, one in Hanford, Washington and the Hanford Nuclear Reservation near the Tri-Cities and one in Livingston, Louisiana, which is where I'm talking to you from tonight. I am here. MIT and Caltech jointly operate the LIGO experiment with funding from the National Science Foundation. The two detectors are located like this. It takes light about 10 milliseconds to travel the 3,000 kilometers or so between the sites. The reason we did it like this is that we want to see a signal at one detector and then within the slight travel time. So within 10 milliseconds, we want to see the same signal at the other detector. That will tell us that we have seen something real and not just an artifact of our electronics or the local environment because the local environment here is not like the local environment 3,000 kilometers away and things that might correlate between the two, we have instruments that can measure those and correct. LIGO is not the only gravitational wave detector in the world. We have four operating detectors. The Geo 600 detector is in Germany. It's a British or UK-German collaboration. It's mainly a test bed for new technologies. It's where the advanced LIGO suspensions were designed and tested. We have the two LIGO detectors and then in Italy, we have Virgo which also just completed an upgrade and it joined an observation run with LIGO on the 1st of August of this year. And I'll talk more about that later. There's a detector being built in Japan which is still a few years away from operation. They've decided to build an underground to get away from some seismic noise that afflicts the other observatories that are built on the surface. They're also going to cool down their test masses to reduce the molecular vibration in the coating on the optics themselves. This molecular vibration, this Brownian motion, this thermal noise, again, also affects our measurement. And then LIGO India, which we hope will come online in the mid 2020s, is a clone of LIGO Hanford and LIGO Livingston. The reason we want to network is we want to have something looking all the time so we don't miss anything, but it also allows us to triangulate the position of an event. And that's important and I'll talk about that later too. The LIGO scientific collaboration of which the LIGO lab is a member contains, I think it must be over a hundred institutions by now. And I see lots of people joined from all over the country. There's probably a university near you with a LIGO group and many of these groups do outreach activities related to LIGO. So we're quite a large collaboration. I think there's something like 800 authors from LIGO on the papers, maybe more at the stage. Virgo has about 280 members. It's made up of scientists from six European countries and these institutions. And it's built in Caschina. It's near Pisa, the famous leaning tower of Pisa Pisa in Italy. Okay, so what have we seen? So some of you have probably heard the story already, but I will tell it again. So 1.3 billion years away, like light years away. So 1.3 billion years ago, we had two black holes. Their orbit decays due to the radiation of gravitational waves and they're going to merge into a bigger black hole. And when they merge, they're going to radiate gravitational waves, powerful gravitational waves. About three solar masses of energy goes into radiating these waves. And at the peak of the merger, this is the brightest object in the universe. It emits more power in that brief instant than the power from all the visible stars in the universe. It's quite amazing. Of course, the signal takes a while to get to earth. And when it does, I hope people can hear this. This is what it sounds like without a frequency. And then we've upped the frequency a little bit in the second one. So you can hear that blip sound. That's what we call a chirp. And a chirp is the signal as the two masses rotate faster and faster until they merge somewhere around here. This is the final moments. This is less than two-tenths of a second, the final moments of the star of this binary black hole system. Existence. And so let me just play this again. So this, and I'll mention the website later, is available on the website. You can hear it much better if you have better speakers than my computer, especially the normal low frequency one. So how do we learn from the strain? I call it HT here. This is the strain as a function of time. So this waveform here is the strain signal as a function of time detected by LIGO. So there's different phases. There's the in spiral where the two black holes are circling each other, slowly losing energy by radiating gravitational waves. And therefore coming closer together, there's the merger phase where they finally collide. This is the most violent part of the process and gives you the maximum amplitude of strain. And then there's this ring down at the end where the final object rings like a bell. It's like a vibration, which is quite hard to see and has not yet been definitively seen. So the phase evolution of these initial waveforms gives you the chirp mass. The chirp mass is something, it's a quantity that's derived from the masses of both of the black holes. And it tells you how fast the object will coalesce. The higher the chirp mass, the faster the final coalescence stage will be. The amplitude of this signal will be modulated by the non-aligned spin components. So if the black holes are spinning, but their spin axes are not aligned with each other, then that will show up here. The ring down