# Black Holes May Not Be What We Thought | World Science Festival

## Метаданные

- **Канал:** World Science Festival
- **YouTube:** https://www.youtube.com/watch?v=bhK849bDqik
- **Источник:** https://ekstraktznaniy.ru/video/40074

## Транскрипт

### Segment 1 (00:00 - 05:00) []

a kind of radical rethinking of what a black hole actually is called fuzzballs. But the dominant perspective in the field is not the fuzzball picture right now. What do you think it would take for everyone to finally get on the same page one way or another? So I think we are at the point where if people really want to sit down and just clearly ask exactly the kind of questions I was just asking you there are enough theorems there that everything is completely boxed in — interesting and so if everybody sat down in a room it would be over. Hey everyone, welcome to today's conversation which is going to focus on black holes. An arena of physics that is rapidly developing and one that has been a focus of physicists now for oh goodness you know 50 60 years you know it depends how far back you want to go. And I'm so pleased that we have as our guest an old friend of mine, not an old friend, a friend who I've known for a long time, Samir Mather from Ohio State uh University. So, welcome Samir. So nice to see you. — Thank you. It's great to be here. — You know, I was thinking just before we got online here that you and I shared an office when we were posttocks at Harvard. That must have been what the mid 1980s or mid to late 1980s something like that. — Yeah. So, so it's going back what 40 years or so. And even back then I recall black holes were a fascination of your research. So you've kind of I mean I know you've done many things but black holes have really been a centerpiece of your research. Is that would you call that accurate? Yeah, I think that is very true. Right from the second year of grad school when I first heard about Hawkings puzzle somehow that really got me and I first worked in astrophysics for my PhD learning all this stuff and then I realized that astrophysics didn't really have all the tools required to tackle this problem. So I moved into string theory and then you know putting all the tools we have from various places it's just been a fascinating and long ride the long — it's fantastic you know and you know among your most famous works is an idea that I'd like to get to in due course in this conversation a kind of radical rethinking of what a black hole actually is and so we'll get there the idea is called fuzz balls in case any of our audience have read ahead and are aware of this development. But I should say at the outset that if true this is a profound upheaval in our understanding of one of the most mysterious objects to emerge from fundamental physics. One that we now know is actually real. We have data that shows these things are real. So let's kind of step through things a little bit. um in the classical non-quantum mechanical approach to black holes comes right out of the general theory of relativity. It's a very simple process, right? You take any mass according to general relativity. You squeeze it down spherical mass to a sufficiently small size. There's a little formula for it that you can look up and that's how you get a black hole. So, were you taken by the simplicity of this weird object coming from this very straightforward algorithm? — Well, absolutely. I mean, the way you said it is exactly what happens. It's the real miracle of gravity because gravity is attractive. Suppose you take a star and you don't even have to squeeze it. Suppose it just exhausts all its fuel. So, it doesn't have the pressure to stay up against gravity, it starts shrinking. Yeah. And if the star was heavy enough, then it's a runaway process. The more it shrinks, the denser it gets. So the more it tries to shrink. And once you get into this runaway phase, there is just no choice in classical Einstein's equations but to squeeze all the way down to a point and that means you get to infinite density. So that by itself is already very remarkable because most of the time in nature things come to a balance. you squeeze and something presses out and sort of you come to a balance somewhere but here one can show that the process is really a runaway process and you can't stop it and that's the beauty of Einstein's equations at some point it tells you that if anything were to hold this up the things which are at the surface of this squeezing object would have to move faster than the speed of light — and as long as you say that cannot happen this thing has to keep squeezing.

### Segment 2 (05:00 - 10:00) [5:00]

So, it's really that basic and you get all the way down to a point of infinite density and that is very surprising. — And so, infinite density of course can be viewed as a clue that something's not quite right with what we're describing. Anytimes infinity, it's kind of a diagnostic tool that we physicists use to test our understanding. So, how do you think about the Well, I don't want to jump the gun. You're going to give us ways of thinking about in a moment, but you know, in the classical history of black holes, what did people say when they had this embarrassing fact that the center of a black hole had infinite density? So, what people said in the beginning is what we'll see doesn't seem to be true now. But what they said in the beginning was the following. Indeed, we don't want something to be infinite because then we can't even use any mathematics to describe it. But then we have quantum mechanics in the real world and quantum mechanics makes everything a little bit fuzzy but acts on very small distances. So perhaps what we thought was a tiny point would become just the scale of what quantum fluctuations would tell us it should be. The scale is called the plank length which is 10 ^ minus 33 cm. It's really very tiny. And so that at least would remove the infinite density but it wouldn't really change much about the picture of the black hole. And the important picture for what we are going to discuss is that everything has gone to the center whether center is a point or whether it's just a plank length in size the whole black hole is actually a lot bigger. So if you make a black hole with the mass of the sun its boundary would be about 3 km in radius. And so that's huge. And so what the picture that people had was everything would be just empty because everything got sucked in and everything would be down not exactly to a point but perhaps within a plank distance of a central point. And the important point was that the boundary of this which we call the horizon around there is nothing. It's just empty. So the picture we're trying to actually get which we got from classical physics which is the one we'll see leads to a problem is the one where there is nothing at the horizon or inside the horizon till you all the way till you get right near the center. So, in a way, black holes, as mysterious as they may be, are incredibly simple in their ultimate description because what you're describing is independent of the detailed features of the matter that may have collapsed downward. Once it gets into this runaway process, the kind of end state is pretty much the same. If you give the mass, you know, other small details like charge and angular momentum that we can put to the side, but if you give the mass, then every black hole the same mass is pretty much the same. Absolutely. And I think that really fascinated people in the beginning because if everything goes to a point and then you all you have left is the gravitational field of this object, you get something which is just completely round with no features. And this was such a striking uh phenomena that John Wheeler named this with the statement black holes have no hair. They just have no features at all. They completely bald. And so people started thinking of this as a fundamental theorem that black holes would really never have any features at all. Doesn't matter what you start with. Once you make this little point in the center, all the features are gone and you always see the same thing who's only described by the gravitational field and the field is determined by only one number, the total mass of the object. — Right. Yeah. I remember just as a small footnote, I believe, if I'm remembering correctly, you and I and a student worked on trying to find black hole solutions that did have some features on the outside. — That's right. — And we did sort of found them, but they were unstable. you sort of kick the system and it all dissipates away as I recall years and years ago. But yeah, so there's no distinguishing characteristics between one of these objects and the other. They just have their mass. And so it was a kind of beautiful clean story until people began to think about this a little bit more deeply. And I guess uh Jacob Beckenstein and Stephven Hawking famously began to question what happens when you start to bring a little bit of thermodynamics or quantum mechanics as the terminology is into the story. So can you just sort of give us a sense of the new ideas that began to emerge say in the 1970s as people began to probe black holes yet more fully. — Yeah. So around 1972, Beckinstein uh who

