Lex Fridman PodcastJanna Levin: Black Holes, Wormholes, Aliens, Paradoxes & Extra Dimensions | Lex Fridman Podcast #468
EVERY SPOKEN WORD
150 min read · 30,161 words- 0:00 – 2:03
Episode highlight
- JLJanna 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 as these 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 the 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, 'cause 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, and 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 form a 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.
- LFLex Fridman
Mm-hmm.
- JLJanna 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. 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.
- 2:03 – 3:03
Introduction
- LFLex Fridman
The following is a conversation with Janna Levin, a theoretical physicist and cosmologist specializing in black holes, cosmology of extra dimensions, topology of the universe, and gravitational waves in spacetime. 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 Turing 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 a Lex Fridman podcast. To support it, please check out our sponsors in the description, and now, dear friends, here's Janna Levin.
- 3:03 – 10:37
Black holes
- LFLex Fridman
I should say that you sent me a message about not starting early in the morning-
- JLJanna Levin
(laughs)
- LFLex Fridman
... and that made me feel like we're kindred spirits.
- JLJanna Levin
(laughs) Yeah.
- LFLex Fridman
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.'"
- JLJanna Levin
Yeah, (laughs) classic.
- LFLex Fridman
So... (laughs)
- JLJanna Levin
Sidney was beloved.
- LFLex Fridman
I think all the best thoughts, honestly-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... maybe the worst thoughts too, are all come at night.
- JLJanna Levin
Yeah.
- LFLex Fridman
There's something, there's something about the night. Maybe it's the silence.
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
Maybe it's the peace all around. Maybe it's the darkness and you just-
- JLJanna Levin
Mm.
- LFLex Fridman
You can be with yourself and you can think deeply.
- JLJanna 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 (laughs) 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. (laughs)
- LFLex Fridman
Okay, so in that way and many other ways, we're kindred spirits.
- JLJanna Levin
Mm.
- LFLex Fridman
So let's go. In the... 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.
- JLJanna Levin
Mm, 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, is, 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, Karl Schwarzschild, who's, who solved Einstein's equations. You know, between sitting in the trenches (laughs) and like cannon fire, um, it was joked that he was calculating ballistic trajectories. He's also perusing the proceedings of the Prussian Academy of Sciences, (laughs) as you do.
- LFLex Fridman
Thing. (laughs)
- JLJanna Levin
(laughs) And he was an astronomer, um, 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. Um, 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. Um, but let's just pretend, and let's just imagine that- 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 te- were telling everybody at the time was that matter and energy curve space and time, and then curved spacetime tells matter and energy how to fall once the spacetime's 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 decade... 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- or a region in spacetime. It's actually, in this case, a surface in spacetime, and it marks, uh, 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. Um, sometimes we talk about the black holes being nothing because at the event horizon there's really nothing there. Uh, sometimes when we, 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- 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 Karl Schwarzschild did either. Um, I think they thought they were, uh, solving theoretical mathematical problems, um, but not describing this... what turned out to be the end state of gravitational collapse.
- LFLex Fridman
And maybe the purpose of the thought experiment was to find the limitations of the theory, so you- you find the most-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... extreme versions in order to understand where it breaks down.
- JLJanna Levin
Yeah.
- LFLex Fridman
And it just so happens in this case that might actually predict these extreme kinds of objects.
- JLJanna 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. It's almost overkill. (laughs) You don't really need to be that precise as relativity. Um, 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- (laughs) you go all the way towards the center, and yeah, this doesn't sound right anymore. Um, sometimes I liken it to, you know, it's like a dying man marking in the dirt (laughs) that something's gone wrong here, right? It- it- it's signaling that- that there's some culprit, there's something wrong in the theory. And, um, and even Roger Penrose who did this general work trying to understand, uh, 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 we'll understand this, um, differently.
- 10:37 – 21:28
Formation of black holes
- JLJanna Levin
we understand it.
