“The 100-Year Quest for Einstein’s Elusive Gravitational Waves”

Nergis Mavalvala speaking at a podium

Nergis Mavalvala is a physicist whose research focuses on the detection of gravitational waves and quantum measurement science; and the Curtis and Kathleen Marble Professor of Astrophysics at the Massachusetts Institute of Technology and a 2010 recipient of a MacArthur Fellowship.

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See the honorary degree citation read by President Biddy Martin at the 2019 Commencement ceremony.


Kate Follette: Hi everyone. Thanks for joining us today. I'm Kate Follette. I'm an assistant professor of astronomy here and it's an honor and a great personal pleasure to welcome astrophysicist Nergis Mavalvala to Amherst today. Professor Mavalvala is the Curtis and Kathleen Marble Professor of Astrophysics and Associate Head of the Department of Physics at the Massachusetts Institute of Technology. She's a leading member of the team at LIGO, the laser interferometer gravitational wave observatory that on February 11, 2016 announced that they had detected gravitational waves from colliding black holes. They've also had several interesting discoveries since then. Although their existence was predicted by Einstein and his theory of general relativity, prior to the LIGO detection, gravitational waves had not been directly observed. Professor Mavalvala's work toward this breakthrough and since has helped to usher in a new era of astrophysics, opening a window into parts of the universe that do not emit light and allowing us to look back into the very first seconds after the big bang.

Kate Follette: Since her days as a graduate student at MIT, Professor Mavalvala has been a pioneer in conducting experiments aimed at rendering abstract and theoretical phenomena, tangible. The glee and creativity with which she does this is inspiring. I imagine many of you will leave this lecture today with a greatly enhanced understanding of and appreciation for astrophysics and quantum physics. Born in Lahore and raised in Karachi, Pakistan, Professor Mavalvala came to the United States in 1986 to attend Wellesley College where she studied physics and astronomy. After receiving her Ph.D. from the Massachusetts Institute of Technology, she did postdoctoral work at the California Institute of Technology and then joined MIT as a faculty member. She's the recipient of numerous recognitions including a MacArthur Fellowship and along with the rest of the LIGO team, the breakthrough prize in fundamental physics and the Gruber Prize in cosmology. Please join me in giving a warm welcome to Professor Mavalvala for her talk entitled "The 100-year quest for Einstein's elusive gravitational waves. "


Nergis Mavavala: Thank you, Kate, and thank you to colleagues and friends at Amherst for having me here today. So, it's always a bit embarrassing to hear all these things about yourself and say, wait, did I really do that? So I want to preface much of my talk by reminding everybody that the discoveries that I'm going to talk about actually is not the work of myself alone but in fact a team of just a little over a thousand scientists. So please treat me today as the messenger. So the story I'm going to tell you is about this discovery that made a great big splashy in even the general media in 2016 when we announced the first detection of gravitational waves and led to the 2017 a Nobel Prize in physics that went to these gentlemen who were the founders of the experiment that I will describe to you.

Nergis Mavavala: So this was...I like the title of, of this announcement of by the Nobel Committee that said, the 2017 Nobel prize in physics goes for a discovery that shook the world. And I like this cause it's both literally and figuratively true. Literally because gravitational waves are ever so imperceptibly shaking us and our world and literally because it led to a big storm of attention to these discoveries. So what I want to do for you today is to sort of unpack this beyond just the headlines and give you a sense of what was done and why even you should be excited. Okay, very good. I will come back to Nobel laureate Ray Weiss at MIT and Barry Barish and Kip Thorne at Caltech. But that sets the stage. Now I stop. I titled my talk...I said, we know we claim we are opening a new window into the universe, gravitational waves

Nergis Mavavala: have opened a new window to the universe. Headline number one. What does that mean? Well, if you open a new window into a universe, the first thing you have to know is what was the old window and the old window it turns out is light. And we know that humans have been looking into the sky for millennia at light. The starlight and light reflected off the moon and the planets in the sky. And that has been our messenger until recently for what the universe is made of. So here, this used to be one of my favorite pictures in astronomy. It isn't anymore. I'll come back to what's my new favorite. But this used to be my favorite picture astronomy because it's very colorful and everybody likes color. But this is a real object. This is an object called Cassiopeia A and it's very special as an image because it's actually made with three different space telescopes, in that, that look at three different colors of light.

