Exploring the Mysteries of Black Holes

Leonardo Silva Reviewer's The American Pronunciation Guide Presents Imagine we're traveling toward a black hole, like the one in these images, so the image behind me. Imagine we actually are going there. If you like it, we're in a starship, if you're so inclined. A small starship going there, if you're so inclined, we're in a broom flying in the interstellar space. And we... I've chosen to... We will travel toward this black hole here, which is Sagittarius A star, because we have a picture of it. This is a picture that was on all the front pages of the newspapers. A few years ago. Plenty of black holes. There are millions of black holes. Billions, in fact, of black holes. And I would say that this is a... That's a both surprising and most beautiful discovery of the last years. that the universe is actually filled with black holes. We didn't know that. When I was a student, nobody even imagined that. In fact, people didn't believe that black holes could actually exist in the sky. I could have chosen a much closer black hole. There are much closer black holes to go to, but this one is big. It's big and we have a picture. We have a picture because it's big. It's far, the center of the galaxy, so it's a long way to go there. But I want to take you there, and I want to take you next to it, and then into it. So what we are going to see if we get close, of course, something like this image that you see and you have seen in newspapers. This image was taken by radio telescopes. In fact, some of you may know that. By having a number of radio telescopes on Earth working together. All around the Earth, half a dozen or more telescopes working as a single radio telescope. So like a radio telescope as large as the Earth and combining all the data. The more it's large, the antenna, the more you can focus on a small angle so you can something like that. So if you go there, if you're going there, we're going to see something like that of course without the need of a huge radio telescope because we are going closer and closer and closer. But when we go closer, we see it better and that's what we're going to see. That is exactly the same images as the previous one, except that this one, the one we actually have seen, is out of focus, because so far the radio telescope cannot focus better. If you use more on focus, or what we expect to see with a technique for getting images gets better, is this one. In fact, this one, this image was computed, was what was expected, was theoretically predicted before the observation. And this is why when we saw this one, it was so spectacular that. This is clearly something like that out of focus. So, why a circle? The reason it's a circle, it's far from obvious. First of all, just to make it clear. It would be a circle from whatever direction you would look at it. So it's not really a circle. It's an optical effect due to the funny way light moves around the black hole. Light around the black hole bends because that's what we know light does next to masses, and that's how Einstein's theory that predicts all that, that tells us how all this works, was first confirmed by measuring the bending of the black hole. the light by the star. And it bends in such a way that a ray of light could actually bend so much to be on an orbit around the black hole, any possible angle, whatever angle. So around the hole, the hole is actually smaller than the ring. The ring, roughly, the radius of the ring is one and a half times the radius of the the radius of black hole inside. So you imagine, black hole imagine for a moment like a black sphere, and around there is actually a sort of sphere of light, which is all the light that comes from all over that starts making some rotation around. So if you have, around this thing there is a lot of matter, attracted by black holes, spiraling, doing funny things, and becoming very hot just because there is friction, this thing clashing against. one another. And imagine you have some light coming from here, this is black hole, you get the light directly, but you also get the light that bends around the black hole roughly at one radius and a half and then get to our eyes. And light from here, you get the direct one and the one come here. So you get a little bit of light from all over and a lot of light from tangentially to that sphere, wherever you are. So wherever you are, you see this ring. which is a sort of meridian or equator of this sphere from which tangentially the light comes to you from wherever. So this is the optical effect that we see when we go there. So we get closer and we start seeing more clear this beautiful ring. We also approach all this matter moving around, but let's imagine that our starship or whatever... sufficiently solid that we're not going to be disturbed by that and we get closer we get closer and let's do it slowly we have rockets we can hold okay there's a pool of the black hole but we can hover at certain distance from it um by keeping the the the rocket firing so we're keeping height and we can look down and down there is a hole and this is big this one say the actual dark thing is the size of the orbit of the moon it's a huge thing okay it's big big things it's bigger than the sun And we keep at a distance, we look down, we mostly see black, and we see things falling. Now, more precisely, imagine we're in our starship, we open a window, sort of, we let Let a stone fall down. We close rapidly because... The stone falls down. We see what it goes down. What do we see? We don't see the fall entering the black hole, right? We see the stone... slowing slowing slowing down And never getting to the to the bottom Slow is lower slower slower slower slower slower slower, okay? Not only that the light that come from the from the stone is light there because it's all just burning things. It's not dark It becomes dimmer and dimmer and dimmer and more red More red so like the the wavelength that light gets to us is more and more long which means the frequency and It's slower and slower. It's like not only the stone is slowing down, but even its atoms are vibrating more slowly. And in fact, if you throw a clock down, I'll never throw my father's clock, but watch, but if you throw a clock down, we would see the clock slow down, slow, slow down, slower and slower and slower, almost stopping at some point when it goes there. Now, we are there. This is slow. Now, suppose we look back. We look up. From Earth, from where we came. And imagine, for instance, that from Earth we're getting messages. Say, every day we get a message from our friend's home. How is it going? Is it beautiful there? Once every day. We get a message once every day, but when we're a distance from the black hole, we start getting messages every 20 hours. Then we come down a little bit. toward the black hole, and we get messages every few hours. And then we come closer, and we get messages every minute. And then we get closer again, and we get one message every second. But from Earth, it was sent once a day. So we see the future, okay? We come closer and closer, we get all these messages, with messages from 100 years in the future, 1,000 years in the future. The American empire is finished. I mean, Congo is dominant, it's the most prosperous, there's peace on Earth, but it's loving everybody. It's the future. Maybe. So that's time dilation. That's exactly the same phenomenon that on Earth we can measure in a laboratory. When we take two clocks, we move one up, wait a bit, come down, and the clock up measures a little bit more time, the clock down measures a little bit less time. Okay? That's done in a laboratory. Except that on Earth it's just... teeny teeny teeny fraction difference the fraction of fraction of fractions of a millisecond that can be measured near a black hole which is a huge mass this is enormous but time dilation doesn't mean of course we're in the rocket hoovering there For us, everything is normal. Time goes normal. Clock beats normally. My heart beats normally. But what we see closer to black hole, we see it slower. What we see far away, we see it faster. Okay? Fine. But I didn't... Take you all the way down just to stay outside. We want to go in. Take you down Because I'm going soon into the side black hole, so let's Decrease our rockets and start getting in inside the black hole now wait a minute In the textbook All the textbook of physics today is written that if you enter a black hole, that's it. You can never come out. Forever. So do we still want to get down? If we're going to die, we're going to die. We're also going to die if we stay out, because we're going to die anyway. But we have less time to live if we go in. I'm Italian and the greatest Italian poet, Dante, Dante Alighieri, has written this marvellous sort of mind travel book, which is a divine comedy, in which he goes into the head. And then actually, this is an incredible story. And when he gets to the dark portal of hell, it's written there, perdete ogni speranza voi che entrate, which means give up any hope. you who come in thou who come in but he goes nevertheless okay so let's go in nevertheless he's gonna come out so let's go now we entered the black hole good so we're in we're inside the black hole now and um We're in, and let's look around. First, nothing particular has happened, right? Crossing the horizon, going into the black hole, we are the same as before. Our starship is the same as before. If you look back at the stars, they're still the stars, all sort of out of position because light goes funny turns. But the lights come in, and we see the star. But something is different. When we were outside, sort of hovering around, there was a sphere around the black hole, another sphere, another sphere, and everything was there static. And in the center of this sphere, there was this black hole, big, but certain size. Now we're inside. Imagine we can look around and see what is the geometry there. What we see is still there are spheres and spheres, but believe me, Below us is not just the volume we would expect. It's an immensely big volume. So spheres and spheres and spheres and spheres and spheres, down, down, down, down, very far. A black hole, it's a... You know these fairy tales that, you know, the two brothers, the sister brother walk in the wood, and then there's this little hut, and there's a door, they go in, and inside is a huge, huge room, and another huge room, and then a garden, fountains, and so on. A black hole. is something similar. It has a size outside, but inside there's much more volume of what we are taught at school, you know, if there's a sphere of this radius, what is the volume inside, you can compute it. Okay, I never believe what they tell you at school, it's not true. There is a sphere in which the volume is much more than what they tell you it's inside the series. Why? Because space is distorted, that's what generativity describes. The distortion of space is enormous. So, inside is sort of... You see, imagine that this is a two-dimensional model. Imagine that this is the sphere around the black hole, the actual black hole. We were before, we were here. Now we move inside, we are in. And inside is much more than you would expect if this was flat. It's longer, longer, longer, longer, longer. It's this long, long tube, enormously, enormously long. The more the black hole is old, the more the tube is long. If there's some person in the public here, or watching through a camera, who is... who knows General TV very, very well, might ask what I'm talking about. I'm describing a black hole in a peculiar affiliation, which is the one I think is best to understand what's inside of the black hole. hole, which is the foliation that maximizes the volume and covers the entire black hole. So the first surprise is that there is this pit, this well, very, very profound. But that's not the biggest surprise. The biggest surprise, what we see there, is that it's moving. While outside, nothing was happening. We could have been hovering there forever. Inside, it's changing. The geometry of space