SciCafe: When Black Holes Collide

SciCafe: When Black Holes Collide - Transcript Narrator: You're listening to Public Programs at the American Museum of Natural History. When black holes collide, the energy of the event generates intense gravitational waves. These waves were predicted by Einstein in his theories, but scientists have only recently been able to detect them experimentally. In this SciCafe, Barnard College professor and astronomer Janna Levin shares her scientific research on the first recordings of a gravitational wave from the collision of two black holes 1.3 billion years ago. This lecture took place at the Museum on December 7, 2016. To watch a video version, visit the AMNH YouTube Channel, or the SciCafe section of AMNH.tv. Janna Levin (Astronomer, Barnard College): It's so nice to be here. I love this venue. It's such a romantic venue. We're literally under the planetarium. I love it. Yeah, this building has a special place in my heart. It's really nice to see everyone come out for science. Woo, science. Yeah, we believe in science. Science is great. So, I wanted to talk tonight about this recent gravitational wave discovery on February 11 of this year; an otherwise obscure experiment made an announcement that sort of made the entire world freeze and pause for a second. How many people heard around the February 11 about the gravitational wave detection? Okay, excellent. It was an amazing moment. I don't even think that the experimentalists expected the world to pay that much attention. After about an hour, how many people felt like they understood what the gravitational wave detection was about? [Laughs] Okay, yeah. Oh, yeah, this one guy. This one guy did. [Laughs] It's very, very difficult to understand. And so I want to spend tonight picking apart what that discovery was, why it was so monumental and why it was so moving. Even though very few people had heard about LIGO before that day, it cost 50 years of people's lives. Some of the original LIGO experimentalists started this in the late '60s when people didn't even know if black holes were real, when people didn't even know if gravitational waves were real. Einstein was arguing about the existence of gravitational waves for decades. He kept changing his mind. He famously wrote a paper saying gravitational waves do not exist, and then between acceptance and actual putting it to press, he snuck in a completely different paper that said that they did. So, he was sort of all over the place. And it's an incredibly difficult subject. So, imagine starting this experiment in the late '60s, and now it's 2016. It's the centenary of the day that Einstein—or the year that Einstein first proposed the existence of gravitational waves, and they make this detection. So, I really want to spend some time talking about what it all really means. So, here it is. We're about to listen. And this is already something you should feel is strange. We're going to listen to the discovery. In astronomy, we look at discoveries. This time, we're going to listen to the discovery. This is—it's going to play twice. It's going to play the sound of the gravitational wave recorded, and then it's going to increase it in pitch because the human ear does not do well with the lower notes. So, you're going to hear it better the second two times. Okay, here we go. [audio plays] Want to hear it again? [audio plays] Okay, so in the second time, all that's happened is it's been falsely increased in pitch, and you could hear it again. Now, you should already be wondering why this giant machine, which spans four kilometers of which there are two on two different coasts in the U.S., made a recording at all. Why didn't it take a picture like a telescope? And so that's what we're going to spend some time picking apart. And why can we hear it? It literally lands in the human auditory range, so we'll talk about that as well. So, what was the gravitational wave discovery about? So, let's first start with what we do know. This is actually from the Digital Atlas here at the American Museum of Natural History. Let me start this one again. It's a compilation of known observations of the universe. Essentially, every object that has ever been observed is placed in this atlas. And then you can drive around the atlas if you want, kind of like Google Maps, only bigger. And here is a map. This is not a cartoon or a simulation. It's a map showing what our own galaxy looks like. This is the Milky Way galaxy. It's a collection of 100 billion stars. We have never gotten this far outside of the Milky Way galaxy to take a look at it. This is some hundreds of thousands of light years from our current location. We have never been able to get there. Where we live is inside the Milky Way. So, here we are. Let's go and figure out where we are in the location of the entire universe before we figure out how we observe it. We're coming inside to the spiral arms. New Yorkers are always very offended to find out that they actually live in the galactic suburbs. I know that's terribly disappointing. [Laughter] But it's better than the center of the galaxy because there's a supermassive black hole there. And we are slowing spiraling into that supermassive black hole, but it's 26,000 light years away. It will take a very long time. There's our sun. It's quite an average yellow star. We have gotten not even this far outside of our solar system. Well, here we have. We have. Voyager, which was launched in the '70s just broke out of the sun's magnetic influence and is basically interstellar after decades. And that's the farthest we've flung anything in the universe. Here we are on the third rock from the sun. It's a very nice place, you may have heard. Good real estate. [Laughs] And you just saw the great view we had of the galaxy. Okay, here we are. Most of us are bound to this rock. Very few of us have left this rock. Most of our satellites orbit this rock. Very few things make it away from the earth or outside of the solar system. So, everything we know about the universe comes to us from light. And that's already stunning. It comes to us as we receive light sent to us actress the universe. And because it takes light traveling to us some