Lisa Randall: Dark Matter, Theoretical Physics, and Extinction Events | Lex Fridman Podcast #403
- 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 Fridman Podcast." To support it, please
check out our sponsors in the description. And now, dear friends,
here's Lisa Randall. 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. You also end one of your books
with a Beatles song, quote, "Got to be good-looking
'cause he's so hard to see." (both 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? - 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. 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. There's eight or nine that we
deduce not just the existence of dark matter but 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, et cetera. 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. - So in theory it, it behaves
just like any other matter. It just doesn't interact with light. - So when we say it interacts
just like any other form of matter, we have to be careful, 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, unless it has its own interactions. That's another story. But we know that it's roughly spherical. 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 radiative forces, which makes it hard to
collapse on small scales. So actually, it's dark matter that sort of drives galaxy formation, and then ordinary matter
kind of comes along with it. 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 an ordinary matter. - 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 - Yeah.
- all the various interactions throughout.
- Exactly, I mean, in my book, I make kind
of sort of jokes about, it's like when we think about a building, we think about the architect. We think about the high level, but we forget about all the workers that did all the grunt work. And in fact, dark matter
was really important in the formation of our universe. And we forget that sometimes. - That's a metaphor on top of a metaphor. Okay. (Lisa laughs) The unheard voices that
do the actual work. Okay.
- 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'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. - Yeah, but the things we cannot see that nevertheless have
gravitational interaction with the things we can't
see is, at the layman level, it's just mind-blowing, you know? - It is, and it 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 discover what it is. We've discovered amazing things. In fact, I would say it's more surprising that the matter that we
know about is constitutes as big a fraction of
the universe as it does. I mean, we're limited. We're human. And the fact that we see
5% of the energy density of the universe, about 1/6 of
the energy density in matter, that's kinda remarkable. I mean, why should that be? There could be anything,
anything could be out there, yet the universe that we see
is a significant fraction. - Yeah, but a lot of
our intuition, I think, operates using visualizations in the mind. - That's absolutely true. And it's certainly writing books that I realized also how many of our words are based on how we see the world. 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. - Yeah, how do you
anthropomorphize dark matter? - I just did, I think. I made it the grunt workers. - Oh yeah, that's good. Yeah, you did.
(Lisa laughs) That's why you get paid the big bucks and write the great books. Okay, so you also write in that book about dark matter having to do something with the extinction events, the
extinction of the dinosaurs, which is kind of a
fascinating presentation of how everything is connected. So I guess the disturbances
from the dark matter, they create gravitational
disturbances in the Oort cloud at the edge of our solar
system, and then that increases the rate of asteroids hitting Earth. - So I wanna be really clear, this was a speculative theory. - [Lex] Mm-hm. I love it though. - I mean, and I liked it too. 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, we, being physicists,
that's just one thing. It's just basically
non-interacting, aside from gravity, or very weakly interacting matter. 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 our universe, we have lots of different types of matter. We have nuclei. We have electrons. We have forces. We have lots. It's not a simple model,
the standard model, but it does have some basic ingredients. So maybe dark matter also has some interesting 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 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 disk the same way ordinary matter collapsed into the Milky Way disk. So it's not all the dark
matter, it's a fraction. But it could conceivably be a
very thin disk of dark matter, thin, dense disk 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 Milky Way is. 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, 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. - It wasn't so amazing for the dinosaurs. - Or for two-thirds of
the species on the planet. - Yeah, but as it gets amazing for humans, we wouldn't be here. - What really is amazing. I mean, I do, I mean, I talk about this in "Dark Matter and the Dinosaurs." It's just an amazing scientific story because it really is
one of the real stories that combine together
different fields of science. Geologists at the time, people thought that things happen slowly, and this would be a cataclysmic event. And also, I have to say, if you think about it,
it sounds like a story like a five-year-old would make up. Maybe the dinosaurs were
killed by some big rock that came and hit the earth.