is changing. Space around us is changing. And two. Things are happening. One is that it's becoming longer and longer and longer and longer with time. So that's why a black hole, it's bigger. Because with time this becomes longer and longer and longer. It keeps growing, growing, growing, growing. We're not talking of kilometers. We're talking about millions of light years. There's a huge, huge, immense space inside an old black hole. Sagittarius A star presumably is at least 10 billion years old. It's very old. So it's a cosmological age. So it's an immense thing. But the other thing is that it's shrinking. It's becoming longer and shrinking. Now, the fantastic people here at the ARA built this beautiful thing, which actually... becomes longer, but I didn't know how to make it shrink at the same time. Maybe next time I'll get it to shrink. So if you are here, we are getting this sphere around us closing up on us. At speed, which is pretty worrying. I mean, we don't have much time. This is closing around us. So here is a picture of... of an idea of this inside the black hole and by the way at the end in this picture you send you see a little dark uh dark spot at the end so down here i said it's very long but it didn't say it's infinite it's not infinite right it's finite and then very bottom here if some of you think there's a singularity forget about it there's no singularity out here what is here here is a star that initially collapsed to form the black hole so if there is some that at some point, most black holes we see, probably not this one, Sagittarius A star, but most stellar black holes that we see, the smaller one, are big stars that when they burn all the hydrogen, they cool down because they stop burning. They don't have fuel anymore. And by cooling down, the pressure of the heat that was sort of keeping the star open diminished. So the weight is very big. The weight is very big. The weight is so strong that they crash everything, and no rock would resist that weight, and they crash, crash, crash, and form the hole. So, create this hole, goes down the horizon, create the horizon, goes down, and the star is still there. Remarkably, millions of years later, this star is still there. Okay? But remember, time depends on what you are. Stars just... So, this is us. Falling down, okay? We don't fall down all the way. We're not going to get to the star. It's far far away. We've got to fall down and Everything is closing around us slower and slower and slower, and we're going to be crushed. But remember, we are mind-travelling. We're not doing this for you. So imagine that we're so strong that we resist this. this pressure. The space is closing around us. We've been pulled in this direction, crashed in this direction. At some point, the space around us becomes very, very, very small. What's going to happen next? That's where I want you to get at. Because everything I've said so far is things we know very well. We have very high confidence in it. All this follows from the theory of Einstein general relativity written in 1915, which has predicted these black holes. We have seen the black holes exactly as it was predicted, has predicted gravitational waves and we have seen the gravitational waves, has predicted expansion of the universe and we have seen expansion of the universe, has predicted the Mercury to behave in that way that contradicts the Newton equation and Mercury behaved that. So we have enormous confidence in this theory. And the theory tells us not only that black holes exist but also what happened inside it also what happened to the very. moment in which everything is going to get squeezed, but not next. Why? Because the more we are squeezed, the more we are pressured, the more there is a curvature, because space is curved, and we know that when we are in that condition, we cannot disregard quantum mechanics anymore. Quantum phenomena become crucial. And quantum phenomena... I have it here. Oh, I don't have it. Sorry. I thought I had it. Quantum phenomena... I had a, I thought I had a copy of a paragraph in a paper by Einstein in 1916. In 1915, Einstein writes a theory, which is our best theory, the most beautiful theory for physics, said Levelandau, the theory of generativity, space and time. In 1916, Einstein writes a paper in which he says, of course, my theory, it's wrong. because it disregards quantum phenomena and the last line says in German the de-annoying gravitation theory must be corrected because of quantum phenomena. So he knew and we know that the theory is incomplete. It doesn't describe what happened to space and time when curvature is so high. So there, we don't have Einstein theory anymore, which means that we lost our guide. We don't know anymore what's going to happen. Dante, when he goes down into hell, goes all the way through, and then... All through his path is guided by Virgil in his poem, and he's very much attached to Virgil because Virgil explained him how to do, what everything happened, Dante's afraid of being in hell, all the devils and so on. Virgil tells him what to do. But at some point, Virgil says, I cannot accompany you more because I don't have the right to go to paradise. And so Dante's alone, not in an easy place, in the other side of the universe. And he's on his own. And we are on our own down there. We don't have a theory anymore that tells us what to do. So how do we? behave when we lose our stars that tell us our masters and I think that's that's beauty of life perhaps When we are on our own and we lose our master our guide. That's when Dante says Here's where you're going to show what you're capable of doing. I've spent all my life, and that's why I wanted to take you to the center of Black Hole, trying to construct a theory with other, with many colleagues, of course. to understand what is the quantum, how are the quantum properties of gravity. So to change the theory of Einstein, generativity, to keep into account the quantum phenomena. And the theory I've developed with colleagues, it's loop quantum gravity, which is a tentative theory of quantum gravity. A tentative means that we haven't yet made measurement observations. to confirm that that's a good theory, like we have so much for general relativity. And I think it's crucial when scientists talk today to distinguish what we know with confidence from our speculations, okay? It's wonderful to speculate. That's what scientists are supposed to do. This is what a place like that is meant to be for. To think and try to think where we don't know. And... Walk a little bit outside the realm of our knowledge to learn something new so to go through that Final passage which is called the singularity the singularity is not the center of the black hole So the light is what's going to happen when everything shrinks down. We need quantum gravity Okay, so what is quantum gravity telling us? What is loop quantum gravity telling us in particular? Loop quantum gravity is a quantum theory. So like all quantum theories, it predicts two things, which are going to be crucial here. One is that things are granular. Light, we can describe light to some precision as a wave, continuous. But to better precision, light has a granular. Structure is made by photons, but it particles of line Line arrives on my hand So the depositing energy continuously, but if I make precise measurement what really happens I see little dots one after the other one coming In fact, if you deem the lie if we deem the light and we have a precise screen That is exactly what we see. We see one dot forming another dot forming another dot forming This is a grain the photons one by one arriving. So this is granularity and all things. That's all over quantum mechanics The energy of harmonic oscillator is granular, one, two, three. The orbits around the atoms are discrete, one orbit, another orbit, another orbit, and so on. So the main result of loop quantum gravity is that space is granular. Space itself is granular. So between my hands, again, at school they tell us there's continuous space. So again, don't believe what they tell at school. The space, if you keep into account the quantum property of gravity, and if you believe this tentative theory, it can be divided in half, in half, in half, but not forever. At some point you get to the bottom. You get to the individual grains of space. So this granularity of space, that's a calculation that one does in loop quantum gravity, have a certain size, very small, extremely small, but finite. Which means, if you go back to the black hole, that this thing cannot shrink forever because at some point the granularity is going to hold it. You finish. You cannot continue shrinking forever. You cannot go to the singularity before something might happen. What is going to happen? Now, second input from quantum mechanics. Jumps. The quantum jumps. The quantum leaps. Jumps were at the very beginning of quantum mechanics. They're core in quantum mechanics. Total effect is achieved. quantum jumps, quantum leaps. Quantum mechanics was born when in Copenhagen, Niels Bohr made his first model of atom and understood that the electron-tonal atom could only be in orbit on certain discrete quantized orbits. So an electron could go roughly this way or this way, okay? But it could jump from one to the other. And when it jumps, it emits one photon, one grain of. of jump down or jump up. If you put some energy, some photon arrives, jump up. And in between, that's the magic of quantum mechanics, is not really like a particle. It's something funny. We don't have a good intuition of what happened from one quantum value to another quantum value. There's these quantum jumps. And these quantum jumps can take you from one orbit to a different orbit. So you can view them from one classical solution to another classical solution. So the shrinking black hole is like one classical solution. So we go there, it becomes narrower and narrower and narrower, and maybe it can jump. But to which classical solution? Maybe it can jump to something else. But what could happen next? Now, The black hole was formed by a star falling into itself. We went to the black hole by falling, turning off the engine and letting ourselves fall down. The shrinking of space itself is like falling. So these are all solutions, Einstein's equation, classical, and our falling. What happens if something falls, typically at the end of a fall? It bounces. And what is the trajectory after the fall? Well, this is going down. If you film it going up, it's like filming going down in reverse. reversing the field. It's the same, but sort of time reversed, with the velocity reversed. Careful, the acceleration is always down, right? Because it goes faster and faster and faster and faster, velocity down, acceleration down. When it jumps up, the velocity is up now, reversed. But the acceleration is still down, because it goes slower, slower, slower, slower. So you change the velocity. Now. The black hole was become longer and longer and narrower and narrower. If you change the velocity, you make it bounce, what does it do? Okay, it becomes longer, longer, longer, and then it becomes shorter and shorter and shorter. And it opens up. Now, is there... Is there a classical solution of the Einstein equation that describes that? Or is there a solution of Newton's equation that describes this moving up? Is there a solution of the Einstein equation, classical, that describes this? It's peculiar geometry. Something very, very long and narrow that becomes shorter and larger, and instead of things going up, things going out, yes, it's white holes. White holes is a solution of Einstein equations, sort of like black holes, a solution of Einstein equations. So it's a possible happening, compatible with Einstein equations, predicted with Einstein equations, where you have a horizon, but not... is a horizon where you can only enter. It's an horizon from which you can only come out. And where inside, you have this enormous tube. But instead of becoming longer, an hour, it's becoming shorter and opening up. And that's. That's predicted by Einstein's