### Segment 3 (10:00 - 15:00) [10:00]

I believe was at Princeton working with Wheeler — uh he started asking this question which goes back to what you said a minute before. If you can make a black hole out of lots of different things then all those things they have somehow gone into the black hole. So there must be many possible configurations of the black hole somewhere inside there. We don't quite know where they are, but should there be a way of counting how many configurations there can be for a black hole with a given mass? Let's say the mass equal to one solar mass. And it's very hard to pose the question because if you ask the same thing for a box of gas, you could say, well, here are the atoms. I could put them here or there and you could count. And the number of counts, the number of states that we get by this counting, it goes in the name of entropy. So we know how to find the entropy of pretty much everything around us. But the black hole was a little bit mysterious because everything has gone to the center and sort of vanished from view into this infinite density singularity. So where are all these configurations? Now biggest couldn't really see where they were but he made some indirect arguments and gave an argument that the number of configurations should be given by the surface area of the black hole measured in plank units. So that would be the entropy of the black hole. And that was quite fascinating because these arguments were just based on thermodynamics which is a very universal law for all matter. And indeed on this surface there is nothing to see. It's all the vacuum. So it very mysterious to everybody. There's nothing to see anywhere. It's all in the center. And yet thermodynamical arguments are suggesting that if you want to count the number of states they are given by the area of the surface measured in blank units. So I should mention that if you do measure the area of the surface in plank units, you do get an answer for the entropy — but it's a huge number. — Yeah, — it's much bigger than the entropy of let's say a star which collapsed to make that black hole. So a lot of new puzzle started from right there. What is actually going on inside the black hole? How does it have so many states? Where are all those states? Are they all hiding in this infinite density point near the center? Are they somehow on the surface or are we just thinking about this whole idea of entropy in a wrong way? — And just quickly, this is the antithesis of the no hair theorem which seems to say there's no states because they're all the same. They all look identical. — Exactly. You can see the conflicts already emerging. On the one hand, you want the black hole to have many internal states because after all, you could have made the black hole in many ways. And then thermodynamics tells you yes they should have an entropy and the number is even larger because if it's given by this kind of calculation it's an even larger number bigger than the number of ways you could have imagined making the black hole from ordinary matter and yet there's nothing to see black holes have no hair so actually the field was quite split on this some people were saying argument doesn't make sense it's just like a postulate but there aren't really any states some people were saying that no uh maybe the number states inside the black hole is potentially infinite. Some people were saying it's zero and Pakistan's number. So things were all over the place. — And then Hawking came along and I think if I remember the history corre correctly, Hawking was very skeptical of what Beckenstein was saying. He sort of set out to prove him wrong. — That's right. — But that's not where it went. Yeah, — that's right. uh so he was very skeptical in the beginning and then he came across a very interesting calculation of his own which strongly supported what Beckistan was saying. So what Hawking found was that if you look at this gravitational field produced by the black hole this is all that you have all the masses in the center and all you have is its gravitational effect. Then if you look uh at this grav at just this empty space the vacuum actually becomes unstable when the gravitational field is so strong. And so the instability is very interesting like imagine a piece of spaceime near the horizon which just looks like a vacuum in this classical picture. But if you look at quantum effects then what would really happen is that empty space just starts bubbling out with particles very slowly but particles just emerge just out of this vacuum. And if you ask where's the energy for all that coming from it's somehow being extracted out of the energy which was in the black hole gravitational field of the black hole. So the black hole slowly loses energy. Energy is same as mass by E= MC². the black hole is losing energy and these particles are just popping out of the vacuum and then drifting off to infinity and so the black hole is slowly evaporating. So it's a very mysterious phenomena because of course everything in the world can evaporate. You can have a glass of water and if you leave it out it evaporates. You can have a star just burns away and sends photons out to

