Instant Genius - How black holes’ even stranger siblings could be the source of dark matter, with Carlo Rovelli

Episode Date: November 6, 2023

These days, largely thanks to science fiction movies, most of us will be familiar with the idea of black holes – regions of spacetime where gravity is so strong that nothing, not even light, can escape it. But what about white holes? In this episode we catch up with theoretical physicist Carlo Rovelli, author of the book White Holes: Inside the Horizon. He tells us all about his ground-breaking work investigating what is happening inside black holes, how they can give birth to white holes and how white holes may be the best candidate for dark matter yet. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Music just as the artist intended. Visit Name, Audio, to learn more. Hello and welcome to Instant Genius, a bite-sized masterclass in podcast form. I'm Jason Goodyear, Kimmerston-Editor at BBC Science Focus magazine. These days, largely thanks to science fiction movies, most of us will be familiar with the idea of black holes. Regions of space-time where gravity is so strong that nothing not even light can escape it.

But what about white holes? In this episode, we catch up with theoretical physicist Carlo Rivelli, author of the the book White Holes Inside the Horizon. He tells us all about his groundbreaking work, investigating what is happening inside Black Holes, how they can give birth to White Holes, and how white holes may be the best candidate for Dark Matter yet. So you begin the book by saying that in order to understand white holes, we first need to understand black holes. So I think that's the best place to start then. So first off, what is a black hole? You know, what's the sort of Cliff Notes version? Black Hole is

first of all, something we actually today see in the sky. And I think that's the best definition we have of black holes today. We see these things, many of them, in fact, millions, billions of them. And some we see very well. We even pictures of them. We have indirect evidence of them. We see things orbiting around black holes. We see the x-rays that come from the matters surrounding black holes. We see the gravitation waves produced by black holes that merge that fall into one another. So there are these things out there. And the best explanation we have about these things out there is the mathematics of black hole. Having said so, I think this is the best way of thinking about black hole, but it hides the main point, which that these black holes were

predicted a century ago by a theory, which is Einstein general relativity. So we actually knew that this could exist much before seeing them. In fact, there was a debate. whether there were just a prediction of the theory that is realized in nature, or whether they were just a prediction of the theory is not realized in nature. So a theory says this could be possible, but it doesn't have to be realized actually in nature. And I remember when I was a student at the university, my textbook, written while the main scientist of the time, Stephen Weinberg, Nobel Prize, great, great scientist, the text were very good, very accurate in explaining the mathematics

and what in principle kind of black wall be. But then it said something like, it's very unlikely that something like that exists there. And that was the early 70s. There was not yet any evidence of their reality. In fact, it was just beginning at that time, first hints that this could exist. So what is this thing?

It's a hole in space, in a very literal sense. So the name hole, not just metaphorical or, or to make it beautiful. It's truly a hole in sense, in the sense in which you can imagine of, you know, walking in the street and there's a hole you can fall down in very much that sense.

So imagine you have a large square in the center of your town and it's all, you know, flat or maybe a little bit hilly, but you can walk everywhere, but somewhere there's a hole. And a hole is a passage that can be narrow, the throat can be narrow, through which you can go into an inside, which can actually be very large.

Imagine you are a little ant that is walking around, and you go inside, and inside maybe there's a parking lot. There's a huge immense space inside, which from the outside is closed inside this little throat. Now, that's exactly what the black holes in the sky are. So, from the outside, they are just little spheres, spheres. A sphere, they can be small, they can be large. The actual black holes we've seen in the sky have very different sizes. Some are big, some are huge, so immensely large.

Maybe there's small ones, we don't know. But the point is that this sphere, imagine a sphere which is larger, I don't know, a kilometer, a big thing that a kilometer. But if you go inside, it's not just one kilometer cube. It's much, much, much, much, much bigger. So there's an enormous amount of space where you're going to fall in. And this is permitted by the distortion of space, which I instance is. in the midst. So that's basically what a black hole. From the outside, it's just a sphere with a mass.

