Lisa Randall: Dark Matter, Theoretical Physics, and Extinction Events | Lex Fridman Podcast #403

1. 0:00 – 0:24

   Introduction

   1. LFLex Fridman0:00

      The following is a conversation with Lisa Randall, a theoretical physicist and cosmologist at Harvard. Her work involves improving our understanding of particle physics, supersymmetry, baryogenesis, cosmological inflation, and dark matter. This is the Lex Friedman Podcast. To support it, please check out our sponsors in the description, and now, dear friends, here's Lisa Randall.

2. 0:24 – 19:16

   Dark matter

   1. LFLex Fridman0:25

      One of the things you work on and write about is dark matter. We can't see it, but there's a lot of it in the universe. Uh, you also end one of your books with a Beatles song quote, "Got to be good-looking 'cause he's so hard to see."

   2. LRLisa Randall0:40

      (laughs)

   3. LFLex Fridman0:41

      (laughs) What is dark matter? How should we think about it, given that we can't see it? How should we visualize it in our mind's eye?

   4. LRLisa Randall0:47

      I think one of the really important things that physics teaches you is just our limitations, but also our abilities. So the fact that we can deduce the existence of something that we don't directly see is really a tribute to people that we can do that. But it's also something that tells you you can't overly rely on your direct senses. If you just relied on just what you see directly, you would miss so much of what's happening in the world.

   5. LFLex Fridman1:15

      Hmm.

   6. LRLisa Randall1:15

      And we can generalize this, but we're, just for now, to focus on dark matter. It's something we know is there, and it's not just one way we know it's there. In my book, Dark Matter and the Dinosaurs, I talk about the many different ways, you know, there's eight or nine, that we, we deduce not just the existence of dark matter, but how much i- how much is there, and they all agree. Now, how do we know it's there? Because of its gravitational force. And individually, a particle doesn't have such a big gravitational force. In fact, gravity is an extremely weak force compared to other forces we know about in nature. But there's a lot of dark matter out there. It carries a lot of energy, five times the amount of energy as the matter we know that's in atoms, etcetera. So you can ask, how should we think about it? Well, it's just another form of matter that doesn't interact with light, or at least as far as we know. So it interacts gravitationally. It clumps. It forms galaxies. But it doesn't interact with light, which means we just don't see it. And most of our detection, before gravitational wave detectors, we only saw things because of their interactions with light in some sense.

   7. LFLex Fridman2:25

      So in theory, it, it behaves just like any other matter, just it, it just doesn't interact with light.

   8. LRLisa Randall2:30

      So when we say it interacts just like any other form of matter, we have to be careful, um, because gravitationally, it interacts like other forms of matter. But it doesn't experience electromagnetism, which is why it has a different distribution. So in our galaxy, it's roughly spherical, uh, un- unless it has its own interactions. That's another story. But we, we know that it's roughly spherical. Um, whereas ordinary matter can radiate and clumps into a disk, and that's why we see the Milky Way disk. So on large scales, in some sense, yes, all the matter is similar, in some sense. In fact, dark matter is, in some sense, more important because it can collapse more readily than ordinary matter because ordinary matter has, has radiative forces, which makes it hard to collapse on small scales. So actually, it's dark matter that sort of drives, um, galaxy formation, and then ordinary matter kind of comes along with it. Um, and there's also just more of it. And because there's more of it, it can start collapsing sooner. That is to say, the energy density in dark matter dominates over radiation earlier than you would if you just had ordinary matter.

   9. LFLex Fridman3:43

      So it's part of the story of the origin of a galaxy, part of the story of the end of the galaxy, and part of the story of-

   10. LRLisa Randall3:48

       Yeah.

   11. LFLex Fridman3:48

       ... all the various interactions throughout.

   12. LRLisa Randall3:50

       Exactly, I mean, in my book, I make kind of, sort of jokes about, you know, it's like when we think about a building, we think about the architect. We think about, you know, the high level, but we forget about all the workers that did all the grunt work. And in fact, dark matter was really important (laughs) in the formation of our universe, and we forget that sometimes.

   13. LFLex Fridman4:07

       That's a, a metaphor on top of a metaphor. Okay.

   14. LRLisa Randall4:11

       (laughs)

   15. LFLex Fridman4:12

       The, the, the unheard voices that do the actual work. Okay.