### Segment 4 (15:00 - 20:00) [15:00]

infinity. It could you could just imagine it evaporating away. The difference here is that as this black hole is evaporating, the particles which are coming out are not coming out of the material which made the black hole. They're coming out of the vacuum because all the material which made the black hole all went to the center. But this welling up of stuff from the vacuum is happening closer to the horizon. — Yeah. — Which is quite far away from the center. It's 3 km away. Had nothing to do with the center. — Right. And in fact, — and so it's somehow evaporating by a different process. So and one point on that we don't have to dwell on it but because it takes so long for this radiation to come out 3 kilometers is even an underestimate in some sense of how far away the radiation is from the singularity the center because there's a time element as well. So this is ex in space time the distance between that radiation and the singularity is enormous. — Absolutely. So it takes an extremely long time for the black hole to evaporate. It's much longer than the age of the universe for a solar mass black hole. And so some of we are talking of distances which are enormous compared to the plank length where quantum mechanics would possibly come and change anything. But the whole net effect of this was that when Hawking noticed the black hole was radiating he went back to Beckerstein's idea of thermodynamics because in thermodynamics things have entropy and once they have entropy they must have a temperature and if they have a temperature they must radiate. So there are laws which relate all these things to each other in rather inflexible ways. Yeah. — And what Hawking found was indeed the rate of radiation he was finding and the whole spectrum of the radiation which he was getting was exactly in accord with thermodynamics and fitted very beautifully with Beckerstein's idea of entropy. So the whole idea of black hole thermodynamics emerged from here once you add quantum mechanical effects onto the classical picture of the black hole together with this what we call semiclassical gravity little bit of quantum mechanics added onto the classical picture of Einstein we get a beautiful thermodynamics of black holes where black holes have an entropy they have a temperature they radiate at that temperature and in some way it all seems to make sense. Yeah. And just colloquially speaking, Hawking liked to frame it as black holes ain't so black, right? Because once radiation is coming out from them, all of a sudden they're looking like a more conventional object like you say that is slowly burning off its surface and the material is going to infinity. But there's a catch here which is the vital one. uh and it has to do with the information content of this radiation. So this is something now we've been struggling with since the 1970s and perhaps with ever greater focus in recent decades. Just give us a feel for what the issue here is. — Yes, as I was saying this discovery of Hawking that black holes radiate is now called Hawking radiation. This was in 1974. And initially he was very happy with it because you got this nice consistent picture of black hole thermodynamics. But right the next year 1975 he realized there was actually a very serious problem. And the problem was the following. This radiation was as we said before just welling out of the vacuum because the strong gravitational field in the vacuum. If something comes out of the vacuum it has no information. It is featureless because the vacuum had featureless. So the energy balance was working out fine. The black hole's energy went down. The energy was all collected in radiation which showed up at infinity. But the information in the black hole which told you what the black hole was made of now that's not in the radiation. So if the black hole is gone and now you're left with the radiation, well good. You have recovered all the energy but you have lost all the information. And the reason this is so bizarre is that it is not possible to write down any equations in physics where you lose information. Even in classical physics, if I give you the equation for bouncing a ball off this floor and hitting the ceiling, I can look at the final state of the ball and trace it back through the equation and say, "Hey, this is where it came from. " And the same is true in quantum mechanics. If I just look at the Schroinger equation, if I look at the wave function at a later time, I can trace it back and say, hey, it came from the same from this initial wave function. But now you can't do that anymore because suppose you make the black hole in one way with one kind of a star and suppose you could make it a different way with a different material making up the star, but they had the same total mass. When they evaporate, the radiation which comes out by Hawkings process has lost all information of what you started with. So there is no possible law of physics that you could write down which would take you from this initial configuration where you had a star it made a black

