Sean Carroll: Quantum Mechanics and the Many-Worlds Interpretation

Lex Fridman (02:16.480)

It does, but that was one of Newton's worries.

Sean Carroll (02:20.140)

You're actually right in a slightly different way about the religious worries.

Sean Carroll (02:24.800)

He was smart enough, this is off the topic but still fascinating, Newton almost invented

Sean Carroll (02:29.680)

chaos theory as soon as he invented classical mechanics.

Sean Carroll (02:33.120)

He realized that in the solar system, so he was able to explain how planets move around

Sean Carroll (02:37.600)

the Sun, but typically you would describe the orbit of the Earth ignoring the effects

Lex Fridman (02:43.100)

of Jupiter and Saturn and so forth, just doing the Earth and the Sun.

Sean Carroll (02:46.400)

He kind of knew, even though he couldn't do the math, that if you included the effects

Sean Carroll (02:50.640)

of Jupiter and Saturn and the other planets, the solar system would be unstable, like the

Sean Carroll (02:54.560)

orbits of the planets would get out of whack.

Lex Fridman (02:57.120)

So he thought that God would intervene occasionally to sort of move the planets back into orbit,

Sean Carroll (03:01.160)

which is the only way you could explain how they were there presumably forever.

Lex Fridman (03:05.560)

But the worries about classical mechanics were a little bit different, the worry about

Sean Carroll (03:08.540)

gravity in particular.

Lex Fridman (03:09.540)

It wasn't a worry about classical mechanics, it was a worry about gravity.

Lex Fridman (03:12.960)

How in the world does the Earth know that there's something called the Sun, 93 million

Lex Fridman (03:16.720)

miles away, that is exerting gravitational force on it?

Lex Fridman (03:20.240)

And he literally said, you know, I leave that for future generations to think about because

Lex Fridman (03:24.760)

I don't know what the answer is.

Lex Fridman (03:26.720)

And in fact, people under emphasized this, but future generations figured it out.

Sean Carroll (03:31.520)

Pierre Simone Laplace in circa 1800 showed that you could rewrite Newtonian gravity as

Sean Carroll (03:38.100)

a field theory.

Lex Fridman (03:39.480)

So instead of just talking about the force due to gravity, you can talk about the gravitational

Sean Carroll (03:43.640)

field or the gravitational potential field, and then there's no action at a distance.

Sean Carroll (03:48.280)

It's exactly the same theory empirically, it makes exactly the same predictions.

Lex Fridman (03:52.260)

But what's happening is instead of the Sun just reaching out across the void, there is

Sean Carroll (03:55.880)

a gravitational field in between the Sun and the Earth that obeys an equation, Laplace's

Sean Carroll (04:01.880)

equation, cleverly enough, and that tells us exactly what the field does.

Lex Fridman (04:05.700)

So even in Newtonian gravity, you don't need action at a distance.

Sean Carroll (04:09.900)

Now what many people say is that Einstein solved this problem because he invented general

Lex Fridman (04:13.880)

relativity.

Lex Fridman (04:14.880)

And in general relativity, there's certainly a field in between the Earth and the Sun.

Lex Fridman (04:19.380)

But also there's the speed of light as a limit.

Sean Carroll (04:21.720)

In Laplace's theory, which was exactly Newton's theory, just in a different mathematical language,

Sean Carroll (04:26.720)

there could still be instantaneous action across the universe, whereas in general relativity,

Sean Carroll (04:31.300)

if you shake something here, its gravitational impulse radiates out at the speed of light

Lex Fridman (04:36.000)

and we call that a gravitational wave and we can detect those.

Lex Fridman (04:39.340)

So but I really, it rubs me the wrong way to think that we should presume the answer

Lex Fridman (04:45.280)

should look one way or the other.

Sean Carroll (04:47.160)

Like if it turned out that there was action at a distance in physics and that was the

Lex Fridman (04:51.800)

best way to describe things, then I would do it that way.

Sean Carroll (04:54.640)

It's actually a very deep question because when we don't know what the right laws of

Sean Carroll (04:58.920)

physics are, when we're guessing at them, when we're hypothesizing at what they might

Sean Carroll (05:03.040)

be, we are often guided by our intuitions about what they should be.

Sean Carroll (05:07.160)

I mean, Einstein famously was very guided by his intuitions and he did not like the

Sean Carroll (05:11.840)

idea of action at a distance.

Lex Fridman (05:15.760)

We don't know whether he was right or not.

Sean Carroll (05:17.120)

It depends on your interpretation of quantum mechanics and it depends on even how you talk

Lex Fridman (05:21.800)

about quantum mechanics within any one interpretation.

Lex Fridman (05:24.040)

So if you see every force as a field or any other interpretation of action at a distance,

Sean Carroll (05:32.200)

just stepping back to sort of caveman thinking, like do you really, can you really sort of

Lex Fridman (05:40.800)

understand what it means for a force to be a field that's everywhere?

Lex Fridman (05:45.520)

So if you look at gravity, like what do you think about?

Sean Carroll (05:47.880)

I think so.

Sean Carroll (05:48.880)

Is this something that you've been conditioned by society to think that, to map the fact

Sean Carroll (05:58.240)

that science is extremely well predictive of something to believing that you actually

Lex Fridman (06:04.380)

understand it?

