Janna Levin: Black Holes, Wormholes, Aliens, Paradoxes & Extra Dimensions | Lex Fridman Podcast #468

Black holes curve space and time around them. In the way that we've been describing, things fall along the curves in space. If the black holes move around, the curves have to follow them, right? But they can't travel faster than the speed of light either. So what happens is, as black holes, let's say, move around, maybe I've got two black holes in orbit around each other. That can happen. It takes a while. A wave is created in the actual shape of space, and that wave follows the black holes. Those black holes are undulating. Eventually, those two black holes will merge. And as we were talking about, it doesn't take an infinite time, even though there's time dilation, because they're both so big. They're really deforming spacetime a lot. I don't have a little tiny marble falling across an event horizon. I have two event horizons. And in the simulations, you can see it bobble, and they merge together. They make one bigger black hole. And then it radiates in the gravitational waves. It radiates away all those imperfections, and it settles down to one quiescent, perfectly silent black hole that's spinning beautiful stuff. And it emits E equals MC squared energy. So the mass of the final black hole will be less than the sum of the two starter black holes. And that energy is radiated away in this ringing of spacetime. It's really important to emphasize that it's not light. None of this has to do literally with light that we can detect with normal things that detect light. X rays, formal light, Gamma rays are a form of light. Infrared, optical, all this whole electromagnetic spectrum, none of it is emitted as light. It's completely dark. It's only emitted in the rippling of the shape of space. A lot of times it's likened closer to sound. Technically, we've kind of argued. I mean, I haven't done an anatomical calculation, but if you're near enough to two colliding black holes, they actually ring spacetime in the human auditory range. The frequency is actually in the human auditory range that the shape of space could squeeze and stretch your eardrum, even in vacuum. And you could hear, literally hear, these waves ringing.

The following is a conversation with Jana Levin, a theoretical physicist and cosmologist specializing in black holes, cosmology of extra dimensions, topology of the universe, and gravitational wa in space time. She has also written some incredible books, including how the Universe Got Its Spots. On the topic of the shape and the size of the universe, A madman dreams of touring machines. On the topic of genius, Madness, and the limits of knowledge. Black Hole Blues and Other Songs from Outer Space. On the topic of LIGO and the detection of Gravitational waves and Black Hole Survival Guide all about black holes, this was a fun and fascinating conversation. This is the Lex Friedman podcast. To support it, please check out our sponsors in the description. And now, dear friends, here's Jenna Levin. I should say that you sent me a message about not starting early in the morning, and that made me feel like we're kindred spirits. Yeah, you wrote to me when the great physicist Sidney Coleman was asked to attend a 9am meeting, his reply was, I can't stay up that late.

Yeah, classic Sidney was beloved.

I think all the best thoughts, honestly, maybe the worst thoughts too, are all come at night. There's something about the night. Maybe it's the silence, maybe it's the peace all around, maybe it's the darkness and you just, you could be with yourself and you could think deeply.

I feel like they're stolen hours in the middle of the night because it's not busy. Your gadgets aren't pinging. There's really no pressure to do anything. But I'm often awake in the middle of the night. And so it's sort of like these extra hours of the day. I think we were exchanging messages at 4 in the morning. Okay.

So in that way, in many other ways, we're kindred spirits. So let's go in. One of the coolest objects in the universe, black holes, what are they? And maybe even a good way to start is to talk about how are they formed.

Yeah, in a way. People often confuse how they're formed with the concept of the black hole in the first place. So when black holes were first proposed, Einstein was very surprised that such a solution could be found so quickly, but really thought nature would protect us from their formation. And then nature thinks of a way, nature thinks of a way to make these crazy objects, which is to kill off a few stars. But then I think that there's a confusion that dead stars, these very, very massive stars that die, are synonymous with the phenomenon of black hole. And it's really not the case. Black holes are more general and more fundamental than just the death state of a star. But even the history of how people realize that stars could form black holes is quite fascinating because the entire idea really just started as a thought experiment. If you think of it's 1915, 1916, when Einstein fully describes relativity in a way, that's the canonical formulation. It was a lot of changing back and forth before then. And It's World War I and he gets a message from the Eastern front, from a friend of his, Carl Schwarzschild, who solved Einstein's equations between sitting in the trenches and like cannon fire, it was joked that he was calculating ballistic trajectories. He's also perusing the proceedings of the Prussian Academy of Sciences, as you do. And he was an astronomer who had enlisted in his 40s. And he finds this really remarkable solution to Einstein's equations. And it's the first exact solution. He doesn't call it a black hole, it's not called a black hole for decades. But what I love about what Schwarzschild did is it's a thought experiment. It's not about observations, it's not about making these things in nature. It's really just about the idea. He sets up this completely untenable situation. He says, imagine I crush all the mass of a star to a point. Don't ask how that's done, because that's really absurd. But let's just pretend and let's just imagine that that's a scenario. And then he wants to decide what happens to spacetime if I set up this confounding but somehow very simple scenario. And really what Einstein's equations were telling everybody at the time was that matter and energy curve space and time, and then curved space time tells matter and energy how to fall. Whence is spacetime shaped? So he finds this beautiful solution. And the most amazing thing about his solution is he finds this demarcation, which is the event horizon, which is the region beyond which not even light can escape. And if you were to ask me today, all these decades, over a hundred years later, I would say that is the black hole. The black hole is not the mass crushed to a point. The black hole is the event horizon. And the event horizon is really just a point in spacetime or a region at spacetime. It's actually, in this case, a surface in spacetime, and it marks a separation in events, which is why it's called an event horizon. Everything outside is causally separated from the inside, insofar as what's inside the event horizon can't affect events outside. What's outside can affect events inside. I can throw a probe into a black hole and cause something to happen on the inside, but the opposite isn't true. Somebody who fell in can't send a probe out. And this one way aspect really is what's profound about the black hole. Sometimes we talk about the black holes being nothing, because at the event horizon there's really nothing there. Sometimes when we think about black holes. We want to imagine a really dense dead star. But if you go up to the event horizon, it's an empty region of spacetime. It's more of a place than it is a thing. And Einstein found this fascinating. He helped get the work published. But he really didn't think these would form in nature. I doubt Carl Schwarzschild did either. I think they thought they were solving theoretical mathematical problems, but not describing what turned out to be the end state of gravitational collapse.

And maybe the purpose of the thought experiment was to find the limitations of the theory. So you find the most extreme versions in order to understand where it breaks down.

Yeah.

And it just so happens in this case, that might actually predict these extreme kinds of objects.

It does both. So it also describes the sun from far away. So the same solution does a great job helping us understand the Earth's orbit around the sun. It's incredible. Does a great job. It's almost overkill. You don't really need to be that precise as relativity. And yes, it predicts the phenomenon of black holes, but doesn't really explain how nature would form them. But then it also, on top of that, does signal the breakdown of the theory. I mean, you're quite right about that. It actually says, oh, man. But you go all the way towards the center. And, yeah, this doesn't sound right anymore. Sometimes I liken it to, you know, it's like a dying man marking in the dirt that something's gone wrong here. Right. It's signaling that there's some culprit, there's something wrong in the theory. And. And even Roger Penrose, who did this general work, trying to understand the formation of black holes from gravitational collapse, he thought, oh, yeah, there's a singularity that's inevitable. It's in every. There's no way around it once you form a black hole. But he said this is probably just a shortcoming of the fact that we've forgotten to include quantum mechanics and that when we do, will understand this differently.