equation. So that's, we are back in the terrain which we're confident that could exist. It's okay for what we know about nature. The difficult part was the jump. Only the jump, only that particular moment. That's where the Einstein equation do not hold, but that's quantum mechanics, that's what Einstein knew the Einstein equation would not hold there. And that quantum mechanics tell us it is possible. So here is the hypothesis. of what could happen. Every line here, it's one possible moment of time, some slicing of time. The black hole forms, becomes longer and longer. There is this quantum jump, this quantum tunneling effect also, and then you come back. And what is next is a white hole. From the outside... What do you see? Well, you see the outside of a black hole, and then you see the outside of a white hole. And this is the magic of just classical Einstein theory. The outside of a black hole, that's what, this was actually the... the technical key that allowed us to realize that this is possible, which is strongly counterintuitive, but is mathematically correct. The outside of a black hole is the same thing as the outside of a white hole. From the outside, you don't distinguish. You always see the surface. Remember that I told you that if I look from the outside, I throw a stone, I don't see it falling down. I just see it, you know, slowing, slowing, slowing, slowing, slowing, slowing. And if it's a white hole, I throw a stone, I just see it slowing, slowing, slowing, slowing, slowing. Exactly the same. Okay? The difference is actually on the surface itself. If I am falling into a black hole, I go through. If there's a white hole I don't go through, what happened? Well, I mean, time slowed down for me. So I go very, very fast to the future until the white hole is finished. It's not there anymore. Or until something come out of the black hole and clashed against me. So outside is the same. Nothing happens in the same. Nothing outside. Inside, there is this whoo-hoo. That's it. The magic is in the jump. In the jump, and this is quantum gravity. See, in the moment in which Einstein, in which Dante is left alone by Virgil, it's alone, it's a moment of, he says, oh, padre mio, my father, because he's of this respectful thing with this great poet of antiquity. He's scared. He's alone. He knows his own. But in that moment, he sees Beatrice, which is his great love. And he, Dante. has been in love with this woman all through his life and he sees there and you know it's just joy light colors fantastic this is beautiful moment of the poem so there in the in the moment of his Light and desperation and he finds his great love. For me it's been the same. Going there in the moment in which we lose Einstein theory, that's what quantum gravity is, it's what I've been looking all my life to. And I think the beauty and the hard part also to understand is this jump, because this jump is not a movement of space or evolution in time. And it's not a jump of an electron from here to here, where space remains space, time remains time. It's just a funny electron that does funny things. It's time itself and space itself that jump from one trajectory to another trajectory. So we have to conceive a way of thinking about physics where you can describe a phenomenon, a normal phenomenon of the universe, not as happening in space-time, but space-time itself does something funny. That's something. So here's a picture. Here's a complete picture. Star collapses, black hole forms, becomes very long, and quantum jump, white hole, and slowly it comes out, everything comes out, and the entire process is finished. Now, first question, how long does it take? I think you're quite on board in the fact that how much time... is a funny question here, it's a tricky question. How long does it take? If we are outside... It might take very long. In fact, it might take extremely long. When we started doing these calculations of the quantum jump, because the classical part is just Einstein's equation, it's just technical. We used quantum gravity to do the calculation. We realized that the probability jumps happen with certain probability, quantum mechanics about probabilities. The probability for the jump to happen... Scene from the outside. is completely negligible if the black hole is big. It's only possible if the horizon is small. Now, this seems to be a big problem, because, oh my God, but most black holes we see in the sky, they're big. They're not going to ever happen there. But black holes become smaller with time. That's a big discovery of Stephen Hawking, that black holes evaporate. If you keep... feeding it becomes bigger but if at some point they eat whatever there is to eat at some point they're isolated and slowly they emit energy and they become smaller and smaller and smaller and smaller so if you wait long enough even such a star you say star which is this gigantic black hole even the biggest one the billion solar masses black hole are going to shrink shrink shrink shrink and become very small once they're small the probability of jumping it's very high in fact becomes order one and it jumps. So the correct story, if you want, cosmologically or astrophysically, the likely story is that whatever black hole you have, it's going to take some time. And the time was computed by Hawking to become small. Then it jumps. And then it becomes a white hole. The white hole is teeny, teeny, teeny, because it's at the end of the apparition. But careful, it's teeny the bottleneck. The inside is big and takes a long time to come out. The inside. That's the full story. Now, the question is how long does it take? Well, a black hole, you have to wait for it to become small. This was computed by Hawkins. For a black hole the size of Sagittarius, it takes much longer than the life of the Universe. Long, long, long, long. So, this happens, and there are billions and billions of years. But... Remember, I said we went inside, inside the black hole, we go through the things, we're in the white hole. The time it takes in Sagittarius A star, from the moment we cross the horizon, for us, for us, we're inside the starship, we're going down. Again, assuming that we're not squeezed and squashed, okay, we have a super powerful kryptonite starship that is going through all quantum jumps of the world. The time it takes for us to go to the jump is minutes. for a big black hole. For a small black hole, it's seconds, even less. The jump is instantaneous, and the time to get out from the white hole is very short, again, minutes. So outside, the time is billions of years. Inside is just seconds or minutes. Which means that the full process is something that lasts billions of years for a clock outside, minutes for a clock inside. Which means if you want to see what is the universe, billions from you in the future, just jump to black hole and you'll get it in a moment. Again, assuming you're not squeezed and squashed and destroyed. So one way of viewing what the... Black-white hole is is considering it as a shortcut to the future You just go in but you come out in the future there in the same place Okay, use this whole going a bloop you come out from the same hole long long away in the future That's this enormous time distortion. It's the same kind of time distortion of my clock, except that it's gigantic, right? Another way of thinking about that is to think... that what's really going on is that you have a star. It goes down, collapses, bounces, come up. But because of time distortion, you're seeing this at slow motion, unbelievable slow motion. For billions of years, just wait, wait, wait. But it's you who are seeing it in slow motion because it's time of the election. So that's a picture of... That's the likely picture of.....of what is going on. Now... One question is, do we believe this? Can we check this? How can we check this? How do we hope to see if this is right or wrong? And as I often say, I've been working this for the last years. sometimes I'm completely sure it's right usually on Mondays I'm completely sure usually on Friday I'm totally desperate no no this is not going to work how do we check this if this is right what could we do to find out it's correct one thing is that um We might see some indirect effect of that and once we could actually see these things directly so the indirect effect In fact, let me make a small parenthesis about black holes black holes the first evidence, the first time astronomers could see something in the star, and they thought, well, that's maybe a black hole, was in the 70s. But it was just a few astronomers. Nobody was taking them seriously. There were some stars that were wiggling. We could see the Doppler effect, they're moving back and forth, like if we're rotating around something, but there was nothing around which to rotate. And somebody says, maybe there is a double star, and one of the two is a black hole. John Wheeler had a beautiful image. If you have a, you know, John Wheeler is another generation, a couple dancing, the waltzer, and he's dressed in black and she's dressed in white, and it's very dim. You only see her. moving around like that. But you know there should be somebody else. So the white-dressed girl is the star that you see. The black is the black hole. That was the 70s, and still in the 80s, nobody would believe black holes. It's only really this century that people, the scientific community at large, started taking the idea of black holes real seriously. I say in the book that when I moved from America back to Europe, the director of my department in France, who was not a relativist, was a particle physicist, in 2000, told me, you don't really believe black holes exist. So still in 2000, many very serious, very good scientists wouldn't take this seriously. Now of course he changes mind, but that's not a criticism of him, not at all, because changing mind is what science is about. I mean that's exactly the best of science. Now why I'm saying all this? Because in the 30s... much, much, much before, this guy constructed this antenna because he was working for the Bell Laboratory for studying noise in communications. And he picked up a signal from a point in the sky, which was in the center of Sagittarius galaxy, the center constellation, was called at the time Sagittarius A star. He was seeing the black hole. The signal, the radio signal of the black hole was received for the first time on Earth by somebody in the 30s, much, much before anybody even thought that that could be a black hole. It took... half a century to recognize that that was a black hole. So it is not totally impossible that white holes is something we have already seen. And in fact, imagine that in the early universe, we know the universe is very hot. And... very agitated. And there's a lot of speculations that in the early universe, a lot of black holes could be formed at the time. They're called primordial black holes. Or imagine even, there's a lot of speculations that Big Bang was not the beginning. There was a big bounce. So previous universe compressing and bouncing. And in fact, the bounce of the universe is a phenomenon which is sort of vaguely similar to the bounce of black holes that becomes white holes. So imagine that either before or in the early universe, little units, many black holes formed, these already become white holes. Now, small white holes, because as I said, the transition happens only when they're small. So imagine there are a lot of these small white holes. The mass we can compute it. The mass, the theory gives it. It's a Planck mass. It's a fraction of microgram, which is the weight of my hair. So one hair, two hair. That's a Planck mass. So a... A white hole is something which has a weight of this, much, much smaller than my hair, but has a weight of that, which means it's a gravitational attraction like this, teeny, which you couldn't see it because it doesn't interact electromagnetically. You could not touch it because touch is electromagnetism. But you could, in principle, see it's a gravitational force. So imagine there was many of these