Speaker 1Hello everyone and welcome to the Mindscape Podcast. I'm your host Sean Carroll. I'm not sure if you know this, but if you're listening to this podcast the week it is being published, it is Black Hole Week. I don't know if it's Black Hole Week all over the world or even all throughout the galaxy, but in Copenhagen it is Black Hole Week. Black Hole Week is a thing that is sponsored by the Center of Gravity. Maybe it's the Center for Gravity, but it sounds sexier if it's the Center of Gravity at the Niels Bohr Institute in Copenhagen. It started two years ago when they realized that two years ago was the 50th anniversary of Stephen Hawking showing that black holes actually emitted radiation and really changing our view of what black holes are, and they decided to make it a biannual thing. Every two years they're going to have a celebration of black holes that lasts a week long in Copenhagen, and there's various events and so forth. Hopefully it catches on worldwide. I think the Black Hole Week is going to be a big event. I think it's going to are important enough that we could use that as an excuse to celebrate science and all the different ways that science intersects with our lives. And today's guest is the perfect person to talk to us about black holes. Vitor Cardoso is the director of the Center of Gravity at the Niels Bohr Institute in Copenhagen, and the reason why he's the perfect person besides his title is that he is a theorist who sort of grew up thinking about how black holes work in general relativity and string theory and physics and things like that. And in this modern era where we're having enormous amounts of data come in from gravitational wave observatories, from the Event Horizon Telescope and elsewhere that are teaching us about properties of black holes, he has been very active in that observational program as well, thinking about what we can learn about black holes from the data that we're collecting. And I think that's why it's worth having a podcast episode about black holes in the landscape is changing. You know, black holes are a different kind of thing now in the practice of science than they were just 20 years ago. Not only do we have enormous progress theoretically in thinking about how black holes work with quantum gravity and with other classical theories of gravity, the behavior of black holes in astrophysical situations, but we have all these new telescopes and observatories that are teaching us about them. So we're going to do the basics of what we're learning. Like, what do you know that you didn't know when you see two black holes spiral together and you're catching their gravitational waves? What are the future prospects for new observatories? How does this all intersect with quantum gravity and other theories like that? The scope of black hole physics is enormous and it's moving forward very quickly. So there's a lot to talk about. Let's go. Peter Cardoso, welcome to the Mindscape podcast. Thank you. Pleasure to be here. So every time I have a biologist on the podcast, I ask them to define what a gene is. And they always give me different answers. So for astrophysicists, I got to ask how you define what a black hole is.
Speaker 2That's a wonderful question. Because I think it really depends also on the type of astrophysicist you ask to. I think for somebody doing observations, a black hole is a point like object, which is very massive and dark. And that's the end of it. It kind of controls the gravitational interaction with nearby matter. But if you ask somebody a bit more like me, who works on theory and is interested in the fundamental concepts, then a black hole is a very different piece. It's an object that curves space-time to the extent that time stops at the event horizon. It has an event horizon. So it's a very special geometry in the universe. It's a very special place out there in the universe. And, you know, this question, therefore, goes to the small scale structure of this type of object. I'm interested. Event horizons means I'm actually zooming in on the structure of the object, rather than zooming out as an astrophysicist. And I think that's a good question. I think that's a good question. I think that's a good question. I think that's a good question. I think that's a good question. I think that's a good question. I think that's a good question.
Speaker 1It's already very interesting because there are different kinds of scientists out there, different kinds of physicists and astrophysicists. And I try to like, I enjoy giving people the lay of the land. And you're kind of in between as someone who's a theoretical physicist thinking about black holes, but also thinking about the observations and what we can learn from the data.
Speaker 2Yeah, I think that's, there's a reason for that. Our field, if there is such a thing as our field, changed dramatically in the last 10 years. Yeah, I was raised to the theory to look at mathematical equations, solve them numerically, and don't really even care about observations or about practical applications of that in observations, because there were no observations, in essence. I mean, black holes were really distant from our everyday experience. In the last 10 years, maybe more, of course, but everything changed. We started seeing black holes with gravitational waves in 2015. And, you know, in the last five, six years, we've seen images of black holes of matter close to the horizon of a black hole. We're doing interferometry, so you're seeing stars passing really close to black holes. So all of a sudden, somebody that was doing purely theory is now thinking, hey, wait a minute, what I'm doing actually has an application. I want to know if we're seeing the stuff I predict.
Speaker 1Yeah, yeah. Are there a lot of people like you who grew up thinking about space-time metrics and quantum fields and things like that near black holes who are now thinking about ringdowns and templates and observational constraints?
Speaker 2I think we're many, yes. And those who are not yet here are trying to do the transition, and we need that. We need new ideas. We need new people that come with fresh concepts. You know, just the very concept of a black hole. How can we know? That we're looking at a black hole, other than just saying, oh, it looks very massive and it looks dark. Just this question, how do we test the concept of a black hole, in itself requires a lot of effort, a lot of thinking and understanding what observations are giving us. So, you know, it does require a transition from one field to the other, and that we speak both languages, I think.