- Yeah. - But then there really was
a scientific story behind it. And that's also why I like the dark disk, 'cause 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, for the observational consequences, to test whether it's true. - 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. Humans seem to be somewhat special, but it seems like so many
fascinating events at all scales, at all scales of physics
had to happen for. - 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 as you point out,
it took many operations on many different scales. We had to have the formation of structure, the formation of galaxies, the formation of the solar system, the formation of our planet, 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, yes, something else might come of it, but it won't be us if we
mess with it too much. - But it is, on a grand scale, Earth is a pretty resilient system. Can you just clarify,
it's just fascinating, the shape of things? So the shape of the Milky Way, 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 disk. - So you want to hear
about the shape of things. - [Lex] Yes. (laughs) Yes, please. - So structure formed early on, and now our structure that we live in is, so we know about the Milky Way galaxy. So the Milky Way galaxy has the disk you can see in a dry, dark place. That's where stars and light is. But you can also measure, in some ways, that dark matter. And we believe that dark matter is more or less spherically distributed. 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 it's actually, because of
conservation angular momentum, 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 a
disk, they see some evidence of some disks, of certain densities. But
- I see. - these are all questions
that are worth asking. Basically, if we can figure it out from existing measurements, why not try? - Okay, so not all dark
matter's made the same. - Well, that's a possibility. 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, especially if those give rise to some observational consequences. I mean, 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. - And the way you detect
it is gravitational effects on things we can see. - 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, particles that have mass about the same as the Higgs boson mass. And 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. - Is that one of the hopes
of the Large Hadron Collider? - That was originally one of the hopes of Large Hadron Collider. I'd say at this point,
it would be very unlikely given what they've already accomplished. But there are these underground detectors, XENON detectors, that look
for dark matter coming in. And they are going to try to achieve a much stronger bound than exists today. - Just take that tangent,
looking back now, what's the biggest, to you, insight to humanity that the LHC
has been able to provide? - It's interesting. It's both a major victory. 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 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. And I think here, people were
more cautious about that. They want to have a more
comprehensive search that could get to higher
energies, more events, so that we could really, more
definitively rule it out. But in that case, many people thought they
knew what would be there. It 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 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 the space. So I'd say, for humanity, it's both a tribute to
the ability of discovery 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 wanna assume things before they've been actually found. So you wanna do things, 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. - But it's also an illustration
of grand engineering efforts that humanity can take
on and maybe a lesson that you could go even bigger. (chuckles) - I'm really glad you
said that though, too, because that's absolutely true. I mean, it really is an impressive. It's impressive in so many ways. It's impressive technologically. It's impressive at engineering level. It's also impressive that so
many countries work together to do this. It wasn't just one country,
and 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. - But also in the United
States, maybe a lesson that bureaucracy can slow things down to what everything.
- Bureaucracy and politics. - Politics
- And economics. Many, many things can make them
faster and make them slower. - So science is the way to make progress. Politics is the way to
slow that progress down. And here we go.
- Well, I don't wanna overstate that
because without politics, - Yeah,
- the LHC - I wouldn't either.
- wouldn't happen either. So.