time, the further away we see something, the further in the past we're looking. So, we have a map not only of a universe 90-some billion light years across, but nearly 14 billion years old. And we get it all by taking pictures. Since Galileo first pointed a telescope at the sky, we've gotten this silent movie. And what we're doing differently now is trying to detect something about the cosmos in complete darkness; in the absence of light. And to do that, we really use gravity. This is another, again, Digital Atlas. It's not a cartoon. Every object you see in this Digital Atlas is an entire galaxy. So, there are as many galaxies in the observable universe as there are stars in the Milky Way. There are hundreds of billions of galaxies, each one with hundreds or more billions of stars. And although this is a lot of stuff that we've seen, most of the universe is actually dark. Ninety-five percent of the universe is not luminous at all. It will never send us light. We will never see it with a telescope. The cosmos is largely darkness. We're lucky we can do astronomy now because the future is getting even darker. So, here we are—and I didn't mean that as a political statement or anything. [laughter] Don't read too much into it people. So, here we are. What do we do with a universe that is largely dark when we cannot take pictures of the sky? We want to use pure gravity. And that's where Einstein comes in. So, Einstein proposes his most radical, most important idea. It's finally written down beautifully in 1915. Did I say 1916? I lied. Nineteen-fifteen. It's all right, you can look it up. Einstein once said he didn't know his phone number because why memorize something you could look up. So, that's kind of how I feel about some of these things. So, in 1915 Einstein writes down his most radical idea: the general theory of relativity. And in this idea, he expresses his entire theory of spacetime. He says—and we're going to say it in one sentence very simply—that mass and energy, like the sun, curves space and time around it. So, things fall under natural curves in space. If I was floating in empty space and I were to throw my clicker, what path do you imagine it would take? I mean, totally empty space; no earth, no nothing. What path do you imagine it would take? Straight line. Because what else is it going to do? Straight line. If I throw the clicker in this room, what path is it going to take? You again. It's going to go down. It is literally tracing for you a curve in the shape of spacetime. It's a stunning idea. It's an absolutely stunning idea. Mathematically it's very hard, but the intuition is obvious once you realize that it's my hand that's in the way of gravity right now. And once I let go, it's going to trace for me the natural curves in spacetime. And so when the International Space Station is orbiting the earth, it is falling freely along a circular curve in spacetime. It's absolutely falling. It's just thrown so fast that it always clears the horizon. It never crashes into the surface of the earth. But it's actually the astronauts in the space station are falling. That's what they're doing all the time. So, there's Einstein's theory in a nutshell. Not bad. Now, you don't have to go to graduate school for five years. Here's the most extreme example of Einstein's theory. It's the black hole. Here's my portrait of a black hole. [laughs] So, in the same year that Einstein publishes his great theory, Karl Schwarzschild, who's an infantry soldier during the war on the Russian front, is between calculating cannon fire trajectories. Starts reading the proceedings of the Prussian Academy of Sciences, as you do, and solves for the curved spacetime around a mass, imagining—pretending really, just as a fantasy—that all the mass is crushed to a point. It's not a physical or real solution. It's just an idea. Imagine that it wasn't the earth, but everything was crushed to a point. He writes down this solution. It's very obscure. Einstein's very impressed with it. He helps him get it published, but Einstein thinks these things will never form. This is not reality. Nature will protect us from such strange things. The strangeness we now know as the black hole, it didn't earn its name until the '60s. Nineteen-sixty-seven, I believe it earned the name from the famous relativist John Wheeler. So, here is a black hole. This is not a cartoon. It's a mathematical model by a physicist Andrew Hamilton who does these stunning visualizations of black holes. In this computer simulation, a black hole about the size of our sun has formed. And if you took the entire sun and made it a black hole, it would be about six kilometers across. It would comfortably fit in Manhattan. Six kilometers across the entire mass of the sun. And what do we mean by six kilometers across? We mean that there's a shadow cast because the space is so strongly curved that even light takes a path that always points inward. There are no paths that point outward. No curves in spacetime. Saying it another way, you would have to travel faster than the speed of light to escape from that shadow. If you were to go up to the black hole, there's nothing there. We have this myth that black holes are dense crushes of matter. Black holes are empty spacetime. They're like places more than they are things. And here we're seeing a little model of the earth, which is self-illuminated because the sun's gone. So, the little model of the earth is self-illuminated, and notice it's looking very warped as it passes behind the black hole sun. And that's just an effect of the light following bent paths as well. So, all of this is about the bending of light. We're almost at gravitational waves. So, here Einstein begins to think immediately, look, if the sun and the earth and these strange things, which don't have a name yet we now call black holes, can curve the space and time around them, surely when they move they curves have to follow them. And if the curves have to follow them, nothing can travel faster than the speed of light, so those curves must follow them at the speed of light. And what you create is a wave literally in the shape of spacetime, and the curves you follow along as the objects move. So, in this