What is a white hole? – with Carlo Rovelli

Imagine that we are traveling towards a black

hole

like the one in these images, so the image behind me imagines that we are actually going there. If you like, we're in a spaceship, if you will, a little star, a spaceship going there. If you lean on a broomstick flying in interstellar space, uh, and we, I've chosen to travel towards this black

hole

here, uh, which is SAR Sagittarius. A star because we have a photo of it. This is a photo that was on everyone. the front pages of the newspapers a few years ago many black holes there are millions of black holes billions of black holes and I would say that this is the most surprising and beautiful discovery in recent years that the universe is actually full of black holes, no we knew that when I was a student nobody imagined that fact, people didn't believe that black holes could really exist in the sky, so I could have chosen a much closer black hole there are much closer black holes uh to go to but this one is uh big it's big and we have a picture we have a picture because it's big it's far in the center of the galaxy so it's a There's a long way to go there, um, but I want to take you there and I want to take you next to it and then inside him, so

what

we're going to see if we zoom in, of course, is something like this image that, uh, uh, that.

You see and have seen in the newspapers, this image was taken by radio telescopes, in fact, some of you may know, by having several radio telescopes on Earth working together, around the Earth, half a dozen or more telescopes working as one radio telescope, like a radio telescope as big as the Earth and combining all the data and the bigger the antenna, the more you can focus at a small angle so we can see something like that, so if you go there, if going there, we're going to to see something like that, of course, without the need for a big radio telescope because we get closer and closer, but when we get closer, we see it better and that's

what

we're going to do. to see um, that's exactly the same as the previous one, except this one, the one we've actually seen, is out of focus because so far that radio telescope can't focus better if it were more focused or what we hope to see with a The technique for obtaining images improvement is this.

In fact, this image was calculated. It was what was expected. It was theoretically predicted before the observation and that's why when we saw this one it was so special you know. This is clearly something out of focus, so why a circle? The reason is a circle. It's far from obvious, first of all, just to make it clear that it would be a circle from any direction you looked at it, so it's not actually a. circle is an optical effect due to the curious way light moves around the black hole the light around the black hole bends because that is what we know light does along with masses and that is how Einstein's theory which predicts everything that tells us how All of this was first confirmed by measuring the curvature of light by the star and it bends in such a way that a ray of light could actually bend so much to be in an orbit around the hole black. any possible angle, whatever the angle, around the hole, the hole is actually smaller than the ring.

The ring approximately the magnitude of the Sun is one and a half times the radius of the ring is one and a half times the radius of the black hole inside it, so you can imagine. black hole imagine for a moment as a black sphere and around it there is actually a kind of sphere of light that is all the light that comes from everywhere and that starts to rotate, so if you have around this thing, there is a lot of matter attracted to a black hole spiraling doing weird things and getting really hot uh just because there's friction there this thing crashes into each other and imagine you have some light coming from here this is a black hole you get the light directly but you also get the light that it curves around the black hole about a radius and a half and then it reaches our eyes and the light from here you get the direct one and the one that comes here to get a little bit of light from everywhere and a lot of tangential light to that sphere wherever you are , so wherever you are you see this ring that is a kind of meridian or or or or equator of this sphere from where tangentially the light comes towards you from wherever.

So this is the optical effect that we see when we go there, so we get closer and we start to see this beautiful ring more clearly. We're also getting close to this, all this matter moving around, but you know, let's imagine our starship or whatever is solid enough. that that's not going to bother us and we get closer we get closer and uh um let's do it slowly uh we have rockets that we can hold well, there is a puddle of the black hole but we can aim at a certain distance from it um keeping the rocket firing so that we maintain the height and we can look down and down there is the hole and this one is big, this one says that the real 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 uh um we uh we stay at a distance we look down we see mostly black and we see things falling now more precise we imagine we're in our starship we open a window like we leave a stone falls we close quickly because the stone falls we see what goes low what we see we do not see the fall entering the black hole right we see the stone decelerating decelerating decelerating um and never reaching the bottom slower slower slower slower slower slower slower slower is okay, not only the light coming from the stone there lights up because it's all this Burning things so it's not dark uh it gets dimmer and dimmer and dimmer and redder redder So the wavelength that The light reaching us is getting longer, which means that the frequency is getting slower, it is as if not only the stone is slowing down, but even its atoms are vibrating more slowly and in fact, if you throw a clock down.