So according to him, the closer you get to the singularity, the more quantum mechanics comes into play, and therefore there's no singularity. There's something else.

I think everybody would say that. I think everybody would say the closer you get to the singularity, for sure, you have to include quantum mechanics. You just can't consistently talk about magnifying such small scales, having such enormous ruptures and curvatures and energy scales and not include quantum mechanics. That. That's just inconsistent with the world as we understand it.

So you've described the brain breaking idea that a black hole is not so much a super dense matter as it's sometimes described, but it's more akin to, you know, a region of space time, but even more so. Just nothing. Yeah, it's nothing. That's the thing you seem to like to say.

I do, I do like to say that black holes are. No, nothing.

Okay, so what does that mean?

That's what I mean. That's the more profound aspect of the black hole. So you asked originally, how do they form? And I think that even when you try to form them in messy astrophysical systems, there's still nothing at the end of the day left behind. This was a very big surprise. Even though Einstein accepted that this was a true prediction, he didn't think that they'd be made. And it was quite astounding that, that people like Oppenheimer, actually, it's probably Oppenheimer's most important theoretical work who are thinking about nuclear physics and quantum mechanics. But in the context of these kind of utopian questions, why do stars shine? Why is the sun radiant and hot and this amazing source of light? And it was people like Oppenheimer who began to ask the question, well, could stars collapse to form black holes? Could they become so dense that eventually not even light would escape? And that's why I think people think that black holes are these dense objects. That's often how it's described. But actually what happens, these very massive stars, they're burning thermonuclear fuel. You know, they're earthfuls of thermonuclear fuel. They're burning and emitting energy in E equals MC squared energy. So it's fusing, it's a fusion bomb, it's a constantly going thermonuclear bomb. And eventually it's gonna run out of fuel, it's gonna run out of hydrogen, helium, stuff to fuse. It hits an iron core, iron. To go past iron with fusion is actually energetically expensive. So it's no longer going to do that so easily. So suddenly it's run out of fuel. And if the star is very, very, very massive, much more massive than our sun, maybe 20, 30 times the mass for the sun, it'll collapse under its own weight. And that collapse is incredibly fast and dramatic. And it creates a shockwave. So that's the supernova explosion. So a lot of these, they rebound because once they crunch, they've reached a new critical capacity where they can reignite to higher elements, heavier elements. And that sets off a bomb essentially. So the Star explodes helpfully, because that's why you and I are here, because stars send their material back out into space, and you and I get to be made of carbon and oxygen and all this good stuff. We're not just hydrogen. So the suns do that for us. And then what's left sometimes ends at a neutron star, which is a very cool object, very fascinating object, super dense, but bigger than a black hole, meaning it's not compact enough to become a black hole. It's an actual thing. A neutron star is a real thing. It's like a giant neutron, literally. Electrons get jammed into the protons and make this giant nucleus in this superconducting matter. Very strange, amazing objects. But if it's heavier than that, the core, and that's heavier than twice the mass of the sun, it will become a black hole. And Oppenheimer wrote this beautiful paper in 1939 with his student saying that they believed that the end state of gravitational collapse is actually a black hole. This is stunning and really a visionary conclusion. Now, the paper is published the same day the Nazis advance on Poland. And so it does not get a lot of fanfare in the newspapers.

We think there's a lot of drama today on social media. Imagine that. Yeah. Here's a guy who predicts how actually in nature would be the formation of this most radical of object that broke even Einstein's brain, while one of the most evil, if not the most evil humans in history, starting the first steps of a global war.

What I also love about that lesson is how agnostic science is, because he was asking these utopian questions, as were other people of the time, about the nuclear physics and stars. You might know this play, Copenhagen, by Michael Frayn. There's this line that he attributes to Bohr, and Bohr was the great thinker of early foundations of quantum mechanics, Danish physicist, where Bohr says to his wife, nobody's thought of a way to kill people using quantum mechanics. Now, of course, then there's the nuclear bomb. And what I love about this was the pressure scientists were under to do something with this nuclear physics and to enter this race over a nuclear weapon. But really, at the same time, 1939, really, Oppenheimer's thinking about black holes. There's even a small line in Chris Nolan's film, it's very hard to catch. There's a reference to it in the film where they're sort of joking. Well, I guess nobody's gonna pay attention to your paper now, you know, because of the Nazi advance on Poland.

That's the Other remarkable thing about Oppenheimer is he's also a central figure in the construction of the bomb. Right, so it's theory and experiment clashing together with the geopolitics.

Exactly. So of course, Oppenheimer, now known as the father of the atomic bomb, he talks about destroyers of worlds, but it's the same technology. And that's what I mean by science is agnostic. Right? It's the same technology overcoming a critical mass, igniting thermonuclear fusion. Eventually there was a fission. The original bomb was a fission bomb. And fission was first shown by Lise Meitner, who showed that a certain uranium, when you bombarded it with protons, broke into smaller pieces that were less than the uranium. Right. So some of that mass that E equals MC squared energy, had escaped. And it was the first kind of concrete demonstration of this, Einstein's most famous equation. So all of this comes together, but the story of they still weren't called black holes. This is 1939 and they had these very long winded ways of describing the end state, the catastrophic end state of gravitational collapse. But what you have to imagine is as this star collapses, so what's the sun? The sun's a million and a half kilometers across. So imagine a star much bigger than the sun, much bigger radius, and it's so heavy it collapses, it's supernovas. What's left is still maybe 10 times the mass of the sun, just what's left in that core and it continues to collapse. And when that reaches about 60 kilometers across, like just imagine 10 times the mass of the sun, city sized, that is a really dense object. And now the black hole essentially has begun to form, meaning the curve in space time is so tremendous that not even light can escape. The event horizon forms, but the event horizon is almost imprinted on the space time because the star can't sit there in that dense state any more than it can race outward at the speed of light, because even light is forced to reign inwards. So the star continues to fall, and that's the magic part. The star leaves the event horizon behind and it continues to fall and it falls into the interior of the black hole. Where it goes nobody really knows, but it's gone from sight, it goes dark. There's this quote by John Wheeler, who's like granddaddy of American relativity, and he has a line that's something to the effect the star, like the Cheshire cat, fades from view. One leaves behind only its grin, the other only its gravitational attraction. And he was giving a lecture. It's actually above Tom's restaurant, you know, from Seinfeld, near Columbia in New York. There was a place, or there still is a place there where people were giving lectures about astrophysics. And it's 1967. Wheeler is exhaustively saying this loaded term, the end state of catastrophic gravitational collapse. And rumor is that someone shouts from the back row, well, how about black hole? And apparently he then foists this term on the world. Wheeler had way of doing that.