Sean Carroll (06:06.120)

Like you can intuitively, the degree that human beings can understand anything that

Lex Fridman (06:12.080)

you actually understand it.

Lex Fridman (06:13.080)

Or are you just trusting the beauty and the power of the predictive power of science?

Lex Fridman (06:19.240)

That depends on what you mean by this idea of truly understanding something, right?

Lex Fridman (06:25.000)

You know, I mean, can I truly understand Fermat's last theorem?

Sean Carroll (06:29.360)

You know, it's easy to state it, but do I really appreciate what it means for incredibly

Lex Fridman (06:34.680)

large numbers, right?

Sean Carroll (06:37.440)

I think yes, I think I do understand it, but like if you want to just push people on well,

Lex Fridman (06:41.680)

but your intuition doesn't go to the places where Andrew Wiles needed to go to prove Fermat's

Lex Fridman (06:46.440)

last theorem, then I can say fine, but I still think I understand the theorem.

Lex Fridman (06:50.120)

And likewise, I think that I do have a pretty good intuitive understanding of fields pervading

Sean Carroll (06:56.200)

space time, whether it's the gravitational field or the electromagnetic field or whatever,

Sean Carroll (07:00.640)

the Higgs field.

Sean Carroll (07:03.000)

Of course, one's intuition gets worse and worse as you get trickier in the quantum field

Sean Carroll (07:08.080)

theory and all sorts of new phenomena that come up in quantum field theory.

Lex Fridman (07:12.960)

So our intuitions aren't perfect, but I think it's also okay to say that our intuitions

Lex Fridman (07:17.080)

get trained, right?

Lex Fridman (07:18.080)

Like, you know, I have different intuitions now than I had when I was a baby.

Sean Carroll (07:21.340)

That's okay.

Lex Fridman (07:22.340)

That's not, an intuition is not necessarily intrinsic to who we are.

Sean Carroll (07:26.000)

We can train it a little bit.

Lex Fridman (07:27.400)

So that's where I'm going to bring in Noam Chomsky for a second, who thinks that our

Sean Carroll (07:33.080)

cognitive abilities are sort of evolved through time, and so they're biologically constrained.

Lex Fridman (07:40.320)

And so there's a clear limit, as he puts it, to our cognitive abilities, and it's a very

Sean Carroll (07:45.520)

harsh limit.

Lex Fridman (07:47.280)

But you actually kind of said something interesting in nature versus nurture thing here, is we

Sean Carroll (07:52.520)

can train our intuitions to sort of build up the cognitive muscles to be able to understand

Lex Fridman (07:58.200)

some of these tricky concepts.

Lex Fridman (07:59.400)

So do you think there's limits to our understanding that's deeply rooted, hardcoded into our biology

Lex Fridman (08:07.320)

that we can't overcome?

Lex Fridman (08:09.960)

There could be limits to things like our ability to visualize, okay?

Lex Fridman (08:15.840)

But when someone like Ed Witten proves a theorem about, you know, 100 dimensional mathematical

Sean Carroll (08:20.200)

spaces, he's not visualizing it.

Lex Fridman (08:22.640)

He's doing the math.

Sean Carroll (08:23.640)

That doesn't stop him from understanding the result.

Sean Carroll (08:27.600)

I think, and I would love to understand this better, but my rough feeling, which is not

Sean Carroll (08:32.480)

very educated, is that, you know, there's some threshold that one crosses in abstraction

Lex Fridman (08:37.800)

when one becomes kind of like a Turing machine, right?

Sean Carroll (08:41.520)

One has the ability to contain in one's brain logical, formal, symbolic structures and manipulate

Lex Fridman (08:48.440)

them.

Lex Fridman (08:49.440)

And that's a leap that we can make as human beings that dogs and cats haven't made.

Lex Fridman (08:55.240)

And once you get there, I'm not sure that there are any limits to our ability to understand

Sean Carroll (09:00.440)

the scientific world at all.

Lex Fridman (09:02.320)

Maybe there are.

Lex Fridman (09:03.320)

There's certainly limits in our ability to calculate things, right?

Sean Carroll (09:07.080)

You know, people are not very good at taking cube roots of million digit numbers in their

Sean Carroll (09:10.880)

head.

Lex Fridman (09:11.880)

But that's not an element of understanding.

Sean Carroll (09:14.320)

It's certainly not a limit in principle.

Lex Fridman (09:15.880)

So of course, as a human, you would say there doesn't feel to be limits to our understanding.

Lex Fridman (09:21.680)

But sort of, have you thought that the universe is actually a lot simpler than it appears

Lex Fridman (09:30.240)

to us?

Lex Fridman (09:31.240)

And we just will never be able to, like, it's outside of our, okay.

Lex Fridman (09:36.200)

So us, our cognitive abilities combined with our mathematical prowess and whatever kind

Lex Fridman (09:43.240)

of experimental simulation devices we can put together, is there limits to that?

Lex Fridman (09:50.760)

Is it possible there's limits to that?

Sean Carroll (09:52.960)

Well, of course it's possible that there are limits to that.

Sean Carroll (09:57.260)

Is there any good reason to think that we're anywhere close to the limits is a harder question.

