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Black Holes and Neutron Stars: A Merger in Space

The first detection of colliding black holes rocked the scientific world, establishing that gravitational waves are real and that we are able to measure them. More recently, scientists have achieved the first detection of colliding neutron stars, rocking the scientific world again and inaugurating the era of multi-messenger gravitational wave astronomy. In this program, astronomers and astrophysicists discuss the implications of these colossal cosmic mergers, what they create, and how they’re changing our view of the universe.

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MARIO LIVIO: So our first panel participant is a Professor of Physics at Princeton University. Uh, in addition to his studies of gravitational collapse, black hole mergers and cosmic singularities, he designs algorithms to efficiently solve Einstein’s equations on large computer clusters

LIVIO: The next participant is an assistant professor in the Gravitational Wave Physics and Astronomy Center of California State University, Fullerton. She has developed a widely used model for dense matter inside neutron stars. She’s the senior lead on the Extreme Matter group within the LIGO Scientific Collaboration Please welcome Jocelyn Read.

LIVIO: Also joining us. He is the lead astrophysicist in the LIGO Scientific Collaboration. She directs the Center for Interdisciplinary Exploration and Research in Astrophysics at Northwestern University where she is the Distinguished Professor of Physics and Astronomy. Please welcome Vicky Kalogera.

LIVIO: And our final participant this morning is a professor of physics at Syracuse University whose research involves using gravitational-waves observations to understand the nature of the universe. Please welcome Duncan Brown.

LIVIO: You know, suppose that some, somebody from some alien civilization, a very advanced civilization comes to earth and asks us how good are you, what have you achieved? Then, one very good way to answer them would be we have detected gravitational waves from merging black holes and neutron stars. And the reason that this would summarize in an extraordinarily good way our achievement is that number one, on the theoretical side, it would tell these people that we have reached a situation where we understand already that space time is not just the backdrop of cosmic events, as was once thought, but is actually an active participant. And spacetime can stretch. It can curve and warp. Uh, and that gravity is not, we also understood that gravity is not some mysterious force acting across distances in space, but rather it is an expression of the curvature of spacetime.

LIVIO: So the sun, you know, as if it’s standing on a trampoline or something, it’s warping space and then the planets find the shortest paths in that curved space. That’s what gravity is all about. So this is on the theoretical side. Then on the experimental side, the fact that we have detected these gravitational waves, we will tell this alien civilization that we have reached such a level of technology that we are able to detect displacements that are thousands of times smaller than the size of the proton, which is really astounding when you think of this. So with this one sentence, we have detected gravitational waves from merging black holes, we would tell these civilizations where we have reached.

LIVIO:. So let me start with Frans. And we could say, I will make this statement and you will tell me if I’m wrong, that, by the LIGO discovery of gravitational waves from merging black holes, uh, this actually was the very first direct discovery of an Einstein black hole. Am I right?

FRANS PRETORIUS: Yes, yes, I agree. And it’s actually the, the, the history of black holes is fascinating. As probably most people here know, Einstein came up with, with the Theory of Relativity finally after many years in 1915 and within a few months Karl Schwarzschild discovered the first black solution, the theoretical solution that describes what we know as black holes today. And no one took that seriously. It was such an absurd solution that predicted so many things that people couldn’t understand. Even Einstein thought it was just a mathematical oddity. Um, and then in the sixties and seventies though, when perhaps what several people, including Kip Thorne called the golden age of black hole physics, theoreticians finally sort of got an understanding what the solution represents. And yes, there are a lot of bizarre things about it, but we sort of understood it that it’s actually perhaps not quite as strange as what people thought.

PRETORIUS: And astronomers also discovered various objects in the universe, which was very mystifying if they weren’t black holes.

LIVIO: Or good candidates.

PRETORIUS: Good candidates, but then a sort of a strange thing happened if you think that the doubt that was there in the scientific community about black holes and then from the sixties and seventies when these objects are to be discovered, it almost went completely the opposite. That black holes became this thing which almost had to be there. It was almost, for what else could they be out there? But if you think about it, until this LIGO discovery, what evidence, what scientific evidence that we actually have, that there really are black holes that are described by Einstein’s theory.