Speaker 1Is it worth going backward even a little bit more? We went backward 10 years already, but could you explain to us a little bit about the historical reception of the idea of black holes? I mean, at least in the last, since general relativity, you know, we don't need to go back to Laplace or whatever, but Einstein went to his grave not knowing about black holes, and it did take a while for the concept to really catch on among physicists and astronomers. Well, actually, that's fascinating
Speaker 2because, you know, Eddington didn't believe that nature would allow something like eternal growth, like gravitational collapse, so black holes to form. Einstein was aware of something like a coordinate singularity or a singularity at the horizon. He didn't like it either. In fact, he has a famous work where he tries to show that these objects just don't form. And the way he tried to show it was that he was trying to build one of these objects, and he showed that matter would just reach the speed of light outside the horizon, and then he would conclude, therefore, it's because we cannot go beyond the speed of light, this stuff doesn't form. Nature finds some way out of that. But then in the 70s, observations were giving us stuff that we just couldn't explain in any other way, right? So there was this immense theoretical effort to dissect, to understand all the physics of black holes that finally, in the last 10, 20 years, has been met by observations
Speaker 1and experiments. It is kind of fascinating to me how scientists throughout history, at least the parts of history that I know about, let's say the last 200 years, will often, you know, derive a wonderful equation, and the equation fits the data perfectly well, and the equation has some implications they don't like. So they just won't believe it, right? Whether it's black holes, or antimatter, or the Big Bang, or many worlds in quantum mechanics, you know, like, we don't often have the courage to face up to the implications of our own equations.
Speaker 2That's a beautiful question. But there's also something mysterious about it. I get asked this question so much. How can you study black holes with pen and paper and the supercomputer? And then you're trying to tell me that the object and the implications you find on your screen, tell us something about what nature produces millions of light years away. There's something intense about this, that the universe is not the same. It's not the same. It's not the same. It's not the same. There's also something else in that, which is, well, in this particular case, we also know that the theory itself breaks down inside of black holes. There's a case to go after and try to check detail by detail whether the predictions of general relativity hold true close to black holes.
Speaker 1And yeah, we will, we will definitely get there. Let me, let me just say this. But one more sort of preparatory question, because you remind me of I was actually an astronomy major as an undergraduate and a graduate student. In fact, I have no degrees in physics. I don't know anything about it. It's all astronomy. But I was shocked at how because the professors I worked with as an undergraduate were just doing photometry, not even spectroscopy. Right. They would take a light curve of some eclipsing binary star and you could show them this light curve. And they would read off this elaborate story about, oh, there's an accretion disk and it's comospherically active star and whatever. And they're able to milk all of this detail out of very little data. And I'm wondering, is that the impression you get these days from much more sophisticated observations?
Speaker 2Well, not yet. I do get the impression we're moving in that direction. But because this field, gravitational wave astronomy, is built, the foundations is called. Much filtering, which means we need to have extremely accurate predictions from general relativity to go and dig the signal under the noise. It's still I would call it a precision science. We know to 10 percent level what we're going after. And so this is not a hand waving kind of science.
Speaker 1Yeah. OK, good. So let's back up then and let's dig into some of the details about what someone like you does for a living. Famously, black holes are not that varying in their structure, right? Like there's a no hair theorem that says that all black holes are kind of simple. Can you tell us about that?
Speaker 2Yeah. So there's a there's a result, a mathematical result in general relativity that says if everything is devoid of matter, so vacuum, OK, then black holes need to belong to a certain family. We call it black holes. The Kerr family and the Kerr family basically is specified entirely by two parameters, the mass of the black hole and the rotation of the black hole, how fast the black hole is rotating any black hole in the universe. That means of all the trillions of black holes we think are out there, all of them are specified entirely by just two parameters. And kind of maybe it doesn't look like much, but it is it's like saying, you know, we have nine billion people in the planet. And all of them and any of them are totally specified by their height and their mass, which is obviously not true.
Speaker 1Right. And there's also charge in there, but you left that out.
Speaker 2Yes, there's also charge, but we think that the universe doesn't really like charge very much. Whenever there's electromagnetic charge somewhere, we go and get another to neutralize it because the universe, as we see it, is neutral to a very good extent. But let me just say, this is one of the things where people like me and thousands of colleagues are important, because this no hair theorem, the fact that black holes are fully specified by two parameters, as you were saying, it has ingredients. The proof, the mathematical proof requires some ingredients, and one of them is the geometry has to be stationary, which means it's not varying in time and it's vacuum. And so we need to think beyond this. Clearly. Clearly, you and I, Sean, are talking here. So vacuum is not a good assumption. And because we are talking, stationarity is also, things happen in the universe. And so one of our jobs is to go and say, OK, so let's now try to break these assumptions. How is that going to change the object? How is that going to change the dynamics of these guys?
Speaker 1I remember reading a quote, and I think it was from Chandrasekhar, I'm not sure, but how struck he was with this. I think it's a beautiful fact that literally every black hole in the universe is precisely described by the Kerr metric and just the Kerr metric. The Kerr, by the way, is K-E-R-R for those listening at home. But I remember when I read the quote, my immediate response was, except no, none of them are, because there's stuff around them. You're not embedded in empty space. There's a warping of space-time because there's extra stuff, so those details might someday matter.
Speaker 2That is true. So I use that quote very often in my talks. But I think Chandrasekhar has a history of dealing with stars and planets. I mean, he's one of the big names of people doing stellar physics. And when you do stellar physics, you realize that the equation of state, the relation, the exact relation between pressure and density of matter does change the global structure of a star, for instance. Whereas black holes, because gravitational collapse acts. So efficiently, it just cleans them of any detail, of any other detail. They're really just simple, curled-up vacuum.
Speaker 1Fair enough. No, I mean, I got the spirit of it. And Chandrasekhar obviously is someone we all should admire for the right reasons. Maybe one thing that I should have said earlier, but let's get it right for the audience. For what you're talking about and what we're going to talk about for the rest of the episode, what happens inside the event horizon is completely irrelevant. Is that correct?
Speaker 2As far as we know, it's, by definition, it's totally irrelevant. By definition, the horizon is this surface beyond which we have no access. So it's causally disconnected from us. There's no experiment we can do in the exterior that let us see inside the black hole.
Speaker 1Yeah. So this is kind of what inspired some people to say, like, you can think of the black hole as just this two-dimensional boundary at the event horizon. And it doesn't matter what's going on inside it. You made me think of it because, of course, for stars, it matters a lot what's going on inside.
