The Origins Podcast with Lawrence Krauss - Carlo Rovelli: From Dante to White Holes

Episode Date: November 3, 2023

Carlo Rovelli is well known as a popularizer of science. His short book, Seven Brief Lessons on Physics, was an international bestseller. I have known Carlo as a physicist ever since he used to visit my Physics Department colleague, Lee Smolin, at Yale, when I was a Professor there. Carlo and Lee were part of a small group of physicists pioneering an idea called ‘Loop Quantum Gravity’ as a way to try and unify General Relativity and Quantum Mechanics. Less well known among the public than its chief competitor, String Theory, and also less popular among physicists as a whole, Loop Quantum Gravity is nevertheless an equally serious attempt to address the vexing paradoxes associated with of quantizing General Relativity.Black Holes are the place in physics where the various problems of quantum gravity become manifest. If Stephen Hawking was correct, and black holes do completely evaporate through quantum processes that result in the emission of thermal radiation, then it appears that the information about what fell into the black hole in the first place will be forever lost. But this violates a central feature of quantum mechanics, which preserves information. At the same time, the final state of classical black hole collapse involves a singularity of infinite density. Most physicists expect this singularity to be removed in a quantum theory of black holes. Rovelli argues that near the singularity of a black hole quantum processes can change a black hole to be a ‘white hole’, the time reversed version of a black hole. While anything that falls into a black hole stays there, everything inside a white hole eventually reappears. If Carlo’s ideas were correct, they could go a long way toward potentially resolving black hole paradoxes. It is a big ‘If” however, and I remain skeptical. Nevertheless I wanted to discuss these ideas with Carlo on this podcast for a variety of reasons. First, any such discussion will illuminate a lot about the physics of black holes. Secondly, I think it is useful for laypeople to listen to physicists debate and discuss ideas at the forefront, presenting challenges to each other, being willing to openly question, and doing all of this with a sense of mutual respect. At the same time, because I share Carlo’s great interest in both popularizing science, as well as connecting science and culture, I was extremely interested in discussing his motivations and thoughts about these important areas, and I was not disappointed. I hope listeners will find our discussions about science, literature, and politics equally enlightening. As always, an ad-free video version of this podcast is also available to paid Critical Mass subscribers. Your subscriptions support the non-profit Origins Project Foundation, which produces the podcast. The audio version is available free on the Critical Mass site and on all podcast sites, and the video version will also be available on the Origins Project Youtube channel as well. Get full access to Critical Mass at lawrencekrauss.substack.com/subscribe

Hi and welcome to the Origins Podcast. I'm your host Lawrence Krauss. Carlo Rovelli is well known throughout the world for his popular science writing. His book, Seven Brief Lessons on Physics was a runaway bestseller. It's a very brief book talking about a variety of ideas in modern physics. It came out of articles he wrote for an Italian newspaper, and he's written numerous small short monographs on different areas of physics that have captured the public's imagination, not least because he loves to introduce ideas from literature, in particular in art, into his discussions of physics. Now, I've known

Carlo for a long time as a physicist. Back from way back when I taught at Yale University, he would come and visit as a collaborator of my colleague, then colleague Lee Smullen. And they were talking in the nascent stages about ideas that a group of physicists have been exploring called loop quantum gravity, which is an alternative to string theory as a potential theory of quantum gravity. It's caught on less among physicists and among the public than string theory. But nevertheless, it's a serious attempt to deal with many of the problems and paradoxes associated with the developing quantum theory of gravity. And that's particularly relevant for the new book that he's just written, called White Holes.

I wanted to talk to Carlo about that, but it also gave me a chance in this podcast to have a discussion about his early life and what got him interested in science and literature in particular and also politics, which I hadn't realized he was quite very interested in as a young person. So we had a fun discussion there and then talked about loop quantum gravity where it came from and how it's where it's going and that allowed us to move into the idea of white holes. Now, black holes are among the most exotic objects we know in physics and they present various paradoxes. Things fall into a black hole, nothing comes out, but if they evaporate by quantum processes, as Stephen Hawking suggested, then information gets lost as they evaporate, and that

violates a fundamental law of quantum mechanics. Also, at the center of a black hole, essentially at the center, there's a singularity in classical general relativity, a region of infinite mass density, where the laws of physics break down. Most physicists think that a theory quantum gravity will resolve that and smooth over singularities and might also solve the information loss problem. When thinking about that and imagining the process of falling into a black hole and thinking about the physics of what that entails, Carlo likens that to Dante's journey into hell, which is normally for most people a one-way trip, at least in mythology, but of course Dante gets out. And Carlo imagines something similar might happen. He argues that near the singularity of

black hole, quantum processes might turn a black hole into a white hole. White holes have been around for a long time in theoretical physics. They're the time-reverse solution of general relativity from a black hole. Everything falls into a black hole. For white hole, everything comes out. And while they've been discussed, no one's really had any evidence that they exist or good ideas where they might come from. But Carlo argues that perhaps in the latter stages of its collapse, it turns in a white hole, which might resolve many of the problems of black hole collapse and information loss. It's an interesting idea. I'm skeptical of it, and I express that skepticism when we discussed these ideas. And I hope you'll find this a kind of interesting discussion

to see how physicists talk to one another, even if they disagree, which often happens at the forefront of physics, but ask each other questions and try and propose ideas that might allow one to at least test in one's mind the consistency of the ideas. Either way, I hope you really enjoy this fascinating discussion with Carlo Rovelli. You can watch it ad-free on our Critical Mass Substac site if you become a paid subscriber. And of course, you can watch it later on on our YouTube channel, our Origins Project podcast YouTube channel, or of course you can listen to it on any podcast listening site. No matter how you listen to it or watch it, I hope you enjoy it as much as I did. And I hope you'll consider supporting the Origins Project Foundation, either by

subscribing to the podcast or by donating directly because the foundation supports the podcast, makes it possible, and also public events and other opportunities for the public to interact with leading scientists and scholars and thought leaders who were discussing the most interesting ideas in the 21st century. No matter how you watch it or listen to it, I really hope you enjoy this dialogue with Carlo Rovelli and with no further ado, here's Carlo. Well, Carlo, thank you. you for taking the time to spend time with me. It's been a long time since I think we were together physically. The last time I can remember was in Germany, I think. Long ago. Maybe 10 years ago. But we're now both located in the same country, it turns out, different parts of the same country.

You're in London, Ontario. I'm here in Prince Edward Island. So we're both moved to Canada. And you were just telling me you like London, which is nice. I love Canada. Oh, you like Canada, love Canada. Well, we can talk about that. I move back here for mostly to escape the United States, but that's a different question. I want to talk about where you've been, but in fact, what you may not know is that since this is an origins podcast, what I like to do when I talk to people I'm interested in talking to is to find out about their origins, how they got to where they are before we then talk about the things they're now doing that are of such great interest. So I want to explore a little bit about you, and I don't know that much.

I know that you're born in Verona. Is that correct? That's correct. I'm Italian. I was born in Verona. That's right. And is that the city of Dante? I notice in your book you keep talking about it.

So is that where Dante's from? No, Dante is from Florence, but he was exiliated from Florence because he was politically minded and got involved in all sorts of troubles. So he had to run away, and he spent time. traveling and he spent most of its exile time in Verona. So he wrote his main things in Verona. And one point in the new book, we're going to be at some

point talking, I'll show it anyway. A new book, Whiteholtz. Which is one of the reasons we picked this time to talk together. But I notice in the book at one point, you reminisce about a fountain and stairway and thinking that he was there. Of course, you'd love to

quote Dante. And we'll get to that. I think you argue that going into Black Hole is somewhat like Dante's trip into hell. But then just like Dante, you can come out again. Very much. So the book, the book you just showed is it's a, it's plunging into Black Hole, going down, down like Dante got into into the hell and then coming out like Dante actually did through purgatory and paradise.

So it's a similar trip. Yeah, it's true. I copied that. It's a very short story. It's a very, very short story. story. And embedded in that short story is a lot of stuff, not just literature, but physics. And I want to unpack that as I unpack your physics career. But so in that case, you're born in Verona. What I don't know about is your parents. Were either of them academics or were they? No. No. My father in engineering

and my mother, I was born in the late 50s. So it's a different universe. She was a, a, a, a, a, woman at the house she was working in in in the house she was proud of not working that's a different did she go to university was she was she educated at university or or not she she she had she was he had a good education she was a um education of a teacher so uh she she started uh for being a teacher uh she worked a little bit when she was a young woman but then she met my father and uh it was her decision at the time to give up working and be the wife of my father and taking care of the house and this little little thing, this little boy, which was me at the time. Sure, sure. I understood. Well, I was trying to think of what your influences were. So she had been a teacher. I was trying

to think, and your father was an engineer. So I was wondering where your interest in science came from. Did you, was it coming for your father talking about science? Was it from your mother giving you books or or were books in the important part? Yeah, books were important part of my life. They were both very much, I would say, respectful, but more than respectful. They had a sort of adoration for anything cultural. There were books in my, you see, this is my house. It's still books all over. And when I grew up, yeah, exactly. When I grew up of the kids, there were books all over where I, in fact, my room and my bedroom was covered by book on three sides. I loved really. There was an enormous respect for culture. My father perceived himself as an engineering as a cost of second level with respect to the real people, the important people who for him were sort of university professors

of all kind that were dealing with knowledge in general. But I was not a kid fascinated by science. Not at all. I was just curious, curious about everything. And I've been curious all my kids. life. I got interest in science very late. I would say already well into the university at the time which in the in America system would be the beginning of graduate studies. So what did you want, what did you do in undergraduate? What did you do your undergraduate studies?

And I know where you went. Did you do, did you go to the same university for undergraduate as well as graduate? No. The Italian system was a bit different to the time. didn't have exactly this division. Let me start a step back. I went to high school. High school is very important in Europe, much more than in the US. It's more formative.

High school was, my high school was mostly based on humanities, classics, history, philosophy, literature. Then at the end of high school, when I went to college, that I sort of didn't know what to do. I went here and there and I focused on physics already, but not really because I wanted to do that.

Mostly just by excluding everything else, and I was not very interesting. I was not a good student. I mean, I was a good student, but I didn't pay much attention to study. I was much more interested in traveling, doing politics, meeting people,

listening to music, staying with my friends, and learning, reading. That was, you know, the time of the youth rebellion,

so it was part of these waves that we don't want to be like our fathers. And I was very much into that. Full disclosure, you're two years younger than me,

so I know the era. You know the era, right. It was part of that era. So I don't long hair and was traveling all around the planet and spent a lot of time in Canada, hitchhiking. I did cost to cost alone hitchhiking in Canada when it was 20. Why Canada? Did you have family here? Why Canada? Oh, no, no, no. Because the big fashion in Italy was to make a big

trip east. All my friends were going to, you know, Afghanistan, India. And of course, I wanted to be different than anybody else. So I did I decide to a big trip west. And the Canada sounds sounded great. Actually, no, because I looked for the price of tickets. And flying to Montreal was much cheaper than flying to New York. Okay. But when you say coast to coast, this is irrelevant, but it's a bone to pick for me because I live on the Far East Coast. A lot of people say coast to coast and they start in Montreal and they go to Vancouver.

That's not coast to coast. Did you actually start? And did he go to the coast of Canada? Did people forget that part? You're right. I apologize. You're absolutely right. I did Montreal to Vancouver. Yeah, it's okay. You're in good company, although... Which is long enough. It's long enough, especially for Italian. We used to a hundred miles. Now, the fact that your high school was, you know, humanities oriented, it's clear the literature had a huge impact on you.

and the literary allusions in all your writing are as extreme. And we'll get to, did you, well, maybe I'll ask a little bit now. So did you have to write a lot in school? Where did your interest in writing begin? Well, I want to talk about that later again as well. I, a good question. I was, again, it was not to school. I've been writing all through my life.

I had a sort of restless adolescence, not very happy, full of troubles, just internal. Yeah, a lot of people have that. Yeah, exactly. So I had one of these adolescents that's a big, fine life, living hard. And I used to write, even now, I mean, I have boxes with no dogs. books with just writing, writing, writing. I never imagined that that would become something I would do, not just for myself, in write, but I've always been writing.

And I always been reading. I mean, always been reading literature, I always been reading all sorts of things in my life. The actual book writing started very late in my life in the 50s when I was 50, sorry, not in the 50s. It began by your writing for newspapers and then you sort of put that together or did you, was your first? Well, if you want to see, yeah, if you want to trace this, it actually, there is a first chapter of that, which is when it was at the university in Bologna, in Italy, in Bologna, San Diego, that was 77. There was a big student revolt in Italy, sort of like a. The last phase of May 68 in Paris, the government were doing something pretty fascist at the time, and the students were on the street.