Nergis Mavavala: So the red, the red and orangey stuff is actually infrared light. So it's the lowest energy light that's in this picture. The greens and the yellows are our optical emission, which is what our own human eyes can see. For the red part, you would need snake eyes. And then the blues and greens over here for the blues or purples over here are extra emission. Now, why is this so important? Well, it's important because by piecing together observations with these three colors of light, we can actually say something about what this object was. So Cassiopeia A, until about 350 years ago, was a star that was happily living its life, but then it ran out of nuclear fuel as most stars do. And when it does that, it stops shining. And because they stopped shining, the pressure of the light going outwards doesn't hold up the star against its own gravity anymore.

Nergis Mavavala: So this star imploded. And in that implosion, it has a shockwave that goes out. And this is the material that was spit out from that process. It's called a supernova. Now, remarkably, if you look very carefully at the very center of this object, you see this little blue dot. Well, this little blue is only visible with the x-ray emission. And what this is this is the remnant. This is the new star that was born when this old large star went supernova and it's called a neutron star. It's a star, a very, very peculiar object. It's an object that's got the mass of about our sun, but it has a radius of about 10 kilometers. So about the size of a little bit bigger than Amherst. Okay. Now if this parent star that that exploded had been heavier than it was, this one happened to be a couple of times the mass of our sun.

Nergis Mavavala: If it had been three, four, five or 10 times the mass of our sun, this object would have continued to collapse under its own gravity and it would have turned into a black hole. So lesson number one, neutron stars and black holes are cousins, black holes, heavy, neutron stars lighter. Okay. Now imagine for a moment that instead of being a neutron star, this was a black hole. Would this x-ray emission have been emitted from this object? And the answer is no because, by definition, the black hole is an object that has so much gravity that even light can escape its gravitational pull. So that's the other, distinction. Neutron stars can emit light whereas black holes usually don't, okay. There are some exceptions, but mostly not. So how might we then go about looking for black holes and why might we care to look for black holes?

Nergis Mavavala: Well, the first question is easy to answer. Black holes are among the building blocks of our universe. So if we want to understand what, how the universe came to be, we have to add black holes to that recipe. And so that's why we like to study them. But how? They're dark. And then comes our next messenger. And that is gravity's messenger. We know that black holes may not emit light, but they have a lot of gravity, they have a lot of mass, and therefore we should be able to observe them through their gravity. So let's look a little bit at how gravity works. Now, gravity is one of the most successful early triumphs of quantitative science. And Sir Isaac Newton in the 17th century already had a universal. Note the grandioseness of the name: the universal law of gravitation.

Nergis Mavavala: And it was quantified in terms of a force that two objects of mass m one and m two feel mutually among themselves, that's proportional to their masses and inversely proportional to the square of the distance between them. It's a formula we all learned even by high school physics. Now Newton himself worried about something that never got resolved in his lifetime. He asked the question, how does mass one know about mass two? Today that's some distance. They are usually at astronomical distances. How do they know about each other? In the modern language of physics, we would say what mediates that force, but really that was not the answer. Turns out even Aristotle and the ancients worried about this action at a distance problem. How do we influence things at a distance? What carries that information? Now that, for gravity, did not get solved until our next hero of gravity shows up in the early 20th century.

Nergis Mavavala: And that was Einstein. And Einstein told us something very radical. Einstein said gravity's not a force. Gravity is geometry. So what does this mean? This basically says, Einstein's view of the universe is that the whole universe is all of spacetime acts like a rubber sheet. And when you put a massive object in the center of this rubber sheet, as you might put a bowling ball on the center of a trampoline, that rubber sheet gets deformed by the massive object. And gravity is the manifestation of that curvature, that deformation. So in the case of the bowling ball on the trampoline, then if you put a little plain marble at the edge of that trampoline, what's the plain marble going to do? It's going to fall into the bowling ball because it's following the curvature of the trampoline. And now take away the trampoline.