Speaker 2Totally. But I think that's also what makes black holes such a special thing. I mean, so suppose you're told there's a box with all sorts of things and precious materials in, unimaginable things, but no one can open it. You have no access to that box. This is exactly what a black hole is, right? All the secrets. The secrets that we think there are about quantum gravity or quantum effects in strong gravitational fields.
Speaker 3Yeah.
Speaker 2The fate of the star that collapses, all of that is hidden from us. So it's very different from anything else we know of around us.
Speaker 1Yeah. Okay, good. So we have mass and we have spin. So let me just ask questions about the actual values of these things in the real world. Is it one of these things where spin could exist in principle, but in fact, most black holes are pretty stable? Is it stationary or the other way around?
Speaker 2Well, I mean, so rotation is inherited from the progenitor. So if I have a star that's suddenly runs out of fuel and it starts collapsing, it will carry the rotation. We call it angular momentum. It will carry the rotation as it collapses. It's true, on the other hand, that black holes spin slowly. There's also a result in general. There's a little bit of relativity that says that the rotation of a black hole is limited by an upper number. Okay. And if it goes beyond that, it cannot be a black hole. It has to be something weird. That means that as stars, on the other hand, carry a lot more on the average, a lot more angular momentum than a black hole with the same mass can hold. So as gravitational collapse proceeds, the star has to get rid of the angular momentum. It's expelled. It expels angular momentum in winds and so on. Maybe it's good to have an idea of what exactly we're talking about. So if a child takes a spinning top, okay, like just a toy, and makes the spinning top, puts it to spin on a table, the angular momentum that this toy has is orders of magnitude above that which a black hole could carry. Okay. So from this way of measuring. You mean, I mean, sorry.
Speaker 1You mean like per mass or something like that. A black hole is very big.
Speaker 2Yeah, per mass. So what I mean is there's a dimensionless combination of angular momentum, which is over rotation, angular momentum over the square of the mass. Okay. This number for a black hole is very small. And what I'm saying is a child can produce something which is orders of magnitude above this.
Speaker 4Okay.
Speaker 1And you did sort of say something provocative in there that I'll let. You elaborate on, you said, like, if it did have more angular momentum, it would be something very weird. Does that mean it just can't have more angular momentum than that? Or are there weird things that.
Speaker 2That's an open question. That's an open question. Maybe let me be again, very practical. If our planet, the Earth, would suddenly decide to collapse to a black hole and everything that composes the Earth would fall into the black hole. It could not be a black hole. It could not be a black hole. Because it's spinning too fast. And what the equations would tell us is there would be no horizon.
Speaker 4Yeah.
Speaker 2Right. So we would be able to see whatever happens inside. And we don't have a theory for that. Right. We don't have a theory that takes us all the way in a collapsed object with too much angular momentum. So we try to protect ourselves from this. We call it the cosmic censorship, which says, you know, there will always. be an horizon protecting us from when gravitational collapse happens. But this is pretty much an open issue in physics. We do not know if the conjecture is true or not.
Speaker 1That's why we do both theory and experiment, right? We've got to figure these things out. It's a journey.
Speaker 2That's true. And we saw when we do physics, gravitational physics, in a number of space-time dimensions higher than four, we have seen cosmic censorship failing. We have seen all of this goes to, yeah, it doesn't work.
Speaker 1Okay, let's go back to the real world then a little bit. When I was young, when I was your age, we thought that black holes would arise from explosions of massive stars. And, you know, the typical black hole would be a few times the mass of the sun. But now, of course, we have data. We're not just guessing. What is the distribution of different kinds of masses of black holes?
Speaker 2Yeah, so in gravitational wave science, we have seen black holes, which are more massive. Then we'd expect, we would expect some mass, we call it the mass gap, no black holes in a certain mass range. We see black holes exist all the way up to 120 solar masses. We don't really know how they form. But if we look at the beginning of the universe, we are also seeing objects that seem to be black holes way more massive than we thought they could be, way earlier than we thought they would form. Possibly they form out of the gravitational collapse of clouds. Possibly they form out of the gravitational collapse of clouds. Possibly they form out of the gravitational collapse of clouds. Possibly they form out of the gravitational collapse of clouds. Possibly they form out of the gravitational collapse of clouds. Possibly they form out of the gravitational collapse of clouds. Possibly they form out of the gravitational collapse of clouds. Possibly they form out of the gravitational collapse of clouds. Yeah. Of dust. But that's still pretty much an ongoing debate.
Speaker 1Maybe you can give the audience some feeling for why physicists are so surprised at all these black holes with different masses. Because, like, how hard can it be to make a black hole?
Speaker 2Well, that's a wonderful point. So if I'm given stars across a mass range, if I'm given a star which has 10 solar masses or 100 solar masses, to 1,000 solar masses, I can easily ask the star, just burn the fuel and collapse the black hole. The problem is that if I try to form one of these stars above 80 solar masses, for example, the star becomes unstable. It doesn't want to be there. And we don't really have an elegant way to get across this gap to tell the star, you know, just stay put, be stable, burn the fuel, and then collapse. That's one of the issues that we're having.
Speaker 1So it would, if you had a big cloud that you were trying to make a 100 solar mass star from, our current state of the art says it would just break up into several stars. That's correct. That's correct. So, of course, you can always say, well, okay, that's true, but then why don't I assemble a black hole via, I collapse the last massive star, then I form a black hole, then I collide it with another black hole, and I grow this way.
Speaker 2The problem is that we don't know exactly how to do it, and we don't know how to do it, and we don't know how to do it, and we don't know how to do it, and we don't know how to do it. The problem with this is that, you know, the black hole is not such a populated place, so it's hard to merge two black holes. Black holes are really tiny for the mass they have, so it's hard to make them come together and just start growing in this way.