frequency, so the frequency of this ring down stage and the cue, like how sharp it is, how damp it is, tell us the mass and spin of the final black hole. And so here on the left is the Hanford, Washington signal that we observed. And here on the right is the Livingston signal overlaid with the inverted Hanford signal. The second row here are the numerical relativity predictions. The third row are the residuals. And I think I say that in text, so let me say that. So there's the time trace from Hanford. Signal is in band for about 0.2 seconds and it's an amplitude of about 10 to the minus 21. So if you remember from earlier in the talk, we said 10 to the minus 21 was easy or at least detectable. We could imagine detecting this. You can see when we invert the Hanford trace because the orientation of the detectors is differs by a minus sign. So we have to invert one signal to match, which is a nice proof that this is a real signal. And then we shifted by 7.1 milliseconds. So the signal arrived at Livingston first and then arrived at Hanford within this light travel time window. Another proof that this was a real signal. These are the best fit waveforms using analytical calculations and numerical relativity. The same fit to the waveform matches the signal at both observatories. The residual shows no evidence of signal if you subtract out the best fit waveform. And then you see this chirp signal. This is a frequency here and this is time. So as time passes, the frequency goes from low to high as these objects go faster and faster in that final two tenths of a second before they merge into a new black hole. This was an important discovery. I'm pleased to say that the 2017 Nobel Prize in physics, half of the prize was awarded to Ray Weiss who really is the father, I think, of this field. And then Barry Barish, the first director of the LIGO lab and Kipthorne who did a lot of the theoretical work that formed the basis for what LIGO does. So after the first detection during our first observation run, we saw a second clear event and a third event which is quite likely to be a BBH, a binary black hole merger. In our just finished second observation run, we have so far, and I've emphasized so far because analysis is still ongoing, we've published three more binary black hole events and the first observation of a binary neutron star merger. Now this binary neutron star merger was one of the class of events that we really built LIGO to observe. LIGO is most sensitive at the frequencies where we expect to see the signal. And so what happened is that there was a neutron star merger on the 17th of August and it was observed by the two LIGO observatories and the Virgo Observatory was also online. And not as sensitive as the LIGO observatories but still able to make a significant contribution as I'll show you in a moment. One of the dreams of gravitational wave physics is that we would see an event that other astronomers could see. These astronomers would then be able to localize the event more precisely than we could and allow us to do more with our signal. It would also tell us a host of other information about what happened. So we call this multi-messenger astronomy to see something in x-rays, gamma rays, neutrinos, radio waves, visible in infrared light and of course gravitational waves. So it had long been speculated that some gamma ray burst signals were from neutron star mergers but had not been proven. This is the signal we saw at Hanford and you can see here, I hope you can see my cursor tracing out this chirp shape at the very end here. The Livingstone detector was more sensitive at the time and so you can see for much longer for the whole width of this graph basically, you can see this gradually curving chirp signal and in Virgo you can't see anything by eye. One thing to note here is that this bottom axis starts at minus 28 seconds. So we see the signal in this graph for close to 30 seconds. It actually was in band for the LIGO detectors for about a hundred seconds. Most of our black holes are a few tenths of a second in time for the signal. This one was a hundred seconds. It's 50 times longer than anything we've seen before. That's because the mass of a neutron star is 1.4 times the mass of our sun. The smallest black hole we've detected is about nine times the mass of our sun and its companion was heavier. So those coalesced much more rapidly in a couple of seconds compared to the neutron stars which took a long time to coalesce, which meant the signal was there but weak for a very long time. But then we were able to accumulate signal for a long period of time and this ended up being the loudest gravitational wave event ever seen at a signal to noise of about 33, 34. We learned that the Fermi gamma ray burst monitor, this is a satellite orbiting the Earth, detected a gamma ray burst two seconds after our signal. You can see here, this is the light curve from the gamma ray burst monitor. Down here at the third panel is from integral another satellite. And so this was a clear signal of a faint gamma ray burst. Normally in LIGO to triangulate to find the position of an object, each detector can basically make this annulus in the sky of where the object might be. And so LIGO Hanford and LIGO Livingston on their own can carve out an annular patch of the sky. Usually it's two disjoint patches because we can use amplitude information to get rid of some pieces. But it's usually many hundreds of square degrees in size. When we use Virgo information, we can shrink