teeny, teeny, teeny things in the universe formed in the past and were floating around. They would behave like matter because attractions have gravity, but they would be dark because they don't interact with light. So they would be dark matter. Astronomers see around galaxies halos of something that they call dark matter, which only interact gravitationally, doesn't interact electromagnetically, and behaves exactly as if It was many little teeny things with a Planck mass moving around. So could it be, and this is not something we know, it's a speculation, it's a possibility, could it be that perhaps we've already seen these things? This is a possibility. zillions of little white holes, which are ancient black holes that have bounced, and now are these teeny things, and they'll be detected by the astronomers, and that's what they call dark matter, which nobody knows what it is. It's one of the greatest mysteries of contemporary astronomy and science, that in the universe we see stars, we see galaxies, but then we have clear evidence. Very strong evidence that there is something else, because we see its gravitational effect. These clouds, big clouds around galaxies, larger than the galaxy, of this sort of powder that only interacts gravitationally. Okay, so maybe we've seen then how do we get convinced that this is the case. That's the way science works, slowly in a complicated manner. We have to make a model of how they were formed before the universe, in the early universe, how had they interacted with the full story of the universe, cosmology, and so on and so forth. Are there alternatives for dark matter? One alternative until short ago was supersymmetric particles, but that was wrong because supersymmetric particles don't exist of that kind. particular kind that was expected one of the great i shouldn't go late with time i just stay within one hour i shouldn't go but let me say that um some physicists even in this room have been talking about a crisis of physics and recently because what they expected didn't happen for instance super similar But you know, crisis physics is like a big defeat in a battle, in a war. The French talk about the big defeat of Waterloo, but the other side of the channel is... was not a big defeat for Waterloo. Crisis of somebody is good news for somebody else. So when super simility was not found, my part of the community was just jumping around, yeah. It's a little bit more than that, because for many people, the next frontier of physics should be the theory of everything. I think that the theory of everything is a bad idea. We don't know that. what's out there. There's so much more about the universe which we don't know. I don't think it's a good idea. It's a good scientific program to write the final theory of everything. We should solve the problems one by one, like Faraday was doing, or Maxwell, or Einstein. We don't have a quantum theory of gravity. We should find a quantum theory of gravity, because we don't know what happens when gravity becomes quantum. That's a concrete problem. So let's solve that. Let's put together quantum mechanics with generativity. It's complicated enough because it's a quantum property of space-time. It's changing the way we think about reality. I think we're in a spectacularly good, positive moment in the history of physics. What we have learned about the universe, black hole, gravitational waves, about gravity and cosmology, is wonderful. Science is going very, very well. It's not in a crisis, not at all. It's a crisis for those who ask the wrong questions. I think trying to figure out the quantum property of gravity and what happened to black hole, what happened to the early universe, these are the right questions we should do. address. I said one possibility is to work out all the cosmology and maybe convince ourselves, we are not convinced at all, it's one possibility that this dark matter is in fact this white hole. The other possibility is to make an actual detection. So could we see one of them? Well, let's see. Imagine this is really dark matter. We can compute how many there are, because we know how many dark matter there are. Astronomers know exactly how much dark matter there is. There's more dark matter than matter. Sort of a few times more in a galaxy. So the density should be comparable a bit higher, but dark matter is more diffused. So we can compute how many there are. We know the weight of these little things. I just told you, a fraction of a micrograph, so we know how many there are. And so it's a simple calculation to see how many of these little things there are. And the result is that presumably in a room like this one, every once in a while, I mean, one goes by. So maybe there is no one going by with this. We couldn't see it. We couldn't touch it. The only interaction would be gravitational, the pull. So if we had something extremely sensitive, okay, imagine we build a machine, a detector, where something goes by, so intuitively, a little pendulum, something goes by, whoop, start penduling. Okay, now, pendulum mechanics. is complicated. But maybe something electronic, something quantum electronic that could detect very. That's one of the things I'm doing right now, trying to figure out what's best chance we have to build a detector that could detect one thing flying by. Is this going to happen? I don't know. I'm working on that. What I know that When I became a faculty at Pittsburgh in 1990, and my friends were saying, we're going to build a gravitational wave detector, that seemed really crazy. And they did it, and they got a Nobel Prize. So, you know, it doesn't mean that all projects are going to work. Many projects don't work, but some do work. So in principle, yes, it's all possible to detect. In principle, it's possible to detect one of these things. So in principle, it's clearly possible to detect them. Practice. building machine and there as i said one particle flying by every once in a while there's not a lot of them so it's not easy to um to detect now let me let me go toward the conclusion um i've asked you to follow me inside the black hole all the way to the center in a sort of land we know, strange but known, through the jump, hypothetical, we don't know, and then out of the white hole, and then we're back to the stars, like say Dante. We came back to see back the stars. What is this? This is mind traveling, right? It's traveling with mind, and it's what Dante does. Dante takes us to this incredible trip into hell, purgatory. paradise, through all the forms of morality, through all the stages of human psyche and mind. Beautiful. But I think it's also what science does. What the best science does, the best theoretical science. Science has made all sorts of things. There are mathematicians, very good equations. There are the people who do experiments, there are people who find out experiments. But then there are the people who think the new, like Faraday, who invented the fields, who just created something completely new. And how do you do that? You mind travel. Think for a moment. I'll give you just a few examples in my last minutes. Anaximander, which is one of my great passions, great thinker in the 26th century ago, who figured out that Earth is like a stone floating in the nothing, right? Before, people thought that the sky is above us. He understood that the sky is all around us, and Earth is just a thing like there. How did he do that? He mind-traveled outside it. He saw it from the outside. In fact, Aki, same as Anaximander, is credited in antiquity to be the one who invented the geographical maps. Which is funny, you know, because people have been traveling and trading and traveling long distance for centuries, millennia. Nobody thought to take a piece of paper writing a map. Why? Because it needs to be creative to change perspective. To imagine, what is a map? A map is how the land would look seen from an eagle, super, super high. But take imagination to think you're an eagle. and grab there. Or to think you're just like the astronauts on the moon looking at the Earth from somewhere. He did that. So he was capable of mind travelling. Hipparchus, the greatest astronomer of antiquity, has a calculation of the distance of the moon, which is a subtle geometrical argument, very, very beautiful. But it starts by saying, imagine the Earth is the sun there. and there's a shadow of the Earth, there's a cone, and imagine to go to the tip of the shadow of the cone of the Earth and look back, and then there's a dramatic angle you would see. I mean, I'm not time to go into that. So again, his mind travelling to the interplanetary space, this is two millennia ago, thinking how would Sun and Earth and Moon would look from the tip of the space. And... Copernicus imagined the solar system seen from the Sun. That's what he did. Changed perspective on the solar system. And Kepler has this fantastic book. He wrote this book all through his life. It's called the dream the title and was published after his death in which he tell the story that He and his mother are taken by a demon to the moon And then he goes to the moon and described the moon and described how the solar system looks from the moon So what tell is doing well, he's doing something fantastic. He's taking out a different perspective Um and see how venus and the earth looks from a moving body the moon To convince us that what we see on earth is nothing else that something seen from a moving body. So it's a profoundly rhetorical, powerful argument for Copernicanism based on change in perspective. Because when you change perspective, what is it, I mean, who cares how... how the solar system looks from the moon. That's not the point. When you change perspective, you realize that your own perspective is partial. That's the point about changing perspective. It's in science, like in politics, like in economy, like in everything. You put yourself in other shoes somewhere else, so you realize that what you usually think might be wrong because you have a different point of view. And so on. Einstein famously traveled on a... Talked about traveling on a... ray of light, riding a ray of light, or falling in an elevator. Maxwell also had a demon, right? Maxwell thought of being small like a molecule, and this demon seeing the individual molecules, what Maxwell's demon is called, opening and closing the door, letting the molecules go, in his work, not on atomities, his work on the kinetic field of gas, on thermodynamics. But how do we mind travel? How do we know what's going to see from a different perspective? I think that the answer is to stay on a subtle balance between what we keep with us, the Einstein equation that tell us up to there, what we keep with us is the knowledge that tell us something, and what we leave home. Because to make a step ahead, we have to leave home something, right? Einstein leave home simultaneity. Kepler leaves home the circles. Copernicus leaves home the idea that the earth is the center of the universe. We left home the idea that you can always describe things in space and time. Something basic, okay? This... How do you know what to bring yourself and what to leave home? I think there is no method in science. There is no recipe for that. Science is just trial and error. You keep walking on the boundary of what you... what we know. We know quantum mechanics. no generativity, we know what happens in the black hole up to there, and you keep turning things around until you find the little hole that allows you to put together a puzzle in a different manner in such a way that... It makes sense and there's a possible step ahead and then you know experiment tell you whether it's right or wrong So I think this mind traveling is science Did better and I think that science is a imagination besides just being calculation and numbers. And in this, science is very similar to Dante, poetical imagination, and to the rest of our life, which is a life of emotion, of... imagination. And I think this part of science is often forgotten. Science has that image of coldness. But this is science, I think. Thank you.