Speaker 1And so, okay, but so what is the population census of the black holes? So we have these 30 to 100 mass black holes, and I know that we have supermassive black holes in the centers of galaxies, so, like, what does the distribution look like?
Speaker 2So we have a pretty good understanding up until when there are 200 solar masses, we know the rate, we know how many they are, how many collide per year, the LIGO-Virgo gravitational wave network has been giving us these numbers. We have also a very good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass. So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass. So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass. So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass. So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass.
Speaker 1So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass. So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass.
Speaker 2So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass. So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass.
Speaker 1So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass. So we have a pretty good understanding that all the galaxies have supermassive black holes, and I mean by this the 1 million to 1 billion solar mass black holes, they sink towards the center of the star as the millions of years pass.
Speaker 2So those are harder to form with the traditional classical gravitational collapse, just because it's hard to form, we know how to form planets like the Earth, the Moon and whatnot, but those are not going to collapse into black holes because pressure there is just going to hold the system forever. So we think we might be able to form tiny black holes out of, we call it quantum fluctuations. In the beginning of the universe, we have not seen any of these yet, there's claims flying around that maybe we have seen a couple of them, but the signal to noise ratio is really just too small, meaning there seems to be something there, it's really just claims. This would be an extraordinary claim, to say we've seen a pair of objects colliding, and these guys have a mass smaller than, say, one solar mass. Then I think the least radical explanation would be this should be two black holes that have to have been formed in the beginning of the universe out of some quantum process. This is amazing, this would be amazing.
Speaker 1That would be amazing. What is our best chance for making that happen someday?
Speaker 2Continue observations. I mean, I think if they're out there, as years go by, the evidence, the signal to noise ratio, we call it, would just keep on growing. Right. So if we keep the instruments on, and they exist at some point, we will have substantial evidence for them, if they are there.
Speaker 1Do you have any credence that the dark matter might be tiny black holes?
Speaker 2That's a tough question, because, so I was raised in high energy physics, I quickly moved to gravitational physics, gravitational science, and I realized that, maybe just as you, Sean, that dark matter is kind of a world on its own.
Speaker 4Oh, yeah.
Speaker 2So, you know, everything is possible, anything is possible, because the only measurement, if you wish, of dark matter has been with gravity, has been with the way things move via the gravitational interaction. And I think some of us try to hold on to everything we've known, traditional matter, and that means baryonic matter and black holes, as an explanation for dark matter, but as years go by, all or many of the possibilities have been ruled out. So most of the black hole range that could explain dark matter has been ruled out by microlensing. So if they were under the form of black holes, once in a while, micro black holes, one of these black holes would pass in front of a star, and it would lens the light from that star, so we would see the light from that star changing, and we haven't seen this, so that kind of rules a large fraction of parameter space. You can still hold on to that explanation. There's a tiny corner in parameter space where black holes would still be an explanation. To me, it sounds too good to be true, or it sounds a bit desperate to be true.
Speaker 1Right, right. No, this is a very good point. I want to dwell on this, because scientists do think this way, and it's good to sort of let the broader public in on how we think. It could be, if I understand what you just said, that dark matter is black holes. When you propose that explanation, you have to be more specific. Like you say, what mass black holes are you talking about? And then you can start ruling out different possibilities from the data, and what you're telling us is we've ruled out most of the possibilities. There's a little sliver left, but come on, like how unlikely would it be that the universe lives exactly where we just haven't looked yet?
Speaker 2Exactly. So if most of dark matter came under the form of very massive black holes, these would lens the light from stars, and we would see the star kind of bleeping. If the black hole was just too small, so very light black holes, they would evaporate under Hawking radiation. So they wouldn't be here today for us to see it. Yeah. as years go by, you use scientific method, you start excluding possibilities. So right now, I would say it's possible, but it's just unlikely. Okay, good. And this is fascinating,
Speaker 1because you've already given us a couple of glimpses into how we do collect data on black holes. You just mentioned microlensing, previously talked about gravitational waves, LIGO and Virgo. But there's also this thing called the Event Horizon Telescope, which is kind of sexy and
Speaker 2interesting. Tell us about how that works. Yeah. So in fact, in the last, I would say, decade or so, we realized that we can... So the way telescopes, traditional telescopes work is they gather light from distant objects. And so the more light they gather, so the larger they are, the better. So of course, ideally, we would have a huge global telescope that just gathers the most amount of... Of light that we can. But we realized that we can replace a mega telescope with just an array of different telescopes spread throughout the globe and combine the data, combine what we actually combine is the electric field that each of those telescopes measures, so as to produce an equivalent super large telescope. So the Event Horizon Telescope, in essence, is something like that. It's... Yeah. It's a... It's a... It's a... It's a global array of telescopes that observes mostly two galaxies, M87 and the center of our galaxy. And the idea is to observe the central black hole. It's not the only one. There's a different instrument. It's called the gravity instrument.
Speaker 4Okay.
Speaker 2That's focused on the center of our galaxy. The working principle is similar, but the Event Radio, the gravity instrument works on the infrared. So the oscillations in the infrared, oscillations of the electric field, are just too rapid for us to collect the electric field. So they need to combine on the spot. So these guys take four telescopes and combine the observations in one go on the spot, okay?
Speaker 4Okay.
Speaker 2But so, yeah, I'm getting technical, but it's important because there's been a huge progress that's due to this technological understanding of how we can combine different measurements. And so what we've been doing is actually observing matter, and by matter I mean either stars or hot spots, hot material close to large black holes. And they need to be large because the telescopes are good, but they still need a large angular thing on the sky. So we need large black holes. And the two largest black holes we know of are the one in the center of our galaxy, just because it's very close, and the one in the M87 galaxy. It's a thousand times larger, but the galaxy is also a thousand times farther. So roughly the position on the sky is similar.
