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

Lex Fridman 0:00 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.

Lex Fridman 0:25 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."

Lisa Randall 0:40 (laughs)

Lex Fridman 0:41 (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?

Lisa Randall 0:47 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.

Lex Fridman 1:15 Hmm.

Lisa Randall 1:15 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.

Lex Fridman 2:25 So in theory, it, it behaves just like any other matter, just it, it just doesn't interact with light.

Lisa Randall 2:30 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.

Lex Fridman 3:43 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-

Lisa Randall 3:48 Yeah.

Lex Fridman 3:48 ... all the various interactions throughout.

Lisa Randall 3:50 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.

Lex Fridman 4:07 That's a, a metaphor on top of a metaphor. Okay.

Lisa Randall 4:11 (laughs)

Lex Fridman 4:12 The, the, the unheard voices that do the actual work. Okay.

Lisa Randall 4:16 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.

Lex Fridman 4:35 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?

Lisa Randall 4:50 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-

Lex Fridman 5:12 Mm-hmm.

Lisa Randall 5:13 ... we're limited. We're human.

Lex Fridman 5:14 Mm-hmm.

Lisa Randall 5:14 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.

Lex Fridman 5:31 Yeah, but a lot of our intuition, I think, operates using visualizations in the mind.

Lisa Randall 5:36 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.

Lex Fridman 5:58 Yeah, how do you anthropomorphize dark matter?

Lisa Randall 6:01 How do... I just did, I think. I made it the grunt work- workers.

Lex Fridman 6:04 Oh, yeah. That's good.

Lisa Randall 6:05 (laughs)

Lex Fridman 6:05 Yeah, you did. Yeah, that's why you get paid the big bucks with the-

Lisa Randall 6:09 (laughs)

Lex Fridman 6:09 ... and write the great books. Okay, so, um, you also write in that book about, uh, dark matter, matter having to do something with the extinction events, the extinction of the dinosaurs, um, which is kind of a fascinating presentation of how everything is connected. So I guess the disturbances from the dark matter, they create gravitation disturbances in the Oort cloud at the edge of our solar system, and then that increases the rate of asteroids hitting Earth.

Lisa Randall 6:42 So I want to be really clear. This was a speculative theory.

Lex Fridman 6:45 Mm-hmm.

Lisa Randall 6:46 Um...

Lex Fridman 6:46 I love it though. (laughs)

Lisa Randall 6:47 I mean, and I liked it too. And, um, and we still don't know for sure, but we can... What we liked about it, so let me take a step back. So we usually assume that dark matter is one... We, being physicists, that it's just one thing. It's just basically non-interacting, uh, aside from gravity, or very weakly interacting matter. Um, but again, we have to get outside this mindset of just humans and ask, "What else could be there?" And so what we suggested is that there's a fraction of dark matter, not all the dark matter, but some of the dark matter, maybe it has interactions of its own. Just the same way in, in our universe we have lots of different types of matter. We have nuclei, we have electrons, we have muon, we, we have forces. Um, we have lots of... It's, it's not a simple model, the standard model, but, but it does have some basic ingredients. So maybe dark matter also has some interesting, um, structure to it. So maybe there's some small fraction. And the interesting thing is that if some of the dark matter does radiate, and, you know, I like to call it dark light because it's light that we don't see but dark matter would see. It could radiate that, and then it could perhaps collapse into a disc the same way ordinary matter collapsed into the Milky Way disc. So it's not all the dark matter. It's a fraction. But it could conceivably be a very thin disc of dark matter, thin, dense disc of dark matter. And so then the question is, do these exist? And people have done studies now to think about whether they can find them. I mean, it's an interesting target. It's something you can measure. By measuring the positions and velocities of stars, you can find out what the structure of the, of the Milky Way is. Um, but the fun proposal was that the solar system orbits around the galaxy, and as it does so, it goes a little bit up and down, kind of like horses on a carousel. And the suggestion was, every time it goes through, you have an enhanced probability that you would dislodge something from the edge of the solar system-

Lex Fridman 8:48 Mm-hmm.

Lisa Randall 8:49 ... in something called the Oort cloud. So the idea was that at those times, you're more likely to have these cataclysmic events, such as the amazing one that actually caused the last extinction that we know of for sure.

Lex Fridman 9:01 It wasn't so amazing for the dinosaurs.

Lisa Randall 9:04 Or for two-thirds of the species on the planet.

Lex Fridman 9:06 Yeah. But it gets amazing for humans. We wouldn't be here.

Lisa Randall 9:08 What, what really is amazing, I mean, I, I do f- I mean, I talk about this in Dark Matter and the Dinosaurs. I, it's just an amazing scientific story because it really is one of the real stories that combined together different fields of science. Geologists at the time, or, you know, people thought that things happen slowly, and this would be a cataclysmic event. And also, I have to say, you know, if you, if you think about it, it sounds like a story like a five-year-old would make up.

Lex Fridman 9:32 Mm-hmm.

Lisa Randall 9:32 "Maybe the dinosaurs were killed by some big rock that came and-"

Lex Fridman 9:35 Yeah.

Lisa Randall 9:35 "... hit the Earth." But then there really was a scientific story behind it. And that's also why I like the dark disc because there's a scientific story behind it. So as far-fetched as it might sound, you could actually go and look for the experimental consequences or the observational consequences to test whether it's true.