### Segment 5 (20:00 - 25:00) [20:00]

hole the black hole evaporated the whole process if the end part doesn't know what the initial state was no equation can describe this and this was very shocking because we really believe that physics should be described by equations — and now suddenly we can't describe it by at least the equations of quantum mechanics that we know and so we are completely lost so Hawkings claim was that the process of black hole formation and evaporation destroys quantum mechanics — which is a huge statement. I mean quantum mechanics has been around since however you want to calculate it. You could say 1900 1905 or 1927 whichever date you want to and it has always worked. There's no example in which a prediction of quantum mechanics has not correctly described the data. But now in this esoteric realm of black holes and Hawking radiation coming from them, Hawking is saying that if you push quantum mechanics to that extreme, it does break and so what did people say about this idea? So I must say that people were very shocked and they didn't know what to say but the calculation was very simple and it was clear and so uh the field actually split again. Some people actually went along with Hawking and said yes we indeed lose quantum mechanics. We have to go back to the drawing board and start with a whole new process of describing physics where losing information which is sort of described by having entropy some kind of disorder is built into our equations in a fundamental way. So thermodynamics and basic physics which somehow be combined and some new kinds of equation should emerge. People tried that. It wasn't very easy to do. It wasn't very successful but that was one set of people. — Did you try that at all? approach yourself? — Uh, no. I did study it a little bit, but it didn't seem to make any greater mathematical sense. — So, no, I haven't done much with that. In fact, what people found was if they tried to do that and they try to mix this ideas of entropy with the with quantum mechanics, you end up violating energy conservation. — And that is something you really don't want to give up. So, that's why that sort of progress in that direction sort of stalled. But the other set of people what they did was they said well if we have to save quantum mechanics we can do it in the following way. Let's assume the black hole evaporates become smaller smaller but when it gets down to a very small size which is of order the plank length then quantum effects will become essentially quantum gravitational effects because that's scale of quantum gravity. And since we don't really understand quantum gravity, maybe there's some magic in quantum gravity that says, well, don't evaporate any further. Let's just stop here. That kind of an object is called a remnant. So if you say that the whole evaporation process doesn't lead to the black hole evapting away like Hawking thought, but stops when the black hole becomes this tiny remnant, in one way you have gotten away from your puzzle because you could say all the information of the star is now locked up in this tiny remnant. So you haven't really lost it. Now it's very hard to get out of the remnant or to see what it's doing there by still locked up in that remnant. So that's called a remnant scenario. And a lot of people actually believe that because there was nothing else they could do, — right? — But there was a lot of challenges with the remnant scenario. And most people don't really like it. And the reason is this that small remnant has to be plank size because that's where quantum gravity is going to kick in. But you could have started with a black hole of any size. this big or that big. So the amount of information you can trap in that remnant is really infinite. Now in no normal theory of physics can you put an infinite amount of information in a finite volume at finite energy. — Yeah. — And so you would have to change your physics in some drastic way anyway. So it wasn't a very good solution. Yeah. And uh other directions though that people went were to try to think of more exotic ways in which quantum mechanics would not break and yet the information might in some way, shape or form actually come out of a black hole. What of those approaches? what do you which struck you as the most promising way to go? — So I must say one thing here on which I will probably touch upon later again when we talk about the story in string theory. A lot of people had the following thought. Uh a black hole as we just said emits a large number of particles before it evaporates. So suppose the black hole wasn't exactly the vacuum but there was some small corrections to this vacuum. So now you're almost having Einstein's theory

### Segment 6 (25:00 - 30:00) [25:00]

because Einstein theory said it's just this featureless vacuum. We've made just a very tiny correction to it. But suppose this tiny deformations of the spacetime could introduce delicate correlations among this very large number of emitted particles. Then in these delicate correlations perhaps we might be able to encode all the information of the matter in the black hole into the outgoing radiation because Hawking had assumed that the spacetime was just exactly the vacuum. And if a small change to the vacuum could encode the information in what was going out then you could have your cake and eat it too. You're close to your classical physics because you've almost got empty space and yet you've got your information encoded in delicate correlations among the emitted particles. So a lot of people tried this in various variants. Okay. Uh — and I remember you argued convincingly that it did not work as I recall. — So interestingly in 2009 I proved a theorem using some results in quantum information theory. — Yeah. — That you can't do this. — Yeah. And so you can actually prove the following that if you make the corrections let's say of 1% strength compared to the strength of the background gravitational field you can only recover 1% of the information. Now that of course doesn't help. So it shows you that you must make a complete change to the black hole to get out of here the puzzle. — And what was the reaction in the community to that argument? I think a lot of people were very confused because a lot of people even in the string theory community had been somehow pinning their hopes on this idea of small corrections because on the one hand in string theory people had believed that information would be recovered. — Yeah. And the reason for that belief went to the fact that string theory is something which actually allows us to understand quantum gravity uh almost completely clearly because you can just see that there isn't any mysteries hiding at the plank scale. At the plank scale you just have a few strings, you have a few brains, you know how they interact, you know how to count them, you know what to do with them. So the mysteries of the plank scale were no longer mysteries because we had a very good theory for them. And then Strowmaner and Waffa building upon some work of Suskind and S they even showed very beautifully that if you try to make a big bunch of strings and brains and you actually try to count how many states that gives you find its entropy. It exactly matches back to the number that Beckenstein had postulated. So somehow it seemed to be that string theory and these strings and brains are really describing black holes beautifully. — Yeah. And just quickly for the audience hold on to that thought. So brains you're referring to the membranes the higher dimensional extended objects that accompany strings within the structure of string theory itself. So strander and vafera they took strings and various membranes and intertwined them in just the right way to create something that looked like a black hole but now you could count the number of configurations because you knew the ingredients that you used to build it up in the first place. — Exactly. So the whole difference between string theory and all the theories of particle physics that came before that is that normally in particle physics you just have pointlike particles and all they can do is buzz around. — With string theory you get what are called extended objects like a string can stretch. A brain is a two-dimensional sheet which can stretch. And once you start taking bound states binding together objects which can stretch you get a much larger number of configurations that you can make with the same energy. It's only with this much larger number of possible configurations that you can actually match onto the Beckistan entropy. In fact, there's a you can easily prove that if you had any theory of just point particles, you could never get to the large number that Beckistan had postulated for the entropy of a black hole. And so the fact the entropy matched exactly down to the last factor of two in Strumman Wafa's work with the prediction of Beckenstein, it was a remarkable success for string theory that here we have a theory which has really no free parameters. It just tells you a number. This has to be the answer and it's exactly on the dot down to the last factor of 2 and pi. It really validates the idea that this is the right way to think about quantum gravity. — Yeah, that's an important point that I try to emphasize to people. Well, I don't know if it always gets through that. Yeah, we don't have experimental evidence for string theory, but the deep mathematical consistency of a theory based on strings and membranes with other ideas that predated string theory had nothing to do with string theory that just emerged from the careful study of black holes and thermodynamics, which are these universal qualities. The consistency between the two isn't quite an experimental verification, but it's pretty compelling. — Very compelling. Yes. and so and so once stravafa had this beautiful way of explicitly counting the states within an object that really can be thought of as

