In this captivating episode of the Beyond Einstein series, renowned physicist Carlo Rovelli joins the discussion to explore the possibility of white holes, an intriguing concept in theoretical physics. White holes are the time-reversed version of black holes, theorized regions in space where matter comes out but cannot enter. Although no observational evidence currently exists for white holes, they are a logical prediction of general relativity and may offer insights into solving the dark matter mystery.

The episode delves deeply into time reversal in physics, suggesting that if black holes exist, then white holes could logically also exist as per the laws of physics. Rovelli shares how the concept of white holes emerged from loop quantum gravity, a theoretical framework he has extensively worked on. He highlights the idea that the end of a black hole could potentially be the birth of a white hole, thereby offering an innovative perspective linking quantum mechanics and general relativity.

Main takeaways from the video:

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Key Vocabularies and Common Phrases:

1. enigmatic [ˌɛnɪɡˈmætɪk] - (adjective) - Difficult to interpret or understand; mysterious. - Synonyms: (mysterious, puzzling, unfathomable)

And together, we are going to delve into a topic that's as enigmatic as it is captivating the notion of white holes.

2. temporal [ˈtɛmpərəl] - (adjective) - Relating to time or the material world; as opposed to eternal or spiritual matters. - Synonyms: (secular, worldly, chronological)

The laws of physics seem to allow any process that can occur in the usual temporal order to also occur in the reverse temporal order.

3. invariance [ɪnˈvɛriəns] - (noun) - The property of remaining unchanged regardless of changes in the conditions of measurement. - Synonyms: (consistency, stability, uniformity)

And this quality that they have, which we often in the field refer to as a sort of time reversal invariance.

4. singularity [ˌsɪŋɡjuˈlærəti] - (noun) - A point in space-time where density becomes infinite, as predicted in some models of gravitational collapse. - Synonyms: (anomaly, peculiarity, oddity)

singularity, they bandy it about as if it is a thing. It's really just a message saying that we don't know what's going on.

5. horizon [həˈraɪzn] - (noun) - In physics, particularly in general relativity, the boundary beyond which events cannot affect an observer. - Synonyms: (boundary, limit, edge)

The horizon is not a black hole horizon, it's a white hole horizon.

6. speculation [ˌspɛkjʊˈleɪʃən] - (noun) - The forming of a theory or conjecture without firm evidence. - Synonyms: (guess, hypothesis, conjecture)

So if everything comes out, who put the stuff in? Where does it come from? In fact, there were some speculations by some Russians

7. quantum regime [ˈkwɑːntəm rɪˈʒiːm] - (noun phrase) - A domain or scale where quantum mechanical effects become significant. - Synonyms: (quantum domain, quantum scale, quantum zone)

Careful, because this jump is a quantum jump. So in order to happen, you have to be in a quantum regime.

8. hypothesis [haɪˈpɒθɪsɪs] - (noun) - A supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation. - Synonyms: (theory, thesis, assumption)

The hypothesis is that if we could do higher order, it wouldn't change much.

9. super symmetry [ˈsuːpər ˈsɪmɪtri] - (noun) - A hypothetical symmetry between fundamental particles, predicting partners for each particle. - Synonyms: (SUSY (abbreviation), particle symmetry, symmetry pairing)

So these are natural candidates for dark matter. Of course, we don't know what dark matter is. There are three or four or five candidates. Best one we had is died super symmetry.

10. astrophysicists [ˌæstroʊˈfɪzɪsɪsts] - (noun) - Scientists who study the physical properties and processes of celestial objects and phenomena. - Synonyms: (cosmologists, astronomers, stargazers)

And when I talk to the astrophysicists, say, they're teeny. They're so teeny.