Well, I love terms like that. Big bang, black hole. There's some. I mean, it's just pointing out the elephant in the room and calling it an elephant. It is a black hole. That's a pretty accurate and deep description. I just wanted to point out that the. Just looking for the first time. It's a 1939 paper from Oppenheimer. It's like two pages. It's like three pages.

Oh, yeah, it's gorgeous.

The simplicity of some of these. That's so gangster. Just revolutionize all of physics with this, with. You know, Einstein did that multiple times in a similar year. When all thermonuclear sources of energy are exhausted sufficiently heavy star will collapse. That's an opener. Unless fission due to rotation, the radiation of mass, or the blowing off of mass by radiation reduce the star's mass to orders of that of the sun, this contraction will continue indefinitely. And it goes on that way.

Yeah. Now I have to say, Wheeler, who actually coins the term black hole, gives Oppenheimer quite a terrible time about this. He thinks he's wrong. And they entered what has sometimes been described as kind of a bitter, I don't know if you would actually say feud, but they were bad feelings. And Wheeler actually spent decades saying Oppenheimer was wrong. And eventually with his computer work, that early work that Wheeler was doing with computers when he was also trying to understand nuclear weapons and in peacetime world found themselves returning again to these astrophysical questions, decided that actually Oppenheimer had been right. He thought it was too simplistic, too idealized a setup that they had used. And that if you looked at something that was more realistic and more complicated, that it just simply, it just would go away. And in fact, he draws the opposite conclusion. And there's a story that Oppenheimer was sitting outside of the auditorium when Wheeler was coming forth with his declaration that in fact, black holes were the likely end state of gravitational collapse for very, very heavy stars. And when asked about it, Oppenheimer sort of said, well, I've moved on to.

Other things because you've written in many places, about the human beings behind the science. I have to ask you about this, about nuclear weapons. Whereas the greatest of physicists coming together to create this most terrifying and powerful of a technology. And now I get to talk to world leaders for whom this technology is part of the tools that is used, perhaps implicitly on the chessboard of geopolitics. What can you say, as a person who's a physicist and who have studied the physicists and written about the physicists, the humans behind this, about this moment in human history when physicists came together and created this weapon that's powerful enough to destroy all of human civilization?

I think it's an excruciating moment in the history of science. And people talk about Heisenberg, who stayed in Germany and worked for the Nazis in their own attempt to build the bomb. There was this kind of hopeful talk that maybe Heisenberg had intentionally derailed the nuclear weapons program. But I think that's been largely discredited that he would have made the bomb, could he, had he not made some really kind of simple multiple errors in his original estimates about how much material would be required or how they would get over the energy barriers. And that's a terrifying thought. I don't know that any of us can really put ourselves in that position of imagining that we're faced with that quandary, having to take the initiative to participate in thinking of a way that quantum mechanics can kill people and then making the bump. I think overwhelmingly physicists today feel we should not continue in the proliferation of nuclear weapons. Very few theoretical physicists want to see.

This continue that moment in history. The Soviet Union had incredible scientists, Nazi Germany had incredible scientists, and the United States had incredible scientists. And it's very easy to imagine that one of those three would have created the bomb first, not the United States. And how different would the world be? The game theory of that, I think, say the probability is 33% that it was the United States. If the Soviet Union had the bomb, I think they would have used it in a much more terrifying way in the European theater and maybe turn on the United States. And obviously with Hitler, he would have used it. I think there's no question he would have used it to kill hundreds of millions of people.

In the game theory version, this was the least harmful outcome. Yes, but there is no outcome with no bomb that any game theorist, I think, would play.

But I think if we just remove the geopolitics and the ideology and the evil dictators, all of those people are just scientists, I think they don't necessarily even think about the ideology. And it's a deep lesson about the connection between great science and the annoying, sometimes evil politicians that use that science for means that are either good or bad. And the scientists perhaps don't. Boy, do they even have control of how that science is used. It's hard.

They don't have control. Right. Once it's, once it's made, it's no longer scientific reasoning that dictates the use or restraint. But I will say that I do believe that it wasn't a 31/3 down the line because America was different. And I think that's something we have to think about right now in this particular climate. So many scientists fled here, they fled to here. Americans weren't fleeing to Nazi Germany. They came here and they were motivated by. It's more than a patriotism. I mean, it was a patriotism, obviously, but it was sort of more than that. It was really understanding the threat of Europe, what was going on in Europe and how quickly it turned, how quickly this free spirited Berlin culture was suddenly in this repressive and terrifying reg. So I think that it was a much higher chance that it happened here in America.

Yeah. And there's something about the American system, the, you know, it's cliche to say, but the freedom, all the different individual freedoms that enable a very vibrant, at its best, a very vibrant scientific community. And that's really exciting.

Absolutely.

To scientists. And it's very valuable to maintain that the vibrancy of the debate, of the funding, those mechanisms.

Absolutely. The world flocked here and that won't be the case if we no longer have intellectual freedom.

Yeah. There's something interesting to think about. The tension, the Cold War between China and the United states in the 21st century, some of those same questions, some of those ideas will rise up again. And we want to make sure that there's a vibrant, free exchange of scientific ideas. I believe most Nobel Prizes come from the United States, right?

Oh, yeah, I don't have the number.

But it's disproportionately so.

Disproportionately so. In fact, a lot of them from particle physics came from the Bronx and they were European immigrants.

How do you explain this?

Fled Europe precisely because of the geopolitics we're describing. And so instead of being Nobel Prize winners from the Soviet Union or from the Eastern Bloc, they were from the Bronx.

And that's the thing you write about. And we'll return to time and time again that, you know, science is done by humans and Some of those humans are fascinating. There's tensions, there's battles, there's. Some are loners, some are great collaborators, some are tormented, some are easygoing. All this kind of stuff. And that's the beautiful thing about it, we forget sometimes is it's humans, and humans are messy and complicated and beautiful and all of that.

Yeah.

So what were we talking about?

Oh, the star is collapsing.

Okay, so can we just return to the collapse of a star that forms a black hole? At which point does the super dense thing become nothing? If we can just linger on this concept.

Yeah. So if I were falling into a black hole and I tried really fast right as I crossed this empty region. But this demarcation, I happened to know where it was. I calculated, because there's no line there, there's no sign that it's there, there's no signpost. I could emit a little light pulse and try to send it outward exactly at the event horizon. So it's racing outward at the speed of light. It can hover there because from my perspective, it's very strange. The space time is like a waterfall reigning in. And I'm being dragged in with that waterfall. I can't stop at the event horizon. It comes, it goes, it's behind me. Really quickly, that light beam can try to sit there because it's like a fish swimming against a. The Niagara, you know, sitting against the waterfall. It's like stuck there, but it's, like stuck there. And so that's one way you can have a little signpost. You know, if you fly by, you think it's moving at the speed of light, it flies past you at the speed of light, but it's sitting right there at the event horizon.

So you're falling back across the event horizon. Right at that point, you shoot outwards a photon.

Yes.

And it's just stuck there.