cartoon due to LIGO—courtesy of LIGO—you see how the curves in spacetime follow the object. And if I have two objects moving, they create a wave in spacetime. Those are the gravitational waves that LIGO detected. They're literally curves in the shape of space. If you were floating freely near the two black holes as they collided, you would kind of bob on the wave like something floating on the ocean. And it would be darkness, completely darkness, but you would bob on the wave as it passed. Now, we'll talk about what was exactly detected in a minute. So, here's how LIGO works, this crazy idea—imagine it's the late '60s. Ray Weiss is a young professor at MIT. He's working on other projects, but he has this mad idea to measure these waves in the shape of spacetime that people don't even know are real. People don't even think black holes are real. Black holes are crucial for the conversation because only the most cataclysmic events can ring spacetime loud enough for anybody to have any kind of hope of detecting it. So, you need something like the intensity of black holes, which as they orbit each other are like mallets on a drum. And they will literally ring spacetime, emanating these waves outward while we wait to receive them here on earth. Now, Ray's idea was kind of like in this cartoon. Imagine spacetime is changing shape where you are. If you hang mirrors at the end points of this L-shaped instrument, and you bounce light along the L-shaped instrument, the light will come back at different times because it will have traveled different lengths as the mirrors bob on the wave. And in that way, you could record basically the floating mirrors and the bobbing wave. In some ways, LIGO is like the body of an electric guitar recording the shape of the guitar string. If you pluck an electric guitar, it technically doesn't make a sound. You attach it to the body of the electric guitar, and it records the ringing shape and plays it back through an amplifier. This is how LIGO works. It's like a giant musical instrument. As the mirrors bob on the wave, LIGO records the shape of the bobbing and it literally plays it back through a conventional speaker system. Bear in mind that these are billion-dollar machines that they took 50 years to build, and now a team of nearly 1,000 people work on them. That they span four kilometers. When the light is going down the arms, each one of these arms is four kilometers long. And there's two of these machines: one on the coast of Louisiana and one in Hanford, Washington. So, when the experimentalists are in the control room, they are literally listening to the detector. Let's take a look at the real LIGO. This is the real LIGO. So, here are the long arms of the instrument. This must be Hanford. You can tell Louisiana because there are swamps along the sides. I swear, swamps. There's Louisiana, and there are alligators in the swamp and mysteriously bass. No one knows how the bass got there. So, you see these four-kilometer-long arms. We're about to go into a cartoon where we're inside the tunnel. There's a laser that's shot down this tunnel, and the light has to travel the four kilometers. It's a powerful laser. This is a vacuum inside here. That vacuum represents the largest holes in the earth's atmosphere. There's less stuff in the LIGO arms than there are in regions of intergalactic space. And these are the beer chambers, as they're sometimes fondly known. There's not actually beer in them, but there are the instruments, the mirrors, the suspension systems. And this is what the instrument sounds like in the control room. [audio plays] It just sounds like noise. It sounds like noise unless a signal hits. So, there is one instrument on the Gulf Coast, one on the West Coast. And if something happens in the universe—some cataclysm event that rings spacetime—these waves will travel and they'll travel all the way to the earth. And they'll strike the instruments, the mirrors will bob, they'll be recorded. That's the aspiration. Now, in the year 2000 the first LIGO instrument was built and listened for 15 years nearly and heard nothing. So, this was an incredibly successful technological achievement in the year 2000. And the skies were silent. And they needed to build an advanced machine, and then things like this happened. The Hanford machine is on the same site as the plutonium separation facilities that were used for the original atom bombs. And so there's security patrols that drive around, apparently with a poor knowledge of geography, [laughter] in the middle of the night no lights or anything and crashed into one of the LIGO arms. Luckily, he didn't puncture the vacuum. If you ask the experimentalists what would have happened, they said we'd all go home. It's over. The vacuum was drawn in 1998 and has never been brought up to atmosphere since. The vacuum is punctured, the experiment would have been over. That would have been it. He broke his arm, but fortunately not the LIGO arm. [Laughs] And it might have been like opening a vent in space. I don't know how deadly it would have been had he broken the vacuum for him. It's like opening a door on the space shuttle, but the vacuum's on the inside, not the outside. There was also an incident in Louisiana were hunters started to shoot up the Louisiana machine. They said by accident, but people didn't really buy that story. One of the main experimentalists on the Louisiana site said the Europeans just think we're so American. Now, all we need is a hamburger incident or something like that. But we survived all of this. The year 2015 comes. They've taken out of the beer chambers, which you can separately isolate from the vacuum, and installed new components. They basically replaced everything but the nothing, everything but the vacuum. And by 2015, the machine was operational. That year in August, Ray Weiss, who's now in his 80s who's onsite all the time, said to me if we don't detect black holes, this thing is a failure. If you asked anybody else on the ground when or if we would ever detect black holes, they said not until 2018, 2020. Just don't even think about it. It's not coming anytime soon. Ray was pushing, pushing for the centenary. He's like I want