Never throw my father's watch down, but he looks, but if you throw a watch down, we would see the watch. Slow down slower, slower, slower, slower, we almost stop at some point when it goes there, now we are there, since it is SL, now suppose we look back, we look up from the Earth where we came from and imagine For example, we receive messages from Earth. We say that every day we receive a message from our friends' house. How's it going? Is it beautiful there? Okay, once a day. Well, we receive a We send messages once a day, but when we are far from the black hole we start receiving messages every 20 hours, then we go down a little bit towards the black hole and receive messages every few hours and then we get closer and receive messages every minute. and then we get closer again and we get a message every second, but from Earth it was sent once a day for us to see the future, okay, we get closer and closer and we get all these messages with messages from 100 years in the future .

A thousand years. future, the American PIR is finished, I mean, the Congo is Domin, it is the most prosperous, it is peace on Earth, everyone loves everyone, the future maybe, so, that is time dilation, it is exactly the the same phenomenon that on Earth we can measure in a laboratory when we take two clocks. we move one Hum, wait a little, go down and the clock up measures a little more time, the clock down measures a little less time, okay, that's done in the laboratory, except on Earth it's just a tiny fraction of difference of a fraction of fraction of fractions of milliseconds uh that can be measured near a black hole which is a huge mass but time dilation doesn't mean of course we're in the rocket who's there for us , everything is normal, time passes normally, the clock beats normally. the heart beats normally but what we see closer to the Black Hole we see slower what we see far away we see faster okay, but I didn't take you down just to stay outside we want to go in, take you down um because soon I will go next to the black hole, so let's slow down our rockets and start entering inside the black hole.

Now wait a minute. In today's physics textbooks it is written that if you enter a black hole, that is. you'll never be able to leave forever so we still want to go down we're going to die we're going to die too we're going to die if we stay outside because we're going to die anyway but uh but we have less time to leave if I go in. I'm Italian and the greatest Italian poet, Dante Dante Leeri, has written this wonderful book of mind journeys, which is a Divine Comedy in which he goes to hell and then, actually, this. amazing story and when he reaches the dark portal of hell it is written there, which means give up any hope, you who enter, who enter, but he goes, anyway, okay, so let's enter, however , he's going out, so let's go. we go down, we enter the black hole, so we're inside, we're inside the black hole now and uh um, we're inside and let's look around, okay first, but nothing in particular has happened, cross the horizon and enter the black hole It's the same, we are the same as before.

Our spaceship is the same as before. If you look back at the stars, the stars are still a little out of position because the light makes strange turns, but the lights come in and we see the star, but something. It's different when we were outside floating around uh there was a sphere around the black hole another sphere another sphere and everything was there static well and in the center of this sp there was this big black hole but of a certain size now that we are inside Imagine that we can look at around us and see what it is, um geometry, what we see is that there is still sphere and spheres, but being below us is not just the volume that we would expect, it is an immensely large volume, so spheres and spheres and spheres and spheres and spheres down down down very far the black hole is um you know these fairy tales that you know the two brothers the sister brother walk in the forest and then there is this little hat and there is a door through which they enter and inside huge, huge room and another huge room and then Gard fountains and so on a black hole is something similar it has a size outside but inside there is much more volume than what we have talked about in school, do you know if the sphere of this radius exists what is the volume inside you can calculate it well never believe what they say in religious school it is not true there is a sphere in which the volume is much greater than what they tell you it is inside the C why because space is distorted that is what generativity described the distortion of the space This is huge, so um inside is kind of um, so imagine this is um, wow, this is a two-dimensional model, imagine this is the sphere around the black hole, the real black hole that we were before being here. now we move in we're in and inside there's a lot more than you would expect if this were flat okay, it's long, long, long, long, there's this long tube, uh, enormously, uh, enormously long, the older the black hole, the more the tube is. a long time, okay, if there is any person here in the audience here or looking through the camera, who is it, who knows General TV very, very well, you can ask what I'm talking about.