this rapidly. And so here I'm showing you this a purplish quasi-circular or elliptical patch on the sky is the localization from the Fermi satellite. This gray band is the localization from the integral satellite. This small green ellipse is from the LIGO and Virgo collaborations localization of the event. Whereas before, I think our best localization had been something like 230 square degrees. The size of this patch of sky is about 30 square degrees. It means that astronomers can look and have a decent chance of finding something. And so we sent an alert to many astronomers who have collaborated with LIGO for this purpose, telling them that we had found this neutron star signal. And I'm showing you again, so this you won't be able to hear, I think even with good speakers for quite a while. This is the neutron star signal compared to five of the previously discovered signals. Just to give you an idea of how long this coalescence takes. So we had a solid, clear, unambiguous neutron star signal associated with the gamma ray burst. We informed the astronomers and the astronomers started pointing their telescopes to this patch of sky and going down through the list. And as I'll show you in a few moments, they quickly discovered, I think we're about to hear something again. I hope people could hear that, especially at the end. That's that chirp sound again. But again, as I said, much longer than the black hole signal and a much cleaner chirp, excuse me, sound at the end. So the probability that the gamma ray and the gravitational wave were within two seconds of each other is a few parts in terms of the minus eight. So, you know, tiny. So almost guaranteed to not be an accidental coincidence. That tells us that binary neutron stars are indeed the progenitors of at least some of the short hard gamma ray bursts which people have been observing for decades now. And this time delay was 1.74 seconds. And that tells us something important as well. We don't know exactly what the delay is between the gamma ray burst or between the gravitational wave from the merger and the gamma ray burst which comes from some of the matter that's been torn apart in that process. But this 1.74 seconds for something that's basically 130 million light years away tells us that gravitational waves and light basically travel at the same speed. Because this, even if there's some physics that has to happen for the gamma ray burst to come, it's going to be of the order of this two seconds. So you can get the speed of light and the speed of gravity to be the same to like a few parts in 10 to the 16. This is a plot showing all of the observatories that did follow-up analysis. The event arrived during the morning in Livingston. It was a little before 8 a.m. local time. So a lot of the Chilean telescopes had to wait basically for nightfall, the next nightfall. There were some problems in Western Australia, actually from a computer hacker that stopped them from controlling their telescopes or they might have been able to observe right away. But six teams found the source of the gravitational waves about 12 hours later. And you can see here where this reticule is. You can see this source. It's in a galaxy, I think NGC 4993. It's about 40 megaparsecs, 130 million light years away. It's in that localized ellipse that LIGO and Virgo gave. And then it fades very fast. You can see it goes away in two weeks. This is one of the companion papers that we published with this discovery, showing all the lines show observations that were made by various detectors. Anytime there's a circle, it means that there was an observation. So you can see we have the gravitational waves and the gamma rays, of course. The X-rays didn't show up until nine days later, seen by the Chandra satellite. We saw UV. This is a logarithmic scale, by the way. It's not linear. So we saw UV and optical and infrared quite soon. And then we finally saw radio about 16 days after the event. And the radio people continue to monitor this. And you can see here the six different discoveries. When we published this paper, about 70 other papers, and I think the archive, which collects physics papers, but something like 130 papers on the day of the announcement, all related to this event. It's one of the most studied astrophysical events ever, if not the most study. GRB 170817A, shown here as the green star. It's the closest short GRB that we've ever observed. Now, it was pretty weak. And one of the reasons it was weak is that we didn't see the gamma ray burst full on. We saw it kind of from an angle from the side. We think the inclination of the orbital plane of the merging binary was about something like 30 degrees to our line of sight. So we didn't see the full jets. Now, that's bad in the sense that you don't get a very strong gamma ray signal, but it's good in other ways. You don't get blinded by that jet that comes out of the merger and you can look behind it. So, I think I put this one in the wrong place. This is just the detector again. So let me... This is it. Yes, it's working. So you can see, I'll pause this a couple of times. You can see this gamma ray burst jet because now the two neutron stars are made of matter. And so when they collide together, they're going to form a radioactive cloud, if you like, from ejected matter. They're going to tear each other apart before they merge and either make a heavy neutron star or a black hole. We can't really tell which the expectation is that they'll make either a low mass black hole