Carlo Rovelli and Brian Greene on Black Holes and White Holes

In this third part of our Beyond Einstein series of conversations, we're venturing fairly far beyond the bounds of established science and into the exciting but tentative realm of scientific possibility. Joining me in this intriguing exploration will be renowned physicist Carlo Rovelli. And together, we are going to delve into a topic that's as enigmatic as it is captivating the notion of white holes. Here is the basic idea. The laws of physics seem to allow any process that can occur in the usual temporal order to also occur in the reverse temporal order. Imagine, for example, a glass shattering on the floor, shards flying this way and that. Now picture the shards leaping from the floor and reassembling the glass back into your hand. This reverse process, unfamiliar though it is, is fully in keeping with our understanding of physics.

Now, this concept of time reversal brings us to our main topic. Because the time reversal of a black hole is a white hole. While black holes are regions of space where nothing, not even light, can escape, white holes are theoretically the exact opposite. Nothing can enter as they spew out matter and light. As yet, there is no observational evidence for white holes. But Carlo Rovelli suggests that white holes might not only be real, but could also be the key to understanding one of the most persistent mysteries in the nature of dark matter. So let's now explore the possibilities that lie at the intersection of black holes, white holes, and dark matter with physicist and author Carlo Rovelli. And let us begin with Carlo Rovelli, the director of the Quantum Gravity Group of the center for Theoretical Physics at Aix Marseille University in France. He is a co-founder of the Loop approach to Quantum gravity and author of the forthcoming book entitled White Holes.

So, Carlo, thanks for joining us. Wonderful to speak to you again. Thank you, Brian. It's a pleasure. So you heard we were talking a lot about some dark energy black holes, and you have been spending some time thinking about a more exotic version of a black hole, so to speak, which is a white hole. Before we get to it, can we motivate it by thinking more fundamentally about the laws of physics and this quality that they have, which we often in the field refer to as a sort of time reversal invariance, that the laws don't really know so much about a directionality in time? And that's at least one way to motivate the possibility of white holes. So is that a fundamental feature of physical law, this kind of insensitivity to forward in time versus backward in time?

Yeah, as far as we know, yes. All the laws of physics, the fundamental laws, are, such as if they allow something to happen and you take a movie of this and you project the movie backward, this is also allowed. Now, that's a little bit strange based upon everyday experience because there's friction, there's dissipation, there's all the irreversible phenomena. But these come about when there are a lot of stuff that get. That mix it up. And when we're talking about black holes, in some sense they're very simple, right? In some sense they're very simple, yes. And so you can reverse them. Yes. And when you reverse them, you get a white hole. You get a white hole. So I will leave it to you to give us a clearer sense of what white holes are and why we should think about them.

Well, let me start from this. Black holes are a fantastic story, right? Because when I was a student, in my textbook was written that they probably don't exist. My teacher said if they were written about at all, most texts wouldn't even mention, would not even mention them. My teacher of general relativity was saying, yeah, but there's nothing like that in the universe. And a few people were studying them. It is a prediction of Einstein's theory, Namely, Einstein's theory says that impossible they could exist, but they also might not exist. And then slowly we started recognizing things in the sky which are black holes. In fact, we have been detecting a black hole long before recognizing it.

Because since the 30s, the people, the first people who put a radio antenna pointing up Jansky in the 30s, got the signal from the Sagittarius constellation, a very strong signal, which nobody knew what it was. And that's the center of our Milky Way. At the center of our Milky Way. It was a big impression at the time it came out on the New York Times. Is that right? I didn't know that. Yeah, yeah, yeah, yeah. It came out of the New York Times. A super strong signal coming from the stars, from the center for the galaxy. There was a NBC radio broadcast in which the sound of this really was. And Jansky was there. And they asked what it is. And they said, don't know, we don't know.

And the person on the radio said it should be an incredibly powerful source because our radio stations are not going to be heard in the center of the galaxy. In fact, it was. Thankfully nobody knew what it was. And now we know. It's this thing that we have just heard about the big black hole, the center of the galaxy. So black holes were not recognized for a while. Then in the 70s, they were first hints, and now we have this spectacular confirmation. White holes today are like black holes 20 years ago. Open possibility in the predict are predicted by general relativity. It is a solution of the equation of generativity. Actually, as you said, it's not a different solution black hole. It's the same solution backward, backwards in time. Backwards in time, yes. And they might be out there. And I think there are reasons to believe that they're there.

