Inside a black hole, space and time swap places: the singularity is not a location in space but a point in the future that the infalling observer cannot avoid.

(00:00:00) The following is a conversation with Jana Levin, a theoretical physicist and cosmologist specializing in black holes, cosmology of extra dimensions, topology of the universe, and gravitational waves in space-time. (00:00:14) 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, (00:00:28) 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. (00:00:46) This was a fun and fascinating conversation. (00:00:51) And now a quick few second mention of each sponsor. (00:00:54) Check them out in the description. (00:00:55) It's the best way to support this podcast. (00:00:57) We got Brain FM for focus, better health for mental health, NetSuite for your business, Shopify for selling stuff, and the AG1 for your health. (00:01:08) Choose wiser, my friends. (00:01:10) I do these longer ad reads up in the beginning. (00:01:12) I try to make them interesting, but I do also make it super easy to skip with timestamps on screen and in the description. (00:01:19) I do, however, (00:01:20) try to make them personal, often related to stuff I'm reading or thinking about. (00:01:26) Also, if you want to get in touch with me for whatever reason, go to lexfreedman.com/contact. (00:01:30) And now onto the full ad reads. (00:01:33) Let's go. (00:01:35) This episode is brought to you by Brain.FM, a platform that offers music specially made for focus. (00:01:42) And when I say music, I mean audio experience. (00:01:46) If you ever see me out in the wild, (00:01:49) like a Starbucks. (00:01:51) I'm usually either writing or programming deeply in focus with headphones. (00:01:56) In those headphones are layers of audio. (00:02:01) A mixture of some noise, beats, rain, layers. (00:02:08) Many layers that help me deeply, deeply, deeply focus. (00:02:14) Speaking of audio, did you know that the Roman Empire (00:02:18) used synchronized war drums to coordinate legions. (00:02:24) Just imagine the sound of those drums. (00:02:27) I need to do a lot more episodes on ancient Rome, on ancient Greece, on ancient China. (00:02:35) Anyway, I'm not listening to war drums. (00:02:38) I'm listening to Brain FM, but I'm focusing. (00:02:41) You too can increase your focus and try brain.fm free for 30 days by going to brain.fm/lex. (00:02:47) That's brain.fm/lex for 30 days free. (00:02:52) This episode is also brought to you by BetterHelp, spelled H-E-L-P. (00:02:56) Help. (00:02:57) It's raining outside. (00:02:59) Thunderstorms. (00:03:00) Like somebody's knocking on the window. (00:03:03) If that's not a metaphor for prodding the subconscious mind, I don't know what is. (00:03:08) Alan Turing comes up in this episode. (00:03:11) He was crucial in the whole code-breaking effort in World War II. (00:03:14) I should probably do an episode on that. (00:03:18) His work, his person, his mind, has been a presence in my life. (00:03:25) What an incredible human being. (00:03:27) But anyway, I think of the human mind, the conscious and the subconscious, as a kind of code. (00:03:35) And therapy is a kind of code-breaking process. (00:03:39) I wonder if AI will be able to help with that. (00:03:42) Not just basic therapy, but ultra-deep personalized therapy. (00:03:48) Boy, that's a dangerous world. (00:03:50) Anyway, check out a human therapist at betterhelp.com slash Lex and save in your first month. (00:03:57) That's betterhelp.com slash Lex. (00:04:01) This episode is also brought to you by NetSuite, an all-in-one cloud business management system. (00:04:05) 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. (00:04:24) 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. (00:04:33) and learning from him. (00:04:34) And he's one of the historians that does look at the logistics, does look at the details of how everything is run. (00:04:42) And NetSuite in the company setting is doing exactly that, the details of how everything is run. (00:04:48) 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. (00:05:02) Nope, it's also all the other stuff that actually make the thing work, make the thing efficient, have great tools to do so. (00:05:09) Download the CFO's Guide to AI and Machine Learning at netsuite.com slash Lex. (00:05:14) That's netsuite.com slash Lex. (00:05:17) This episode is also brought to you by Shopify, a platform designed for anyone to sell anywhere with a great looking online store. (00:05:25) Since I mentioned history, the merchant networks were crucially important. (00:05:31) in ancient Greece, were crucially important in the Roman Empire. (00:05:35) And of course, Genghis Khan, very, very, very important. (00:05:39) Of course, Genghis Khan was well known for protecting the merchants. (00:05:45) 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 (00:06:01) empire, successful civilization. (00:06:04) Because trade is really, really important. (00:06:07) It's a kind of a financial freedom. (00:06:10) So it's nice when in the 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. (00:06:24) Sign up for a $1 per month trial period at shopify.com slash Lux. (00:06:28) That's all lowercase. (00:06:29) Go to shopify.com slash Lux to take your business to the next level today. (00:06:36) This episode is also brought to you by AG1, an all-in-one daily drink to support better health and peak performance. (00:06:43) Because I mentioned peak performance, I'm reminded of Nietzsche. (00:06:48) In the book I read maybe freshman, maybe sophomore year in college. (00:06:54) Thus spoke Zarathustra. (00:06:58) It's been forever. (00:06:59) I've been reading summaries of Nietzsche way more than Nietzsche directly since college. (00:07:09) 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. (00:07:23) that you don't want to go to the original sources because it's a lot of work. (00:07:28) But you must, of course, if you want to understand. (00:07:32) As the meme goes, but have you been there? (00:07:38) That never gets old. (00:07:40) And anyway, I think about that with some of the classics, but even some of the 20th century, 19th century works, you know, you want to read Marx directly. (00:07:50) You want to read Nietzsche directly, you want to read Sigmund Freud and Carl Jung directly. (00:07:56) Because of course there is great books about them, about their ideas, summarizing their ideas, elaborating on their ideas, putting them in the proper context, but there's nothing quite like reading it directly. (00:08:09) But anyway, I brought that up because in Thus Spoke Zarathustra, there's the pursuit of peak human potential. (00:08:17) And we in the West, on the health front, (00:08:20) have a times taking that to an almost ridiculous place. (00:08:24) I think it's still really useful, but sometimes it's also useful to fuck off a bit, to relax a bit and not care. (00:08:36) Funny enough, AG1 helps me in a certain kind of way, relax and not care. (00:08:42) I got my nutrition handled. (00:08:44) I can do all kinds of crazy physical stuff, mental stuff, because I'm drinking AG1. (00:08:50) They'll give you a one-month supply of fish oil when you sign up at drinkag1.com/lex. (00:08:56) This is a Lex Friedman podcast. (00:08:58) To support it, please check out our sponsors in the description. (00:09:01) And now, dear friends, here's Jenna Levin. (00:09:21) 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. (00:09:29) You wrote to me when the great physicist Sidney Coleman was asked to attend a 9 A.m. (00:09:34) meeting, his reply was, I can't stay up that late. (00:09:39) Yeah, classic. (00:09:40) Sydney was beloved. (00:09:42) I think all the best thoughts, honestly, maybe the worst thoughts too, are all come at night. (00:09:46) There's something about the night. (00:09:47) Maybe it's the silence. (00:09:49) Maybe it's the peace all around. (00:09:51) Maybe it's the darkness. (00:09:52) And you just, you can be with yourself and you can think deeply. (00:09:56) I feel like there's stolen hours in the middle of the night because it's not busy. (00:10:00) Your gadgets aren't pinging. (00:10:03) There's really no pressure to do anything, but I'm often awake in the middle of the night. (00:10:08) And so it's sort of like these extra hours of the day. (00:10:10) I think we were exchanging messages at 4:00 in the morning. (00:10:14) Okay. (00:10:15) So in that way, many other ways we're kindred spirits. (00:10:18) So let's go. (00:10:18) In one of the coolest objects in the universe, black holes, what are they? (00:10:24) And maybe even a good way to start is to talk about how are they formed. (00:10:30) Yeah. (00:10:31) In a way, people often confuse how they're formed with the concept of the black hole in the 1st place. (00:10:37) 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. (00:10:48) And then nature thinks of a way. (00:10:50) Nature thinks of a way to make these crazy objects, which is to kill off a few stars. (00:10:54) 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. (00:11:04) And it's really not the case. (00:11:05) Black holes are more general and more fundamental than just the death state of a star. (00:11:12) But even the history of how people realize that stars could form black holes is quite fascinating