- LFLex Fridman
So you've described the brain-breaking idea that a black hole is, uh, not so much a super dense matter, as it's sometimes described-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... but it's more akin to, you know, a region, you know, spacetime, but even more so just nothing.
- JLJanna Levin
Yeah. (laughs)
- LFLex Fridman
Just nothing. That- that's a thing you seem to like to say. (laughs)
- JLJanna Levin
(laughs) I do. I do like to say that bl-... (laughs) Black holes are no thing.
- LFLex Fridman
No thing.
- JLJanna Levin
They're nothing. Um...
- LFLex Fridman
Okay, so what- what does- what does that mean?
- JLJanna Levin
And that's- that's what I mean. That's the more profound aspect of the black hole. So you asked originally, um, how do they form, and I think that- that- that even when you try to form them in messy astrophysical systems, there's still nothing, at the end of the day, left behind. And, um, this was a very big surprise. Even though Einstein accepted that this was a true prediction, he didn't think that- that they'd be made. And- and it was quite astounding that- that people like Oppenheimer... Actually it's probably Oppenheimer's most important theoretical work... um, who were thinking about nuclear physics and quantum mechanics but in the context of these kind of utopian questions, "Why do stars shine?" um, "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, uh, 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 Earth full of thermonuclear fuel, they're burning, um, and emitting energy and it equals MC squared energy. So it's fusing, it's a fusion bomb, it's a constantly going thermonuclear bomb. And, um, 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 of our sun, it'll collapse under its own weight. And that collapses 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, uh, 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, 'cause stars send their material back out into space, and you and I get to be made of carbon and oxygen and (laughs) all this good stuff. We're not just hydrogen. So, (laughs) 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, uh, but bigger than a black hole, meaning it's, it's, 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 super conducting matter. Very strange, amazing objects. But if it's heavier than that, the core, and that's, you know, heavier than twice the mass of the sun, um, it will become a black hole. And Oppenheimer was... wrote this beautiful paper in 1939 with his student, uh, saying that they believed that the end state of gravitational collapse is actually a black hole. This is stunning and really, um, a visionary conclusion. Now the paper is published the same day the Nazis advance on Poland. (laughs) And so it does not get a lot of fanfare in the newspapers. (laughs)
- LFLex Fridman
Yeah, well, we think there's a lot of drama today on social media.
- JLJanna Levin
(laughs)
- LFLex Fridman
Imagine that.
- JLJanna Levin
Yes. (laughs)
- LFLex Fridman
Like, 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-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... while one of the most evil, if not the most evil humans in history is starting a, uh, the first steps-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... of a global war.
- JLJanna Levin
And what I also love about that lesson is how agnostic science is.
- LFLex Fridman
Yeah.
- JLJanna Levin
Because he was asking these utopian questions, as were other people at the time, about the nuclear physics in 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, and to enter this race over, um, a nuclear weapon. But really at the same time, 1939, really, uh, Oppenheimer's thinking about black holes. There's a, 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 he... they're sort of joking, "Well, I guess nobody's gonna pay attention to your paper now," you know, because, uh, because of the Nazi advance on Poland.
- LFLex Fridman
That's the other remarkable thing about Oppenheimer is he's also a central figure in the construction of the bomb.
- JLJanna Levin
Right.
- LFLex Fridman
So it's theory and experiment clashing together-
- JLJanna Levin
Right.
- LFLex Fridman
... with the geopolitics.
- JLJanna Levin
Exactly. So, of course, Oppenheimer, now known as the father of the atomic bomb, um, he talks about destroyers of worlds. Um, 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, um, 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, um... They still weren't called black holes. This is 1939, (laughs) 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 now... 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 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 spacetime is so tremendous that not even light can escape. 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 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, um, "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 (laughs) you know, from Seinfeld, near Columbia in New York.
- LFLex Fridman
(laughs) Nice.