LIVIO: And by that you mean really this extreme warping of space time.

PRETORIUS: The extreme warping of space time. So it’s in the sense that. So what the evidence that there was before was, it was, I’d say there was incontrovertible evidence that there were very dense, compact, massive objects out there, but that there was no direct evidence that they were black holes of general relativity. In other words, these manifestations of space and time were space and time that were so warped that event horizons formed.

PRETORIUS: But then with LIGO’s discovery, we saw the gravitational waves coming from this warping of space time around two merging black holes. To me, that was the most astonishing thing about seeing their data for the first time, is that it was the first direct evidence for black holes as described by the theory of relativity.

LIVIO: Right? Just to continue on the trampoline example, I mean black holes, they warped the thing so extremely that they essentially punch a hole through the drum. So this was the evidence for that. Jocelyn, neutron stars are extraordinarily dense object. In fact, one cubic inch of neutron star matter has a mass of maybe a billion tons or thereabouts. Now you take such two neutron stars and collide them. So it’s a little bit like a, an accelerator experiment only on a cosmic scale. Tell us a little bit about that.

JOCELYN READ: Yeah. So, uh, so neutron stars are um the remnants left over after massive stars, not quite massive enough to collapse to a black hole uh, reached the end of their lives and they compact down into some. The core of the star is more than the mass of the sun, about one and a half times the mass of our sun, but compacts down into a region about the size of Manhattan. So it’s this phenomenally dense object. And one of the interesting things about these dense objects is that we don’t exactly know what happens at their cores. So the matter in neutron stars is denser than, than any physical matter you might be familiar with. So dense that we don’t anymore have nuclei with electrons kind of fuzzing around them, but the nuclei have been compressed so close together that a neutron star is almost like an atomic nucleus the size of a, of a city.

LIVIO: Let me just stop you for one second. So basically if you take a normal atom, the size of the atom is about 100,000 times bigger than the size of the nucleus, but in this case you have compressed everything so much that the nuclei touch each other. There is no more that space of 100,000 times. Please go on.

READ: Yeah, so, so we have this, this bizarre form of matter and it’s possible when we get to these densities we’re still describing standard model physics, but we don’t understand how to translate that into what happens. And it might not be that there are protons and neutrons like nuclei anymore, but there are just direct quark matter interacting with itself in ways that we can’t create and reproduce in any terrestrial laboratory. So these cosmic accelerators smash neutron stars together, they take some of the densest matter and the gravitational waves trace out the dynamics of this collision. And then they collide and they throw out the source of an array of, of electromagnetic counterpart observations, uh, in this really dramatic, this dramatic kind of event that we now, for the first time observed. So we see the traces of the densest stable objects in the universe, in this panonopoly of observation

LIVIO: Quarks just for those who don’t know are the fundamental particles that make up protons. For example, there are three quarks that make up a proton and so on, and in this case, you really reach those densities where the quarks seem to interact with each other and so on.

LIVIO: Vicky, so we’re talking about collisions of black holes, of neutron stars in this, but how do these systems form at all? How do we get to the situation which, you know, then we have something to collide.

VICKY KALOGERA: Well, um, theoretical astrophysicists have ideas about how these systems form. But we don’t have the perfect answer to your question. So, um, over the years we, uh, we have developed some hypothesis for how this can happen in nature. What we know for sure is that it happens in nature. We know that these binary systems, two black holes in orbit, one around the other, just like the earth is going around the sun. But now imagine two black holes or two neutron stars, like as dense as Jocelyn was describing in orbit one around the other exist in nature, and the way they might form, um, is, uh, there are two classes of potential formation mechanisms. One is you take two stars that, that form, they are born in pairs. They are born one going around the other, unlike our sun, which is a single star as we are.

LIVIO: About 50 or so of all stars are in such pairs.

