Lex Fridman Podcast
#468 – Janna Levin: Black Holes, Wormholes, Aliens, Paradoxes & Extra Dimensions
Mon, 05 May 2025
Janna Levin is a theoretical physicist and cosmologist specializing in black holes, cosmology of extra dimensions, topology of the universe, and gravitational waves. Thank you for listening ❤ Check out our sponsors: https://lexfridman.com/sponsors/ep468-sc See below for timestamps, transcript, and to give feedback, submit questions, contact Lex, etc. Transcript: https://lexfridman.com/janna-levin-transcript CONTACT LEX: Feedback - give feedback to Lex: https://lexfridman.com/survey AMA - submit questions, videos or call-in: https://lexfridman.com/ama Hiring - join our team: https://lexfridman.com/hiring Other - other ways to get in touch: https://lexfridman.com/contact EPISODE LINKS: Janna's X: https://x.com/JannaLevin Janna's Website: https://jannalevin.com Janna's Instagram: https://instagram.com/jannalevin Janna's Substack: https://substack.com/@jannalevin Black Hole Survival Guide (book): https://amzn.to/3YkJzT5 Black Hole Blues (book): https://amzn.to/42Nw7IE How the Universe Got Its Spots (book): https://amzn.to/4m5De8k A Madman Dreams of Turing Machines (book): https://amzn.to/3GGakvd SPONSORS: To support this podcast, check out our sponsors & get discounts: Brain.fm: Music for focus. Go to https://brain.fm/lex BetterHelp: Online therapy and counseling. Go to https://betterhelp.com/lex NetSuite: Business management software. Go to http://netsuite.com/lex Shopify: Sell stuff online. Go to https://shopify.com/lex AG1: All-in-one daily nutrition drink. Go to https://drinkag1.com/lex OUTLINE: (00:00) - Introduction (00:51) - Sponsors, Comments, and Reflections (09:21) - Black holes (16:55) - Formation of black holes (27:45) - Oppenheimer and the Atomic Bomb (34:08) - Inside the black hole (47:10) - Supermassive black holes (50:39) - Physics of spacetime (53:42) - General relativity (59:13) - Gravity (1:15:47) - Information paradox (1:24:17) - Fuzzballs & soft hair (1:27:28) - ER = EPR (1:34:07) - Firewall (1:42:59) - Extra dimensions (1:45:24) - Aliens (2:01:00) - Wormholes (2:11:57) - Dark matter and dark energy (2:22:00) - Gravitational waves (2:34:08) - Alan Turing and Kurt Godel (2:46:23) - Grigori Perelman, Andrew Wiles, and Terence Tao (2:52:58) - Art and science (3:02:37) - The biggest mystery PODCAST LINKS: - Podcast Website: https://lexfridman.com/podcast - Apple Podcasts: https://apple.co/2lwqZIr - Spotify: https://spoti.fi/2nEwCF8 - RSS: https://lexfridman.com/feed/podcast/ - Podcast Playlist: https://www.youtube.com/playlist?list=PLrAXtmErZgOdP_8GztsuKi9nrraNbKKp4 - Clips Channel: https://www.youtube.com/lexclips
Chapter 1: What are black holes and how are they formed?
Anyway, check out a human therapist at betterhelp.com slash lex and save in your first month. That's betterhelp.com slash lex. This episode is also brought to you by NetSuite, an all-in-one cloud business management system. The more I study war, of course, the more I study business too, but war...
the more I realize the importance of the organizational layer, of the supply chain, of the logistics, the stuff that nobody talks about, the stuff that most historians don't talk about. And actually, I've read a lot of James Holland recently and spoken with him, had the great honor of speaking with him, had the great joy of speaking with him and learning from him.
And he's one of the historians that does look at the logistics, does look at the logistics details of how everything is run. And NetSuite in the company setting is doing exactly that, the details of how everything is run. Because a business is not just a CEO with a bunch of sexy ideas or the late night engineer crouching over a table trying to fix a bug, trying to find a breakthrough idea. Nope.
It's also all the other stuff that actually make the thing work, make the thing efficient. I have great tools to do so. Download the CFO's Guide to AI and Machine Learning at netsuite.com. That's netsuite.com. This episode was also brought to you by Shopify, a platform designed for anyone to sell anywhere with a great-looking online store.
Since I mentioned history, the merchant networks were crucially important. In ancient Greece. Or crucially important in the Roman Empire. And of course Genghis Khan. Very very very important. Of course Genghis Khan. Is well known for protecting the merchants. And I think any empires. Any civilizations. Any state of the global affairs. That protects the merchants.
from the friction of geopolitics, of military tensions and military conflicts, is a successful empire, successful civilization. Because trade is really, really important. It's a kind of a financial freedom.
So it's nice when in a digital age, we build systems like Shopify that allows you to exercise that financial freedom by buying stuff, selling stuff, create the market at scale in the digital world. Sign up for a $1 per month trial period at shopify.com slash lux. That's all lowercase. Go to shopify.com slash lux to take your business to the next level today.