I am describing a black hole in a peculiar foliation which is I think it is better to understand what is inside the black hole, what is the foliation that maximizes the volume and covers the entire black hole, so the first surprise is that there is this well this good. very deep but that is not the biggest surprise the biggest surprise what we see there is that it is moving while outside nothing was happening we could have been floating there forever inside the geometry of the space is changing the space around us is changing and two things are happening one is that it is getting longer and longer with time, that's why the black hole is bigger because with time it gets longer and longer and longer and longer it keeps growing, growing, growing, growing, we are not talking about kilometers, we are talking about millions. of light years there is a huge, huge space inside an old black hole, uh Sagittarius.

A star is presumably at least 10 billion years old, it's very old, so this is up to the cosmological age, so it's something immense, but the other thing is that it's shrinking, it's getting longer and shrinking now. , the fantastic people here at a build this beautiful thing that actually gets longer, but I didn't know how to make it shrink at the same time, maybe so next time I'll make it shine. If you're here, we're getting this, the SPH On Us is approaching us, well, at a speed that's quite concerning. I mean, we don't have much time, this is coming up around us, so here's a photo of uh uh of. an idea of ​​the inside of a black hole and by the way, at the end of this image that you sent, you see a small dark U dark spot at the end, so down here, I said it's very long, but it didn't say it's infinite, no It's infinite. right, it's finite and then it's way down here, if some of you think there's a singularity, forget it, there's no Singularity out here, whicha funny electron doing something funny is time itself and space itself jumping from one trajectory to another trajectory so We have to conceive of a way of thinking about physics in which you can describe a phenomenon, a normal phenomenon of the universe, not as if It was happening in space-time, but space-time itself does something funny, so here's a picture, here's a full picture, a star collapses, a black hole forms. because it's very long um and uh uh Quantum leap

white

hole and uh uh slowly everything comes out and uh the whole process is finished now first question how does this work?

How long it takes? Well, I think you are quite in agreement with the fact. that how long is a fun question here is a complicated question how long does it take if we are outside? um it could take a long time in fact it could take a long time when we start doing these quantum leap calculations because the classical part is just Einstein's equation it's just technical uh we use quantum gravity for the calculation um we realized that probability jumps occur with some quantum mechanical probability over probabilities the probability of the jump occurring as seen from the outside um is completely insignificant if the black hole is large, it is only possible if the black hole, if the horizon is small, now this It seems like a big deal because, gosh, but most of the black holes we see in the sky are big.

They're not going to let it happen there, but black holes get smaller over time, that's big. Stephen Hawking's discovery that black holes evaporate if you keep feeding them. The black hole gets bigger, but if at some point they eat anything. to eat at some point they are there isolated and slowly they emit energy and they become smaller and smaller, so if you wait long enough, even if you are a star, you say star, which is this gigantic black hole, even the biggest , billion. um solar masses, the black hole is going to shrink, it shrinks, it shrinks, it shrinks and it becomes very small once they are small, the probability of jumping is very high, in fact, it turns into water and jumps, like this that the correct story, uh, if you want cosmologically astrophysically, the likely story is that whatever black hole you have, it will take some time and the time was calculated by the Hulk to become small, then it jumps and then it becomes a hole

white

, the white hole is tiny, tiny, tiny, because it is the end of the operation, but be careful, it is tiny.

The bottleneck inside is big and it takes a long time to get out of the inside, that's the whole story. Now the question is how long does it take for a black hole to wait for it to become small and this was calculated by Hawkins. It's H for a Sagittarius-sized BL that's been around much longer than the life of the universe. things we are in the white hole the time it takes in Sagittarius A star from the moment we cross the Horizon by we are inside the Starship we are going to go down again assuming we are not squeezed and crushed, okay, we have a Starship of super powerful kryptonite that is going through all the quantum leaps the time it takes us to get to the jump is minutes for a large black hole for a small black hole it is seconds even less uh the jump is instantaneous and the time to get out of white space is very short again, minutes, so outside time is billions of years, inside it's just seconds or minutes, meaning the entire process is something that takes billions of years for a clock outside, minutes for a clock inside .