or a heavy neutron star. And so this part here, and I maybe should have paused at this part, this central part, people call this the cocoon. This is a very dense area of matter, region of matter after the merger. So we can't see inside because either a black hole or a neutron star is in there, but this is the ejecta from the merger. And in here, something very interesting is going on. It's called an R process where neutron capture is happening at a very, very rapid rate. And this is responsible for the production, and I'll show you the periodic table in a minute, of many of the heavy elements that we're familiar with. Having a NASA satellite be part of your detection, so the NASA people, of course, are very good friends. So this is a periodic table of the elements showing you which of these elements and how much of them, if you like, are produced in this R process after a neutron star merger. And so most of gold, most of iodine, surprisingly, a lot of the lanthanides and the transuranic, well, the uranium, palladium. So these elements come from this process. It's for some of them, it's the only means of production, except maybe for a few like small tens of a percent that may come from elsewhere, but small enough that they don't appear in this graph. So this is, again, an important part of physics that comes from observing this and knowing how it came about and formed this what people call a kilonova or a macronova, and being able to study the light emitted by this promptly after the merger and look for the signature of these elements being formed. So this is the masses, so the smaller ones here, of course, you can see merging into the bigger ones. These are the binary systems that merge that we've discovered so far. There's six of them, including this one with the dotted line which had weaker evidence, but we're about 88% sure that it's a black hole merger. You can see the masses range all the way from up in the mid-60s, around 60, down to less than 10. So the lightest one was one of the ones we detected from the second observation run. And one of our hopes is to fill in this mass spectrum because the stellar graveyard, as people call it, where stars go to die, we have black holes, or if they're not completely done, they can merge with each other and make bigger black holes. These purple circles are the black holes that are part of these X-ray binaries. These are the only black holes in this mass range that were known before LIGO. And you can see we're expanding this mass spectrum and learning more about it. There were many known neutron stars, you can see here at the bottom, but then these two orange dots go into a question mark because we're not exactly sure what it forms, is the merging neutron star system that we discovered. And we hope this is one of many that we will find as LIGO proceeds. What else might we hear? We say hear because the chirps happen in the 100 to 200 Hertz region, which is audio frequency. Well, we're going to have more coalescing binary systems. We're hoping to see something from an asymmetric core collapse supernova, although that has to be pretty close by. There's an object called a cosmic string, which is fairly speculative, which if it develops a kink can emit gravitational waves. Neutron stars that spin very fast, if they have some kind of a deformity on their surface, can emit gravitational waves, maybe continuously. And then of course, what we'd really like is to be able to go back to the first picosecond after the Big Bang. So the cosmic microwave background radiation comes from about 300,000 years after the Big Bang. If we can ever see the gravitational wave background from the Big Bang, that will take us back to the first like instant basically after the Big Bang, to the period of what we believe is inflation. For more information, you can look at www.ligo.caltech.edu, especially the gallery has lots of these pictures and videos more than I've shown you today, and educational resources. So there's lots of nice things there. You can also go to the LIGO scientific collaboration webpage, which again has educational resources on multimedia and information on the detections that we've made so far. So I think that's all I had for you this evening. So here's the... Thank you for passing. Thank you so much. I know everybody's laughing wherever they are. So Dr. O'Reilly, I need to ask here real quickly. And so we were running a little bit late. Do you mind going a little over past our normal time so that we can get to a few more of the questions? Do you mind? That's fine. Okay. So we'll try to be protective of your time because I know it's later there and later still for some of our members on the East Coast. So we do have quite a few questions. And we had a couple, I'll just ask this right off. We had several people asking whether or not you will make your PowerPoint available to us. We can put it on the website for people to download if that's possible. If not, we understand. Yes, I intend to make it public in our DCC, but yes, there's nothing proprietary in here so I can share that. Okay, fantastic. Thank you so much. And so you and I can touch base about that later. So a long time ago, we had that. Let's see. Jenna asked, if we had a large space-based, Lagrange-based LIGO, would it be able to detect reverberations from the Big Bang? But it looks like in one of those last slides, perhaps you can do it with the ground-based ones. Well, some of the, so LIGO, Advanced LIGO is currently what we call a second-generation