Speaker 1Yeah, so when you say we need a large thing, you want the angular... The angular size of it to be large. We know that out there in the universe there's even bigger black holes than our galaxy has, but they're further away.
Speaker 2That's right, but they're just further away.
Speaker 1And I want to dig into a little bit more what exactly it is we're seeing when we do these observations. They're called the Event Horizon Telescope, but we're not really seeing the event horizon. The event horizon is not giving off any light.
Speaker 2That's true. I mean, the event horizon, by definition, as we kind of discussed, is impossible to see. Yeah. But I still think... It's a sexy name. It's a good name. It's a very good name. It should be called the Light Ring Telescope. Tell us what that is. So around black holes, or at least around the black holes that we think that gravity produces, there's a region where if you send a laser beam, okay, if you shine a laser beam very close to the black hole in this so-called light ring, the laser beam is going to orbit. It's going to orbit in a circle around the black hole. Okay. So you can think of this as extreme light deflection. Light is always falling in the same way that the moon is falling to the earth, but it's always falling. So therefore, it's orbiting. So light can orbit black holes in this closer trajectory. So this light ring is actually what defines the things we can see when we look at a black hole. What this means is the... The following. If you take a black hole and you place a source of light behind the black hole, so you're going to do, you're going to see some black hole shadow. The shadow of the black hole is governed by this light ring. Any photon, any light that goes within the light ring just gets trapped by the black hole. It falls into the horizon. Okay. Anything that's pointed outwards of this light ring is going to eventually come to an observer. Such as us. So what's in an image? I guess my point is, why should we choose a certain name or another? And what exactly is in an image? And I think there was a lot of discussion. I've been in some of the discussions of what an image even means, what the Event Horizon Telescope and similar instruments are seeing. And so in the end, I do think it's a wise choice to name it the Event Horizon Telescope, because in the end, it is, I think, a good description of what we're after. And we're after physics close to the Event Horizon of a black hole, because that's where we expect new things to occur. If there's new physics, it's going to show up close to the horizon. We're trying to go as close as we can.
Speaker 1And so is the light coming from stars behind the black hole or from stuff in the accretion disk? Or what is the actual light that we're seeing in the Event Horizon Telescope?
Speaker 2Yeah. The actual light that we're seeing comes from possibly remains of stars that were tidally disrupted and formed what we call it an accretion disk around black holes. And friction heats up the material, the material heated up gets rightened. And so that's exactly what we see. We also see in both the center of our galaxy and the galaxy M87, that there's transience. It's not a perfectly smooth, stationary, boring disk. There's flaring episodes where we see things happen, which possibly is what you would expect, but this flaring and all this activity in the future. So this is new science for the next 10 years or so are going to be used to make videos, no longer images of the center of galaxies, but videos. So it's going to be, there's amazing stuff coming, coming on. And again, the purpose is to understand what exactly is gravity doing close to black holes.
Speaker 1I think people are a little bit. Spoiled by these ultra high precision images of black hole accretion disks, which are actually from the movie interstellar, not from actual data. The real event horizon telescope images are kind of blurry.
Speaker 2Oh, they're extremely blurry and, you know, they're reconstructed. And that means, in fact, they're reconstructed from, I would say, roughly 10%. So 90% of the image is reconstructed and it's reconstructed. It's reconstructed based on thousands of simulations that we do of matter around black holes. So there's prior knowledge that goes into these images. Ideally, we would have instruments that see much, much better and that don't require prior knowledge. And I think that's where we want to go in the future.
Speaker 1Do we think that essentially all black holes have accretion disks around them to help us see them? Or are we just looking at the ones that happen to be lit up that way?
Speaker 2I think the lore is that supermassive black holes should have dead stars because it's easy to smash a star when it approaches the black hole. Stellar mass black holes, less so. And we have no, so we've seen two or 300 of these guys merging and we have never seen an electromagnetic counterpart to the gravitational wave signature. So if they do have matter around them, it's really weak. And we don't have many mechanisms to produce a lot of a substantial accretion disk around stellar mass black holes.
Speaker 1So just to be clear, my impression is we have seen electromagnetic counterparts for LIGO events, but that's because there was a neutron star involved. You're saying that whenever it's just two black holes, we've seen no photons.
Speaker 2We have seen no photons.
Speaker 1Yes.
Speaker 2The only big event was in fact a neutron star binary merger. There was a beautiful event. We saw light. In fact, we used that beautiful event to understand that gravitational waves travel at basically the same speed as light. Because the interval between the arrival of a photon and of a gravitational wave was basically zero.
Speaker 1I think that's worth digging into even more deeply. You're just emphasizing because to we theoretical physicists, of course, gravitational waves and electromagnetic waves travel at the same speed. It would be absolutely flabbergasting if they did not. But therefore, we should test it because we like to be flabbergasted by big discoveries. So, I mean, and we did. That's what goes on. We test all these big ideas. I mean, what is your... How do you think about results like this with the speed of gravity versus the speed of light?
Speaker 5To be honest, Sean, I dislike them because they're... Good, be honest. It's time to be honest. Well, I think most of my career were after the unexpected.
Speaker 2So maybe we're going to see in our lifetime something that's totally against the law, something that's going to say general relativity is wrong or black holes are not what we thought they were. And maybe that would be a good start. Gravitational waves are not traveling at the speed of light. So I'm always expecting some news along that side of things. So I always got to be not depressed. It's traveling at the speed of light with 15 decimal digits. Okay, good.
Speaker 1So you don't like it in the sense that you love the experiment. You don't really like the result that we've gotten.
Speaker 2It's a beautiful result. It's also somewhat unexpected and lucky in the sense that we measure that speed by observing gravitational waves. And 1.4 seconds afterwards light from the same event. So that constrains a lot. It's also somewhat lucky. But it's a beautiful result. It's not exciting. Yeah, not the one you wanted to get. Because that's what we expected.