   16. LRLisa Randall4:16

       Exactly. No, but it is a metaphor, but it also captures something because the fact is, we don't directly see it. So we forget it, it's there, or we don't understand it's there, or we think it's not. The fact that we don't see it makes it no less legitimate. It just means that we have challenges in order to find out exactly what it is.

   17. LFLex Fridman4:35

       Yeah, but things we cannot see that nevertheless have, uh, gravitational interaction with the things we can see is, at the, uh, layman level, is just mind-blowing, you know?

   18. LRLisa Randall4:50

       It is and isn't because I think what it's teaching us is that we're human. The universe is what it is, and we're trying to interact with that universe and, and discover what it is. We've discovered amazing things. In fact, I would say it's more surprising that the me- the matter that we know about is, constitutes as big a fraction of the universe as it does. I mean-

   19. LFLex Fridman5:12

       Mm-hmm.

   20. LRLisa Randall5:13

       ... we're limited. We're human.

   21. LFLex Fridman5:14

       Mm-hmm.

   22. LRLisa Randall5:14

       And the fact that we see 5% of the energy density of the universe, um, you know, about one-sixth of the energy density of matter, that's kind of remarkable. I mean, why should that be? There could be anything e- anything could be out there, yet the universe that we see is a significant fraction.

   23. LFLex Fridman5:31

       Yeah, but a lot of our intuition, I think, operates using visualizations in the mind.

   24. LRLisa Randall5:36

       That's absolutely true, and certainly writing books, I realized also how many of our words are based on how we see the world. Um, and that's true, and that's actually one of the fantastic things about physics, is that it teaches you how to go beyond your immediate intuition to develop intuitions that apply at different distances, different scales, different ways of thinking about things.

   25. LFLex Fridman5:58

       Yeah, how do you anthropomorphize dark matter?

   26. LRLisa Randall6:01

       How do... I just did, I think. I made it the grunt work- workers.

   27. LFLex Fridman6:04

       Oh, yeah. That's good.

   28. LRLisa Randall6:05

       (laughs)

   29. LFLex Fridman6:05

       Yeah, you did. Yeah, that's why you get paid the big bucks with the-

   30. LRLisa Randall6:09

       (laughs)
3. 19:16 – 30:16

   Extinction events

   1. LRLisa Randall19:16

   2. LFLex Fridman19:16

      Given the complexity, all cosmological scales involved here that led to the extinction of the dinosaurs, when you look out at the future of Earth, do you worry about future extinction events?

   3. LRLisa Randall19:29

      I, I do think that we might be in the middle of an extinction right now, if you define it by the number of species that are getting killed off. And it's subtle, but, you know, it's a complex system. The way things respond to events is sometimes things evolve. Sometimes animals just move to another place, and the way we've developed the Earth, it's very hard for species just to move somewhere else, and we're seeing that with people now too. I mean, I know people are worried just about AI taking over, and that's a totally different story. We just don't think about the future very much. We think about what we're doing now. And we certainly don't think enough about all the animals that we're destroying, all the things that are precursors to humans that we sort of rely on.

   4. LFLex Fridman20:14

      It's interesting to, to think whether the, the, the things that threaten us is the stuff we see that's happening gradually or the stuff we don't really see that's gonna happen all of a sudden. I sometimes, like, think about what is... what should we be more worried about 'cause it seems like, like with asteroids or nuclear war, it could be stuff that just happens one day, you know. When I say one day, meaning over a span of a few days or a few months, but on a, you know, not on a scale of decades and centuries 'cause we sometimes mostly talk about stuff that's happening gradually, but we can be really surprised.

   5. LRLisa Randall20:57

      It's actually really interesting, and that was actually one of the reasons it took a while to determine what it was that had caused the last extinction because people did think at the time, or many people thought that things were more gradual. And the idea of extinction was a very... was actually a novel concept at some point-

   6. LFLex Fridman21:13

      Mm-hmm.