KALOGERA: Are in binaries. Exactly. So we know from observations with regular telescopes that most of the stars out there actually are born in pairs. So they are born in pairs and they start burning the nuclear fuel. They’re producing a light, uh, they’re gonna go through the nuclear revolution. Eventually their cores are going to collapse and depending on their mass, they’re extremely massive, um 20, 30, 50 times the mass of the sun. Then they’re gonna form black holes. If they’re lucky enough, then at the time of the formation, the two black holes would stay in orbit and the system will not disrupt. And then you’re going to get a binary or you might get a binary with two neutron stars and then eventually they’re gonna come together in this spiral dance. They’re going one around the other, emitting gravitational waves which now we have detected, and then they have no other option but to collide together. Uh, so this can happen with black holes or neutron stars. There’s a whole other class of formation channels that broadly speaking can be described in the following way.

KALOGERA: You can be in a extremely dense stellarly junk, not the kind of region we live in, thankfully. Otherwise we won’t have a quiet planet like we have it. But there are parts of the galaxy where stars are extremely packed together in a small volume. So you have a collection of stars, we call them clusters of stars. And the stars may be individual stars and they’re going to go through their nuclear revolution. And eventually they’re gonna form the death remnants. Neutron stars or black holes. Those, especially the black holes, are going to be, end up being the heaviest objects in that cluster. And as uh, imagine, um, I guess in a glass of, I don’t know, I’m thinking a glass of milk with heavy, I don’t know.

LIVIO: The heavy stuff sinks to the bottom.

KALOGERA: I’m thinking of coffee, coffee beans covered with chocolate. So, uh, so they’re going to sink in the bottom because they are the heaviest things. Then as they sink in the bottom, they’re also moving at high velocity and they’re interacting together and they can actually, even though the black holes were formed separately, they can actually combine together into pairs and then eventually with the emission of gravitational waves they are going to end up colliding. So you have pairs that were born as stars together and they stayed together all their lives and others that went through the dance floor. They had many, many partners along the time, the time of their lifetime. And eventually they collided with some partner.

LIVIO: Let me just add one thing to both of their comments and that is, I mean, they both describes stellar evolution, in stellar evolution and stars basically spend their lives trying to fight gravity. If you don’t have any opposing force, everything because of gravity will collapse to the center. So stars spend their life trying to fight gravity, and they do that by doing nuclear reactions that produce a lot of energy and they, which builds a lot of pressure and that holds them against gravity. But as they go through their nuclear fuels, at some point they ran out of fuels and then gravity finally has the upper hand. And that leads it eventually to these collapse, which forms, neutron stars or black holes.

LIVIO: Duncan, I say to that experimentally. You, you are at the closest from these groups to the experimental side. What does LIGO do, and how does it do it?

DUNCAN BROWN: Sure. So, so LIGO is a, it’s basically a very, very precise ruler for measuring, for measuring length. And to give you an idea of how precise, think about the, you know, you have these two black holes or neutron stars going around each other, many, many, many billions of light years away, producing these ripples in space time. And the physical effect of these ripples in space time is to stretch and squeeze space itself. So gravitational waves, you know, weak gravitational waves are probably passing through this room right now, stretching and squeezing everything in this room, but that’s so tiny that we don’t notice. You know, fortunately for us, there’s no nearby binary’s producing gravitational waves that you would feel and otherwise we don’t live in a quiet planet.

BROWN: so the, the, the stretching and squeezing. So you gave an analogy that that over a couple of miles you’re measuring a distance that is a change in distance that let, that is less than the size of a proton. Or another way of saying that if you think about the distance between the sun and the nearest star to the sun, which is about four light years or so, you’re measuring that distance to the width of a human hair. So that’s how precise the measurement has to be to detect these incredibly weak gravitational waves. And so LIGO itself is both simple and complicated. It’s simple in the sense that it’s an experiment that you can build as an undergraduate in the lab. You take a laser beam, you shine it through a mirror that’s partially silvered, and so half of the laser beam goes this way and half of the laser beam goes the other way.

BROWN: You put another mirror at the end, another mirror this end in this L shape, you bounce the light back so the light comes back towards the what we call the beam splitter. This, this mirror that split the beam, and then the light leaks out towards a a, a, a light detector here, a photodetector right here, and if you set this up so the lengths of those two arms are perfectly balanced, then you can set it up. So if in LIGO we think classically we think of light as a wave and you set this up so the the peaks and troughs of the waves line up and so you get what we call constructive interference coming back towards the laser and destructive interference going towards the photo detector.