Nergis Mavavala: And imagine all of empty space does that. You put massive objects there. The space deforms. You put another object nearby. It feels that the other, the initial object because of the curvature of spacetime, Einstein, like Newton, wrote a formula and like Newton's formula, it looks very benign. It turns out to be one of the most difficult to solve, rather beastly formulas ever written. And in part because it's a formula that because of all these indices has many parts to it. And in part also because the left-hand side is included also in the right-hand side. So you have to really solve it with rather sophisticated math and that has been one of the challenges of why it's taken so long to fully understand Einstein's theory of general relativity.

Nergis Mavavala: Now Einstein also--so here is a picture of a star that's deforming the spacetime around it and it could be our star cause there's a little sweet planet there. Einstein also saw something else that came out of that formula that really popped out of his equations. And that was the question he asked was what happens if you actually take the massive object and instead of it sitting still, what if you accelerated? What if it's vibrating, moving, orbiting something? And then he found, much to his surprise, alarm and dismay, that the spacetime wouldn't just deform, it would ripple. So this is the concept of a gravitational wave. It's a wave. That's the actual rippling of spacetime itself that's caused by massive objects that are accelerating. In this case, it could be a pair of neutron stars and black holes orbiting each other.

Nergis Mavavala: But there are other ways to generate them as well. So gravitational waves then are the ripples of spacetime. It's south of you. Come back to the trampoline analogy. It would be just if you took that trampoline and you took a pair of drumsticks and started to hammer on it and little waves would go out, okay. And they emanate outwards from where you're drumming or if you dropped a rock on a surface of a still pond and the waves on the surface travel outwards. Those are sort of the two-dimensional analogies you can come up with for these ripples of spacetime. Now, Einstein himself was very ambivalent about these in between 1915 and 1918 he gave us a complete period of general relativity, including the formulation of the gravitational wave. In 1918 another physicist, Schwarzschild, took Einstein's equations and proposed stars that have so much gravity that light can't escape their pull.

Nergis Mavavala: Today we know these as black holes, at the time they were called dark stars. There's correspondence between Einstein and Schwarzschild where Einstein didn't think nature would do something that absurd, even though it came out of his own equations. And he had, he had a lot of doubt about it. The math said one thing, but he didn't fully appreciate that the universe and nature would do the things that the math said. So he vacillated about gravitational waves. And in 1936, he actually submitted a paper retracting their very existence, and then in the same year he retracted the retraction. And so you can see that he had a very sort of ambivalent relationship with it. But I have to say his ambivalence was justified and his ambivalence was justified because already by this very early 1916 paper, he had understood that gravitational waves are extremely weak, very, very faint.

Nergis Mavavala: And in fact, in the 1916 paper, he wrote something to the effect and in German, they will have no practical purpose ever. Okay. So he dismissed, them but his vacillation actually was, he wasn't even sure they existed, you know, leave alone be detectable. And the reason was that if you take ordinary stars like the stars that were known in his time, stars like our own sun, and you ask what amount of gravitational waves with these emit, it is really laughably, laughably small and so small that you shouldn't contemplate it. But when you get to things like black holes and neutron stars that are much, much more compact, all that gravity compressed into a smaller volume, then you start to see larger amounts of gravitational waves. And so his ambivalence was justified. Black holes and neutron stars we're not actually discovered until the 1960s and he died in 1955. So that was on [inaudible]

Nergis Mavavala: But the most remarkable thing when I look at this history, the most remarkable thing is that those equations that he wrote actually gave us a very detailed description of space and time and black holes. And so now we fast forward to 2006 or so, a little over 10 years ago. And it took all of those years from 1916 till 2006 to solve Einstein's equations numerically using super computers to get our first models of what would happen if two black holes were orbiting each other. So here is a movie. Initially in the early days, people used to make cartoons and now this is an actual movie, not of an observation of a black hole, but of the output of a computer code that solves Einstein's equations. Okay, numerically. So here we go. This is two black holes orbiting each other. And when they're far apart, notice they each have their own little funnel of space, time curvature.