detector. It's an upgrade from the initial detectors, quite a bit more sophisticated. There are plans, people are already thinking about third-generation ground-based detectors. Some of those might be able to probe the stochastic background, this hiss of noise that comes from the Big Bang. I think the real promise for that though will be something like LISA, which is a space-based gravitational wave detector. And I saw, I think somebody said something about a Lagrange point. This actually orbits the sun. It's about 30 degrees behind the Earth orbit. It's designed to orbit the sun with like arms that are several million kilometers long. And that should be able to probe down to very low frequencies and directly see this stochastic background. Advanced LIGO itself may see incoherent sum of unresolved black holes and neutrons to our mergers. So we don't see the individual ones, but there's this kind of a buzz for many faint ones going off all of us. Okay, well, kind of related to that, or at least it seems somewhat related, if gravitational waves shrink or expand space-time, wouldn't the speed of light in the LIGO arms vary as the space-time changes? This is from Bruce. Light goes at a finite speed, but if the mile is shorter, wouldn't the second be shorter as well? Yeah, so this is a tricky point and came up quite often in the, and there's still people out there who, I think we've all seen the websites where Einstein is wrong, Newton is wrong, that we'll never believe that we can detect anything. But it's a good question. The short answer is that the wavelength of the gravitational waves is much larger than the size of the detector. And we can see the stretching and squeezing is a much larger effect in the spatial components of the metric than in the time components. And so what we're actually measuring is we're measuring the time it takes to go down each arm. You can think of it as the gravitational wave, in this case, is only stretching and squeezing space in these two polarizations. And that's effectively the right answer. If you get to very high frequencies, very high frequency gravitational waves, like 37 kilohertz, then LIGO can't actually see those because the time it takes the wave to transit the detector basically overlaps with itself and we can't distinguish the two. So for the wavelengths we're looking at, we can measure the time delay, just because the wavelengths are so much longer than the size of the detector. Okay, so Alan asked a question, and I'm not entirely certain that I understand what it is, but I'm guessing you will. So Alan asked regarding the resolution for displacement or change in the length of the arms, that it's a very small fraction of the size of a proton, how might one reconcile that with the Heisenberg uncertainty principle? So the distance measurement that we make comes from, if you want to think in terms of quanta, it's trillions of photons hitting the test mass optic. So it's basically an average of the position measured by a large ensemble of photons in our laser beam. So while individually each photon cannot give you a precise measurement, the mean of all of those photons actually gives you a precise measurement. So the average position is what you look at. And that's what of course changes as well when the gravitational wave comes through. And so that's the reason why we can go to these incredibly small distances like 10 to the minus 18 meters. I saw some cool questions about how we determine the size and distance of these objects. Can you talk a little about that? So the size is a prediction of general relativity. The mass of the black holes comes from the wave form. And I don't know, am I still sharing my screen? No. So the way we detect gravitational waves is we do something called matched filtering. So we basically make a template bank of thousands and thousands of waveforms for all conceivable combination of black hole masses, for example, between one solar mass and 99 solar masses. And so when we analyze our data, we basically compare that template to our data and we see if any of them match what's in the data. And so when we find one that matches, and usually we find several dozen that match when we look at what we call our low latency analysis when we're analyzing in real time. But then we go back and we do many, many analysis runs and we find which waveform best matches the signal we've seen. And then we can look and we can say, okay, this waveform which comes from some combination of analytical calculation and numerical relativity had these masses as input parameters. So they had two black holes say of 30 solar masses each and they had spender, they didn't have spin and the orbit was aligned this way and not that way. And so you basically take the best match waveform and that tells you what your signal was to begin with. Does that make sense? Yeah, that's pretty incredible. That's a lot of options when you're looking at it. How about the distance? Is that from other observations that are not gravitational waves? So the distance, if you look at the papers is fairly poorly determined. So the amplitude of the gravitational wave decreases linearly with distance. It doesn't decrease as R squared, it's an amplitude not an intensity. So the amplitude of the gravitational wave that we see because we know, again, if we were close to the merger, we know what the amplitude of the gravitational waves are from our calculations. So that gives us some idea of how far away it is. What we don't know is we don't know