Lex Fridman Podcast
Reformatted for readability — timestamps removed, lightly restructured. Not verbatim.
Janna Levin
"…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 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 as black holes are undulating. Eventually those two black holes will merge."
Janna Levin
"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."
Janna Levin
"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."
Lex Fridman
"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. You wrote to me, 'When the great physicist Sidney Coleman was asked to attend a 9:00 AM meeting his reply was, I can't stay up that late.'"
Janna Levin
"Yeah, classic. Sidney was beloved."
Lex Fridman
"I think all the best thoughts, honestly, maybe the worst thoughts too, 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."
Janna Levin
"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 4:00 in the morning."
Lex Fridman
"So in that way, many other ways were 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."
Janna Levin
"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 a way to make these crazy objects, which is to kill off a few stars."
Janna Levin
"And 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, Karl Schwarzschild, who solved Einstein's equations between sitting in the trenches and cannon fire."
Janna Levin
"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."
Janna Levin
"And really what Einstein's equations were telling everybody at the time was that matter and energy, curved space and time, and then curved spacetime tells matter and energy how to fall once the spacetime is shaped. So he finds this beautiful solution."
Janna Levin
"And the most amazing thing about a 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 decade, over a hundred years later, I would say that is the black hole."
Janna Levin
"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."
Janna Levin
"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."
Janna Levin
"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."
Lex Fridman
"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."
Janna Levin
"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. It does a great job."
Janna Levin
"Sometimes I liken it to 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."
Lex Fridman
"So according to him, the closer you get to the singularity, the more quantum mechanics comes into play, and therefore there is no singularity. There's something else."
Janna Levin
"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."
Lex Fridman
"So you've described the brain breaking idea that a black hole is not so much as super dense matter as it's sometimes described, but it's more akin to a region of spacetime, but even more so just nothing. It's nothing. That's the thing you seem to like to say."
Janna Levin
"I do. I do like to say that black holes are no thing."
Lex Fridman
"No thing."
Janna Levin
"They're nothing."
Janna Levin
"And 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."
Janna Levin
"And it was quite astounding that people like Oppenheimer, actually it's probably Oppenheimer's most important theoretical work, who were thinking about nuclear physics and quantum mechanics but in the context of these kind of utopian questions."
Janna Levin
"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?"
Janna Levin
"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."
Janna Levin
"There are earth fault of thermonuclear fuel they're burning, and emitting energy and 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 going to run out of fuel."
Janna Levin
"It's going to 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."
Janna Levin
"And if the star is very, very, very massive, much more massive than our sun, maybe 20, 30 times the mass of our sun, it'll collapse under its own weight. And that collapse is incredibly fast and dramatic, and it creates a shockwave."
Janna Levin
"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."
Janna Levin
"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."
Janna Levin
"And then what's left sometimes ended 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 object."
Janna Levin
"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."
Janna Levin
"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."
Lex Fridman
"We think there's a lot of drama today on social media. Imagine that. 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."
Janna Levin
"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."
Janna Levin
"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."
Janna Levin
"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."
Janna Levin
"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 going to pay attention to your paper now,' because of the Nazi advance on Poland."
Lex Fridman
"That's the other remarkable thing about Oppenheimer is he's also a central figure in the construction of the bomb."
Janna Levin
"Right?"
Lex Fridman
"So it's theory and experiment clashing together with the geopolitics."
Janna Levin
"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."
Janna Levin
"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."
Janna Levin
"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."
Janna Levin
"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…"
Janna Levin
"So now 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 supernovas what's left. It still maybe 10 times the mass of the sun, just what's left in that core. And it continues to collapse."
Janna Levin
"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 spacetime is so tremendous that not even light can escape."
Janna Levin
"The event horizon forms but the event horizon is almost imprinted on the spacetime, 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 rain inwards."
Janna Levin
"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."
Janna Levin
"There's this quote by John Wheeler, who's 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.'"
Janna Levin
"And he was giving a lecture. It's actually above Tom's restaurant from Seinfeld near Columbia in New York."
Lex Fridman
"Nice."
Janna Levin
"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."
Janna Levin
"'Well, how about black hole?' And apparently he then foists this term on the world. Wheeler head wave of doing that."