So I was involved in that, and after that, I wrote a book on that. So that's my first book, actually. Yeah, I'd heard about that, and I was going to ask you about your politics. You've always been quite political, I guess. Yeah, I've always been quite political, even now. I mean, there's always this side interest of me, which is interest in the sort of common affairs of humankind. Yeah, I want to talk about politics later on

because there's so many interesting questions going on. But yeah, you were part of that student revolt. And your first book, and it got published in Italy, that book? It got published. It got published in an interesting manner because police was not happy for the book being published. So police tried to stop it repeatedly. So we found a publisher who actually used a printing house hidden in the countryside far away from Verona.

And the publishers from Verona as a left-wing publisher to avoid the police stopping it. When the book came out, it was charged with all sort of, so I got in trouble with judiciary, but then everything was dismissed. It was charged by sort of opinion crimes like instigation to commit crimes like

despising the flag despising the Italian whatever. I mean it didn't get to an actual trial when it went for the police to a judge A judge says, come on. After all, Italy is still a pre-country when you can say what you want.

So everything was dismissed. Okay. And so that was, yeah. So as I say, we'll talk about politics. And so that was your first book. But then it was a while before you wrote another one. It was very long. And I would somehow, in my late 20s at some point, politics and, you know, the dream of changing the world and making a better world seemed to be too hard.

And how would I put it? The majority of people didn't seem to be interested in that. That was the point. So we lost the revolution. And I was searching for something else to do. And in that moment, I just fell in love with science. I just was a, it was, let me put it this way. I was going to college as a second or third activity for me, but I had to take exams once in a while.

And at some point I started taking exams on advanced physics, relativity in quantum mechanics. And it just blew my mind. It's at this point. It blew my mind. And I said, wow, this is fantastic. So without even relying that, I was spending more and more time studying physics. And...

So that's intriguing to me. So you got interested in physics at an advanced stage. It's not like the rest where you had to take physics at an early stage physics. first in order to be able to do the advanced courses? I did take physics. I did take early exams on classical mechanics. But that was

boring. That was not particularly interesting. I did it. I was good. I just sort of was flying through these things and then think about something else. But modern physics was great. And I was not

in the Italian system you can you don't have to go to classes if you want. I used to go to the first one or two lectures, and then if I found the teacher boring, which was happening most of the time, I would just go see him and say, please tell me what books I should study. I was studying at home.

And usually what I would do is just not take one or two books, but take four or five to get a larger perspective, different stories. And then I was taking the exam and usually going pretty well and doing something else. But again, you say now, okay, I was going to, this answers the question. I was wondering why physics. But you say this was not as an undergraduate, this was more at what you might call the graduate stage? Or was it in?

Yeah, Italy at the time had a different system than the graduate and the graduate. So there was a first longer part, which would be sort of undergraduates, first year of master's. You did all sorts of things, literature, histories, what are... I was already focused, I mean, my university curriculum was centered on physics. Oh, so you made that to say, so let me go back again. Why after a school of humanities and literature did you choose to focus on physics when you entered university? That's what surprises me. Because I actually was not interested in studying.

My dream was to be a beggar, to go around the world and do nothing, which you shouldn't think was underestimating myself because my model was Buddha, which is not underestimating yourself. And then I failed that. I'm a failed beggar. and I wanted to so I had to actually what happened what really happened is that I didn't want to enroll in university and I want to start again traveling

but police find some Marihuana Samar Johanna in my motorbike so there was a moment so I could not leave Italy and I said okay so I'll go to university for the moment to see what happens next And then I had to choose.

So there was a long list of topics. At that point, you have to choose sort of your major in. And I said, not this, not this, not this. I got to the end and there was nothing left. So I got back and I tried to leave something out. Philosophy was one thing I was interested. Physics, it was interested because physics seemed to me something fundamental about reality that humankind has sort of worked out,

which was relevant from understanding the world. So that was the aspect of physics that were intriguing me. And I would say I want to know about that. I was a very pretentious adolescent, if you want. I mean, you know, Uda was my model or Alexander the Great or Aristotle, right? Yeah. So I wanted to know everything. And I thought about philosophy because it seemed to be.

that philosophers are those who ask the big questions. Well, why not philosophy then? Right. And there were very specific reason I discarded philosophy, because after all, in the sort of a little bit mad mindset of those years, I did not trust the system and I did not trust university. So philosophy was too important. important to left it to my to the university system. And we were a bit, we were a bit wild at that time now that I think about it.

But really that was now, you are, if I'm not mistaken, you're now at a school of philosophy, are you not? I am. I am. The interest of philosophy always remain there. When I, when I, when I moved to Pittsburgh much later in my life, which is one of the greatest, the philosophy of physics. school in the world, I was spending my time in the philosophy, in the sort of history philosophy. You're jumping for me perfectly. So after, I was wondering why you came to the United States and when, I knew you later. But so after graduate school, what did you? After graduate school, after graduate school, I sort of bouncing around in Italy from one city to the other with, I got some postdoc position in Rome.

Had you focused already in general relativity at that point? Yeah. What happened is that after I sort of got full immersion in physics, I was very fascinated by that. So I was studying, starting, starting. One day I went to the physics department in the library. That is before the internet. And the way you would get information about research going on is to prep print.

preprints were this copies of articles that were in a shelf in a shelf in the library so you would scheme through and there were all this boring thing like you know measurement of a cross section at CERN or things like that

but then there was this one by Chris Ayschen and it was a review paper on the problem of quantum gravity and I of course I couldn't understand a word what was the side. But what I understood

is that at the core of modern physics, there is a fundamental open problem, which is the, you know, understanding quantum space time, the disconnection between generativity and quantum mechanics, which was open, and because

of that, we don't know what is space, we don't know what is time. And, you know, the young kid, I was fantastic, that's what I want to do. I want to study that. I want to spend my life doing that. And unbelievably, you know, this is, you know, 45 years later, and that's what I've done. Well, no, it's not, yeah, you certainly focused on that.

And so you got a postdoc? You say, where was it in Italy? Yeah, I got a postdoc in Rome. I was studying alone. Even my PhD, I continued this very solitary habit of studying. I was not really inserted in a research group. Also, from this perspective, my path as a physicist is not the most common one because I didn't really come out from a school. I had good teachers.

There were some good teachers in Bologna, which I followed, and in Padova where I did my Ph.D., which I followed, where I followed some classes. But my advisor was a great scientist, but doing something completely different. And he was saying, well, if you want to study that, Carla, go ahead. I mean, I was chatting with him once in a while, but I was just on my own. So when I got my postdoc, I got a scholarship from the sort of Italian institution that does this thing. And it was not for me joining a group. It was just for me continuing doing what I was doing in a very solid way. So I went to Rome for a while, Trieste for a while.

And I start, at this point I started traveling because I decided to go to meet the people who were actually doing quantum gravity. Like Chris Isham, I went to London to see Chris Isham. I went to the States to meet Abayashikar. And then that's how I met Liz Molling later on. So on my own, I would just, you know, jump on a plane and, you know, send a letter because at the time was letters. I'm very much interested in what you're doing. Do you mind if I come and see you and go? well so and then what caused you to move to the states

that's when I started actually writing papers after months and months and years of being alone in my in my little office redoing calculations and the copying and trying to understand during my first trip to the US that's when I started working on

And quite rapidly, we got with LIS morning to some major results. That's 85, 86 is already when the first papers on Lukeontogravity came out. That was quite a shock at the time. Was that before Pittsburgh? Yeah, yeah, before Pittsburgh. Just shortly before Pittsburgh. So this is probably where we first met, right, because I was a professor at Yale and Lees Mullen was headed the office next to me next door to me. Oh, that's what

we met the first time. I came to Yale in 85, just visiting this way. That's when I started being a professor there. I began as a professor there in 85. Yes. And Lee was also a sort of young, young faculty who helped recruit me in a way. Oh, okay, okay. So that's that's a connection. So maybe we met at that time first time. I'm not sure I met you in his office as a visitor. I think you were visiting. Yeah. I was visiting him and that's what we sort of stumbled on this family of solutions of the Will-Dweed equation.

It was a major result with a big impression at the time. I was back in Italy. Italy, of course, nobody was interested in quantum gravity. So nobody was paying any attention to me. My post-doc had expired. I was just, you know, calling my father and dad, could you please, please send me a little bit of money because I have nothing to eat. That was the life.

And one day I got a phone call from Pittsburgh saying, are you interested in a faculty position here? And this was in the physical department. Yeah, it was a physics department of Pittsburgh. Who was to get that? Ted Newman. Okay, so it was a general altivist. Okay, Ted Newman.

Okay, I know him. Yeah. Okay. Yeah. Yeah. Ten Newman is, you know, for the people, you know, you know him of the people who are listening to us is one of the great relativists, I would say, of the generation before others. It's the one who wrote the general solution for Black Holtz, is the one who wrote the most general description of station in Black Holes.

I see. Okay. And a lot of other things. He's a close collaborator of Roger Penrose. Roger was often in Pittsburgh to seeing us, so I got to know Roger there. And TED is a fantastic human being also. So for me, it was great. So Pittsburgh was a, I changed. It was a big face transition for me from, you know, being strange kid to doing things and producing ideas and more or less solitary jumping around into now the American professor, yeah, junior professor with a credibility position.

And then, okay, so yeah, it must have been a shock. And okay. And then you, I think I knew Lee would visit you there, I think, as I remember. Yeah. And he left after a few years after Yale. He went to Syracuse, whereby Ashikah was there. And I was often visiting to Syracuse. That's right.

And then at some point in the 90s, both Ashikar and this morning moved from Syracuse to Bay State College to Penn State University, which is near Pittsburgh. So we were not far from one another. And then, but then you moved back to Europe. Was it because of, well, what did it have to do with wanting to be back in Europe or was it more pedestrian reasons? You don't have to answer questions, by the way. No, let me put it this way. America, US was fantastic for me.

It was a great experience. I was taken seriously by the US. Europe is much more complicated culturally. If you have new ideas, it's hard to be listened to in Europe. Everything is more slow and cold and you're constantly pushed down. While the US is a place where if you have ideas, it's easy to go up, you're constantly pushed up. And it was a great experience culturally. I learned a lot.

It's a different culture. It's really a different culture, more than it what looks like between Europe and the US. There was 10 human and there were the philosophers. The philosopher fantastic. It was a great opportunity for me to get to know this world, philosophy of science in the Anglo-Saxon world. But I was not totally, I was never feeling home somehow. at the time in the American system.

There were so many things were too hard for me to digest in the culture. Somehow, let me put it in this way. It's the other side of the coin. If you are young and bright and white and male and whatever, America is fantastic. You just go up. I was going to say all of that has changed a little bit,

I think, in the United States, in terms of people being pushed up and who's being pushed up. But anyway, we're going on. Right, but it's a, for me, it was a very, you know, coming from Europe, America is a very violent place, extremely violent. Okay. Violent, both in the, you know, they're killing the street,

which in Europe don't happen. There is, there's a, there's a, violence in society. People are fired like that, which, so they fall down and socially, which in Europe doesn't happen, is violent politically. There are weapons all over the world. So all this, I didn't, I never felt home. So when there was an opportunity for going back to Europe, I felt that that was, I had 10 years in the U.S.

It was good to learn. It was time to come back. Okay. And then, yeah, I went to Europe. We don't have to go through their whole life, although you ended up in Canada. You say it came back to Canada, which I guess you're funny. I spent 20 years in France. Yeah, you spent 12.

Most of the time I, the last time I saw you, You were still in France, I think, yeah. Yeah, yeah. I'm still, my official affiliation is still the University of Marseilles in France. This is a great period there also. It was fantastic. I was sad. I never visited there.

I've always liked to visit Marseilles and I never got to. Oh, Marseille is marvelous. The Mediterranean, the sea, blue sky is great. I love it down there. And I remember you were there and I was thinking, oh, maybe I should go visit there. But anyway. But then, you know, life is complicated. I jumped the Atlantic one more time and now in Canada and I love it again.

Maybe I'll come back. Who knows? Yeah, you never know. Okay, well, okay, enough of the travelogue. But we'll get back to some of the other ideas. Now I want to focus on physics. Both the physics that led up more generally in your life and then the physics associated with white holes. And I will, as you, you know, I'm an eternal skeptic just so you're aware, but you know that about me, I think.

That's what we need. We need also those people. But, okay. But let's first talk about luke quantum gravity. What, what, again, for the, I think most people who listen to this, but maybe not, realize that there are two major thrusts in the effort to reconcile these two fundamental areas of physics. General relativity, there are space and time and quantum mechanics, which sort of the theory of how the universe works on the very small scales. And both of which have been tested on their own regimes and work unimaginably well in there, but they're fundamentally inconsistent. And so there have been lots of...