A small starship going there, if you're so inclined, we're in a broom flying in the interstellar space. And we... I've chosen to... We will travel toward this black hole here, which is Sagittarius A star, because we have a picture of it. This is a picture that was on all the front pages of the newspapers.

A few years ago. Plenty of black holes. There are millions of black holes. Billions, in fact, of black holes. And I would say that this is a...

That's a both surprising and most beautiful discovery of the last years. that the universe is actually filled with black holes. We didn't know that.

When I was a student, nobody even imagined that. In fact, people didn't believe that black holes could actually exist in the sky. I could have chosen a much closer black hole. There are much closer black holes to go to, but this one is big. It's big and we have a picture.

We have a picture because it's big. It's far, the center of the galaxy, so it's a long way to go there. But I want to take you there, and I want to take you next to it, and then into it. So what we are going to see if we get close, of course, something like this image that you see and you have seen in newspapers. This image was taken by radio telescopes.

In fact, some of you may know that. By having a number of radio telescopes on Earth working together. All around the Earth, half a dozen or more telescopes working as a single radio telescope. So like a radio telescope as large as the Earth and combining all the data. The more it's large, the antenna, the more you can focus on a small angle so you can something like that.

So if you go there, if you're going there, we're going to see something like that of course without the need of a huge radio telescope because we are going closer and closer and closer. But when we go closer, we see it better and that's what we're going to see. That is exactly the same images as the previous one, except that this one, the one we actually have seen, is out of focus, because so far the radio telescope cannot focus better. If you use more on focus, or what we expect to see with a technique for getting images gets better, is this one. In fact, this one, this image was computed, was what was expected, was theoretically predicted before the observation.

And this is why when we saw this one, it was so spectacular that. This is clearly something like that out of focus. So, why a circle? The reason it's a circle, it's far from obvious. First of all, just to make it clear.

It would be a circle from whatever direction you would look at it. So it's not really a circle. It's an optical effect due to the funny way light moves around the black hole.

Light around the black hole bends because that's what we know light does next to masses, and that's how Einstein's theory that predicts all that, that tells us how all this works, was first confirmed by measuring the bending of the black hole. the light by the star. And it bends in such a way that a ray of light could actually bend so much to be on an orbit around the black hole, any possible angle, whatever angle. So around the hole, the hole is actually smaller than the ring. The ring, roughly, the radius of the ring is one and a half times the radius of the the radius of black hole inside.

So you imagine, black hole imagine for a moment like a black sphere, and around there is actually a sort of sphere of light, which is all the light that comes from all over that starts making some rotation around. So if you have, around this thing there is a lot of matter, attracted by black holes, spiraling, doing funny things, and becoming very hot just because there is friction, this thing clashing against. one another.

And imagine you have some light coming from here, this is black hole, you get the light directly, but you also get the light that bends around the black hole roughly at one radius and a half and then get to our eyes. And light from here, you get the direct one and the one come here. So you get a little bit of light from all over and a lot of light from tangentially to that sphere, wherever you are. So wherever you are, you see this ring. which is a sort of meridian or equator of this sphere from which tangentially the light comes to you from wherever.

So this is the optical effect that we see when we go there. So we get closer and we start seeing more clear this beautiful ring. We also approach all this matter moving around, but let's imagine that our starship or whatever... sufficiently solid that we're not going to be disturbed by that and we get closer we get closer and let's do it slowly we have rockets we can hold okay there's a pool of the black hole but we can hover at certain distance from it um by keeping the the the rocket firing so we're keeping height and we can look down and down there is a hole and this is big this one say the actual dark thing is the size of the orbit of the moon it's a huge thing okay it's big big things it's bigger than the sun And we keep at a distance, we look down, we mostly see black, and we see things falling.

Now, more precisely, imagine we're in our starship, we open a window, sort of, we let Let a stone fall down. We close rapidly because... The stone falls down.

We see what it goes down. What do we see? We don't see the fall entering the black hole, right?

We see the stone... slowing slowing slowing down And never getting to the to the bottom Slow is lower slower slower slower slower slower slower, okay? Not only that the light that come from the from the stone is light there because it's all just burning things.

It's not dark It becomes dimmer and dimmer and dimmer and more red More red so like the the wavelength that light gets to us is more and more long which means the frequency and It's slower and slower. It's like not only the stone is slowing down, but even its atoms are vibrating more slowly. And in fact, if you throw a clock down, I'll never throw my father's clock, but watch, but if you throw a clock down, we would see the clock slow down, slow, slow down, slower and slower and slower, almost stopping at some point when it goes there.

Now, we are there. This is slow. Now, suppose we look back.

We look up. From Earth, from where we came. And imagine, for instance, that from Earth we're getting messages. Say, every day we get a message from our friend's home.

How is it going? Is it beautiful there? Once every day.

We get a message once every day, but when we're a distance from the black hole, we start getting messages every 20 hours. Then we come down a little bit. toward the black hole, and we get messages every few hours. And then we come closer, and we get messages every minute.

And then we get closer again, and we get one message every second. But from Earth, it was sent once a day. So we see the future, okay?

We come closer and closer, we get all these messages, with messages from 100 years in the future, 1,000 years in the future. The American empire is finished. I mean, Congo is dominant, it's the most prosperous, there's peace on Earth, but it's loving everybody. It's the future.

Maybe. So that's time dilation. That's exactly the same phenomenon that on Earth we can measure in a laboratory. When we take two clocks, we move one up, wait a bit, come down, and the clock up measures a little bit more time, the clock down measures a little bit less time.

Okay? That's done in a laboratory. Except that on Earth it's just... teeny teeny teeny fraction difference the fraction of fraction of fractions of a millisecond that can be measured near a black hole which is a huge mass this is enormous but time dilation doesn't mean of course we're in the rocket hoovering there For us, everything is normal.

Time goes normal. Clock beats normally. My heart beats normally. But what we see closer to black hole, we see it slower.

What we see far away, we see it faster. Okay? Fine.

But I didn't... Take you all the way down just to stay outside. We want to go in. Take you down Because I'm going soon into the side black hole, so let's Decrease our rockets and start getting in inside the black hole now wait a minute In the textbook All the textbook of physics today is written that if you enter a black hole, that's it. You can never come out.

Forever. So do we still want to get down? If we're going to die, we're going to die.

We're also going to die if we stay out, because we're going to die anyway. But we have less time to live if we go in. I'm Italian and the greatest Italian poet, Dante, Dante Alighieri, has written this marvellous sort of mind travel book, which is a divine comedy, in which he goes into the head. And then actually, this is an incredible story. And when he gets to the dark portal of hell, it's written there, perdete ogni speranza voi che entrate, which means give up any hope.

you who come in thou who come in but he goes nevertheless okay so let's go in nevertheless he's gonna come out so let's go now we entered the black hole good so we're in we're inside the black hole now and um We're in, and let's look around. First, nothing particular has happened, right? Crossing the horizon, going into the black hole, we are the same as before. Our starship is the same as before.

If you look back at the stars, they're still the stars, all sort of out of position because light goes funny turns. But the lights come in, and we see the star. But something is different. When we were outside, sort of hovering around, there was a sphere around the black hole, another sphere, another sphere, and everything was there static.

And in the center of this sphere, there was this black hole, big, but certain size. Now we're inside. Imagine we can look around and see what is the geometry there. What we see is still there are spheres and spheres, but believe me, Below us is not just the volume we would expect. It's an immensely big volume.

So spheres and spheres and spheres and spheres and spheres, down, down, down, down, very far. A black hole, it's a... You know these fairy tales that, you know, the two brothers, the sister brother walk in the wood, and then there's this little hut, and there's a door, they go in, and inside is a huge, huge room, and another huge room, and then a garden, fountains, and so on. A black hole.

is something similar. It has a size outside, but inside there's much more volume of what we are taught at school, you know, if there's a sphere of this radius, what is the volume inside, you can compute it. Okay, I never believe what they tell you at school, it's not true. There is a sphere in which the volume is much more than what they tell you it's inside the series. Why?

Because space is distorted, that's what generativity describes. The distortion of space is enormous. So, inside is sort of...

You see, imagine that this is a two-dimensional model. Imagine that this is the sphere around the black hole, the actual black hole. We were before, we were here.

Now we move inside, we are in. And inside is much more than you would expect if this was flat. It's longer, longer, longer, longer, longer. It's this long, long tube, enormously, enormously long.

The more the black hole is old, the more the tube is long. If there's some person in the public here, or watching through a camera, who is... who knows General TV very, very well, might ask what I'm talking about.

I'm describing a black hole in a peculiar affiliation, which is the one I think is best to understand what's inside of the black hole. hole, which is the foliation that maximizes the volume and covers the entire black hole. So the first surprise is that there is this pit, this well, very, very profound. But that's not the biggest surprise. The biggest surprise, what we see there, is that it's moving.

While outside, nothing was happening. We could have been hovering there forever. Inside, it's changing.

The geometry of space is changing. Space around us is changing. And two. Things are happening. One is that it's becoming longer and longer and longer and longer with time.

So that's why a black hole, it's bigger. Because with time this becomes longer and longer and longer. It keeps growing, growing, growing, growing.

We're not talking of kilometers. We're talking about millions of light years. There's a huge, huge, immense space inside an old black hole.

Sagittarius A star presumably is at least 10 billion years old. It's very old. So it's a cosmological age.