### Segment 7 (30:00 - 35:00) [30:00]

a black hole. We seemed to take a giant step forward toward understanding the structure and the makeup of black holes. But still the information problem puzzle persisted. And so there were approaches maybe you want to spend a moment on black hole complimentarity which is effectively what you were making reference to before is that was sort of a primary approach and then if we can change gears and talk about your alternative way of thinking about this. — Yeah. So what happened with the calculation of strumming waffa is that they did count the number of states of the black hole the number of configurations but without actually knowing what those configurations were and that might seem a little surprising but it was an indirect argument. So what happens is that in string theory you can go to a limit where you switch gravity off and in that limit it's rather easy to count the number of states of these strings. they're just strings just you know bouncing around without any gravity and you count them and then there is an argument that at least for a special class of black holes that they were working with the number of states will not change if you switch gravity back on. So this is an indirect way of counting because once you switch gravity back on which is when you really get the black hole you don't actually know what the states look like. Yeah. — Now what you really want to know is what happens when you switch gravity back on. And so at that point they said well we just get the usual classical black hole and we'll try to count its area divide units of blank length and compare with that entropy. So we hadn't really changed the picture of the black hole at that stage. But the obviously the next step to deal with was let's take the same strings and brains that storm and buffer counted when they have switched gravity off and then slightly and slowly try to increase the gravitational coupling and actually ask what happens when you make the coupling strong enough to make a black hole. What do the states look like? Now there were two obvious choices and most people had thought that all these strings and brains when the gravity is switched back on would all be sitting right in the center of the black hole at this tiny plank length little blob and everything else would be empty space just like the classical picture. — Y — but when we did that calculation we got a surprise that's not what happened. As you increase the coupling away, the gravitational strength all the way back from zero all to the value it actually has, you find this bunch of strings and brains starts fluffing up and there's something very peculiar to these extended objects. They just start fluffing up and as they fluffed up, if you notice what size they had, it was very interesting. The size was always of the order of the size of the horizon. So somehow in string theory when you try to make a black hole by squeezing things in they don't want to squeeze they always maintain a size which is at least the size of the horizon that you would have gotten from the classical theory. So then what we found was if you can never squeeze things inside you never get to the situation of runaway collapse you never actually get to the situation where the gravitational field becomes strong enough to have this welling of particles out of the vacuum. These things don't radiate in the way that Hawking thought. The entire picture of the black hole is different. In a way, it's actually much simpler because now the black hole has just been replaced by what you might call a string star. Just a bunch of strings and brains making a star-like object with a radius which is the order of what we thought for the horizon was. So the entire picture of the black hole interior has radically altered. And the reason this change is radical is because you normally thought that string theory or any theory of quantum gravity would only change your picture of how things should be at very microscopic scales. But what we are now finding is that making a change at completely microscopic scales. You can take a black hole as big as you want. But with that much mass with that many strings, the size of the ball that you end up making will always have a size which is of order the horizon size. So that's what we call the first B. — And so in that picture, how does the notion of Hawking radiation work? Is there an analog that happens? And how do you describe it in the fuzzball language? — Yes. So we could compute the radiation from this object in a few simple explicit cases. And it's very beautiful. It the rate of radiation matches exactly the rate you would have got from Hawking's calculation. But the method by which the radiation is produced is completely different. In Hawke's calculation, the radiation had emerged by being pulled out of the vacuum by the strength of the gravitational field. But now it comes out just the way any other normal body radiates. Like the way a star radiates is because the atoms near the surface of the star are in some excited state. They come down to a lower energy state and the extra energy is given off as a photon that comes to us. But now exactly the same thing was happening. A string would be vibrating. It would lose some energy and drop to a