Lex Fridman 9:52 I wish you could know, like, high resolution details of where that asteroid came from, like where in the Oort cloud, why it happened, is it in fact because of dark matter, just like the full, tracing back to the origin of the universe. Because it, humans seem to be somewhat special. But it just, it seems like so many fascinating events at all scales, at all scales of physics had to happen for it to be so complicated.

Lisa Randall 10:16 So I'm really, really glad you mentioned that because actually, that was one of the main points of my book, Dark Matter and the Dinosaurs. One of the reasons I wrote it was because I really think we are abusing the planet. We're changing the planet way too quickly. And just like anything else, when you alter things, it's good to think about the history of what it took to get here. And it, and as you point out, it took many operations on many different scales. You know, we had to have the formation of, of structure, the formation of galaxies, the formation of the solar system, the formation of our planet, the formation of humans. I mean, there's so many steps that go into this. And humans, in some part, were the result of the fact that this big object hit the Earth, made the dinosaurs go extinct, and mammals developed. I mean, it is an incredible story. And (laughs) yes, something else might come of it, but it won't be us if we mess with it too much.

Lex Fridman 11:05 But it is on a grand scale. Earth is a pretty resilient system. Uh, can, can you just clarify, which is fascinating, uh, the shape of things? So the shape of the Milky Way's, of the observable stuff, is mostly flat. And you said dark matter tends to be spherical, but a subset of that might be a flat disc.

Lisa Randall 11:31 So you want to hear about the shape of things?

Lex Fridman 11:33 Yeah. (laughs) Yes, please.

Lisa Randall 11:35 So structure formed early on, and now our structure that we live in is... So we know about the Milky Way galaxy.

Lex Fridman 11:42 Mm-hmm.

Lisa Randall 11:43 So the Milky Way galaxy has the disc you can see in a dry, dark place. That's where stars and light is. But you can also measure, in some ways, the dark matter. And we believe that dark matter is more or less spherically distributed.Um, and like we said, there's a lot of it. Not necessarily in the disk, but just because it's a sphere, there's a lot of it sitting there. And the reason it doesn't collapse, as far as we know, is that it doesn't really... It can't radiate the same way. So because it can radiate, ordinary matter collapses. And this actually, because of conservation of angular momentum, it become-

Lex Fridman 12:23 Yeah.

Lisa Randall 12:23 ... it stays a disk, and it doesn't just collapse to the center. So our suggestion was that maybe there are some components of dark matter that also radiate. Like I said, that's far from proven. People have looked for disks. They see some evidence of some disks of certain densities, but...

Lex Fridman 12:38 I see.

Lisa Randall 12:39 These are all questions that are worth asking. Basically, if we can figure it out from existing measurements, why not try?

Lex Fridman 12:44 Okay, so there's... Not all dark matter's made the same. This is...

Lisa Randall 12:47 Well, that's a possibility.

Lex Fridman 12:49 Right.

Lisa Randall 12:49 We actually don't know what dark matter is in the first place. We don't know what most of it is. We don't know what a fraction is. I mean, it's hard to measure. Why is it hard to measure? For exactly the reason you said earlier. We don't see it. So we wanna think of possibilities for what it can be if, especially if those give rise to some of observational consequences. I mean, it's, it's a tough game because it's not something that's just there for the taking. You have to think about what it could be and how you might find it.

Lex Fridman 13:16 And the way you detect it is gravitational effects on things we can see.

Lisa Randall 13:22 That would be the way you detect the type of dark matter I've been talking about. People have suggestions for other forms of dark matter. They could be particles called axions. They could be other types of particles. And then there are different ways of detecting it. I mean, the most popular candidate for dark matter, probably until pretty recently because they haven't found it, is something called WIMPs, weakly interacting massive particles. Um, particles that have mass about the same as the Higgs boson mass. Um, and it turns out then you would get about the right density of dark matter. But then people really like that, of course, because it is connected to the standard model, the particles that we know about. And if it's connected to that, we have a better chance of actually seeing it. Fortunately or unfortunately, it's also a better chance that you can rule it out because you can look for it. And so far, no one has found it. We're still looking for it.

Lex Fridman 14:08 Is that one of the hopes of the Large Hadron Collider?

Lisa Randall 14:11 That was originally one of the hopes of the Large Hadron Collider. I'd say at this point, it would be very unlikely given what they've already accomplished.

Lex Fridman 14:20 Mm-hmm.

Lisa Randall 14:21 Um, but there are these, um, underground detectors, xenon detectors that look for dark matter coming in. And they, they are going to try to achieve a, a much stronger bound than exists today.

Lex Fridman 14:35 Just a- take of that tangent, looking back now, what's the biggest, to you, insight, uh, to humanity that the LHC has been able to provide?

Lisa Randall 14:47 It's interesting. It's both, um, a major victory. Um, the Higgs boson was proposed 50 years ago, and it was discovered. The Higgs mechanism seemed to be the only way to explain elementary particle masses, and it was right. So on the one hand, it was a major victory. On the other hand, I've been in physics long enough to know it was also a cautionary tale in some sense because, um, at the time I started out in physics, we had some, proposed something in the United States called the Superconducting Super Collider. A lot of physicists, I'll say particularly in Europe, but I'd say a lot of physicists, were saying when that the Large Hadron Collider would have the energy reach necessary to discover what underlies the standard model. We don't wanna just discover the standard model. We wanna know what the next step is.