And why were they? Or have they been really till today? I think that most physicists that haven't done a survey are of the point of view that is expressed in many of the famous textbooks of general relativity, that white holes, sure, there's a mathematical formula, but they're not with. Did I say. Yeah, I mean, white holes because. Because a black hole, we have a plausible story of how it's formed, right? Actually, we don't know how it dies, but we have a plausible story how it's born, it starts, which is, you know, matter that collapses. A big star. If the star is very big, it's sort of kept open by the fact it's burning and nuclear fuel and propping it up makes pressure, so it keeps it up. But as soon as the hydrogen is consumed, it cools down. And then the weight, if the thing is so big, it squashes a rock, everything, and falls into itself and produces a black hole. And that's at least the big black holes we see around. We think that at least many should be born with it. And Oppenheimer was part of the. Oppenheimer was the first model. Just before other work. Yes, just before other works, yes. Oppenheimer, Schneider, they have the people, white holes, we don't know how they could. Wait, wait. We didn't know until a few years ago how they could be born. Because black holes, everybody knows a black hole, everything falls inside, nothing can come out. A white hole, it's immediate. If you just think of reversing the movie in your head, everything comes out and nothing can go inside. So if everything comes out, who put the stuff in? Where does it come from?

In fact, there were some speculations by some Russians. There's always a Russian theoretical physics, who suggested that from the Big Bang, there were some little big bangs at the Big Bang, which produced the white hole. But that was not very convincing. So people thought, well, they're not white holes because we don't know how they could be born. Right? And yeah, please. And here it's a novelty. And that's what I've been doing in the last five, ten years almost now, which is the following all this is classical generativity. But we know, we just heard a moment ago that classical generativity is wrong in the same sense in which Newton theory is wrong, or in the sense that it needs to be superseded. Not that it makes wrong predictions, but it's limited range of validity. It's like Newton's theory, super right, but within a certain domain. And then when you go out.

And we know that since the beginning, Einstein wrote his generativity main paper in 1915, and in 1916, one year later, there is a paper by Einstein that says, of course my theory is wrong, because quantum mechanics, there's something missing. It cannot be the end of the story about gravity. So there's a problem of quantum gravity. Now, from black hole, there are two questions. First question, we see all this thing falling in. Okay. We practically see the matter spiraling and going. Yep. Where does it go? What happened next? We know that it can cross the horizon. I trust general activity completely at this point. Some people say, well, it may happen something. I don't believe it. So things go in, falls inside. I trust general relativity entirely. It goes toward the small. It gets squashed by very strong forces. At that point, quantum mechanics has to come in because it's so small at that point that because it's so small, it's so much curvature, so much pressure, so much energy density, everything tell us that that's where quantum mechanics comes in.

Yes. Even more, if we don't think that quantum mechanics comes in and we think the generativity is right, we get to the singularities, we get to nonsense, the time stop. And it's important just to emphasize, because people use the word singularity, they bandy it about as if it is a thing. It's really just a message saying that we don't know what's going on, that the mathematics breaks down. So when we talk about the center of a black hole as a singularity, it is a place where we throw up our hands in the classical Einsteinian theory of general relativity. Yeah. So it's, as you say, it's a message that the theory goes wrong. But this is a weak message. We already knew that the theory goes wrong there. Yes, because we expect quantum phenomena there. So we expect the Einstein equation not to be valid there. That's one open question. And the other open question which turns out to be connected is that we know, thanks to Stephen Hawking, which has, I guess, convinced all of us because it's very convincing. His theory, his calculations, has been redone all sorts of ways, that if you Take a black holes, you wait there for very long, it slowly evaporates, right? It radiates this. So it becomes smaller, smaller, smaller, smaller, smaller, smaller. And then what people say, it disappears. But that, what does that mean? Yeah, yeah. The inside is still very large. Okay. There's a huge inside. So it pops up, nothing to.