LIVIO: Constructive interference is when the light from the two things amplifies each other and destructive is when it kills each other.

BROWN: Right, and so all the light comes back towards the laser when the thing is perfectly balanced, as the gravitational wave passes through, it stretches one arm and squeezes the other arm, and so it upsets that perfect length balance between these two arms. Now in reality, these arms are two and a half miles long with 40 kilogram mirrors at the end of each arm, so as a gravitational wave goes past, it changes this delicate balance of the two arm lengths and some light just leaks out towards this photo detector and that light. The light that’s leaking out towards the photo detector encodes the length change which encodes the ripples in space time, which are the gravitational wave signal

LIVIO: So they hit these things with miles long arms and they measure the thing changing by the length of a proton. This is the experiment. This is why I said if you tell somebody we have achieved this, you know, there is nothing more dramatic that you can say that we have achieved from a technological perspective.

KALOGERA: It is my, it is actually the most precise measurement we’ve ever made. Humans in any, in any field of science. Technology.

BROWN: Yes, and I think coming back to your question to Frans, you know, have we directly detected black holes? You know, we, I think everyone believes that there was firm evidence for black holes, for observing the light given off as material falls into black holes or the light given off by stars orbiting black holes. But with LIGO, you actually, those gravitational waves, those ripples in space came from the black holes themselves. So this machine reached out and touched the event horizon of those black holes with a machine that we built here on earth.

LIVIO: Right? So Frans, we’ve now talked a little bit about how these things emit gravitational waves and what not and so on. And these are these perturbation in spacetime. Tell us a little bit about that, and also tell us a little bit about how do you simulate what happens in these things.

PRETORIUS: Right? Right. So I guess as people have talked about and Duncan, so these gravitational waves or ripples in space and time. So Einstein’s theory of general relativity is not a theory about a force of gravity. It’s a theory about space and time and it says that space, space and time or space time can be described by a geometric structure, distances by things separated by a certain amount of space or distances in time. We are moving in time as we speak now, um, and what I…

LIVIO: Too fast unfortunately.

PRETORIUS: And so, and what Einstein’s theory says. So, you know, like what Mario described in the beginning. I mean if you think something like the earth or the sun is producing this sort of bend sheet, which sort of represents the curvature of spacetime. But when, uh, when objects accelerate, when they move. And one example of acceleration is two black holes or two stars orbiting each other, well the earth going around the sun, would actually be producing gravitational wave right now, but that kind of acceleration produces these little ripples in the geometry of space and time. They propagate out at the speed of light and as Duncan has mentioned, the exact details of how, of the shapes of these ripples encodes, what produced them, so you can imagine there’s just some two things going around like that. That produces a very nice sinusoidal pattern in the wave.

LIVIO: Let. Let me just stop you for one second. One of the things about Einstein’s theory is that gravity doesn’t act instantaneously. It propagates at the speed of light, so these disturbances propagate at the speed of light. That is a very important feature of Einstein’s general relativity. Please go on.

PRETORIUS: We so so that that perhaps brings to one problem with something like LIGO in the sense that as with Duncan described, it’s not a telescope, it’s not focusing gravitational waves onto an image, so we can sort of see what things look like in gravitational waves. Perhaps a better analog is a seismometer. There’s something that produces an earthquake in certain space and time. These ripples propagate and LIGO measures these little fluctuations, so now say, okay, we’ve seen some ripple. What does that mean? What does it represent? How can we say that it’s two black holes that collided. It was two neutron stars collided, and the way that we do that is we try to solve the Einstein equations using various theoretical tools, pencil and paper methods, computer simulations, and try to predict what Einstein’s theory says for each one of these systems and vary the parameters of the system. And then when then for the events at LIGO saw, we take these waveforms, these little ripples, and we try to match them up to our various predictions and when we get good matches we can say that the parameters acquainted the simulations. That is how we interpret the ripples.