- 21:28 – 27:50
Oppenheimer and the Atomic Bomb
- JLJanna Levin
- LFLex Fridman
Because you've written in many places about the human beings behind the science-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... I have to ask you about this, about nuclear weapons, whereas it's 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 (sighs) , what can you say, a- as a person who's a physicist and who have studied the physicists and written about the physicists-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... 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?
- JLJanna Levin
I think it's an excruciating moment in, in the history of science, and, um, people talk about Heisenberg who stayed in Germany and, and, uh, 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 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. (laughs) Um, I, 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 bomb. I think overwhelmingly physicists today feel we should not per- continue in the proliferation of nuclear weapons. Very few, um, theoretical physicists want to see this continue.
- LFLex 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 have created the bomb first, not the United States.
- JLJanna Levin
Yes.
- LFLex Fridman
And how different would the world be, the game theory of that-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... I think, say if the probability is 33% that it was the United States, if the Soviet Union had the bomb, I think, I think they would have used it in a much more terrifying way in the, 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 (sighs) to, to, to, to kill hundreds of millions of people.
- JLJanna Levin
In the game theory version, this was the least harmful outcome.
- LFLex Fridman
Yes.
- JLJanna Levin
Yes. But there is no outcome with no bomb.... that-
- LFLex Fridman
Yeah.
- JLJanna Levin
... that any game theorist would, uh, I think, would play. (laughs)
- LFLex Fridman
But I, I think if we just remove the geopolitics and the ideology and the-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... evil dictators, all of those people are just scientists. I think they don't necessarily even think about the ideology, and it, it's a, it's a, 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.
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
And the scientists perhaps don't... Boy, do they even have control of how that science is used? It's hard.
- JLJanna Levin
They don't have control, right. Once it's, once it's made, it's no longer scientific reasoning that dictates the use or, um, its restraint. But I will say that I do believe that it wasn't a 30, 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. So many scientists fled here.
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
They fled to here. Americans weren't fleeing to Nazi Germany. They came here, and, and they were motivated, um, by, uh, it's more than a patriotism, you know? It was a... I mean, it was a patriotism, obviously, but it was sort of more than that. It was really understanding the threat of Europe, uh, what was going on in Europe, and, um, and what that life s- how quickly it turned, how quickly this free-spirited Berlin culture, (laughs) you know, was suddenly in this repressive and terrifying, uh, regime. So I think that it was a much higher chance that it happened here in America.
- LFLex Fridman
Yeah, there's something about the American system. The, you know, it's cliché to say, but the freedom, all the different individual freedoms that enable a very vibrant, at its best, a very vibrant scientific community, that's really exciting-
- JLJanna Levin
Absolutely.
- LFLex Fridman
... to scientists, and it's very valuable to ma- maintain that.
- JLJanna Levin
Right.
- LFLex Fridman
The, the vibrancy of the debate, of the funding those mechanisms.
- 27:50 – 40:53
Inside the black hole
- LFLex Fridman
So can we just return to the collapse of a star that forms a black hole? At which point does a super dense thing become nothing, if we can just, like-
- JLJanna Levin
Yeah.
- LFLex Fridman
... linger on this concept?
- JLJanna Levin
Yeah. So if I were falling into a black hole, and I, I, I tried really fast, right as I crossed this empty region, but this demarcation, I happened to know where it was. I calculated 'cause there's no line there. There's no sign that it's there. (laughs) There's no signpost. Um, 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-
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
... 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 'cause it's like, it's like a fish swimming against the Niagara, (laughs) you know, swimming against a waterfall. (laughs)
- LFLex Fridman
But it's like stuck there.
- JLJanna Levin
But it's like stuck there.
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
Um, and so that's one way you could have a little signpost, (laughs) you know? If you fly by, you think it's moving at the speed of light.
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
It flies past you at the speed of light, but it's sitting right there at the event horizon the whole time.
- LFLex Fridman
So you're falling back across the event horizon.
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
Right at that point, you shoot outwards a photon.
- JLJanna Levin
Yes, and-
- LFLex Fridman
And it's just stuck there.