And the inside people can think about is the long indentation, if you will, in the fabric of space. So a black hole is really a long, long tube. Not infinite, but very long. But with time it becomes longer and longer, longer and narrower and narrower and. Careful, the singularity is not down there. The singularity is when the tube shrinks to zero. So the outside, the surface of the black hole, the thing we see from the outside, this little sphere becomes smaller, smaller, smaller, smaller. At some point, it's deep into the quantum regime. What happened next? Question mark. Okay, so here's what I told you. You're a good scientist. We have two problems. We don't know how white hole are born and we don't know what happened to black hole at the end of this story. Right? Here's the idea. The death of the black hole is the birth of the white hole. And that's the idea of the transition.

So this tube becomes very, very long, very narrow. We entered a quantum regime. Classical theory doesn't work anymore. Now we have, in the quantum regime, quantum jumps. Oh, that's a. We are falling inside. This is the black hole that becomes longer and longer and longer. The quantum regime is the red line there. And what happened next, there is a jump to the opposite process. Namely, it bounces back. Now we're in a white hole, the same process. Here it is, here it is. We keep going up and the white hole is the next phase where these long tubes come out. And now the horizon is not a black hole horizon, it's a white hole horizon. I mean, if there were a white hole horizon, if it was actually out there, if this isn't just theoretical ideas, what would it look like? Would there be a specific signature that we could use to detect it? Okay, so after the red transition, we are back in classical relativity. So we know everything, right? We're home. Because so we know everything about how the thing would look from the outside. I'm going to tell you in a moment, but careful, because this jump is a quantum jump. So in order to happen, you have to be in a quantum regime. So I would expect that's a model we're working out, that it happens only when the black hole is very small. Right? So now we are very.

It's not small inside. It's kind of huge stuff. But this small throat. They're getting a small white hole. Presumably it becomes a small white hole. Right? So from the outside, a small white hole. It's a little thing, very teeny, that lives for very long because all the things have to come out very slowly. Has to come out. So it's a very long life. It's called a remnant because the black hole dies and it leaves this remnant which very, very slowly emits this and this very, very slow radiation. And it's what? It's just a teeny, teeny thing with a mass. The mass is easy to compute because this is quantum gravity. It's a Planck mass, which is about my hair. There it is. You have more than I. I would not waste one of my remaining ones. I do not do everything for science. Okay, okay. So imagine one of these things flies by very fast. But here we can see it. Well, I can see it, you can see it. But it is here because it interacts.

Because as electron, proton, so it interacts electromagnetically. A white hole would have this mass, but would not interact electromagnetically. So it's only the gravity. And let me just quickly point out, because having a sense of scale, I think helps people have an intuition. When we talk about the Planck length, where quantum mechanics and general relativity come together, it's a very small length, about 10 to the -33 centimeters. The Planck mass that you're referring to is tiny by everyday scales, of course, a piece of hair is so light, but that's enormous on the scales of elementary particles, the kinds of things for which the quantum world is usually applied to. So that's huge by the mass of an electron. And that's why it has this dual role in what you're describing.

Exactly. So when I talk about this white hole that we are studying, when you talk to the particle physicists, they say, oh, but they're huge. Planck mass is enormous. And when I talk to the astrophysicists, say, they're teeny. They're so teeny. It's the same. But careful. Look, there's another way of saying that. Planck mass, Planck energy, Planck length, Planck area, Planck, all that stuff immediately. This is quantum gravity, extreme energy, extremely small. But a Planck mass. No, Planck mass is that. It's a. It's a. It's a. So if there is a quantum gravity phenomena phenomenon that we have a chance to observe, it has to do with the Planck mass for sure. And here it is a tiny white hole. Big white holes might be unstable, there's some difficulty with them. But small black holes are most likely stabilized by gravity, by quantum gravity. And if you think quantum gravity is a theory with a scale, the Planck mass. Sure. And theories with a scale typically have a particle at that scale. So there should be Planck scale, Planck mass particles around. And if we detect them, we have observation of quantum gravity. Now, the transition in your approach from the black hole side to the white hole side, as you emphasize, has to pass through this quantum domain famously.