You can be in orbit around it, or you can fall inside. And it's a huge distortion of space. Next to it, what it does is a huge distortion to time, which is what makes them particularly interesting. So you sort of touched on it there. The key to this whole idea is Einstein's famous theory of relativity. This predicts some fairly strange counterintuitive behaviors of space and time. So how does that relate to a black hole? A lot, because Einstein talked us through generativity that space can be deformed and time can be deformed. I'll say in a moment what it means time to be deformed.

Not only that, but the gravity, the phenomenon of gravity, the best way to understand it is through us a deformation of space and time, the consequences of deformation of space in time. A deformation of space is sort of comprehensible, right? you take a rubber sheet and you push it and then you get deformed. And in some sense, three-dimensional space can be deformed in the same manner. Information in time, it's rarely discussed, but it's not very complicated either. Information in times means that time is not the same for all of us.

So while for me, one hour passes, it's perfectly possible for you a longer time passes or shorter time passes. So we meet and then we separate, we meet again. and you've lived one hour, I've lived five days. And this is perfectly possible, and not only is perfectly possible, we measure that, we measure this in the laboratory. And what happens roughly is that wherever there is a mass, time slows down. So if you go closer to the earth, time goes slower. So literally, your feet have a shorter life than your head, okay, literally.

And this can be measured, in this measure in the laboratory. These distortions of time are very small on earth near an even bigger star, and a colossal near a black hole. And the way to visualize this is to imagine to go close to a black hole, not inside, close. What I do in the book is I take the reader first close to the black hole and then through the horizon and then inside and then so on, continue. But if you get just close to the black hole, you sort of stop there. You hover there with a rocket.

keep yourself at the distance. And then you look around, you're going to be surprised by a number of things. And the thing that's going to mostly surprise you is if you look back at Earth. Because if you look back at Earth and you look what's going on, you see everybody moving stupendously fast. You know, the day is going very, very fast and Earth rotating very faster around itself. You see the Earth that rotate around itself not every 24 hours, but much less. Depending on the distance you are from the Black Hole.

The more you get close to the surface of Blackwood itself, the faster you see the spinning and everybody moving and a civilization going around. So you see what's going to happen in the future, so to say. If you want to know what's going to happen on Earth, who is going to win the Third World War, and whether there would be humankind after nuclear war or not, and whether we will live in peace forever, all that. Just go close to Whitehall, stay a half an hour, you see the full thing, to see the future.

And vice versa, obviously, because it's the same thing. If you stay on Earth and you look at your friends who are just hovering there next to the Black Hole, you see them moving very, very slowly, incredibly slowly,

and speaking very slowly, and the clock's ticking very slowly, and the more they get close to the surface of the Black Hole, the more everything slows down. So this is a relative fact. In other words, on Earth's time is normal, and their time is normal.

Well, it's not normal. It's a relative speed of time of one place with respect to the other one. And that's the time distortion. The enormous time distortion when you go close to a black hole. And in fact, from Earth, if somebody got to the horizon,

you see everything stop, literally stop. slowing down so much that it stops. The light that you see becomes more and more red because the frequency of the light goes down, down. It's like the atoms oscillating, producing the light oscillates slow and slow and slow, and then they stop.

So you see everything's stopping. Because if you are there, everything is normal, and you can go through the horizon. So that was going to be my next question then. Let's say we've reached this point. What happens then? What do we know about that? We know everything. That's a remarkable thing.

That's part of the note. So everything I'm saying now, it's sort of, I would say, established science, solid science. Which is not the case for the Whitehall themselves, for the rest of the book. Because what I'm going to say after, it's what we think is going to happen, but we have far from short. But what's going to happen on the horizon? Like, all that we know. We know very well. And because it's predicted by the theory by Einstein,

and we're very confident because so far, sort of everything that the theory predicted has done out to be right. So it's a lot of confidence, very reliable, reliable in theory. So when we cross the horizon, as I said, what happened is basically nothing at all. It's just normally moving through some regional space. And that's actually the reason of the name horizon, right? Because if you are on the cost and look at the sea at the ocean, you see the horizon.