So it's an immense thing. But the other thing is that it's shrinking. It's becoming longer and shrinking. Now, the fantastic people here at the ARA built this beautiful thing, which actually... becomes longer, but I didn't know how to make it shrink at the same time.

Maybe next time I'll get it to shrink. So if you are here, we are getting this sphere around us closing up on us. At speed, which is pretty worrying.

I mean, we don't have much time. This is closing around us. So here is a picture of...

of an idea of this inside the black hole and by the way at the end in this picture you send you see a little dark uh dark spot at the end so down here i said it's very long but it didn't say it's infinite it's not infinite right it's finite and then very bottom here if some of you think there's a singularity forget about it there's no singularity out here what is here here is a star that initially collapsed to form the black hole so if there is some that at some point, most black holes we see, probably not this one, Sagittarius A star, but most stellar black holes that we see, the smaller one, are big stars that when they burn all the hydrogen, they cool down because they stop burning. They don't have fuel anymore. And by cooling down, the pressure of the heat that was sort of keeping the star open diminished.

So the weight is very big. The weight is very big. The weight is so strong that they crash everything, and no rock would resist that weight, and they crash, crash, crash, and form the hole. So, create this hole, goes down the horizon, create the horizon, goes down, and the star is still there. Remarkably, millions of years later, this star is still there.

Okay? But remember, time depends on what you are. Stars just... So, this is us. Falling down, okay?

We don't fall down all the way. We're not going to get to the star. It's far far away. We've got to fall down and Everything is closing around us slower and slower and slower, and we're going to be crushed.

But remember, we are mind-travelling. We're not doing this for you. So imagine that we're so strong that we resist this.

this pressure. The space is closing around us. We've been pulled in this direction, crashed in this direction.

At some point, the space around us becomes very, very, very small. What's going to happen next? That's where I want you to get at.

Because everything I've said so far is things we know very well. We have very high confidence in it. All this follows from the theory of Einstein general relativity written in 1915, which has predicted these black holes.

We have seen the black holes exactly as it was predicted, has predicted gravitational waves and we have seen the gravitational waves, has predicted expansion of the universe and we have seen expansion of the universe, has predicted the Mercury to behave in that way that contradicts the Newton equation and Mercury behaved that. So we have enormous confidence in this theory. And the theory tells us not only that black holes exist but also what happened inside it also what happened to the very.

moment in which everything is going to get squeezed, but not next. Why? Because the more we are squeezed, the more we are pressured, the more there is a curvature, because space is curved, and we know that when we are in that condition, we cannot disregard quantum mechanics anymore. Quantum phenomena become crucial. And quantum phenomena...

I have it here. Oh, I don't have it. Sorry. I thought I had it.

Quantum phenomena... I had a, I thought I had a copy of a paragraph in a paper by Einstein in 1916. In 1915, Einstein writes a theory, which is our best theory, the most beautiful theory for physics, said Levelandau, the theory of generativity, space and time. In 1916, Einstein writes a paper in which he says, of course, my theory, it's wrong. because it disregards quantum phenomena and the last line says in German the de-annoying gravitation theory must be corrected because of quantum phenomena.

So he knew and we know that the theory is incomplete. It doesn't describe what happened to space and time when curvature is so high. So there, we don't have Einstein theory anymore, which means that we lost our guide. We don't know anymore what's going to happen.

Dante, when he goes down into hell, goes all the way through, and then... All through his path is guided by Virgil in his poem, and he's very much attached to Virgil because Virgil explained him how to do, what everything happened, Dante's afraid of being in hell, all the devils and so on. Virgil tells him what to do.

But at some point, Virgil says, I cannot accompany you more because I don't have the right to go to paradise. And so Dante's alone, not in an easy place, in the other side of the universe. And he's on his own.

And we are on our own down there. We don't have a theory anymore that tells us what to do. So how do we?

behave when we lose our stars that tell us our masters and I think that's that's beauty of life perhaps When we are on our own and we lose our master our guide. That's when Dante says Here's where you're going to show what you're capable of doing. I've spent all my life, and that's why I wanted to take you to the center of Black Hole, trying to construct a theory with other, with many colleagues, of course. to understand what is the quantum, how are the quantum properties of gravity. So to change the theory of Einstein, generativity, to keep into account the quantum phenomena.

And the theory I've developed with colleagues, it's loop quantum gravity, which is a tentative theory of quantum gravity. A tentative means that we haven't yet made measurement observations. to confirm that that's a good theory, like we have so much for general relativity.

And I think it's crucial when scientists talk today to distinguish what we know with confidence from our speculations, okay? It's wonderful to speculate. That's what scientists are supposed to do. This is what a place like that is meant to be for.

To think and try to think where we don't know. And... Walk a little bit outside the realm of our knowledge to learn something new so to go through that Final passage which is called the singularity the singularity is not the center of the black hole So the light is what's going to happen when everything shrinks down. We need quantum gravity Okay, so what is quantum gravity telling us?

What is loop quantum gravity telling us in particular? Loop quantum gravity is a quantum theory. So like all quantum theories, it predicts two things, which are going to be crucial here. One is that things are granular.

Light, we can describe light to some precision as a wave, continuous. But to better precision, light has a granular. Structure is made by photons, but it particles of line Line arrives on my hand So the depositing energy continuously, but if I make precise measurement what really happens I see little dots one after the other one coming In fact, if you deem the lie if we deem the light and we have a precise screen That is exactly what we see.

We see one dot forming another dot forming another dot forming This is a grain the photons one by one arriving. So this is granularity and all things. That's all over quantum mechanics The energy of harmonic oscillator is granular, one, two, three.

The orbits around the atoms are discrete, one orbit, another orbit, another orbit, and so on. So the main result of loop quantum gravity is that space is granular. Space itself is granular. So between my hands, again, at school they tell us there's continuous space.

So again, don't believe what they tell at school. The space, if you keep into account the quantum property of gravity, and if you believe this tentative theory, it can be divided in half, in half, in half, but not forever. At some point you get to the bottom. You get to the individual grains of space. So this granularity of space, that's a calculation that one does in loop quantum gravity, have a certain size, very small, extremely small, but finite.

Which means, if you go back to the black hole, that this thing cannot shrink forever because at some point the granularity is going to hold it. You finish. You cannot continue shrinking forever. You cannot go to the singularity before something might happen. What is going to happen?

Now, second input from quantum mechanics. Jumps. The quantum jumps.

The quantum leaps. Jumps were at the very beginning of quantum mechanics. They're core in quantum mechanics. Total effect is achieved. quantum jumps, quantum leaps.

Quantum mechanics was born when in Copenhagen, Niels Bohr made his first model of atom and understood that the electron-tonal atom could only be in orbit on certain discrete quantized orbits. So an electron could go roughly this way or this way, okay? But it could jump from one to the other. And when it jumps, it emits one photon, one grain of.

of jump down or jump up. If you put some energy, some photon arrives, jump up. And in between, that's the magic of quantum mechanics, is not really like a particle.

It's something funny. We don't have a good intuition of what happened from one quantum value to another quantum value. There's these quantum jumps.

And these quantum jumps can take you from one orbit to a different orbit. So you can view them from one classical solution to another classical solution. So the shrinking black hole is like one classical solution.

So we go there, it becomes narrower and narrower and narrower, and maybe it can jump. But to which classical solution? Maybe it can jump to something else.

But what could happen next? Now, The black hole was formed by a star falling into itself. We went to the black hole by falling, turning off the engine and letting ourselves fall down.

The shrinking of space itself is like falling. So these are all solutions, Einstein's equation, classical, and our falling. What happens if something falls, typically at the end of a fall?

It bounces. And what is the trajectory after the fall? Well, this is going down.

If you film it going up, it's like filming going down in reverse. reversing the field. It's the same, but sort of time reversed, with the velocity reversed. Careful, the acceleration is always down, right? Because it goes faster and faster and faster and faster, velocity down, acceleration down.

When it jumps up, the velocity is up now, reversed. But the acceleration is still down, because it goes slower, slower, slower, slower. So you change the velocity.

Now. The black hole was become longer and longer and narrower and narrower. If you change the velocity, you make it bounce, what does it do?

Okay, it becomes longer, longer, longer, and then it becomes shorter and shorter and shorter. And it opens up. Now, is there...

Is there a classical solution of the Einstein equation that describes that? Or is there a solution of Newton's equation that describes this moving up? Is there a solution of the Einstein equation, classical, that describes this?

It's peculiar geometry. Something very, very long and narrow that becomes shorter and larger, and instead of things going up, things going out, yes, it's white holes. White holes is a solution of Einstein equations, sort of like black holes, a solution of Einstein equations. So it's a possible happening, compatible with Einstein equations, predicted with Einstein equations, where you have a horizon, but not...

is a horizon where you can only enter. It's an horizon from which you can only come out. And where inside, you have this enormous tube. But instead of becoming longer, an hour, it's becoming shorter and opening up.

And that's. That's predicted by Einstein's equation. So that's, we are back in the terrain which we're confident that could exist. It's okay for what we know about nature.