Lex Fridman
"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."
Lex Fridman
"I just wanted to point out that just looking for the first time, it's a 1939 paper from Oppenheimer. It two pages. It's like three pages."
Janna Levin
"Oh yeah. It's gorgeous."
Lex Fridman
"The simplicity of some of these, that's so gangster, just revolutionize all of physics with Einstein did that multiple times. In a simple year when all thermonuclear sources of energy are exhausted, a sufficiently heavy star will collapse."
Lex Fridman
"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, the orders of that of the sun, this contraction will continue indefinitely. And it goes on that way."
Janna Levin
"Yeah. Now I have to say that 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 there were bad feelings."
Janna Levin
"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 found themselves returning again to these astrophysical questions, decided that actually Oppenheimer had been right."
Janna Levin
"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."
Janna Levin
"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.'"
Lex Fridman
"Because 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 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."
Lex Fridman
"What can you say as a person who's a physicist and who have studied the physicist 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?"
Janna Levin
"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."
Janna Levin
"Had he not made some really kind of simple 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 are faced with that quandary, having to take the initiative to participate in thinking of a way that quantum mechanics can kill people."
Janna Levin
"And then making the bomb, 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."
Lex Fridman
"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've created the bomb first, not the United States. And how different would the world be?"
Lex Fridman
"The game theory of that, I think say the probability is 33% that it was in the United States. If the Soviet Union had the bomb, I think they would've 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've used it. I think there's no question he would've used it to kill hundreds of millions of people"
Janna Levin
"In the game theory version. This was the least harmful outcome."
Lex Fridman
"Yes,."
Janna Levin
"Yes. But there is no outcome with no bomb. That any game theorist would, I think would play."
Lex Fridman
"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."
Lex Fridman
"Boy, do they even have control of how that science is used. It's hard."
Janna Levin
"They don't have control, right? Once it's made, it's no longer scientific reasoning that dictates the use or it's restraint. But I will say that I do believe that it wasn't [inaudible 00:25:16] one third down the line, because America was different. And I think that's something we have to think about right now in this particular climate."
Janna Levin
"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 what that life."
Janna Levin
"How quickly it turned, how quickly this free spirited Berlin culture was suddenly in this repressive and terrifying regime. So I think that it was a much higher chance that it happened here in America."
Lex Fridman
"And there's something about the American system, 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 to scientists."
Janna Levin
"Absond it's very valuable to maintain that, the vibrancy of the debate of the funding, those mechanisms."
Janna Levin
"Absolutely. The world flocked here. And that won't be the case if we no longer have intellectual freedom."
Lex Fridman
"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?"
Janna Levin
"Yeah. I don't have the number, but-"
Lex Fridman
"But it's disproportionately so."
Janna Levin
"It's disproportionately so. In fact, a lot of them from particle physics came from the Bronx, and they were European immigrants."
Lex Fridman
"How do you explain this?"
Janna Levin
"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."
Lex Fridman
"And that's the thing you write about. And we'll return to time and time again that 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 that it's humans. And humans are messy and complicated and beautiful and all of that."
Janna Levin
"Yeah."
Lex Fridman
"So what were we talking about? Oh-"
Janna Levin
"The stars collapsing."
Lex Fridman
"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."
Janna Levin
"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 spacetime is like a waterfall raining 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 like a fish swimming against the Niagara, sitting against a waterfall."
Lex Fridman
"It's like stuck there."
Janna Levin
"But it's stuck there. And so that's one way you can have a little signpost, 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 like that."
Lex Fridman
"So you're falling back, cross the event horizon, right at that point you shoot outwards a photon."
Janna Levin
"Yes."
Lex Fridman
"And it's just stuck there."
Janna Levin
"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. I can't know what happens to it. It could be pure matter and anti-matter 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."
Lex Fridman
"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."
Lex Fridman
"But what can we know and what do we know about the physics of spacetime inside black hole?"
Janna Levin
"I don't mind being incautious about thinking about what the math tells us."
Janna Levin
"I'm not such 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."