And if you see a boat going toward the horizon after the horizon disappears. So it seems to there's a line there. And that's the line where the ocean ends. But if you go there, there's no line whatsoever. You just cross it and nothing happened at all. Maybe you make a party because you cross the horizon. but it's a rise of seen from the Earth. Nothing special is happening there.

So similarly, we can enter the black hole, nothing happened. We look back and we still see the stars. We still can get the information from Earth just going faster and faster and faster, but we still get the messages from us. We cannot send back messages because they will arrive far too much in the future. That is normal. However, if you look around, we find out that there is something strange. First of all, there's this immense space, which we didn't expect, because it looked just a

kilometer outside and inside there are millions and millions of kilometers or miles you can go in. And the space has a peculiar shape. So it's like being inside a pit, a huge pit, a huge well which go down, down, not infinite, but very, very, very long. So this is tube, long, long, long there. Now, in a third surprise, not only there's this huge space very long, but it's getting longer, longer, longer with time, and that's bad news, it's narrowing around you. So it's like being in a deep pit, which, okay, is becoming longer, but it's also squeezing around you. And in fact, the space itself, you have less and less space around you, you're going to be compressed on the sides and squeezed in the, in,

your height, in the length, more and more. And that's the part we are confident about. So what causes that narrowing? The theory of generativity, Einstein theory, not only tells us that gravity is a bending of space time and is a distortion of space and distortion of time, but also that distortion of space and distortion of time are dynamical. They can change.

There's some equation that describe how they change. So time and space are all together, like a rubber band, which not only stretches, but there's some equations that tell how it stretches. Like a wave, they can... And this equation are the Ler Eastern equations. So this equation tells us how the dynamics of

the form space works, and they tell us that the dynamic inside is this squeezing. So it's gravity itself. The intuitive way of thinking about that, I would say, is that space itself is falling down, so to say, to closing up

along this increasing narrow tube. It's gravity. It's another way of saying that things fall. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building. Fit for your ambition for Citizens Bank. You said this place was steps from the water.

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Since day one, this mantra has shaped every innovation in hi-fi design, technology and acoustic engineering, balancing craftsmanship and tradition with pioneering thinking. Name audio pushes cutting-edge technology to ensure digital precision whilst sustaining Pratt, pace, rhythm and timing, the elusive quality that makes music feel alive and gives it a emotional texture. Today, in partnership with French acoustic specialist focal, name audio creates systems that deliver exceptional sound, and unforgettable listening experiences at home. Try it for yourself at a focal powered by name boutique. Visit focal powered by name.com for more information. So then one of your key ideas then is the process of a black hole becoming a white

hole. So can you talk me through that, please? That's a novelty, so to say, in the sense that here there is a step and it's a major step. So far, it's Einstein equations and they're reliable. If I had a lot of money in the bank, I would make a bet and I would be pretty confident that I'm not going to lose it. The reason we are not confidence anymore is because when the tube becomes very narrow, very curved into itself, the curvature is very high, it means the energy density is very high. The ice and theory is not good anymore because we expect quantum phenomena to kick in. In quantum phenomena, quantum mechanics, it's been the other great discovery of 20th century physics

besides gender nativity. Quantum mechanics tell us that there are quantum phenomena. So peculiar things happen, describe your quantum mechanic, which you usually disregard when you talk about big object because they're too small effect. But when you're deep inside the black hole, they're not too small effect. So quantum mechanics become important. So we need a theory that is consistent with high-stained theory, so to extend science and theory,

but it keeps quantum mechanics into account. And we have theories that do that. I've spent all my life working on luke quantum gravity, it's a theory that has exactly that, but they're tentative theories, namely they are not being supported by experience, by measurements, by observations, that have given full confidence in them.