This episode is also brought to you by AG1, an all-in-one daily drink to support better health and peak performance. Because I mentioned peak performance, I'm reminded of Nietzsche. In the book, I read maybe freshman, maybe sophomore year in college. Thus spoke Zarathustra. It's been forever. I've been reading summaries of Nietzsche way more than Nietzsche directly since college.
That's one of the worries I have with AI is the summaries, the talking about the talking about the talking is so damn efficient and fun and easy and even insightful that you don't want to go to the original sources because it's a lot of work. But you must, of course, if you want to understand. As the meme goes, but have you been there? That never gets old.
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Chapter 2: How did scientists historically struggle with the idea of black holes?
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. 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 realized that stars could form black holes is quite fascinating because the entire idea really just started as a thought experiment. If you think of, it's 1915, 1916, when Einstein fully describes relativity in a way that's the canonical formulation. It was a lot of changing back and forth before then. And it's World War I, and he gets a message
From the Eastern Front, from a friend of his, Karl Schorchild, who solved Einstein's equations, you know, between sitting in the trenches and like cannon fire. It was joked that he was calculating ballistic trajectories. He's also perusing the proceedings of the Prussian Academy of Sciences, as you do. Interesting. He was an astronomer who had enlisted in his 40s.
And he finds this really remarkable solution to Einstein's equations. And it's the first exact solution. He doesn't call it a black hole. It's not called a black hole for decades. But what I love about what Churchill did is it's a thought experiment. It's not about observations. It's not about making these things in nature. It's really just about the idea.
He sets up this completely untenable situation. He says, imagine. I crush all the mass of a star to a point. Don't ask how that's done, because that's really absurd. But let's just pretend, and let's just imagine that that's a scenario. And then he wants to decide what happens to spacetime if I set up this confounding but somehow very simple scenario.
And really what Einstein's equations were telling everybody at the time was that matter and energy curve space and time. And then curved space-time tells matter and energy how to fall once the space-time is shaped. So he finds this beautiful solution.
And the most amazing thing about his solution is he finds this demarcation, which is the event horizon, which is the region beyond which not even light can escape. And if you were to ask me today, all these decades, over 100 years later, I would say that is the black hole. The black hole is not the mass crushed to a point. The black hole is the event horizon.
And the event horizon is really just a point in spacetime or a region in spacetime. It's actually, in this case, a surface in spacetime. And it marks a separation in events, which is why it's called an event horizon. Everything outside is causally separated from the inside insofar as what's inside the event horizon can't affect events outside. What's outside can affect events inside.
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Chapter 3: What role did Oppenheimer play in understanding black holes?
Chapter 4: How does general relativity describe space-time?
That's what I mean. That's the more profound aspect of the black hole. So you asked originally, how do they form? And I think that even when you try to form them in messy astrophysical systems, there's still nothing at the end of the day left behind. This was a very big surprise, even though Einstein accepted that this was a true prediction. He didn't think that they'd be made.
And it was quite astounding that that people like Oppenheimer, actually it's probably Oppenheimer's most important theoretical work, who are thinking about nuclear physics and quantum mechanics, but in the context of these kind of utopian questions. Why do stars shine? Why is the sun radiant and hot? this amazing source of light.
And it was people like Oppenheimer who began to ask the question, well, could stars collapse to form black holes? Could they become so dense that eventually not even light would escape? And that's why I think people think that black holes are these dense objects. That's often how it's described. But actually what happens, these very massive stars, they're burning thermonuclear fuel.
You know, they're earthfuls of thermonuclear fuel they're burning. And emitting energy in E equals mc squared energy. So it's fusing. It's a fusion bomb. It's a constantly going thermonuclear bomb. And eventually it's going to run out of fuel. 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. 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. So that's the supernova explosion.
So a lot of these, they rebound because once they crunch, they've reached a new critical capacity where they can reignite to higher elements, heavier elements, and that sets off a bomb, essentially. So the star explodes, helpfully, because that's why you and I are here, because stars send their material back out into space, and you and I get to be made of carbon and oxygen and all this good stuff.
We're not just hydrogen. So the suns do that for us. And then what's left sometimes ends at a neutron star, which is a very cool object, very fascinating object, super dense, but bigger than a black hole, meaning it's not compact enough to become a black hole. It's an actual thing. A neutron star is a real thing. It's like a giant neutron.
Literally, electrons get jammed into the protons and make this giant nucleus and this superconducting matter. Very strange, amazing objects. But if it's heavier than that, the core, and that's heavier than twice the mass of the sun... It will become a black hole.
And Oppenheimer wrote this beautiful paper in 1939 with his student saying that they believed that the end state of gravitational collapse is actually a black hole. This is stunning and really a visionary conclusion. Now, the paper is published the same day the Nazis advance on Poland. And so it does not get a lot of fanfare in the newspapers.