It just gets stuck there. Now it's very unstable. So the star can't sit there is the point. It just can't. So it rains inward with this waterfall. But from the outside, all we should ever really care about is the event horizon, because I can't know what happens to it. It could be pure matter and antimatter thrown together, which annihilates into photons on the inside and loses all its mass into the energy of light. Won't matter to me because I can't know anything about what happened on the inside.

Okay, can we just linger on this? So what models do we have about what happens on the inside of the black hole at that moment? So I guess that one of the intuitions, one of the big reminders that you're giving to us is like, hey, we know very little about what can happen on the inside of a black hole. And that's why we have to be careful about making. It's better to think about the black hole as an event horizon. But what can we know, and what do we know about the physics of space time inside a black hole?

I don't mind being incautious about thinking about what the math tells us. So I'm not such a. An observer. I'm very theoretical in my work. It's really pen on paper a lot. These are thought experiments that I think we can perform and contemplate. Whether or not we'll ever know is another question. So one of the most beautiful things that we suspect happens on the inside of a black hole is that space and time in some sense swap places. So while I'm on the outside of the black hole, let's say I'm in a nice comfortable space station. This black hole is maybe 10 times the mass of the sun. 60 kilometers across. I could be 100 kilometers out. That's very, very close. Orbiting quite safely, no big deal, you know, hanging out. I don't bug the black hole, black hole doesn't bug me. It won't suck me up like a vacuum or anything crazy. But my astronaut friend jumps in as they cross the event horizon, what I'm calling space. I'm looking on the outside at this spherical shadow of the black hole. Cast by maybe light around it. It's a shadow. Cause everything gets too close, falls in. It's just this contrast against a bright sky. I think, oh, there's a center of a sphere. And in the center of the sphere is the singularity. It's a point in space. From my perspective. But from the perspective of the astronaut who falls in, it's actually a point in time. So their notions of space and time have rotated so completely that what I'm calling a direction in space towards the center of the black hole, like the center of a physical sphere. They're going to tell me. Well, they can't tell me, but they're going to come to the conclusion, oh, no, that's not a location in space, that's a location in time. In other words, the singularity ends up in their future. And they can no more avoid the singularity than they can avoid time coming their way. So there's no shenanigans you can do once you're inside the black hole to try to Skirt it. The singularity. You can't set yourself up in orbit around it. You can't try to fire rockets and stay away from it, because it's in your future, and there's an inevitable moment when you will hit it. Usually, for a stellar mass black hole, we think it's microseconds.

Microseconds to get from the event horizon to the.

To the singularity.

To the singularity. Oh, boy.

Oh, boy.

So that's describing from your astronaut friend's.

Perspective, yes, from their perspective, the singularity's in their future.

But from your perspective, what do you see when your friend falls into the black hole and you're chilling outside and watching?

So one way to think about this is to think that as you're approaching the black hole, the astronaut's space time is rotating relative to your space time. So let's say right now my left is your right. We're not shocked by the fact that there's this relativity in left and right. It's completely understood. And I can perform a spatial rotation to align my left with your left. Right now I've completely rotated left out right. If I just want to draw a kind of compass diagram. Not a compass diagram, but, you know, at the top of maps, there's a north, south, east, west. But now time is up, down, and one direction of space is, let's say, east, west. As you approach the black hole, it's as though you're rotating in space. Time is one way of thinking about it. So what is the effect of that? The effect of that is as this astronaut gets closer and closer to the event horizon, part of their space is rotated into my time, and part of their time is rotated into my space. So in other words, their clocks seem to be less aligned with my time. And the overall effect is that their time seems to dilate the spacing between ticks on the clock of their watch, let's say on the face of their watch is elongated, dilated relative to mine. And it seems to me that their watches are running slowly. Even though they were made in the same factory as mine. They were both synchronized beautifully. And they're excellent Swiss watches. It seems as though time is elapsing more slowly for my companion. And likewise for them, it seems like mine's going really fast. So years could elapse in my space station. My plants come and go. They die. I age faster. I've got gray hair, and they're falling in, and it's been minutes in their frame of reference. Flowers in their little rocket ship haven't rotted. They don't have gray hair. Their biological clocks have slowed down relative to ours. Eventually at the event horizon, it's so extreme, it's so slow. It's as though their clocks have stopped altogether from my point of view. And that's to say that it's as though their time has completely rotated into my space. And this is connected with the idea that inside the black hole, space and time have switched places. So I might see them hover there for millennia. Other astronauts could be born on my space station. Generations could be populated there watching this poor astronaut never fall in.

So basically, time almost comes to a standstill, but we still. They do fall in, right?

They do fall in eventually. Now that's because they have some mass of their own. So they're not a perfectly light particle. And so they deform the event horizon a little bit. You will actually see the event horizon bobble and absorb the astronaut. So in some finite time, the astronaut will actually fall in.

So it's like this weird space time bubble that we have around us. And then there's a very big space time curvature bubble thing from the black hole and there's a nice swirly type situation going on. That's how you get sucked up.

Yeah.

So if you're a perfect infinitely small particle, you would just be. It'd take longer and longer and probably just be stuck there or something. But no, there's quantum mechanics.

Eventually you'll fall in. Any perturbation will only go one way. It's unstable in one direction, in one direction only. But it's really important to remember that from the point of view of the astronaut, not much time has passed at all. You just sail right across as far as you're concerned, and nothing dramatic happens there. You might not even realize you've come to the event horizon. You might not even realize you've crossed the event horizon because there's nothing there. Right. This is an empty region of space time. There's no marker to tell you you've reached this very dangerous point of no return. You can fire your rockets like hell when you're on the outside and maybe even escape. Right. But once you get to that point, there's no amount of energy. All the energy in the universe will not save you from this demise.

You know, there's different sized black holes and maybe. Can we talk about the experience that you have falling into a black hole? Depending on what the size of the black hole is?

Yeah.

Because as I understand, if the, the, the bigger it is, the less drastic the experience. Of falling into it.

Yeah. That might surprise people. The bigger it is, the less noticeable it is that you've, you've crossed the event horizon. One way to think about it is curvature is less noticeable, the bigger it is. So if I'm standing on a basketball, I'm very aware I'm balancing on a curved surface. My two feet are in different locations and I really notice. But on the Earth, you actually have to be kind of clever to deduce that the Earth is curved. The bigger the planet, the less you're gonna notice the curvature, the global curvature. And it's the same thing with a black hole. Huge, huge black hole. It just kind of feels like just flat. You don't really notice.

I'm trying to figure out how the physics. Because if you don't notice and there's nothing there, but the physics is weird.

In your frame of reference.

No.

Well, so another cool thing. So I like to dispel myths.

Yeah.

Do you need a minute? You're holding your head.

There's a sense like you should be able to know when you're inside of a black hole, when you've crossed the event horizon. But no, from your frame of reference, you might not be able to know.