- JLJanna Levin
... it does get stuck there. (laughs) Now it's very unstable, so the star can't sit there is the point. It- 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 'cause 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.
- LFLex Fridman
Okay, can we just like 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-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... we have to be careful about making... It's better to think about the black hole as an event horizon.
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
But what can we know and what do we know about the physics of s- of spacetime inside a black hole?
- JLJanna Levin
I don't mind being incautious about thinking about what the math tells us, so.
- LFLex Fridman
Okay, great.
- JLJanna Levin
I'm not such a, uh, uh, an observer, right? (laughs) I'm very theoretical in my work. It's really pen on paper a lot. Um, these are thought experiments that I think we, we can perform and contemplate. Um, whether or not we'll ever know is another question, and, um, 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 a black hole, let's say I'm in a nice, comfortable space station, this black hole's maybe 10 times the mass of the sun, 60 kilometers across. I could be a 100 kilometers out. That's very, very close. (laughs)
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
Orbiting quite safely. No big deal. You know? Hanging out. Uh, I, I don't bug the black hole, the black hole doesn't bug me. (laughs) It won't suck me up like a vacuum or anything crazy. But, uh, some ... But my astronaut friend jumps in. Um, as they cross the event horizon, what I'm calling space, I'm looking on the outside of 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, um, uh, just contrast against a bright sky. I think, "Oh, there is 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. (laughs) 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 gonna tell me ... Well, they can't tell me, but they're gonna (laughs) 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, (laughs) 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 and there's an inevitable moment when you will hit it. (laughs) Usually for a stellar mass black hole we think it's microseconds.
- LFLex Fridman
Microseconds to get from the event horizon to the-
- JLJanna Levin
To the singularity.
- 40:53 – 44:22
Supermassive black holes
- JLJanna Levin
- LFLex Fridman
So yeah. What's, uh... How are small black holes versus super massive, uh, black holes formed? Just so people can kinda load that in.
- JLJanna Levin
Hmm. Yeah.
- LFLex Fridman
Are they, are they all... Is it always a star?
- JLJanna 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. Um, making them with stars is the 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, um, end their lives in, in the, in a death state that is a black hole. But we now see, and this was really quite a surprise, that there are super massive black holes. They're billions or even hundreds of billions of times the mass of the sun, and, um, uh, millions to, 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 hun- uh, a hundred, few hundred million years after the Big Bang, and that they're formed directly by collapsing out of primordial stuff-
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
... that there's a direct collapse right into the black hole.
- LFLex Fridman
So like in the, in the very early universe, these are primordial black holes from the-
- JLJanna Levin
Well, they're different.
- LFLex Fridman
... the stars not quite...
- JLJanna Levin
(laughs) Yeah.
- LFLex Fridman
Wait. How, how do you get from that soup-
- JLJanna Levin
(laughs)
- LFLex Fridman
... black holes right away?
- JLJanna Levin
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 talking about. So in some sense, if they're just allowed to directly collapse very early in the universe's history, they can do that more easily. Um, and it's so much so that we think that there's one of these super massive black holes in the center of every galaxy. (laughs)
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
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 um, in a really surprising and deeply connected way.
- LFLex Fridman
I wonder if the, like, the chicken or the egg, is it, uh, like how critical, how essential are the super massive black holes to the formation of galaxies?
- JLJanna Levin
Hmm. Yeah. I mean, it's ongoing, right? It's ongoing. Which came first-
- LFLex Fridman
Yeah.
- JLJanna Levin
... the black hole or the galaxy? Um, probably, um, big early stars, which were just made out of hydrogen and helium from the Big Bang. Um, there wasn't anything else, not much of anything else. Um, those early stars were forming and then maybe the black holes and kinda the galaxies were, like, these gassy clouds around them. Um, but there's probably a deep relationship between the black hole powering jets, these jets blowing material out of the galaxy, that, that shaped galaxies, maybe kind of curbed their growth. Um, and so I think the mechanisms are still, are still ongoing, i- i- attempts to understand exactly the ordering of these things.