Speaker 1Right. That's right. And just so the audience has a lay of the land on the experimental side, we're still working, as far as I can tell, with LIGO and Virgo, which were the two big gravitational wave observatories that won the Nobel Prize for stuff 10 years ago. I know that everything takes very, very long. What does the future horizon look like for different kinds of gravitational wave detectors?
Speaker 2So I think, first of all, gravitational wave, physics, science, I think, is going to be the future because it gives us an entirely new channel. So there's a bunch of new detectors programmed. Some of them are online, like CAGRA in Japan is working in late. So around 2030, India is going to have one detector that actually came from the U.S. So they should also be operating elsewhere in the globe. Europe is planning the Einstein telescope. Europe and U.S. are about to launch LISA. And they're going to launch LISA into space. So I think there's -- and then there's new technology, like atom interferometry, that's also aiming at seeing gravitational waves, but not with, you know, light interferometry, but atoms.
Speaker 1But tell us what this is. What is LISA? What is the Einstein telescope?
Speaker 2So they are, if you wish, upgraded versions of LIGO. So the working principle of LIGO is really just to send light beams between two mirrors, and they record the distance by seeing how the mirrors move, if they move, and if it's not noise. But the ability to see the gravitational wave clearly depends on roughly the distance between these mirrors. So LISA is going to fly into space with a length between the mirrors of the order of a million kilometers, which we should contrast with LIGO's four kilometers. So that's an increase in sensitivity. But it also -- it gives us access to the low frequency regime. So LIGO is measuring events, gravitational waves that have somewhere between 20 hertz and a kilohertz. So the size of the detector roughly mimics the size of the source we're looking for. So LISA is going to look for supermassive things, very big things, lower frequency things. And in parallel, we're thinking about the Einstein telescope, which is really a better version of LIGO. The arms are going to be slightly longer. The technology is evolving. So that's also going to be better. And the hope is maybe we see something we were not expecting. We're going to probe different scales. We're going to see better. Something has to give. You know, something has to come up.
Speaker 1So Einstein telescope is here on the ground? It's an interferometer just like LIGO?
Speaker 2Yes. It's here on the ground. It's not decided yet where exactly it's going to be. Germany? Where? Italy? Italy. But that's still under discussion.
Speaker 1Okay. Very, very good. I mean, what would LISA, which is looking at different, you said supermassive things. So like, what do you hope to measure about supermassive things?
Speaker 2Well, first, we hope to see if supermassive black holes exist out there and if they're interacting. We know we've seen supermassive black holes in isolation. We want to know if galaxies merge and if we can see them. But there's a number of other things. There's a number of other things that happen at low frequency, or at least that we expect that happen at low frequencies. And one of them relates to the birth of the universe, right? When the universe is forming, you might expect things to happen, non-homogeneous things that get redshifted, that get pushed to lower frequencies as the universe expands. So that's one of the things we might be able to see.
Speaker 1Okay. Yeah, that's cool. So we do have good evidence. For the existence of these supermassive black holes, but we don't have a lot of data. I mean, it's all kind of indirect, right? I mean, we see things nearby. We see the accretion disk, et cetera. The Event Horizon Telescope has helped us a bit.
Speaker 2That's right. I mean, it's also based on expectations, right? You expect that as things get more massive, they go down and pile up at the center of the galaxy. So we do have some formation mechanisms, robust ways of growing black holes and piling them at the center of galaxies. I think the question is slightly more interesting. The question is, first of all, how did these guys form and grew to be so massive, a billion solar masses? How does that happen? But also, and that's something that we do not have a good answer to, how do I take two of these supermassive black holes and I get them close enough together that they merge and they meet gravitational waves? We don't have yet a very road map. We don't have a very robust way of getting them across the last parsec, actually, of distance.
Speaker 1Okay, I see. So it's, you know, there's still some mysteries out there, but these mysteries are all, even if we think general relativity is the right theory of gravity. I mean, the other fun thing to think about is, could we finally possibly discover something that is not consistent with general relativity? I mean, do you have a favorite, either theoretical possibility for what that could be? Or experimental possibility, or how we could find that?
Speaker 2I think, yeah, so I think each of us has their own favorite thing. But if you, if we think back to the beginning of our conversation, which is how, just how grotesque and special black holes are, my favorite item is, can we quantify the evidence that the things we're looking at are black holes? How do we do that? The only thing we're doing is measuring how mirrors move. How can we measure? The intellectual, you know, challenge is enormous. How can we, from the motion of two mirrors, understand if we're looking at black holes and how deep into the gravitational well can we probe? I think this is a fascinating issue that takes most of my time, I would say.
Speaker 1Okay. So you're, you're kind of model independent in some way. It's not that you have a favorite alternative to general relativity. You just want to see how close can we push the data to figuring out whether GR is on the right track or not.
Speaker 2That's right. Actually, I would say the opposite. I don't think there's any alternative to general relativity that's more elegant or that solves any of the problems that general relativity has in a better way.
Speaker 1Well, you know, again, you and I are theoretical physicists, but for the people out there on the street, I mean, they've all heard that quantum gravity is hard to do. How do we know that quantum gravity won't change the predictions of general relativity for black holes?
Speaker 2That's a very interesting question. So if we had predictions from quantum gravity of what would happen, and that would tell us you go out there, you measure this, you're going to find this, but those predictions don't exist. And I think it's also an interesting stage in physics somehow. Physics used to be, and I want to think it still is, driven by data and by observations. And so hopefully, as precision in data gathering increases, we're going to find more and more data, and we're going to find something that's really not consistent with the paradigm. And that's when you and I need to sit down and think, how can we model this in a better way?