We're not quite sure what that quantum theory of gravity is. You've worked a number of years, a good fraction of your professional life on loop quantum gravity. I've worked on string theory, distinct approach. Have you been able to use loop quantum gravity to make this story more complete, or does this still stand beyond the reach of the ideas that are still in progress to build quantum gravity? No, this story come out because, in fact I, I told you the story of white holes. But the way I came into it is through loop quantum gravity. And in fact it's the opposite. I mean, loop quantum gravity, I would have put it your string theory. So I should choose my words carefully here. So let me say string theory is a wonderful theory. It's a unification of everything. It's fantastic, it's beautiful. Let me put it nicely here at the World Science Festival, you had the big guys of the string theory saying, yeah, I'll do all that. Actually would be great if we could understand, could have a theory of quantum space and quantum time. Okay, now, loop quantum gravity doesn't do unification, doesn't do all the goodies of string theory.

But you said that pretty quietly just now. But I just, you know, he slipped in what it doesn't do. But anyway, I'll just be quiet, you can keep going. It doesn't do unification. It's not so ambitious. But it is a theory of quantum space and a theory of quantum time. It definitely is. And I think it's a very good theory of quantum space. I don't know if it is a right theory. I don't know if the good Lord likes it. But it's a good theory, I think, of quantum space and quantum time. If those old guys would study the books of loop quantum gravity, I think they would like it. So it allows us to exactly disappear the image, study what happened this transition. And in fact, you can do a calculation with loop quantum gravity which gives you the transition amplitude, you see? I mean, the likelihood of the transition taking place.

Yes, the probability of jumping, you see, all around is classical space time. Sure. So this is a typical quantum jump. Like, you know, you use classical theory to one point, classical theory after, and in between. You want to know, can this happen and what does the probability happen? Sure. Does it happen likely? Does it not happen likely. So you can do the calculation and we're doing these calculations, approximating, throwing away piece using computer. I mean all, it's not easy calculations. And the calculations we do step by step, order by order, but we can do these calculations. And in fact the calculation has been telling us. I was hoping that the calculation would tell us that this can happen when the black hole is large. But the calculation is saying, no, no, that's extremely suppressed, that's extremely improbable. But when the black hole is small, the probability of this happening goes to one when it goes to.

But can you trust your calculations in a regime where, say, an order by order approximate approach may run into trouble because the. Oh, oh. Can you trust the calculation? I don't know. I am truncating the theory I'm computing to first order in this truncation. It's not really an expansion, it's a truncation. And the bet is that if we could do higher order, it wouldn't change much. Okay, it's quite a bit. That's one of the things we're doing. Right, exactly. But look, this is exactly what we're doing. We are testing the theory. The theory is predicting this jump. And so if there are primordial black holes small enough. I'm not a cosmologist, so. But there are people who are working out this. Or if there is a big bounce and there were big black holes in the previous phase that go through the big bounds. So if something in the early universe has produced this remnants, they should be around and we should be able to test it.

And so this would be Planck scale remnants. Yes. And do you propose a specific way of looking for them? Well, yes, but let me first add another idea, which is speculative, but is what really motivating me. Suppose there are many of these. So you have small particles, grains, powder, that only interacts gravitationally. There's a lot of them. Sounds like dark matter is coming out very much. Sounds like dark matter. So these are natural candidates for dark matter. Of course, we don't know what dark matter is. There are three or four or five candidates. Best one we had is died super symmetry. Well, it's not quite dead yet, but yeah, okay. But we don't know. Right. So we really don't know. That's beauty of science. Let's see. And so I have this hope that like the black hole that was observed from the 30s to the 70s for half a century without knowing what it is, perhaps we have been observing these things in the form of dark matter for decades without knowing what they are. And we are in the process. I don't know. Right. I've written this book on white holes. And in the book I say at page two, I don't know if white holes exist in the universe. So these are tentative.