- 44:22 – 47:25
Physics of spacetime
- JLJanna Levin
- LFLex Fridman
Can we get back to spacetime? Just going back to the beginning of the 20th century, how do you imagine spacetime? How do we, as human beings, supposed to visualize and think about spacetime-
- JLJanna Levin
Mm.
- LFLex Fridman
... where, you know, time is just another dimension in this 4D space-
- JLJanna Levin
Mm.
- LFLex Fridman
... that combines space and time? Because we've been talking about morphing in all kinds of different ways, the curvature of spacetime. Like how do you, how are we supposed to conceive of it? How do you think of it-
- JLJanna Levin
Yeah.
- LFLex Fridman
... that time's just another dimension?
- JLJanna 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. But our, we're limited because as three-dimensional beings, we can't really draw four dimensions, which is what I'd require. Three spatial, 'cause I'm pretty sure there's at least three. I think there's probably more. But, um, I'm happy just talking about the large dimensions, this, the three we see. (laughs) Up, down, right? East, west, uh, north, south. Three, three spatial dimensions. And time is the fourth. Nobody can really visualize it. Um, 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. Euclidean spacetime is when all the dimensions are orthogonal and are treated equally. Time is not another Euclidean dimension. It's actually a Minkowskian spacetime. But it means that the spacetime, 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, um, 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, I would know to use the correct rules for Minkowski spacetime. And, and that will allow me to, to, to, to calculate how long, uh, time has elapsed, which is now a kind of a length, a spacetime length on my map, um, between two relative observers. And I will get the correct answer, um, but only if I use these different rules.
- 47:25 – 52:56
General relativity
- JLJanna Levin
- LFLex Fridman
So then what does, according to general relativity, does, uh, objects with mass do to the spacetime?
- JLJanna Levin
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 gonna call that the right-hand side of the equation. (laughs) Everything on the right-hand side of Einstein's equations is how matter and energy are distributed in spacetime. 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, like, a month (laughs) of reading, um, 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 spacetime 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, for the shape of the spacetime. 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, 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. Um, so all of these consequences of his theory, you know, came with revelations (laughs) that were not at all obvious when he first wrote down, um, the general theory.
- LFLex Fridman
And he was afraid to take the consequences of that theory seriously, which is a-
- JLJanna Levin
Often.
- LFLex Fridman
The theory itself in its scope and grandeur and power is scary.
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
So I can understand.
- JLJanna Levin
(laughs)
- LFLex Fridman
Then there's, you know, the, the edges of the theory where it falls apart, the consequences of the theory that are extreme.
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
It's hard to take seriously.
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
So you can-
- JLJanna Levin
Yeah.
- LFLex Fridman
... sort of empathize.
- JLJanna Levin
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 (laughs) you know, as a physicist, and he writes all of these remarkable papers on relativity and quantum mechanics. Um, and then even in 1915, '16, he does not know that there are other galaxies out there. This, this was not known. People had mused about it. Um, 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, um, that it's confirmed that there are other galaxies.
- LFLex Fridman
Wow.
- JLJanna Levin
Yeah.
- LFLex Fridman
He didn't obviously... (laughs) There's so much we think of now that he didn't think of. So there's no big bang.
- JLJanna Levin
Right.
- LFLex Fridman
Static universe.
- JLJanna Levin
Mm. But these are all connected.
- LFLex Fridman
Wow. Yeah. So he's operating in very little information.
- JLJanna Levin
Very little information. That's absolutely true. Actually, one of the things I like to point out is the idea of relativity w- 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. (laughs) And, um, the way Einstein got there with so little information, um, 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, uh, discovered, really. That was observed through experiments trying to figure out, um, you know, what would the relative speed of light be? It's the only, re- really, only massless particles have this property that they have an absolute speed. And if you think about it, it's incredibly strange.
- LFLex Fridman
Yeah. It's really strange.