Lex Fridman (10:02.880)

Look, imagine asking this question 500 years ago to the world's greatest thinkers, right?

Lex Fridman (10:07.240)

Like are we approaching the limits of our ability to understand the natural world?

Lex Fridman (10:12.320)

And by definition, there are questions about the natural world that are most interesting

Lex Fridman (10:17.180)

to us that are the ones we don't quite yet understand, right?

Lex Fridman (10:19.840)

So there's always, we're always faced with these puzzles we don't yet know.

Lex Fridman (10:23.280)

And I don't know what they would have said 500 years ago, but they didn't even know about

Sean Carroll (10:26.140)

classical mechanics, much less quantum mechanics.

Lex Fridman (10:28.800)

So we know that they were nowhere close to how well they could do, right?

Sean Carroll (10:33.560)

They could do enormously better than they were doing at the time.

Lex Fridman (10:36.160)

I see no reason why the same thing isn't true for us today.

Lex Fridman (10:39.240)

So of all the worries that keep me awake at night, the human mind's inability to rationally

Lex Fridman (10:44.560)

comprehend the world is low on the list.

Sean Carroll (10:47.560)

Well put.

Lex Fridman (10:49.880)

So one interesting philosophical point that quantum mechanics bring up is the, that you

Sean Carroll (10:54.320)

talk about the distinction between the world as it is and the world as we observe it.

Lex Fridman (11:02.040)

So staying at the human level for a second, how big is the gap between what our perception

Sean Carroll (11:08.200)

system allows us to see and the world as it is outside our mind's eye sort of, sort of

Sean Carroll (11:15.720)

not at the quantum mechanical level, but as just our, these particular tools we have,

Sean Carroll (11:21.720)

which is the few senses and cognitive abilities to process those senses.

Lex Fridman (11:26.320)

Well, that last phrase, having the cognitive abilities to process them carries a lot, right?

Sean Carroll (11:31.400)

I mean, there is our sort of intuitive understanding of the world.

Sean Carroll (11:36.640)

You don't need to teach people about gravity for them to know that apples fall from trees,

Lex Fridman (11:41.040)

right?

Lex Fridman (11:42.040)

That's something that we figure out pretty quickly.

Sean Carroll (11:43.960)

Project permanence, things like that, the three dimensionality of space, even if we

Sean Carroll (11:47.480)

don't have the mathematical language to say that, we kind of know that it's true.

Lex Fridman (11:52.000)

On the other hand, no one opens their eyes and sees atoms, right?

Lex Fridman (11:56.280)

Or molecules or cells for that matter, forget about quantum mechanics.

Lex Fridman (12:00.240)

So but we got there, we got to understanding that there are atoms and cells using the combination

Lex Fridman (12:07.520)

of our senses and our cognitive capacities.

Lex Fridman (12:11.080)

So adding the ability of our cognitive capacities to our senses is adding an enormous amount

Lex Fridman (12:16.860)

and I don't think it is a hard and fast boundary.

Sean Carroll (12:19.640)

You know, if you believe in cells, if you believe that we understand those, then there's

Lex Fridman (12:23.480)

no reason you believe we can't believe in quantum mechanics just as well.

Lex Fridman (12:29.840)

What to you is the most beautiful idea in physics?

Lex Fridman (12:36.960)

Conservation of momentum.

Lex Fridman (12:38.500)

Can you elaborate?

Lex Fridman (12:39.760)

Yeah.

Lex Fridman (12:40.760)

So if you were Aristotle, when Aristotle wrote his book on physics, he made the following

Lex Fridman (12:44.240)

very obvious point.

Lex Fridman (12:45.240)

We're on video here, right?

Lex Fridman (12:46.240)

So people can see this.

Sean Carroll (12:47.240)

Yeah.

Lex Fridman (12:48.240)

So if I push the bottle, let me cover this bottle so we do not have a mess, but okay.

Lex Fridman (12:51.400)

So I push the bottle, it moves, and if I stop pushing, it stops moving.

Lex Fridman (12:56.040)

And this kind of thing is repeated a large number of times all over the place.

Sean Carroll (13:00.940)

If you don't keep pushing things, they stop moving.

Lex Fridman (13:03.640)

This is an indisputably true fact about our everyday environment, okay?

Lex Fridman (13:09.040)

And for Aristotle, this blew up into a whole picture of the world in which things had natures

Lex Fridman (13:15.040)

and teleologies, and they had places they wanted to be, and when you were pushing them,

Sean Carroll (13:19.600)

you were moving them away from where they wanted to be, and they would return and stuff

Lex Fridman (13:23.040)

like that.

Lex Fridman (13:24.360)

And it took a thousand years or 1500 years for people to say, actually, if it weren't

Sean Carroll (13:31.880)

for things like dissipation and air resistance and friction and so forth, the natural thing

Lex Fridman (13:37.280)

is for things to move forever in a straight line, there's a constant velocity, right?

Lex Fridman (13:42.960)

Conservation of momentum.

Lex Fridman (13:44.680)

And the reason why I think that's the most beautiful idea in physics is because it shifts

Lex Fridman (13:51.200)

us from a view of natures and teleology to a view of patterns in the world.