They look, as they are, they seem to be inconsistent. Yeah, yeah, yeah, okay, exactly. The assumption is, yeah, it appears as if they're inconsistent. Many people, I will say, it's true, in my life, in my period, from a graduate student on, there have been many people who say, well, really that inconsistency is an illusion, and general relativity is somehow consistent. I don't think those people, general relativity unmodified is somehow consistent. I'm not sure that.

Oh, no, no, no, no. Classical generativity is not consistent. I've, I remember writing, I agree. I think it is. I know, but there are people who say that. There are some people who still fight against the current. Yes, there are, but they're wrong. Fighting against the current.

So that's okay. We encourage that. Anyway, but they appear to be, you know, inconsistent. And I think what has emerged, and there's been a plethora of lots of ideas, and there's still lots of ideas. People are throwing out lots of ideas, emergent gravity, this or that. But the two that appear to be more entrenched, I guess, or more, is loop quantum gravity and string theory. But it's not fair, I think, and I think you'll agree with this, to put them on the same level.

It is interesting that string theory took off and as hundreds, and over time it may be thousands, but certainly hundreds of people working on at major institutions devoted to it. Loop quantum gravity 1000, thousands, okay, I would agree. Loop quantum gravity

has had dozens, I would say. No, no, 100. What? No, no, no, typical conference on loops

as 200, 300, 300 people. Okay, okay, great. But that doesn't change what you're saying. Yeah, that doesn't change what you're saying.

There is a one order of magnitude of difference. Yeah, okay, I agree. The numbers are, you know, and I'm a cosmologist, so, you know,

numbers. But anyway, so, so it's fair to say that, and when people, for the public, they hear about string theory, probably they don't hear about loop quantum gravity for wet or worse. Maybe, you know, as you write books, they do hear more. But so the question is, why do you explain the difference between the two or what loop quantum gravity is based on? And then we can follow it up. But I thought, why don't you explain the basis of

loop quantum gravity, why it was developed and what its central features are and how those differ from string theory? Can you do that or no? Yeah. Yeah, absolutely. Let me start from two differences and then tell you more about By doing the differences, by doing the differences, I think it's easy to frame the situation. One main difference is that loop quantality is much less ambitious

that string theory. So string theory is not just bringing generativity and quantum mechanics together. It's much more. than that. It's an attempt to write a single equation which unify all the forces, all the particles, all everything from a single theory. Okay. That's not at all what look quantum gravity is. Look quantum gravity is a much more, much less ambitious effort, which is just to understand what are the quantum properties of gravity. We understand the quantum property for electromagnetic

pretty well. We understand the quantum properties of the strong interaction that make the, the, the nuclear, the nuclear forces pretty well, without unification. So, Lump quantum gravity is just that. There is a hole in our knowledge of the world, which prevents us from understanding what happened at the Big Bang and the side of black holes, is that we don't understand the quantum property of gravity, that's what we want to understand. So that's, in that sense, the ambition of string series huge, the ambition of quantum gravity is small. The second difference is that both theories have definitely difficulties in predicting verifiable or suggesting verifiable consequences.

Look quantum reality has definitely these difficulties, this weakness. String theory has given some suggestion verifications. They are all come out wrong, especially in the last years. So nowadays, string theory, I would say, is very much in difficulty, because string theories were all convinced that there were, you know, supersymmetric particles to be discovered at CERN. They were not. It's a little unfair to call them wrong.

They haven't yet been verified. I think, I mean, it's true. The fact that they haven't been discovered at CERN is not, the same as they don't exist. Absent of evidence isn't evidence of absence, necessarily. So it's true. It's caused problems for low energy theories of supersymmetry, which are the most attractive ones, I would say. But you know, so they certainly rely on supersymmetry and there's no evidence of supersymmetry. I think we can agree on that. No, that's not the point. I'm making a different point. I make a point that there was a suggestion that was believed by most practically old-string

theories, they would expect something and what they would expect did not happen repeatedly. This doesn't mean that the theory is falsified. Falsification is a completely different story. That's the point you're making. And I agree. But this maybe is a point of philosophy of science. I don't think science goes ahead just by falsification. It goes ahead by piling up, you know, when you suggest something happened and it happened,

you say, oh, maybe on the right track. When you suggest something else, it happens, you're in the right track. When you suggest something is happening, it does not happen. Your credibility goes down a little bit. And this has happened with supersymmetry, it has happened with a sign of the cosmological constant. Everybody expected it to be negative. So when you guys, cosmologists came out saying, look, it's expanding. That's like a positive cosmologist.

First they tried to explain why it was zero. And then it was, and then after it became positive, it's negative. Exactly. consistent theory. You're right. It certainly caused, it always is interesting. I have to say that about, I will say that, I don't want to be too provocative early on, but one of the reasons I've always been skeptical about most approaches to quantum gravity, and I don't, you know, I follow it and I've thought about tests of quantum gravity, but I've never actively pursued either string theory or loop quantum gravity, so I try to be agnostic in that sense. But one of the things that always made me

suspicious is because I've spent a lot of my time over 20, 30 years thinking about the cosmological constant, which I proposed was there. I know. Yeah, but one of the things that I would have thought that a good theory of quantum gravity, one of the key predictions of it might be to explain the cosmological constant. Of course, nothing has, which makes it even more interesting. But it's certainly one of the reasons why I've been suspicious, because ultimately I do think we need a theory of a real understanding of quantum gravity if we're probably going to

understand the cosmological constant. And the fact that we don't yet has always been of some concern to me. Let me just say it that way. Probably you agree with that. I don't know. I agree. I can, if you want, later on, we can go and talk about the cosmological constant loop to gravity.

But let's get to loop quantum gravity. Sure. Sure. Sure. I agree. Somehow that I wanted to train it. Absolutely. So what it is, it is a theory, it's a well-defined theory, right?

So it's not just a collection of pieces. Sorry, let me interrupt you one more time. Have we finished all the negatives of string theory? There's a cosmological constant. Oh, I could go forever, but it better stop here. Well, no, no, give me a few more because I think it's world. People hear all about string theory. It'd be nice if they hear some of the problems.

So there's the supersymmetry. There's the cosmological constant as the wrong sign. What's two or three more? In terms of suggestions that came out from string theory, which again are not falsification of string theory, but suggestions that turn out to be wrong, for instance, there was a moment in which everybody was expecting black holes being produced at CERN. Yeah, well, I remember all sorts of thesis of the argument and, and that's sort of a semi, that was one of my students actually, but anyway.

Oh, sorry. No, it's okay. I got him, it got him a tenure, so I don't really care. I never liked it. Okay. Okay. Okay. But I would say that the trouble of string theory is not just that.

The trouble of string theory is that in the age, it raised a huge amount of enthusiasm with some promises. And the promises was, for instance, to be able to compute all the parameters of the standard model from just a theory with one constant. Predict why there are three generations of fundamental particles. And also write a fundamental theory clear and clean with some basic equations. So this is it. That's a fundamental theory.

Nothing of that has been realized in half a century. Absolutely. As my colleague. That's a real problem more than the, yeah. That's the real problem. String theory is promising and it keeps promising and promising and promising. But it also caused your colleague, I think, Lisa Mollender, write a book called The Trouble with Physics, which got a lot of people, which got a lot of people.

Infuriated a lot of people. Okay. string theory indeed look like it would have a theory of everything and look very like it would explain exactly the things we see. And those aspects of string theory have not been forthcoming. There are successes besides the mathematics, which has been incredibly useful in the rest of physics and ideas of duality maybe. Oh, yeah, yeah, yeah. But also one might say later on that some people would say, I'm more dubious, but I know when I've talked to maybe even on the air to some string theorists, you know,

the calculation of black hole entropy, they think, is a big success. Maybe we can talk about that later. So, but in any case, it has had problems. Now we can go back to Unquant Gravity. Sorry to interrupt you, but I wanted to put it in perspective. Yeah. So, as I said, Lekwendo gravity is much less ambitious things.

It's not, it doesn't have, it doesn't explain why there are fermions and their young wheels field and everything coming from a single thing. Nothing like that. It's just the idea that there's general activity, which explained everything gravitational remarkably well, as immense success, generativity. But it's clear since the very beginning.

In fact, Einstein, in 1916, a wrote a paper saying generativity cannot be exactly right because of quantum mechanics. So there are quantum effects that affect gravity, and it's a theory of quantum effects in gravity. That's what quantum gravity are. What makes it fascinating and what fascinated me from the very beginning from this paper by Chris Aisham is that generativity interprets gravity as the dynamic of space itself and space and time themselves. So it's space that bends and time that stretches that do gravity.

So therefore, the quantum feature of gravity are the quantum feature of space. and the quantum feature of time. And that's mind-blowing because once again, in the history of our human thinking, we have to change something fundamental, what is space and what is time. So look quantum gravity, it's a mathematical theory. It's a Hilbert space, a set of operators or something that gives the dynamics, the transition amplitudes, which describes not just how things move in space as time passes, okay, but describes, but describes the way of the dynamics of the transition amplitudes, which describes, describes some quantum stuff, right, which looks like space time only in the classical limit when you average many things. So it describes the elementary quanta of space. So the main

result of loop quantum gravity, I would say that the core result, which is a result of a calculation actually, is that space is granular. And the calculation is a standard calculation of one does in quantum mechanics to find granularity. For instance, if I had a pendulum, something that, you know, that oscillates, the amplitude of the pendulum, the energy that how much, whether it's not moving or moving a lot, in quantum mechanics, right, it's quantum mechanics one a yon, you do a calculation and you discover that it can be either

zero or one or two or three, but not in between. There's this granularity in the energy, as one quantum energy, two, one, generally three quantum energy. And the same with light, it's a, it's granular. So there is calculation, which technically is called the spectral analysis of an operator, finding the eigenvalues. And when you apply that to generativity, you discover this quantum space, this grain of space. So the space in which you are immersed is just granular.

And the fascinating point, for me, at least, is that this quantum space are not moving in space. They're space themselves. They make up space. So what we call space around us is sort of the large-scale picture of many little things which have plank size, which are roughly, which are the quantum space. When you say space, you mean space time? Yeah.

In fact, I'm cheating. Yeah, yeah, okay. I was just one. It's really space time, of course. Yeah, the quantum space and time, but they're both quantized. The quantum space and time, right, exactly. And why is it called loop? I'm asking questions.

I'm asking questions. I don't answer you, but why is it called loop quantum gravity? Are the quantum loops? Yeah. It's called loop quantum gravity for historical reasons. By the way, also string theory is historical reasons because now it's not just strings. Exactly.

So the point is the following. If you have one of this quantum space, the theory knows. which one of the other quantum space sort of next to him. So the way to describe that mathematically is that at this point connected by lines, which make a network. In fact, the state of the theory are called spin the network state. Spin because in a sense, this is so simple rotating. So you have this network.

And so a generic state, you can imagine of a network. And in particular, there's a super simple network. which is one quanta, you jump another one, another one, and you loop back. Okay, a single loop. Okay. Now, historically, these were the first that came out for the equations. So when these loops came out for the equation, there were loop states. And the idea was, oh, can we write these states as a basic element of the theory?

So that's where a super simple universe will space is just a single loop. But then it turned out that that's not enough. you have to go this network much more complicated but the name loop remained and it's also you see when you go around let me give you a little bit more technical

explanation I was going to go technical so as good you are yeah slightly more technical space is curve right what does it mean that it is curve it means that if you make a loop if you start from somewhere you make a loop

you turn out rotated, okay? And the way to think about is to think about that the earth is curved, right? The surface of the earth is curved. So you start for the North Pole and you move down to the equator, you rotate around the equation and you come back, you turn out rotated, right?

You start this way, this way, this way, and you have rotated. And this rotation captured the curvature of the earth because on a flat space, you make a loop and you come back with the same direction. So loops are a way, looping, it's a way to capture curvature, and curvature is exactly the quantity that characterize curvature of space time, namely gravity. Yeah, and intellectually, I think it, I'm pretty sure it came from another direction in the sense that I was going to ask, in the standard picture of normal particle physics,

what are so-called gauge theories are the basis of all of these things, and what are called gauge invariant variables in the, theories which describe all the rest of the electromagnetism, the strong force, the weak force, involve loops. Exactly. And in fact, in electromagnetism, when you consider a loop, you know, in fact, even in Maxwell's equations, it's what tells you if there's really an electric field there or a magnetic field there comes from thinking about loops.

Absolutely. And the gauge in very quantities instead of what are called. And so loops have a long history in gauge there. and gravity, which I first learned when I was a graduate student from my friend Jim Gates at the time, is also a gauge theory. It's exactly that, Louie. It's not unusual if you're thinking of what.