Lex Fridman 15:30 Mm-hmm.

Lisa Randall 15:31 And I think here, um, people were more cautious about that. They wanted to have a more comprehensive search that could get to higher energies, um, more events so that we could, you know, we could really more definitively rule it out.

Lex Fridman 15:42 Mm-hmm.

Lisa Randall 15:43 But in that case, many people thought they knew what would be there, which happened to be a theory called supersymmetry. So a lot of physicists thought it would be supersymmetry. I mean, it's one of the many factors, I think, that went into the fact that the Large Hadron Collider became the only machine in town. And, um, the Superconducting Super Collider would've just been a much, if it had really had achieved what it was supposed to, would've been a much more robust test of, of the space.

Lex Fridman 16:06 Mm-hmm.

Lisa Randall 16:07 So, so I'd say for humanity, it's both a tribute to the ability to discover and the ability of really believing in things so that you have the confidence to go look for them. But it's also a cautionary tale that you don't want to, you know, assume things before they've been actually found. So you wanna do things in, you know, you, you wanna believe in your theories, but you also wanna question them at the same time in ways that you're more likely to discover the truth.

Lex Fridman 16:32 But it's also an illustration of grand engineering efforts that humanity can take on and, uh, maybe a lesson that you could go even bigger? (laughs)

Lisa Randall 16:42 Um, I- I'm really glad you said that though, too, because that, that's absolutely true. I mean, it's, it really is an impressive... It's, it's impressive in so many ways. It's impressive technologically. It's impressive at an engineering level. It's also impressive that so many countries worked together, um, to, to do this. It wasn't just one country. And how it was, it was also impressive in that it was a long-term project that people committed to and made it happen. So it is a demonstration that when people set their minds to things, and they commit to it, that they can do something amazing.

Lex Fridman 17:18 But also in the United States, uh, maybe a lesson that bureaucracy can slow things down to where everything-

Lisa Randall 17:24 Bureaucracy and, and politics.

Lex Fridman 17:26 Politics.

Lisa Randall 17:27 And economics. Many, many things can make them faster and make them slower.

Lex Fridman 17:32 So science is the way to make progress. Politics is the way to slow that progress down. And here we are.

Lisa Randall 17:38 Um, well, I don't wanna overstate that because without politics, the-

Lex Fridman 17:42 Yeah.

Lisa Randall 17:42 ... you know, the LHC-

Lex Fridman 17:42 No, we need it.

Lisa Randall 17:43 ... wouldn't happen either. So, um-

Lex Fridman 17:44 Yeah.

Lisa Randall 17:44 But I do-

Lex Fridman 17:45 (laughs) You need broccoli. Um- (laughs)

Lisa Randall 17:49 (laughs) But sometimes I do think, what, I mean, you're not asking this question, but sometimes I do think when I, you know, think about some of these conflicts, you know, sometimes it's just good to have a project that people work on together. And-There were some efforts to do that with s- in science too, to have Palestinians and Israelis work together, a project called SESAME. Um, I think it's not a bad idea when you can do that, um, when you can get, you know, uh, sort of, for- forget the politics and just focus on some particular project. Sometimes that can work.

Lex Fridman 18:25 Some kind of forcing function, some kind of deadline that gets people to sit in a room together, and you're working on a thing. But as part of that, you realize the common humanity, that you all have the same concerns, the same hopes, the same fears, the same... That you're all human, and that's an accidental side effect of working together on a thing.

Lisa Randall 18:45 I, that's absolutely true, and it's one of the reasons CERN was formed, actually. It was post-World War II, and a lot of European physicists had actually left Europe, and they wanted to see Europeans work together and, and sort of rebuild, and, and it worked. I mean, they, they did. And it's true. And I often think that, that, you know, one of the major problems is we just don't meet enough people so that everyone thinks... seems like... when they seem like the other, it's more easy to forget their humanity. So I think it, it is important to have these connections.

Lex Fridman 19:16 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?

Lisa Randall 19:29 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.

Lex Fridman 20:14 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.

Lisa Randall 20:57 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-

Lex Fridman 21:13 Mm-hmm.

Lisa Randall 21:14 ... 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?

Lex Fridman 23:23 It certainly shouldn't be a source of division. It should be a source of grand collaboration probably.

Lisa Randall 23:29 Wouldn't that be nice?

Lex Fridman 23:30 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.

Lisa Randall 23:48 You know-

Lex Fridman 23:48 It's just to watch it.

Lisa Randall 23:48 ... 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.

Lex Fridman 23:54 So you just experience it.

Lisa Randall 23:55 (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.

Lex Fridman 24:01 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.

Lisa Randall 24:13 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?

Lex Fridman 24:29 Yeah.

Lisa Randall 24:30 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?"

Lex Fridman 24:47 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.

Lisa Randall 25:01 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.

Lex Fridman 25:23 Mm-hmm.

Lisa Randall 25:23 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.