LIVIO: Maybe we can have the the video? So here we have these two objects and they spiral around each other and you can see that they are getting closer and closer. And they emit these ripples which are these gravitational waves. And the frequency of the waves becomes higher and higher the closer they get together. And eventually boom, they merge together. And in the case of neutron stars, they produce this thing which we happen to call a kilonova simply because it’s about a thousand times brighter than what we call a normal nova. And basically that’s what we’ve seen. So please go on.

PRETORIUS: The problem with these simulations. The Einstein equations are very, very complicated and that’s perfect for computers. They don’t care about complication except it just takes a lot of processing power, a lot of computers. So for example, to do an actual simulation, like something like that, which lasts for perhaps a few dozen orbits, the merger calculates how the gravitational waves propagate. It might take anywhere from say 100,000 to a few million CPU hours. So your typical, like the power in your cell phone these days, cellphones are.

LIVIO: So don’t try this at home.

PRETORIUS: So if you have like a million cell phones all working together for an hour. They’d be able to produce one waveform.

LIVIO: Right. Can you just address this? I mean there is this thing that is known as the Chirp.

PRETORIUS: Yeah. So right, so let me explain. So I said I get to the structure that signal tells us sort of what’s going on, um, and for, for, for binary black holes, that’s called this chirp waveform. And so what’s happening is, so now we’ve got these two black holes of in orbit. So when they’re pretty far apart, um, it’s a, it’s a very nice, essentially circular orbit. And I said, you know, we get through the sinusoidal wave and you can actually also, it’s nice for these black hole mergers because the frequency that they’re emitting at it is in the audio range. You can almost, you can say, LIGO is listening to the sounds of the universe, so say two black holes, they were orbiting at a certain frequency. So it’s at, when they’re far apart, it’s essentially a monotonic frequency. So just a very pure tone. Um, but this, this orbital motion produces gravitational waves and that drains energy from the system. And so that’s why two black holes that are in orbit, they’re not going to stay in orbit because they lose the energy to gravitational waves. And the way that that affects the system is that they start, they start getting more, more bound, they start moving, they’re spiraling closer to each other. So when they’re closer it to each other, um, they, they have to go faster in some sense, you know, now there’s a stronger, if you will, gravitational force, so they have to start moving faster. So the sine wave that they’re emitting increases in frequency, so with the period decreases and it also increases in amplitude because they’re going faster, but now they’re going faster so they emit more gravitational wave energy.

LIVIO: So the sound is stronger but the pitch is higher, right?

PRETORIUS: So, so, so it’s sort of a runaway process. And so they start spiraling around faster and faster and faster. And eventually they collide and it’s at, it’s, if you think of like what, what happens to this monotonic, well almost monotonic wave that’s increasing in amplitude and frequency. And it’s as this woop sound like, that’s what’s called the Chirp. And then finally when they collide together there’s that final burst of the Chirp. And then very, very quickly, in fact, astonishingly quickly, the black hole, the, the two black holes, they merge into a single larger black hole and it sells down to a stationary black hole that’s completely quiet. It doesn’t emit gravitational waves and they’ve had final stages called the ring down phase. And you can almost think of it’s like these two black holes. They’re busy chirping because of the motion. They smashed together. It’s like having taken a bell and like giving it a big sort of. You hit it with a hammer. Now bells are very efficient at ringing. They ring, they make a good sound for a long time. Black holes are terrible bells. It’s like throwing two pieces of party together. They almost stopped ringing immediately, which is actually an astonishing prediction. Like how can objects stop ringing that quickly? Be it black holes or like that.

LIVIO: Thank you. Now Jocelyn, I want to ask you something but I’ll just make a small introduction to my question so that it will become clear. So for example, on earth we have tides and the tides are because of the moon. Basically what happens is that the point that is closest to the moon feels a little bit of a stronger gravity than the center of the Earth, and so the sea goes a little bit higher. Similar thing happens at the farthest from the moon point because there, it’s the center of the earth that moves a little bit farther. So still the, the sea is higher, now believe it or not, these two neutron stars, even though they only are, they have a mass, a little bit larger, larger than the sun, but they are just six miles, you know, in radius. They still, when they get very close, they can raise tights like this. So tell us a little bit about this and how can you tell whether there are tides or not?