Yeah, at first, at least, you might not realize what's happened. There are some hints. For instance, black holes are dark from the outside, but they're not necessarily dark on the inside. So this is a kind of fascinating that your experience could be that it's quite bright inside the black hole because all the light from the galaxy can be shining in behind you and it's focusing down because you're all approaching this really focused region in the interior. And so you actually see a bright white flash of light as you approach the singularity. You know, I kind of, I joke that it's a, you know, it's like a near death experience. We see the light at the end of the tunnel. So you would see millennia pass on Earth. You could see, see the evolution of the entire galaxy. You know, one big bright flash of light. So it's like a near death experience, but it's definitely a total death experience.

It goes pretty fast. But you looking out, you looking out, everything's going super fast.

Yeah, the clocks on the Earth, on the space station seem to be progressing very rapidly relative to yours. The light can catch up to you and you get this bright beam of light as you see the evolution of the galaxy unfold. And I mean, it sort of depends on the size of the black hole and how long you have to hang Around. The bigger the black hole, the longer it takes you to expire in the center.

Obviously, the human sensory system are not able to process that information correctly.

Right. It would be a microsecond and. Right. That would be too fast.

Yeah, but it would be. Wow. It'd be so cool to get that information.

But a big black hole, you could actually, you know, hang around for some months.

So. Yeah. How are small black holes versus supermassive black holes formed just so people could kind of load that in? Is it always a star?

No. So this is also why it's important to think of black holes more abstractly. They are something very profound in the universe. And there are probably multiple ways to make black holes. Making them with stars is most plentiful. There could be hundreds of millions, maybe even a billion black holes in our Milky Way galaxy alone. That many stars, it's only about 1% of stars that will end their lives in a death state. That is a black hole. But we now see, and this was really quite a surprise, that there are supermassive black holes. There are billions or even hundreds of billions of times the mass of the sun and millions to tens of billions, maybe even hundreds of billions. So extremely massive. We don't think that the universe has had enough time to make them from stars that just merge. We know that two black holes can merge and make a bigger black hole, and then those can merge and make a bigger black hole, but we don't think there's been enough time for that. So it's suspected that they're formed very early, maybe even a few hundred million years after the Big Bang, and that they're formed directly by collapsing out of primordial stuff, that there's a direct collapse right into the black hole.

So, like in the very early universe, these are primordial black holes from the stars? Not quite. Wait, how. How do you get from that soup black holes right away?

Right. So it's odd, but it's weirdly easier to make a big black hole out of something that's just the density of air. If it's really, really as big as what we're talk some sense. If they're just allowed to directly collapse very early in the universe's history, they can do that more easily. And it's so much so that we think that there's one of these supermassive black holes in the center of every galaxy. So they're not rare. And we know where they are. They're in the nuclei of galaxies. So they're bound to the very early formation of entire galaxies in a really surprising and deeply connected way.

I wonder if the chicken or the egg, is it like, how critical, how essential are the supermassive black holes to the formation of galaxies?

Yeah, I mean, it's ongoing, right? It's ongoing. Which came first, the black hole or the galaxy? Probably big early stars which were just made out of hydrogen and helium from the Big Bang. There wasn't anything else, not much of anything else. Those early stars were forming, and then maybe the black hole black holes and kind of the galaxies were like these gassy clouds around them. But there's probably a deep relationship between the black hole powering jets. These jets blowing material out of the galaxy that shaped galaxies, maybe kind of curbed their growth. And so I think the mechanisms are still ongoing attempts to understand exactly the ordering of these things.

Can we get back to space time, just going back to the beginning of the 20th century, how do you imagine space time? How do we as human beings supposed to visualize and think about space time? Where, you know, time is just another dimension in this 4D space that combines space and time? Because we've been talking about morphing in all kinds of different ways the curvature of space time. Like, how do you. How are we supposed to conceive of it? How do you think of it? Yeah, time's just another dimension.

There are different ways we can think about it. We can imagine drawing a map of space and treating time as another direction in that map. But we're limited because as three dimensional beings, we can't really draw four dimensions, which is what I'd require, three spatial, because I'm pretty sure there's at least three. I think there's probably more, But I'm happy just talking about the large dimensions. The three we see up, down, right? East, west, north, south, three spatial dimensions, and time is the fourth. Nobody can really visualize it, but we know mathematically how to unpack it. On paper. I can mathematically suppress one of the spatial dimensions and then I can draw it pretty well. Now, the problem is that we'd call it a Euclidean spacetime. A Euclidean spacetime is when all the dimensions are orthogonal and are treated equally. Time is not another Euclidean dimension. It's actually a Minkowski in spacetime. But it means that the space time, we're misrepresenting it when we draw it. But we're misrepresenting it in a way that we deeply understand. I can give you an example. The Earth I can project onto a flat sheet of paper. I am now misrepresenting A map of the Earth. And I know that, but I understand the rules for how to add distances on this misrepresentation, because the Earth is not a flat sheet of paper, it's a sphere. And as long as I understand the rules for how I get from the North Pole to the South Pole, that I'm moving along really a great arc, and I understand that the distance is not the distance I would measure on a flat sheet of paper, then I can do a really great job with a map and understanding the rules of addition, multiplication and the geometries, not the geometry of a flat sheet of paper. I can do the same thing with spacetime. I can draw it on a flat sheet of paper, but I know that it's not actually a flat Euclidean space. And so my rules for measuring distances are different than the rules I would use that, for instance, Cartesian rules of geometry. I would know to use the correct rules for Minkowski spacetime. And, and that will allow me to calculate how long time has elapsed, which is now a kind of a length, a space time length on my map between two relative observers. And I will get the correct answer, but only if I use these different rules.

So then what does, according to general relativity, does objects with mass due to the space time?

Right, Exactly. So Einstein struggled for this completely general theory, not a specific solution like a black hole or an expanding spacetime, or galaxies make lenses or Those are all solutions. That's why what he did was so enormous. It's an entire paradigm that says over here is matter and energy. I'm going to call that at the right hand side of the equation. Everything on the right hand side of Einstein's equations is how matter and energy are distributed in space. Time on the left hand side tells you how space and time deform in response to that matter and energy. And it can be impossible to solve some of those equations. What was so amazing about what Churchill did is he found this very elegant, simple solution within like a month of reading this final formulation. But Einstein didn't go through and try to find all the solutions. He sort of gave it to us, right? He shared this. And then lots of people since have been scrambling to try to ah, I can predict the curvature of the space time. If I tell you how the matter and energy is laid out. If it's all compact in a spherical system like a sun or even a black hole, I can understand the curves in the spacetime around it. I can solve for the shape of the space time. I can also say, well, what if the Universe is full of gas or light and it's all kind of uniform everywhere. And I'll find a different and equally surprising solution, which is that the universe would expand in response to that, that it's not static, that the distances between galaxies would grow. This was a huge surprise to Einstein. So all of these consequences of his theory, you know, came with revelations that were not at all obvious when he first wrote down the general theory.

And he was afraid to take the consequences of that theory seriously, which is a. Often the theory itself, in its scope and grandeur and power is scary. So I can understand. Then there's, you know, the, the edges of the theory where it falls apart. The consequences of the theory that are extreme, it's hard to take seriously. So you can sort of empathize.