So we have this tentative theory. And working with the tentative theories, I and my colleague think it's happened, what this tentative theory suggests, is that at the end of the squeezing, the squeezing stops and slows down because there's a sort of quantum pressure that prevents the space to squeeze too much, become too small, cannot become too small. And then what happens? Well, what happens, what happens is, what happens is actually the usual thing that happens when things are stopped by falling. Maybe they bounce. they bounce back. Okay, if a soccer ball that falls, then bounces up. And when it bounce, it sort of repeat the same path as before,

but with a inverse velocity. Velocity was going down and now velocity is going up. This reversed velocity, in a sense, it's doing the same process backward in time. If you film a falling ball and you project the field backward, you see a ball that goes up, which is exactly what happened after the bounce. So if this is correct, and this is what equation of blue quantum gravity seem to indicate,

what happened after the balance is easy. It's just the same as a black hole, but sort of filmed and projected backward. And this is again a solution of Einstein equation. In fact, it doesn't name this thing. It's a white hole. A white hole is a black hole sort of seeing backward in time, velocity is reverse, so to say. So instead of a tube that is going to be longer and squeezing,

it's a tube that come back, become shorter and opening up. And most importantly, from the throat, you don't see things falling in, you see things falling out. So this is the full complete story, or at least this is the first sketch of the complete stories. Black hole is formed, everything folds in for a while. Then the squeezing become too strong. There is this quantum bounce. So for a while, the Einstein equations don't work anymore because it's quantum phenomena. But then they work again, and the Einstein equation itself tell us what happened next,

namely everything that's inside now is free to come out and to, in fact, come out from the hole. And if you're outside, what we see is very simple. We see just this little sphere, the first things go in and then things come out. So you mentioned earlier then that for many years, black holes were simply a theory, a hypothesis, and many people didn't believe in them. When did we first observe a black hole and how did we do that? It happened in steps. The first observation was very nice, in fact.

Observing some stars, stars not too far away in our galaxy, astronomers saw one of the star in the constellation of sinus that was wiggling, moving back and forth. In fact, the light was a double effect. You see it moving, sometimes coming towards us, sometimes away from us. like it was dancing around something else. That's not pretty too surprising because we see a lot of stars which are a double star, namely there are two stars which are very close and orbit around each other. So if you look just at one of them, it wiggles exactly that way.

Sometimes it comes toward us, sometimes comes away from us. So people said, oh, okay, this is a double star. It's orbiting around something else. But you didn't see there's something else. So what is this other thing? So there had to be something there, which is very massive, massive enough to have the other star orbiting around. So not just a rock, but some big stuff, but not a star, not making light, not visible to light. So what could that be?

And somebody said, well, could that be a black hole? That was the first element of evidence. Another element of evidence was strange signals that were coming from far away called quasars. they're very, very strong signals and nobody could make any sense of them because they seem to be coming from something enormously energetic with a huge amount of mass, but very small. So how could be so massive and so small?

Piling out things, but what really broke the hesitations and is two astronomers that got the nobiblies for that, who we're looking at the center of our galaxy, you know, most stars in the sky don't move much. I mean, apart from this wiggling and sometimes dancing around one another, most stars don't move. If you look at the center of the galaxy, really the center, and they had some technique to look well through

because there are a lot of powder there you don't see well. They could see the star well, and they saw that they move. They don't stay put. So they decided to trace the movement, and it took various years to take pictures of them and how they move months after month, and they found out that they move around Keplerian orbits, ellipses, all the stars, 5 or 6 were very visible in the movement,

all around the same center. And from the Keplerian orbit, they could reconstruct the mass of the object there. It turned out to be 4 million times the mass of the sun, far bigger than any star that we know. So there should be something 4 million, dark, and then that was recognized by one of the sources of radio waves that was known since long time. It was called Sagittarius A-star. This is a constellation of Sagittarius.