it, goddammit. Oh, Ray loves to swear, by the way. He loves to swear. I want it, goddammit. I want the centenary of Einstein's paper, and pushing for it. But even he started to think, oh, it'll never happen. So, here it is, it's September 13. Ray's onsite looking for radio interference. And a lot of the experimentalists are interrupting the machine. They feel they're not ready yet. So, they decide to do tests. They're running these aggressive tests on the machine, postponing their science run. And these tests really disturbed the machine. They really basically ruin any potential for detection. But they just didn't feel they were ready yet. It comes early Monday morning September 14 and in Louisiana these—a lot of young graduate students and post-docs working on this instrument at 4:00 in the morning. They get tired. They decide to go home. They put down their tools. They're done for the day. Same thing happens in Washington state. They were going to run all night, but they just get fed up. They leave the machines locked, though, in observing mode. Within the span of an hour, this signal, which had been travelling for 1.3 billion years—I know, it's crazy—strikes Louisiana, rings the machine there. It's beautifully recorded. Middle of the night, nobody's awake. Nobody hears it in the control room because it's too fast. It's like seven or five milliseconds later. It cruises across the continent and rings the machine in Washington and is beautifully captured there. Eight-thirty a.m., Ray wakes up and looks at the logs, as he always does, and thinks what the hell is going on here. What's this? And here's what they detected that morning. So, this is a computer simulation of two black holes, each one about 30 times the mass of the sun. One was a little bit bigger than that. So, they were big. We were surprised how big they were. One was maybe 29, one was maybe 35 times the mass of the sun. This is drastically slowed down, so that we can watch it. But we caught the final few orbits where these two black holes, which might have been orbiting for a billion years for all we know, are in their final few orbits. These orbits took one-fifth of a second. That's what was caught by the instrument; one-fifth of a second. In that final one-fifth of a second, the ringing of spacetime was finally loud enough for the instrument to record. They merged. They formed one bigger black hole. It's about 62 times the mass of the sun. It very quickly sheds off its imperfections and goes quiet. I think we have to see that again because it's so beautiful. No telescope could see this event. It happened in utter darkness. And, again, it's drastically slowed down. This means that the original LIGO instrument very well could have had these very gravitational waves passing over them when the black holes were further apart, going slower, but it was too quiet to detect. It was only the final fifth of a second that was loud enough because what LIGO measured was a deviation in the mirrors swinging over four kilometers of less than 1/10,000 of the width of a proton. And it was not until they could do that that they could even catch that final instant. So, there it is, that stunning result. So, now what you should be listening to—I'm going to play you two different things. What you should be listening to when you're listening to the gravitational waves, you should be thinking, oh, it's like ringing drum. And the LIGO instrument's like a musical instrument that's recorded the shape of the ringing drum and that's why we're listening to it, just like you listen to a guitar in some ways. And also you should be listening for the sweep up. The reason why it sweeps up is because the black holes go faster and faster as they get closer together. And so the ringing sweeps up in pitch. And we're going to listen to two different detections. Because on December 22 there was a second detection of two black holes colliding on Boxing Day. Two smaller black holes. And so we're going to listen to the Boxing Day detection and, again, the original detection from September 14. And then we're going to hear them again scooped up in frequency—I mean, in pitch. And the reason we do that is because the human ear's not so good at the low notes. So, here we go. The low notes are particularly bad on a computer. [audio plays] So is it going again? Oh here we go, increased pitch. [audio plays] Who thought black holes would sound like that? It's not what you expect. But it'll be your ring tone any day now. So, when you think about how remarkable that was, let's think about that. These black holes came from somewhere in the southern sky. Right now, they formed a big black hole that's dark and quiet. We cannot see it with a telescope. We cannot find it with a telescope and we can't hear it anymore. And it's out there moving away from us with the expansion of the universe. And when we start to ask ourselves how many are out there, we're already—once LIGO's operational all the time, we're going to start recording black hole collisions monthly, and now many neutron stars, other dead stars. Maybe stellar explosions. Maybe other kinds of collisions. But out here, this was the Hubble deep field. Again, every one of those objects is a real galaxy. And this is a patch of sky about a tenth of the moon. And this is how many galaxies we see. In every one of these galaxies there might be a billion black holes. There's a supermassive black holes, millions or billions times the mass of the sun, in probably every one of those galaxies. And so we're looking for those future—not just when we're going to be recording sounds of space, but maybe something we haven't even thought of yet. I mean, that's what we're all really after in truth. Scientists love nothing more than the unexpected. Being confirmed right after $1 billion is kind of a drag. [laughs] But with the first LIGO detection we already made new discoveries. Those black holes were huge. We have never detected two black holes. We've never detected black holes that were completely dark. We've never detected gravitational waves. So, it was like a clean sweep. But what we're looking forward to ultimately in the future in the life of the universe, when you think about where we're