Yeah, he very much resisted the expansion. So if you think about 1905, when he's writing these sequence of unbelievable papers as a 25 year old who can't get a job as a physicist, and he writes all of these remarkable papers on relativity and quantum mechanics, and then even in 1915, 16, he does not know that there are other galaxies out there. This just was not known. People had mused about it. There were these kind of smudges on the sky that people contemplated, what if there are other island universes? You know, going back to Kant, thought about this. But it wasn't until Hubble, it really wasn't until the late 20s that it's confirmed that there are other galaxies. Wow.

Yeah, he didn't. Obviously there's so much we think of now that he didn't think of. So there's no Big Bang static universe.

But these are all connected.

Wow. Yeah. So he's operating on very little information.

Very little information. That's absolutely true. Actually, one of the things I like to point out is the idea of relativity was foisted on people in this kind of cultural way. But there's many ways in which you could call it a theory of absolutism. And the way Einstein got there with so little information is by adhering to certain very strict absolutes, like the absolute limit of the speed of light and the absolute constancy of the speed of light, which was completely bizarre when it was first discovered. Really, that was observed through experiments trying to figure out, you know, what would the relative speed of light be. It's the only, really only massless particles have this property that they have an absolute speed. And if you think about it, it's incredibly strange.

Yeah, it's really strange.

Incredibly strange.

And so from a Theoretical perspective. He takes that seriously.

He takes it very seriously. And everyone else is trying to come up with models to make it go away, to make the speed of light be a little bit more reasonable. Like everything else in the universe. If I run at a car, two cars coming at each other, they're coming at each other, each other faster than if one of them stops. It's really a basic observation of reality right here. This is saying that if I'm racing at a light beam and you're standing still relative to the source, we'll measure the same exact speed of light. Very strange. And he gets to relativity by saying, well, what's speed? Speed is distance. It's space over time. It's how far you travel. It's the space you travel in a certain duration of time. And he said, well, I bet something must be wrong then with space and time. So this is an enormous leap. He's willing to give up the absolute character of space and time in favor of keeping the speed of light constant.

How was he able to intuit a world of curved space time? I think it's like one of the most special leaps in human history, right?

It's amazing.

Like, it's very, very, very difficult to make that kind of leap.

I'll tell you, it took me, I think, a long time to. I can't say this is how he got there exactly. It's not as though I studied the historical accounts of. Or his description of his internal states. This is more, having learned the subject, how I try to tell people how to get there in a few short steps. One is to start with the Equivalence principle, which he called the happiest thought of his life. And the Equivalence principle comes pretty early on in his thinking. And it starts with something like this. Right now, I think I'm feeling gravity because I'm sitting in this chair, and I feel the pressure of the chair, and it's stopping me from falling. And lie down in a bed, and I feel heavy on the bed. And I think of that as gravity. And Einstein has this beautiful ability to remove all of these extraneous factors, including atoms. So let's imagine instead that you're in an elevator and you feel heavy on your feet because the floor of the elevator is resisting your fall. But I want to remove the elevator. What does the elevator have to do with fundamental properties of gravity? So I cut the cable. Now I'm falling, but the elevator is falling at the same rate as me. So now I'm floating in the elevator. And if this happened to me, if I woke up in this state of falling or floating in the elevator. I might not know if I was in empty space just floating or if I was falling around the Earth, there would actually. They're equivalent situations. I would not be able to tell the difference. I'm actually. When I get rid of the elevator in this way, by cutting the cable, I'm actually experiencing weightlessness. And that weightlessness is the purest experience of gravity. And so this idea of falling is actually fundamental. It's how we talk about it all the time. The Earth is in a free fall around the sun. It's actually falling. It's not firing engines. Right. It's just falling all the time, but it's just cruising so fast.

So actually, yeah, God, you said so many profound. So one of them is really one of the ways to experience space time is to be falling.

To be falling. That is the purest experience of gravity. The experience of gravity unfettered, uninterrupted by atoms, is weightlessness. Yeah, that observation, no, it has an unhappy ending, the elevator story, because of atoms. Again, that's the fault of. Of the atoms in your body interacting electromagnetically with the crust of the Earth or the bottom of the building or whatever it is. But this period of free fall. So the first observation is that that is the purest experience of gravity. Now, I can convince you that things fall along curved paths, because I could take a pen and if I throw it, we both know it's going to follow an arc. And it's going to follow an arc until atoms interfere again and it hits the ground. But while it's in free fall, experiencing gravity at its purest, what the Einsteinian description would say is it is following the natural curve in spacetime inscribed by the Earth. So the Earth's mass and shape curves the paths in space, and then those curvatures tell you how to fall, the paths along which you should fall when you're falling freely daily. And so the Earth has found itself on a free fall that happens to be a closed circle, but it's actually falling. The International Space Station uses this principle all the time. They get the space station up there and then they turn off the engines. Can you imagine how expensive it would be if they had to fuel that thing at all times? Right. They turn off the engines. They're just falling. Yeah, they're falling and they're not that far up. There are certainly people sometimes say, oh, they're so far away they don't feel gravity. Oh, absolutely. If you stopped the space station, it's going like 17,500 miles an hour, something like that. If you were to stop that, it would drop like a stone right to the Earth. So they're in a state of constant free fall and they're falling along a curved path. And that curved path is a result of curving space time.

And that particular curved path is calculated in such a way that it curves onto itself. So you're orbiting.

Right. So it has to be cruising at a certain speed. So once you get it at that cruising speed, you turn off the engines.

But yeah, to be able to visualize at the beginning of the 20th century that not, you know, that free falling in curved space, time. Time, boy, the human mind is capable of things. I mean, some of that is constructing thought experiments that collide with our understanding of reality. Maybe in the collisions, in the contradictions. You try to think of extreme thought experiments that exacerbate that contradiction and see, like, okay, what is actually. Is there another model that can incorporate this? But to be able to do that, I mean, it's kind of inspiring because, you know, there's probably another general relativity out there.

Yeah.

In all. Not just in physics, in all lines of work, in all scientific pursuits, there's certain theories where you're like, okay, I just explained like a big elephant in the room here that everybody just kind of didn't even think about. There could be for stuff we know about in physics that could be stuff like that for the origin of life on Earth.

Yeah.

Everyone's like, yeah, okay. Everyone's like in polite companies, like, yeah, yeah. Somehow it started.

Right.

Nobody knows.

I find it wild that that's so elusive.

Yeah, it's. It's strange. In the lab, you can.

Strange that it's so elusive.

I think it's a general relativity thing. There's going to be something. It's going to involve aliens and wormholes and dimensions that we don't quite understand or some field that's bigger than, like, it's possible, maybe not. It's possible that it's a field that is different, that will feel fundamentally different from chemistry and biology. It'll be maybe through physics again, maybe the key to the origin of life is in physics. And the same there. It's like a weird neighbor is consciousness. It's like, all right, a weird neighbor.

Yeah.