Janna Levin
"And 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 a hundred kilometers out. That's very, very close, orbiting quite safely, no big deal hanging out. I don't bug the black hole. The black hole doesn't bug me. It won't suck me up like a…"
Janna Levin
"I don't bug the black hole. A black hole doesn't bug me. It won't suck me up like a vacuum or anything crazy, but some … 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, just contrast against a bright sky."
Janna Levin
"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. 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 where 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.'"
Janna Levin
"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. 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, 'cause it's in your future. There's an inevitable moment when you will hit it. Usually for a stellar mass black hole, we think it's microseconds."
Lex Fridman
"Microseconds to get from the event horizon to the-"
Janna Levin
"To the singularity."
Lex Fridman
"To the singularity. Oh boy. Oh boy, so that's describing from your astronaut friend's perspective."
Janna Levin
"Yes, from their perspective, the singularity's in their future."
Lex Fridman
"From your perspective, what do you see when your friend falls into the black hole and you're chilling outside and watching?"
Janna Levin
"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. 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. If I just want to draw a kind of compass diagram, not a compass diagram, but 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."
Janna Levin
"As you approach the black hole, it's as though you're rotating in space-time is one way of thinking about it. 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. In other words, their clocks seem to be less aligned with my time. 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."
Janna Levin
"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. 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."
Janna Levin
"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 other clocks have stopped altogether from my point of view. That's to say that it's as though their time is completely rotated into my space. This is connected with the idea that inside the black hole space and time have switched places."
Janna Levin
"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."
Lex Fridman
"Basically, the time almost comes to a standstill, but we still, they do fall in."
Janna Levin
"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'll actually see and event horizon bobble and absorb the astronauts. In some finite time, the astronaut will actually fall in."
Lex Fridman
"It's like this weird space-time bubble that we have around us. 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. If you're a perfect infinitely small particle, you would just be-"
Janna Levin
"Take longer and longer."
Lex Fridman
"Probably just be stuck there or something, but no, there's quantum mechanics."
Janna Levin
"Eventually, you'll fall in. Any perturbation will only go one way. It's unstable in one direction, in one direction only. 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 are concerned. 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."
Janna Levin
"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? Once you get to that point, there's no amount of energy, that all the energy in the universe will not save you from this demise."
Lex Fridman
"There's different size black holes. 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?"
Janna Levin
"Yeah."
Lex Fridman
"As I understand, the bigger it is, less drastic the experience of falling into it."
Janna Levin
"That might surprise people. The bigger it is, the less noticeable it is that you've crossed the event horizon. One way to think about it is curvature is less noticeable the bigger it is. 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. On the earth, you actually have to be kind of clever to deduce that the earth is curved."
Janna Levin
"The bigger the planet, the less you're going to notice the curvature, the global curvature. It's the same thing with a black hole, A huge, huge black hole. It just kind of feels like just flat. You don't really notice."
Lex Fridman
"I'm trying to figure out how the, because if you don't notice-"
Janna Levin
"There's nothing there."
Lex Fridman
"The physics is weird."
Janna Levin
"In your frame of reference."
Lex Fridman
"No."
Janna Levin
"Well, so another cool thing. I like to dispel myths. Do you need a minute? You're holding your head."
Lex Fridman
"There's a sense 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."
Janna Levin
"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. This is a kind of fascinating that your experience could be that it's quite bright"
Janna Levin
"Inside the black hole because all the light from the galaxy can be shining in behind you. It's focusing down because you're all approaching this really focused region in the interior. You actually see a bright white flash of light as you approach the singularity. I joke that it's like a near-death experience. We see the light at the end of the tunnel. You would see millennia pass on earth. You could see the evolution of the entire galaxy, one big bright flash of light. It's like a near-death experience, but it's definitely a total death experience."
Lex Fridman
"It goes pretty fast, but you looking out, you looking out, everything's going super fast."
Janna Levin
"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. 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."
Lex Fridman
"Obviously, the human sensory system, we're not able to process that information correctly."
Janna Levin
"It would be a microsecond in a, right, that would be too fast,"
Lex Fridman
"It would be, while it'd be so cool to get that information."
Janna Levin
"A big black hole, you could actually hang around for some months."