I think it's very important to say what we understand about the universe. These beautiful things we heard about black holes, that solid that we know it is. Einstein equations were believable on the horizon, beyond the horizon, beyond the horizon. White holes is an idea speculation, but it would be an explanation of dark matter which doesn't need extra particles, change of equations. It just needs quantum mechanics and generativity. It doesn't even need strings. And so I'll just ignore that last part, but. Well, actually, maybe I won't. So there's still an issue of the singularity in the black hole solution. So where do you address that? Because that will be lurking behind any of these proposals if it isn't somehow resolved by whatever theory of quantum gravity, string theory, loop quantum gravity that you're dealing with. No, the reason.

Can we have back. Yeah, I can bring back the image, in fact. So I think there is something actually. Can we have the previous one, the one with. Yes, that this is, in my opinion, this is the best way of viewing the interior of a black hole. There are different ways of viewing it because you can choose your time surface the way you want the foliation. So that's the best picture, mental picture one can have. And look, this is the horizon is there, right. And then there's this long thing and down at the tip is where the star that formed this is falling. Now, intuitively, we think that the singularity is down at the tip, but that's not true. The singularity is not there. The singularity is when this tube shrinks. So it's not singularity in the future. It's a singularity in time, not space. Right. So it's in time there. So if the singularity is replaced by a quantum jump, it's not there. It's just like the singularity in the atom of the electron falling into the proton. It doesn't. There is a quantum.

You see, what loop quantum gravity tells us is that space is discrete. There is a minimal size. There's nothing smaller than something that's the main result of loop. That's the result of calculation. Look, quantum gravity, like the energy of harmonic oscillator, you cannot go down arbitrary small, there's minimum one. And here the same, the size is a minimum one. So what happened when you go there? Something must stop the right. Now, of course, there's a similar statement in string theory. Not that we're trying to trade achievements. No, no, I know it's a similar string theory and that's what makes me think that these ideas are correct. Because to some extent, because if it agrees with string theory, it's probably right. I like that. I like that. If there's something that is both true in string theory, quantum gravity has a better chance to be right than it's just true in one. We only have a couple minutes and I want to jump off from that. So both you and I, let's put the possibility of white holes. It's really an interesting.

And if they're found, spectacular. I mean, it really would be wonderful. But you and I have both been working on highly abstract fields for a long time that for most of their development are pretty divorced from experiment. And some certainly would say, and they say it loudly, that the kind of science that we do is more metaphysics than physics because it's not something that's directly tied to experiment. Now, I have my own particular way of addressing that. What's your view on that? What do you say to people who describe things in those terms? To some extent, I agree with them. Namely, I think that what I have been doing, it's going to be wasted unless at some point there is a clear connection with experiments. There is nothing wrong in doing theoretical. Copernicus did a purely theoretical thing. It took a century and a half before it turned out that his ideas matched, ended up changed a little bit, matching reality better than previous ideas. So there's nothing wrong in this speculative search. And there's some science that is nourished by direct experiments. Quantum mechanics is an example.

But there is certainly science which is not nourished by direct experiment. Copernicus, Einstein, other. However, it becomes credible only when it gets tested. In some sense, testing doesn't mean yes and no means piling up success. Yeah. So you definitely come at your work from the traditional standpoint of I need this ultimately to describe the world. Yes, because otherwise. Because that's a strength of science. Otherwise we go free into speculation and we can go forever. Or you're just doing mathematics, which is why I'm in both the math department and the physics department. That's called hedging your bets right there. Look, I am also in the philosophy department. There you go. Just to be sure, an even bigger hedge. So please join me in thanking Carlo Rovelli.

Thank you. White holes as dark matter. Now, that would be a poetic resolution to one of the great mysteries of physics. But as we have emphasized, there is, as yet, no observational evidence for white holes. But having said that, one can't help but note that there was a time when it would have been accurate to say there's no evidence for black holes. There's no evidence for gravitational waves. There's no evidence for dark energy. There's no evidence for the Big bang. And yet, as science has marched on, we have uncovered evidence for all of these once exotic features of general relativity. Will the same pattern play out for white holes? Only time will tell.

Science, Technology, Innovation, Quantum Gravity, White Holes, Carlo Rovelli, World Science Festival