You see, and the reason, these sound like words, but the reason to think of gauge invariant variables is that the physical quantities, the things you measure, should be independent of how you describe the system, what variables use to describe the system. That's what we call gauge invariant. So in these theories, you can describe the many different ways, like you can general relativity with coordinate systems,

many different ways. But the physical things, the actual things you measure should be independent of that. And so it's not, it's not, it's sensible to think at least of defining the theory, especially if it's incredibly complicated like general relativity in our sense, in terms of the kind of variables that might relate to observations. So I always assume that that's what, that was the motivation behind thinking of loops and loop quantum gravity. But I could be wrong. Yeah, no, no, no, you're exactly right. So one way of sort of derive the mathematics of loop quantum gravity is exactly start from the variable you're referring to,

this loop variables, so olonomies, which describe what I was saying. If you're in a point in space time, there is a symmetry, which is a rotation, in a single point, if you rotate, you see the same physics. But if you move around the loop, you come back rotated. them. And so one way of capturing the gravitational field is to say how much you're rotated for every possible loop. And these are the loop variables that describe the gravitational field. And a loop quantum gravity can be obtained by taking these variables as the fundamental variables to jump to the quantum theory. Okay. So that describes, I think loop quantum gravity,

at least in a way, in a mini way. We're not going to spend hours on it. But so, and that got you guys interested. What are its successes? What are its chief failures? Well, I would say the success, the main success is, in my opinion, after 30 years of development of lukemental gravity more, is that there is a possible theory of quantum gravity. It's not a complete, perfectly complete. There are theorems which are not being demonstrated with the thing. But there's a theory. I could write down four equations and say, this is it. This is the main theory. It's very hard to do calculations, but it's well-defined, and it is a quantum theory of gravity. Now, what is the difficulty? Why are we not happy and jumping and say we have solved a problem? Well, because

it's not sufficient to have a possible theory of quantum gravity. I mean, the good lord may have chosen something else. How do we know? So we need to we need something to to compare it to observation and that's where in my opinion the all the interesting current work is. Maybe that's too much because I have a lot of colleagues which are still working on refining the theory, filling up the gap and the gaps and cleaning up and there are different formulations of the theory to you know how it's radical physics. It's got longer complicated. But there been really, I think, is it fair to say then, or is it unfair to say, that unlike sort of string theory, which in principle had a

calculational success, or so people think, for very specific, weird kind of unphysical black holes, they got an answer which is, which was, which of the entropy of black holes, which is relevant to quantum physics of black holes. And they call that a big success. Is it fair to say that loop quantum gravity hasn't had similar calculational successes? No, no, no, that definitely would not be fair to say.

No, no. There were analogous theoretical successes. And in particular, the calculation of the end of the black hole was done in loop quantum gravity. In fact, more or less at the same time in which Strominger and Vafa

did the calculation in string theory. and it's like often in this case on one hand it's weaker than the string one on the other hand is much stronger it's stronger because it actually works for physical black holes for charge for care

for three dimensional real stuff that we have in the sky and for the ones which are not extreme or nearly extreme like the storming of alpha calculation of their extensions So that's a much stronger result. It's weaker because one can get the proportionality area entropy, which is what one gets with the right audio of magnitude. But there's a free parameter floating around the theory. It doesn't get the pies.

Exactly. It doesn't get the pies. It's much stronger the string theory because they get the four pi, the two pi, whatever. Yeah. Right. So again, you know, one, one. I saw Andy Stromigan the day, two days ago, in fact, in New York. He's the one who got, and, you know, we're still bickering about which one of the two is better.

Andy was a graduate student with me at MIT. Oh, was he? I see. I see. In the neighboring office, actually. I see. Both had the same supervisor, in fact, for a while before I changed. Who was the supervisor?

For a while, Roman Jakif is his name. Oh, I eventually left Roman, and so I could work on gravity and cosmology, actually. But anyway, but Andy stayed. But in any case, yeah, so we go back. Let me make one point. It was slow, this building up of the theory, right? Because in the 80s, was the theory was defined. In the 90s, there was this calculation of the discreteness, sort of the eigenvalues area and volume operators. And then 10 years later was what seemed to be the better version of the dynamics, the covariant way of computing transition amplitudes.

And then after that, there was the first result that was a big missing point before of the classical limit, how to connect that to classical generativity. Because one thing is to have a theory that looks like quantum GR. Another thing is to show that in the light limit. It better result in general relativity because general relativity has been pretty well

at a classical level. It has better. Whatever quantum theory we have. has to reduce the relative. In the regimes where generatively works so well, it should give the same result. Which is, by the way, I should want to be fair, one of the motivations of string theory in some sense

is that it automatically, it automatically, I mean, a central part of it was that the physics of strings automatically gives you something it looks like gravity. Now, what are the, so, okay, so what are the major outstanding challenges in loop quantum gravity? There is a serratical,

one and a substantial one. The theoretical one is that there are this transition amplitude that one can define. They're well defined. But if you try to sit down and compute them, it's horrible. These are horrendously complicated integrals over special functions of which we have no idea

where to do. So it's much more complicated than the integrals of QED. Feynman was doing there. There you, you know, we have learned how to do these integrals. Here, we haven't learned yet, and I don't know how easy it is, the groups that try to do these calculations numerically, computers, but, you know, it's very slow and going very slow. So the challenge of extracting actual physics from the theories, it's pretty there. But that's sort of internal. I mean, after all, you know, the Einstein equation seems very complicated at the beginning.

Nobody knew how to extract physics. And then things go ahead. The real, right. The real problem is, as I said before, it's connect to reality. You know, we want to test this theory. And there are definitely phenomena in the universe where quantum gravity, we expect it to be very relevant. but this faraway phenomenon, Big Bang, Black holes. And so applying the theory to this phenomena, it's complicated,

and we don't have enough, in a sense, we put it this, in a sense, the quantum gravity effects are very much hidden in secret details. So there's a part of the community which has spent a lot of time trying to apply the theory to your field, to cosmology, and try to figure out a way you're thinking about the Big Bang, where you actually bring in the quantum aspect of gravity, and this is a big chunk of work. There's another part. That's what I've been doing with others recently, who tries to see what the theory tells us

about black holes. And that's what I tell in the book that you have shown in the beginning, the white holes. The two places that quantum gravity, if it ever comes to, if it's ever relevant to our universe, our beginning of time, or at least the beginning of our universe, and black holes. And everyone's hoping to use, different people are hoping to use those as different laboratories to test their ideas. And that's, and by the way, while you saw my field's cosmology, I'm a particle physicist by training, but the reason I do cosmology is because it's a laboratory to understand.

Yeah, for the same reason after all. Yeah, it's the same reason. Yeah. And I was less ambitious. I just wanted to test our ideas of the other four-fourth, three-forces, and understand them initially before understanding, you know, there's a lot we still have to learn. But in any case, okay, so, yeah,

so it would be nice to come up with something relevant to understand the Big Bang and black holes, and that does lead in some sense to your white holes, which we'll get to. Okay, let's, let, before we go about white holes specifically, because this will be relevant to white holes. You wrote a book about time, but time is an interesting concept, and I wrote about it in my recent book.

A lot of people think time's an illusion. And I say, well, you know, and in some sense, of general relativity, time is an illusion, but as I point out, that doesn't do much for people who miss the 710 train into London and don't get their job interview

and lose their job. I mean, so for them, time's pretty real. And so let's talk about, the nature of time because you have some interesting thoughts on it that's relevant to black to white holes and your your discussion in the book and you've written another book about it and and let's point out that in some sense i mean from a very basic perspective let me let me say something you can say whether you disagree or agree let me see my perspective on it which is a theory of quantum gravity if i think of the way feyman made me you know not made me but the way fineman thought about

quantum field theory is to think of some over paths. And if I think of quantum gravity, I'm summing over space-time paths in some sense. And in some sense, in order to understand the universe, then I'm summing over all space-time paths, and from the beginning of the universe to the end, and from all over all space. And so it sounds like all of time is already there. And therefore, this illusion that the universe is proceeding in time-stead, appears to be like an illusion in that picture because you're you know it's all it's already all part of the of the playing field already and so it's already all there just like space is already there so's time

already there and you just try and calculate things in space time so in some sense then that's probably the language of wheeler to wit or at least the the idea of it is that and is that in some sense when you think of quantum gravity you already automatically have to think of time as sort of an But why don't you take off from there and tell me where that might be wrong or right or how that relates to your picture of time? I basically agree with what you said. And I also agree that in spite of that, if you miss the 7-10 train, it's still a problem. That doesn't resolve the fact that you miss your meeting. The way I think about this is to realize.

that when we say time, we're actually meaning a lot of different stuff. So the notion of time is a composite notion, if you want. In our everyday life, we give a lot of attributes to time. Time passes, it goes from the past to the future. There's the same time for everybody. You know, now is whatever this time. I'm in a different time than you, but that's okay. But that's about, yeah, exactly.

And we know that we are in the same instant of the universe. The universe is a common instant all over, and so on and so forth. There are all these features of time, which are very much ingrained in our intuition about time. And then when we start studying physics, we realize that a lot of this is just not part of the fundamental reality, but a specific feature of some variables, what we measure by clock, around us where we're moving slowly and not faster, there's not strong gravitational field,

there is nothing heavy quantum mechanical visible and so on and so forth. So as soon as we get out of our common domain, we realize that time works differently. Already in special relativity, times works completely differently because simultaneity is not defined and so on. And in general, relativity, even more.

I used to make this point in public talks that if you take two clocks and you move one up and one down, you wait a little bit when you bring them together, they're not together anymore. The one that was higher has measured more time. It's actually physically more time passing here than there, higher than lower.

So this is general relativity. So this changes our intuition of time. So once we go all the way down to quantum gravity, which is a very extreme regime far from our everyday life, there is very little of the common temporality that we're used to that we bring with us. And because of the reason that you say, In a sense, let me put it this way.

In loop quantum gravity, I said there is a well-defined set of equations that define the theory. In this equation, there is no variable T. There's no variable time at all. Can this equation describe a world? Yes, of course, because some solution, some limit, some approximation, blah, blah, blah, blah. you also describe a world in which you and I are discussing in a common time and in which if you miss the 10 o'clock train, you're in trouble. So the point is not that time exists or do not exist. Time is a layered thing.

All the properties of time exist, but they are relevant for describing situations which are more and more particular. And down there, at the most general level that we think we begin to understand, maybe, which is quantum gravity, there's definitely not a flow, a common flowing time in which the universe is in one second and then next second and the next second. There's no priori difference between past and future, but that does not mean that everything is static. I think it means that temporality is much more fluid and generic and local and complicated. And out of that mess, this quantum gravity mess, there are situations that can organize themselves in the temporality we're used to. Okay, that's great. Thank you.

But I want to think now, okay, that's great as a general picture. I want to think in terms of the kind of things that will become relevant when we talk about going into a black hole and coming out of a white hole, which is your picture, which is which is what you talk about. And I have issues, but, but it's an interesting idea. You point out that the time that we think of time is really why, look, everyone says, look, and you point out in your book,

time is obvious. Look, the past is different than the future. Past, I can't affect, I can affect the future. I have memories of the past, I don't have memories of the future. So clearly, past and future are different. And you talk about, you address that issue, as you have to, and talk about it in some sense as a nature of sort of time is related to a departure from equilibrium.

As a, you know, and the example you use is a tank of water, you let the water flow from one tank to another. if you run the film backwards, it looks weird because you don't see all the water coming back into the original hole. You see it, you know, and the waves and everything else. And, you know, there are lots of films you can run backwards in time. And people have often talked about thermodynamics and the era of time. Certainly, the entropy is some people have argued they're related. And Stephen Hawking once made a famous error about that.

But you point out as a departure that if there were no departure, from equilibrium, we would have no memories. There would be no experiences. Anyway, why don't you elaborate on that a little bit? I don't want to, I can paraphrase what you said, but let you paraphrase it. Yeah, I think that one of the aspects of time, which is more obvious to us, is exactly what you're saying, which is the difference between past and future, the fact that some processes happen, but their time reveals does not happen, and so on.