### Segment 8 (35:00 - 40:00) [35:00]

configuration with a slightly lower vibration and the extra energy would come off as a graviton. So now it radiates just like a star would radiate. And so now there is no information puzzle because this object is radiating from the strings making it up. And so if the string had this shape, it would radiate in this way. The string had this shape. It would radiate in that way. And that's how the information is preserved when a star radiates itself because the information on the surface in the atoms is carried out by the radiation. And now similarly the information in the strings at the surface of the fuzz ball is carried out by its radiation. A — and is there an event horizon here? In other words, is there a point of no return for a fuzz ball or if you fall in can you somehow pull away? So it's very important that there is no horizon because if you squeeze things if you manage in any theory to squeeze things where they fall inside the event horizon is described as the point of no return so that even light cannot escape out from there. So even in string theory I would say we have not found any evidence that you can ever have anything traveling faster than the speed of light. And so if you could really squeeze something inside the horizon then nothing could come out and the information would again be trapped. So the really beautiful thing we are finding is that string theory somehow is very clever. It doesn't allow you to squeeze things so much that you would ever create a horizon. New effects come up just at the time that things become that dense and they start creating these fuzz balls instead. — And so what would it be like to fall into a fuzzball versus falling into a black hole? — Good. So in the classical black hole the picture is that as you drift in through the horizon you feel nothing because you're just wafted in and there's nothing there. Only when you reach the center where everything is very dense you will get pulled apart and crushed. But I think in the fuzz ball it is different. As you come near the surface of the fuzz ball, I think strings materialize out of the vacuum just outside the fuzz ball and they grab you and they tear you also apart into little strings which then join up with the strings which were already there in the entire fuzz ball and then you just become part of the fuz ball. So it's like as if you're falling into a star and like little flares reached out and grabbed you and tore you apart and then like merged you into the star and then everything gets eaten up and settles back there. So I don't think that you would really continue right away all the way through the surface of the object as if there was no problem. — And how then does this compared to another idea that was developed and is still studied the idea of a firewall. So there is this notion that came out of a famous paper with the acronym AMPs from the four authors. And in trying to really make sense of the information puzzle, they were led to the possibility that there is no real interior to a black hole. Rather, there's kind of a shell, a firewall where the old boundary, the old event horizon would be. And if you were to fall in, you'd kind of get incinerated. You you'd slam into this surface. on the surface description that I just gave, sorry to use the word surface twice, it sounds somewhat similar to the fuzzball picture. Can you sort of give us a sense of how they talk to each other, these two ideas? Right. So the fuzzball is an object like we're trying to describe the black hole and in some theory the black hole might be have a vacuum and just something in the center or it could be like a string star which is the fuzzball. So that's an actual object and actual theory and what you actually get depends on the theory. Yeah. — Now firewall is not an object. The firewall is like is a behavior. — So the firewall is trying to say if something is falling into an object which would come from some theory. If you go through smoothly through the horizon you would say there is no firewall. If you get caught up and burnt you would say there is a firewall. So the question would be phrased as does the fuzzball have firewall behavior or vacuum infall behavior. — Right? And in fact the example that the firewall people gave when making their arguments was well we expect the fuzzball to behave like a firewall. So the question is that argument correct? It's very fascinating. I do think that in the end the claim they are making that things would burn up at the surface of a fuzz ball. I feel it is correct. — Mhm. — But the argument they gave it turned out is actually not correct. So it turns out there was an internal loophole in the argument and uh because of that loophole it's actually possible to make a counter example quite easily. You can make what we call a bit model but you just imagine some dynamics for this fuz ball. Not the dynamics will necessarily come from quantum gravity because we don't even understand it completely. But you could make a toy model of something where the information comes out and yet somebody falling in notices almost nothing as he goes in. So the catch in the argument is that what they were using was prop were

### Segment 9 (40:00 - 45:00) [40:00]

properties of Hawking radiation. They were trying to turn Hawkings argument around. Hawking had said that if you have a vacuum around the horizon you lose information. So they said well if you don't want to lose information then you can't have the vacuum there. So up to here same statement as hawking. But then they went a little further and said that with adding a few extra assumptions we can prove that if something is falling in this thing which is not the vacuum will be very sharp and will burn you up. It can't be a gentle change. It has to be a sharp change. But to do that they made some extra assumptions and it turned out those extra assumptions actually violate causality. — So we actually wrote a paper myself and David to call the flaw in the firewall argument. And so we showed that their basic goal was to keep causality because if you violate causality the fact you can't get travel faster than speed of light. If you can violate causality there's never any information puzzle. You can take something from center of the black hole and you can put it outside. But there's a catch in the set of assumptions that it was implicitly violating causality. So the argument wasn't quite right. But I think the claim that you would actually a fuzball would show you firewall behavior, I do think that is likely to be the case. You can't prove it or disprove it because you don't understand enough about the dynamics of fuzz balls and string theory at this stage. And as I said, you can make a toy model of the dynamics where you don't see firewall behavior. But I think it's unlikely because the model is has to behave in a certain way. And I see no reason why fuzball should behave in that way. So I think the firewall people did us a great service by bringing to the community the idea that black holes don't have to be this featureless things. Before that, not many people were focusing on the issue. But I did want to point out the argument had a loophole in it. Uh even though it raised a lot of uh interest and I do think that it's likely that fuzzballs would behave like firewalls — and so it's very unlikely of course but can you imagine I mean now we actually use radio telescopes a consortium of them to image black holes. I mean until the event horizon telescope all the evidence in favor of black holes is very indirect either mathematical you know Roger Penrose giving mathematical arguments of the inevitability of black holes or observations of stars whipping around the center of the Milky Way galaxy giving evidence for there being a enormous black hole but when you take a photograph and you actually see you know these things are real is there any chance of a difference in predictions from the conventional story and the fuzzball story that we might one day be able to see through observation. So I think observations of black holes or the purpose of distinguishing the traditional classical black hole from fuzzballs are going to be extremely hard — and the reason is the following. — Whenever we say we actually image a black hole like we say we have an event horizon telescope that sees the event horizon of a black hole — seeing the stuff outside. Yeah. — Yeah. It's not really true because what we are seeing is the last what's called the last stable orbit of photons. It's roughly — two or three times the radius of the black hole depending on how much spin it has because anything that gets too close to the horizon actually then just spirals in. So any light that you want to get out of the black hole to us as an observation doesn't come from near the horizon but from something which is about three times further out. And the same is true for gravitational waves. When two black holes are merging, a lot of waves are produced in the vicinity of the merger. But some part of the waves, those inside this last stable orbit, they just fall into the final black hole. And only the part which is from outside for roughly twice the radius, they float out to us and that's what we see. On the other hand, there are abstract rough calculations or estimates we can do with fuz balls which suggest that the size of a fuzz ball even though it's larger than the horizon would only be of the order of a few plank lengths outside the horizon radius. And that again goes back to the fact that the plank length really is a basic length scale in quantum gravity in some way. — Sure. So uh if the and it's very interesting if you take a solar mass black hole and you have a surface which is let's say one plank length outside the horizon radius and suppose it tries to emit some light to us to say hey I'm a surface here the are my features most of the light rays emitting emerging from there they'll turn around and fall right back into the object only a light ray which is very close to being radially emitted would ever get out of there and the little angle the solid angle which can actually emerge from that close is 10 ^ -77. So only one part in 10 ^ 77 of the light coming out from that point can