Speaker 1OK, but I mean, do we have expectations for quantum gravity? Is there any hopes of seeing a hint of it in data from LIGO or LISA or Einstein or anywhere else?
Speaker 2I think the most serious problem we face in gravitational physics, maybe you can correct me if you can, but to me is the existence of gravitational singularities. We really don't know how to work with a theory that contains singularity. Now, as we were discussing, these are hidden or seem to be hidden from us within horizons, within black holes. So I think it's a natural expectation that if there's a theory of quantum gravity that's going to change, that's going to resolve, that's going to do away with singularities, I think it's also reasonable. It's a reasonable expectation that it's going to do something to the region close to the horizon. In fact, I think many of the hand-waving things that we've seen in the last couple of decades tell us that there are issues in doing quantum mechanics around the horizon. So it's a natural expectation to search for changes close to the horizon, changes relative to what the theory of Einstein predicts. Good.
Speaker 1So then let's dig just a little bit more specifically into what the data can tell us. I mean, I remember when I was hearing colloquia about LIGO before it existed, right? You know, when they were still planning, there was these pictures that you would be shown, okay, there's two black holes, they're spiraling in. And the claim was, we understand very well what it should look like when the black holes are spiraling together. We don't understand what it's going to look like after that. It's sort of messy, and there's angular momentum, and we don't know what's going to happen. But my impression, which is a quasi-outsider here, is that it wasn't all that surprising, actually, when we collected the data. That ring-down phase is better understood than we thought it was a few decades ago.
Speaker 2Totally. I think that's our, in hindsight, but again, hindsight is always 20-20, that I don't think we could ever have expected anything grotesquely different from a boring relaxation stage. Two black holes come together, they merge, a single horizon, a single black hole is born, and the only thing left for this guy to do is to relax. To the final quiet stage. And that's what we've been seeing in numerical simulations, and that's what we've been seeing in observation. But I think it's also easy to discard just how revolutionary it is, the stage we're in. We are, for the first time in the history of humankind, seeing two black holes relaxing in the gravitational wave channel. I mean, it is amazing. They relax at the speed of light. So a black hole... A black hole that's 10 kilometers wide, so the size of Copenhagen, relaxes in a fraction of a millisecond. How amazing is that, that we have technology to measure this? So, you know, I think we should tap ourselves in the shoulder at least for once.
Speaker 1It is a funny thing. Like, things that we thought were completely mysterious a little while ago, we figure out the answer, and then suddenly it's old hat. Like, of course it's like that. Like, let's move on.
Speaker 2Yeah, and then we move on and forget how amazing it is.
Speaker 1It is very amazing. So we... And again, I'm sort of showing my age here because I actually heard more talks about LIGO, I think, before it collected data than after. There was also the claim that we needed to really understand the templates. You know, we needed to understand the prediction ahead of time so that when we collected data on some event, we could say, oh, yes, the mass of this black hole was this. The angular momentum was the other thing. I mean, is that still true? Is that the right way of going about taking the journey from the data to the story that we tell about what happened?
Speaker 2Totally. So actually, my first international school, this was back in 2001, and all the experts, these are big names, I heard them at lunch saying, you know, I think in the end, these detectors will only see earthquakes. They'll never see gravitational waves. And then, you know, 15 years after, here we are, and we see all of it. So... So, yes, totally the way it goes. So we call it match filtering.
Speaker 4Okay.
Speaker 2And really, it means we match the signal, the output in the detector we have with some theoretical expectation for the signal. And that means solving Einstein equations and finding a very precise and accurate waveform, a prediction for the signal as a function of time. We have other ways of checking that there are events. It's not the only way. We have other ways of checking that there are events. We have to search for it. It's just the best. Banks and other companies have been using this for decades. Even just to know if it's your signature, we do match filtering as well. We compare what you write down in the paper with prior expectations because somewhere, at least back in the day, we used to have a signature somewhere in the bank of banks. So this is still the way to go. And you can imagine the unbelievable effort that it took us to have template, banks of millions of waveforms. We need to have a template bank for each of the possibility because it could be that a 10 solar mass flakal is merging with a 20 solar mass flakal. But we don't know. Maybe it's a 10 versus an 11 solar mass flakal. And we need to build templates, accurate templates that describe the full history of the merger until they collide. So it's a colossal effort. And as a community, I think we've done brilliantly. Yeah.
Speaker 1And is that still sort of the future? I guess I'm still a pencil and paper theorist. I still write down equations. Maybe I use my iPad now rather than literal paper. But other people just sit at the computer and write a code and make predictions. Is there still room in black hole physics for pencil and paper theory? Or have we turned it over to the computers?
Speaker 2I don't think so. I think there's room and there's the need for people. Like you. I'm also on that camp, I would say. We need both sides. We need people that sit down and say, we need to run supercomputer simulations for five years until we have this bank of templates. But you know, you only get what you feed the machine. Then you need other people doing their job, which is to say, look, that's all fine. But I'm afraid you're only doing vacuum black holes. The universe is full of plasma. The universe is full of dark matter. How can you do that? And you start including that. Let me tell you a few things. Okay. And so that's where you and I should come in and say, let's join hands. It's a larger scale effort. And now you see having two black holes merging, you need to specify mass. You need to specify rotation of each of these black holes. So it's still doable. We need millions of templates, but it's still doable. When you start adding dirty astrophysics, now I need an accretion disk. Now I need the dark matter halo. Suddenly building banks. I need a lot of templates for this. It's not feasible. So the search will have to be hierarchical. We need to find a way, template-based, for instance, that sees something happens. And then we take this event off the grid and we say, okay, now let's scan this for maybe there's environments here. Maybe there's some dark matter halo around the black hole.