Lex Fridman (13:58.140)

So when you were Aristotle, you needed to talk a vocabulary of why is this happening,

Sean Carroll (14:05.000)

what's the purpose of it, what's the cause, etc., because, you know, it's nature does

Sean Carroll (14:08.380)

or does not want to do that, whereas once you believe in conservation of momentum, things

Lex Fridman (14:12.280)

just happen.

Sean Carroll (14:13.760)

They just follow the pattern.

Lex Fridman (14:15.320)

You give me, you have Laplace's demon, ultimately, right?

Sean Carroll (14:17.920)

You give me the state of the world today, I can predict what it's going to do in the

Lex Fridman (14:21.120)

future, I can predict where it was in the past.

Sean Carroll (14:23.180)

It's impersonal, and it's also instantaneous.

Sean Carroll (14:26.880)

It's not directed toward any future goals, it's just doing what it does given the current

Sean Carroll (14:30.920)

state of the universe.

Sean Carroll (14:32.320)

I think even more than either classical mechanics or quantum mechanics, that is the profound

Sean Carroll (14:37.080)

deep insight that gets modern science off the ground.

Lex Fridman (14:40.660)

You don't need natures and purposes and goals, you just need some patterns.

Lex Fridman (14:45.400)

So it's the first moment in our understanding of the way the universe works where you branch

Lex Fridman (14:51.160)

from the intuitive physical space to kind of the space of ideas.

Lex Fridman (14:57.820)

And also the other point you said, which is, conveniently, most of the interesting ideas

Lex Fridman (15:03.380)

are acting in the moment.

Sean Carroll (15:05.940)

You don't need to know the history of time or the future.

Lex Fridman (15:09.000)

And of course, this took a long time to get there, right?

Sean Carroll (15:12.000)

I mean, the conservation of momentum itself took hundreds of years.

Sean Carroll (15:16.260)

It's weird, because like, someone would say something interesting, and then the next interesting

Lex Fridman (15:19.080)

thing would be said like 150 or 200 years later, right?

Lex Fridman (15:22.200)

They weren't even talking to each other, they were reading each other's books.

Lex Fridman (15:25.060)

And probably the first person to directly say that in outer space, in the vacuum, a

Sean Carroll (15:30.640)

projectile would move at a constant velocity was Avicenna, Ibn Sina in the Persian Golden

Sean Carroll (15:35.880)

Age, circa 1000.

Lex Fridman (15:38.200)

And he didn't like the idea.

Sean Carroll (15:39.700)

He used that, just like Schrodinger used Schrodinger's cat to say, surely you don't believe that,

Lex Fridman (15:44.600)

right?

Sean Carroll (15:45.600)

Ibn Sina was saying, surely you don't believe there really is a vacuum, because if there

Lex Fridman (15:48.760)

was a really vacuum, things could keep moving forever, right?

Lex Fridman (15:52.300)

But still, he got right the idea that there was this conservation of something impetus

Lex Fridman (15:56.040)

or mile, he would call it.

Lex Fridman (15:58.300)

And that's 500 years, 600 years before classical mechanics and Isaac Newton.

Lex Fridman (16:03.100)

So Galileo played a big role in this, but he didn't exactly get it right.

Lex Fridman (16:07.000)

And so it just takes a long time for this to sink in, because it is so against our everyday

Lex Fridman (16:12.280)

experience.

Lex Fridman (16:13.280)

Do you think it was a big leap, a brave or a difficult leap of sort of math and science

Lex Fridman (16:21.580)

to be able to say that momentum is conserved?

Sean Carroll (16:25.280)

I do.

Lex Fridman (16:26.280)

You know, I think it's an example of human reason in action.

Sean Carroll (16:31.420)

You know, even Aristotle knew that his theory had issues, because you could fire an arrow

Lex Fridman (16:36.020)

and it would go a long way before it stopped.

Lex Fridman (16:38.700)

So if his theory was things just automatically stop, what's going on?

Lex Fridman (16:42.720)

And he had this elaborate story.

Sean Carroll (16:43.720)

I don't know if you've heard the story, but the arrow would push the air in front of it

Sean Carroll (16:48.440)

away and the molecules of air would run around to the back of the arrow and push it again.

Lex Fridman (16:53.300)

And anyone reading this is going like, really, that's what you thought?

Lex Fridman (16:56.860)

But it was that kind of thought experiment that ultimately got people to say like, actually,

Sean Carroll (17:00.660)

no, if it weren't for the air molecules at all, the arrow would just go on by itself.

Lex Fridman (17:04.460)

And it's always this give and take between thought and experience, back and forth, right?

Sean Carroll (17:09.980)

Theory and experiment, we would say today.

Sean Carroll (17:13.740)

Another big question that I think comes up, certainly with quantum mechanics, is what's

Lex Fridman (17:20.420)

the difference between math and physics to you?

Sean Carroll (17:25.720)

To me, you know, very, very roughly, math is about the logical structure of all possible

Sean Carroll (17:30.280)

worlds and physics is about our actual world.

Lex Fridman (17:35.100)

And it just feels like our actual world is a gray area when you start talking about interpretations

Lex Fridman (17:40.820)

of quantum mechanics, or no?

Lex Fridman (17:43.060)

I'm certainly using the word world in the broadest sense, all of reality.