READ: Yeah. So, so as, as Frans said, so the, the gravitational wave is draining energy from the system and that’s ah, and then so that’s changing how the orbit happens. And then slowly as the stars get closer and closer together, um, different features of the stars come into play in the dynamics of how they’re orbiting. So when they’re far apart, the only thing that really matters is their mass, uh, depending on the mass of the star, they’ll, they’ll orbit at a particular frequency and the frequency changes in a particular way. As they get closer, the influences of their spins and how different the masses starts to come into play. And then as they get very close to each other, the stars of tides become more and more significant. And what happens is, in addition to energy being drained away by the gravitational waves, some of that energy goes into deforming the star.

READ: So you can think of a star at rest has a certain energy and to deform the star to raise the tides, that changes the energy. It pulls the star out of equilibrium and that, that takes energy to do. So, the forming the tides is another drain on energy. And so it causes the inspiral to accelerate. Um, and then of course, you know, there, there’s also a factor that depending on the size of the star, they eventually crash into each other. And if they’re very compact, they can do a few extra orbits, if they are large, they’ll, they’ll interfere with each other more quickly. So the Chirp at the very end of the pattern of the ripples will either get accelerated by tides and then cut off by the stars smashing together. Or if the stars are very compact, the tides are weaker, the, it’s harder to disrupt a very compact star than a larger, fluffier star. So that more compact stars will continue their, their black hole like orbit, and then eventually merge in a more simplified way. So that’s encoded in the pattern.

LIVIO: Basically, how, how much you can deform the star by these tides depends on well we called these, the equation of state. Basically, it depends on how hard it is to deform it, how, you know, how does it respond to trying to change, you know, by applying a force to it out, trying to change that. So basically, isn’t it true that what happens is that these tides at some level tell us this equation of state, I mean, how the pressure depends on the density of the star and so on.

READ: Right. So, so we’re, we’re trying to find out what are the properties of the mysterious matter in the core of the stars. And the key property that we’re interested in is at a given density, how much pressure does that matter provide? So this, uh, this is matters’ last stand against the crush of gravity, so it has to be providing a lot of pressure at a very high density, but if that pressure is low, the collapse happens a little further and you have a compact star. If the pressure is high, you have a larger star. And the larger the star is the larger the impact of the tides, the outer material of the star is farther away, and it’s easier to be disturbed by the tidal force of the other star. So a larger star deforms more, pulls more energy from the system.

LIVIO: Vicky, we mentioned this a little bit, but I want to get a little bit into the more nitty gritty. Black holes are, you should understand that black holes are actually very simple objects. They are characterized by two numbers. Well three in principle, but one is the mass. The second is the spin, how fast they rotate, and the third principle is their electric charge, but in astrophysical objects, there is no real electric charge. So it’s two numbers basically. But from these things we can try to determine the mass and the spin and also the orientation of the spin, namely is it rotating around these axes or that axis, and so on. Well and also things like the distance and things like- Walk us a little bit through how do you determine all these parameters from the observations?

KALOGERA: Yes. Um, so it goes back to what, uh, I think Frans mentioned it first, the chirp. Okay. So, uh, all this information about the mass and the spin and the distance of the source of these, uh, uh, dense spiral dances and eventual, uh, collisions and the tides that are all encoded in that chirp we’re observing with the gravitational wave detectors. What we measured directly is basically the squeezing and stretching and squeezing and stretching of the space time here on earth, which is the, uh, is the propagation of the wave that started from the source, when the wave is reaching us it’s disturbing our spacetime here. And what we measure is the amplitude of the wave. We measure the frequency of the wave and we also measure the duration of the wave. And during that duration of the signal, the frequency is not steady, but it actually changes.

KALOGERA: So we can measure what we call a frequency derivative, so the frequency of the wave is not a simple sinusoid as you were saying, but the peaks of the, um, of the sinusoid that are coming closer and closer together.

LIVIO: The rate of change.