- JLJanna Levin
Incredibly strange. Yeah.
- LFLex Fridman
And then so, so from, from a theoretical perspective, he, he s- he takes that seriously.
- JLJanna Levin
He takes it very seriously, and everyone else is trying to come up with models to make it go away. (laughs)
- LFLex Fridman
Yeah.
- 52:56 – 1:09:29
Gravity
- LFLex Fridman
a world of curved spacetime? Like-
- JLJanna Levin
Mm. Yeah.
- LFLex Fridman
I, I think it's like one of the most special-
- JLJanna Levin
Oh, my gosh.
- LFLex Fridman
... leaps-
- JLJanna Levin
Yeah.
- LFLex Fridman
... in human history, right? 'Cause you're-
- JLJanna Levin
It's amazing. (laughs)
- LFLex Fridman
Like, it's very, very, very difficult to make that kind of leap.
- JLJanna Levin
I'll, 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. Um, one is to start with the equivalence principle, which he called the happiest thought of his life. (laughs)
- LFLex Fridman
(laughs)
- JLJanna Levin
And the equivalence principle comes pretty early on in his thinking. And, and, um, it starts with something like this. Like, right now, I think I'm feeling gravity 'cause I'm sitting in this chair, and I feel the pressure of the chair, and it's stopping me from falling. And, um, l- lie down in a bed and I feel heavy on the bed, and I think of that as gravity. And Einstein has a 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 'cause the floor of the elevator's resisting your fall. But I wanna 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 the state of falling or floating in the elevator, I might not know if I was in empty space, just floating, um, or if I was falling around the Earth. There would actually... They're equivalent situations. I would not be able to tell the difference. And 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, um, 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, it's just falling all the time, but it's just cruising so fast.
- LFLex Fridman
So actually, you have... Oh God, you just said so many profound things.
- JLJanna Levin
(laughs)
- LFLex Fridman
So, o- one of them is, really one of the ways to experience spacetime is to be falling.
- JLJanna Levin
To be falling. That is the purest experience of gravity. The experience of gravity, uh, unfettered, uninterrupted by atoms is weightlessness.
- LFLex Fridman
Yeah.
- JLJanna Levin
That observation... Now, it has an unhappy ending, the elevator story. (laughs)
- LFLex Fridman
Yeah.
- JLJanna Levin
Right? 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 it is. Um, 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, uh, you know, a pen, and if I throw it, (laughs) we both know it's gonna follow an arc. And it's gonna 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. And so the Earth has found itself on a free fall that happens to be a closed circle, but it's, 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? (laughs) They turn off the engines. They're just falling.
- LFLex Fridman
Yeah, they're falling.
- JLJanna Levin
And they're not that far up. Um, they're, they're... 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. (laughs) If you were to stop that, it would drop like a stone, right to the Earth. (laughs) 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 spacetime.
- LFLex Fridman
And, uh, that particular curved path is calculated in such a way that it curves onto itself-
- JLJanna Levin
Right.
- LFLex Fridman
... so you, you're orbiting.
- JLJanna Levin
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. (laughs)
- LFLex Fridman
But yeah, to be able to visualize at the beginning of the 20th century-
- JLJanna Levin
Mm-hmm.
- LFLex Fridman
... that not... you know, that free falling in, in, in curved spacetime.
- JLJanna Levin
Mm-hmm.
- 1:09:29 – 1:17:59
Information paradox
- JLJanna Levin
(laughs)
- LFLex Fridman
Well, on that topic, I have to ask you about the paradox, the information paradox of black holes. What is it?
- JLJanna 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, (laughs) 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, um, is intriguing is this idea that you can never say the vacuum's actually completely empty. We, 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 fou- foundational. So it's 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. (laughs) 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." Um, maybe two particles come into existence, and they're entangled in such a way that they cancel out each other's properties, so they, they have the properties of the vacuum, you know? They don't, they don't destroy the kind of properties of the vacuum 'cause 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 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. (laughs) 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 that's no longer the property of that vacuum it came from. It, 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. (laughs) And if it es- it'll escape, radiate out to infinity, and look like, f- to an observer far away, that the black hole has actually radiated a particle and 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 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 in time. This is energy." It is negative energy, and it is, 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. And, um, 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 information-less 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.