Lex Fridman (17:47.940)

So I think that reality is specific.

Lex Fridman (17:50.740)

I don't think that there's every possible thing going on in reality.

Sean Carroll (17:54.420)

I think that there are rules, whether it's the Schrodinger equation or whatever.

Lex Fridman (17:58.520)

So I think that there's a sensible notion of the set of all possible worlds and we live

Sean Carroll (18:03.180)

in one of them.

Sean Carroll (18:04.440)

The world that we're talking about might be a multiverse, might be many worlds of quantum

Sean Carroll (18:07.980)

mechanics, might be much bigger than the world of our everyday experience, but it's still

Lex Fridman (18:10.940)

one physically contiguous world in some sense.

Lex Fridman (18:15.940)

But so if you look at the overlap of math and physics, it feels like when physics tries

Sean Carroll (18:24.220)

to reach for understanding of our world, it uses the tools of math to sort of reach beyond

Sean Carroll (18:30.620)

the limit of our current understanding.

Lex Fridman (18:34.320)

What do you make of that process of sort of using math to, so you start maybe with intuition

Sean Carroll (18:41.380)

or you might start with the math and then build up an intuition or, but this kind of

Lex Fridman (18:45.500)

reaching into the darkness, into the mystery of the world with math.

Sean Carroll (18:49.540)

Well, I think I would put it a little bit differently.

Sean Carroll (18:51.700)

I think we have theories, theories of the physical world, which we then extrapolate

Lex Fridman (18:57.900)

and ask, you know, what do we conclude if we take these seriously well beyond where

Lex Fridman (19:02.380)

we've actually tested them?

Sean Carroll (19:03.700)

It is separately true that math is really, really useful when we construct physical theories

Lex Fridman (19:09.180)

and you know, famously Eugene Wigner asked about the unreasonable success of mathematics

Lex Fridman (19:13.060)

and physics.

Sean Carroll (19:14.060)

I think that's a little bit wrong because anything that could happen, any other theory

Sean Carroll (19:20.140)

of physics that wasn't the real world, but some other world, you could always describe

Lex Fridman (19:24.700)

it mathematically.

Sean Carroll (19:25.700)

It's just that it might be a mess.

Sean Carroll (19:28.180)

The surprising thing is not that math works, but that the math is so simple and easy that

Lex Fridman (19:33.660)

you can write it down on a t shirt, right?

Lex Fridman (19:35.660)

I mean, that's what is amazing.

Sean Carroll (19:37.640)

That's an enormous compression of information that seems to be valid in the real world.

Lex Fridman (19:44.020)

So that's an interesting fact about our world, which maybe we could hope to explain or just

Sean Carroll (19:48.340)

take as a brute fact.

Lex Fridman (19:49.620)

I don't know.

Lex Fridman (19:50.880)

But once you have that, you know, there's this indelible relationship between math and

Lex Fridman (19:56.100)

physics, but philosophically I do want to separate them.

Lex Fridman (19:59.380)

What we extrapolate, we don't extrapolate math because there's a whole bunch of wrong

Lex Fridman (1:00:04.180)

and quantum gravity and holography and space time doing things like that.

Lex Fridman (1:00:09.280)

And when you take any of the other versions of quantum theory, they bring along classical

Sean Carroll (1:00:14.540)

baggage, all of the other versions of quantum mechanics, prejudice or privilege some version

Lex Fridman (1:00:21.820)

of classical reality like locations in space, okay?

Lex Fridman (1:00:26.020)

And I think that that's a barrier to doing better at understanding the theory of everything

Lex Fridman (1:00:31.220)

and understanding quantum gravity and the emergence of space time.

Sean Carroll (1:00:34.460)

Whenever if you change your theory from, you know, here's a harmonic oscillator, oh, there's

Sean Carroll (1:00:38.660)

a spin, here's an electromagnetic field, in hidden variable theories or dynamical collapse

Lex Fridman (1:00:43.640)

theories.

Sean Carroll (1:00:44.640)

You have to start from scratch.

Sean Carroll (1:00:45.640)

You have to say like, well, what are the hidden variables for this theory or how does it collapse

Lex Fridman (1:00:48.220)

or whatever?

Lex Fridman (1:00:49.220)

Whereas many worlds is plug and play.

Sean Carroll (1:00:50.900)

You tell me the theory and I can give you as many worlds version.

Lex Fridman (1:00:53.900)

So when we have a situation like we have with gravity and space time, where the classical

Sean Carroll (1:00:58.900)

description seems to break down in a dramatic way, then I think you should start from the

Lex Fridman (1:01:04.100)

most quantum theory that you have, which is really many worlds.

Lex Fridman (1:01:07.160)

So start with the quantum theory and try to build up a model of space time, the emergence

Lex Fridman (1:01:14.940)

of space time.

Sean Carroll (1:01:15.940)

That's it.

Lex Fridman (1:01:16.940)

Okay.

Lex Fridman (1:01:17.940)

So I thought space time was fundamental.

Lex Fridman (1:01:21.020)

Yeah, I know.

Lex Fridman (1:01:22.360)

So this sort of dream that Einstein had that everybody had and everybody has of, you know,

Lex Fridman (1:01:28.860)

the theory of everything.