And from the feature of the radio signal, the astronomers deduced that it had to be very small. So this is a four million mass thing, very small around which other stars are a orbit, like planets, I know it around the sun. Nobody had any alternative idea than the black hole. And finally, well, finally, the next proof, I would say, or more and more, sorry, there was detection of gravitational waves, because people had computed the shape of the wave emitted by two black holes falling to one another. And when the people who built the detector of gravitational wave turn it on, the final one, the big one, LIGO, the final version of LIGO, bingo, they found exactly the signal that

was expected from Blackhalls. And as this was not enough, there was the photograph, the picture. This famous, marvelous, what is 2019. It was all the first pages of all the major newspaper of Earth. Radio astronomers got together, put together a lot of radio antennas telescopes

around the Earth and actually took a picture. And the picture is a funny picture because you see this ring, this red ring, the red ring is due to the distortion of light around the, the black hole, then it turned out to be exactly what was expected, using the Einstein equation, to be the shape of the light that comes from a black hole. So at this point, the amount of

evidence is overwhelming. It comes from star, from the center of the galaxy, from black hole, very, very far away colliding from these pictures. So yes, black hole there and are very, very described by the mathematics of Einstein. So having said that then, how might we go about observing a white hole? We might be in the same situation of black hole 40 years. ago, right? We have a possible prediction of science and theory. We have a sort of story, how it could be born. It could be born from a black hole transforming to a white hole. Is that real? Well, we have to do the same thing that we did with black holes.

Slowly pile up evidence. And when the evidence is enough, we'll cheer. Now, nothing has happened so far. It's only recently that scientists have been going around the idea of white holes. I see two possible directions for confirming the existence. One is direct detection because for reasons that maybe we don't have time to go into, I expect most of white holes to be very small, not big white holes, small white holes. And there may be many around and produced by black hole very much in the past,

and they might be flying around us. So we might detect one and they interact only gravitationally. So we might be able to detect us that detects teeny teeny things. flying by and interacting gravitationally. I'm working with some colleagues who know more than me on these things, try to design. Well, design is too early,

trying to see whether we can design a machine and does that. But the second idea, the one which I am more excited about, is this. Let me put it in this way. I said that the center of the galaxy Sagittarius radio astronomer

had already detected this signal from the much before understanding that it was black hole. So in a sense, the black hole has been seen. In fact, the signal was first detected in the 30s, almost a century ago, when people didn't even talk about black holes, because it was too early. People haven't yet studied the Einstein equation well enough to understand these things. And it was this mysterious signal.

So in a sense, we see black holes since the century, which didn't know that they were black holes. And it took all this time to figure out that that signal is a signal of the matter's parallel a black hole. So could it be that we have already seen the white hole? Well, it could. Again, it's a speculation. Don't take it as a discovery, but it's a speculation. Because if there were a lot of black holes in the past that have become small white holes, we should expect that many of these little things, sort of powder, which only interact gravitation. So you cannot see them. You cannot touch them. They don't collide against things. Just go through things. The only thing that

have is a gravitational pool. So imagine there's a lot of them, there's a cloud of them around the galaxy. So how would they manifest themselves? Like matter, because matter, dark, because we don't see it.

Well, it turned out that astronomers see very well, very mysterious something that they call dark matter. It's probably the biggest mystery in astronomy. I think it is the bigger mystery to astronomy. There's evidence that in the universe is a vast amount of something which is not usual matter. It's not protons, electrons,

photons, something else, because it's not visible. Everything interact with light and that thing doesn't interact with light and interacts gravitationally. So here is the obvious hypothesis. Could this be many little teeny white holes? Now, for the moment it's a speculation. It requires a lot of work to see whether it matches with a cosmology or the rest. And people are beginning to do the work to see if this is possible. We have to understand the dynamics of white hole much better. We have to understand whether black holes could have been produced in the past and had the time to become white holes. So this is all work which is being done together with a scientist. But if this goes well, maybe the evidence that dark matter is the teeny white hole will grow,

and then we will say, okay, these are the things. Thank you for listening to this episode of Instant Genius, brought to you from the behind BBC Science Focus. That was theoretical physicist Carlo Rovelli. To read more about the topics we've just discussed, check out his latest book, White Holes, Inside the Horizon. The current issue of BBC Science Focus magazine is out now. Pick up a copy wherever you buy your favourite magazines,

or download us on your preferred app store. You can, of course, also find us online at sciencefocus.com. This podcast is sponsored by name, audio and focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with analog warmth. Alongside French acoustic specialist vocal, Name creates high-end audio systems combining innovation with craftsmanship, so you can listen to music, just as the artist intended.

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