   7. LRLisa Randall21:14

      ... you know. I mean, these aren't predictable events necessarily. They're only predictable on a grand scale. Um, but sometimes, sometimes they are, and, and I think people were pretty aware that nuclear, um, weapons were dangerous. I'm not sure people are as aware now as they were, you know, say, 20 or 30 years ago, and that certainly worries me. Um, I have to say, I was not as worried about AI as other people, but now I understand. And it's not... I mean, it's more that as soon as you create things that we lose control over, it's scary. And the other thing that we're learning from the events today is that, is that it takes a few bad actors. It takes everyone to sort of make things work well. It takes not that many things to make things go wrong. It's, it's the issue with disease. You know, we can find out what causes a disease, but to make things better is not necessarily that simple. Sometimes it is, but for things to be healthy, a lot of things have to work. For things to go wrong, only one thing has to go wrong. And so it's amazing that we do it, and the same is true for democracy. For democracy to work, a lot of people have to believe in it. A few bad actors can destroy things sometimes. So a lot of the things that we really rely on are delicate equilibrium situ- some of... You know, and there is some robustness in the systems. We try to build in robustness, but a few extreme events can sometimes alter things, and, um, I think that's what people are scared of today in many ways. They're scared of it for democracy. They're scared of it for peace. They're scared of it for AI. I think they're not as scared as they should be about nuclear weapons, to be honest. (laughs) Um, I think that's a more serious danger than people realize. Um, I think people are a little bit more scared about pandemics than they were before. Um, but I still say they're not super scared about it. So you're right. There are these major events that can happen, and we are setting things up so that they might happen, and we should be thinking about them. The question is, who should be thinking about them? How should we be thinking about them? How do you make things happen on a global scale 'cause that's really what we need?

   8. LFLex Fridman23:23

      It certainly shouldn't be a source of division. It should be a source of grand collaboration probably.

   9. LRLisa Randall23:29

      Wouldn't that be nice?

   10. LFLex Fridman23:30

       Yeah. I just wonder what it'd be like a- to be a dinosaur. It must have been beautiful to, like, look at the asteroid just enter the atmosphere until, like, everything just... Man, would I... That'd be one of the things I would travel back in time to.

   11. LRLisa Randall23:48

       You know-

   12. LFLex Fridman23:48

       It's just to watch it.

   13. LRLisa Randall23:48

       ... that's also one of the things that I think you probably could do with virtual reality. I don't think you have to be there and get extinct.

   14. LFLex Fridman23:54

       So you just experience it.

   15. LRLisa Randall23:55

       (laughs) I think there is something. You know, it's an event. You're just watching. You're not doing anything. You're just looking at it. So maybe you could just recreate it.

   16. LFLex Fridman24:01

       I actually heard that there's a, there's a nuclear weapon explosion experience in virtual reality that's good to remind you-... about, like, what it would feel like.

   17. LRLisa Randall24:13

       I have to say, you know, so I got a, I got an, a award from the Museum of Nuclear History and Technology in the southwest. And I went to visit the museum, which turned out to be mostly a museum of nuclear weapons. And the scary thing is that they look really cool, you know?

   18. LFLex Fridman24:29

       Yeah.

   19. LRLisa Randall24:30

       It's true that you have that, "Yes, this is scary," but you also have this, "This is cool" feeling. And I think we have to get around that because, yeah, I'm s- I kind of think that, yes, you can be in that, but I'm not sure that's going to make people scared. Has it, ha-have they actually asked afterwards, "Are you more or less scared?"

   20. LFLex Fridman24:47

       Yeah. That's a good, uh, th- th- that's a really good point. I mean, that's a good summary of just humanity in general. We're attracted to creating cool stuff even though it's, can be dangerous.

   21. LRLisa Randall25:01

       And actually that was the really interesting thing about visiting that museum actually. I, it was very nice 'cause I had a tour from people who had been working there in the Cold War and actually one or two people from the Manhattan Project. It was a very cool tour. And you just realize just how, just the thing itself gets you so excited. I think that's something that sometimes these movies miss, just the thing itself. You're not thinking about the, the overall consequences.

   22. LFLex Fridman25:23

       Mm-hmm.

   23. LRLisa Randall25:23

       And it was kind of like, in some ways, it was like the early Silicon Valley. You know, people were just thinking, like, "What if we did this? What if we did that?" And, you know, not t-keeping track of, like, what the peripheral consequences are. And you definitely see that happening with AI now. I mean, I think that was the moral of the battle that just happened, that, you know, it's just full speed ahead.