because the entire idea really just started as a thought experiment. (00:11:23) If you think of, it's 1915, 1916, when Einstein fully describes relativity in a way that's the canonical formulation. (00:11:33) It was a lot of changing back and forth before then. (00:11:35) 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, you know, between sitting in the trenches and like cannon fire. (00:11:49) It was joked that he was calculating ballistic trajectories. (00:11:52) He's also perusing the proceedings of the Prussian Academy of Sciences, as you do. (00:11:59) And he was an astronomer who had enlisted in his 40s. (00:12:03) And he finds this really remarkable solution to Einstein's equations. (00:12:06) And it's the first exact solution. (00:12:09) He doesn't call it a black hole. (00:12:10) It's not called a black hole for decades. (00:12:13) But what I love about what Schwarzschild did is it's a thought experiment. (00:12:17) It's not about observations. (00:12:18) It's not about making these things in nature. (00:12:22) It's really just about the idea. (00:12:23) He sets up this completely untenable situation. (00:12:27) He says, imagine I crush all the mass of a star to a point. (00:12:32) Don't ask how that's done, because that's really absurd. (00:12:35) But let's just pretend. (00:12:37) And let's just imagine that that's a scenario. (00:12:39) And then (00:12:40) He wants to decide what happens to space-time if I set up this confounding, but somehow very simple scenario. (00:12:48) And really what Einstein's equations were telling everybody at the time was that matter and energy curved space and time, and then curved space-time tells matter and energy how to fall once the space-time is shaped. (00:13:00) So he finds this beautiful solution. (00:13:02) And the most amazing thing about his solution is he finds this demarcation, which is the event horizon. (00:13:08) which is the region beyond which not even light can escape. (00:13:13) And if you were to ask me today, all these decade, over 100 years later, I would say that is the black hole. (00:13:19) The black hole is not the mass crushed to a point. (00:13:22) The black hole is the event horizon. (00:13:25) And the event horizon is really just a point in space-time or a region at space-time. (00:13:30) It's (00:13:31) actually, in this case, a surface in space-time. (00:13:34) And it marks a separation in events, which is why it's called an event horizon. (00:13:39) Everything outside is causally separated from the inside insofar as what's inside the event horizon can't affect events outside. (00:13:48) What's outside can affect events inside. (00:13:51) I can throw a probe into a black hole and cause something to happen on the inside. (00:13:56) But the opposite isn't true. (00:13:57) Somebody who fell in can't send a probe out. (00:14:00) And this one-way aspect really is what's profound about the black hole. (00:14:06) Sometimes we talk about the black holes being nothing because at the event horizon, there's really nothing there. (00:14:12) Sometimes when we think about black holes, we want to imagine a really dense, dead star. (00:14:18) But if you go up to the event horizon, it's an empty region of space-time. (00:14:22) It's more of a place than it is a thing. (00:14:26) And Einstein found this fascinating. (00:14:28) He helped get the work published, but he really didn't think these would form in nature. (00:14:33) I doubt Karl Schwarzschild did either. (00:14:36) I think they thought they were solving theoretical, mathematical problems, but not describing what turned out to be the end state of gravitational collapse. (00:14:49) And maybe the purpose of the thought experiment was to find the limitations of the theory. (00:14:54) So you find the most extreme versions in order to understand where it breaks down. (00:14:59) Yeah. (00:15:00) And it just so happens in this case that might actually predict these extreme kinds of objects. (00:15:06) It does both. (00:15:07) So it also describes the sun from far away. (00:15:11) So the same solution does a great job (00:15:14) helping us understand the Earth's orbit around the sun. (00:15:17) It's incredible. (00:15:18) It does a great job. (00:15:19) It's almost overkill. (00:15:20) You don't really need to be that precise as relativity. (00:15:24) And yes, it predicts the phenomenon of black holes, but it doesn't really explain how nature would form them. (00:15:30) But then it also, on top of that, does signal the breakdown of the theory. (00:15:34) I mean, you're quite right about that. (00:15:35) It actually says, oh man, but you go all the way towards the center and yeah, this doesn't sound right anymore. (00:15:43) Sometimes I liken it to, it's like a dying man marking in the dirt that something's gone wrong here, right? (00:15:52) It's signaling that there's some culprit, there's something wrong in the theory. (00:15:57) And even Roger Penrose, who did this general work trying to understand the formation of black holes from gravitational collapse, he thought, oh yeah, there's a singularity that's inevitable. (00:16:10) It's in every, there's no way around it. (00:16:13) once you form a black hole. (00:16:15) 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 differently. (00:16:26) So according to him, the closer you get to the singularity, the more quantum mechanics comes into play, and therefore there's no singularity, there's something else. (00:16:33) I think everybody would say that. (00:16:34) I think everybody would say the closer you get to the singularity, for sure you have to include quantum mechanics. (00:16:40) 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. (00:16:53) That's just inconsistent with the world as we understand it. (00:16:56) So you've described the brain-breaking idea that a black hole is (00:17:02) not so much a super dense matter as it's sometimes described, but it's more akin to a region of space-time, but even more so just nothing. (00:17:13) Yeah. (00:17:14) It's nothing. (00:17:15) That's the thing you seem to like to say. (00:17:17) I do. (00:17:17) I do like to say that black holes are no thing. (00:17:21) They're nothing. (00:17:22) Okay, so what does that mean? (00:17:23) And that's what I mean by that's the more profound aspect of the black hole. (00:17:27) So you asked originally, how do they form? (00:17:31) 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. (00:17:41) This was a very big surprise, even though Einstein accepted that this was a true prediction. (00:17:46) He didn't think that they'd be made. (00:17:49) 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, why do stars shine? (00:18:06) Why is the sun radiant and hot and this amazing source of light? (00:18:10) And it was people like Oppenheimer who began to ask the question, well, could stars collapse to form black holes? (00:18:18) Could they become so dense that eventually not even light would escape? (00:18:25) And that's why I think people think that (00:18:29) Black holes are these dense objects. (00:18:30) That's often how it's described. (00:18:32) But actually what happens, these very massive stars, they're burning thermonuclear fuel. (00:18:36) You know, they're earth fulls of thermonuclear fuel they're burning and emitting energy in E equals MC squared energy. (00:18:44) So it's fusing. (00:18:45) It's a fusion bomb. (00:18:46) It's A constantly going thermonuclear bomb. (00:18:49) And eventually it's going to run out of fuel. (00:18:52) It's going to run out of hydrogen, helium stuff to fuse. (00:18:55) It hits an iron core. (00:18:57) Iron (00:18:58) to go past iron with fusion is actually energetically expensive. (00:19:02) So it's no longer going to do that so easily. (00:19:05) So suddenly it's run out of fuel. (00:19:07) 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. (00:19:15) And that collapse is incredibly fast and dramatic and it creates a shockwave. (00:19:20) So that's the supernova explosion. (00:19:22) So a lot of these, they rebound because once they crunch, they've reached a new critical (00:19:28) capacity where they can reignite to higher elements, heavier elements, and that sets off a bomb, essentially. (00:19:36) 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. (00:19:49) We're not just hydrogen. (00:19:51) So the suns do that for us. (00:19:53) 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. (00:20:08) It's an actual thing. (00:20:09) A neutron star is a real thing. (00:20:10) It's like a giant neutron. (00:20:12) Literally, electrons get jammed into the protons and make this giant nucleus and this superconducting matter. (00:20:18) Very strange. (00:20:19) amazing objects. (00:20:21) 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. (00:20:30) 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. (00:20:42) This is stunning and really (00:20:46) a visionary conclusion. (00:20:48) Now, the paper is published the same day the Nazis advance on Poland. (00:20:52) And so it does not get a lot of fanfare in the newspapers. (00:20:57) We think there's a lot of drama today on social media. (00:20:59) Imagine that. (00:21:01) Like, here's a guy who predicts how actually in nature would be the formation of this most radical object that broke even Einstein's brain, while (00:21:13) one of the most evil, if not the most evil humans in history, starting the first steps of a global war. (00:21:20) What I also love about that lesson is how agnostic science is. (00:21:24) Because he was asking these utopian questions, as were other people at the time, about the nuclear physics and stars. (00:21:30) You might know this play, Copenhagen, by Michael Frayn. (00:21:33) There's this line that he attributes to Bohr. (00:21:35) And Bohr was the great thinker of early foundations of quantum mechanics. (00:21:41) Danish physicist, where Bohr says to his wife, Nobody's thought of a way to kill people using quantum mechanics. (00:21:48) Now, of course, then there's the nuclear bomb. (00:21:51) 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. (00:22:02) But really, at the same time, 1939, really, Oppenheimer's thinking about black holes. (00:22:08) There's even a small line in Chris Nolan's film. (00:22:11) It's very hard to catch. (00:22:13) 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, you know, because of the Nazi advance on Poland. (00:22:22) That's the other remarkable thing about Oppenheimer is he's also a central figure in the construction of the bomb. (00:22:28) Right. (00:22:28) So it's theory and experiment clashing together with the geopolitics. (00:22:32) Exactly. (00:22:33) So of course, Oppenheimer, now known as the father of the atomic bomb, he talks about destroyers of worlds. (00:22:42) But it's the same technology. (00:22:44) And that's what I mean by science is agnostic, right? (00:22:46) It's the same technology, overcoming a critical mass, (00:22:50) igniting thermonuclear fusion. (00:22:53) Eventually, there was a fission. (00:22:54) 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? (00:23:06) So some of that mass, that E equals MC squared energy, had escaped. (00:23:11) And it was the first kind of concrete demonstration of this, Einstein's most famous equation. (00:23:17) So all of this comes together, but the story of, they still weren't called black holes. (00:23:23) This is 1939. (00:23:25) And they had these very long-winded ways of describing the end state, the catastrophic end state of gravitational collapse. (00:23:32) But what you have to imagine is as this star collapses, so now, so what's the sun? (00:23:36) The sun's a million and a half kilometers across. (00:23:40) So imagine a star much bigger than the sun. (00:23:43) much bigger radius. (00:23:44) And it's so heavy, it collapses, it's supernovas. (00:23:47) What's left is still maybe 10 times the mass of the sun, just what's left in that core. (00:23:52) And it continues to collapse. (00:23:53) And when that reaches about 60 kilometers across, like just imagine 10 times the mass of the sun, city sized. (00:24:00) That is a really dense object. (00:24:03) And now the black hole essentially has begun to form, meaning the curve in space-time is so tremendous that not even light can escape. (00:24:11) The event horizon forms, but the event horizon is almost imprinted on the space-time, because the star can't sit there in that dense state any more than it can race outward at the speed of light, because even light is forced to rain inwards. (00:24:26) So the star continues to fall, and that's the magic part. (00:24:30) The star leaves the event horizon behind. (00:24:33) And it continues to fall, and it falls into the interior of the black hole. (00:24:38) Where it goes, nobody really knows, but it's gone from sight. (00:24:43) It goes dark. (00:24:45) 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. (00:24:52) The star, like the Cheshire Cat, fades from view. (00:24:56) One leaves behind only its grin, the other only its gravitational attraction. (00:25:02) And he was giving a lecture. (00:25:04) It's actually above Tom's restaurant, from Seinfeld near Columbia in New York. (00:25:10) There was a place there, still is a place there where people were giving lectures about astrophysics and it's 1967. (00:25:18) Wheeler is exhaustively saying this loaded term, the end state of catastrophic gravitational collapse. (00:25:26) And rumor is that someone shouts from the back row, well, how about black hole? (00:25:30) And apparently he then foists this term on the world. (00:25:36) Wheeler had a way of doing that. (00:25:37) Well, I love terms like that. (00:25:38) Big bang, black hole. (00:25:40) There's some, I mean, it's just pointing out the elephant in the room and calling it an elephant. (00:25:46) It is a black hole. (00:25:47) That's a pretty accurate and deep description. (00:25:50) I just wanted to point out that the, just looking for the first time, it's a 1939 paper from Oppenheimer. (00:25:56) It's like 2 pages. (00:25:57) It's like 3 pages. (00:25:58) Oh yeah, it's gorgeous. (00:26:00) The simplicity of some of these, that's so gangster. (00:26:03) Just revolutionize all of physics with, Einstein did that multiple times in a single year. (00:26:09) When all thermonuclear sources of energy are exhausted, a sufficiently heavy star will collapse. (00:26:14) That's an opener. (00:26:16) Unless fission due to rotation, the radiation of mass or the blowing off of mass by radiation reduced the star's mass to orders of that of the sun, this contraction will continue indefinitely. (00:26:28) And it goes on that way. (00:26:30) I have to say that Wheeler, who actually coins the term black hole, gives Oppenheimer quite a terrible time about this. (00:26:37) He thinks he's wrong. (00:26:38) And they entered what has sometimes been described as kind of a bitter, I don't know if you would actually say feud, but they were bad feelings. (00:26:47) And Wheeler actually spent decades saying, (00:26:52) Oppenheimer was wrong. (00:26:53) And eventually, with his computer work, that early work that Wheeler was doing, the 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. (00:27:12) 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. (00:27:24) And in fact, he draws the opposite conclusion. (00:27:27) 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. (00:27:41) And when asked about it, Oppenheimer sort of said, well, I've moved on to other things. (00:27:46) Because you've written in many places about the human beings behind the science. (00:27:50) I have to ask you about this, about nuclear weapons. (00:27:53) Where is the greatest of physicists coming together to create this most terrifying and powerful of a technology? (00:28:00) And now I get to talk to world leaders for whom this technology is part of the tools that is used (00:28:08) perhaps implicitly on the chessboard of geopolitics. (00:28:12) 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? (00:28:32) I think it's an excruciating moment in the history of science. (00:28:37) And (00:28:39) people talk about Heisenberg, who stayed in Germany and worked for the Nazis in their own attempt to build the bomb. (00:28:48) 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 (00:29:04) errors in his original estimates about how much material would be required or how they would get over the energy barriers. (00:29:10) And that's a terrifying thought. (00:29:14) I don't know that any of us can really put ourselves in that position of imagining that we're faced with that quandary. (00:29:22) Having to take the initiative to participate in thinking of a way that quantum mechanics can kill people and then making the bomb. (00:29:28) I think overwhelmingly physicists today feel we should (00:29:33) not continue in the proliferation of nuclear weapons. (00:29:37) Very few theoretical physicists want to see this continue. (00:29:42) That moment in history, the Soviet Union had incredible scientists, Nazi Germany had incredible scientists, and the United States had incredible scientists. (00:29:50) And it's very easy to imagine that one of those three would have created the bomb first, not the United States. (00:29:59) And how different would the world be? (00:30:01) The game theory of that, I think, say if the probability is 33% that it was in the United States, if the Soviet Union had the bomb, I think they would have used it in a much more terrifying way in the European theater and maybe turn on the United States. (00:30:22) And obviously with Hitler, he would have used it. (00:30:25) I think there's no question he would have used it. (00:30:28) to kill hundreds of millions of people. (00:30:31) In the game theory version, this was the least harmful outcome. (00:30:35) Yes. (00:30:36) But there is no outcome with no bomb that any game theorist would, I think, would play. (00:30:43) But I think if we just remove the geopolitics and the ideology and the evil dictators, all of those people are just scientists. (00:30:53) I think they don't necessarily even think about the ideology. (00:30:57) 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. (00:31:12) And the scientists perhaps don't, boy, do they even have control of how that science is used? (00:31:18) It's hard. (00:31:18) They don't have control, right? (00:31:20) Once it's made, it's no longer scientific reasoning that dictates the use. (00:31:26) or it's restraint. (00:31:29) But I will say that I do believe that it wasn't a 31 third down the line because America was different. (00:31:37) And I think that's something we have to think