going—as we will fall into black holes eventually. It's going to take a very, very, very long time, but eventually everything that can will fall into black holes. Eventually, all of those black holes will evaporate, as Stephen Hawking told us they must evaporate into radiation. And the universe will expand and it will go both dark and quiet, which is why a friend of mine says we have to do astronomy now. [Laughter] But I think what we have to look forward to is really a remarkable future and really what we're all hoping for—LIGO just came online, I think, this week maybe; I've got to check in, to do more detections—is what we've always gotten when we've done astronomy, which is something totally unexpected. Thank you. Kira Lacks (Public Programs Manage, AMNH):  A huge thank you, again, to Janna Levin. Levin: Was that 20 minutes? Lacks: Yeah, you're good. So, we'll now open it up to the audience. We'll be taking as many questions as we can and, of course, encourage you to also use Twitter at #amnhscicafe, and we'll take some questions that way as well. And I'm going to try so hard to get to you. Let's see. Levin: Should I make up questions in the mean time? Hi. Question: Hi. Is it possible that if you travel through the black hole and you'd end up in some other part of the universe, is it possible? Levin: I would say hypothetically it's possible. I mean, we don't know what happens in the center of a black hole. We know a lot very confidently about what happens at the shadow. That whole range of both our theoretical understanding at least is very, very solid. But even though I said a black hole is not a crush of matter, one way black holes form is by crushing matter. So, the death state of a star is the only scenario we know of when a black hole actually forms, if the star's heavy enough. So, yes, you get this crush of matter. Eventually, the spacetime gets so strongly curved that the shadow is cast, but then the stuff keeps falling. So, it keeps falling towards the center of the black hole. So, what happens to it then? We just don't know. We don't know. Does it create a new Big Bang? Is all that stuff blown out into a new universe? Or is there some way of sewing worm holes on? These are all kinds of hypothetical possibilities that may or may not be true. We really don't understand that that well. And we might not until we know more about quantum gravity and how to quantize the theory of gravity. We might just not know what's going on in the center there. Lacks: Janna, we have a question on your left this way. Levin: Yes? Question: Good evening. By the way, great presentation. Levin: Oh, thank you. Question: Just wonderful. Levin: Thank you so much. Question: How do you know that it was two black holes that collided and not something else? Levin: No, it's an important question. So, in the decades while LIGO was really getting tooling up, theorists spent an enormous amount of time modeling the different sounds. And so I can theoretically predict that black holes will sound exactly like that. If anything, it was so stunningly exactly like what two black holes sound like when they collide that people started to suspect each other of cheating and injecting false data. So, they did a little CSI where they started isolating and interrogating people until they concluded there was no one single person or group of people who could possibly have pulled it off. It would have been too difficult. But it does have a very distinct sound. So, it's kind of like saying how do you know it's your phone ringing when you hear your phone? We can recognize sounds and we can reconstruct let's say from the shape of a ringing drum the motion, size, of the mallets that are creating the waves. And so it had all the hallmarks of black holes. Now, it could have been something like neutron stars, but neutron stars are never that big. It has to do with the size and shape of the mallets. So, neutron stars are always much, much smaller. These were much bigger than neutron stars. And you see them—I mean, you hear it really ring down to one black hole. There's a really impressive ending there where you just—it's just spot on. On the other hand, you heard the sound. It's not that complex. You could imagine maybe something else would mimic it or even that general relativity isn't exactly right and something else sounds similar. But it's as close as we could have dreamt really for the signal. Lacks: We have another question for you mid-center. Levin: Mid-center. Lacks: Over here. Levin: Okay. Yes? Question: Hi. Levin: Hi. Question: Fascinating lecture. Thank you very much. Can you clarify how the Louisiana and Washington sites were selected? And the second really brief question is how are the arms protected? When you show that car crashing in, I'm assuming they're protected quite well. Levin: Yeah. So, let's start with the site selection. Site selection was a bitter and difficult problem. You need to be very geographically isolated. And so there were several sites. And it has to do with the two detectors working in concert. If only one detector had recorded the sound, I think that nobody would believe it. You needed two to have—and with exactly the travel time you expect for a gravitational wave passing between them. So, the two act not only as two ears to a little bit narrow location, but also to confirm the detection. So, site selection was a blood bath honestly of a battle. And it went through Congress and there were senators that fought for it and there were political battles. And some people say we'll never completely understand why these two sites were chosen. That some people have taken that to their graves basically. But I did talk to Walter Massey, who was the head of the National Science Foundation at the time, and he insist it was never a political element to the decision. Nonetheless, politicians were involved because they took it politically whether or not they would get the machines in their state. If you look at the actual instruments, the beam tubes are millimeters thin of spooled steel that unraveled to make the tubes. They're incredibly delicate. Had a car crashed into it, or a bullet been shot at it, it would have been over. So, they even debated whether or