It means that if you want to see what the billions of the universe are for you in the future, just jump into the black hole and you'll be back in in a moment, assuming you're not crushed and destroyed, so one way to see what a white is black. The hole is being considered as a shortcut to the future, you go in, Boop, you come out in the future, there, in the same place, okay, you go in this hole and you come out of the same hole, far, far in the future, eh, that one This is a huge time.

The distortion is the same type of time distortion on my watch, except it's gigantic. Another way to think about that is to think that what's really happening is that you have a star falling, collapsing, bouncing, but because of time warp you're watching this in slow motion amazing slow motion for billions of years just wait wait wait but it's you who's watching it's slow motion because it's addition time so that's an image um of um that's the likely image of um um uh of what's happening um now a question is um um do we believe this ? Can we verify this?

How can we verify this? How do we wait uh to see if this is right or wrong and as I usually say? I've been working on this for the last few years. Sometimes I'm completely sure it's right. Usually on Monday I'm completely sure. Usually on Friday I am totally desperate. No, no, this still isn't going to work. How do we check that this could if this? is true, what could we do to find out that it is correct? One thing is that we could see some indirect effect of that and we could actually see these things directly, so the indirect effect, in fact, let me make a little parenthesis about black holes black holes uh uh the first evidence of the The first time astronomers could see something in the star and thought maybe it was a black hole was in the '70s, but it was just a few astronomers, no one took them seriously, um, there were some stars that were moving, we could see the Doppler effect, they were moving back and forth, like we were spinning around something but there was nothing around to spin around and someone says maybe there's a double star and one of the two is a black hole um, John Wheeler had a beautiful image, if you have a you know, John Willow, another generation, a couple dancing the vzer and he's dressed in black and she's dressed in white and there, very dark, you just see her moving like that, uh, but you know . there should be someone else because she is, so, that's the girl in the white dress is a star you see, the black one is a black hole, that was in the 70s and, still, in the 80s, no one would believe, black holes , it's just, it's really In this century people in the scientific community at large began to take seriously the idea of ​​the black real.

I say in the book that when I moved from the United States to Europe, the head of my department in France, who was not a relativist, he was a particle physicist in 2000 told me that you don't really believe black holes exist, so still in the year 2000 many very good and very serious scientists would not take this seriously now, of course, he changed his mind, but that is not a criticism of him at all. because changing your mind is what science is all about, I mean that's exactly the best thing about science, now what's the reason I'm saying all this because in the '30s, long, long before this guy built this antenna because he was working for Laboratory B to study noise in Communications and he picked up a signal from a point on the star in this in the sky that was in the center of the Sagittarius galaxy in the central constellation and was called at that time moment sag arus a star, was seeing the black hole, well, the signal, the radio signal from the black hole was first received on Earth by someone in the 1930s, long before anyone thought it could be a hole black, took half time. century to recognize that it was a black hole, so it's not totally impossible that white holes are something that we've already seen and in fact, imagine that in the early Universe we know the very hot and very choppy universe and there's a lot of speculation. that nowadays many black holes could form at the time they are called primordial black holes or imagine, there are even many speculations that the big bang was not a beginning, there was a big bounce, so the previous Universe was compressed and bounced and in fact, the bounce universe is a phenomenon that is vaguely similar to the bounce of the black body that becomes a white hole, so imagine that before or in the universe many black holes formed, these are already became white holes, now they are small white holes because, as I said, the transition occurs only when they are small, so imagine that there are many of these small white holes and the mass that we can calculate, the mass that the theory gives, is a board mass which is a fraction of a microgram, which is the weight of my hair, so one hair two hairs uh, that's a board mass, so a white hole is something that has a much smaller weight than my hair, but it has a weight of that, which means that it is a gravitational attraction like this little one that you cannot see because it does not interact electromagnetically, you cannot touch it because it is electromagnetism, but in principle you can see that it is a gravitational force, so imagine that there were a lot of these tiny things in the universe that formed in the past and that were floating around and would behave like they matter because the attractions have gravity but they would be dark because they don't interact with light so they would be dark matter Dark Matter astronomers look around of galaxies Halos of something they call dark matter that only interacts gravitationally does not interact electromagnetically and behaves exactly like many small things were with a board mass moving around, so it could be and this is not something we know, it is speculation, it is one possibility, could it be that maybe we've already seen these things, this uh um zillions of little white holes that are former black holes that have bounced around and now these things and they've been detected by astronomers and that's what they call dark matter, nobody knows what it is, it is 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 are big clouds around galaxies larger than the galaxy, the kind of dust that only interacts gravitationally, so maybe we've seen. So how can we convince ourselves that this is the case? This is how science works slowly in a complicated way. We have to make a model of how they formed before the universe in the early Universe, how they interacted with the entire history of the universe. cosmology, etc., etc., are there alternatives to Dark Matter?