about right now in this particular climate. (00:31:41) So many scientists fled here. (00:31:44) They fled to here. (00:31:46) Americans weren't fleeing to Nazi Germany. (00:31:49) They came here and they were motivated by (00:31:55) it's more than a patriotism, I mean, it was a patriotism, obviously, but it was sort of more than that. (00:32:01) It was really understanding the threat of Europe, what was going on in Europe, and what that life, how quickly it turned, how quickly this free-spirited Berlin culture, you know, was suddenly in this repressive and terrifying regime. (00:32:21) So I think that it was a much higher chance that it happened here in America. (00:32:26) Yeah, there's something about the American system. (00:32:27) The, it's cliche to say, but the freedom. (00:32:31) all the different individual freedoms that enable a very vibrant, at its best, a very vibrant scientific community. (00:32:37) And that's really exciting to scientists. (00:32:39) And it's very valuable to maintain that, the vibrancy of the debate, of the funding those mechanisms. (00:32:47) Absolutely. (00:32:48) The world flocked here. (00:32:50) And that won't be the case if we no longer have intellectual freedom. (00:32:55) Yeah, there's something interesting to think about. (00:32:57) 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. (00:33:04) And we want to make sure that there's a vibrant, free exchange of scientific ideas. (00:33:10) And I believe most Nobel Prizes come from the United States, right? (00:33:15) Oh, yeah, I don't have the number, but I- But it's disproportionately so. (00:33:18) It's disproportionately so. (00:33:19) In fact, a lot of them from particle physics came from the Bronx. (00:33:26) And they were European immigrants. (00:33:27) How do you explain this? (00:33:28) fled Europe precisely because of the geopolitics we're describing. (00:33:33) And so instead of being Nobel Prize winners from the Soviet Union or from the Eastern Bloc, they were from the Bronx. (00:33:40) 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. (00:33:47) There's tensions, there's battles, there's some are loners, some are great collaborators, some are tormented. (00:33:53) Some are easy going, all this kind of stuff. (00:33:55) And that's the beautiful thing about it we forget sometimes is it's humans. (00:33:59) And humans are messy and complicated and beautiful and all of that. (00:34:02) Yeah. (00:34:04) So what were we talking about? (00:34:05) Oh. (00:34:06) The star is collapsing. (00:34:08) Okay. (00:34:09) So can we just return to the collapse of a star that forms a black hole? (00:34:16) At which point (00:34:18) does the super dense thing become nothing if you can just like linger on this concept? (00:34:22) Yeah. (00:34:23) 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. (00:34:37) There's no sign that it's there. (00:34:39) There's no signpost. (00:34:40) I could emit a little light pulse and try to send it outward exactly at the event horizon. (00:34:46) So it's racing outward at the speed of light. (00:34:48) It can hover there because from my perspective, it's very strange. (00:34:52) The space-time is like a waterfall raining in and I'm being dragged in with that waterfall. (00:34:57) I can't stop at the event horizon. (00:34:58) It comes, it goes, it's behind me really quickly. (00:35:02) That light beam can try to sit there because it's like a fish swimming against the Niagara, you know, swimming against the waterfall. (00:35:09) It's like stuck there. (00:35:10) But it's like stuck there. (00:35:13) And so that's one way you can have a little signpost. (00:35:15) if you fly by, you think it's moving at the speed of light. (00:35:17) It flies past you at the speed of light, but it's sitting right there at the event horizon. (00:35:21) So you're falling back across the event horizon, right at that point, you shoot outwards a photon. (00:35:26) Yes. (00:35:27) And it's just stuck there. (00:35:28) It just gets stuck there. (00:35:30) Now it's very unstable. (00:35:32) So the star can't sit there is the point. (00:35:34) It just can't. (00:35:35) So it rains inward with this waterfall. (00:35:38) But from the outside, all we should ever really care about is the event horizon, because I can't know what happens to it. (00:35:44) 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. (00:35:53) Won't matter to me because I can't know anything about what happened on the inside. (00:35:57) Okay, can we just like linger on this? (00:35:58) So what models do we have about what happens on the inside of the black hole at that moment? (00:36:03) So I guess that one of the intuitions, one of the big reminders (00:36:06) that you're giving to us is like, hey, we know very little about what can happen on the inside of a black hole. (00:36:13) And that's why we have to be careful about making, it's better to think about the black hole as an event horizon. (00:36:20) But what can we know and what do we know about the physics of space-time inside a black hole? (00:36:27) I don't mind being incautious about thinking about what the math tells us. (00:36:31) So I'm not such a (00:36:35) an observer. (00:36:36) I'm very theoretical in my work. (00:36:38) It's really pen on paper a lot. (00:36:40) These are thought experiments that I think we can perform and contemplate. (00:36:46) Whether or not we'll ever know is another question. (00:36:49) 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. (00:36:59) So while I'm on the outside of the black hole, let's say I'm in a nice, comfortable space station. (00:37:05) This black hole is maybe 10 times the mass of the sun, 60 kilometers across. (00:37:10) I could be 100 kilometers out. (00:37:11) That's very, very close. (00:37:13) Orbiting quite safely. (00:37:15) No big deal, you know, hanging out. (00:37:18) I don't bug the black hole. (00:37:19) Black hole doesn't bug me. (00:37:20) It won't suck me up like a vacuum or anything crazy. (00:37:23) But my astronaut friend jumps in. (00:37:29) 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. (00:37:40) It's a shadow because everything gets too close, falls in. (00:37:42) It's just this contrast against a bright sky. (00:37:47) I think, oh, there's a center of a sphere. (00:37:50) And in the center of the sphere is the singularity. (00:37:52) It's a point in space from my perspective. (00:37:55) But from the perspective of the astronaut who falls in, it's actually a point in time. (00:38:00) So their notions of space and time have rotated so completely that what I'm calling a direction in space towards the center of the black hole, like the center of a physical sphere, they're going to tell me what they can't tell me, but they're going to come to the conclusion, oh no, that's not a location in space. (00:38:17) That's a location in time. (00:38:19) 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. (00:38:29) So there's no shenanigans you can do once you're inside the black hole to try to skirt it, the singularity. (00:38:36) You can't set yourself up in orbit around it. (00:38:39) You can't try to fire rockets and stay away from it, because it's in your future. (00:38:44) And there's an inevitable moment when you will hit it. (00:38:47) Usually for a stellar mass black hole, we think it's microseconds. (00:38:51) Microseconds to get from the event horizon to the singularity. (00:38:54) To the singularity. (00:38:55) Oh boy. (00:38:55) Oh boy. (00:38:57) So that's describing from your astronaut friend's perspective. (00:39:03) Yes, from their perspective, the singularity's in their future. (00:39:06) But from your perspective, (00:39:08) What do you see when your friend falls into the black hole and you're chilling outside and watching? (00:39:14) So one way to think about this is to think that as you're approaching the black hole, (00:39:23) The astronaut's space-time is rotating relative to your space-time. (00:39:29) So let's say right now, my left is your right. (00:39:33) We're not shocked by the fact that there's this relativity in left and right. (00:39:37) It's completely understood. (00:39:38) And I can perform a spatial rotation to align my left with your left. (00:39:43) Right now, I've completely rotated left out, right? (00:39:48) If I just want to draw a kind of (00:39:52) a compass diagram, not a compass diagram, but at the top of maps there's a north-south, east, west. (00:39:56) But now time is up, down, and one direction of space. (00:40:00) 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. (00:40:08) So what is the effect of that? (00:40:10) 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. (00:40:24) So in other words, their clocks seem to be less aligned (00:40:29) with my time. (00:40:31) And the overall effect is that their time seems to dilate. (00:40:34) 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. (00:40:46) 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. (00:40:56) It seems as though time is elapsing more slowly for my companion. (00:41:00) And likewise, for them, it seems like mine's going really fast. (00:41:06) So years could elapse. (00:41:09) In my space station, my plants come and go, they die, I age faster, I've got gray hair. (00:41:15) And they're falling in and it's been minutes in their frame of