- Yeah. - [Lisa] But. - You need broccoli. (both chuckles) - But sometimes I do think, I mean, you're not asking this question, but sometimes I do think when I think about some of these conflicts, sometimes it's just good to have a project that people work on together. And there were some efforts
to do that in science too, to have Palestinians and
Israelis work together, project called SESAME. I think it's not a bad
idea when you can do that. When you can get, sort of forget the politics and just focus on some particular project. Sometimes that can work. - Some kind of forcing
function, some kinda 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 are all human. And that's a accidental side effect of working together on a thing. - 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 sort of rebuild. And it worked. I mean, they did. And it's true. I often think that, that
one of the major problems is we just don't meet enough
people so that everyone thinks, when they seem like the other, it's more easy to forget their humanity. So I think it is important
to have these connections. - 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? - 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
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. - It's interesting to think whether 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. And sometimes, like think about what should we
be more worried about. Because it seems like with
the 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 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. - It's actually really interesting. And that was actually one of
the reasons it took a while to determine what it was that
it caused the last extinction because people did think, at the time, many people thought that
things were more gradual. And the idea of extinction
was actually a novel concept at some point. - Mm-hm. - I mean, these aren't
predictable events necessarily. They're only predictable on a grand scale. But sometimes they are. And I think people were pretty aware that nuclear weapons were dangerous. I'm not sure people are
as aware now as they were, say, 20 or 30 years ago, and that certainly worries me. 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
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 the issue with disease. 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 situations. And there is some
robustness in the systems. We try to build in robustness. But a few extreme events
can sometimes alter things. And 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. I think that's a more serious
danger than people realize. I think people are a
little bit more scared about pandemics than they were before. 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. - It certainly shouldn't
be a source of division. It should be a source of
grand collaboration, probably. - [Lisa] Wouldn't that be nice? - Yeah. I just wonder what it'd
be like to be a dinosaur. It must have been beautiful
to like look at that asteroid just enter the atmosphere, until like everything just, man, that would be one of the things I would travel back in time to. It's just to watch it.
- 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] So you just experienced it? - I think there's something. 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. - I actually heard that there's a nuclear
weapon explosion experience in virtual reality
that's good to remind you about like what it would feel like. - I have to say, so I got an 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?
- Yeah. - 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 kind of think that, yes, you can be in that, but I'm not sure that's
going to make people scared. Have they actually asked afterwards? Are you more or less scared? - Yeah, that's a good, it'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. - And actually that was the
really interesting thing about visiting that museum actually. It was very nice 'cause
I had a tour from people who had been working there on 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 overall consequences. And it was kind of like, in some ways, it was like the early Silicon Valley. People were just thinking
like, what if we did this? What if we did that? And not 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
it's just full speed ahead. - Which gives me a really great transition to another quote in your book. So you write about it, the
experience of facing the sublime in physics, and you quote Rainer Rilke. Quote, "For beauty is nothing
but the beginning of terror, which we are still just able to endure, and we're so odd because
it's serenely disdains to annihilate us." It's pretty intense. It, I think, applies to nuclear weapons. - But it also, I mean, at a more mundane, perhaps, level, I think it applies. It's really interesting. One of the things I found
when I wrote these books is some people love certainty. Scientists kind of, many,
revel in uncertainty. It's not that you wanna be uncertain. You wanna solve it. But you're at this edge
where it's really frustrating because you don't really
wanna 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 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 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 wanna be clear, I'm doing theoretical
physics, not very dangerous, but sometimes things end up
having bigger consequences than you think. - Yeah, but dangerous in
a very pragmatic sense. But isn't it still, in part, terrifying when you think
of just the size of things, like the size of dark matter, like the power of this thing in terms of, is potential gravitational effects. Just this cosmological
objects of a black hole at the center of our galaxy. - So this might be why I'm a
physicist or why I'm different from other people.
- Yeah. - Because I'm not such a big
fan of humanity in some ways. 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? - Yeah, and the full
diversity of other stuff. It's pretty interesting. - We have no idea how much there is. We know 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. - Well, let me ask you out-there question. Okay, so if you think
of like humans on earth, life on earth, as this pocket
of complexity that emerged, and there's a bunch of
conditions that came to be, and there's 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?
- Well, so that's possible. - Chemistry and biology
evolving in different ways. - 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'd suggest. I mean, it would need more complex ones. And we don't know. I will say that the
conditions that give rise to life and complexity, they're complex. They're unlikely. 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. - So the dark matter sector
doesn't have all the forces of the standard model of physics? - Right, as far as we know, it doesn't have any. It might have it at some low level. - Yeah.
- But it could have its own forces just like, - Yeah.