Nergis Mavavala: And those two funnels don't talk to each other. These are far enough apart. But now as these black holes orbit each other, they are emitting gravitational waves and those waves are carrying away energy and that energy comes from the orbits. So the two black holes are getting closer and closer to each other. And on down below here you see how the signal is building up. These are spacetime ripples and eventually, the movie will slow down because you'll see these two funnels start to notice each other. And as the black holes get closer and closer, spacetime gets very, very deformed. Movie slows down. And then you'll see at the moment that the horizons or the shrouds of the two black holes touch, you'll get this incredible distortion. It's destruction of spacetime. You get a maximum in the signal here and then the two black holes merge into a single black hole that wobbles for a little bit and then just becomes a quiescent object sitting darkly, never to be seen again.

Nergis Mavavala: Okay? That's the story of this particular pair of black holes. And you can see a long list of institutions that were involved in making this movie. And this is one of the simplest black holes you can make. They don't really spin around each other. They don't really point in different directions and it's taken that long to solve those equations of Einstein. But it wasn't always. So I feel now go back to like the 1960s this man Kip Thorne, one of our Nobel Laureates, was among the early people...this was shortly after neutron stars were first discovered observationally using radio telescope. Kip Thorne asked the question, what would gravitational wave signals look like from a pair of neutron stars that are orbiting each other in a galaxy not too far away from our own? And he was among the first to show that the strain, by the way, is the amplitude of the gravitational wave versus time that it would have this characteristic signature, which is this pattern is called a chirp because it basically gets faster in frequency and grows louder. The amplitude and the frequency grow with time. And he was also among the first to put a scale on it, that the amplitude of this wave would be 10 to the minus 21 which is an awfully small number. But until now I've put no units and I've given you no scaling with which to think about it. So you should be sanguine. Okay.

Nergis Mavavala: Then around the same time the question came up of how one might detect gravitational waves. Remember there's this storm of activity. Neutron stars have been discovered. Black holes were discovered only two or three years later. And so people are thinking about it. So to detect gravitational waves, we need to say a little bit about what do they do here on the earth. Not that spacetime distortion that I showed you in the simulation, but here on the earth. So what do they do? They are ripples of spacetime. So you have already that picture in your mind. The waves traveling away from their source and they travel at the speed of light. That's part of Einstein's theory of general relativity. Now, most importantly for our purposes, what they do is they stretch and compress spacetime itself. So if a gravitational wave from a distant source were to pass right through me, and I'm a spacetime object, we all are, what it would do to me is it would basically stretch me in one direction and compress me in the other and vice versa and the other half part of the cycle.

Nergis Mavavala: So as the gravitational wave goes through me, if I'm just a spacetime object, you can think of this as a grid. I'm getting squat and spread out and tall and thin. So I get shorter in this dimension longer here, and then vice versa. That's what the gravitational wave does. So if you want it to make a measurement, you have to measure changes in distances between objects. But let's see, look at what that means. So the distance between two objects that are separated by a distance L would change by an amount that's proportional to the amplitude of the wave and the length between us. So now let me put the...now you won't be sanguine. Let me put a scale on the number that I showed you. We already said Thorne, even by the late sixties, had told us we might expect an amplitude of about 10 to the minus 21.

Nergis Mavavala: So you can see, if you now put that gravitational wave going through me, I'm an object of about one meter of spacetime object in that dimension, hopefully only. And then what would happen to my length and my height is that it would change by 10 to the minus 21 meters as that gravitational wave goes through me. Now 10 to the minus 21 meters, just to put into perspective, is a million times smaller than a single proton. Okay. So it's a very, very, very small quantity. That did not deter this man. Ray Weiss, our second Nobel Laureate, who at the time at MIT. Now another historical thing to keep in mind was in 1960, the laser was invented and Weiss was working with lasers and other things, and he had the idea that you could make a device like this, which is called an interferometer.

Nergis Mavavala: That's the "I" in LIGO and it basically is a device where you take some laser light and you split it into two halves and you reflected off of the mirror here and a mirror there. And then as the gravitational wave goes by, it changes this distance relative to this distance and that will control how much light comes out of the outputs here from no light to all the light will come out. So by measuring the pattern of light at the output of this interferometer, you can tell what the relative spacing of the two arms was. That was his idea. He also did one other very, very important thing. Others had this idea, but the most important thing he did was he understood that making a measurement of the changes and the distances between these mirrors have 10 to the minus 21 meters was impossible.