The difficult part was the jump. Only the jump, only that particular moment. That's where the Einstein equation do not hold, but that's quantum mechanics, that's what Einstein knew the Einstein equation would not hold there.

And that quantum mechanics tell us it is possible. So here is the hypothesis. of what could happen.

Every line here, it's one possible moment of time, some slicing of time. The black hole forms, becomes longer and longer. There is this quantum jump, this quantum tunneling effect also, and then you come back. And what is next is a white hole. From the outside...

What do you see? Well, you see the outside of a black hole, and then you see the outside of a white hole. And this is the magic of just classical Einstein theory. The outside of a black hole, that's what, this was actually the... the technical key that allowed us to realize that this is possible, which is strongly counterintuitive, but is mathematically correct.

The outside of a black hole is the same thing as the outside of a white hole. From the outside, you don't distinguish. You always see the surface.

Remember that I told you that if I look from the outside, I throw a stone, I don't see it falling down. I just see it, you know, slowing, slowing, slowing, slowing, slowing, slowing. And if it's a white hole, I throw a stone, I just see it slowing, slowing, slowing, slowing, slowing.

Exactly the same. Okay? The difference is actually on the surface itself. If I am falling into a black hole, I go through.

If there's a white hole I don't go through, what happened? Well, I mean, time slowed down for me. So I go very, very fast to the future until the white hole is finished. It's not there anymore.

Or until something come out of the black hole and clashed against me. So outside is the same. Nothing happens in the same.

Nothing outside. Inside, there is this whoo-hoo. That's it. The magic is in the jump. In the jump, and this is quantum gravity.

See, in the moment in which Einstein, in which Dante is left alone by Virgil, it's alone, it's a moment of, he says, oh, padre mio, my father, because he's of this respectful thing with this great poet of antiquity. He's scared. He's alone. He knows his own.

But in that moment, he sees Beatrice, which is his great love. And he, Dante. has been in love with this woman all through his life and he sees there and you know it's just joy light colors fantastic this is beautiful moment of the poem so there in the in the moment of his Light and desperation and he finds his great love.

For me it's been the same. Going there in the moment in which we lose Einstein theory, that's what quantum gravity is, it's what I've been looking all my life to. And I think the beauty and the hard part also to understand is this jump, because this jump is not a movement of space or evolution in time.

And it's not a jump of an electron from here to here, where space remains space, time remains time. It's just a funny electron that does funny things. It's time itself and space itself that jump from one trajectory to another trajectory. So we have to conceive a way of thinking about physics where you can describe a phenomenon, a normal phenomenon of the universe, not as happening in space-time, but space-time itself does something funny.

That's something. So here's a picture. Here's a complete picture. Star collapses, black hole forms, becomes very long, and quantum jump, white hole, and slowly it comes out, everything comes out, and the entire process is finished.

Now, first question, how long does it take? I think you're quite on board in the fact that how much time... is a funny question here, it's a tricky question. How long does it take?

If we are outside... It might take very long. In fact, it might take extremely long.

When we started doing these calculations of the quantum jump, because the classical part is just Einstein's equation, it's just technical. We used quantum gravity to do the calculation. We realized that the probability jumps happen with certain probability, quantum mechanics about probabilities. The probability for the jump to happen...

Scene from the outside. is completely negligible if the black hole is big. It's only possible if the horizon is small.

Now, this seems to be a big problem, because, oh my God, but most black holes we see in the sky, they're big. They're not going to ever happen there. But black holes become smaller with time. That's a big discovery of Stephen Hawking, that black holes evaporate. If you keep...

feeding it becomes bigger but if at some point they eat whatever there is to eat at some point they're isolated and slowly they emit energy and they become smaller and smaller and smaller and smaller so if you wait long enough even such a star you say star which is this gigantic black hole even the biggest one the billion solar masses black hole are going to shrink shrink shrink shrink and become very small once they're small the probability of jumping it's very high in fact becomes order one and it jumps. So the correct story, if you want, cosmologically or astrophysically, the likely story is that whatever black hole you have, it's going to take some time. And the time was computed by Hawking to become small.

Then it jumps. And then it becomes a white hole. The white hole is teeny, teeny, teeny, because it's at the end of the apparition. But careful, it's teeny the bottleneck.

The inside is big and takes a long time to come out. The inside. That's the full story.

Now, the question is how long does it take? Well, a black hole, you have to wait for it to become small. This was computed by Hawkins.

For a black hole the size of Sagittarius, it takes much longer than the life of the Universe. Long, long, long, long. So, this happens, and there are billions and billions of years. But...

Remember, I said we went inside, inside the black hole, we go through the things, we're in the white hole. The time it takes in Sagittarius A star, from the moment we cross the horizon, for us, for us, we're inside the starship, we're going down. Again, assuming that we're not squeezed and squashed, okay, we have a super powerful kryptonite starship that is going through all quantum jumps of the world. The time it takes for us to go to the jump is minutes.

for a big black hole. For a small black hole, it's seconds, even less. The jump is instantaneous, and the time to get out from the white hole is very short, again, minutes. So outside, the time is billions of years.

Inside is just seconds or minutes. Which means that the full process is something that lasts billions of years for a clock outside, minutes for a clock inside. Which means if you want to see what is the universe, billions from you in the future, just jump to black hole and you'll get it in a moment.

Again, assuming you're not squeezed and squashed and destroyed. So one way of viewing what the... Black-white hole is is considering it as a shortcut to the future You just go in but you come out in the future there in the same place Okay, use this whole going a bloop you come out from the same hole long long away in the future That's this enormous time distortion.

It's the same kind of time distortion of my clock, except that it's gigantic, right? Another way of thinking about that is to think... that what's really going on is that you have a star. It goes down, collapses, bounces, come up.

But because of time distortion, you're seeing this at slow motion, unbelievable slow motion. For billions of years, just wait, wait, wait. But it's you who are seeing it in slow motion because it's time of the election.

So that's a picture of... That's the likely picture of.....of what is going on. Now...

One question is, do we believe this? Can we check this? How can we check this? How do we hope to see if this is right or wrong?

And as I often say, I've been working this for the last years. sometimes I'm completely sure it's right usually on Mondays I'm completely sure usually on Friday I'm totally desperate no no this is not going to work how do we check this if this is right what could we do to find out it's correct one thing is that um We might see some indirect effect of that and once we could actually see these things directly so the indirect effect In fact, let me make a small parenthesis about black holes black holes the first evidence, the first time astronomers could see something in the star, and they thought, well, that's maybe a black hole, was in the 70s. But it was just a few astronomers.

Nobody was taking them seriously. There were some stars that were wiggling. We could see the Doppler effect, they're moving back and forth, like if we're rotating around something, but there was nothing around which to rotate.

And somebody says, maybe there is a double star, and one of the two is a black hole. John Wheeler had a beautiful image. If you have a, you know, John Wheeler is another generation, a couple dancing, the waltzer, and he's dressed in black and she's dressed in white, and it's very dim.

You only see her. moving around like that. But you know there should be somebody else. So the white-dressed girl is the star that you see.

The black is the black hole. That was the 70s, and still in the 80s, nobody would believe black holes. It's only really this century that people, the scientific community at large, started taking the idea of black holes real seriously.

I say in the book that when I moved from America back to Europe, the director of my department in France, who was not a relativist, was a particle physicist, in 2000, told me, you don't really believe black holes exist. So still in 2000, many very serious, very good scientists wouldn't take this seriously. Now of course he changes mind, but that's not a criticism of him, not at all, because changing mind is what science is about.

I mean that's exactly the best of science. Now why I'm saying all this? Because in the 30s... much, much, much before, this guy constructed this antenna because he was working for the Bell Laboratory for studying noise in communications. And he picked up a signal from a point in the sky, which was in the center of Sagittarius galaxy, the center constellation, was called at the time Sagittarius A star.

He was seeing the black hole. The signal, the radio signal of the black hole was received for the first time on Earth by somebody in the 30s, much, much before anybody even thought that that could be a black hole. It took... half a century to recognize that that was a black hole. So it is not totally impossible that white holes is something we have already seen.

And in fact, imagine that in the early universe, we know the universe is very hot. And... very agitated. And there's a lot of speculations that in the early universe, a lot of black holes could be formed at the time.

They're called primordial black holes. Or imagine even, there's a lot of speculations that Big Bang was not the beginning. There was a big bounce.

So previous universe compressing and bouncing. And in fact, the bounce of the universe is a phenomenon which is sort of vaguely similar to the bounce of black holes that becomes white holes. So imagine that either before or in the early universe, little units, many black holes formed, these already become white holes.

Now, small white holes, because as I said, the transition happens only when they're small. So imagine there are a lot of these small white holes. The mass we can compute it. The mass, the theory gives it.

It's a Planck mass. It's a fraction of microgram, which is the weight of my hair. So one hair, two hair. That's a Planck mass. So a...

A white hole is something which has a weight of this, much, much smaller than my hair, but has a weight of that, which means it's a gravitational attraction like this, teeny, which you couldn't see it because it doesn't interact electromagnetically. You could not touch it because touch is electromagnetism. But you could, in principle, see it's a gravitational force.

So imagine there was many of these teeny, teeny, teeny things in the universe formed in the past and were floating around. They would behave like matter because attractions have gravity, but they would be dark because they don't interact with light. So they would be dark matter.