- the dark matter might not experience our light. Maybe it has its light
that we don't experience. - So there could be other kinds of forces. - 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 exist at different scales than we know about. I mean we detect what
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. - So maybe let's zoom out and look at the standard
model of particle physics. How does dark matter fit into it? First of all, what is it? Can you explain what
the standard model is? - 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. 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, 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 in their interactions. Dark matters 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. It's 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 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 at some level. We're looking for the precision
measurement or the energy, or whatever it will take,
where the standard model is no longer working. Like, 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 wanna see next. - Where can you find the places where the standard model breaks down? Like where are the places you can see those tiny little deviations? - So we don't know yet, but we know the kinds of things
you wouldn't wanna look for. So one obvious place to
look is at higher energy. We're looking at the
Large Hadron Collider, but we'd love to go beyond that. Higher energy means shorter distances, and it means things that we
just couldn't produce before. I mean, E equals mc-squared. 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. The standard model has
been tested exquisitely. So if it's been tested 1%, you wanna look at a 10th of a percent. And there are some processes that we know shouldn't even happen at
all in the standard model or happen at 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. - When you just step back and
look at the standard model, the quarks and all the different
particles and neutrinos, isn't it wild how this like
little system came to be and creates, is underpins
everything we see? - Absolutely, and that's why we'd like to understand it better. We wanna know, is it part
of some bigger sector? Why are these particles, why did they have the masses they do? - [Lex] Yeah. - Why is the Higgs boson so light compared to the mass it could have 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 they originate. There's like some mechanism
that created the whole thing. - That's one of the things
we're trying to study. Why is it what it is? - 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 (imitates wind blowing) embryogenesis, like
there's the whole thing, like you build up this thing. All of it, this whole thing, comes to be from just like a- - Well, don't forget it is
interacting with the environment. - Sure. Okay, right. Right, right. It's not incidentally right. - [Lisa] It's important. 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 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 versus some other thing acting on it? - These are big questions. These are big questions in
pretty much every field. For the universe, we do consider it, it's everything there is by definition. But people now think about it,
is it one of many universes? 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 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. - One interesting way to
explore the standard model is to look at your fun
nuanced disagreement with Carlo Rovelli. (Lisa laughs) 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 kinda 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 a tree fall in the forest
if nobody's there to see it? - 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. - [Lex] There's a lot of weird stuff. - I mean, I could believe that
the Middle East doesn't exist because I'm not there now. - Yeah.
- I mean, that would be kind of ridiculous. I think we would all agree on that. So I think there's only so
much that we can attribute to our own powers of seeing. 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 not that every electron
has to be in the universe. I mean there can be. I mean that's what happens at colliders. Particles get created and destroyed. But that doesn't mean that if
I have electron in an atom, it's not there. It's certainly there,
and we know about it. It's charge is there. - So physics is the kinda
way to see the world. So at the bottom, what's
the bottom turtle? Do you have a sense that
there's a bottom reality that we're trying to
approximate with physics? - I think we always have in our head maybe that we'd like to find
that, but I have to, 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. - I just wonder how far
away we are, you know? - We all wonder that. 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 different. Maybe we'll measure. The notion of distance itself might break down at some point. - But also to push back on
the, we've measured everything, maybe there's stuff we
haven't even considered is measurable. For example, consciousness, or there might be stuff,
just like you said, forces unseen, undetected. - So it's an interesting thing. 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 we'll call like an
effective theory at some level. - [Lex] Mm-hm. - 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 of
the matter that we know about, in terms of the stuff we know about. And when I say we've measured everything, I say that with a grain of salt. - Right, of course.
- I mean I measure everything. - That's true.
- I understand them all. So there's lots of phenomena
that we don't understand, but often there are complex
phenomena that will be given in terms of the fundamental
ingredients that we know about. - 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 think that consciousness
is panpsychus, right? That there's going to be
almost like a fundamental force of physics, that's
cautiousness, that permeates all that, right?
- So usually, when you have a crazy, sorry, okay, when you have a far-out theory, - Yes.