KALOGERA: The frequency is changing. And we can measure that rate with which the frequency is changing. So we have amplitude, frequency and frequency derivative, uh, so that rate of change of the frequency. So these three are the fundamental measurements we make, uh, and, and we can use these three fundamental properties with a little bit extra information that is hard to get into right now. Uh, but these are the three main things that allow us to decode masses, spins and the distance because the amplitude of the wave is telling us something about how far away is the source and also how massive the object are. And the change of the amplitude with time and the change of the frequency with time is also telling us something about the spins. Sometimes, not always. Sometimes we go straight in the spins, but sometimes we get very weak constraints on the spins.

LIVIO: Maybe we can have an image just so in the case of the neutron stars in particular, we also observed this in ultraviolet light, optical light and so on. And we have an image of what was actually observed with a swift experiment. So look in that box and you will see that thing which suddenly, well you see the period disappearing ah, it’s in a galaxy that’s, well has a telephone number of 4993 and so on, but um, so just realize that everything they told you, these are things that happen sometimes billions of light years away. So this is the type of thing we see and it is from that type of information that you get all these details that you see here. And that leads me to you Duncan because in this case of the neutron star merger are, there was somewhat of a mishap in that in one of, you see they have this alert system where when something happened because they want to alert other telescopes and this and that, when this neutron star merger happened, it, the alert worked in one of the locations but not in the other. Tell us a little bit of what happened then.

BROWN: That’s right. So, so to set the context is, as Frans said, gravitational wave detectors are like listening to the universe. You’re listening to the sounds of space time, the ripples and gravitational wave detectors don’t just look in one direction. They look in all different directions. So just like my ears can hear sounds from all over this room. If someone over there made a noise, then I could say, okay, my, my brain can localize that. The sound is coming from over there and I can turn, I can point my eyes and I can look over there and I can see what’s going on. And that’s what we wanted to do with, with gravitational wave detectors and electromagnetic telescopes. We wanted to say, let’s listen out for gravitational wave signal coming from somewhere on the sky, somewhere out there in the universe. And then when we hear this signaling gravitational ways to then say, point telescopes over there and look over there and see if you can see light in all its different forms coming from these two objects colliding.

BROWN: So we had a system that was set up that would alert us to these, these, uh, these gravitational wave detections. And in order to figure out where sounds are coming from, I have two ears and a very complicated signal processing unit inside my head. So I can, I can detect and I can localize sources. With gravitational wave detectors, we have three detectives. There’s the two US LIGO detectors, one in Washington state, one in Livingston. And then there’s a French Italian detector just outside Pisa in Italy. So these three detectives make a network and the idea is they triangulate on the sky, where were the sources? And so you can say gravitation wave over there, go look. And so they all have to work together. All three detectors have to work together in order to figure out where the source is, like my two ears have to work together.

BROWN: Unfortunately what happened was these gravitational wave detectors are incredibly sensitive instruments, meaning as we said, you’re measuring changes in length that are smaller than a proton. And the real world is a noisy, messy, dirty place. Things glitch, things ping. There is all these complicated control loops that keep the detector stable. And what happened on the morning of August 17th when we, uh, when we detected the binary neutron star is the LIGO Hanford detector, one of the detectors as computer software that scans continuously for these patterns, these chirp patterns of gravitational waves looking for these signals in the detector data and the software that was looking at the Hanford data in Washington state said, I think I’ve seen something really interesting. You should take a look at this. And it sends out alerts to people’s cell phones and emails and people start to get text messages saying, hey, hey, come and take a look at this. And we looked and I used to tell my students, you know, when we finally see a binary neutron star signal, you know, won’t be that obvious. We’ll be pulling out the noise. We’ll look. And it was just obvious. It was just a beautiful thing. I’m like, okay, well that’s clearly. There you go.

BROWN: I’m happy to be wrong. That look, you can see a chirp. There it is. Um, but it was, the computers only found that in the Hanford detector, they didn’t find it in Virgo and they didn’t find it in Livingston, which is key to knowing where on the sky that it is. And so this is where the humans come in. This is why people get PhDs and spent many years. I tried to understand what’s going on in these detectors and figuring out what’s going on, so the humans came along and said, okay, we have to take over, the computers have failed us. What’s going on? And and we realized very quickly a team of people who’ve dedicated, large fractions of their lives to understanding how these detectors work, realize that there was a glitch, what you call a glitch in the Livingston detectors, like having a scratch on a record or a or a CD.