what the orientation is. So if the orientation is such that we're looking at it basically down on top of the orbital plane, that's the maximum signal. Whereas if we're looking at an edge on, that can be a minimal signal. And so there can be some angle in there. We don't know that very well. And that contributes. So as you rotate the orbital plane, the signal gets stronger or weaker, which is the same as the object's been closer or further away. So there's quite a large uncertainty in the distance. The really nice thing about the neutron star merger is that it was observed by the astronomers. And now you have light. And so we know the galaxy it was in. And so we have a very, very precise measurement of the distance. And it turns out that, again, with some assumptions about the inclination angle, you can actually do a measurement of the Hubble constant using gravitational waves that doesn't require using any of these, it's called a cosmic distance ladder. It's how people determine the distance to objects. Many of you probably have heard of this in astronomy. How do you know how far away a quasar is? How do you step out to larger and larger distances? Well, gravitational waves can do this in a model-independent way in one step. And one of the papers that came from this neutron star merger is a measurement of the Hubble constant. It's not competitive with the people who do the supernova measurements. Not yet. May never be quite as precise with them, but it does agree with those measurements. And it's a completely different way of getting that answer. One of the questions, and so there's so many questions here. And so this one is interesting. And so with the multi-modal, I guess, detection of this, why no detection of neutrinos? Well, I think we don't know how beamed any neutrinos are from this. We don't know how energetic. We wouldn't expect very many neutrinos anyway. I mean, for supernova 1987A, we might have seen about a dozen neutrinos. We have better neutrinos detectors now. They're difficult to detect. And it could be that there's very few produced in this process. It could be that they're produced in their low energy. It could be that they're isotropic. So your chances of seeing one are very small. If they go in the jet, then they would have been pointed away from us. So that's still an open question. People still hope to see, and they do look, several of the neutrino detectors are part of our alert system. And when we see a gravitational wave, they look for neutrinos in coincidence. All right. I think we have time for maybe one more because it's getting quite late for a lot of you on the East Coast. Okay, it's a two-parter. I can't help myself. I can't help myself. So, Ron Schmitz asking, why does gravity travel at the speed of light? And kind of as a final thought on that, Stuart Myers had a great question about, can this studying gravitational waves help us learn about the true nature of gravity? Okay, so why does gravity travel at the speed of light? I think so the best way of, well, so one way of thinking about it is that gravity interacts with all mass and energy. And if it traveled slower than the speed of light, you would quickly have a lot of contradictions. It's very hard to build a consistent theory of massive gravity, which interacts with itself and still satisfies the tenets of relativity. Namely that no matter what your coordinate system is, you see the same physics happen. So that's kind of one way of looking at it. Another way of looking at it is that the graviton is massless, that when you do the field equations for gravity, the propagator of that field, the thing that carries the force. And if there were a quantum particle called the graviton, it would be required to be massless. If something is massless, it has to travel at the speed of light. Again, if it doesn't travel at the speed of light and it has zero mass, it would quickly violate the tenets of special relativity. The second question, can you learn about the nature of gravity? So what we will learn is we will learn how good Einstein's theory is in the very strong field. So it's very difficult for us to do any tests of gravity where the fields are really strong. So gravity is really weak or space is really stiff, whichever way you want to look at it. So anything you would do in the solar system, for example, is weak field gravity. And so if you're trying to look for discrepancies between Einstein's theory and experimental data, you want to go to where the gravitational forces are really strong and then test the theory there. There are some speculations where people are trying to tie in what we see and maybe some after effects now that we can directly see a black hole form and try to advance some quantum gravity ideas. But that's quite speculative and there's people who think that there's no such thing as quantum gravity, that gravity is just an emergent, if you'd like, force from space time itself. But what we will really learn for sure and what we are already learning is we're testing the general theory of relativity in the strong field limit. All right. Well, thank you very much, Dr. O'Reilly. This is unfortunately we've got a, I apologize to everyone whose questions that we weren't able to get to, but we're run quite long here. And so thank you very much, Dr. O'Reilly, for sharing an evening with us. This is fantastic. I think that we generated a lot of excitement about what you're doing and the future of looking and understanding more about the universe through.