Lex Fridman
"Yeah. How are small black holes or supermassive black holes formed just so people can kind of load that in? Is it always a star?"
Janna Levin
"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."
Janna Levin
"We now see, and this was really quite a surprise, that there are supermassive black holes. They 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. Extremely massive."
Janna Levin
"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. We don't think there's been enough time for that. It's suspected that they're formed very early, maybe even a hundred, 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."
Lex Fridman
"In the very early universe, these are primordial black holes from the star's, not quite … Wait, how do you get from that soup? Black holes right away."
Janna Levin
"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 talking about. In sense, if they're just allowed to directly collapse very early in the universe's history, they can do that more easily. It's so much so that we think that there's one of these supermassive black holes in the center of every galaxy. They're not rare and we know where they are. They're in the nuclei of galaxies. They're bound to the very early formation of entire galaxies in a really surprising and deeply connected way."
Lex Fridman
"I wonder if the chicken or the egg, is it, how critical, how essential are the supermassive black holes of the formation of galaxies?"
Janna Levin
"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. Then maybe the black holes and kind of the galaxies were like these gassy clouds around them. 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. I think the mechanisms are still ongoing attempts to understand exactly the ordering of these things."
Lex Fridman
"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 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. How are we supposed to conceive of it? How do you think of it? Time's just another dimension?"
Janna Levin
"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. We're limited because as three-dimensional beings, we can't really draw four dimensions, which is what I'd require. Three spatial, 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, 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."
Janna Levin
"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 space-time. Euclidean space-time is when all the dimensions are orthogonal and are treated equally. Time is not another Euclidean dimension. It's actually a Minkowski in space-time."
Janna Levin
"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. 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. 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 in the geometries, not the geometry of a flat sheet of paper."
Janna Levin
"I can do the same thing with space-time. I can draw it on a flat sheet of paper, but I know that it's not actually a flat Euclidean space. 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 from Minkowski's space-time, 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. I will get the correct answer, but only if I use these different rules."
Lex Fridman
"Then what does, according to general relativity, does objects with mass do to the space-time?"
Janna Levin
"Right, exactly. Einstein struggled for this completely general theory, not a specific solution like a black hole or an expanding space-time 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 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."
Janna Levin
"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 Schwarzschild did is he found this very elegant, simple solution within 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."
Janna Levin
"He shared this. Then lots of people since have been scrambling to try to, '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 space-time 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.' I'll find a different 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. All of these consequences of his theory came with revelations that were not at all obvious when he first wrote down the general theory."
Lex Fridman
"He was afraid to take the consequences of that theory seriously, which is-"
Janna Levin
"Often."
Lex Fridman
"The theory itself in its scope and grandeur and power is scary. I can understand. Then there's 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 empathize."
Janna Levin
"He very much resisted the expansion. If you think about 1905 when he's writing these sequence of unbelievable papers as a twenty-five- 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? Going back to Kant thought about this, but it wasn't until Hubble. It really wasn't until the late twenties that it's confirmed that there are other galaxies."
Lex Fridman
"Wow, and obviously, there's so much we think of now that he didn't think of, so there's no Big Bang static universe,"
Janna Levin
"Right, but these are all connected."
Lex Fridman
"Wow. Yeah, so he's operating on very little information."
Janna Levin
"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. There's many ways in which you could call it a theory of absolutism. 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 what would the relative speed of light be? It's really only massless particles have this property that they have an absolute speed. If you think about it's incredibly strange."
Lex Fridman
"Yeah. It's really strange."
Janna Levin
"Incredibly strange."
Lex Fridman
"From a theoretical perspective, he takes that seriously."
Janna Levin
"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 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. He gets to relativity by saying, 'Well, what 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.' He said, 'Well, I bet something must be wrong then with space and time.' 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."
Lex Fridman
"How was he able to intuit a world of curved space-time? I think it's one of the most special leaps in human history, right?"
Janna Levin
"It's amazing."
Lex Fridman
"It's very, very, very difficult to make that kind of leap."
Janna Levin
"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 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. The equivalence principle comes pretty early on in his thinking. It starts with something like this. Right now, I think I'm feeling gravity because 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. I think of that as gravity."