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
You know the mass of a 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. (laughs)
- LFLex Fridman
(laughs)
- JLJanna Levin
He did. Um, but some people who love quantum mechanics were really annoyed. Okay? 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. Um, and the reason why it says there's something fundamentally wrong with quantum mechanics is 'cause 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. In this example, more sacred 'cause you can violate conservation of energy with Heisenberg's uncertainty principle a little tiny bit. (laughs) Um, but so sacred that it created what became, um, coined as the black hole wars where people were saying, "Look, (laughs) general relativity's 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.
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
But as they get smaller and smaller, they get hotter and hotter. So as the 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, um, it will be as though the curtain, the event horizon's yanked up.
- LFLex Fridman
Mm.
- JLJanna Levin
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? 'Cause it's not in the radiation 'cause the radiation has no information.
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
And, um, and so it was an incredibly productive debate (laughs) because in it are the signs of what will make gravity and quantum mechanics play nice together, you know, some quantum theory of gravity. Um, whatever these clues are, and they're hard to assemble, uh, 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.
- LFLex Fridman
But it still has not been resolved.
- JLJanna Levin
It has not been fully resolved. (laughs)
- LFLex Fridman
I, I looked up all the different ideas for the resolution, so there's the information loss, which is what you refer to.
- JLJanna Levin
Right.
- LFLex Fridman
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."
- JLJanna Levin
I'm unhappy with that. I'm, I would not be happy with information loss. I love that it's telling us that there's this crisis 'cause I do think it's giving us the clues, and we have to take them seriously.
- LFLex Fridman
For you, the, the gut is like, uh-
- JLJanna Levin
Unitarity's gonna be preserved.
- LFLex Fridman
Preserved, so quantum mechanics is holding strong.
- JLJanna Levin
We have to come to the rescue. As Lenny Susskind in his book Black Hole War says, uh, his subtitle is, um, My Battle with Stephen Hawking to Make the World Safer for Quantum Mechanics. (laughs)
- LFLex Fridman
(laughs) Quantum mechanics, I love it.
- JLJanna Levin
Uh, something to that effect.
- 1:17:59 – 1:21:10
Fuzzballs & soft hair
- JLJanna Levin
(laughs)
- LFLex Fridman
So then from string theory, one of the resolutions is called fuzzballs. I love physicists so much.
- JLJanna Levin
(laughs)
- LFLex Fridman
"Originating from string theo- theory, this proposal suggests that black holes aren't singularities surrounded by empty space and an event horizon. Instead, they are horizon-less, complex, tangled objects, AKA fuzzballs, made of strings and branes roughly the size of the would-be event horizon."
- JLJanna Levin
Yeah.
- LFLex Fridman
"There's no single point of infinite density and no-"
- JLJanna Levin
Yeah.
- LFLex Fridman
"... true horizon to cross."
- JLJanna Levin
In some sense, it says there's no interior to the black hole, nothing ever crosses. 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 kinda shmear around."
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
"And then they get caught like a tangle-"
- LFLex Fridman
Mm.
- JLJanna Levin
"... around the event horizon, and they just actually never fall through." Um, I don't think that either.But it was interesting. (laughs)
- LFLex Fridman
Yeah. So it's just adding a very large number of extra complex-
- JLJanna Levin
Degrees of freedom.
- LFLex Fridman
... yeah.
- JLJanna Levin
There are no teeny tiny marbles to fall through.
- LFLex Fridman
But it's similar to what we already have with quantum mechanics. It's just giving a- a deeper, more complicated-
- JLJanna Levin
But it- it's really saying the interior's just not there ever, nothing falls in. So the information gets out because it never went in in the first place.