Okay. So we all know. Know that life started on Earth somehow. Nobody knows how. We all know that. We're conscious. We have a subjective experience of things. Nobody understands that people have ideas and so on. But it's such a dark sort of. We're entering a dark room where a bunch of people are whispering about like, hey, what's in this room? But nobody, nobody has a effing clue. So, so. And then somebody comes along with a general relativity kind of conception where like reconceives everything and you're like, ah, it's.

Like a watershed moment.

Yeah, yeah, yeah. And it's there. And until we're living in the. We're living in a time until that theory comes along and it'll be obvious in retrospect, but right now, we're right.

Well, this. It was obvious to no one that spacetime was curved. But even Newton understood something wasn't right. So he knew there was something missing. And I think that's always fascinating when we're in a situation where we're pressure testing our own ideas. He did something remarkable, Newton did with his theory of gravity. Just understanding that the same phenomenon was at work with the Earth around the sun as the apple falling from the tree. That's insane. That's a huge leap. Understanding that mass, inertial mass, what makes something hard to push around is the same thing that, that feels gravity, at least in the Newtonian picture, in that simple way. Unbelievable leap. Absolutely genius. But he didn't like that the apple fell from the tree even though the Earth wasn't touching it.

Yeah, the action at a distance thing. The action at a distance thing, that is weird too. Well, but that is a really weird one.

It's really weird. But see, Einstein solves that. Relativity solves that because it says the Earth created the curve in space. The apple wants to fall freely along it. The problem is the tree's in the way. The tree's the problem. The tree's actually accelerating the apple. It's keeping it away from its natural state of weightlessness in a gravitational field. And as soon as the tree lets go of it, the apple will simply fall along the curve that exists.

I would love it if somebody went back to Newton's time him and told him all this. Probably some like, some like hippie would be like, it's a. Gravity is just the curvature in space time. Man, I wonder if he would be able to. I don't think there's, you know, every idea has its time. He might not, he might not even be able to load that in. I mean, sometimes even the greatest geniuses.

I mean, you can't too out of context.

You need to be standing on the shoulders of giants and on the shoulders of those giants and so on.

I heard that Newton used that as an unkind remark. To his competitor Hook.

Oh, no. The people talk shit even back then.

Yeah, trash talking.

It's one of the hilarious things about humans in general. But scientists too, like these huge minds. There's these moments in history where you'll see this in this in universities, but everywhere else too. Like you have gigantic minds, obviously also coupled with. Everybody has an ego. And like sometimes it's just the same soap opera that played out amongst humans everywhere else. And so you're thinking about the biggest cosmological objects and forces and ideas and you're still like jealous and.

Right.

I know your office is bigger than my office. I know this chair this. Or. Or maybe you got married to this person that I was always in love with. The betrayal or something.

The one woman in the.

One woman in part, yeah. And it's just, I mean, but that is also the fuel of innovation. That jealousy, that tension. That's.

That's, well, you know, the expression. I'm sure the battles are so bitter in academia because the stakes are so low.

That's a beautiful way to phrase it, but also like, we shouldn't forget. I mean that I love seeing that even in academia because it's humanity, the silliness, it's. There's a degree to academia where the reason you're able to think about some of these grand ideas is because you still allow yourself to be childlike.

Oh yeah.

There's a childlike nature.

There's no question.

Oh, children can also be like children children. So like, you don't. I think when in, in. In a corporate context and maybe the world gets. Forces you to behave. You're supposed to be a certain kind of way. There's some aspects, and it's a really beautiful aspect to preserve and to celebrate in academia is like you're just allowed to be childlike in your curiosity and your exploration. You're just exploring, asking the biggest questions.

The best scientists I know often ask the simplest questions. They're really. First of all, there's probably some confidence there. But also they're never going to lie to themselves that they understand something that they don't understand. So even this idea that Newton didn't understand the apple falling from the tree, had he lived another couple hundred of years, he would have invented relativity because he never would have lied to himself that he understood it. He would have kept asking this very simple question. And I think that there is this childlike beauty to that. Absolutely.

Yeah. Just some of the topics. I don't know why I'm stuck to those two topics of origin of life. And consciousness. But there's some of the most brilliant people I know are stuck. Just like with Newton and Einstein, they're stuck on that. This doesn't make sense. I know a bunch of brilliant biologists, physicists, chemists, that are thinking about the origin of life. They're like, this doesn't. I know how evolution works. I know how the biological systems work, how genetic information propagates. But like this, this part, the singularity at the beginning doesn't make sense. We don't understand what keeps. They can't create in the lab. They're bothered. Every single day, they're bothered by it. And that being bothered by that tension, by that gap in knowledge is. Yeah, that's the catalyst. That's the fuel. Catalyst for the discovery. For discovery.

Yeah, absolutely. The discovery is going to come because somebody couldn't sleep at night and couldn't rest.

So in that way, I think black holes are a kind of. Of portal into some of the biggest mysteries of our universe. So it's a good terrain on which to explore these ideas. So can you speak about some of the mysteries that the black holes present us with?

Yeah, I think it's important to separate the idea that there are these astrophysical states that become black holes from being synonymous with black holes, because black holes are kind of this larger idea, and they might have been made primordially when the Big Bang happened. And there's something flawless about black holes that makes them fundamental, unlike anything else. So they're flawless in the sense that you can completely understand a black hole by looking at just its charge, electric charge, its mass and its spin. And every black hole with that charged mass and spin is identical to every other black hole. You can't be like, oh, that one's mine, I recognize has this little feature, and that's how I know it's mine. They're featureless. You try to put Mount Everest on a black hole and it will shake it off in these gravitational waves. It will radiate away this imperfection until it settles down to be a perfect black hole again. So there's something about them that is unlike. And another reason why I don't like to call them objects in a traditional sense, unlike anything else in the universe. That's macroscopic. It's kind of a little bit more like a fundamental particle. So an electron is described by a certain short list of properties. Charge, mass, spin, maybe some other quantum numbers. That's what it means to be an electronic. There's no electron that's a little bit different. You can't recognize your electron. They're all identical in that sense. And so in some very abstract way, black holes share something in common with microscopic fundamental particles. And so what they tell us about the fundamental laws of physics can be very profound. And it's why even theoretical physicists, mathematical physicists, not just astronomers who use telescopes, they rely on the black hole as a terrain to perform their thought experiments. And it's because there's something fundamental about them.

Yeah. General relativity meets quantum mechanics meets singularity. And sadly, heartbreakingly. So it's out of reach for experiment at this moment, but within reach for theoretical.

It's in reach for thought experiments, for thought experiments, which are quite beautiful.

Well, on that topic, I have to ask you about the paradox, the information paradox of black holes. What is it?