Nergis Mavavala: So he was the first to say, you've got to make the detectors long. That's the knob you turn the length of the detector. And he proposed that we make two and a half mile or four-kilometer long detectors. And then the measurement we would have to make is just 10 to the minus 18 meters. And he said, Oh yeah, that we can do. Okay. And that was in, he had formulated the complete proposal for how to do this by 1972. Okay. All right. At that then, you know, the, a couple of decades later turned into LIGO the laser interferometer gravitational wave observatory. Throne and Weiss met in 1975 somewhat by accident, and then sort of became the champions of the US building this device to directly detect gravitational waves. So you can see there's the similarity between the cartoon here and their actual observatories.

Nergis Mavavala: One in Washington state, east of Seattle, and one in Louisiana, just roughly between band rouge and New Orleans. These are four-kilometer long l shapes. The laser lives in the center and there are mirrors four kilometers away. Okay. So that's what happened. Now, how does LIGO work? How can you possibly make such a ridiculous measurement? So I'm going to tell you about two factors. Of a trillion. Okay. So the first factor for trillion is if you just take the mirrors of LIGO and you ask how much does a mirror if I just plopped a mirror down here in this room, does anyone want to guess how much it moves by? No one wants to guess.

Nergis Mavavala: Oh, I like that.


Nergis Mavavala: Oh my goodness, I wish it were true. It moves by about 10 to the minus six meters. So a trillion times more than the gravitational wave would move it. So vibrations of the earth, air currents, all of those things move at by trillion times more than the measurement we're trying to make. So the first thing we have to do is take the mirrors of LIGO and isolate them from vibrations, which are done by devices like these vibration isolation systems that are very much like a noise canceling headphones. You measure the vibration and then you push on the mirror to cancel out the vibration that comes from the earth. Okay. And so when you do that, you can sort of reach this level of 10 to the minus 18 meters. That's sort of the two levels of isolation. Now imagine you've done that successfully. That would do you really no good if you didn't also know how to measure such a small distance. What kind of ruler can you use that lets you measure such a small distance, and it turns out no ruler that we know of except for light itself. The wavelength of the light becomes the tick marks of our ruler. Now the light that we use is near infrared, very close to visible. Anybody want to guess as to what the wavelength of near infrared red light might be? Or even visible?


Nergis Mavavala: It's many hundreds of nanometers. In our case, it's a thousand nanometers, which is 10 to the minus six meters. That's your second factor of a trillion. We have a ruler whose tick marks are a trillion times farther apart than the measurement we wish to make. Okay? The way you do that is you use a very powerful laser and you use lots and lots of photons. So you average over each of those photons wavelengths and construct something that's more precise than just the wavelength of the photon. So in LIGO, we have a megawatt of laser power circulating, compare that to this laser pointer, which I promise is within the legal limits. What do you guess this power would be? [inaudible] A few milliwatts. Five milliwatts is within the legal limit, so you can see that's a million watts compared to a thousandth of a watt. So it's lots and lots of photon. But once you do that, you can actually, you're ready to make this measurement. And there comes a...this was actually funded in the United States for over many decades by the National Science Foundation and also our third Nobel, Roy Barry Barish, is the one credited with having these observatories, getting them built and operating.

Nergis Mavavala: Okay?

Nergis Mavavala: Now what?...LIGO is not the only observatory around. You can see the two LIGO detectors in the US and those are four kilometers long. And there's another operating observatory in Europe called Virgo. It's a three-kilometer detector. There is a planned three-kilometer detector in Japan and in India. And there's even proposals for a space observatory. So this is a global network of detectors. And now you fast forward to September 14, 1915, which was the discovery of the first binary black hole. This is the thing that you saw in the newspapers. Okay. And the picture here is two black holes that are orbiting each other and giving off these gravitational waves. And after they merge, when they collide, and these waves travel outward from where the black holes were. And in the case of these, this particular system, these waves traveled for 1.3 billion years and remarkably they found our earth.