not to build the cement encasement around it. The cement encasement was thankfully added at great expense, of course, because it's long. But it also created a place where vermin have infested. So, when Ray Weiss was the first person to walk the four kilometer tunnels after many years, and he found wasps, black widows, rats and chlorine, which is in their urine, apparently corrodes stainless steel. So, you'll never see a swimming pool made out of stainless steel. And so they were making tiny leaks in the vacuum. So, these were problems that they had to solve as they went along. He also got fungal pneumonia for walking the four kilometers. I've been in those tubes. It's intense. The tunnels at least. Lacks: Janna, do we have another— Levin: I feel like I'm picking, but I'm not picking. I'm sorry. I'm looking at you like I can pick you, but I can't. Yes, here. Lacks: I have another on your left up here. Levin: Okay. Yeah? Question: I have a two-fold question. First, how would they be able to pick the signal out of ambient vibrational noise? And when they began the project in the late '60s, how did they talk people into funding this? Levin: Yes. Okay, so these are both—I mean, really strong questions. The first question is just the physical question. There's been a lot of work on how to pull a signal out of the noise. It's called the cocktail party effect. So, let's say you're in a loud cocktail party, but you know you want to listen to your friend's voice. You can actually hone in on a signal that's much quieter than the ambient noise if you know what you're listening for. And so we thought that we would have to pull the signal out of the noise using this kind of extraction process. But actually, the first detection was loud enough. It was above the noise by a lot. And it was just this gold-plated event. It just was there. It was just blindly there. You didn't have to—so, there is an attempt to study the past noise and see if there are other signals underneath the noise that we just didn't notice. So, that is the whole industry. The second question, which was how did they have it funded, that was also—I mean, this was an arduous and painful campaign. And there were bitter battles. There were like bodies along the road. I mean, not everyone made it to the summit. This was really a climbing Mount Everest story where not everyone makes it. So, funding it was also an incredibly difficult battle. It didn't really secure funding until the '80s. And then it created a lot of negativity from a lot of people in the astronomy community who felt why is this experiment no one had ever heard of getting $200 million when black holes aren't even real and gravitational waves don't even exist. So, again, there were lots of fights and battles in Congress to secure the money. And there are real champions who pushed it forward. Yeah, it's one of those stories of persistence and tenacity. Lacks: We have another question all the way in the center in the back. You won't even be able to see us. Levin: Okay, that's all right. I can hear you. Question: Hi. I have a question about Hawking radiation, if that's okay. Levin: Sure. Question: So, if I understand Hawking radiation correctly, it's basically a copy of the information of whatever gets into the black hole is basically bled out of the black hole. So, is it possible that let's say I do a swan dive into the black hole, and after billions of years, that whole information can come out of the black hole and be reconstructed or you can figure out what exactly went into the black hole? Levin: Yeah. Question: From the radiation. Levin: So, this is the whole—what Hawking did was just incredibly remarkable, not least for being counterintuitive and not least for being provocative. So, the papers that he wrote in succession in the '70s have never been completely resolved. So, the first statement is that black holes evaporate. That is uncontroversial. Every physicist who studies quantum mechanics and relativity agrees with Hawking on that. That if you look at the quantum phenomenon near the event horizon of a black hole, it does this magic trick where it steals some energy from empty space and it allows some energy to escape and the net consequence is the black hole gets smaller. It's true. The black hole radiates and gets smaller. The spooky thing is that it's not coming from inside the black hole. The radiation is not coming inside and then traversing the event horizon and coming out. That's not how the mechanism works. So, then the second thing he said, which is the one that's been endlessly controversial, is, well, I guess we just lost all the information about what fell into the black hole. It's just gone. It's just gone. Because when the black hole disappears, when the curtain's yanked up and the event horizon is gone and the whole thing evaporates, whatever fell in—maybe it was a star, maybe it was a bunch of chairs or encyclopedias or whatever went in there, is lost. And proponents of fundamental physics freaked out because you cannot destroy information. There is not physics in a world where there's no information. There's no predictability. There's no evolving equations. That's what physics is. So, it was an absolute slap in the face to the entire enterprise to suggest that information was lost. And so this battle has been going on for decades where there are proponents who say there must be a way where the black hole somehow lets the information out somehow. And so this is still hotly debated. And one of the solutions is based on an idea called holography where the entire interior of the black hole, to some extent, is just a projection of the surface. So, in a holographic system, you take a two-dimensional image and it projects a three-dimensional volume. And the black hole has features of holography, which is that all of the information can be encoded on the surface. There's no more information content in the black hole than what can be encoded on the surface. So, some people argue in this holographic approach that the information never technically went in in the first place. And other people argue, well, there's two different realities where you swan dive into the black hole and you went through. But according