Lex Fridman 25:43 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.

Lisa Randall 26:12 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.

Lex Fridman 27:10 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.

Lisa Randall 27:22 So-

Lex Fridman 27:22 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-

Lisa Randall 27:36 So this might be where, why I'm a physicist or why I differ from other people.

Lex Fridman 27:39 Yeah.

Lisa Randall 27:39 Because I'm not such a big fan (laughs) of humanity in some ways. (laughs) In some ways I am, but the idea that we were everything would be really boring to me. I love the idea that there's so much more out there, that there's a bigger universe, and there's lots to discover and that we're not all there is. Wouldn't it be disappointing if we were all there is?

Lex Fridman 27:58 Yeah, and, and the, the full diversity of other stuff, it's pretty interesting.

Lisa Randall 28:04 You know, we have no idea how much there is. You know, we, we know what we s- what we can observe so far. So the idea that there's other stuff out there that we yet have to figure out, it's exciting.

Lex Fridman 28:13 Let me ask you a-a-out there question.

Lisa Randall 28:16 Uh-oh.

Lex Fridman 28:16 Okay. (laughs) So if you think of, like, humans on Earth, life on Earth as this pocket of complexity that emerged.

Lisa Randall 28:25 Mm-hmm.

Lex Fridman 28:27 You know, and there's a bunch of conditions that came to be, and there's, uh, Darwinian evolution and however life originated. Do you think it's possible there's some pockets of complexity of that sort inside dark matter that we can't see?

Lisa Randall 28:42 Well, so that's possible.

Lex Fridman 28:44 Mm-hmm.

Lisa Randall 28:45 Um, I th- I-

Lex Fridman 28:47 Chemistry and biology evolving in different ways.

Lisa Randall 28:49 And that's one of the reasons we suggest. I mean, it's not the reason, but it would be true if there were the type of interactions we suggest. I mean, it would need more complex ones. And we don't know. Um, I will say that the conditions that give rise to life and complexity, they're complex. They're unlikely. Um, so it's not like there's great odds that would happen. But there's no reason to know that it doesn't happen. It's worth investigating. Are there other forces that exist in the dark matter sector? That's exactly it.

Lex Fridman 29:20 So the, the dark matter sector doesn't have all the forces of the, the standard model of physics?

Lisa Randall 29:26 Right. As far as we know, it doesn't have any. It might have it at some low level.

Lex Fridman 29:29 Yeah.

Lisa Randall 29:29 But it could have its own forces, just like-

Lex Fridman 29:32 Yeah.

Lisa Randall 29:32 ... the dark matter might not experience our light. Maybe it has its light that we don't experience.

Lex Fridman 29:38 So there could be other kinds of forces.

Lisa Randall 29:40 I mean, there could be other kinds of forces even within our sector that are too weak for us to have discovered so far, or that exists at different scales than we know about. I mean, we detect what (laughs) interacts strongly enough with our detectors to detect. So it's worth asking. And that's one of the reasons we build big colliders to see are there other forces, other particles that exist, say, at higher energies, at shorter distance scales than we've explored so far. So it's not just in the dark matter sector. Even in our sector, there could be a whole bunch of stuff we don't yet know.

Lex Fridman 30:16 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?

Lisa Randall 30:27 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.

Lex Fridman 31:25 Mm-hmm.

Lisa Randall 31:25 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.

Lex Fridman 32:46 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?

Lisa Randall 32:53 So, we don't know yet.

Lex Fridman 32:54 Mm-hmm.

Lisa Randall 32:54 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.

Lex Fridman 33:26 Mm-hmm.

Lisa Randall 33:27 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.

Lex Fridman 33:42 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?

Lisa Randall 33:58 (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?

Lex Fridman 34:07 Yeah.

Lisa Randall 34:08 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...

Lex Fridman 34:21 And they originate, there's like some mechanism that created the whole thing?

Lisa Randall 34:25 That's one of the things we're trying to study. Why is it what it is?

Lex Fridman 34:28 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...

Lisa Randall 34:47 Well, don't forget, it is interacting with the environment.

Lex Fridman 34:49 Sure.

Lisa Randall 34:50 (laughs)

Lex Fridman 34:50 Okay, right. Right, right.

Lisa Randall 34:51 It's important.

Lex Fridman 34:51 It's not incident- uh, right.

Lisa Randall 34:52 It's important.

Lex Fridman 34:53 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?

Lisa Randall 35:14 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.

Lex Fridman 35:25 Mm-hmm.

Lisa Randall 35:25 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.

Lex Fridman 35:49 One interesting way to explore the standard model is to look at your, uh, fun, nuanced disagreement with Carlo Rovelli.

Lisa Randall 35:59 (laughs)

Lex Fridman 36:00 (laughs) When, when you talked about him writing in his book, "Electrons don't always exist. They exist when they interact. They materialize in a place when they collide with something else." And you wrote that, well, I'll just read the whole thing 'cause it's, it's kind of interesting. "Stocks may not achieve a precise value until they are traded."... but that doesn't mean we can't approximate their worth until they change hands. Similarly, electrons might not have definite properties, but they do exist. It's true that the electron doesn't exist as a classical object with definite position until the position is measured, but something was there which physicists use a wave function to describe. It's a fascinating, nuanced disagreement. So do electrons always exist or not? Does it, does a tree fall in the forest if nobody's there to see it?