Astronomers see around galaxies halos of something that they call dark matter, which only interact gravitationally, doesn't interact electromagnetically, and behaves exactly as if It was many little teeny things with a Planck mass moving around. So could it be, and this is not something we know, it's a speculation, it's a possibility, could it be that perhaps we've already seen these things? This is a possibility. zillions of little white holes, which are ancient black holes that have bounced, and now are these teeny things, and they'll be detected by the astronomers, and that's what they call dark matter, which nobody knows what it is.

It's one of the greatest mysteries of contemporary astronomy and science, that in the universe we see stars, we see galaxies, but then we have clear evidence. Very strong evidence that there is something else, because we see its gravitational effect. These clouds, big clouds around galaxies, larger than the galaxy, of this sort of powder that only interacts gravitationally.

Okay, so maybe we've seen then how do we get convinced that this is the case. That's the way science works, slowly in a complicated manner. We have to make a model of how they were formed before the universe, in the early universe, how had they interacted with the full story of the universe, cosmology, and so on and so forth.

Are there alternatives for dark matter? One alternative until short ago was supersymmetric particles, but that was wrong because supersymmetric particles don't exist of that kind. particular kind that was expected one of the great i shouldn't go late with time i just stay within one hour i shouldn't go but let me say that um some physicists even in this room have been talking about a crisis of physics and recently because what they expected didn't happen for instance super similar But you know, crisis physics is like a big defeat in a battle, in a war. The French talk about the big defeat of Waterloo, but the other side of the channel is...

was not a big defeat for Waterloo. Crisis of somebody is good news for somebody else. So when super simility was not found, my part of the community was just jumping around, yeah.

It's a little bit more than that, because for many people, the next frontier of physics should be the theory of everything. I think that the theory of everything is a bad idea. We don't know that. what's out there.

There's so much more about the universe which we don't know. I don't think it's a good idea. It's a good scientific program to write the final theory of everything.

We should solve the problems one by one, like Faraday was doing, or Maxwell, or Einstein. We don't have a quantum theory of gravity. We should find a quantum theory of gravity, because we don't know what happens when gravity becomes quantum. That's a concrete problem. So let's solve that.

Let's put together quantum mechanics with generativity. It's complicated enough because it's a quantum property of space-time. It's changing the way we think about reality. I think we're in a spectacularly good, positive moment in the history of physics.

What we have learned about the universe, black hole, gravitational waves, about gravity and cosmology, is wonderful. Science is going very, very well. It's not in a crisis, not at all. It's a crisis for those who ask the wrong questions.

I think trying to figure out the quantum property of gravity and what happened to black hole, what happened to the early universe, these are the right questions we should do. address. I said one possibility is to work out all the cosmology and maybe convince ourselves, we are not convinced at all, it's one possibility that this dark matter is in fact this white hole.

The other possibility is to make an actual detection. So could we see one of them? Well, let's see.

Imagine this is really dark matter. We can compute how many there are, because we know how many dark matter there are. Astronomers know exactly how much dark matter there is. There's more dark matter than matter.

Sort of a few times more in a galaxy. So the density should be comparable a bit higher, but dark matter is more diffused. So we can compute how many there are.

We know the weight of these little things. I just told you, a fraction of a micrograph, so we know how many there are. And so it's a simple calculation to see how many of these little things there are. And the result is that presumably in a room like this one, every once in a while, I mean, one goes by. So maybe there is no one going by with this.

We couldn't see it. We couldn't touch it. The only interaction would be gravitational, the pull.

So if we had something extremely sensitive, okay, imagine we build a machine, a detector, where something goes by, so intuitively, a little pendulum, something goes by, whoop, start penduling. Okay, now, pendulum mechanics. is complicated. But maybe something electronic, something quantum electronic that could detect very. That's one of the things I'm doing right now, trying to figure out what's best chance we have to build a detector that could detect one thing flying by.

Is this going to happen? I don't know. I'm working on that. What I know that When I became a faculty at Pittsburgh in 1990, and my friends were saying, we're going to build a gravitational wave detector, that seemed really crazy.

And they did it, and they got a Nobel Prize. So, you know, it doesn't mean that all projects are going to work. Many projects don't work, but some do work. So in principle, yes, it's all possible to detect. In principle, it's possible to detect one of these things.

So in principle, it's clearly possible to detect them. Practice. building machine and there as i said one particle flying by every once in a while there's not a lot of them so it's not easy to um to detect now let me let me go toward the conclusion um i've asked you to follow me inside the black hole all the way to the center in a sort of land we know, strange but known, through the jump, hypothetical, we don't know, and then out of the white hole, and then we're back to the stars, like say Dante.

We came back to see back the stars. What is this? This is mind traveling, right? It's traveling with mind, and it's what Dante does.

Dante takes us to this incredible trip into hell, purgatory. paradise, through all the forms of morality, through all the stages of human psyche and mind. Beautiful.

But I think it's also what science does. What the best science does, the best theoretical science. Science has made all sorts of things. There are mathematicians, very good equations. There are the people who do experiments, there are people who find out experiments.

But then there are the people who think the new, like Faraday, who invented the fields, who just created something completely new. And how do you do that? You mind travel. Think for a moment. I'll give you just a few examples in my last minutes.

Anaximander, which is one of my great passions, great thinker in the 26th century ago, who figured out that Earth is like a stone floating in the nothing, right? Before, people thought that the sky is above us. He understood that the sky is all around us, and Earth is just a thing like there. How did he do that?

He mind-traveled outside it. He saw it from the outside. In fact, Aki, same as Anaximander, is credited in antiquity to be the one who invented the geographical maps.

Which is funny, you know, because people have been traveling and trading and traveling long distance for centuries, millennia. Nobody thought to take a piece of paper writing a map. Why?

Because it needs to be creative to change perspective. To imagine, what is a map? A map is how the land would look seen from an eagle, super, super high. But take imagination to think you're an eagle. and grab there.

Or to think you're just like the astronauts on the moon looking at the Earth from somewhere. He did that. So he was capable of mind travelling. Hipparchus, the greatest astronomer of antiquity, has a calculation of the distance of the moon, which is a subtle geometrical argument, very, very beautiful.

But it starts by saying, imagine the Earth is the sun there. and there's a shadow of the Earth, there's a cone, and imagine to go to the tip of the shadow of the cone of the Earth and look back, and then there's a dramatic angle you would see. I mean, I'm not time to go into that.

So again, his mind travelling to the interplanetary space, this is two millennia ago, thinking how would Sun and Earth and Moon would look from the tip of the space. And... Copernicus imagined the solar system seen from the Sun. That's what he did.

Changed perspective on the solar system. And Kepler has this fantastic book. He wrote this book all through his life.

It's called the dream the title and was published after his death in which he tell the story that He and his mother are taken by a demon to the moon And then he goes to the moon and described the moon and described how the solar system looks from the moon So what tell is doing well, he's doing something fantastic. He's taking out a different perspective Um and see how venus and the earth looks from a moving body the moon To convince us that what we see on earth is nothing else that something seen from a moving body. So it's a profoundly rhetorical, powerful argument for Copernicanism based on change in perspective.

Because when you change perspective, what is it, I mean, who cares how... how the solar system looks from the moon. That's not the point.

When you change perspective, you realize that your own perspective is partial. That's the point about changing perspective. It's in science, like in politics, like in economy, like in everything. You put yourself in other shoes somewhere else, so you realize that what you usually think might be wrong because you have a different point of view.

And so on. Einstein famously traveled on a... Talked about traveling on a... ray of light, riding a ray of light, or falling in an elevator. Maxwell also had a demon, right?

Maxwell thought of being small like a molecule, and this demon seeing the individual molecules, what Maxwell's demon is called, opening and closing the door, letting the molecules go, in his work, not on atomities, his work on the kinetic field of gas, on thermodynamics. But how do we mind travel? How do we know what's going to see from a different perspective?

I think that the answer is to stay on a subtle balance between what we keep with us, the Einstein equation that tell us up to there, what we keep with us is the knowledge that tell us something, and what we leave home. Because to make a step ahead, we have to leave home something, right? Einstein leave home simultaneity. Kepler leaves home the circles. Copernicus leaves home the idea that the earth is the center of the universe.

We left home the idea that you can always describe things in space and time. Something basic, okay? This...

How do you know what to bring yourself and what to leave home? I think there is no method in science. There is no recipe for that.

Science is just trial and error. You keep walking on the boundary of what you... what we know.

We know quantum mechanics. no generativity, we know what happens in the black hole up to there, and you keep turning things around until you find the little hole that allows you to put together a puzzle in a different manner in such a way that... It makes sense and there's a possible step ahead and then you know experiment tell you whether it's right or wrong So I think this mind traveling is science Did better and I think that science is a imagination besides just being calculation and numbers.

And in this, science is very similar to Dante, poetical imagination, and to the rest of our life, which is a life of emotion, of... imagination. And I think this part of science is often forgotten. Science has that image of coldness.

But this is science, I think. Thank you.

Transcript for:Exploring the Mysteries of Black Holes

Transcript for:
Exploring the Mysteries of Black Holes