Janna Levin
"I think it has a beautiful ability to remove all of these extraneous factors, including atoms. Let's imagine instead that you're in an elevator and you feel heavy on your feet, 'cause 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? I cut the cable, now I'm falling, but the elevator is falling at the same rate as me. Now, I'm floating in the elevator. 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 an 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."
Janna Levin
"I'm actually, when I get rid of the elevator in this way, by cutting the cable, I'm actually experiencing weightlessness. That weightlessness is the purest experience of gravity. 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, it's just falling all the time, but it's just cruising so fast."
Lex Fridman
"Actually, yeah, God, you said so many profound things. One of them is really one of the ways to experience space- time is to be falling?"
Janna Levin
"To be falling. That is the purest experience of gravity. The experience of gravity, unfettered, uninterrupted by atoms is weightlessness."
Lex Fridman
"Yeah."
Janna Levin
"That observation, no, it has an unhappy ending. The elevator story because of atoms, again. That's the fault of the atoms in your body interacting electromagnetically with the crust of the earth or the bottom of the building or whatever. 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. 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 space-time inscribed by the earth."
Janna Levin
"The earth's mass and shape curves the paths in space. Then those curvatures tell you how to fall, the paths along which you should fall when you're falling freely. 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."
Lex Fridman
"Yeah, they're falling."
Janna Levin
"They're not that far up. 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 or something like that. If you were to stop that, it would drop like a stone right to the earth. 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."
Lex Fridman
"That particular curved path is calculated in such a way that it curves onto itself, so you're orbiting."
Janna Levin
"It has to be cruising at a certain speed, so once you get it at that cruising speed, you turn off the engines."
Lex Fridman
"Yeah, to be able to visualize at the beginning of the 20th century that not that free-falling in curved space-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, 'Okay, actually, is there another model that can incorporate this?' To be able to do that, I mean, it's kind of inspiring because there's probably another general relativity out there 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 a big elephant in the room here that everybody just kind of didn't even think about.'"
Lex Fridman
"There could be, for stuff we know about in physics, there could be stuff like that for the origin of life on earth. Everyone's like, 'Yeah, okay.' Everyone's like in polite companies. Yeah, yeah, yeah, yeah. Somehow it started. Nobody knows."
Janna Levin
"Yeah, I find it wild that that's so elusive."
Lex Fridman
"Yeah, it's strange. In the lab, you can't replicate-"
Janna Levin
"Strange that it's so elusive."
Lex Fridman
"I think it's a general relativity thing. There's going to be something, it's going to involve aliens and worm holes and dimensions that we don't quite understand, or some field that's bigger than … 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. The same there, it's like a weird neighbor is consciousness. It's like, all right."
Janna Levin
"A weird neighbor."
Lex Fridman
"It's like, okay, so we all know that life started on earth somehow. Nobody knows how. We all know that we're conscious. We have a subjective experience of things and nobody understands. That people have ideas and so on, but it's such a dark, we're entering a dark room where a bunch of people are whispering about, 'Hey, what's in this room?' Nobody has an effing clue, and then somebody comes along with a general relativity kind of conception where it reconceives everything and you're like, 'Ah.'"
Janna Levin
"It's like a watershed moment."
Lex Fridman
"Yeah."
Janna Levin
"Yeah."
Lex Fridman
"Yeah. It's there."
Janna Levin
"It's there."
Lex Fridman
"We're in a time until that theory comes along and it'll be obvious in retrospect, but right now we're-"
Janna Levin
"Right. Well, this, it was obvious to no one, that space-time was curved, but even Newton understood something wasn't right. He knew to his something missing. 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 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."
Lex Fridman
"Yeah. The action-at-a-distance thing."
Janna Levin
"The action-at-a-distance thing."
Lex Fridman
"That is weird too."
Janna Levin
"Well, but-"
Lex Fridman
"That is a really weird one."
Janna Levin
"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. When the tree …"
Janna Levin
"… Along it. The problem is the tree's in the way. The tree is the problem. The tree is 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."
Lex Fridman
"I would love it if somebody went back to Newton's time…"
Janna Levin
"And told him all this?"