Lex Fridman (1:01:30.660)

So how do we build up from many worlds from quantum mechanics, a model of space time model

Lex Fridman (1:01:37.460)

of gravity?

Sean Carroll (1:01:38.460)

Well, yeah, I mean, let me first mention very quickly why we think it's necessary.

Sean Carroll (1:01:42.580)

You know, we've had gravity in the form that Einstein bequeathed it to us for over a hundred

Sean Carroll (1:01:47.420)

years now, like 1915 or 1916, he put general relativity in the final form.

Lex Fridman (1:01:52.600)

So gravity is the curvature of space time and there's a field that pervades all the

Sean Carroll (1:01:57.020)

universe that tells us how curved space time is.

Lex Fridman (1:02:00.100)

And that's a fundamentally classical.

Sean Carroll (1:02:01.980)

That's totally classical.

Lex Fridman (1:02:02.980)

Right.

Sean Carroll (1:02:03.980)

Exactly.

Lex Fridman (1:02:04.980)

But we also have a formalism, an algorithm for taking a classical theory and quantizing

Sean Carroll (1:02:09.980)

it.

Lex Fridman (1:02:10.980)

This is how we get quantum electrodynamics, for example.

Lex Fridman (1:02:13.900)

And it could be tricky.

Sean Carroll (1:02:14.900)

I mean, you think you're quantizing something, so that means taking a classical theory and

Sean Carroll (1:02:19.500)

promoting it to a quantum mechanical theory.

Lex Fridman (1:02:22.400)

But you can run into problems.

Lex Fridman (1:02:23.940)

So they ran into problems and they did that with electromagnetism, namely that certain

Lex Fridman (1:02:27.460)

quantities were infinity and you don't like infinity, right?

Lex Fridman (1:02:30.300)

So Feynman and Tominaga and Schwinger won the Nobel Prize for teaching us how to deal

Lex Fridman (1:02:35.200)

with the infinities.

Lex Fridman (1:02:36.200)

And then Ken Wilson won another Nobel Prize for saying you shouldn't have been worried

Lex Fridman (1:02:39.320)

about those infinities after all.

Lex Fridman (1:02:41.380)

But still, that was the, it's always the thought that that's how you will make a good quantum

Lex Fridman (1:02:45.400)

theory.

Sean Carroll (1:02:46.400)

You'll start with a classical theory and quantize it.

Lex Fridman (1:02:47.860)

So if we have a classical theory, general relativity, we can quantize it or we can try

Sean Carroll (1:02:51.760)

to, but we run into even bigger problems with gravity than we ran into with electromagnetism.

Lex Fridman (1:02:57.780)

And so far, those problems are insurmountable.

Sean Carroll (1:03:00.060)

We've not been able to get a successful theory of gravity, quantum gravity, by starting with

Lex Fridman (1:03:04.980)

classical general relativity and quantizing it.

Lex Fridman (1:03:08.020)

And there's evidence that, there's a good reason why this is true, that whatever the

Lex Fridman (1:03:12.700)

quantum theory of gravity is, it's not a field theory.

Sean Carroll (1:03:17.020)

It's something that has weird nonlocal features built into it somehow that we don't understand.

Sean Carroll (1:03:22.300)

We get this idea from black holes and Hawking radiation and information conservation and

Sean Carroll (1:03:27.540)

a whole bunch of other ideas I talk about in the book.

Lex Fridman (1:03:30.180)

So if that's true, if the fundamental theory isn't even local in the sense that an ordinary

Sean Carroll (1:03:34.580)

quantum field theory would be, then we just don't know where to start in terms of getting

Lex Fridman (1:03:39.060)

a classical precursor and quantizing it.

Lex Fridman (1:03:42.400)

So the only sensible thing, or at least the next obvious sensible thing to me would be

Sean Carroll (1:03:46.220)

to say, okay, let's just start intrinsically quantum and work backwards, see if we can

Sean Carroll (1:03:50.020)

find a classical limit.

Lex Fridman (1:03:51.020)

So the idea of locality, the fact that locality is not fundamental to the nature of our existence,

Sean Carroll (1:04:03.900)

I guess in that sense, modeling everything as a field makes sense to me.

Lex Fridman (1:04:07.460)

Stuff that's close by interacts, stuff that's far away doesn't.

Lex Fridman (1:04:12.300)

So what's locality and why is it not fundamental?

Lex Fridman (1:04:15.580)

And how is that even possible?

Sean Carroll (1:04:16.580)

Yeah.

Sean Carroll (1:04:17.580)

I mean, locality is the answer to the question that Isaac Newton was worried about back in

Lex Fridman (1:04:21.260)

the beginning of our conversation, right?

Lex Fridman (1:04:22.580)

I mean, how can the earth know what the gravitational field of the sun is?

Lex Fridman (1:04:27.300)

And the answer as spelled out by Laplace and Einstein and others is that there's a field

Lex Fridman (1:04:31.340)

in between.

Lex Fridman (1:04:32.340)

And the way a field works is that what's happening to the field at this point in space only depends

Lex Fridman (1:04:38.660)

directly on what's happening at points right next to it.

Lex Fridman (1:04:41.580)

But what's happening at those points depends on what's happening right next to those, right?