So it's direction of time. Where does the direction of time come from? Is that intrinsic in nature? And that seems, I guess, for the people who are not in physics, that seems to be a silly question because the obvious answer is, yeah, of course, it must be different for the future. It becomes an interesting question for a physicist because historically, the effort to sort of go down to the grammar

of how the world works from Newton on, the Newton equation, the Maxwell equation, of the Einstein equation, the Schrodinger equation, the quantum field theory equations, they all, the particle, the standard model equation, they are all,

they all have the properties that if you, if you consider a story and a suitable time reversed story, the standard model would be CPT, they both are possible. So it seems like the fundamental physics is saying, look, what is possible in one,

sense of time is also possible the other sense of time. And yet, this is obviously wrong because, you know, things break and don't come up back together. So that has, that puzzle has prompted this big debate of what is the origin of the direction of time. And this is discussed. It's discussed by physicists, discussed with philosophers. I would say it's not an issue on which there is a total consensus. and everybody has clear ideas. I am convinced that the distinction between the past and the future

only makes sense when we look at microscopic variables, average variables. It's really a thermodynamical issue in this sense. and I think that we would not have this sense of time-oriented if it wasn't because of something related to this use of macroscopical variable and in particular to the fact that when you have microscopical variables you can talk about equilibrium and you can talk about equilibrations and let me put in this way if the world was really in equilibrium so you imagine that the world wouldn't expand forever. You can put it in a box and you wait until it gets completely. I am convinced. I'm not sure I can convince anybody, but I'm convinced everybody. But I'm convinced that if we were part of that world,

really there would be no distinction between past and future. Because there could be no memory, no. Well, that's because in such a world there'd be no distinction. There'd be no events. There'd be no in such a world where everything is in equilibrium. Nothing happens. No, nothing macroscopic happens. You have fluctuations at the quantum mechanical level.

But in equilibrium, you're in equilibrium. And so it's departures from equilibrium that would produce what we might think of as events. I mean, in a world, if everything was in equilibrium, there'd be no free energy to do work. I wouldn't, I wouldn't be able to have a metabolism. We wouldn't have this computer conversation. I couldn't have power. I couldn't get power to run my computer. So it's more than just, it's true that time

might cease to exist, but then so would anything of any interest in some sense, if everything, that's what some people would call the heat death of our universe, where there's nothing left, there's no source of anything that can allow you to do anything interesting. Yes, but interest, I completely agree, but interest is in the eyes of the beholder, whatever. Namely, there will, if, imagine I'm God,

outside and I'm a mechanical god and I'm interested in how each particle moves. Oh, very interesting things happening. This particle is hitting that particle and is moving. Look how fast is moving this and how slow is moving this other one. So there will still be a very rich mechanical series of event. Okay. But it's a mechanical series of event that doesn't have the property that seen in one

direction of time or the other direction of time looks different. And so, but your argument in some say, okay, so let's, no sense to debate. So wait, let me, let me just go back to saying that we have memories because of the no, no, no, let me come into exactly what, what you're saying. In order to see the direction of time, we have to go to the quantities that you were referring to, namely all the quantities work as distinct for, from thermal energy, free energy, and so as distinct from total energy, all the things which make sense

once you have a course grading and microscopic violence. Yeah, okay. But then you might say the memories are somehow memories of the ripples associated with a previous departure from equilibrium, which proves events.

And, you know, that's not a bad, I mean, that resonates with me to some extent, but I do have issues. For example, in the future, there can be extreme departures from equilibrium. And why don't we have memories of those? I have a paper in which I try to connect memory and entropy.

It's not a long ago paper. It's two or three years ago. I spent years asking how the hell memory works. How is it possible that there are memories? What is a mechanism that is such that, in other words, In our words, in our words, which is the second principle of the dynamics and work and heat and so on, what is the general feature of a property of the present that talks you about the past and not the future.

So I have a little model of that, which I think give some hints of that, and I think that what you need, I think that pretty generically, if there is a low entropy in one side of a time span, generically, you have memories in that direction. But you, yes, but you can, but the problem is, of course, locally you can, locally you can reduce entropy. That's what life is. And by, you know, a system that isn't, that isn't isolated. Anyway, it's an issue that I have, you know, maybe it's too complicated to talk to here, but I have issues because locally you can imagine a direction which, entropy, where there's lower entropy in the future in a local system, not globally, but locally. And then I don't,

then that argument seems to break down to me because it seems to me that, if they think of that, the future should become the past in that way. But anyway, that's an issue. So these are things we can table for future conversations when we're not talking for the public. But, but I want to just, it's fun for people to see physicists. Yeah, it's a very, talk to each other and have, and have questions. Let me, as I said, this is a really open question in contemporary foundations of physics. And let me just one of the most interesting one of the problems. And let me go to one of the theories you love, of course, which is relativity, and we all

loves, but it's pretty special relativity. The beauty of special relativity is it's designed so that causality isn't destroyed. You know, the first thing you learn when you're, maybe not the first thing, but when you're undergraduate, you learn that, you know, one person's future can be another person's past. And you say, what, that's impossible because, you know, if I, you know, if I, you know, if I shoot you with a bullet, you don't die before the bullet hits you. And of course, the wonderful thing about relativity, as I like to say my students, is just kind of this cosmic Catch-22. Catch-22 is one of my favorite pieces of American literature. But Catch-22 that always

frustratingly arranges things so that if one person's future is another person's past, an event in one place can never affect the other place. So you never screw up causalism. So causality, which really implies in some sense a direction of cause and effect is central to relativity. And therefore, in some sense, the question once again is how causality merges with this notion of time as related to entropy. I don't get it myself and I don't know if you thought about it, but it's an issue. Well, I talked a lot. That's my penultimate paper. It's causality.

the physics of causality. I think these are the terrains which the metaphysics should address more. And I think they are addressed by some works. Philosophers are also interested in these questions, because they're really foundational. And that's one of the reasons I talk to philosophers, and some kind of philosophers. The philosophers who talk about physics, who know physics. Okay, I'm more skeptical of talking to philosophers about physics. I haven't ever gotten anything out of it, but that's just me.

Yeah, I think I have got something out of them, some philosopher that John Herman or some Richenbach. I have a paper exactly on the issue that you just discussed, which is causality, the origin of causality and its connection to relativity on the one hand and to on the other hand. See, the connection that you're referring to is definitely there, but the special relativity,

the structural space in special relativity, does what you do, but it's remarkably does what you do without having anything that distinguish the past or the future. Because I could in the same way, you know, shoot the causality in the past and the future, if I

just look at relativity, but then I think with my own understanding of the world, wait a minute, I can shoot to the future, not to the past. So relativity by itself doesn't seem to be sufficient to give this distinction between past and future, which on the other hand is there. And once again, I understand you're skeptical about that, but I got convinced, I would say in the last years, and I've been writing these papers about that, technical. The last one is exactly causality and entropy, with idea that the who is connected to who

doesn't depend on entropy, okay? But who influences who? One of the two is oriented, one of the two is not oriented. Yeah, yeah, yeah. So if I, if these and these are connected, that's something which has nothing to do with entropy. But every time we say that this is a cause and this is a fact, and not vice versa, we are bringing him some entropic thinking inside.

Okay, it's interesting. Again, I'm skeptical, but specifically for the reason I said, you can imagine a future with lower entropy, locally, not globally, but locally, and that's certainly allowed, and then you have a problem. By the way, I was amused. You brought in some quotes about free people worry about free will, and you point that out, but, you know, I just finished three hours with Robert Sapolsky talking about free will in a book he's written. Oh, what did you go to then?

There's no such thing. But very, it was something I've always accepted too. But I'm intrigued. I didn't know the quotes from Spinoza. They're very interesting. I didn't realize Spinoza had hit that so much. But, but you argue that, of course, actually you argue in a way that I'm not sure if you get your cake

and eat it too. You seem to suggest at macroscopic scales, we have free will, but at microscopic scales, we don't. I would argue that at neither scale do we have free will, But maybe that, I don't know if you've, just out of interest. Do I understand you correctly that you're saying it, macroscopic scales,

we have free choice and microscopic scales we don't? No, I think fundamentally we agree. If I understand you, we fundamentally agree. I am objecting about the use of the words because by free will, we might mean what you and I are agreeing about in this situation, but we might also agree, sorry, but we might also use freewheel in a much weaker sense, which is, it's still the fact that I can say, oh, I choose what to do tomorrow, and I've chosen, I've chosen freely.

It's just not meaningless talk. It just has to be reinterpreted properly in the complicated process. You feel you've chosen freely, certainly, yeah. Yeah, I have a little story about the little paper by the old fishermen. Oh, yeah, you talked about in the book. Yeah, the story of the old fisherman, just in one second, is the following. This old fisherman that loves sunset. And he sees the sun going down in the ocean, okay?

And then his whole life is organized about this sunset. He goes to sleep and so on. And then one day, the band of the city comes and tell him that the sun doesn't move. Okay. And he says, oh, my God, there's no sunset because the sun doesn't set, right? There's no sunset. So he becomes crazy because he's saying, okay, so this is all illusion. I shouldn't look at it. It's illusion. I'm eluding myself.

It doesn't go to sleep and so on. Okay. Now he's making a mistake because of course there's no sunset in the sense of the sun setting. There is sunset in another sense. Yeah, of course. And I think we can say, we can tell the same story for free will. So there's no free will in the sense of our mind changing the laws of physics and doing something, right? Our mind doesn't change the law of physics. What has to happen, happen. Okay. But there's free will in the sense there's a sunset. I mean, there is something happening in my mind. You have the illusion of a sunset and you have the illusion of free will.

You're very similar. And we also have the illusion that this table is solid. And lots of important physics has taught us that much of the world we see is an illusion. And in fact, we wrote some of the voice about that at the same time. But why calling it illusion? I mean, this is solid. If I, the sun is setting. I'm not going to follow through it no matter what I think,

no matter what movies I see to tell me the opposite. Exactly. So if you redefine the table is solid as meaning simply, not that is not mega atom, there's no simply that you cannot go through it, you can still say there. And you can still say, oh, there's a sunset. In the back of your mind, you know that this means that, you know, we are rotating and so on and so on and so forth. Well, there's a sunset. Let me tell you a story, in fact, related to the sunset, which is a different, I think I just said it on stage the other day in English. Well, I learned it from my friend Sidney Coleman, late Sidney Coleman, from who I learned a lot at Harvard. But it's a story about Wittgenstein from the Tom Stopper play about Wittgenstein. Wittgenstein's in Cambridge, and he's staring at it. For a long time, he's puzzled before he crossed the streets and someone sees him and says, hi, what's going on?

He said, I'm puzzled. And I said, people say that the earth goes around the sun. And you can tell that because you can see the sunsets and things like that. And someone says, yeah, yeah. And he says, well, what would it be, what would it look like if it were the other way around? Of course, it would look identical. And I think that's the point. That's the key point.

Yeah, absolutely. In any case, we both told little stories. but now I want to get to white holes. I'd like to finally get there. But I thought it'd give some perspective of where you were coming from, and I hope for people it's useful. And some of these ideas will become relevant.

So let me paraphrase in a brief sentence or two what I see as the central thrust of this. Black holes are strange and interesting objects. They've fascinated physicists as long as ever since we first realized they could exist, when people debated about whether they could even exist or not.

And some of us, by the way, still wonder whether black holes exist. I wrote a paper with my friend Tomnevachaspati a while ago, arguing that they never really form. But in any case, let's say they do. One of the strange things about a black hole is it has this thing called a horizon. You go through it. You can't ever get out. One of the many fascinating things about black holes. And then what happens inside the horizon is even more fascinating,

as you talk about so eloquently in the book. And the idea, however, is in some sense, what happens ultimately to black holes. And in conventional physics, there's a problem. And the problem is that Mr. Hawking told us that black holes evaporate. So big black holes get smaller and smaller and smaller. And then the problem is lots of stuff falls in them. Physics papers falls in them and lots of things fall in them. and then when the black holes evaporate, they evaporate thermally and all the information of what fell in gets lost.

And that violates the fundamental law of quantum mechanics called unitarity. And that's this so-called information problem. But what you argue at some sense is in relativity, and this is well known well before, well, for the first people developed black holes, because of the equations of relativity are kind of invariant under time, you can imagine a time-reversed version. of a black hole, which was called a white hole. I don't know who the first person to call it a white hole was. Maybe you can tell me at some point. But anyway, I don't know.

I should look at it since I didn't read it in your book. And so I don't know who first called. I know who first came up with the name black hole, but I don't know who came the white hole. But and so people have argued, well, why don't you see white holes? And there's been, you know, and we don't see them so far. But your point is that quantum mechanics, that when things collapse, the quantum world takes over because very small scales are relevant. And we all know that at quantum mechanics

scales, strange things happen. Electrons can tunnel through barriers because quantum mechanics allows that. And if gravity is a quantum theory of space time, then why doesn't space time tunnel? And your argument is, as you came up in an afternoon conversation with one of your colleagues, as you allude at the very beginning of your book, that maybe inside the horizon what was a black hole tunnels and becomes a white hole of which only things can only escape,

not fall into. And that's the future of black holes, and that may affect both the information paradox. And you may even argue that it may, there may be lots of them around. So that's the five-cent version of the picture, But let's try and fill in the gap.