Lex Fridman
"Probably some hippie would be like, 'Gravity is just a curvature in space-time, man.' Every idea has its time, he might not even be able to load that in. Sometimes even the greatest geniuses-"
Janna Levin
"It's too out of context."
Lex Fridman
"You need to be standing on the shoulders of giants and on the shoulders of those giants and so on."
Janna Levin
"I heard that Newton used that as an unkind remark to his competitor, Hooke."
Lex Fridman
"Oh, no. So people talked shit even back then."
Janna Levin
"Trash talking."
Lex Fridman
"I love it. It's one of the hilarious things about humans in general, but scientists too, these huge minds… There's moments in history where, you'll see this in universities, but everywhere else too, you have gigantic minds, obviously also coupled with everybody has an ego, and 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 jealous."
Janna Levin
"I know. It's Fascinating."
Lex Fridman
"Your office is bigger than my office."
Janna Levin
"I know"
Lex Fridman
"This chair… Or maybe you got married to this person that I was always in love with, it's a betrayal or something."
Janna Levin
"The one woman in the department."
Lex Fridman
"Yeah, the one woman in the department. But that is also the fuel of innovation, that jealousy, that tension."
Janna Levin
"You know the expression, I'm sure, the battles are so bitter in academia because the stakes are so low."
Lex Fridman
"That's a beautiful way to phrase it. But also we shouldn't forget, I love seeing that even in academia, because it's humanity, the silliness. There is 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, because there's a childlike nature to ask the big question, but children can also be like…"
Janna Levin
"Children."
Lex Fridman
"Children. I think when in a corporate context and maybe the world 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 you're just allowed to be childlike in your curiosity and your exploration. You're just exploring, asking the biggest questions."
Janna Levin
"The best scientists I know often ask the simplest questions. 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've invented relativity, because he never would've lied to himself that he understood it. He would've kept asking this very simple question. And I think that there is this childlike beauty to that, absolutely."
Lex Fridman
"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-"
Janna Levin
"I'll talk about those."
Lex Fridman
"Some of the most brilliant people I know, 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, 'I know how evolution works, I know how the biological systems work, how genetic information propagates, but this part, this singularity at the beginning doesn't make sense. We don't understand, we can't create it in a lab.' Every single day they're bothered by it. And that being bothered by that tension, by that gap in knowledge, that's the catalyst, that's the fuel for the-"
Janna Levin
"Discovery."
Lex Fridman
"The discovery."
Janna Levin
"Yeah, absolutely. The discovery is going to come because somebody couldn't sleep at night and couldn't rest."
Lex Fridman
"So in that way, I think black holes are a portal into some of the biggest mysteries of our universe, so it's a good terrain I wish to explore these ideas. So can you speak about some of the mysteries that the black holes present us with?"
Janna Levin
"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 this larger idea, and they might've been made primordially when the Big Bang happened. And there's something flawless about black holes that makes them fundamental, unlike anything else."
Janna Levin
"So they're flawless in the sense that you can completely understand a black hole by looking at just its electric charge, its mass and its spin. And every black hole with that charge mass and spin is identical to every other black hole. You can't be like, 'Oh, that one's mine, I recognize it. It has this little feature and that's how I know it's mine.' They're featureless."
Janna Levin
"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."
Janna Levin
"So there's something about them that is, 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 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 electron. 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."
Lex Fridman
"Yeah, general relativity means quantum mechanics means singularity."
Janna Levin
"And it happens there."
Lex Fridman
"And sadly, heartbreakingly so, it's out of reach for experiment at this moment, but it's within reach for theoretical."
Janna Levin
"It's in reach for thought experiments, which are quite beautiful."
Lex Fridman
"Well, on that topic, we have to ask you about the information paradox of black holes. What is it?"
Janna Levin
"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. Wasn't going for full-blown quantum gravity, but just asking, 'Well, what if I allowed this nothing, this vacuum, this empty space around the event horizon, the stars gone, there's nothing there, what if I allowed it to possess ordinary quantum properties, just a little tiny bit, nothing dramatic? Don't go crazy.'"
Janna Levin
"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, 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."