Lex Fridman (1:04:45.180)

And so you can build up an influence across space through only local interactions.

Sean Carroll (1:04:50.620)

That's what locality means.

Lex Fridman (1:04:51.860)

What happens here is only affected by what's happening right next to it.

Sean Carroll (1:04:54.780)

That's locality.

Sean Carroll (1:04:57.000)

The idea of locality is built into every field theory, including general relativity as a

Sean Carroll (1:05:01.460)

classical theory.

Sean Carroll (1:05:03.100)

It seems to break down when we talk about black holes and, you know, Hawking taught

Sean Carroll (1:05:07.340)

us in the 1970s that black holes radiate, they give off, they eventually evaporate away.

Lex Fridman (1:05:12.520)

They're not completely black once we take quantum mechanics into account.

Lex Fridman (1:05:17.180)

And we think, we don't know for sure, but most of us think that if you make a black

Sean Carroll (1:05:22.540)

hole out of certain stuff, then like Laplace's demon taught us, you should be able to predict

Lex Fridman (1:05:28.380)

what that black hole will turn into if it's just obeying the Schrodinger equation.

Lex Fridman (1:05:32.660)

And if that's true, there are good arguments that can't happen while preserving locality

Sean Carroll (1:05:37.820)

at the same time.

Sean Carroll (1:05:38.820)

It's just that the information seems to be spread out nonlocally in interesting ways.

Lex Fridman (1:05:44.180)

And people should, you talk about holography with the Leonard Susskind on your Mindscape

Lex Fridman (1:05:49.540)

podcast.

Sean Carroll (1:05:50.540)

Oh yes, I have a podcast.

Lex Fridman (1:05:51.540)

I didn't even mention that.

Sean Carroll (1:05:52.540)

This is terrible.

Sean Carroll (1:05:53.540)

No, I'm going to, I'm going to ask you questions about that too, and I've been not shutting

Sean Carroll (1:05:57.340)

up about it.

Lex Fridman (1:05:58.340)

It's my favorite science podcast.

Sean Carroll (1:05:59.340)

So, or not, it's a, it's not even a science podcast.

Lex Fridman (1:06:02.900)

It's like, it's a scientist doing a podcast.

Sean Carroll (1:06:06.140)

That's right.

Lex Fridman (1:06:07.140)

That's what it is.

Sean Carroll (1:06:08.140)

Yeah.

Lex Fridman (1:06:09.140)

Anyway.

Sean Carroll (1:06:10.140)

Yeah.

Lex Fridman (1:06:11.140)

So holography is this idea when you have a black hole and black hole is a region of space

Sean Carroll (1:06:14.580)

inside of which gravity is so strong that you can't escape.

Lex Fridman (1:06:17.580)

And there's this weird feature of black holes that, again, it's totally a thought experiment

Sean Carroll (1:06:21.980)

feature because we haven't gone and probed any yet.

Lex Fridman (1:06:24.520)

But there seems to be one way of thinking about what happens inside a black hole as

Sean Carroll (1:06:29.960)

seen by an observer who's falling in, which is actually pretty normal.

Lex Fridman (1:06:33.660)

Like everything looks pretty normal until you hit the singularity and you die.

Lex Fridman (1:06:37.020)

But from the point of view of the outside observer, it seems like all the information

Lex Fridman (1:06:41.380)

that fell in is actually smeared over the horizon in a nonlocal way.

Lex Fridman (1:06:47.460)

And that's puzzling and that's, so holography because that's a two dimensional surface that

Lex Fridman (1:06:51.660)

is encapsulating the whole three dimensional thing inside, right?

Sean Carroll (1:06:55.420)

Still trying to deal with that.

Lex Fridman (1:06:56.420)

Still trying to figure out how to get there.

Lex Fridman (1:06:58.100)

But it's an indication that we need to think a little bit more subtly when we quantize

Lex Fridman (1:07:01.340)

gravity.

Lex Fridman (1:07:02.340)

And because you can describe everything that's going on in the three dimensional space by

Sean Carroll (1:07:07.460)

looking at the two dimensional projection of it, it means that locality doesn't, it's

Sean Carroll (1:07:13.900)

not necessary.

Lex Fridman (1:07:14.900)

Well, it means that somehow it's only a good approximation.

Sean Carroll (1:07:18.900)

It's not really what's going on.

Lex Fridman (1:07:20.380)

How are we supposed to feel about that?

Sean Carroll (1:07:22.020)

We're supposed to feel liberated.

Sean Carroll (1:07:25.700)

You know, space is just a good approximation and this was always going to be true once

Sean Carroll (1:07:29.680)

you started quantizing gravity.

Lex Fridman (1:07:31.680)

So we're just beginning now to face up to the dramatic implications of quantizing gravity.

Lex Fridman (1:07:38.820)

Is there other weird stuff that happens to quantum mechanics in black hole?

Lex Fridman (1:07:43.780)

I don't think that anything weird has happened with quantum mechanics.

Sean Carroll (1:07:47.020)

I think weird things happen with space time.

Lex Fridman (1:07:48.620)

I mean, that's what it is.

Sean Carroll (1:07:49.620)

Like quantum mechanics is still just quantum mechanics, but our ordinary notions of space

Lex Fridman (1:07:53.680)

time don't really quite work.