to us on the outside, you never went through, and those two realities are never forced to come into contradiction because there's no one observer who can observe the two realities. And the saga goes on. It's gorgeous stuff. What it really is is that the black hole gives us the only terrain that we know of—that and the Big Bang—to study things like this. It's like a clue. The black hole is a clue and it's the only one we have to find our way in. Lacks: Janna, we have a question. Levin: Yes, please. Finally, you got the mic. Question: Hi. Levin: Hi. Question: What's the most boring event the LIGO can detect? Levin: That—oh, LIGO can detect? The most boring event? Oh, I should have told you something of this event, too, so I'll spruce up your question in a second. So, I don't know. Is any of it boring? The thing is it's hard to be boring with LIGO because gravity is so weak that boring things just don't make it ring. I mean, if you think about it, the entire planet is pulling on me right now. And with my little arm I can still resist it. That's crazy. Gravity is weak. So, in order to make LIGO ring, you need something pretty exciting to happen. Two neutron stars colliding, an entire star exploding, the Big Bang, might have made a bang. I can't really think of anything that's particularly dull. None of that sounds that dull. But I could say about the two black holes that collided, one thing I meant to mention, but forgot to, which I'll take this opportunity, is it was the most powerful event we have detected since the Big Bang itself. And none of it came out as light. It all came out in the gravitational waves. And the power in the gravitational waves was more than the power in all the stars in the observable universe combined at that moment. So, yeah, it's hard to be boring. Who has the mics? Lacks: Another question towards your right in the middle over here. Levin: Okay. Yeah? Question: I believe you said that for the first 15 years of LIGO's operation there weren't any observations made and maybe some changes occurred. And then in the span of those three months between September and December you had two events, and now you're anticipating as many as one a month or something like that. I was just curious what change occurred that all of a sudden you're able to predict more activity? Levin: So, they always planned on two instruments: an initial generation, which they thought might succeed, but might not—they were realistic about it—and an advanced machine, which was very likely to succeed, but still might not. So, the first instrument was a technological achievement. It was amazing. But the mirrors were suspended by these very thin glass wires that could literally break to the touch. They were so delicately suspended. And the point is that they're supposed to be falling freely. So, they can kind of fall in the transverse direction, meaning if the space moves that they'll start to bob a bit. All of the things like earthquakes, trucks driving by, winds, planes overhead, the moon, caused the mirrors to wobble as expected. That's the noise. And that's what you're listening to when you listen to the machine and there's no detection. It's just the wobbling from all of the noise around you. What they did is they created a much better suspension system; these very elaborate four tiers of pendula that cut down the noise a lot. And so it was really cutting down the noise. They put in a more powerful laser. They recoded the mirrors. So, everything got upped. But things like the thermal compensation system, the suspension system, everything was replaced with noise-reducing, advanced components. And even now there's things that they can do with quantum optics and quantum effects in the laser light, squeezing of the laser light, just a particular quantum attempt at reducing noise. But eventually LIGO will hit a floor. It's only so well you can do on the earth. And there's a limit. And eventually we'll have to go to space. And so there are missions proposed for space. Lacks: And we have a Twitter question for you, Janna. Levin: Okay. Where are you? Lacks: You won't be able to see me. I'm somewhere in a black hole. Levin: Okay. Lacks: After the mention of multiple black holes per galaxy, how would rogue black holes be detected? And how many could exist in ours? Levin: In our universe? Galaxy? Lacks: Sure. Levin: Galaxy. Rogue black holes. Well, so I'm not sure what's meant by rogue black holes in that context. Would my astronomer friends have a theory of what's meant by rogue black holes? You mean black holes that are just wandering about the galaxy? I don't know. You're on Twitter. You can't answer me. That's all right. Yeah, it is possible that there are black holes that are just wandering around the universe. There are—we think that a certain percentage of stars form black holes. About one percent of stars are heavy enough to actually make black holes. So, if you have 100 billion stars in our own Milky Way, that's a billion that will eventually become black holes. Now, some of them might have been star systems like our own with planets and things like that, and they'll just die and they'll be alone. And they won't have anything to orbit with, and so they won't create gravitational waves. They'll just slowly spiral into the center of the galaxy as we are. But some of them will be in two star systems, and there's possibilities of black holes being ejected from those systems as one star explodes, ejecting the companion. We also think there are lots of black holes in the center of the galaxy, like tens of thousands that are just clustered around the supermassive black hole. And so there are these different locations in the galaxy where we expect to find black holes. Now, is there ever a chance one is going to slingshot hazardly close to us? Probably not. Pretty unlikely. We're going to collide with Andromeda eventually, which is our neighboring galaxy. And it's much bigger than us. And when we collide with Andromeda, it's unlikely we'll even hit any other stars. Galaxies are largely empty space. The stars will just pass right through each other. So, you can watch the galaxies in these simulations pass through each other a few times before they eventually