An alternative until recently was supersymmetric particles, but that was wrong because supersymmetric particles don't exist of that type, that particular type that was expected to be one of the big ones that should. I'm not late, I'm in 1 hour, I should go, but let me tell you that some physicists, even in this room, have been talking about the crisis of physics and recently because what they expected did not happen because of supersymmetry, but. You know, crisis physics is like a big defeat in a bottle in a war. The French talk about the great defeat of water, but the other side of the canal is not a great defeat, water, so, um someone's crisis.

It's good news for someone else, so when Super Cimity wasn't found, my part of the community was just jumping, yeah, it's a little bit more than that because, for a lot of people, the next frontier of physics should be a big one. UniFi. The Theory of Everything I think the Theory of Everything is a bad idea um we don't know that what's out there there's a lot more about the universe that we don't know um I don't think it's a good idea, it's a it's a good science program to write the final Theory of Everything. We should solve problems one by one like Fara or Maxwell or Einstein did.

We don't have a quantum of gravity. We should find a quantum of gravity because we don't know what will happen when gravity becomes quantum, that's a concrete problem, so let's solve that, let's put quantum mechanics together with generativity, it's quite complicated because it's a quantum property of space- time, it's changing the way we think about reality, um, uh, I think we're in a spectacularly good positive moment in physics what we've learned about the universe black hole gravitational weights on gravity and cosmology it's wonderful science It's going very, very well. It's not in crisis at all. It's a crisis for those who ask the wrong questions.

I think trying to discover the quantum property of gravity and what happened to the black hole, what happened in the universe. these are the right ones, the right question we should address, I said one possibility is to solve all of them. cosmology and maybe convince ourselves that we are not convinced at all that it is a possibility that this dark matter is actually this white hole, the other possibility of making a real detection, so could we see one of them? Well, let's see, let's imagine this really dark matter. uh, we can calculate how many there are because we know how much Dark Matter there is.

Astronomers know exactly how much dark matter there is. There is more Dark Matter than matter a few times more in a galaxy. Okay, so the density should be somewhat comparable. a little higher, but the dark matter is more diffused, so we can calculate how manythere is. We know the weight of these little things. I just told you a fraction of a micro so we know how many there are. So it's a simple calculation to see how many of them. These little things that there are and the result is that presumably in a room like this from time to time, I mean, it depends on one passing well, so maybe there is no one passing with this, we couldn't see it, we could.

If we put it, the only interaction would be gravitational in the pool, so if we had something extremely sensitive, okay, imagine we built a machine, a detector where something passes through so intuitively, a little pendulum, something happens, what it starts pending, okay, now the pendulum is mechanically complicated, but it could be something electronic something quantum electronic that can detect that's one of the things I'm doing now. I'm trying to figure out what's the best chance we have of building a detector that can detect a thing flying. Is this going to happen? I don't know, I'm working on it, but I know, when I became a professor in Pittsburgh in the 1990s and my friends were saying we were going to build a gravitational wave detector that seemed really crazy and they did it and they got a Noel 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, everything is possible to detect, in principle.

It is possible to detect one of these things, so in principle it is clearly possible to detect them, practice building machines and, as I said, a particle flies by every now and then, there are not many of them, so it is not easy to detect them now, let me . Let me come to the conclusion. I asked you to follow me inside the black hole to the center of the kind of earth that we know strange but known through the hypothetical jump and we don't know um and then we get out of the white hole and then we go back to the stars like we see the stars again um what is this?