Janna Levin
"And the uncertainty isn't a lack of ability that we'll technologically overcome. It's foundational. 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. 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 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. They don't destroy the properties of the vacuum. They cancel out each other, spin maybe, each other's charge maybe, things like that, but they froth around. They come, they go, they come, they go.'"
Janna Levin
"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 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. 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 that'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."
Janna Levin
"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 is 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 the separation on other 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.'"
Janna Levin
"You say, 'Well, this is actually motion and time.' This is energy. If it has negative energy and it has absorbs negative energy, it's mass goes down, 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. And it's intriguing because Hawking said, 'Look, this is going to look thermal, meaning featureless. It's going to have no information in it. It's going to be the most informationless possibility you could possibly come up with when you're radiating particles. It's just going to 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.'"
Janna Levin
"And when he first said it, not everyone understood how really naughty he was being. He did. But some people who love quantum mechanics were really annoyed, people like Lenny Susskind, Gerard't Hooft, 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 to preserve information. It's one of the sacred principles. As sacred as conservation of energy."
Janna Levin
"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 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 a black hole nears the end of this cycle, of evaporating away, it takes a very long time, much longer than the age of the universe. It will be as though the current and the event horizon is yanked up, 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."
Janna Levin
"Because not in the radiation, because the radiation has no information. And so it was an incredibly productive debate because in it are the signs of what will make gravity and quantum mechanics play nice together, 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."
Lex Fridman
"But it still has not been resolved."
Janna Levin
"It has not been fully resolved."
Lex Fridman
"I looked up all the different ideas for the resolution. So there's the information loss, which is what you refer to. It's perhaps the simplest yet 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."
Janna Levin
"I'm unhappy with that. 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."
Lex Fridman
"For you the gut is like-"
Janna Levin
"Unitarity is going to be preserved."
Lex Fridman
"Preserved, so quantum mechanics is [inaudible 01:17:47]"
Janna Levin
"We have to come to the rescue. Lenny Susskind in his book, Black Hole Wars, his subtitle is My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics."
Lex Fridman
"Quantum Mechanics, I love it."
Janna Levin
"It's something to that effect."
Lex Fridman
"So then from string theory, one of the resolutions is called fuzz balls. I love physics so much. Originating from string theory, this proposal suggests that black holes aren't singularities surrounded by empty space and an event horizon. Instead, they are horizonless, complex, tangled objects, AKA fuzz balls 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."
Janna Levin
"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 this 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 smear around, and then they get caught like a tangle around the event horizon, and they just actually never fall through.' I don't think that either, but it was interesting."
Lex Fridman
"So it's just adding a very large number of extra complex…"
Janna Levin
"Degrees of freedom."
Lex Fridman
"Yeah."
Janna Levin
"There are no teeny tiny marbles to fall through."
Lex Fridman
"But it's similar to what we already have with quantum mechanics. It's just giving a deeper more complicated-"
Janna Levin
"But it's really saying the interior's just not there ever. Nothing falls in, so the information gets out because it never went in the first place."
Lex Fridman
"Oh, interesting. So there is a strong statement there, okay."
Janna Levin
"There's a strong statement there, yeah."
Lex Fridman
"Okay. Soft hair challenges the classical no-hair theorem by suggesting that black holes do possess subtle quantum, quote, hair. This isn't classical hair like charge, but very low energy quantum excitations, soft gravitons or photons at the event horizon that can store information about what fell in."
Janna Levin
"Worth trying but I also don't think that that's the case. So the no-hair theorems are formal proofs that the black hole is this featureless perfect fundamental particle that we talked about, that all you can ever tell about the black hole is it's electrical charge, it's mass and it's spin, and that it cannot possess other features. It has no hair, is one way of describing it, and that those are proven mathematical proofs in the context of general relativity. So the idea is, well, therefore, I can know nothing about what goes into the black hole, so the information is lost. But if they could have hair, I could say that's my black hole, because it'll have features that I could distinguish and it could encode the information that went in this way. And the event horizon isn't so serious, isn't such a stark demarcation between events inside and outside and where I can't know what happened inside or outside, and I don't think that's the resolution either, but it was worth a shot."