Lisa Randall 36:48 So I like to think of the universe as being out there, whether or not... I mean, it would be really weird if the only time things came into existence was when I saw them or I measured them. I mean-

Lex Fridman 36:57 There's a lot of weird stuff in the universe.

Lisa Randall 36:58 I mean, I could believe that the Middle East doesn't exist because I'm not there now.

Lex Fridman 37:01 Yeah.

Lisa Randall 37:01 I mean, that would be kind of ridiculous. I think we would all agree on that. So I think there's only so much (laughs) that we can attribute to our own powers of seeing, so... And the whole system doesn't come into being because I'm measuring it. And so what is weird, and this isn't even a disagreement about the standard model, this is a disagreement about how you interpret quantum mechanics. I mean, I would say that those wave functions are real. I mean, one of the things that, don't forget, that particle physics does that quantum field theory says, is that electrons can be created and destroyed. It's n- it's not that every electron has to be in the universe. I mean, there can be. I mean, that's what happens at collider, is particles get created and destroyed. Um, but that doesn't mean that if I have an electron in an atom, it's not there. It's certainly there and we know about it. Its charge is there.

Lex Fridman 37:47 So physics is a kind of way to see the world. So what, w- with, at the bottom, what's the bottom turtle? Like what... Do you have a sense that there's a bottom reality that we're trying to approximate with physics?

Lisa Randall 38:01 I think we always have in our head maybe that we'd like to find that, but I have t-... I mean, I might not seem so, but I think I'm kinda more humble than a lot of physicists. I'm not sure that we're ever gonna get to that bottom level. But I do think we're going to keep penetrating different layers and get further.

Lex Fridman 38:16 I just wonder how far aw- away we are, you know?

Lisa Randall 38:20 We all wonder that. Um, it's not even cl-... Like, what's even the measure of how far away we are? I mean, one way you can measure it is just by our everyday lives. In terms of our everyday lives, we've measured everything. In terms of what underlies it, there's a lot more to see. And so part of it has to do with how far we think we can go. I mean, it might be that the nature of reality changes so much that even these terms are, are different. Maybe we'll measure, you know, this, the notion of distance itself might break down at some point.

Lex Fridman 38:48 But also to push back on the, "We've measured everything," maybe there's stuff we haven't even considered as measurable. For example, consciousness, or th- there's... There might be stuff, just like you said, forces unseen, undetected.

Lisa Randall 39:02 So it's an interesting thing. So one... And this is often a confusion that happens. So there's sort of the fundamental stuff underlying it, and then there's sort of the higher levels, what you know, what we'll call, like, an effective theory at some level.

Lex Fridman 39:14 Mm-hmm.

Lisa Randall 39:15 You know, so we're not always working... I mean, when I throw a ball, I don't tell you where every atom is. I tell you there's a ball. And so there might be different layers of reality that are built on terms, in terms of the matter that we know about, in terms of the stuff we know about that. And when I say we've measured everything, I, I say that with a grain of salt. I mean-

Lex Fridman 39:35 Right, of course, yeah, that's true.

Lisa Randall 39:36 ... I measure everything about the standard model. So, so there's lots of phenomena that we don't understand that we... But often, they're complex phenomena that will be given in terms of the fundamental ingredients that we know about.

Lex Fridman 39:47 But that is an interesting question because, yes, there's phenomena that are at the higher level of abstractions that emerge, but maybe, like with consciousness, there is far out people that, you know, think that consciousness is, um panpsychist, right? That it's, that is, that there's going to be almost like a fundamental force of physics that's consciousness that permeates all matter, right?

Lisa Randall 40:09 So, I mean, usually when you have an, a crazy... Sorry. Okay, when you have a-

Lex Fridman 40:13 (laughs)

Lisa Randall 40:13 ... far out theory-

Lex Fridman 40:14 Yes.

Lisa Randall 40:14 ... the thing you do is you test all the possibilities within the constructs that exist.

Lex Fridman 40:19 Yeah.

Lisa Randall 40:19 Uh, so you don't just jump to the most far out possibility. I mean, you can do that, but then to see if it's true, you either have to find evidence of it or you have to show that it's not possible without that.

Lex Fridman 40:30 Uh-

Lisa Randall 40:30 And we're very far from that.

Lex Fridman 40:32 I think one of the criticisms of your theory on the dinosaurs was that it requires, if I remember correctly, for dark matter to be weirder than it already is. And then I think you had a clever response to that. Can you re-, can you remind?

Lisa Randall 40:46 I'm not sure I remember what I said then, but I mean, we have no idea how weird dark matter is. I mean, it's based on everyone thinking they know what dark matter is. I mean, so weirder than it already is. I mean, it's not already anything. We don't know what it is, so there's no normalization here.

Lex Fridman 40:59 (laughs) D- so dark matter, do we know that if dark matter varies in density?

Lisa Randall 41:05 It definitely does in the universe, just like... I mean, so for example, there's more dark matter in galaxies than there is between-

Lex Fridman 41:11 Sure.

Lisa Randall 41:11 ... galaxies. So it, it clumps. I mean, it's... so it's, it's matter, so it's distributed like matter. It is matter.