So did I get the general picture right? Or would you say that I missed? That's a very good, a very good summary. Thank you. I mean, you did better than what I would have done. I appreciate it. I don't know about that. But let's, so the key idea, well, but you know,

one of the nice things about the book is that you will elaborate on, you know, things, and I've tried to do this in my writing too, the strange difference of what goes on inside of horizon and outside and near the horizon. things that you're absolutely right, and you point out, confused physicists for a long time. And one of the interesting confusions comes from the reason that in Russian, black holes are called frozen stars. Yeah. Because one of the strange things, of course, as you pointed out, when you take a clock up here,

it takes a different rate because it's farther away from the center of the earth. Same thing happens with a black hole. You fall in a black hole, your clock slow down and slow down. and slow down, and they appear to stop at the event horizon. Black hole, if you're standing far away and I'm watching you, you freeze on the surface of black hole. I never see you fall in. Never in, what's your history of the universe will I see you fall in the black hole?

By the way, which is the cause of one of our papers on why black holes might not exist. But anyway, aside from that, you freeze. But of course, if you're falling through the black hole, if you're the person there, you don't know. There's not a signpost saying the eventorizes here. And if it's a very big black hole, even the gravitational forces at the horizon may not be so extreme. So you could survive that. And your clock is ticking and nothing strange happens.

And so that's already a really strange thing. And that took, as you point out, time for even physicists to appreciate that difference. So why don't you talk about falling into a black hole and the inside and outside, just to put that in context. And then we'll talk more about the white hole scenario itself. Yeah, and this part, I think it's not speculative. That's what generativity tells us. And generativity seems to be pretty good. And we're not certain, of course,

but we have good reasons to believe that generativity, and we can trust generativity what happened to the horizon. Of course, you know, good luck may have decided differently, but we don't have very strong reasons to put. endowed generativity there. We have a strong reason to put in doubt generativity at the end of the evaporation or down the center of the black hole when it squeezes. But on the horizon, we can rely on generativity because generativity has given all these fantastic predictions. And there's this very funny phenomenon, which is the horizon itself, which is what you said. If I fall inside,

I just go through the horizon and it's just nothing happens. It's completely irregular. It's David Finkelstein, the first to realize that and decades after after the actual equations for the metric of the black hole. You're a lovely picture of David Finkelstein in the book, who I met once. Yeah, yeah, great guy. And yet, if we are outside and if I see you, if you want to go into black hole, you go across the horizon. But if I'm aside, I never see you entering. I see you approaching slowly, slowly, slowly. I see you approaching slow and slow.

I become old and old and old, more and more, super old, and I just see you at the same age being still there, near the black hole, never entering. And that seems completely strange. The way to think about, which I think is correct, is that the light that is emitted from you while you're falling takes a long time to come out. So really light slows down.

because you're very close to the horizon. So when you're very close to the horizon, the light, the picture of you that come from the light, takes very long time to come out. So I receive it when I'm super old. Okay, I think of it in a different way, but the net effect is the same. I think of redshifting rather than slowing down, but okay.

That's another way of viewing. That's another way of... I guess I'm tied to the fact that locally light is always traveling at light speed, but space is compressed. Oh, yeah, you can view it. You can view it. But the end of the result is.

is the same. So what I'm seeing is you slowing down more and more and more. And of course, you're, you become more and more red because the light itself gets slowed down. So it becomes red shifted. So this is a phenomenon, which is very interesting because it's a perspectival phenomenon. So it's because you and I are different positions that this happens. It's not phenomenal just by happening at the horizon or not just up outside, but by the relation between how, from the outside, side one see the rise. Then you go in and in the book I follow somebody who goes in and we might reasonably trust general activity also going in. And so we know what's happened. But the point is that when what happened is that you find yourself more and more pushed and pulled by these

forces and space side itself sort of closes around you and becomes more and more slow in some sense. and at some point we strongly expect that generativity is not valid anymore because there's quantum mechanics and so at this point the prediction of generativity alone are not good. We shouldn't

rely that. And there is another point where we should not trust the prediction of generativity which is you were saying before that Mr. Hawking told us quite convincingly I think we're all convinced of that

that a black hole slowly evaporate. Okay, so it becomes smaller and smaller, smaller. It's a very long process. It can take for a stellar black holes, can take billions of years. I mean, it's become very, very small. And it becomes smaller, smaller, smaller. At some point, it's so small, again,

that the curvature just outside it is so strong, it's Planckian again, that we are deep in a quantum regime. So we cannot think that generativity outside, horizon is still good. Okay. Now we are in quantum gravity. So everybody who says, oh, we know from general activity the black holes evaporate and disappear, boom. Well, the disappear part is that we don't know about. The evaporation part we trust. The evaporation is reliable until they're very small. The disappear is a guess. And so it's a guess in my suggestion that this guess is strong.

They don't disappear, right? And the key point is that the answer is a quantum gravity answer. So it's not just, we need a theory of quantum gravity to say what happened at the end of the evaporation. So it's all a guess, even what you're talking about is a guess, too. We have loop quantum gravity. We have tentative theories, which we're not, we don't know if they're right, we're not if they're right or wrong. Okay, that's the point.

So that's the point. You can apply loop quantum gravity to try to tell you what happened there. I will interrupt. I mean, you talk about calculations you've done in loop quantum gravity to try and support this. Yeah. But nothing about the picture you present seems to me to require loop quantum gravity. at all. It's just a statement about quantum gravity that if space time can tunnel, it can tunnel. And I don't see. And then everything else is words, and you could try and support it with

tentative calculations in a theory, whether it's string theory or luke quantum gravity. But nothing about the idea that suddenly space time changes its nature by a quantum tunneling event is inherent to loop quantum gravity. It's just a statement about what might happen in gravity. And will you agree with me, or am I being too hard-headed? to say that every, no matter what, you can say you can support it at some level with calculations from luke quantum gravity, but it's still talk, it's still just a guess, right? Because we don't have a full theory, so it's still a guess. Yeah, of course.

I mean, there are two different questions here. One is, would another consistent theory of quantum gravity give the same prediction? And I would expect that the answer is yes. Yeah. I would I mean, if it happens, it better. Yeah. One needs another consistent theory of gravity.

I don't know. Let's face it. Any theory of quantum gravity is going to treat space and time as quantum objects. And it will apply fluctuations, tunneling. It'll say that space and time do what quantum mechanics allows things to do. And that's one of the reasons why I go to a universe from nothing. You know, it's quite possible that space time tunnels from nothing. and you get a closed universe, and that's...

Yeah, here is even simpler. Yeah, here is even simpler. Namely, I can describe the entire process as a black hole forming and then become a white hole and then... Just, I could, just as a regular metric, regular space-time geometry, which satisfies the Einstein equations everywhere,

except in a little jump. Except on that. Except where I can write a smooth metric. And then you can come and say, look, any decent quantum theory of gravity should allow this little violations of the classical equation for a short while. Exactly. That's exactly.

Yeah. I mean, that's the good way saying it. The other way, and I think I said this again recently in a podcast, but my favorite cartoon of Sidney Harris, I don't know if you know the cartoonist, the science cartoonist, Sidney Harris. He has these two gentlemen at an equation at a blackboard, and they're writing a long equation, and there's the first part and the second part. And in the middle, it says, and then a miracle occurs.

And then one guy says to another, you should be. a little more specific right there, I think. And in some sense, when we talk about quantum gravity, we're really in that middle piece and we're saying a miracle occurs one way or another. It would be a well-motivated miracle, but until we have a full theory, we basically are saying, we don't understand what happens here, and here's something that might happen, and it plausibly can happen. Yes, now we completely. We don't have a full agreed theory, but we have a tentative theory, which is loop quantum gravity. So what you can do in loop quantum gravity, You say, well, let me take look quantogravity as an hypothesis and see whether look quantogravity in particular would allow that.

Okay, so you can write basically the equation that give you the probability of doing the jump. So you have the metric before the jump, you have the metric after, and you can compute the probability for this to happen as a function of the actual parameters of that. And you do this calculation. This calculation can be done in some approximation. and some truncation, which means, who knows that, you know, doing it better would survive, but within some truncation,

the result is that as long as the horizon is large, slowly shrinking, as long as large, the probability for this jump to happen, it's very small, super suppressed exponentially with the mass of the black hole. But when it becomes small, the probability goes rapidly to one,

and so you expect this to happen. Well, dimensionally, that makes sense. It's what you would expect, but this is less trivial than what, because I was hoping that the theory would tell me that it happened for large black holes. Oh, I could have told you it wouldn't. No, I would have guessed it wouldn't.

That's all. If it had told me that, I would have found the theory suspicious. Let me put that way. Well, you know, I was hoping, because it would, you know, be very interesting experimentally. Yeah, it would be very, obviously, it would be very interesting, but, you know, life doesn't give you what you want. It gives you what it is.

Life doesn't give you no. But look, I put a poor PhD student and said, please do this calculation and give me this answer. And the poor students were coming back and say, it doesn't give you that answer. They try harder. Well, good for the students. Good for the students. Yeah. And I think that's important.

I want to come back to that. So the idea is that when the black hole shrinks enough so that it really becomes quantum mechanical, strange things can happen. But I want to focus, given the amount of time we're going to spend, and on a few facets of this, the horizon is one, this weird horizon. The fact that you point out when you fall in, you know, space seems to press around you, it stretches and you become spaghettified. And you point out also that all of the mass of the star that collapsed is still there.

It's just compressed into a region, region the size of a... Plank Scalph. I think everybody has the wrong intuition here. And one reason I've written the book, including my colleagues, I mean, many of my colleagues, because the intuition that people have, I think it's wrong for the very technical thing, but it's very important for the intuition that people have is that the inside of the black hole is static. And there is a singularity down there at the center.

So there's something like that. And down there in the center, there's a singularity. And it's static, stays there forever. And that's bullshit. That's not what happens. This is evolving inside. Exactly. Exactly.

Exactly. I mean, everybody who... No, I think that's a very good discussion, the fact that case evolving, and you're saying that it's sort of stretching and stretching and stretching, and for a very, very old black hole, it's kind of stressed from a long long. It's huge.

It's huge. Exactly. And you make the statement, and this is a different version of the statement that I've normally heard, but it's important to say that people realize that black hole, do something that is truly miraculous, the horizon of a black hole, from the outside, a black hole can be very small.

But if you're inside the black hole, the space can be extremely big. Now, are you arguing it's extremely big because it's been stretching for a long time? Or the argument I've often heard is that space and time reverse in argument, and space becomes always forward. At time, you know, it becomes spatial

and you can look up and see baths. No, this space and time reversing, I think is just true in the structure coordinates. It's not true. You just write the black hole, different coordinates. Time is time, space, space. There's no reversing of space and time inside.

There is a reversing of the behavioral space. Yeah, space becomes time-like, and time becomes space-like. Right. So outside, it's static. So outside, if you're moving time, nothing happens in the short-sham metric. If you move in space, things change.

Well, inside... Let me push a little bit more on this, because maybe, well, because I think it's interesting and I also want to see if we agree, when space becomes time like, at times it becomes fake-like, since you've argued that time is kind of a... It doesn't.

What do you mean? In a black hole, no matter what you do, you move always forward in space towards the singularity, no matter what you do. No, no, no. Into a black hole, you always move forward in time. It's just the coordinate are, it's a time-like coordinate. move up. If you put your foot on the gas, you don't find yourself moving up in a black hole. Do you agree?

That's right. That's right. I agree. You always, so therefore, space becomes time-like in a sense that it appears to have a unique direction, always forward towards the singularity. No? You, but if you're somewhere inside the black hole, in a little, in a little, region, your metric around you is Minkowski. It's completely normal. Well, yes, but, but you're point out that space time becomes squished. So it becomes harder and harder to think it as Mikowski. It becomes more and more. I agree.

I agree. Like something with just a space having a direction and a time having a past and future. Yeah, yeah, yeah. So space is moving around you in a dramatic way. Anyway, to get back to the question, would you say that the reason that the space inside of a black hole is large while the outside of a small is specifically because it's been stretching? Yeah.

Yeah, yeah. And in fact, you can put it rigorously mathematically. You can define, in general activity, you can take a sphere and ask what is the volume inside. But it's a bit tricky because it can chase different foliations, right? So what is the volume inside? And if you think one way of defining the volume inside in Mikoski, you choose the foliation which maximizes volume. Because if you think in one less dimension, if you have a circle, any other foliation, it is a less area because it like a zero area, right? So the flat one maximize. So if you ask what is the foliation that maximize the volume inside, you can define a volume. And you, if you take Sharshel, for instance, the same curve, but sharpsule is actually not, Sharshel with a, with a

with a falling star inside. When you move along the horizon, you ask how the volume grows inside, you just get a formula. And you have a formula, which is a maximal, the foliation that maximize the volume inside. It just grows. It goes linearly with a syntotic type, with advanced time. So there is a precise, sorry, maybe this is, was technical, no, no, no, no, but there's a, okay, there's a precise mathematical sense in which as time past, the volume inside, it becomes bigger and bigger because it shrinks. Okay, because, sorry, because it becomes longer. The tube becomes longer.