BROWN: You’re listening to the music and you suddenly hear a click and the computers are trained to ignore clips and pings and things that don’t look like gravitational waves. So they said, oh, we’re ignoring that. Nevermind. No signal there. Nothing to see here. So the humans actually went in and said, no, no, no. Look, we can see a signal in Livingston, we can see a signal in Hanford. There’s just this giant glitch in Livingston at the same time that the humans can figure out what the computers couldn’t. So what we did is we leapt into action, understood where the glitch came from. This is kind of a frenzied, maybe four or five hours on that morning trying to figure out where, the where, you know, what had gone wrong in the instrument, how to remove this and excise it from the detector data, reanalyze the data together with Virgo and Hanford, and Livingston, bring the detector from, the data from all three detectors together, so then finally triangulate the source and say it’s there and send an email out to the observing community to say, here’s the patch on the sky where we think the signal is, go point the telescopes. And fortunately we managed to do that quickly enough that we didn’t really lose that much. I think if we’d move very, very quickly, telescopes in South Africa, it would have been overhead in South Africa. We could have caught some very important information from the early parts of the, uh, the signal. So. So next time we’re going to do this better, but we got pretty much everything you could have.

LIVIO: Vicky, you wanted to say something.

KALOGERA: Yeah, I wanted to say that the, that we were, as you said, we were a little lucky because we only took four or five hours, but these early four or five hours. Uh, the source was in the southern sky and those are four or five hours was daylight in the, in the south. So even if we had the early localization, it was still daylight even in southern South Africa. So we were still delayed by a little bit, but we didn’t miss the whole day. We missed only maybe a few hours at the end of the day. Yeah. So it could have been worse.

LIVIO: Can you imagine. Here is an event that was perhaps the most exciting scientific event in the almost history of humankind. And you had the glitch at one of the sites of the experiments. I mean, this is how science works.

KALOGERA: But humans saved the day.

LIVIO: Yes, yes, yes, yes.

LIVIO: I want to thank these wonderful panels. So please join us in a round of applause.

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Black Holes and Neutron Stars: A Merger in Space

The first detection of colliding black holes rocked the scientific world, establishing that gravitational waves are real and that we are able to measure them. More recently, scientists have achieved the first detection of colliding neutron stars, rocking the scientific world again and inaugurating the era of multi-messenger gravitational wave astronomy. In this program, astronomers and astrophysicists discuss the implications of these colossal cosmic mergers, what they create, and how they’re changing our view of the universe.

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Moderator

Mario LivioAstrophysicist, Author

Dr. Mario Livio is an astrophysicist, a best-selling author, and a popular speaker. He is a Fellow of the American Association for the Advancement of Science. He has published more than 400 scientific papers on topics ranging from Dark Energy and cosmology to black holes and extrasolar planets.

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Participants

Duncan BrownAstrophysicist

Duncan Brown is the Charles Brightman Professor of Physics at Syracuse University. Brown has worked in gravitational-wave astronomy for 20 years and played a lead role in LIGO’s discovery of binary black hole and binary neutron star collisions. Brown’s research involves using gravitational-wave observations to understand the nature of the universe.

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Jocelyn ReadAstrophysicist

Jocelyn Read is an assistant professor in the Gravitational Wave Physics and Astronomy Center of California State University, Fullerton. She has spent more than a decade studying neutron star astrophysics and gravitational waves.

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Vicky KalogeraAstrophysicist

Vicky Kalogera directs the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), and is the Daniel I. Linzer Distinguished University Professor in the Department of Physics and Astronomy at Northwestern. Kalogera is lead astrophysicist in the LIGO Scientific Collaboration (LSC)

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Frans PretoriusPhysicist

Frans Pretorius is a Professor of Physics at Princeton University. His primary field of research is general relativity, specializing in numerical solution of the Einstein equations.

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