Lex Fridman 41:18 So it does clump, but the, the, uh, the full details of how it clumps-

Lisa Randall 41:23 Again, it's-

Lex Fridman 41:24 ... and the complexity of the clumping.

Lisa Randall 41:25 It's understood, uh, pretty well. People do simulations. They're... I mean, where, where people are, are always looking for things, including us as particle physics, is sort of at small scales. Are the deviations on small scales, so that indicating other interactions or other processes, or interactions with baryons? That is to say, normal matter that we don't understand. But on large scales, we have a pretty good understanding of dark matter distribution.

Lex Fridman 41:50 You were part of a recent debate on, quote, "Can science uncover reality?" Let, let me ask you this question then. Uh, w- what do you think is the limits of science?

Lisa Randall 42:00 I'm smart enough to know I have no idea. And also, it's not even clear what science means, right? Because there's the science that we do, which is particle physics. We try to find fundamental things and-

Lex Fridman 42:12 Mm-hmm.

Lisa Randall 42:12 ... figure out what their effects are. There's science like biology, where, you know, it's a, at a higher level. The kind of questions you ask are different. The kind of measurements are different. Um, the kind of science that's going to happen......in the sort of more numerical age, I mean, or even AI, or, like, what does it mean to answer a question? Does it mean that we can predict it? Does it mean that we can reproduce it? So I think we're coming up against sort of the definition of what we mean by science, as human beings. So in terms of the sciences that we can do, I, I don't think we'll know it until we get there. Um, you know, we're trying to solve hard problems and, you know, we've made progress. I mean, if you think of how much science has advanced in the last century or century and a half, it's incredible. I mean, you know, we didn't even know the universe was expanding at the beginning of the 20th century. We didn't know about quantum mechanics at the beginning of the century. We didn't know about special relativity.

Lex Fridman 43:03 Mm-hmm.

Lisa Randall 43:03 That's a lot, in a relatively short time on, uh, depending on how you think of time. Um, so I think it would be premature to say we know the limitations.

Lex Fridman 43:14 And at various points throughout that history, we thought we solved everything, or declared, or at least various people declared.

Lisa Randall 43:19 Where "we" was various people, exactly.

Lex Fridman 43:21 Yeah, yeah, yeah. Declared that we've solved everything. Uh, so th- this is also a good place to, maybe could you describe the difference between top-down and bottom-up approaches to theoretical physics that you talked about in the book?

Lisa Randall 43:32 So you could try to jump in and say, "I have a theory that I think is so perfect, um, that I can predict everything from it, or at least predict some salient features from it."

Lex Fridman 43:45 Mm-hmm. That's top-down.

Lisa Randall 43:47 That would be top-down. Bottom-up is more like, you know, like the questions we just asked, like, why are masses what they are? We measure things. We wanna put them together.

Lex Fridman 43:56 Mm-hmm.

Lisa Randall 43:56 And usually a good approach is to combine the two. If you ask a very specific question, but combine it with the methods of knowing that there could be a fundamental theory underlying it, sometimes you make progress. I mean, sometime- you know, the community tends to get segmented or fragmented into people who do one or the other. But there are definitely times, I mean, some of my best collaborations have been with people who are more top-down than I am, so that we come up with interesting ideas that we wouldn't have thought of if either one of us was working individually.

Lex Fridman 44:25 Would you say the truly big leaps happen top-down, like Einstein?

Lisa Randall 44:30 Einstein was not a top-down person in the beginning. He was, um, special relativity was very much him thinking about, you know, they were thought experiments, but he was very much, you know, the original theory about relativity is something like on the nature of electromagnetism. He was trying to understand how Maxwell's laws could make sense when they w- you know, seemed to have different symmetries than what we had thought there were. So he was very much a bottom-up person, and in fact, he resisted top-down for a long time. Then when he tried to do the theory of general relativity or the general theory of relativity, whichever you wanna call it, um, incorporating gravity into the system, where you need some feedback, then he was helped by a mathematician who had developed some differential geometry and helped him figure out how to write down that. And after that, he thought top-down was the way to go, but he actually didn't make that much progress.

Lex Fridman 45:20 Hmm.

Lisa Randall 45:21 Um, but certainly, so I think it's, you know, naïve to think it was just one or the other. In fact, a lot of people who made real progress were rooted in actual

Lisa Randall 45:30 measurements.

Lex Fridman 45:31 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?

Lisa Randall 45:41 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.

Lex Fridman 45:56 Mm-hmm.

Lisa Randall 45:57 Physicists are trying to concentrate on, to some extent, on the consequences for the world. Um, but there is a lot of overlap.

Lex Fridman 46:04 The string theory is an example. There's certain theories where there, there's a, a, a certain kind of mathematical beauty to it.

Lisa Randall 46:12 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.

Lex Fridman 46:20 Mm-hmm.

Lisa Randall 46:21 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.

Lex Fridman 46:37 Mm-hmm.

Lisa Randall 46:38 You wouldn't think it's so beautiful.

Lex Fridman 46:39 (laughs)

Lisa Randall 46:39 You write it in a compact way, it looks nice.

Lex Fridman 46:42 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?