Lex Fridman
"Okay. The pros and cons of that one, the pros that works within the framework of quantum field theory in curved space-time potentially requiring less radical modifications than fuzzballs or information loss. Recent work by Hawking, Perry, Strominger revitalizes this idea. The cons is that the precise mechanism by which information is encoded and transferred to the radiation is still debated and technically challenging to work out fully. And indeed it needs to store a vast amount of information. Okay, another one, this is a weird one, boy is ER equals EPR."
Janna Levin
"This is probably it though."
Lex Fridman
"Oh, boy. So ER equals EPR is an Einstein-Rosen bridge equals Einstein-Podolsky-Rosen bridge. Posits a deep connection between quantum entanglement and space-time geometry, specifically Einstein-Rosen bridge, commonly known as wormholes. It suggests that entangled particles are connected by a non-traversable wormhole, so tiny wormholes connected. Okay."
Janna Levin
"I can say that this is not a situation where we can follow the chalk. We can't start at the beginning and calculate to the end. So it's still a conjecture. I think it's very profound though. I imagine Juan Maldacena, who's part of this, with Lenny Susskind, they were like, 'ER equals EPR.' They couldn't even formulate it properly. It was like an intuition that they had landed on and now are trying to formalize. But to take a step back, one way of thinking about ER equals EPR, you have to talk about holography first. And holography both Juan Maldacena really formalized it, Lenny Susskind suggested it. The idea of a black hole hologram is that all of the information in the black hole, whatever it is, whatever entropy as a measure of information, whatever the entropy of the black hole is, which is telling you how much information is hidden in there, how much information you don't have direct access to, in some sense, is completely encoded in the area of the black hole."
Janna Levin
"Meaning, as the area grows, the entropy grows. It does not grow as the volume. This actually turns out to be really, really important. If I tried to pack a lot of information into a volume, more information than I could pack, let's say, on the surface of a black hole, I would simply make a black hole and I would find out, oh, I can't have more information than I can fit on the surface. So Lenny coined this a hologram. People who take it very seriously say, 'Well, again, maybe the interior of the black hole just doesn't exist. It's a holographic projection of this two-dimensional surface.' In fact, maybe I should take it all the way and say so are we. The whole universe is a holographic projection of a lower dimensional surface. And so people have struggled, nobody's really landed it to find a universe version of it."
Janna Levin
"Oh, maybe there's a boundary to the universe where all the information is encoded and this entire three-dimensional reality that's so compelling and so convincing is actually just a holographic projection. Juan Maldacena did something absolutely brilliant. It's the most highly cited paper in the history of physics. It was published in the late 90s. It has a very opaque title that would not lead you to believe it's as revelatory as it is, but he was able to show that a universe in a box with gravity in it, it's not the same universe we observed. Doesn't matter. It's just a hypothetical called an anti-de Sitter space. It's a universe in a box. It has gravity, it has black holes, it has everything gravity can do in it."
Janna Levin
"On its boundary is a theory with no gravity, a universe that can be described with no gravity at all, so no black holes and no information loss problem, and they're equivalent, that the interior universe in a box is a holographic projection of this quantum mechanics on the boundary, pure quantum mechanics, purely unitary, no loss of information. None of this stuff could possibly be true. There can't be loss of information if this dictionary really works, if the interior is a hologram, a projection of the boundary. I know that's a lot."
Lex Fridman
"Yeah. So there's some mathematics there, there's physics, and then there's trying to conceive what that actually means practically for us."
Janna Levin
"The idea was, well, maybe the stuff that's interior to the black hole is, like EPR, quantum entangled, with the Hawking radiation outside the black hole that's escaping. And that quantum entanglement is what allows you to extract the information because it's not actually physically moving from the interior to the exterior. It's just subtle quantum entanglement. An, in fact, I can think of the entire black hole. If I look at it and it looks like a solid shadow cast on the sky, some region of space-time, if I look at it very closely, I will see, oh no, it's actually sown from these quantum wormholes, like embroidered. And so when I get up close, it's almost as though the event horizon isn't the fundamental feature on the space-time. The fundamental feature is the quantum entanglement embroidering the event horizon."
Lex Fridman
"The embroidering is just tiny wormholes. So the quantum entanglement is when two particles are connected at arbitrary distances?"