Stretches. The word is stretches. Thank you. Stretches, stretches. The tube becomes longer, longer. If we had this discussion in Italy, I'd be much less coherent than you are in English. Thank you. Don't worry about it.

Okay. Thank you. Okay. So this is important. It becomes longer and the same time it shrinks. Now the shrinks is the right word. I suppose in the radial direction. At some point, it shrinks so little that along the entire length of it, if you want to call it, length.

Yeah. You're now in the quantum regime. Exactly. Exactly. And you can suddenly tunnel to a classical picture, which is exactly the same as the old classical picture with time reversed, if you want to call it that. Exactly. Where instead of shrink, instead of stretching, it's shrinking.

Exactly. And that's a plausible quantum mechanical. process and you argue that that's interesting. Exactly. First it's possible and it's interesting. And so I want to talk about in the last few minutes why it may be interesting and why it may be interesting and why I have concerns about that. Okay.

And then and then we might leave it there. Then I want to come back to your, I want to come back at the end to art and literature and politics again, believe it or not. Okay. That's where we're heading. So one of your points. points is that this solves the information paradox. Yeah, entirely. Look, because once it's made that's tunneling, everything comes out. Yeah. And all at now. So here are my objections to that,

and I have many of them. Where can I begin? Well, the first one is the simplest one. And it is that at the point this has happened, everything has shrunk down to this plank scale. And all of the mass of star that was originally there, ignoring evaporation for the moment, because really what's happened is a very small fraction of the mass of the star is still there. But it's still there because you talk about it in one case if it is, but really, as you point out later, all this only becomes relevant when the whole, when the black hole is largely evaporated. So it's really very little mass left over. Very little mass. It's another problem. But here's the thing. It's crushed to the point where all the quarks and all of the protons and all of the neutrons and all of the the the the the the

catalanx and the and the and the people and everything else that have fallen into the black whole the alien civilizations the planets have all been crushed down to the plank size so what comes out has no relationship to what fell in so there's no information about what fell in the first place so what's the problem what how do you get around that problem i mean it's already gotten to the point where everything that you might have imagined would give you information about what fell in has disappeared because the character of everything that that's fallen in has now been crushed to the plank scale and comes out as elementary particles or whatever you want but it's not going to come back if a Cadillac fell in it's not going to come out

if a Cadillac fell in is not going to come out but the the minimal size of the squashing is not the plant length it's big. because what determines being in the quantum region is a plank curvature. And the plank curvature is the mass divided by length. Okay, good, good point. So it's actually scaled up by the cubic root of the mass divided by the plank mass. So there is plenty of space for information for going through. The squeezing is not, it's not.

this is a similar story which you might know better in the bouncing universe yeah sure I know picture yeah it doesn't it's not that the universe becomes plank scale it's a curvature because plank yeah yeah yeah I have equal problems with the bouncing universe so let me make that clear okay but but but let me ask you something I hadn't thought of before and maybe if are you really saying suggesting that it's really time reversal invariant in the sense remember you've made a tunneling You made it tunneling so you can match on the curvature and the nature of space. But you're really not, but you're really not suggesting that that white hole,

as it pushes things out, all of these quarks come together to form Lawrence Krauss. No, no, no, no, no, no, no, no. Exactly. I didn't think you were. So it's not a complete time reversal. We're falling in, you see Lawrence Krauss being spaghettified and stretched. And that's it. Bye-bye Lawrence Krause.

Yeah. Coming out, Lawrence Krause isn't there. So I still don't understand exactly where, how you saw the information loss problem. Okay. So, okay. So first of all, think about a bouncing ball. Yeah.

Okay. There's a simple description, sort of mechanics 101, where it bounces and when it comes out, it's just exact time reversal of what goes down. And then you're going to say in real life, it doesn't come up as high because it loses energy. I realize that because, exactly, because there's dissipation.

So the actual mouse itself has a lot of time reversal to first approximation, but actually there is a lot of dissipation. So in the black hole case, obviously there's a lot of dissipation. And in fact, the Hawking radiation is dissipative. It's a very dissipative phenomenon. Oh, it's a dissipative. That's the problem. That's the origin of information loss problem.

It's horrendously. There's no information in thermal radiation except the temperature. Exactly, exactly. So it's very dissipative. It raises entropy randomly. So what comes out, it's definitely not the exact time reversal of what goes in. And in fact, if you start from a black hole, it becomes small, and then the white hole is small. So it doesn't become the big black hole.

Now, the energy doesn't come out for the white hole. The energy is already gone out with the... And we should have... Let me just go back because I don't think we explain that to the public. Your picture is that hawking radiation continues until the black hole from the outside becomes plant-like. Roughly blank, yeah. And then it splits into a white hole and does this thing.

So most of the history of a black hole is the same as a conventional picture. That's correct. Okay, good. That causes problems for me too. But anyway, go on. No, no. Most of the same, sorry, black hole is the same. Sorry, but I'm always the same with the conventional picture.

The next part is different. When the hocking radiation goes out, there is hokey radiation that falls in. Okay. And this can be computed and it has negative energy. Yeah, sure. There's a flux of negative energy. Right, that's why. And the negative energy of the incoming hock irradiation, it's up the energy of the star, if you want.

Okay. So that's why there's no energy inside. Yeah. That's the story of the energy. The story of the information. The information doesn't do that. The information is still there. So you need, that's why the remnant lives so long.

That's why the white hole lives so long. Because you have little energy and a lot of information. And to bring out a lot of information with little energy, you need a long time. Right? It's like... Yeah, well, that's your next argument. And I have problems with that too. But, okay.

So the argument is, your point is that somehow the energy is there. The information is there. and and somehow the information comes out. You haven't explained to me how the information comes out because it's not coming out as Lawrence Krauss or Cadillacs, which have all been destroyed. It's particles. And as far as I know, I can understand how energy can come out,

but nothing about the procedure you've given me tells me where the information comes out, except there's a lot of quanta. There might be a lot of quanta, but then I have a problem with that too. because there are a lot of low energy quantity right so if you think about no but now I don't think you can have your cake and eat it too Carlo but maybe I'm you know I'm happy to be wrong um here here's you've got a very small let's do all of our calculations outside the black hole what you see

coming out is what matters I don't care what's going on whether there's a gazillion long mile mile long neck and you know and and and whatever it inside could be arbitrarily complicated but outside I'm seeing, let's say, of 10 of the 5 gram black hole or something where the plank scale is important. It's already, it's the plank size. And so now, now, now, and the whole point about this is you can,

what you, what you can do in your picture is match on, it's lovely, you do it in general relativity all the time. You match on these two different pictures. You've glued them at the center, and you don't apparently see any problem. But from, as observed from the outside, I see lots of problems.

Okay? One is, here's one, and then we'll talk about the long time and quantum, but here's one. There's a sudden discontinuity because in the Hawking picture, by the time the black hole has gotten down to close to the 10 of the 5 grams or whatever the blank size black hole is, it's incredibly hot. It's unbelievably hot. It's radiating like crazy. But then when your flip happens, it doesn't look the same at all. It's suddenly a white hole that's almost got zero temperature. So suddenly when you look at it from the outside, there's an incredible discontinuity.

There is. And this is the same as if you have a very excited atom. To radiate, it, it goes down to the lowest and then it stops. And then maybe there is a... No, no, no, but with an excited atom, it radiates and radiates and cools down. With a black hole, it's radiating and radiating and heating up. up. And that's a fundamental difference. And so at that stage, you have an incredibly hot thing, which when you suddenly have your white hole picture, you suddenly have a very cold thing.

So something happens to that temperature. There's a discontinuity. And I think it needs to be addressed. And I don't understand how you go from the outside. You know, from the inside, it looks like it's fine. You go from a black hole to white hole, things falling in, things going out. Nothing seems discontinuous or singular. But in this case, when I see the thing evaporating, getting hotter and hotter and hotter, turns the white hole, it's suddenly very cold, and suddenly I have a discontinuity and temperature.

And maybe it's there, maybe I just don't understand it, but it's a problem for me to picture. That's right. I mean, black holes have an inverse heat capacity, right? That's why they have this explosion. Negative specific heat, which is really weird. Right. Gravity allows it, of course.

Gravity allows it. So that's we got used to that. I should say in full disclosure, up there is my PhD thesis, which involved the idea of tunneling black holes in the beginning of the universe. So I've thought a lot about this, okay? Okay, okay, okay, okay. So maybe I should look at what you've done, definitely.

No one's ever read my thesis as far as I can say. Well, maybe there is finally somebody who's reading it. There were a physical little letter papers about it early on, so you can read those. So the picture here is that the black hole becomes hotter and hotter and radiates more and more. As it's area shrink, the area of the rise on shrinks. And remember that this is, we're thinking this terms of quantum gravity.

So the area shrinks, if you want quantum mechanically, by jumping down rapidly to a gain values of area more and more and more until it settles on the, there's an area gap between the minimal area and zero. Yeah. Okay. So it doesn't set to zero. It sets of the minimal area But the probability amplitude of the last jump

It's very very small That's the point because it has all these things inside there So it has burned out everything it can send out And the last thing it could send out It takes very, very long time The last jump, it's very improbable So you're saying that walking radiation stops And it cools down

That's right That's right. It doesn't do it continuously. It doesn't do a dismal. continuously. It does it pretty broadly. It slows down, stops radiating, and cools down. That's your picture. That's a picture.

And what happened next, okay, is that it still has to speed out, if you want, one plank mass of energy and a huge amount of entropy. Now, if you think about just photons, you can't have one pluck mass of entropy and a huge amount of plant mass of energy and a huge amount of entropy is possible with photons. But you need very, very long wavelength photons. You have a very little small energy photons. You have a lot, a lot of them. And way long, very way, very long photons, you know, require a lot of stretch, a lot of time.

Yeah. Yeah. Well, okay. So that's what causes. You introduce the other problem I have. So you would argue that the information loss problem is solved somehow in still ways I still can't see because if it's thermally radiated, you haven't solved anything. No, it's not terminally radiating. So what I'm saying...

If it's very coherent, okay, fine. Let's say it's very coherent. What I'm saying is that this emission that there, which all these photons, okay, of course, you have zillions of different ways of emitting photons. Yeah. And these are unitary related to the zillions of different ways things could fall in inside. Okay, okay. Yeah. So information is concerned. But I see, I know where you're getting. but now let me give you my next problem. And then that's it. I don't want to, I didn't want this podcast to be.

I think it's important for people to see physicists debating, but my purpose is not to try and destroy the picture. It's just I have problems with it. The other is it's a fundamental problem in physics. Again, I like to live outside the black hole where everything's simple and I can understand what's going on. Jumping between frames always causes problems, but let's all stay outside the black hole.

Yeah, yeah. Outside the black hole, you have an obvious. of extremely small area. Yeah. And everything I know about physics tells me that that object cannot emit long wave radiation. In particular, it can't emit wavelengths that far exceed the scale of the horizon. That any vibrate, any mode that's going to be emitted by an extremely small object. It can't.

It's not going to be, not going to be coherent over extremely large scales. If you're in Minkowski space time, that's true. But if you're not in Minkowski space time, that's not true. Look, imagine that you have a very tiny hole, okay? And you're going to say, but this is not possible in Minkowski's this time. It's possible in generativity. And imagine that a variable there oscillates very, very slowly. Yeah.

Okay. That is my producer very slow. Now, the question is, wait a moment. How can it oscillate very slowly if the space in time is small? Yeah. You need some. But if inside, that's generativity, inside the things, there's a long, long tube. I know, but remember, we're only seeing radiation from the horizon.

So we're stupid people. We're not stupid. We're physicists who are far away and we're looking at this object. You're right. Inside, it can be arbitrarily complicated. But outside, I say that object has to obey the laws of physics that I understand. And outside, I see a very small object somehow emitting very slowly varying radiation. and that certainly goes against most of the conventional wisdom that I would say,

that's how you can tell about time variability of stars and other things. Yeah, I know. I know that there's certain size. I know. I know. It goes against what you usually use in astrophysics or in, or in, even in particle physics. Yeah, but if you think, but I'm sure, I'm sure,

Orange said, if you think carefully, you're going to agree with that. Because there is another dimension there. Which isn't there in astrophysics. A star is a star is. Well, it's another dimension inside the horizon. But once again, I think we all agree that you can't see what goes on inside the horizon. Suppose you have a window of a building, okay? And in this window, you see something oscillating, okay?