Lisa Randall 46:54 I think to talk about any science in terms of success and failure often misses the point because-

Lex Fridman 47:00 Yeah.

Lisa Randall 47:00 ...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-

Lex Fridman 47:10 Hmm.

Lisa Randall 47:10 ...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.

Lex Fridman 47:43 Mm-hmm.

Lisa Randall 47:43 So the real physics insights often come from people who are thinking about physics as well as, as math.

Lex Fridman 47:49 '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?

Lisa Randall 48:09 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-

Lex Fridman 48:40 Mm-hmm.

Lisa Randall 48:41 ... 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.

Lex Fridman 49:33 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-

Lisa Randall 49:49 Mm-hmm.

Lex Fridman 49:49 ... which we don't know if it's true or not.

Lisa Randall 49:52 Right.

Lex Fridman 49:52 And that's a big problem for physics.

Lisa Randall 49:54 So large language models are more or less like generalizations of stuff that we have. So the question is, so there's still breakthroughs in AI waiting to happen, and maybe they are happening, and maybe they'll be good, maybe not. But that, that's not quite the same. I mean, maybe just some, in some cases it's just pattern recognition that leads to important things, but sometimes it could be something more insightful than that, that I can't even put my finger on. So it forces us to, I mean, we don't really understand how smart we are. We don't understand how we think about things all that well, actually. But one thing is true, that we're a lot more efficient right now than computers in coming up with things. We require a lot less energy to do that. So if computers figure out how to do that, then it's gonna be a totally different ball game. So, and so there are clearly kinds of connections that we don't know how we're making, but we are making them. And so that's going to be interesting. So, um, you know, I say we're in early stages, but this is changing very rapidly. But right now, I don't think that it's actually, you know, discovered like new laws of physics, but could it in the future? Maybe it can.

Lex Fridman 51:06 It will raise big questions about what is special about humans that we don't quite appreciate. You know, with, there could be things that are like that leap of insight that happens, truly novel ideas, that could potentially be very difficult to do.

Lisa Randall 51:26 So there are sort of abstract questions like that. There's also questions of how is it that we can address to some extent, you know, how will AI be used in the context of the world we live in-

Lex Fridman 51:37 Sure.

Lisa Randall 51:37 ... which is based on, you know, at least our country's based on capitalism and a certain political system. And how will global politics deal with it? How will our capitalist system deal with it? What will be the things that we focus on doing with it?

Lex Fridman 51:51 Mm-hmm.

Lisa Randall 51:51 How much will researchers get control of it, um, to be able to ask different sorts of questions? I mean, you know, while it was starting out, people were doing these kind of toy problems, but what will it actually be applied to and what will it be optimized to do? There's m- there's a lot of questions out there that it's really important we start addressing.

Lex Fridman 52:11 Uh, what to you is the most beautiful unsolved problem in physics and cosmology? What to you is really exciting if we can unlock the mystery of in the next few decades?

Lisa Randall 52:29 Um, so is it, what's the most beautiful unsolved problem or what is the most beautiful unsolved problem I think we can make progress on?

Lex Fridman 52:34 (laughs) Ooh, boy.

Lisa Randall 52:37 I mean, look-

Lex Fridman 52:37 What do we make progress on in the next few centuries?

Lisa Randall 52:41 (laughs) There's, you know, there's, m- most of the questions, the big questions have to do with what underlies things, how things started, what's at the base of it. There's also just basic questions like how, that you asked earlier, how far will science take us?

Lex Fridman 52:53 Mm-hmm.

Lisa Randall 52:54 How much can we understand? Um, there are questions like how we got here, um, what underlies it, are there, you know, but also, I mean, there's really deep questions like, you know, what fraction of, are we actually seeing? Or if there are these other forces, if there is another way of seeing the world, are there galac-, you know, universes beyond our own that are, if they're so totally different, how do we even comprehend them? I mean, how do we detect, like what would we even think about them? So there's a lot about trying to get beyond. It's always just getting beyond our limited vision and limited experience and trying to see what underlies it, both at small scales and at large scales. We just don't know the answers. I mean, I'd like to think that we understand more about dark matter, about dark energy, about are there extra dimensions, things that we actually work on, but there's probably a lot beyond what we work on that's yet to be discovered.

Lex Fridman 53:50 Yeah. Understanding the extra dimensions piece will be really interesting.

Lisa Randall 53:56 Totally. I mean, if it is, you know, how the universe went from higher dimensions to what we see, it, or do the, you know, are the extra dimensions present everywhere? I mean, you know, one of the really interesting pieces of physics we did that I talk about in my first book, Warp Passages, is finding out that there can be a higher dimension, but only locally do you think there's a gravity of a lower dimension. So it could be like only locally do we think we live in three dimensions, and it could be higher dimensions is different. It's not actually the gravity we have, but it, there's all sorts of phenomena that might be out there that we don't know about. All sorts of evolution things, time dependence that we don't know about. Um, and of course, that's from the point of view of particle physics. From the point of view of other kinds of physics, we're just beginning. So who knows?

Lex Fridman 54:40 Yeah, if the physics changes throughout, is, is not, homogeneous throughout the universe, that's, that would be weird.