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Chapter 5: What is the information paradox in black holes?
Oh, yeah, it's gorgeous.
The simplicity of some of these, that's so gangster. Just revolutionize all of physics. Einstein did that multiple times in a single year. When all thermonuclear sources of energy are exhausted, a sufficiently heavy star will collapse. That's an opener.
Unless fission due to rotation, the radiation of mass, or the blowing off of mass by radiation reduce the star's mass to orders of that of the sun, this contraction will continue indefinitely. And it goes on that way.
Yeah. Now, I have to say, Wheeler, who actually coins the term black hole, gives Oppenheimer quite a terrible time about this. He thinks he's wrong. And they entered what has sometimes been described as kind of a bitter, I don't know if you would actually say feud, but there were bad feelings. And Wheeler actually spent decades saying Oppenheimer was wrong.
And eventually, with his computer work, that early work that Wheeler was doing with computers when he was also trying to understand nuclear weapons, and in peacetime, found themselves returning again to these astrophysical questions, decided that actually Oppenheimer had been right.
He thought it was too simplistic, too idealized a setup that they had used, and that if you looked at something that was more realistic and more complicated, that it just simply, it just would go away. And in fact, he draws the opposite conclusion. And
There's a story that Oppenheimer was sitting outside of the auditorium when Wheeler was coming forth with his declaration that, in fact, black holes were the likely end state of gravitational collapse for very, very heavy stars. And when asked about it, Oppenheimer sort of said, well, I've moved on to other things.
because you've written in many places about the human beings behind the science. I have to ask you about this, about nuclear weapons, where 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 can you say as a person who's a physicist and who have studied the physicists and written about the physicists, the humans behind this, about this moment in human history when physicists came together and created this weapon that's powerful enough to destroy all of human civilization?
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Chapter 6: What are potential resolutions to the black hole information paradox?
And the scientists perhaps don't, boy, do they even have control of how that science is used? It's hard.
They don't have control, right. Once it's 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 a 31 third down the line because science America was different. And I think that's something we have to think about right now in this particular climate. So many scientists fled here. They fled to here.
Americans weren't fleeing to Nazi Germany. They came here and they were motivated. It's more than a patriotism. I mean, it was a patriotism, obviously, but it was sort of more than that. It was really understanding the threat of Europe, what was going on in Europe, and how quickly it turned.
How quickly this free-spirited Berlin culture, you know, was suddenly in this repressive and terrifying regime. So I think that it was a much higher chance that it happened here in America.
Yeah, there's something about the American system, the... you know, it's cliche to say, but the freedom, all the different individual freedoms that enable a very vibrant, at its best, a very vibrant scientific community, and that's really exciting. Absolutely. To scientists, and it's very valuable to maintain that. Right. The vibrancy of the debate, of the funding those mechanisms.
Absolutely, the world flocked here, and that won't be the case if we no longer have intellectual freedom.
Yeah, there's something interesting to think about. The tension, the Cold War between China and the United States in the 21st century. Some of those same questions, some of those ideas will rise up again. We want to make sure that there's a vibrant, free exchange of scientific ideas. I believe most Nobel Prizes come from the United States, right?
Oh, yeah. I don't have the number, but it's disproportionately so. In fact, a lot of them from particle physics came from the Bronx. And they were European immigrants.
How do we explain this?
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Chapter 7: How do concepts like 'ER = EPR' and 'holography' relate to black holes?
Yes, from their perspective, the singularities in their future.
But from your perspective, what do you see when your friend falls into the black hole and you're chilling outside and watching?
So one way to think about this is to think that as you're approaching the black hole, the astronaut's space-time is rotating relative to your space-time. So let's say right now my left is your right. We're not shocked by the fact that there's this relativity in left and right. It's completely understood. And I can perform a spatial rotation to align my left with your left.
Right now, I've completely rotated left out. If I just want to draw a 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. 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. So in other words, their clocks seem to be less aligned with my time. And the overall effect is that their time seems to dilate.
The spacing between ticks on the clock of their watch, let's say, on the face of their watch, is elongated, dilated. relative to mine. And it seems to me that their watches are running slowly, even though they were made in the same factory as mine, they were both synchronized beautifully, and they're excellent Swiss watches. It seems as though time is elapsing more slowly for my companion.
And likewise, for them, it seems like mine's going really fast. So years could elapse In my space station, my plants come and go. They die. I age faster. I've got gray hair. And they're falling in, and it's been minutes in their frame of reference. Flowers and their little rocket ship haven't rotted. They don't have gray hair. Their biological clocks have slown down relative to ours.
Eventually, at the event horizon, it's so extreme, it's so slow, it's as though their clocks have stopped altogether from my point of view. And that's to say that it's as though their time has completely rotated into my space. And this is connected with the idea that inside the black hole, space and time have switched places. So I might see them hover there for millennia.
Other astronauts could be born on my space station. Generations could be populated there watching this poor astronaut never fall in.
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