   24. LFLex Fridman25:43

       Which gives me a really great transition to an-another quote in your book. So you, you write about the experience of facing the sublime in, in physics. And you quote, uh, Ryan Aroque. Quote, "For beauty is nothing but the beginning of terror, (laughs) which we are still just able to endure. And we're so awed because it serenely disdains to annihilate us." That's pretty intense. It, I think, applies to nuclear weapons.

   25. LRLisa Randall26:12

       But it also, I mean, at a more mundane, perhaps, level, I think it apply, you know, it's really interesting. One of the things I found when I wrote these books is, you know, s-s-some people love certainty. You know, scientists kind of, many revel in uncertainty. It's not that you want to be uncertain. You want to solve it. But you're at this edge where it's really frustrating because you don't really want to not know the answer, but, of course, if you knew the answer, that would be, it would be done. So you're always at this edge where there's, you're trying to sort things out. And there is something scary. You don't know what is, you don't know if there's gonna be a solution. You don't know if you're going to find it. So it's not something that can destroy the Earth. It's just something that y- you do on your individual level. But then, of course, there are much bigger things like the ones you're talking about, where they could actually be dangerous. The stuff I do, I just want (laughs) to be clear, I'm doing theoretical physics, not very dangerous. Um, but sometimes things end up having bigger consequences than you think.

   26. LFLex Fridman27:10

       Yeah, but dangerous in a very pragmatic sense. But isn't it still, in part, terrifying when you think of a, just the size of things.

   27. LRLisa Randall27:22

       So-

   28. LFLex Fridman27:22

       Like the size of dark matter, like the, the, the, the power of this thing in terms of, uh, its g- its potential gravitational effects, just these, these c-cosmological objects of a black hole at the center of our galaxy. They-

   29. LRLisa Randall27:36

       So this might be where, why I'm a physicist or why I differ from other people.

   30. LFLex Fridman27:39

       Yeah.
4. 30:16 – 45:30

   Particle physics

   1. LFLex Fridman30:16

      So, maybe let's zoom out and look at the standard model of phys- of particle physics. How does dark matter fit into it? First of all, what is it? Can you explain w- what the standard model is?

   2. LRLisa Randall30:27

      So the standard model of particle physics is, basically it tells us about nature's most basic elements and their interactions. And so, it's the substructure as far as we understand it. So if you look at atoms, we know they have nuclei and electrons. Nuclei have protons and neutrons in them. Protons and neutrons have particles called quarks that are held together by something called the strong force. They interact through the strong force, the strong nuclear force. There's something called the weak nuclear force and electromagnetism. So basically, all those particles and their interactions describe many, many things we understand. That's the standard model. We now know about the Higgs boson which is associated with how elementary particles get their mass, so that piece of the puzzle has also been completed. We also know that there are kind of a weird array of masses of elementary particles.

   3. LFLex Fridman31:25

      Mm-hmm.

   4. LRLisa Randall31:25

      There's not just the up and down quark, but there are heavier versions of the up and down quark, charm and strange, top and bottom. There's not just the electron. There's a muon and a tau. There are particles called neutrinos which are under intense study now, which are partnered with the leptons through the weak interactions. So we really do know these basic elements, and we know the forces. We know... I mean, w- when we're doing particle physics experiments, we can usually even ignore gravity, except in exceptional cases that we can talk about. So, those are the basic elements and their interactions. Dark matter s- stands outside that. It's not interacting through those forces. So when we look at the world around us, we don't usually see the effects of dark matter itself, because there's so much of it that we do, and it doesn't have those forces that we know about. But the standard model has worked spectacularly well. It's been ch- tested to a high degree of precision. People are still testing it. And one of the things we do as physicists is we actually want it to break down on some level. We're looking for the precision measurement, or the energy, or whatever it will take, where those, where the standard model is no longer working. Like, w- not that it's not working, approximately, but we're looking for the deviations. And those deviations are critical, because they can tell us what underlies the standard model, which is what we really want to see next.

   5. LFLex Fridman32:46

      Where can you find the, the places where the standard model breaks down? Like, uh, what are the, what are the places you can see those tiny little deviations?

   6. LRLisa Randall32:53

      So, we don't know yet.