Lisa Randall 54:48 I mean, you know, for the observable universe, it's the same, but beyond the observable universe, who knows?

Lex Fridman 54:54 What advice would you give... Uh, you've had an exceptional career. What advice would you give to young people, maybe high school, college, on how to have a career they can be proud of and a life they can be proud of?

Lisa Randall 55:10 I think the weird thing about being a scientist or an academic in general is you have to believe really strongly in what you do while questioning it all the time.

Lex Fridman 55:17 Hmm.

Lisa Randall 55:18 You can't, you know... And that's, that's a hard balance to have. Sometimes it helps to collaborate with people. But to really believe that you could have good ideas, at the same time knowing they could all be wrong, that's, that's, that's a tough tightrope to walk sometimes.

Lex Fridman 55:30 Mm-hmm.

Lisa Randall 55:30 But to really, um, test them out. Um, the other thing is, sometimes, you know, if you get too far buried, you look out and you think, "Oh, there's so much out there." And sometimes it's just good to bring it back home and just think, "Okay, can I have a g- as good idea as the person next to me rather than, you know, the greatest physicist who ever lived?" But right now, like you said, I think there's lots of big issues out there. And it, and it's hard to balance that. And sometimes it's hard t- to forget the role of, of physics. But I think, you know, Wilson said it really well when he said, you know, when they were building Fermilab, it was like, "This won't defend the country, but it'll make it worth defending." You know, there's just the idea that, you know, in all this chaos, it's still important that we still make progress on these things. And sometimes, you know, when major world events are happening, it's easy to forget that. And I think those are important too. You don't wanna forget those. But to try to keep that balance because we don't wanna lose what it is that makes humans special.

Lex Fridman 56:24 So, that's the big picture. Would you also lose yourself in the simple joy of puzzle-solving?

Lisa Randall 56:29 Yeah, yeah. I mean, I, that's... We all like s- solving puzzles. And actually, one of the things that drives me in my research is the inconsistencies. When things don't make sense, it really bugs me. And it just will go in, to, you know, different directions to see, how could these things fit together?

Lex Fridman 56:46 So, it bugs you, but that motivates you.

Lisa Randall 56:48 Yeah, totally.

Lex Fridman 56:49 Until it doesn't. (laughs)

Lisa Randall 56:50 Because I think-

Lex Fridman 56:51 You have to resolve it.

Lisa Randall 56:52 ...I have this underlying belief that it should make sense-

Lex Fridman 56:54 Yeah.

Lisa Randall 56:54 ...even though the world comes at you in many ways and tells you nothing should make sense. But if you believe that it makes sense, then you look for underlying logic. And I think that's just good advice for everything, to try to find, like, why it is the way it is. I think, you know, we talk, I talk about effective theory in my second book, Knocking on Heaven's Door, a lot. You know, it's sort of, rather than ask the big questions, sometimes we just ask the questions about the immediate things that we can measure.

Lex Fridman 57:17 Mm-hmm.

Lisa Randall 57:18 And we can, like I said, we can sometimes tell one that will fail. We can have these effective theories. And sometimes I think, you know, when we approach these big questions, it's good to do it from an effective theory point. You know, why do I find this satisfying? Why is the world we have the way it is? We think things are beautiful that we live in. I mean, you know, I'm not sure if we had different senses or different ways of looking at things, we wouldn't necessarily find it beautiful. But I have to say, you know, it is kind of fantastic that no matter how many times I see a sunset, I will always find it beautiful.

Lex Fridman 57:47 Mm-hmm.

Lisa Randall 57:47 It's like, I don't think I'll ever see a sunset and say, "Ah, whatever."

Lex Fridman 57:50 Mm-hmm.

Lisa Randall 57:50 You know, it's just always beautiful. You'll always... You know. And so there's, there are things that as humans, you know, clearly resonate with us, but, you know, we were maybe evolved that way. That's about us. Um, but in terms of figuring out the universe, it's kind of amazing how far we've gotten. You know, we have discovered many, many wonderful things. Um, but there's a lot more out there. And I hope, I hope we have the opportunity to keep going.

Lex Fridman 58:14 And with effective theories, one small step at a time. Just keep unraveling-

Lisa Randall 58:19 But-

Lex Fridman 58:19 ... the mystery.

Lisa Randall 58:19 ...but also having in mind the big questions, but doing one small step at a time. Exactly.

Lex Fridman 58:23 Yeah, looking out to the stars. You said the sunset. For me, it's the sunset, the sunrise, and just looking at the stars. It's wondering what's all out there and having a lot of hope that humans will figure it out.

Lisa Randall 58:39 Right. I like it.

Lex Fridman 58:42 Lisa, well, thank you for being one of the humans-

Lisa Randall 58:44 (laughs) Well, thank you for having me here.

Lex Fridman 58:45 ...in the world for, uh, that are pushing it forward and, uh, figuring out this beautiful puzzle of ours. And thank you for talking today. This was amazing.

Lisa Randall 58:53 Thank you.

Lex Fridman 58:55 Thanks for listening to this conversation with Lisa Randall. To support this podcast, please check out our sponsors in the description. And now, let me leave you with some words from Albert Einstein. "The important thing is to not stop questioning. Curiosity has its own reason for existence." Thank you for listening and hope to see you next time.