merge into one bigger galaxy. So, I think we have more serious problems like in the coming few months basically. Don't read too much into it. Lacks: Thank you. We have time for two more questions. Levin: Okay. Lacks: Janna, we have one right up front. And it's a young astronomer. Levin: Okay, hi. Question: Hi. Wait, I forgot my question. When a black hole evaporates, is there any way we could detect it? Is it like a supernova or is it really quiet? Levin: Yeah. It's a great question. When Hawking first started thinking about black hole evaporation he was wondering if we could detect these black holes. So, the way it works when you study the mathematics of Hawking radiation, it's bigger the black hole the colder it is, the less it evaporates. So, all the astrophysical black holes that we know of are actually absorbing much more light than they are radiating. So, we've never seen anything like it. However, small black holes as they get smaller, from evaporation, they'll get hotter and they'll start to evaporate more quickly. So, the end of the life of a black hole is like an explosion. Now, none of the black holes that we know of in the universe are anywhere near that phase of their lives. They've got a lot longer of hanging around before they get that small. However, in the very early universe, when the universe was first born, it's very possible that we made what are called primordial black holes, which were not formed as death states of stars, but are these little, tiny black holes that formed just by the intensity and the density of the early universe. Those very well could already have evaporated in bright flashes, and so one very good question that people asked was, well, if they had evaporated we would have seen gamma ray bursts, bursts of light or bursts of gamma rays, I should say, to distinguish them from the astronomical phenomena. And that would have been detectable. So, there must not have been a lot of primordial black holes. But that is still a serious question that scientists are looking at. Maybe they were big enough that they're just evaporating now, and we should be looking for these sparks in the sky as these small black holes—smaller than the moon—start to burn away. Lacks: And we have our last question all the way in the back. Levin: Okay. Question: Hi, good evening. So, you mentioned earlier in your speech that black holes are more spaces than they are things. And since suns collapse onto themselves—ones that are massive enough—and then that's how black holes are made, can you explain how the transition from a very dense, massive body goes into a space that doesn't really contain mass per se? Levin: Yeah. It's actually kind of a phenomenal story really. So, our sun is luminous because it's burning thermonuclear fuel. It's basically like hydrogen bombs going off all the time, fusion bombs, making heavier elements. And that's how we got heavy elements. If you look at the early universe, it was just hydrogen really and some helium. That's it. There's no carbon and oxygen and things like that. So, you need stars—these thermonuclear furnaces—to create the heavier elements, and then expel them in supernova explosions. If the stars didn't expel them, we couldn't be here. So, it's really this ecology that leads to second, third generation inhabitants like us. So, after the stars explode, there's a core that's left over that didn't get blown out in a supernova. And that core has to be heavy enough to overcome the pressures on the interior. And if it is heavy enough, it'll start to form a black hole. And then what will happen is it begins to collapse under its own weight, and eventually—so, imagine the sun just some millions of kilometers across, a couple million kilometers across, collapsing under its own weight. When it gets to about six kilometers, the sun's not heavy enough to become a black hole, by the way. It has to be much heavier than the sun. But when it gets to about six kilometers, the spacetime around it is so strongly curved that even the light from the hot collapsing star can no longer point outwards. It has to fall inwards. And it leaves that shadow behind like an imprint on spacetime. It's like an archeological record that it used to be there. And then it continues to fall inward. One thing that happens inside the black hole, it's almost like a waterfall of spacetime dragging everything down with it. So, even if the light from the star is trying to race outward at the speed of light, the waterfall of spacetime is just going to carry it down. And what happens to it when it gets to the very, very, very center is the great mystery. We don't know. Maybe there's a little remnant left that some crazy quantum ball of information, or maybe like we said earlier, it gets blown out into another universe. Or maybe something we haven't thought of happens in the interior. Maybe it really just ceases to exist. It just falls out of reality, and all that's left is this imprint of the previous mass on the spacetime around it. The amazing thing is from the outside we can't possibly care about any of those alternatives because the event horizon says we can know nothing about the inside. That the world on the outside of the event horizon is causally separated from the world on the interior. Maybe there's a little remnant left that some crazy quantum ball of information, or maybe like we said earlier, it gets blown out into another universe. Or maybe something we haven't thought of happens in the interior. Maybe it really just ceases to exist. It just falls out of reality, and all that's left is this imprint of the previous mass on the spacetime around it. The amazing thing is from the outside we can't possibly care about any of those alternatives because the event horizon says we can know nothing about the inside. That the world on the outside of the event horizon is causally separated from the world on the interior. Lacks: Well, please join me, again, in thanking Dr. Janna Levin. [applause] Narrator: Thanks for listening to Public Programs at the American Museum of Natural History. To listen to our archive of podcasts, visit amnh.org/podcasts. The SciCafe series is proudly sponsored by Judy and Josh Weston. [End of audio]