Nergis Mavavala: And so here they do, the waves come through the earth arriving first at our Louisiana Observatory, seven milliseconds later at the Washington Observatory. Of course, the earth did not do that. It was, you can tell on that four-kilometer scale, it moved by very, very little and this is what the signal looks like. So this on the vertical access is the amplitude of the gravitational wave on the horizontal access is time, which is under one second. So this is another remarkable thing. These bumps and wiggles that you see are the bumps and wiggles of spacetime itself that this is spacetime going up and going down. Another remarkable thing is these black hole pair lived for over a hundred million years and we saw the last 200 milliseconds of their crash into each other. Okay. A third remarkable thing at the time that the black holes collided, which is the maximum of the signal, the gravitational wave amplitude or strain was 10 to the minus 21, that's a huge check mark for Kip Thorne.

Nergis Mavavala: He told us that in 1967 when that gravitational wave was measured by LIGO, the mirrors of LIGOS moved by a few times, 10 to the minus 18, big check mark for Ray Weiss, he told us how to do this in 1972. Okay, so it's a spectacular story. Now, what can the signal tell you? So here is a computer model of this same event. And I just put this up because I want to show you a few things. If you look, if you look at how the frequency is changing with time, that can tell you how heavy the black holes were. If you look at how the amplitude is at the maximum, that will tell you how far the black holes are. And if you look at this last part where the black holes merge and become one single black hole from the frequency indicator, you can tell what the final black hole mass was.

Nergis Mavavala: So from the frequency, you can tell what the two initial black hole masses were from the end part of the signal. You can tell what the final black hole masses where you put that all together and you can tell the story of these two black holes. Once upon a time, 1.3 billion years ago, there existed two black holes. They're orbiting each other exactly as Einstein instructed them to these black holes. You could see the storing the spacetime around themselves. They orbited and eventually they merged and the final object that they made was a new black hole. Now, what do we know? What did we learn? We learned that these black holes were about 30 times the mass of our sun. And now you know, make sure you're sitting firmly planted in your seat because this should really blow your mind. These 30 solar mass black holes when they collided, we're moving at half the speed of light.

Nergis Mavavala: Think of an object that's 250 kilometers big, weighs 30 times the mass of our sun and it's whipping at almost the speed of light. Only in nature can you contemplate something like that? Okay. And now we also, I told you, we also know that these were 1.3 billion light years. Now we know one other remarkable thing, the new black hole that was formed was three solar masses lighter than the two parents. So three times the mass of our sun was radiated away as gravitational wave energy in those, in that fraction of a second. Okay? So another really remarkable thing. Now of course, you will notice that they did not live happily ever after. So are are, you might think this is a story with a better ending, but let me remind you that a new black hole was born a solitary black hole that will never be seen again, albeit, but just like human parents, these two black holes also gave up their lives to bake a new one.

Nergis Mavavala: Okay? All right. Since then, there have been more detections of black holes, a number of them. We have now, 10 confirmed black hole detections. And I'm just putting these up to show you what the Menagerie of black holes is. That's the very first one on the left. Each of these signals is labeled by the date on which it was observed. So the first one happens to still be the loudest one we've seen. There's one that we saw a year later in July, 2017 that happens to be the heaviest. It's the farthest, it's also the spinniest. Black hole spin about their axes. So and so on. And then there was another remarkable event, which was... LIGOS and Virgo saw for the first time in August 2017. So a year, just a little under a year after, two years after the first black hole, we saw the merger of a pair of neutron stars.