   7. LFLex Fridman32:54

      Mm-hmm.

   8. LRLisa Randall32:54

      But we know the kinds of things you would want to look for. So one obvious place to look is that higher energy. Um, we're looking at the Large Hadron Collider, but we'd love to go beyond that. Higher energies means shorter distances, and it means things that we just couldn't produce before. I mean, E=MC². So if you have a heavy particle and you don't have enough energy to make it, you'll never see it. So that's one place. The other place is precision measurements. If you, you know the standard model has been tested exquisitely, so if been s- if it's been tested at 1%, you want to look at a tenth of a percent.

   9. LFLex Fridman33:26

      Mm-hmm.

   10. LRLisa Randall33:27

       And there are some processes that we know shouldn't even happen at all in the standard model, or happen at a very suppressed level. And those are other things that we look for. So, all of those things could indicate there's something beyond what we know about, which of course would be very exciting.

   11. LFLex Fridman33:42

       W- when you just step back and look at the standard model, the quarks and all the different particles, and neutrinos, and... Isn't it wild how this like little system came, came to be and creates, it underpins everything we see?

   12. LRLisa Randall33:58

       (laughs) Absolutely, and that's why we'd like to understand it better. We want to know, is it part of some bigger sector? Um, why are these particles, why do they have the masses they do?

   13. LFLex Fridman34:07

       Yeah.

   14. LRLisa Randall34:08

       Um, why is the Higgs boson so light compared to the mass it could've had, which we might've even expected based on the principles of special relativity and quantum mechanics? So, that's a really big question, why are they what they are and...

   15. LFLex Fridman34:21

       And they originate, there's like some mechanism that created the whole thing?

   16. LRLisa Randall34:25

       That's one of the things we're trying to study. Why is it what it is?

   17. LFLex Fridman34:28

       I mean, even just like the mechanism that creates stuff, like the way a human being is created from a single cell. It's like shh-bea-genesis, like the whole thing is built, you build up this thing, all of it, th- this whole thing comes to be from just like a tiny little...

   18. LRLisa Randall34:47

       Well, don't forget, it is interacting with the environment.

   19. LFLex Fridman34:49

       Sure.

   20. LRLisa Randall34:50

       (laughs)

   21. LFLex Fridman34:50

       Okay, right. Right, right.

   22. LRLisa Randall34:51

       It's important.

   23. LFLex Fridman34:51

       It's not incident- uh, right.

   24. LRLisa Randall34:52

       It's important.

   25. LFLex Fridman34:53

       Well, that's a really good question is how much of it is the environment? Is it just the environment acting on a set of constraints? And, uh, like how much of it is just the information in the DNA or any information? How much is it in the initial conditions of the universe, or, uh, versus the, the, uh, some other thing acting on it?

   26. LRLisa Randall35:14

       These are big questions. These are big questions in pretty much every field. Um, you know, we, for the universe, we do consider it, you know, it's everything there is, by definition.

   27. LFLex Fridman35:25

       Mm-hmm.

   28. LRLisa Randall35:25

       But people now think about it as that one of many universes, um, and of course it's a misnomer, but could there be other places where there are self-contained gravitational systems that we don't even interact with? So, but, those are really important questions, and the only way we're gonna answer them is, you know, we go back as far as we can. We try to think theoretically, and we try to think about observational consequences. That's all we can do.

   29. LFLex Fridman35:49

       One interesting way to explore the standard model is to look at your, uh, fun, nuanced disagreement with Carlo Rovelli.

   30. LRLisa Randall35:59

       (laughs)
5. 45:30 – 59:24

   Physics vs mathematics

   1. LRLisa Randall45:30

      measurements.

   2. LFLex Fridman45:31

      Well, speaking of mathematicians, uh, what to you is the difference, because you've had a bit of foot in both, uh, between physics and mathematics, in the way it helps us understand the world?

   3. LRLisa Randall45:41

      Well, there's, to be frank, there's a lot of more overlap in physics and math I think than has been. I mean, well, maybe not more, but there's certainly a lot. But I think, again, the kinds of questions you're asking are usually different. Um, mathematicians like the structure itself.

   4. LFLex Fridman45:56

      Mm-hmm.