### Segment 10 (45:00 - 50:00) [45:00]

ever come to us. And so I think of this as a in this context as a positive thing. Somehow there is something in the black hole which is trying to maintain a classical part of its dynamics. is trying to shield us from the full quantum gravity so that from outside it will look like a classical black hole but when you go to at the horizon radius it all becomes quantum mechanical. So this is a very radical proposal and just really to summarize for the audience you know there's no information puzzle when you burn uh coal because the radiation is coming from the surface of the coal and therefore it knows about the structure of the coal. The original problem was radiation coming from the event horizon of a black hole. If the mass of the black hole is crushed at the singularity so far away, how could that radiation carry that information? Now you're saying, hey, it isn't coming from the singularity. There's actually structure, brains, and strings that are playing the role of the coal, and that's where the radiation is coming from. And so it it restores in a sense more conventional ways of thinking about how the world works. But I think it's also fair to say and do correct me if you think I'm wrong. The dominant perspective in the field is not the fuzzball picture right now. Why do you think that is? Yes. So I should say that the dominant perspective has been changing in a way which hasn't been very clear to the public. — Yeah. So way back in uh the early 2000s people were indeed split. The people making doing fuzz balls was saying it has to be like this fuzzball while other people were saying no the thing is really like the vacuum outside. Okay. And even people doing string theory so they're really working with the same theory and still there's this difference. — But then you could turn around and ask the people who were saying it's not a fuzzball and ask them then how do you resolve the information puzzle? And so they were pinning their faith in the idea of small corrections that the black hole would really be almost like the classical black hole. It would not be like a fuzz ball which is completely different from the vacuum all the way inside but just gently different and these small gentle differences would then in a delicate way encode all the information in the radiation and so there is no problem. I think things changed in 2009 with the small corrections theorem — because a theorem is uh it's just an absolute fact. You can't get around a theorem. And so the theorem says is that if you don't have a fuzz ball, you can't solve the puzzle unless you violate one of the assumptions of the theorem. And so let's just look at what the theorem is really telling you. The theorem is saying that if you have an approximation to the old classical picture and approximation is fine. Being close to a classical picture is sort of like being in the classical picture. It is saying you cannot solve the puzzle. Not having this vacuum there is what we call a fuzz ball. So that's a fuzz ball is something which is just some mess there but not the vacuum. So the two different things are close to the vacuum or nowhere close to the vacuum. The small correction theorem tells you that the only way that you would not be able to have a fuzz ball and still get the information out is to allow longd distance non-locality in the interactions. So in some sense you have just two choices either you say that the black hole is a fuzz ball or you say that I have a smooth horizon but my theory has non-local transport of information where something inside the black hole can magically couple to something far away. Now these ideas have been thought about before in the idea of like wormholes which also appear in science fiction that you could go from here and magically appear somewhere very far away. And so by far away I mean not up to the horizon. This is really going all the way out to where the radiation is which is billions of miles away. — Yeah. So now I think once you have this theorem you really have to face some stark choices because now if you say you don't want the black hole to be a fuzzball you have to find some source of this very non-local effect in your theory and I think it's fair to say that there is no source of such non-local effects in string theory. So I think around that time and also with the work of the firewall people, the amps people around that time the view changed and people started saying well in its exact description the black hole is really like a piece of gold but so in that sense they say okay it is a fuzzball but and so what is the but