This is traveling with the mind, right, it is traveling with the mind and that is what Dante does. Dante takes us on this incredible journey to Hell, Purgatory and Paradise, through all forms of morality, through all stages of the human psyche and mind, beautiful, but I think it's also what makes the science. does what the best science does the best theoretical science science does all kinds of things there are mathematicians very good equations that people who experiment people who discover experiment but then there are people who think the new like fari who invented the fields that is something completely new um and how do you do that do you care about traveling um think for a moment um let me just give you a few examples in my last few minutes uh anaximander which of my great passions great thinker in the 26th century Who discovered that the Earth is like a floating stone in nothingness just before people thought the sky is above us?

He understood that heaven is all around us and the Earth is just such a thing, how did he do that? could travel outside of it, saw it from the outside, in fact, the same man is credited in ancient times with being the one who invented geographical maps, which is funny, you know, because people have been traveling, trading and traveling long distances for centuries. Millennia, no one thought. to put take a piece of paper writing a map why because you have to be creative to change the perspective to imagine what a map is a map is what the earth would look like seen from a super super high eagle but it takes imagination to think that you are a eagle and you're up there or think you're like you know, like the astronauts on the moon looking at the Earth from somewhere, he did that so he was able to travel mentally um um uh ep parus the greatest astronomer of antiquity is a calculation of a distance from the Moon, um, which is a subtle geometric argument, very, very beautiful, but it starts by saying imagine the Earth, there's a sun there and there's a shadow of the Earth there, a core and imagine going to the tip of the Moon. shadow of the earth's corn and look back and then there is a geometric AR angle that you would see, I mean, it's not time to get into that, so again your mind travels to interplanetary space, this is, you know, two ago millennia, uh, thinking.

What would the Sun, Earth and Moon look like from the edge of space and did Copernicus imagine the solar system SE seen from the Sun? That's what he did. He changed the perspective of the solar system and Kepler Kepler has this. He published a fantastic book, he wrote the book about it throughout his life. It is called The Dream, the title was published after his death and tells the story of him and his mother being taken by a demon to the Moon. And then. he goes to the moon and describes the moon and describes what the solar system looks like from the moon so what the hell is he doing right? he is doing something fantastic he is taking a different perspective and seeing what Venus and Earth look like from a moving body. moon to convince us that what we see on Earth is nothing more than something seen from a moving body, so it is a powerful and deeply rhetorical argument for Copernicanism based on the change of perspective because when you change perspective, what is it?

I mean, who cares how? the solar system is seen from the Moon that is not the point when you change perspective you realize that your own perspective is partial that is the point of changing perspective in science as in politics as in economics as in everything that you put yourself in different shoes in another place Then you realize that what you normally think could be wrong because you have a different point of view etc., I mean, Einstein famously traveled, talked about traveling in a ray of light, riding in a ray of light or falling in an elevator maxw Max also had a certain demon maxw thought of being small as a molecule and this demon scenes the individual molecules what is the name of the demon Maxell opening closing the door letting the molecules go in his work no in electromagnetic his work in the kinetic theory of gas in uh, about thermodynamics, but how do we care about traveling?

How do we know what we are going to see from a different perspective? I think the answer is to remain in a subtle balance between what we carry with us and an equation that tells us how far we are. what we keep with us is a knowledge that tells us something and what we leave at home because to take a step forward we have to leave something correct at home Einstein leave simultaneity at home uh Kepler leaves the circles at home uh Copernicus leaves the idea that the Earth is the center of the universe. We let go of the idea that you can always describe things in space and time.

Something basic. Well how do we know? How do you know what to bring and what to leave at home? That's me. I think there is no method in science there is no recipe for that science it is just trial and error you keep walking at the limit of what we know quantum mechanics we know general relativity we know what happens to Black up to that point and you keep changing the things until you discover that you know the little hole that allows you to put together the puzzle P in a different way, in such a way that it makes sense and there is a possible step forward and then you know.

The experiment tells you if it's right or wrong, so I think this mind trip is science. I did better and I believe that science is imagination as well as being calculation and numbers, and in this science it is very similar to Dante's poetic imagination and the rest of our imagination. the life that is a life of emotion of imagination um and I think this part of science is always U is often forgotten science has an image of coldness but this is science, I think it's okay