reference. (00:41:21) Flowers and their little rocket ship haven't rotted. (00:41:25) They don't have gray hair. (00:41:27) Their biological clocks have slowed down relative to ours. (00:41:31) 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. (00:41:40) And that's to say that it's as though their time is completely rotated into my space. (00:41:46) And this is connected with the idea that inside the black hole, space and time have switched places. (00:41:52) So I might see them hover there for millennia. (00:41:57) Other astronauts could be born on my space station. (00:42:00) Generations could be populated there watching this poor astronaut never fall in. (00:42:07) So basically, the time almost comes to a standstill, but we still, they do fall in. (00:42:15) Right, they do fall in eventually. (00:42:17) Now that's because they have some mass of their own. (00:42:20) So they're not a perfectly light particle. (00:42:23) And so they deform the event horizon a little bit. (00:42:27) You will actually see the event horizon bobble and absorb the astronaut. (00:42:32) So in some finite time, the astronaut will actually fall in. (00:42:36) So it's like this weird space-time bubble that we have around us. (00:42:41) And then there's a very big... (00:42:43) space-time curvature bubble thing from the black hole and there's a nice swirly type situation going on. (00:42:50) That's how you get sucked up. (00:42:51) Yeah. (00:42:52) So if you're a perfect, like infinitely small particle, you would just be... (00:42:56) Take longer and longer. (00:42:57) And probably just be stuck there or something, but no, there's quantum mechanics. (00:43:01) Eventually you'll fall in. (00:43:03) Any perturbation will only go one way. (00:43:05) It's unstable in one direction, in one direction only. (00:43:09) But it's really... (00:43:12) important to remember that from the point of view of the astronaut, not much time has passed at all. (00:43:17) You just sail right across as far as you're concerned, and nothing dramatic happens there. (00:43:22) You might not even realize you've come to the event horizon. (00:43:25) You might not even realize you've crossed the event horizon, because there's nothing there, right? (00:43:31) This is an empty region of space-time. (00:43:34) There's no marker to tell you you've reached this very dangerous point of no return. (00:43:39) You can fire your rockets like hell when you're on the outside and maybe even escape, right? (00:43:44) But once you get to that point, there's no amount of energy. (00:43:48) All the energy in the universe will not save you from this demise. (00:43:54) You know, there's different sized black holes. (00:43:57) And maybe can we talk about the experience that you have falling into a black hole, depending on what the size of the black hole is? (00:44:03) Yeah. (00:44:03) Because as I understand, the bigger it is, (00:44:09) the less drastic the experience of falling into it. (00:44:14) Yeah, that might surprise people. (00:44:16) The bigger it is, the less noticeable it is that you've crossed the event horizon. (00:44:22) One way to think about it is curvature is less noticeable the bigger it is. (00:44:27) So if I'm standing on a basketball, I'm very aware I'm balancing on a curved surface. (00:44:33) My 2 feet are in different locations, and I really notice. (00:44:37) But on the Earth, you actually have to be kind of clever to deduce that the Earth is curved. (00:44:41) The bigger the planet, the less you're going to notice the curvature, the global curvature. (00:44:48) And it's the same thing with a black hole, a huge, huge black hole. (00:44:50) It just kind of feels like just flat. (00:44:54) You don't really notice. (00:44:55) I'm trying to figure out how the physics, because if you don't notice... (00:44:58) And there's nothing there. (00:44:59) But the physics is weird. (00:45:01) In your frame of reference. (00:45:04) No. (00:45:05) Well, so another cool thing, so I like to dispel myths. (00:45:09) Yeah. (00:45:11) Do you need a minute? (00:45:13) You're holding your head. (00:45:14) There's a sense like you should be able to know when you're inside of a black hole, when you've crossed the event horizon. (00:45:20) But no, from your frame of reference, you might not be able to know. (00:45:24) Yeah, at first, at least, you might not realize what's happened. (00:45:28) There are some hints, for instance, (00:45:31) Black holes are dark from the outside, but they're not necessarily dark on the inside. (00:45:36) So this is a kind of fascinating, that your experience could be that it's quite bright inside the black hole because all the light from the galaxy can be shining in behind you and it's focusing down because you're all (00:45:52) approaching this really focused region in the interior. (00:45:56) And so you actually see a bright white flash of light as you approach the singularity. (00:46:02) You know, I kind of, I joke that it's a, you know, it's like a near-death experience. (00:46:06) We see the light at the end of the tunnel. (00:46:08) So you would see millennia pass on Earth. (00:46:10) You could see the evolution of the entire galaxy, you know, one big bright flash of light. (00:46:16) So it's like a near-death experience, but it's definitely a total death experience. (00:46:19) It goes pretty fast, but you looking out (00:46:22) You're looking out, everything's going super fast. (00:46:25) Yeah, the clocks on the earth, on the space station seem to be progressing very rapidly relative to yours. (00:46:34) The light can catch up to you and you get this bright beam of light as you see the evolution of the galaxy unfold. (00:46:42) And I mean, it sort of depends on the size of the black hole and how long you have to hang around. (00:46:48) The bigger the black hole, the longer it takes you (00:46:51) to expire in the center. (00:46:53) Obviously, the human sensory system, we're not able to process that information correctly. (00:46:59) Right. (00:46:59) It would be a microsecond in a, right, that would be too fast. (00:47:02) Yeah, but it would be, wow, it'd be so cool to get that information. (00:47:06) But a big black hole, you could actually, you know, hang around for some months. (00:47:11) So yeah, what's, how are small black holes, or just supermassive (00:47:16) the black holes formed? (00:47:18) Just so people kind of load that in. (00:47:21) Are they all, is it always a star? (00:47:25) No. (00:47:25) So this is also why it's important to think of black holes more abstractly. (00:47:31) They are something very profound in the universe, and there are probably multiple ways to make black holes. (00:47:38) Making them with stars is most plentiful. (00:47:41) There could be hundreds of millions, maybe even a billion black holes in our Milky Way galaxy alone. (00:47:47) That many stars, it's only about 1% of stars that will end their lives in a death state that is a black hole. (00:47:55) But we now see, and this was really quite a surprise, that there are supermassive black holes. (00:48:01) There are billions or even hundreds of billions of times the mass of the sun. (00:48:07) And (00:48:09) millions to 10s of billions, maybe even hundreds of billions. (00:48:12) So extremely massive. (00:48:14) We don't think that the universe has had enough time to make them from stars that just merge. (00:48:20) 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. (00:48:27) We don't think there's been enough time for that. (00:48:29) So it's suspected that they're formed very early, maybe even 100, a few 100 million years. (00:48:36) after the Big Bang and that they're formed directly by collapsing out of primordial stuff. (00:48:43) That there's a direct collapse right into the black hole. (00:48:47) So like in the very early universe, (00:48:50) These are primordial black holes from the stars not quite. (00:48:55) Wait, how do you get from that soup black holes right away? (00:48:59) Right. (00:48:59) 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. (00:49:10) 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. (00:49:19) And it's so much so that (00:49:20) We think that there's one of these supermassive black holes in the center of every galaxy. (00:49:26) So they're not rare, and we know where they are. (00:49:28) They're in the nuclei of galaxies. (00:49:30) So they're bound to the very early formation of entire galaxies in a really surprising and deeply connected way. (00:49:39) I wonder if the, like the chicken or the egg, is it, (00:49:44) Like how critical, how essential are the supermassive black holes of the formation of galaxies? (00:49:49) Yeah, I mean, it's ongoing, right? (00:49:51) It's ongoing. (00:49:52) Which came first? (00:49:53) The black hole or the galaxy? (00:49:56) Probably big early stars, which were just made out of hydrogen and helium from the Big Bang. (00:50:04) There wasn't anything else, not much of anything else. (00:50:07) Those early stars were forming and then maybe the black holes and kind of the galaxies were like these (00:50:12) gassy clouds around them. (00:50:15) But there's probably a deep relationship between the black hole powering jets, these jets blowing material out of the galaxy that shaped galaxies maybe kind of curbed their growth. (00:50:30) And