   5. LRLisa Randall45:57

      Physicists are trying to concentrate on, to some extent, on the consequences for the world. Um, but there is a lot of overlap.

   6. LFLex Fridman46:04

      The string theory is an example. There's certain theories where there, there's a, a, a certain kind of mathematical beauty to it.

   7. LRLisa Randall46:12

      There's also, you know, there's also some really cool ideas that you get in particle physics, where you can describe what's going on and connect it to other ideas. That's also really beautiful.

   8. LFLex Fridman46:20

      Mm-hmm.

   9. LRLisa Randall46:21

      Um, you know, I think, I think basically insights can be beautiful. Um, s- you know, they might seem simple but someti- and sometimes they genuinely are, and sometimes they're built on a whole system that you have to understand before. I mean, if you actually saw Einstein's equations written out in components.

   10. LFLex Fridman46:37

       Mm-hmm.

   11. LRLisa Randall46:38

       You wouldn't think it's so beautiful.

   12. LFLex Fridman46:39

       (laughs)

   13. LRLisa Randall46:39

       You write it in a compact way, it looks nice.

   14. LFLex Fridman46:42

       Uh, what do you think about the successes and the failures of string theory? To what degree do you think it succeeded? To what degree has it not succeeded yet or has failed?

   15. LRLisa Randall46:54

       I think to talk about any science in terms of success and failure often misses the point because-

   16. LFLex Fridman47:00

       Yeah.

   17. LRLisa Randall47:00

       ...there's not some absolute thing. And I think, I do think that string theorists were a bit overly ambitio- not overly ambitious, but a little bit overly arrogant-

   18. LFLex Fridman47:10

       Hmm.

   19. LRLisa Randall47:10

       ...in the beginning, thinking they could solve many problems that they weren't going to solve. That's not to say the methods and, um, advances in string theory don't exist, and, um, but they certainly weren't able to immediately solve all the problems they thought they could solve. But it has given us tools. It has given us some insights. Um, but it becomes almost a sociological question of like how much it should be one or the other. I do think that you can get caught up in the problems themselves, and, and sometimes you can get caught up in the methods and just sort of do other examples.

   20. LFLex Fridman47:43

       Mm-hmm.

   21. LRLisa Randall47:43

       So the real physics insights often come from people who are thinking about physics as well as, as math.

   22. LFLex Fridman47:49

       'Cause you mention AI. Is there hope that AI might be able to help find some interesting insights? I mean, another question, another way to ask this question is, how special are humans that we're able to discover novel insights about the world?

   23. LRLisa Randall48:09

       That's a great question. Um, and it depends on what kind of insights, and we're, we're going to find that out. I mean, you know, it's, uh, because it's hard to think about something that doesn't quite exist yet, I mean, I could just think about something, take a step back. You know, it's a little bit like trying to understand three dimen- four dimensions, you go back to three dimension, you know, so s- to go to something you can imagine. So you can sort of say a lot of the...... things in a very different level about the internet. You could say, you know, has the internet helped do things? And that's-

   24. LFLex Fridman48:40

       Mm-hmm.

   25. LRLisa Randall48:41

       ... you know, it definitely took on a life of its own in some sense. But it's also something that we're able to tame. You know, I know that I myself wouldn't have been able to write books if the internet didn't exist, because I wouldn't have had the time to go to a library and look everything up. And, um, it helped me enormously. And in some sense, AI could be that in a very nice world. It could be a tool that helps us go a step further or than we would and a lot more efficiently. And it's already done that to some extent. Or it could be like the parts of the internet that we can control, the ruining politics or whatever. So, and there's certainly a lot of indications it can do that. Then there are even bigger things that, you know, people speculate about, about AI being able to do its own things. But in terms of actually figuring things out, um, you know, we're in the early stages.

   26. LFLex Fridman49:33

       Yeah, there's several directions here. One is like on the theorem prover side, so Wolfram|Alpha where everything's much more precise, and we have large language model type of stuff. One of the limitations of those is it seems to come up with convincing looking things-

   27. LRLisa Randall49:49

       Mm-hmm.

   28. LFLex Fridman49:49

       ... which we don't know if it's true or not.

   29. LRLisa Randall49:52

       Right.

   30. LFLex Fridman49:52

       And that's a big problem for physics.

Episode duration: 59:24