So this is what catapulted Hawking's fame. When he was a young researcher, he was thinking about black holes and wanted to just add a little smidge of quantum mechanics, just a little smidge. You know, wasn't going for full blown quantum gravity, but kind of just asking, well, what if I allowed this nothing, this vacuum, this empty space around the event horizon. Star's gone, there's nothing there. What if I allowed it to possess sort of ordinary quantum properties just a little tiny bit, you know, nothing dramatic. Don't go crazy, you know. And one of the properties of the vacuum that is intriguing is this idea that you can never see the vacuums actually completely empty. We talked about Heisenberg, but, you know, the Heisenberg Uncertainty Principle really kicked off a lot of quantum mechanical thinking. It says that you can never exactly know a particle's position simultaneously with its motion, with its momentum. You can know one or the other pretty precisely, but not both precisely. And the uncertainty isn't a lack of ability that we'll technologically overcome. It's foundational so that there's, in some sense, when it's in a precise location, it is fundamentally no longer in a precise motion. And that uncertainty principle means I can't precisely say a particle is exactly here, but it also means I can't say it's not okay. And so it led to this idea that, what do I mean by a vacuum? Because I can't 100% precisely know. In fact, there's not really meaningful to say that there's zero particles here. And so what you can say, however, is you can say, well, maybe particles kind of froth around in this seething quantum sea of the vacuum. Maybe two particles come into existence and they're entangled in such A way that they cancel out each other's properties. So they have the properties of the vacuum. You know, they don't destroy the kind of properties of vacuum because they cancel out each other's spin, maybe each other's charge, maybe things like that. But they kind of froth around. They come, they go, they come, they go. And that's what we really think is the best that empty space can do in a quantum mechanical universe. Now, if you add an event horizon, which, as we said, is really fundamentally what a black hole is, that's the most important, important feature of a black hole, the event horizon. If the particles are created slightly on either side of that event horizon, now you have a real problem. Okay? Now, the pair has been separated by this event horizon. Now they can both fall in. That's okay. But if one falls in and the other doesn't, it's stuck. It can't go back into the vacuum because now it has a charge or it has a spin or it has something. It's no longer the property of that vacuum it came from. It needs its pair to disappear. Now it's stuck. It exists. It's like you've made it real. So in a sense, the black hole steals one of these virtual particles and forces the other to live. And if it'll escape, radiate out to infinity and look like to an observer far away, that the black hole has actually radiated a particle. Now, the particle did not emanate from inside. It came from the vacuum. It stole it from empty space, from the nothingness that is the black hole. Now, the reason why this is very tricky is because in the process, because of this separation on either side of the event horizon, the particle it absorbs, it has to do with the switching of space and time that we talked about. But the particle it absorbs, well, from the outside. You might say, oh, it had negative momentum. It was falling in from the inside. You say, well, this is actually motion and time. This is energy. It has negative energy. And as it absorbs negative energy, its mass goes down. The black hole gets a little lighter. And as it continues to do this, the black hole really begins to evaporate. It does more than just radiate. It evaporates away way. And it's intriguing because Hawking said, look, this is gonna look thermal, meaning featureless. It's gonna have no information in it. It's gonna be the most informationless possibility you could possibly come up with when you're radiating particles. It's just gonna look like a thermal distribution of particles, like a Hot body. And the temperature is going to only tell you about the mass, which you could tell from outside the black hole anyway. You know the mass of the black hole from the outside. So it's not telling you anything about the black hole. It's got no information about the black hole. Now you have a real problem. And when he first said it, a lot of people describe that not everyone understood how really naughty he was being. He did. But some people who love quantum mechanics were really annoyed, okay? People like Lenny Susskind, Gerard et Nobel Prize winner. They were mad because it suggested something was fundamentally wrong with quantum mechanics if it was right. And the reason why it says there's something fundamentally wrong with quantum mechanics is because quantum mechanics does not allow this. It does not allow quantum information to simply evaporate away and poof out of the universe and cease to exist. It's a violation of something called unitarity. But really, the idea is it's the loss of quantum information that's intolerable. Quantum mechanics was built, built to preserve information. It's one of the sacred principles. As sacred as conservation of energy. In this example, more sacred because you can violate conservation of energy with Heisenberg's uncertainty principle a little tiny bit, but so sacred that it created what became coined as the black hole wars, where people were saying, look, general relativity is wrong. Something's wrong with our thinking about the event horizon, or quantum mechanics isn't what we think it is. But the two are not getting along anymore. And just to tell you how dramatic it is, so the temperature goes down with the mass of the black hole. Heavier a black hole, the cooler it is. So we don't see black holes evaporate. They're way too big. But as they get smaller and smaller, they get hotter and hotter. So as the black hole nears the end of the cycle of evaporating away, it takes a very long time, much longer than the age of the universe. It will be as though the curtain, the event horizon's yanked up. Like it'll literally explode away, just boom. And the event horizon in principle, would be yanked up. Everything's gone. All that information that went into the black hole, all that sacred quantum stuff, gone. Poof. Okay? Because it's not in the radiation, because the radiation has no information. And so. So it was an incredibly productive debate because in it are the signs of what will make gravity and quantum mechanics play nice together. You know, some quantum theory of gravity. Whatever these clues are, and they're hard to assemble if you want a quantum gravity theory, it has to correctly predict the temperature of a black hole, the entropy of a black hole. It has to have all of these correct features. The black hole is the place on which we can test quantum gravity.

But it still has not been resolved.

It has not been fully resolved.

I looked up all the different ideas for the resolution. So there's the information loss which is what you referred to. It's perhaps the simplest. Yes. Most radical resolution is that information is truly lost. This would mean quantum mechanics as we currently understand it. Specifically unitarity is incomplete or incorrect under these extreme gravitational conditions.

Unhappy with. I would not be happy with information loss. I love that it's telling us that there's this crisis because I do think it's giving us the clues and we have to take them seriously.

For you, the gut is like unitarity.

Is going to be preserved.

Preserved. So quantum mechanics is holding stuff to the rescue.

Lenny Susskind in his book Black Hole War says his subtitle is My Battle with Stephen Hawking to Make the world safe for Quantum Mechanics.

Quantum mechanics. I love it.

Something to that effect.

So then from string theory, one of the resolutions physicists called fuzzballs. I love physicists so much. Originating from string theory, this proposal suggests that black holes aren't singularity surrounded by empty space and an event horizon. Instead they are horizon less complex tangled objects, AKA fuzzballs made of strings and brains roughly the size of the would be event horizon. There's no single point of infinite density and no true horizon to cross.

In some sense it says there's no interior to the black hole, nothing of a cross. So I gave you this very nice story that there's no drama. Sometimes that's how it's described at the event horizon and you fall through and there's nothing there. This other idea says. Well, hold on a second. If it's really strings. As I get close to this magnifying quality and the slowing time down near the event horizon, it is as though I put a magnifying glass on things and now the strings aren't so microscopic. They kind of schmear around and then they get caught like a tangle around the event horizon and they just actually never fall through. Through. I don't think that either, but it was interesting.

So it's just adding a very large number of extra complex degrees of freedom.

Yeah, there are no teeny tiny marbles to fall through.

But it's similar to what we already have with quantum mechanics. It's just giving a deeper.

But it's really saying the interior is just not there ever. Nothing falls in, so the information gets out because it never went in in the first place.

Oh, interesting. So there is a strong statement there.

There's a strong statement there, yeah.

Okay. Soft hair challenges the classical no hair theorem.