so I think the mechanisms are still ongoing, attempts to understand exactly the ordering of these things. (00:50:40) Can we get back to space-time? (00:50:42) Just going back to the beginning of the 20th century, how do you imagine space-time? (00:50:46) 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? (00:50:56) Because we've been talking about morphing in all kinds of different ways, the curvature of space-time. (00:51:00) Like how do you, how are we supposed to conceive of it? (00:51:03) How do you think of it? (00:51:04) Yeah. (00:51:05) Time's just another dimension. (00:51:07) There are different ways we can think about it. (00:51:09) We can imagine drawing a map of space and treating time as another direction in that map. (00:51:18) But we're limited because as three-dimensional beings, we can't really draw four dimensions, which is what I'd require, three spatial, because I'm pretty sure there's at least three. (00:51:28) I think there's probably more. (00:51:30) But (00:51:31) I'm happy just talking about the large dimensions, the three we see up, down, right? (00:51:37) East, west, north-south, three spatial dimensions. (00:51:42) And time is the fourth. (00:51:44) Nobody can really visualize it. (00:51:49) But we know mathematically how to unpack it on paper. (00:51:52) I can mathematically suppress one of the spatial dimensions, and then I can draw it pretty well. (00:51:59) Now, the problem is that we'd call it a Euclidean spacetime. (00:52:02) A Euclidean spacetime is when all the dimensions are orthogonal and are treated equally. (00:52:07) Time is not another Euclidean dimension. (00:52:10) It's actually a Minkowskian spacetime. (00:52:13) 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. (00:52:22) I can give you an example. (00:52:24) The Earth, (00:52:25) I can project onto a flat sheet of paper. (00:52:27) I am now misrepresenting a map of the Earth. (00:52:30) 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. (00:52:40) 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, (00:52:54) 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. (00:53:02) I can do the same thing with spacetime. (00:53:03) I can draw it on a flat sheet of paper, but I know that it's not actually a flat Euclidean space. (00:53:09) And so my rules for measuring distances are different than the rules I would use that, for instance, Cartesian rules of geometry. (00:53:18) I would know to use the correct rules for Minkowski spacetime. (00:53:23) 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 2 relative observers. (00:53:38) And I will get the correct answer, but only if I use these different rules. (00:53:43) So then what does, according to general relativity, does objects with mass due to the space-time? (00:53:51) Right, exactly. (00:53:52) So Einstein struggled for this completely general theory, not a specific solution like a black hole or an expanding space-time or galaxies make lenses. (00:54:04) Those are all solutions. (00:54:06) That's why what he did was so enormous. (00:54:07) It's an entire paradigm that says, over here is matter and energy. (00:54:13) I'm going to call that the right-hand side of the equation. (00:54:17) Everything on the right-hand side of Einstein's equations is how matter and energy are distributed in space-time. (00:54:23) On the left-hand side tells you how space and time deform in response to that matter and energy. (00:54:31) And it can be impossible to solve some of those equations. (00:54:35) What was so amazing about what Schwarzschild did is he found this very elegant, simple solution within like a month of reading this final formulation. (00:54:45) But Einstein didn't go through and try to find all the solutions. (00:54:48) He sort of gave it to us, right? (00:54:50) He shared this, and then lots of people since have been scrambling to try to, ah, I can predict the curvature of the space-time if I tell you how the matter and energy is laid out. (00:55:01) 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. (00:55:09) I can solve for the shape of the space-time. (00:55:13) I can also say, what if the universe is full of gas or light and it's all kind of uniform everywhere? (00:55:18) And I'll find a different and equally surprising solution, which is that the universe would expand in response to that, it's not static, that the distances between galaxies would grow. (00:55:30) This was a huge surprise to Einstein. (00:55:33) So all of these consequences of his theory, you know, came with revelations. (00:55:40) that were not at all obvious when he first wrote down the general theory. (00:55:45) And he was afraid to take the consequences of that theory seriously, which is a... (00:55:50) The theory itself in its scope and grandeur and power is... (00:55:56) scary, so I can understand. (00:55:58) Then there's, the edges of the theory where it falls apart, the consequences of the theory that are extreme. (00:56:04) It's hard to take seriously. (00:56:06) So you can sort of empathize. (00:56:08) Yeah, he very much resisted the expansion. (00:56:10) 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, you know, as a physicist, and he writes all of these remarkable papers on relativity and quantum mechanics. (00:56:25) And then even in 1915, 16, he does not know that there are other galaxies out there. (00:56:30) This was not known. (00:56:32) People had mused about it. (00:56:35) There were these kind of smudges on the sky that people contemplated. (00:56:40) What if there are other island universes? (00:56:41) You know, going back to Kant thought about this, but it wasn't until Hubble, it really wasn't until the late 20s, that it's confirmed that there are other galaxies. (00:56:52) Wow. (00:56:52) Yeah. (00:56:53) And he didn't obviously. (00:56:55) There's so much we think of now that he didn't think of. (00:56:59) So there's no big bang, static universe. (00:57:03) But these are all connected. (00:57:05) Wow, yeah. (00:57:06) So he's operating on very little information. (00:57:10) Very little information. (00:57:12) That's absolutely true. (00:57:13) 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. (00:57:22) but there's many ways in which you could call it a theory of absolutism 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 (00:57:46) when it was first discovered, really. (00:57:48) That was observed through experiments trying to figure out what would the relative speed of light be. (00:57:56) It's the only, really, only massless particles have this property that they have an absolute speed. (00:58:01) And if you think about it, it's incredibly strange. (00:58:03) Yeah, it's really strange. (00:58:04) Incredibly strange. (00:58:05) And then so from a theoretical perspective, he takes that seriously. (00:58:10) He takes it very seriously, and everyone else is trying to come up with models to make it go away. (00:58:16) to make the speed of light be a little bit more reasonable, like everything else in the universe. (00:58:21) 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. (00:58:26) It's really a basic observation of reality, right? (00:58:30) 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. (00:58:40) Very strange. (00:58:42) And he gets to relativity by saying, well, what speed? (00:58:45) Speed is distance, it's space over time. (00:58:50) It's how far you travel. (00:58:52) It's the space you travel in a certain duration of time. (00:58:55) And he said, Well, I bet something must be wrong then with space and time. (00:58:59) So this is an enormous leap. (00:59:01) He's willing to give up the absolute character of space and time in favor of keeping the speed of light constant. (00:59:10) How was he able to intuit (00:59:14) a world of curved space-time. (00:59:17) Like, I think it's like one of the most special leaps in human history, right? (00:59:24) Because you're- It's amazing. (00:59:26) Like, it's very, very, very difficult to make that kind of leap. (00:59:30) I'll tell you, it took me, I think, a long time to, I can't say this is how he got there exactly. (00:59:37) It's not as though I studied the historical accounts or his (00:59:42) description of his internal states. (00:59:45) This is more having learned the subject, how I try to tell people how to get there in a few short steps. (00:59:54) One is to start with the equivalence principle, which he called the happiest thought of his life. (01:00:00) And the equivalence principle comes pretty early on in his thinking. (01:00:05) And it starts with something like this. (01:00:08) Like right now, (01:00:09) 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 lie down in a bed and I feel heavy on the bed. (01:00:19) And I think of that as gravity. (01:00:20) And Einstein has a beautiful ability to remove all of these extraneous factors, including atoms. (01:00:29) 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. (01:00:38) But I want to remove the elevator. (01:00:39) What does the elevator have to do with fundamental properties of gravity? (01:00:43) So I cut the cable. (01:00:45) Now I'm falling, but the elevator is falling at the same rate as me. (01:00:49) So now I'm floating in the elevator. (01:00:52) And if this happened to me, if I woke up in this state of falling or floating in the elevator, I