Speaker 1Well, dark matter halo around the black hole. That's an interesting idea. I should have asked this earlier, but I'm sure that many listeners are thinking, like, what about dark matter? Yeah. Does not knowing too much. About the dark matter get in the way of making predictions for black holes?
Speaker 2Well, it does get in the way. But on the other hand, you know, it's our job. So it gives us a way of keeping things going. So the possibilities are so wide. You're right. That where exactly do we start? Yeah. Is dark matter like normal matter, like dust, that we can somehow model and think that it's going to pile up and orbit around the black hole? Or is it something a bit different? Like a field, like radiation, long wavelength radiation that hovers around the black hole? And how do we model this? There's been progress, I think, on both sides of these possibilities. But it is a challenge. What is dark matter?
Speaker 1So it's late in the podcast, so we can be just a little bit even more technical than we've been being here now. I mean, you mentioned sort of long wavelength waves as dark matter. I know that. Axions are a favorite dark matter candidate, one of my favorites. And I also know that people have put a lot of theory work into the interaction of axions and black holes. Like axions are a certain kind of elementary particle that can kind of hang around black holes and have effects on what we see. And so you know much more about this than I do. Why don't you tell us what that story is?
Speaker 2The story is beautiful. So even without the axions, when the concept of... Black holes was kind of understood, people also realized that if you shine light on a black hole, and if the light is sufficiently low frequency, then you shine light and it's going to come back with a higher amplitude. You get more than you put in. Right? I mean, it's not that surprising. I can extract energy from a carousel. Anything that's rotating, I can use it to extract energy.
Speaker 1It's not magic. You're actually just... Just getting energy out of the black hole by slowing it down.
Speaker 2That's right. You're just slowing the black hole down. The special thing about black holes is that it all happens in vacuum. That's, I think, the feature. But the point is, we also understood that if light had a mass, if light was like a stone, as it tries to escape the black hole, eventually it falls down again. Right? So you shine some light on a rotating black hole. This light extracts rotating energy from the black hole. It tries to escape. It tries to go wherever it wants to go. But it has a weight. So it falls back. If light had a weight, it would fall back. It would be amplified, and so on and so forth. We call this the black hole bomb mechanism.
Speaker 4Okay.
Speaker 2And when actions come into play, when we realize, wait a minute, we can do physics with black holes and actions, actions usually have a mass. They are light. but they have some mass, so they fall back and we realize that what this would lead to is to the condensation of clouds of action clouds around spinning black holes it's a fascinating thing to think that if dark matter would come under the form of axions there might be systems there that look like atoms a nucleus which is a spinning black hole surrounded by a cloud like the hydrogen atom a cloud of axions it's a fascinating thing there's been thousands of work trying to predict the spectrum of this system how do they look like how much energy is in the cloud and so on and so forth and what is
Speaker 1your I don't know this is an unfair question what is the probability you
Speaker 2think that axions exist that's a tough question because zero I always take it
Speaker 3at zero
Speaker 2so I work I think we most do we work because of the theoretical challenge we think there's a non-zero chance that axions exist in the mass range that's going to be giving us something interesting and then there's the challenge of how exactly we solve these mathematical equations and we get a good understanding of the system and I think that drives most of us as we were discussing in the beginning it turns out that the universe is very naughty whenever you do something in an equation it finds a way of making it happen yeah so as I said I start with the assumption that they don't exist but I'm constantly
Speaker 1surprised things are out there that's a very good motto okay so the last question last issue I wanted to talk about is a slight change of path here we're doing this on this particular day this podcast because there's something called black hole week in Copenhagen that you're you're part of and we both know that black holes capture the popular imagination right I mean they're they're things that the person on the street knows about even if they know very little physics and so talk about that kind of public image of black holes you know is it is it good is it bad are we using it are you happy that so many people know about black holes uh do you want more of them to know about axions
Speaker 2like what how do you think about this well so a week ago I took a guest Noah Zilberman to to dinner in a restaurant nearby the Niels Bohr Institute we sat down we were talking about black holes she's doing quantum field theory in uh in black holes space times and after five minutes the manager of the restaurant overheard this and you do black holes oh my God I'm so interested in entanglement entropy around black oh that's good 10 minutes later a customer on the next table got up and said I love black holes can I go to the Institute can I learn more so so the point is I think we are capitalizing black holes still capture the imagination I think because there's things we don't know it's physics has to be completed and we know black holes are a key ingredient in the story so we're we're doing this we're reaching tens of thousands of people in Copenhagen we're just today I got a couple of messages of people strangers volunteering to help in this because they want to learn more about black holes they want to see how exactly are we going to learn what exactly do we know right now in 2026. so I think as a physics community we're doing rather well I think we're doing rather well Sean yes what exactly happens during black hole week many things I don't know so there's going to be an opera uh there's going to be a scientific event called science and cocktails where five experts are going to discuss the status of observations there's going to be an immersive life performance on black holes called your borrowed stardust there's going to be a comic books exposition things for children painting black holes with light all sorts of things that you can imagine it's going to happen so it's going to be wonderful what are the dates uh 22 to 26 uh well to 29 August so the last week and a half of this month okay I'm sorry I'm
Speaker 1not going to be in Copenhagen for this this sounds like oh Sean you have to come back you have to come back is it every year do you have black hole week every two years every two years okay good I I actually Googled it and I realized by mistake I was reading the 2024 one because it was it said like the 50th anniversary of Stephen Hawking's Fantastic Discovery but so was that the first black hole week or is this that was the first black hole week so now you're going to try to
Speaker 2do it every two years yes all right we hope you're here for the next one okay I'll put that on the
Speaker 1calendar tentatively that would be that would be good I love Copenhagen it's not it's not hard to love so uh that's good I'm very glad you're doing this I'm very glad the public is excited and Vitor Cardozo thanks very much for being on the mindscape podcast thank you Sean my pleasure you
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