- LFLex Fridman
Oh, interesting. So there is a strong statement there. Okay.
- JLJanna Levin
It's a strong statement there, yeah.
- LFLex 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 they can store information about what fell in.
- JLJanna Levin
Worth trying, but I also don't think that that's the case. So the no-hair theorems are, um, 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 its electrical charge, its mass, and its 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'd have features that I could distinguish and it could encode the information that went in in this way. And- 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, um, I don't think that's the resolution either, but it was worth a shot. (laughs)
- LFLex Fridman
Okay. The pros and cons of that one. The pros, it works within the framework of quantum field theory in curved spacetime, potentially requiring less radical modifications than fuzzballs, or information loss. Recent work by Hawking, Perry, Strominger revitalized 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.
- 1:21:10 – 1:27:49
ER = EPR
- LFLex Fridman
Okay. Another one, this is a weird one, boy, is, uh, ER equals EPR. (laughs)
- JLJanna Levin
This is- this is probably it, though. (laughs)
- LFLex Fridman
Oh, boy. So ER equals EPR is the Einstein-Rosen bridge equals Einstein-Podolsky-Rosen bridge, posits a deep connection between quant- quantum entanglement to spacetime geometry, uh, specifically Einstein-Rosen bridge, commonly known as wormholes. It suggests that e- entangled particles are connected by a non-traversable wormhole. So tiny wormholes connected. Okay.
- JLJanna Levin
Mm-hmm. I- I can say that this is not, uh, a situation we can follow the chalk. We can't start at the beginning and calculate to the end.
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
So it's, um, it's still a conjecture. I think it's very profound, though. Um, I kind of imagine Juan Maldacena who's part of this with Lenny Susskind, they were kind of like, "Oh, it's like ER equals EPR." They couldn't even formulate it properly. It was like an intuition that they had kind of 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 and Lenny Susskind suggested it. The idea for the black hole hologram is that all of the information in the black hole, whatever it is, whatever, you know, entropy as a measure of information, uh, 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, um, is completely encoded in the area of the black hole, meaning as the area grows, the entropy grows. It does not grow as the volume. This is- 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.
- LFLex Fridman
Mm-hmm.
- JLJanna Levin
The whole universe is a holographic projection of a lower dimensional surface, right? And so people have struggled, nobody's really landed it, to find a universe version of it. 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. Uh, 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, like in a box, with gravity, and it's not the same universe we observe, doesn't matter, it's just a hypothetical called an Anti de Sitter space. There's a universe in a box, it has gravity, it has black holes, it has everything gravity can do in it. On its boundary is, uh, 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.
- LFLex Fridman
Yeah.
- JLJanna Levin
(laughs)
- LFLex Fridman
So there's, uh, there's some mathematics there, there's physics-
- JLJanna Levin
Mm-hmm. Mm-hmm.
- LFLex Fridman
... and then there's trying to conceal what that actually means practically for- for us.
- JLJanna Levin
Mm-hmm. Well, what it would mean for us is that information can't be lost even if we don't know how to show it in the description in which there are black holes. It means it can't possibly be lost because its equivalent-...to this description with no gravity in it at all, no event horizons, no black holes, just quantum mechanics. So, it really strongly suggested that, that quantum mechanics was gonna win in this battle, but it didn't show exactly how it was gonna win. So, then comes ER equals EPR. A visual way to imagine what this means, so ER has to do with little wormholes. EPR, Einstein-Podolsky-Rosen, has to do with quantum entanglement. The idea was, well, maybe the stuff that's interior to the black hole is quantum entangled, 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, it's just subtle quantum entanglement, and in fact, I can kind of 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 spacetime. If I look at it very closely, I will see, oh, no, it's actually sewn from these quantum wormholes, like embroidered. And so when I get up close, it's almost as though (laughs) the event horizon isn't the fundamental, uh, feature on the spacetime. The fundamental feature is the quantum entanglement embroidering the event horizon.
Episode duration: 3:00:50
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