might not know if I was in empty space, just floating. (01:01:04) Or if I was falling around the earth, there would actually, they're equivalent situations. (01:01:08) I would not be able to tell the difference. (01:01:10) I'm actually, when I get rid of the elevator in this way by cutting the cable, I'm actually experiencing weightlessness. (01:01:18) And that weightlessness is the purest experience of gravity. (01:01:24) And so this idea of falling is actually fundamental. (01:01:29) It's how we talk about it all the time. (01:01:30) The earth is in a free fall. (01:01:33) around the sun. (01:01:34) It's actually falling. (01:01:35) It's not firing engines, right? (01:01:37) It's just falling all the time, but it's just cruising so fast. (01:01:40) So actually, yeah, oh God, you said so many profound things. (01:01:43) So one of them is really one of the ways to experience space-time is to be falling. (01:01:51) To be falling. (01:01:51) That is the purest experience of gravity. (01:01:54) The experience of gravity, unfettered, uninterrupted by atoms, is weightlessness. (01:02:02) That observation, no, it has an unhappy ending, the elevator story, because of atoms again. (01:02:07) 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. (01:02:17) But this period of free fall, so the first observation is that is the purest experience of gravity. (01:02:22) Now I can convince you that things fall along curved paths because I could take a pen and if I throw it, (01:02:31) 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. (01:02:38) 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 space-time inscribed by the Earth. (01:02:53) So the Earth's mass and shape curves the paths in space, and then (01:02:59) those curvatures tell you how to fall, the paths along which you should fall when you're falling freely. (01:03:06) And so the Earth has found itself on a free fall that happens to be a closed circle, but it's actually falling. (01:03:15) The International Space Station uses this principle all the time. (01:03:18) They get the space station up there and then they turn off the engines. (01:03:21) Can you imagine how expensive it would be if they had to fuel that thing at all times, right? (01:03:25) They turn off the engines. (01:03:27) They're just falling. (01:03:28) Yeah, they're falling. (01:03:29) And they're not that far up. (01:03:31) There are certainly people sometimes say, oh, they're so far away, they don't feel gravity. (01:03:35) Oh, absolutely. (01:03:36) If you stopped the space station, it's going like 17,500 miles an hour, something like that. (01:03:44) If you were to stop that, it would drop like a stone right to the earth. (01:03:49) So they're in a state of constant free fall and they're falling along a curved path. (01:03:54) And that curved path is a result of curving space-time. (01:03:57) And that particular curved path is calculated in such a way that it curves onto itself, so you're orbiting. (01:04:03) Right. (01:04:03) So it has to be cruising at a certain speed. (01:04:06) So once you get it at that cruising speed, you turn off the engines. (01:04:10) But yeah, to be able to visualize at the beginning of the 20th century, that not, you know, that free falling in curved space-time. (01:04:24) boy, the human mind is capable of things. (01:04:27) I mean, some of that is constructing thought experiments that collide with our understanding of reality. (01:04:36) Maybe in the collisions and the contradictions, you try to think of extreme thought experiments that exacerbate that contradiction and see like, okay, what is actually, is there another model that can incorporate this? (01:04:50) But to be able to do that, (01:04:53) I mean, it's kind of inspiring because, you know, there's probably another general relativity out there. (01:04:59) Yeah. (01:04:59) In all, not just in physics, in all lines of work, in all scientific pursuits. (01:05:06) There's certain theories where you're like, okay, I just explained like a big elephant in the room here that everybody just kind of didn't even think about. (01:05:16) Right. (01:05:16) There could be... (01:05:19) For stuff we know about in physics, there could be stuff like that for the origin of life on Earth. (01:05:24) Yeah. (01:05:25) Everyone's like, yeah, okay. (01:05:26) Everyone's like in polite companies, like, yeah, Somehow. (01:05:31) It started. (01:05:33) Right. (01:05:34) Nobody knows. (01:05:34) I find it wild that that's so elusive. (01:05:37) Yeah, it's strange. (01:05:38) And the lab became up with, I think it's a general relativity thing. (01:05:41) There's going to be something. (01:05:43) It's going to involve aliens and wormholes and dimensions that we don't quite understand or some field that's bigger than like, it's possible, maybe not. (01:05:55) It's possible that it has, it's a field that is (01:06:00) different, that will feel fundamentally different from chemistry and biology. (01:06:03) It'll be maybe through physics. (01:06:06) Again, maybe the key to the origin of life is in physics. (01:06:09) And the same there, it's like a weird neighbor is consciousness. (01:06:14) It's like, all right. (01:06:14) A weird neighbor, yeah. (01:06:16) It's like, okay, so we all know that life started on Earth somehow. (01:06:21) Nobody knows how. (01:06:23) We all know that we're conscious. (01:06:27) We have a subjective experience of things. (01:06:28) Nobody understands that. (01:06:31) The people have ideas and so on. (01:06:33) But it's such a dark sort of, we're entering a dark room where a bunch of people are whispering about like, hey, what's in this room? (01:06:41) But nobody has a effing clue. (01:06:44) So, and then somebody comes along with a general relativity kind of conception where like reconceives everything and you're like, ah. (01:06:53) It's like a watershed moment. (01:06:54) Yeah. (01:06:56) Yeah, it's there. (01:06:56) And until we're living in the, we're living in a time until that theory comes along. (01:07:01) And it'll be obvious in retrospect, but right now we're... (01:07:05) Right. (01:07:06) Well, this, it was obvious to no one that space-time was curved, but even Newton understood something wasn't right. (01:07:15) So he knew there was something missing. (01:07:18) And I think that's always fascinating when we're in (01:07:21) a situation where we're pressure testing our own ideas. (01:07:25) 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. (01:07:36) That's insane. (01:07:38) That's a huge leap. (01:07:39) 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. (01:07:50) Unbelievable leap. (01:07:51) absolutely genius, but he didn't like that the apple fell from the tree, even though the earth wasn't touching it. (01:07:59) Yeah, the action at a distance thing. (01:08:01) The action at a distance thing. (01:08:02) That is weird too. (01:08:04) Well, but- That is a really weird one. (01:08:06) It's really weird, but see, Einstein solves that. (01:08:09) Relativity solves that because it says, (01:08:13) The earth created the curve in space. (01:08:16) The apple wants to fall freely along it. (01:08:19) The problem is the tree's in the way. (01:08:21) The tree's the problem. (01:08:23) The tree's actually accelerating the apple. (01:08:25) It's keeping it away from its natural state of weightlessness in a gravitational field. (01:08:31) And as soon as the tree lets go of it, the apple will simply fall along the curve that exists. (01:08:36) I would love it if somebody went back to Newton's time. (01:08:39) And told him all this. (01:08:40) Probably some, like some, like hippie would be like, it's a, gravity is just the curvature in space-time, man. (01:08:48) I wonder if he would be able to, I don't think there's, you know, every idea has its time. (01:08:53) He might not, he might not even be able to load that in. (01:08:57) I mean, sometimes even the greatest geniuses, I mean, you can't. (01:09:03) It's too out of context. (01:09:05) You need to be standing on the shoulders of giants and on the shoulders of those giants and so on. (01:09:10) I heard that Newton used that as an unkind remark to his competitor hook. (01:09:15) Oh no. (01:09:17) The people talk shit even back then. (01:09:19) Yeah, trash talking. (01:09:22) It's one of the hilarious things about humans in general, but scientists too, like these huge minds. (01:09:29) There's these moments in history where (01:09:31) You'll see this in universities, but everywhere else too. (01:09:36) Like you have gigantic minds, obviously also coupled with everybody has an ego. (01:09:42) And like sometimes it's just the same soap opera that played out amongst humans everywhere else. (01:09:49) And so you're thinking about the biggest cosmological objects and forces and ideas, and you're still... (01:09:56) jealous and I know your office is bigger than my office. (01:10:00) I know. (01:10:01) This chair, this, or maybe you got married to this person that I was always in love with, the betrayal of something. (01:10:11) The one woman in the department. (01:10:12) Yeah, the one woman in the department. (01:10:14) Yeah. (01:10:15) And it's just, I mean, but that is also the fuel of innovation, that jealousy, that tension, that's human. (01:10:21) Well, you know the expression, I'm sure, the