Nergis Mavavala: Okay. So let me just tell you what we've learned so far. So far, we know that Einstein's theory of general relativity seems to be correct. The waves do travel at the speed of light. They seem to have the expected geometry. We didn't know where the black holes would exist in pairs. And we know they exist and they formed pairs. We've seen the signals from those. But now we have the puzzles. How does nature make such heavy black holes? We don't know. How do they form parents? We don't know. We have some guesses and I won't tell you exactly what those are, but we have some ways of looking for that by looking at the spins of black holes. So many puzzles. Now I'm telling you about the discovery of neutron stars because it may be one of the most exciting discoveries. The black hole discovery got a lot of media coverage. It was the first time we [inaudible]black holes. This is just amazing, right? The neutron stars collision. Now one thing we know about neutron stars I already told you compared to black holes is when neutron stars collide, we should see a spectacular light show. And so what happened was, is here's the story of what happened. So the first thing that happened was LIGO and Virgo see a gravitational wave signal. And we know it's a neutron star, not a black hole because the signal lasted a minute instead of just a few seconds cause these are much lighter. We knew from the masses that these were neutron stars. 1.7 seconds later, there was a bright blip in a gamma-ray telescopes. So this collision of neutron stars was followed shortly by a burst of gamma rays. Very, very energetic photons LIGO and Virgo together could pinpoint where in the sky this must've happened.

Nergis Mavavala: It's a little bit, it's a relatively big patch of sky, happens to be in the southern hemisphere. And when this was detected by LIGO in Virgo, it was daytime in the southern hemisphere. So astronomers worldwide had roughly 11 hours to prepare to point their telescopes and look for this object with light telescopes. And they did. And what followed is what I would call a sort of an astronomers night to remember. Over 70 telescopes pointed in that patch of sky and quickly found an object. So let me just tell you, it's a very busy plot. I want to just show you a few things. That was LIGO Virgo, that was the Gamma Ray telescope. And so gap gravitational waves and gamma rays. And now look at this in x rays, something was seen, ultraviolet, something was seen, optical, many, many things. We're seeing infrared radio and every color of light that you looked, you saw a signal.

Nergis Mavavala: What does the signal look like? So here are images from all these different telescopes. This is the galaxy NGC 49 93 and let me just point you look at all the Clark Crosshairs are pointing at this object here. And this was 11 hours after LIGO Virgo sent out the alert. And look at this picture 20 days earlier. Look at the clock. Crosshair. Is there anything there? No. This new object lit up in the sky as we would have expected when the black neutron stars orbiting each other at a great distance. They were not emitting light, but when they collided, this enormous acceleration of neutrons and would give off a tremendous light show. So what does that light show look like? Two neutron stars. These are the gravitational waves that they're emitting and they're getting closer and closer to each other and they'll start to distort and the collide. And then here is the light jet, the gamma rays in a jet. And then all the other, this enormous cloud of nuclear material giving off light, right? Well, this is a reconstruction of the event by an artist. This is not really observed by, by a telescope, but let me just show it to you one more time without disturbing you with my chatter. Gravitational waves only. Gammas and then all the other colors of light. And this is the nuclear material that's fusing new elements. [video plays with dramatic music]

Nergis Mavavala: So I'll just say this, this, this object is still being studied two years later because it has so many interesting, mysterious parts, which I won't get to talk about. But I will say a little bit about this. This is a curve that is made by different colors of light and every one of these peaks over here in the spectrum. Tell us about the chemistry of the object. And you can see as a function of time, but chemistry of the object is changing. And so let me tell you what this is. This helps us solve some decades-long mysteries. Okay? So the first one is that there had long not known what causes these particular class of burst of gamma rays. People thought they would be, collisions of neutron stars, but we didn't know until this observation, the fact that the Gamma Ray telescope saw the signal shortly after gravitational wave detectors.

Nergis Mavavala: This is solving a long time mystery. Another mystery. Believe it or not, you might think, you know, entire civilizations have been won and lost over gold. So you'd find it hard to believe that we have too much gold here on the earth. But it's true when we think about where the elements of the periodic table come from, we like to say they come from stars and of course we don't need to look far. We have our own very own star, the sun, but turns out that the sun and stars like the sun cannot fuse heavier elements. Then iron in the periodic table joins. I rarely have such...it's such a nice prop available here, the periodic table. [points at a poster of the periodic table with a laser pointer] And if you look carefully, you'll see those yet here go the irons and here after about iron and nickel, the sun can't fuse these because it takes more energy to produce the energy that gets released.