- the thing you do is you test all the possibilities within the constructs that exist. - Yeah.
- 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. And we're very far from that. - 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 remind? - 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. - So dark matter, do we know that if dark matter varies in density? - It just certainly does in the universe. Just like, I mean, so for example, there's
more dark matter in galaxies than there's between galaxies.
- Sure. - [Lisa] So it clumps. I mean, so it's matter. So it's distributed like matter. It is matter. - It does clump. But the full details of how it clumps and the complexity of the clumping. - It's understood pretty well. People do simulations. I mean, where people 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. - You were part of a
recent debate on, quote, "Can science uncover reality?" Let me ask you this question then. What do you think is
the limits of science? - 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 figure out what their effects are. There's science like biology where, at a higher level, the
kind of questions you ask are different, the kind of
measurements are different. The kind of science that's going to happen in the sort of more numerical age, I mean, or even AI, 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 science that we can do, I don't think we'll know
it until we get there. We're trying to solve hard problems, and 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, 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. That's a lot in a relatively short time depending on how you think of time. So I think it would be premature
to say we know limitations. - And at various points
throughout the history, we thought we solved
everything or declared, or at least various people declared.
- Where we was various people. Exactly.
- Yeah, yeah. Declared that we've solved everything. So this 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? - So you could try to jump
in and say I have a theory that I think is so perfect, that I can predict everything from it or at least predict some
salient features from it. - Mm-hm, that's top-down. - That would be a top-down. Bottom-up is more like the
questions we just asked, like why are masses what they are? We measure things. We wanna put them together. - Mm-hm.
- 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, 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. - Would you say the truly
big leaps happened top-down, like Einstein? - Einstein was not a top-down
person in the beginning. Special relativity was very
much him thinking about. They were thought experiments,
but he was very much, 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 seemed to have different symmetries than what we had thought they 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, incorporating gravity into the system when
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] Mm-hm. - So I think it's naive to think it was just one or the other. In fact, a lot of people
who made real progress were rooted in actual measurements. - Well, speaking of mathematicians, what to you is the difference, because you've had a bit of foot in both between physics and mathematics, in the way it helps us
understand the world? - Well, to be frank, there's a lot more overlap
in physics and math. I think that, 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. Mathematicians like the structure itself. - Mm-hm.
- Physicists are trying to concentrate on, to some extent, on the consequences for the world. But there is a lot of overlap. - The string theory is an example. There's certain theories where, there's a certain kind of
mathematical beauty to it. - 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] Mm-hm. - I think, basically,
insights can be beautiful. They might seem simple, 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, if you wouldn't
think it's so beautiful, you write in a compact way. It looks nice. - What do you think about the successes and the failures of string theory? To what degree do you think it succeeded, to what degrees it not
succeeded yet or has failed? - I think, to talk about any science in terms of success and
failure often misses the point because there's not some absolute thing. And I think, I do think
that string theorists were a bit overly ambitious. Not overly ambitious but a
little bit overly arrogant in the beginning, thinking
they could solve many problems that they weren't going to solve. That's not to say the methods and advances in string theory don't exist, 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, 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 sometimes you can get
caught up in the methods and just sort of do other examples. So the real physics insights
often come from people who are thinking about
physics as well as math. - 'Cause you mentioned AI, is their 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? - That's a great question, and it depends on what kind of insights, and we're going to find that out. I mean, 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. It's a little bit like
trying to stand three, four dimensions. You go back to three dimensions 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, has the
internet helped do things? It definitely took on a life
of its own in some sense, but it's also something
that we're able to tame. 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 the library and look everything up. And 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 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't control,
ruining politics or whatever. And there's certainly a lot of
indications that can do that. Then there are even bigger things that people speculate about, about AI being able to do its own things. But in terms of actually
figuring things out, we're in the early stages. - Yeah, there's several directions here. One is like on the theorem