### Segment 11 (50:00 - 55:00) [50:00]

they said but there could be some approximate description in which it may still look like the classical black hole. So that would be again like trying to have your cake and eat it too. And a lot of the recent work on wormholes is sort of based on trying to have this your cake and eat it too thing. But in fact we showed in a paper in 2022 that it was not possible. So a small corality of the small correction theorem shows you that all the things that are coming out of the wormholes a lot of them were trying to say uh that you can sort of make some approximation to the fuzz ball which will behave like the semiclassical black hole near the horizon but you can actually show that's not possible and the reason is actually very simple. If there was a description by which you could take some kind of an, you know, approximate low energy dynamics and get a smooth horizon there, you could ask for the evolution of Hawkings particles in that description and you would see the pairs being created and then the small correction theorem tells you that the information is just going to be trapped. So in fact by just making a cor of the old theorem and the new theorem is called the effective small correction theorem because says you can't even make an effective description of the first ball which is going to be approximately like empty space. It actually removes a lot of the things that the wormhole people were saying. So the description is the difference is now rather stark. Either you really believe in long-d distanceance non-local effects in string theory or you have to accept fast balls. A lot of the other things which have been said which confused a lot of people in the field are just not correct. So now given these two choices I don't think it makes sense in string theory to say that there's any alternative to first pulse because then you really have to go and find this non-local effect and I don't think it's there. I have really looked and I don't think so. — How about the work of um Harlo for instance arguing that the non-local effects are somewhat hidden within the complexity of the operations necessary to actually exert the non-locality. So the reason why it's not obvious is we never consider these extraordinarily complex operations. you know, Hawkings calculations were insensitive to these complex operations and that's why they're absent in the traditional framing. Yeah, I have studied those papers in great detail and I must say that they do not make sense to me and so let me explain why. You can ask the question in a very concrete way in this fashion very complicated compared to what? So let's make a black hole of mass 10 blank length. Okay, so that's already much bigger than one plank mass. So 10^ the 5 plank masses. — Sure. — And so you make a black hole here and let's make another black hole of size same size very far away. — And the distance between these I'll keep making it longer and longer. — Let's do that. — Now there's some amount of complexity here because 10^ the 5 blank mass is a complicated black hole and you can make it 10 ^ 10 if you wanted. Okay, you can make it as complicated as you want. But let's make the distance between them longer and longer. Let that be the dominant length scale in the entire argument. And now let us ask everybody the same question. Is there any effect between these two different pieces of you know strings here and strings there? Is there some new effect between them which would surprise somebody who was just doing ordinary textbook quantum mechanics? — Yes. — And now you get different answers from different people. Some people are telling you yes there are effects in gravity between this bunch of strings and that bunch of strings even though the difference between them is arbitrarily large that is real longd distance non-locality yeah okay now some people are saying no there are no such effects in string theory but I will do a very complicated operation here okay I'm sorry they would say there is an effect in the following sense if I do a very complicated operation on the brains on this side I can actually influence something and now that would really be a real non-local effect because normal physics doesn't have such effects. — And now you come to a very simple contradiction. You can describe these same strings by using what is called a gauge theory. That's the gauge gravity duality of Maldosina. So now you could describe them by a field theory which is just an ordinary field theory with a certain number of degrees of freedom. And now away there on the other side you also have some brains with some driven by some field theory. And now if you ask somebody in this field theory language if I do something very complicated with one field theory does it really do something to the other field theory out there? And now everybody says no. And

### Segment 12 (55:00 - 59:00) [55:00]

now you have a problem. If you believe in gravity duality the gravity is nothing but field theory in a different language just a change of variables. In one language we call it open strings. called it closed strings. In terms of brains where you have just open strings and a free theory language, nobody's arguing for any new effect. — Mhm. — Because if they did, the people who do field theory would be at their throats. Like we never seen any non-local effects like this in field theory. But if field theory and gravity are just a change of variables, how can something happen with these brains on this side when the other brains are billions of miles away? What effect is this? It's not there in one language. How is it there in the other language and I have talked to everybody and at this point nobody can get across it. So the whole wormhole paradigm I think was a confusion. People got confused among different people were saying different things and they somehow ended up confusing each other in my opinion. So in fact I wrote a set of lectures at the Sims program we had uh last fall which explains how four different groups got confused having four different similar sounding but actually different thoughts. So there are some people who were coming from what are called ensemble average theories. is called the SYK model where you don't actually have a pure theory but a theory which is statistically averaged and people took results from there where you do get non-local correlations but because of statistical averaging and they applied it to gravity which is not an ensemble average theory the issue got further confused because some people think that gravity should be ensemble averaged as per the old work of Coleman atal where you actually have wormholes from everywhere to everywhere so there were different groups coming from different sides and they somehow confused each other. But if you put everything down on paper at the same time and ask can we find any derivation of the kind which has been claimed that somehow we can prove the information comes out or non-locality. I could not extract a coherent picture or any argument where you could get anything out of the entire one paradigm. — And so what do you think it will take to get consensus? I mean this is a problem that people have been kicking around for a long time and certainly there's been a lot of progress you know 29 2009 till today. What do you think it would take for everyone to finally get on the same page one way or another? — So I think we are at the point where if people really want to sit down and just clearly ask exactly the kind of questions I was just asking you I have some brains here, here. What are you saying will happen? There are enough theorems there that everything is completely boxed in. The small correction theorem and the effective idea of ads CFT and what we know from string theory and the calculations of fuzz balls. If we just put all of these on the board at one place, there's really no way to escape. So I think it's not like there is something to be done or to be found. I think the puzzle is over. But I think it's a little bit like what happened two centuries ago with the second law of thermodynamics and even the first law of thermodynamics. People kept trying to build perpetual motion machines of the first kind and the second kind. — 200 years after we understood all about what can and cannot be done. I think today if somebody sits down clearly understands all the information which is out there clearly all the theorems all that is known from string theory all that's known from you know about black holes there is nothing to be found it's just the clarity just needs to we just have to be clear — interesting — and so if everybody sat on a room it would be over well maybe we should get them all in a room well look obviously this is a fascinating subject and uh you If indeed things turn out the way that you envision, you you'll have rewritten the rules of black hole physics, which is a non-trivial contribution to human understanding. So, uh it's a thrilling prospect, and we'll just have to see where it all goes from here. So, maybe at some point, maybe a year from now, we should get on another one of these conversations and see whether things have gone in that direction, which of course would be enormously exciting. In any event, Samir, so great to see you after all these years and I've really enjoyed this conversation and looking forward to crossing paths sometime soon. — Thank you, brother. This was great. — My pleasure. — Thank you.