The light going on and off, okay? Now, nothing prevents you. The size of the window doesn't limit the, the period of those oscillations. It does not. It depends on the size of the switch. I think you're allowing the switch to be arbitrarily big and the size of the window

to be arbitrarily small. Sure, if I have a little hole in a window, I can see it also. It doesn't matter how big the hole is. I can certainly see slow oscillations. But that's because my switch and whatever is turning it on or off inside is large. And I just don't know if I can buy that.

Anyway, it's something I have to come to grips with. Let's leave it as we I see, for the moment to disagree. I think I see, I see your objection, and I think it's what is intuition of many people bulk at that. But I think that it connected to what we discussed previously, the strangeness of a teeny, teeny object inside which there's a huge volume. Yeah, no, no, I agree that it's, I agree with everything you said up to the last, I mean, and it is incredibly strange inside, but it shouldn't be strange outside, and that's my concern. But anyway, let's leave that. There's only one thing that, I mean, those I kind of mightily disagree with, there's only one thing in the end of your book that I have

to say, as someone who's thought a lot about dark matter, that you kind of sneak in and have your cake and eat it too. You argue that white holes could be somewhat dark matter. Let me tell you, it doesn't add up to me. Because in order to be dark matter, you'd have to have a heck of a lot of white holes, which means you'd have to have a heck of a lot of really big black holes, comparable to the mass of our universe that evaporated down at some early time to produce lots of white holes, a minuscule mass white holes,

which then add up to be the mass, comparable to the mass of galaxies and cluster of galaxies. It's wishful thinking, in my opinion, but let me just point it out that way. I understand you'd like it, and it would be lovely if we're there. A bottle of Italian wine against... I'm happy to bet a bottle of Italian wine

and a lovely Italian meal. You don't like... Because that's one of those things where I would be happy to lose, right? You don't, yeah, sure. You don't like the big bounce, right? You're not a fan of the big bounce. But this is independent of that. I think, I think in order to add up,

no, no, no, no, it's not independent. No, it's not independent. No, it's not independent of that. It is connected to that. Because if you liked the big bounce, the big black holes could be the previous universe. Yeah, yeah, yeah. But yeah, I know.

But then I think it's even weirder. so let's not get there. Okay, great. It's fascinating to think about. It's the descriptions of general relativity and what happens you follow black collar. Lovely. And I enjoyed reading.

It's an easy light read. I'm suspicious about a number of key factors, and I bet an Italian bottle of wine that it's not true, but that's okay. I hope people have seen that physicists can have discussions. And that's part of the reason I wanted to go into the details. So I think it's nice for people to see. And I did this with Roger Pindon,

too, some people may have said a little too aggressively. It physicists argue, and that's the way we make progress, and we don't always agree. Let me just conclude in the last few minutes, and I've really enjoyed the conversation. I hope you have. You know, we're both science communicators at some level now, and certainly levels of success. You spend time, so science communication is important to you. Why? It just happened.

It was not a decision. As you hinted at some point, I was, you know, writing articles for Italian newspapers as somebody suggested to put it in a book, and then people started reading my books, and it was lovely to find so many people. It sort of fell into it. Yeah, and then I loved the reaction of the people. I mean, I really like readers to... Isn't it wonderful when you find people actually read it?

Yeah, and I feel that they find something out in my books, and that's so great. Also, you know, I've been, a lot of my life has been sort of an outsider, one way or the other. Yeah. A little bit an outsider to most of humankind because, you know, it was a rebellious youth. It was a rebellious scientist. I came out with, you know, with friends with Luke Quantongravity,

many people were doing strings. So there always been a sort of, and I always had political ideas, which were not really matching with what everybody thinks. So I always felt myself a little bit disconnected from the rest of humankind. At some point, I started writing, and I got millions of people writing to me and say, wonderful... Suddenly you're the ultimate insider, right? Because you sell best-selling books.

Well, I wouldn't say that. I would hope that. But certainly not disconnected from everybody as I thought before. That's a nice feeling. It's good. Yeah, you have been. I can say, as a physicist, I've always thought of sort of, I've watched you and Lee, and I sort of see it, at least in the physics perspective,

has come up outside the mainstream, and right and proudly so. And it really was interesting to see, you know, to suddenly see, you know, most great physicists of the past world. Yeah, it was interesting to see that. Let me, let me. Most great physicists of the past were outside, right?

Yeah, yeah. You know, one of the things you write about in this book, and you mention is something I've thought a lot about because it's involved, because if one writes about physics, as well as thinks about, physics analogies are important at various levels and one of the big problems not you with you so I'm not saying it but one of the big problems that many people

who write about science have is pushing analogies beyond their domain of validity and it's want to be very careful if one's explaining things that and using an analogy to say it's an analogy and it works and there's a wonderful sentence a few sentences I wanted to just ask you to elaborate and I say making an analogy involves taking an aspect of a concept and using it in another context, preserving something of its original meaning while letting something else go in such a way that the resulting combination produces new and effective meaning. This is how the best science works. I was really intrigued by that, and it's a lovely set of sentences. And it really, I think it captures how, by the way, my cat is

meowing because he wants to eat soon, so we're going to have to end in a bit. but it's different than many people use analogies because the whole point about analogy is that you capture there, which I think is so important, is that you have to let things go. That when I make an analogy of a bed sheet for a curved universe, you know, and put a rock on it, which so many people do, it's not an exact analogy and you have to, and you have to understand where you're letting go, but what many people, too many people do is not make the point that you're, that you have to let things go and explain. what you're letting go.

And if you don't do that, you come up with bad science and bad explanations. And I think you're absolutely right. We proceed by thinking an analogy as scientists to things we know. But at some point, we have to realize those analogies break down. Yeah, yeah, I think it's a very good point. Analogies are fundamental, I think. We think in terms of analogies all the time, both when we do science and when we try to explain science. but the point about analogies is exactly what you're saying,

that they're not, they're partial, but that's not a defect. No, it's not a good analogy. Yeah, it is a good analogy because there's something in common and something different. That's what the good analogy is, right? It's a really important point. And people always stress the similarity of analogies

and not the differences. And I think it's really important if one's going to do science or communicate to stress that. So I just wanted to say, I like the discussion of treating analogies as differences. You also have throughout your writing career, and I didn't know this when I first knew you as a scientist. We didn't spend a lot of time. I haven't spent a lot of time talking science in the past. But obviously art and literature are central to your being.

And I've always argued that that's what makes science worthwhile. It's indistinguishable from art and literature. All of them cause us to have new perspectives of our place in the cosmos. Yeah, that's right. That's the way I think about it. Yeah. And I guess I wanted to ask you to elaborate. I mean, it's clear, you know, not just in this book, Dante, you, you, these are not just, this isn't just window dressing. No, it's something that is central to your being a science from your history.

Yeah, it's just embellishment. I think that the way I view science, it's a part, that's what fascinated me on science, it's part of our constant effort to get to understand the universe better. There are many ways of doing science. Some people just like solving problems. Everybody has its own way, and they're all good. I don't want to challenge the others. But for me, science is we learn more about the universe. We get new eyes to look at things.

Go on. Sorry, I didn't want to interrupt. No, and this is what makes it, that's a similarity with arts and also philosophy in a sense. So these are all efforts to give a larger perspective. And as I've written, you study Einstein and you learn how to better think about the reality, but also in Shakespeare and you learn to have better things about humankind. Of course, with all the difference, it's not the same thing. It's like an analogies, a huge difference in tools and methods and everything, which are extremely important.

And there are different ways of being human. I mean, that's the whole point. I don't think culture, I don't think Revelation is the same as science or does a Picasso painting the same as science? They're very different. No, they're not. And I just, it's the reason, the reason I relate to it so much is central to my being too. I don't know how much you know about me, but it is.

And it's one of the reasons why this podcast is the way it is. I talk to you, but I also talk to writers, actors, directors, because to me, the whole point of my foundation in much of what I've written, much of my life, is that science and culture are not separate. They're part of the same thing. They're all part of humanity's quest to have, have a new understanding of their place in the cosmos.

And that's why I try and bounce around. Yeah. And I find that the gaps, huge gaps in our understanding, but I don't find that there is a contradiction between the scientific picture of the world and the way the world is from the perspective on an artist of a writer. No, no.

I do when it comes to religion. There's complexity there. Sure, sure, sure. No, we can disagree and we can have different opinions. or some can be right, some can be wrong. I'm not saying that. Some ideas can be wrong. But I think that we should not,

we should see the compatibility between things. Oh, yeah, absolutely. And that's, we both agree on. And I wanted to celebrate that, because I kind of figured when we had this podcast, we'd have this disagreements. And I want to point out that, although we have disagreements about some things,

are fundamental pictures and what motivates us, both as scientists and writers and other things are the same in many ways. And I think that's really important. I enjoy the connections to art and literature, and I try in my own books to do that as well. And one of my books, which is about the fascination with extra dimensions, I think it was called hiding in the mirror,

was really written mostly because I want to understand the historical and artistic fascination with extra dimensions as much as science. I see it. And there was a tremendous fascination in the 18th. and 19th centuries. And it gave me an excuse to go back and look at that from, from artistic perspectives and written perspectives. But now politics, which we also, I think, share, we're both from the left initially. I'm not, I'm becoming maybe less left.

And I admire both what you did as a student and some of the other politics you've done. Politics is intruding on science right now, at least in this country and in Canada, in this country. We're both in Canada. But in the United States and in Canada. Any comments? You don't have to say anything if you don't want to, but I've written a lot, as you may be aware, because I'm concerned about the intrusion of ideology into governing science. Unfortunately, while it used to come from the right, it's now coming from the left, and I'm concerned about it. Anything you want to say about that? No, what concerns me more and what is my political engagement right now is in a different direction,

in a different world, which is I see the world getting more and more belligerent and going toward war. So I see a war coming out with China, and I think the West is making mistakes. Yeah. I just was going to pick up my cat. I told you, my cat is starving,

but I want to get this point. Now I've just scared them away, so let's go. So I think that the West is making mistakes. And so this is not. being a pacifist by, you know, being naive and being ideological. I think we are, the West should realize that the world has changed and find a better way to get, get along with the rest of humankind. And it's not doing that. Oh, certainly outside. But I'm also talking about academia and inside the West.

No, no, that's what I'm saying. My attention has been more. Not being able to ask questions in universities, which I think both of you and I, we're both rebels in other ways, but, but being able to say something provocative and rebellious, should be the centerpiece of education. It shouldn't be what gets you removed from education. Do you agree? Well, I personally have never experienced any limitation so far. So both in Europe and in Canada,

I haven't been in the United States for a while after my 10 years in the 90s. So I see ideological battles. I see wars and stuff. And I do agree that there is, a radicalization that sometimes squeezes the debate. I agree. I don't think we are at the moment in which in the West there is limitation of freedom of speech. Do you think there is?

Oh, I see it all the time. But I think that's because I relate to happily for you, you avoid it. But I'm worried, and that's why I write about it. Because I think it's central to not just democracy but science. What I may say is that my, what I say is that my languages, my books have been translated in many languages. Yes. And only two countries have, I have experienced a sort of censorship from the publisher. The publishers say, no, you shouldn't say that because it's not good.

And the only two countries where this has happened are China and the United States. Interesting. Well, that will leave that. That's interesting. Maybe we'll leave that there. What I want to say is I think we both encourage rebels and young people to ask new questions. And I know you try and think outside the box. And I encourage that.

And sometimes you think outside the box you're right. Sometimes you're wrong. We can disagree on things. But I encourage people to ask questions. And that's why I do what I do. And I'm glad you're asking these questions and willing to provoke and provoke me and others. and it's been a pleasure chatting with you. I hope that you've enjoyed it as well.

Yeah, I loved it. I loved it very, very much, all aspect of this. But I think you should take care of your cat. Yeah, I'm going to feed my cat now. Yeah, I feel bad otherwise. It's okay. Well, I knew the animals would be hungry. But just so you know, we talked originally about going two hours.

And you said, no, we can't. Well, we've gone two and a half, just so you know. Oh, I'm sorry for the listener. It was so much fun. And it's just like being inside a black hole or an outside of black hole. Time is different. As Einstein once said when describing relativity, if you're having a fascinating conversation, he didn't use these words.

But if you're having a fascinating conversation, an hour can seem like a minute. If you're sitting on a hot stove, a minute could seem like an hour. So I hope it was a former for us and not the latter. It's very nice. It was very nice. You take care, Carlo. And we'll see you. Thank you very much.

Bye. Bye. Okay. Okay. Bye-bye. I hope you enjoyed today's conversation. This podcast is produced by the Origins Project Foundation, a non-profit organization whose goal is to enrich your perspective of your place in the cosmos

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