prover side, so WolframAlpha, 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, which we don't know if it's true or not. - [Lisa] Right. - And that's a big problem for physics. - 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's not quite the same. I mean, maybe 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 are a lot more efficient right now than computers,
and 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. 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, I say we're in early stages, but this is changing very rapidly. But right now, I don't
think that it's actually, we discovered like new laws of physics, but could it in the future? Maybe it can. - It will raise big questions about, what is special about humans
that we don't quite appreciate? There could be things that are like, that leap of insight that
happens, truly novel ideas that could potentially
be very difficult to do. - So there are sort of
abstract questions like that. There's also questions of
how is it that we can address to some extent, how will
AI be used in the context of the world we live
in, which is based on, at least our countries based on capitalism in 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? How much will researchers
get control of it to be able to ask different
sorts of questions? I mean, 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 a lot of questions out there that it's really important
we start addressing. - 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? - 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 laughs) - Oh, boy. We make progress on in
the next few centuries. (Lisa chuckles) - 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 that you asked earlier, how far will science take us? How much can we understand? There are questions like how we got here, what underlies it? But also, I mean there's
really deep questions like what fraction are we actually seeing, if there are these other forces, if there is another way
of seeing the world? Are there universes beyond our own there? 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. - Yeah, understanding the
extra dimensions piece will be really interesting. - Totally. I mean, if it is how the universe went from higher dimensions to what we see, are the extra dimensions
present everywhere? I mean, one of the
really interesting pieces of physics we did that I
talk about in my first book, "Warped 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. It could be higher
dimensions is different. That's not actually the gravity we have, but 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. 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? - Yeah, if the physics changes throughout, is not homogeneous
throughout the universe, that'll be weird. - I mean, for the observable universe, it's the same, but beyond
the observable universe, who knows? - What advice would you give? 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? - I think the weird thing
about being a scientist or an academic in general is you have to believe
really strongly what you do while questioning it all the time. And 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 a tough tightrope
to walk sometimes. But to really test them out. The other thing is sometimes,
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 as good idea as the person next to me rather
than the greatest physicist who ever lived? But right now, like you said, I think there's lots of
big issues out there, and it's hard to balance that. And sometimes it's hard to
forget the role of physics. But I think, Wilson said it
really well when he said, when they were building Fermilab, it was like, this won't
defend the country, but it'll make it worth defending. There's just the idea
that, in all this chaos, it's still important that
we still make progress in these things. And sometimes, 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. - So that's the big picture. Do you also lose yourself in the simple joy of puzzle solving? - Yeah. Yeah. I mean, we all like 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 I just will go into
different directions to see how could these
things fit together. - So it bugs you, but that motivates you. - [Lisa] Yeah, totally. - Until it doesn't. - Because I think,
- Just to resolve it. - I have this underlying belief
that it should make sense, - Yeah.
- 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 talk about effective
theory in my second book, "Knocking on Heaven's Door," a lot. Rather than ask the big
questions, sometimes we just ask the questions about the immediate things that we can measure. Like I said, we can sometimes
tell one that we'll fail, but we can have these effective theories. And sometimes I think, when we
approach these big questions, it's good to do it from
an effective theory point. 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, 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, it is kind of fantastic that no matter how many
times I see a sunset, I will always find it beautiful. It's like, I don't think I ever
see a sunset as a whatever. It's just always beautiful. And so there are things, as humans, clearly resonate with us, but we were maybe evolved that way. But that's about us. But in terms of figuring out the universe, it's kind of amazing how far we've gotten. We have discovered many,
many wonderful things, but there's a lot more out there. And I hope we have the
opportunity to keep going. - And with effective theories,
one small step at a time. Just keep unraveling the mystery. - But also having in
mind the big questions, but doing one small
step at a time, exactly. - 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. - Right. I like it. - Lisa, well, thank you
for being one of the humans - Well, thank you - in a world
- for having me here. - that are pushing it forward and figuring out this
beautiful puzzle of ours. And thank you for talking today. This is amazing. - Thank you. - 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.