Nergis Mavavala: So anytime you see have elements heavier than iron, they came from somewhere else than our star. And the thought was that maybe they came when stars explode when they go supernovae. Now it turns out that process also does not allow us to...does not give us enough of the really heavy stuff like gold, platinum, the lanthanide series, et cetera. So people had hypothesized that the only place where such heavy elements could be fused and somewhere some part of the universe that's very nutrient rich. Because what is a heavy element? A heavy element is a nucleus with lots of neutrons and protons. So those colors of light that I showed you with those different peaks gave us confirmation. We were watching in real time, the peaks for gold and platinum. And so we now know that heavy elements like gold and platinum are formed.

Nergis Mavavala: And in neutron star mergers, these neutron star mergers are nature's gold mines. It's a very striking thing because it tells us at some point in our own galaxy that were these neutron star mergers that scattered all this gold. The thing we saw with that one neutron star merger made many, many tens of times the mass of our own earth in gold alone. Okay. So that's where all the gold comes from. Carl Sagan famously said we are stardust because we used to think all the elements came from stars. I think it's now fair to say we have to modify that we are neutron stardust. Okay. Another thing we've learned from this one event alone is we have an independent measurement of something called the Hubble constant. Now I'll tell you the Hubble Constant, you know, physicists love to talk about numbers and constants that are very, sort of valuable to us, but the rest of the world kind of wonders why do they care? So I'll tell you why you should care about the Hubble Constant. The Hubble Constant describes the expansion of the universe. So it tells us about our history and our future. So everyone should care. Right? And we have two different measurements of the Hubble Constant using light and they don't agree with each other. So there's this independent measurement using gravitational waves as the first time. We can try to get a handle on that. And I'll give you the answer are the gravitational wave answer lies between the two but the measurement is not very accurate because we only have one neutron star collision yet. So that there's more to say on the subject. Okay. So let me just close up by telling you why we should all be excited. The first observation of gravitational waves, these bumps and wiggles were the first time we saw spacetime actually do it's rippling dance. Einstein's theory of general relativity confirmed yet again in this regime that has never been before.

Nergis Mavavala: Black holes and neutron stars do form. It appears. And, and they, they do collide and we can see these collisions in real time. And as someone who spent my entire career working on the instruments, it's really nice. The damn machine works too. Okay. Because believe me, it's not easy to make a measurement of 10 to the minus 18 meters. And so that was another triumph. But I will put out to you that in a hundred years from now, these are not the things that will be remembered. Okay. Really what we've done is we've turned on a completely new way in which to observe the universe. We can for the first time, use gravity alone or gravity with light for new discoveries. Okay. So I think that's really what with this moment in time will be remembered for in the future, very much like Galileo and what happened after.

Nergis Mavavala: So we believe Galileo was the first person to point a, a telescope at the sky about 400 years ago in 1609. It was actually a very modest telescope, you know, toy store telescopes can compete with that today. It was a little one and a half inch telescope and then, and no one really even remembers what Galileo saw even though for the time it was so important, he saw craters on the moon, and the moon wasn't made of cheese and you know, phases of Venus and many other things. But you know, in the meantime we've made a 100-inch telescope here on the earth. Another century later we put a 100-inch telescope into space. Now we are building 25-meter class telescopes here on the earth. And we've also built telescopes in space with all other colors of light from infrared to gamma ray, to x-ray to radio.

Nergis Mavavala: And this is all happened in the 400 years since Galileo. So you really have to take the long view of this, not what did the universe see when Galileo pointed his telescope, the important thing Galileo did was not what he saw, but the paradigm shift that humans no longer needed to look to the sky with their naked eyes. They could use instruments and they would do better. And that then set up this range of instruments. And the same thing will be true for gravitational waves. The gravitational wave sky cover...gravitational waves covered the same huge range of wavelength as like 20 orders of magnitude. And what we've done is with these terrestrial interferometers, these detectors, we've only seen the fastest of these gravitational waves, which are intrinsically very light objects, like black holes and neutron stars that can rip around each other at, you know, at tens or hundreds of per second. But there's a whole range of techniques for mapping out the gravitational wave sky. And I predict that if we were here 400 years from now, we would have filled in many of these mysteries as well. So thank you.