battles are so bitter in academia because the stakes are so low. (01:10:25) That's a beautiful way to phrase it. (01:10:28) But also, like we shouldn't forget, I mean, that I love seeing that even in academia, because it's humanity. (01:10:36) The silliness, there is a degree to academia where the reason you're able to think about some of these grand ideas (01:10:44) is because you still allow yourself to be childlike. (01:10:47) Oh yeah. (01:10:47) There's a childlike nature. (01:10:48) There's no question. (01:10:50) But children can also be like. (01:10:53) Children. (01:10:53) Children. (01:10:55) So like you don't, I think when in a corporate context and maybe the world gets, forces you to behave, you're supposed to be a certain kind of way. (01:11:05) There's some aspects and it's a really beautiful aspect to preserve and to celebrate in academia is like you're just, (01:11:13) allow it to be childlike in your curiosity, in your exploration. (01:11:18) You're just exploring, asking the biggest questions. (01:11:21) The best scientists I know often ask the simplest questions. (01:11:26) They're really, first of all, there's probably some confidence there, but also they're never going to lie to themselves that they understand something that they don't understand. (01:11:39) So even this idea that Newton didn't understand the apple falling from the tree, had he lived another couple 100 of years, he would have invented relativity, because he never would have lied to himself that he understood it. (01:11:51) He would have kept asking this very simple question. (01:11:55) And I think that there is this childlike beauty to that. (01:11:59) Absolutely. (01:12:00) Yeah, just some of the topics. (01:12:02) I don't know why I'm stuck to those two topics of origin of life and consciousness. (01:12:05) I'll talk about this. (01:12:06) Some of the most brilliant people I know are stuck, just like with Newton and Einstein, they're stuck on that. (01:12:12) This doesn't make sense. (01:12:14) I know a bunch of brilliant biologists, physicists, chemists that are thinking about the origin of life. (01:12:18) They're like, this doesn't. (01:12:21) I know how evolution works. (01:12:22) I know how the biological systems work, how genetic information propagates, but this part, the singularity at the beginning doesn't make sense. (01:12:30) We don't understand, we can't create in a lab, they're bothered, every single day they're bothered by it. (01:12:37) And that being bothered by that tension, by that gap in knowledge is, yeah, that's the catalyst. (01:12:44) That's the fuel for the discovery. (01:12:47) discovery. (01:12:48) Yeah, absolutely. (01:12:49) The discovery is going to come because somebody couldn't sleep at night and couldn't rest. (01:12:55) So in that way, I think black holes are a kind of portal into some of the biggest mysteries of our universe. (01:13:01) So it is a, it's a good terrain on which to explore these ideas. (01:13:05) So can you speak about some of the mysteries that the black holes present us with? (01:13:11) Yeah, I think it's important to (01:13:14) separate the idea that there are these astrophysical states that become black holes from being synonymous with black holes, because black holes are kind of this larger idea. (01:13:28) And they might have been made primordially when the Big Bang happened. (01:13:32) And there's something flawless about black holes that makes them fundamental, unlike anything else. (01:13:42) So they're flawless in the sense that you can completely understand a black hole by looking at just its charge, electric charge, its mass, and its spin. (01:13:51) And every black hole with that charge, mass, and spin is identical to every other black hole. (01:13:57) You can't be like, oh, that one's mine, I recognize it. (01:14:01) has this little feature, and that's how I know it's mine. (01:14:03) They're featureless. (01:14:05) You try to put Mount Everest on a black hole, and it will shake it off. (01:14:10) these gravitational waves. (01:14:11) It will radiate away this imperfection until it settles down to be a perfect black hole again. (01:14:19) So there's something about them that is unlike, and another reason why I don't like to call them objects in a traditional sense, unlike anything else in the universe that's macroscopic. (01:14:29) It's kind of a little bit more like a fundamental particle. (01:14:32) So an electron (01:14:34) is described by a certain short list of properties, charge, mass, spin, maybe some other quantum numbers. (01:14:40) That's what it means to be an electron. (01:14:44) There's no electron that's a little bit different. (01:14:46) You can't recognize your electron. (01:14:49) They're all identical in that sense. (01:14:52) And so in some very abstract way, black holes share something in common with microscopic fundamental particles. (01:15:01) And so what they tell us (01:15:04) about the fundamental laws of physics can be very profound. (01:15:10) 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. (01:15:26) And it's because there's something fundamental about them. (01:15:29) Yeah, general relativity means quantum mechanics, means singularity. (01:15:34) And sadly, heartbreakingly so, it's out of reach for experiment at this moment, but it's within reach for theoretical. (01:15:42) It's in reach for thought experiments. (01:15:45) For thought experiments. (01:15:45) Which are quite beautiful. (01:15:47) Well, on that topic, I have to ask you about the paradox, the information paradox of black holes. (01:15:54) What is it? (01:15:54) So this is what catapulted Hawking's fame. (01:15:59) 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. (01:16:10) 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? (01:16:23) star's gone, there's nothing there. (01:16:24) What if I allowed it to possess sort of ordinary quantum properties? (01:16:28) Just a little tiny bit, you know, nothing dramatic. (01:16:31) Don't go crazy, you know. (01:16:33) And one of the properties of the vacuum that is intriguing is this idea that you can never say the vacuum's actually completely empty. (01:16:43) We talked about Heisenberg, but you know, the Heisenberg uncertainty principle really kicked off a lot of quantum mechanical thinking. (01:16:49) It says that you can never exactly know (01:16:52) a particle's position simultaneously with its motion, with its momentum. (01:16:56) You can know one or the other pretty precisely, but not both precisely. (01:17:01) And the uncertainty isn't a lack of ability that we'll technologically overcome. (01:17:05) It's foundational. (01:17:06) 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. (01:17:14) 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. (01:17:22) Okay. (01:17:22) And so it led to this idea that what do I mean by a vacuum? (01:17:26) Because I can't 100% precisely know. (01:17:31) In fact, it's not really meaningful to say that there's zero particles here. (01:17:36) 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. (01:17:46) Maybe (01:17:47) Two particles come into existence and they're entangled in such a way that they cancel out each other's properties. (01:17:54) So they have the properties of the vacuum. (01:17:57) They don't destroy the kind of properties of vacuum because they cancel out each other's spin maybe, each other's charge maybe, things like that. (01:18:05) But they kind of froth around. (01:18:06) They come, they go, they come, they go. (01:18:09) And that's what we really think is the best that empty space can do in a quantum mechanical universe. (01:18:15) 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. (01:18:25) The event horizon, if the particles are created slightly on either side of that event horizon, now you have a real problem. (01:18:34) Okay. (01:18:35) Now, the pair has been separated by this event horizon. (01:18:40) Now they can both fall in, that's okay. (01:18:42) But if one falls in and the other doesn't, it's stuck. (01:18:47) It can't go back into the vacuum because now it has a charge or it has a spin or it has something. (01:18:53) It's no longer the property of that vacuum it came from. (01:18:56) It needs its pair to disappear. (01:18:58) Now it's stuck, it exists. (01:19:00) It's like you've made it real. (01:19:02) So in a sense, the black hole steals one of these virtual particles and forces the other (01:19:09) to live. (01:19:11) And if it'll escape, radiate out to infinity and look like to an observer far away that the black hole has actually radiated a particle. (01:19:23) And the particle did not emanate from inside. (01:19:26) It came from the vacuum. (01:19:27) It stole it from empty space, from the nothingness that is the black hole. (01:19:33) Now, the reason why this is very tricky is because in the process, because of this separation on either side of the event horizon, the particle it absorbs, it has to do with the switching of space and time that we talked about, but the particle it absorbs, well, from the outside, you might say, oh, it had negative momentum, it was falling in. (01:19:51) From the inside, you say, well, this is actually motion and time. (01:19:54) This is energy. (01:19:55) It has negative energy. (01:19:57) And it absorbs negative energy. (01:19:59) It's mask.