Lex Fridman (1:07:57.620)

And there's a principle that goes hand in hand with holography called complementarity,

Sean Carroll (1:08:03.940)

which says that there's no one unique way to describe what's going on inside a black

Sean Carroll (1:08:10.540)

hole.

Sean Carroll (1:08:11.540)

Different observers will have different descriptions, both of which are accurate, but sound completely

Sean Carroll (1:08:17.220)

incompatible with each other.

Lex Fridman (1:08:18.640)

So depends on how you look at it.

Sean Carroll (1:08:20.900)

The word complementarity in this context is borrowed from Niels Bohr, who points out you

Lex Fridman (1:08:25.060)

can measure the position or you can measure the momentum.

Sean Carroll (1:08:28.260)

You can't measure both at the same time in quantum mechanics.

Lex Fridman (1:08:30.900)

So a couple of questions on many worlds.

Lex Fridman (1:08:34.900)

How does many worlds help us understand our particular branch of reality?

Lex Fridman (1:08:40.740)

So okay, that's fine and good that is everything is splitting, but we're just traveling down

Sean Carroll (1:08:45.100)

a single branch of it.

Lex Fridman (1:08:46.220)

So how does it help us understand our little unique branch?

Sean Carroll (1:08:50.780)

Yeah, I mean, that's a great question.

Lex Fridman (1:08:52.840)

But that's the point is that we didn't invent many worlds because we thought it was cool

Lex Fridman (1:08:56.260)

to have a whole bunch of worlds, right?

Sean Carroll (1:08:57.820)

We invented it because we were trying to account for what we observe here in our world.

Lex Fridman (1:09:02.700)

And what we observe here in our world are wave functions collapsing, okay?

Lex Fridman (1:09:07.500)

We do have a position, a situation where the electron seems to be spread out.

Lex Fridman (1:09:11.500)

But then when we look at it, we don't see it spread out.

Lex Fridman (1:09:13.380)

We see it located somewhere.

Lex Fridman (1:09:15.100)

So what's going on?

Lex Fridman (1:09:16.160)

That's the measurement problem of quantum mechanics.

Sean Carroll (1:09:17.880)

That's what we have to face up to.

Lex Fridman (1:09:19.400)

So many worlds is just a proposed solution to that problem.

Lex Fridman (1:09:22.900)

And the answer is nothing special is happening.

Lex Fridman (1:09:25.660)

It's still just the Schrodinger equation, but you have a wave function too.

Lex Fridman (1:09:30.080)

And that's a different answer than would be given in hidden variables or dynamical collapse

Lex Fridman (1:09:34.380)

theories or whatever.

Lex Fridman (1:09:36.020)

So the entire point of many worlds is to explain what we observe, but it tries to explain what

Lex Fridman (1:09:42.100)

we already have observed, right?

Sean Carroll (1:09:44.280)

It's not trying to be different from what we've observed because that would be something

Lex Fridman (1:09:48.380)

other than quantum mechanics.

Lex Fridman (1:09:50.240)

But you know, the idea that there's worlds that we didn't observe that keep branching

Lex Fridman (1:09:54.220)

off is kind of, it's stimulating to the imagination.

Lex Fridman (1:10:00.000)

So is it possible to hop from, you mentioned the branches are independent.

Lex Fridman (1:10:08.040)

Is it possible to hop from one to the other?

Sean Carroll (1:10:10.260)

No.

Lex Fridman (1:10:11.260)

So it's a physical limit.

Sean Carroll (1:10:14.180)

The theory says it's impossible.

Lex Fridman (1:10:16.260)

There's already a copy of you in the other world, don't worry.

Sean Carroll (1:10:18.620)

Yes.

Lex Fridman (1:10:19.620)

Leave them alone.

Sean Carroll (1:10:20.620)

No, but there's a fear of missing out, FOMO, that I feel like immediately start to wonder

Lex Fridman (1:10:28.780)

if that other copy is having more or less fun.

Sean Carroll (1:10:32.900)

Well, the downside to many worlds is that you're missing out on an enormous amount.

Lex Fridman (1:10:38.620)

And that's always what it's going to be like.

Lex Fridman (1:10:40.460)

And I mean, there's a certain stage of acceptance in that.

Sean Carroll (1:10:44.380)

In terms of rewinding, do you think we can rewind the system back, sort of the nice thing

Sean Carroll (1:10:49.980)

about many worlds, I guess, is it really emphasizes the, maybe you can correct me, but the deterministic

Lex Fridman (1:10:58.800)

nature of a branch and it feels like it could be rewound back.

Lex Fridman (1:11:05.700)

Is it, do you see it as something that could be perfectly rewound back, rewinding back?

Lex Fridman (1:11:11.900)

Yeah.

Sean Carroll (1:11:12.900)

If you're at a fancy French restaurant and there's a nice linen white tablecloth and

Sean Carroll (1:11:17.060)

you have your glass of Bordeaux and you knock it over and the wine spills across the tablecloth.

Sean Carroll (1:11:22.620)

If the world were classical, okay, it would be possible that if you just lifted the wine

Sean Carroll (1:11:28.440)

glass up, you'd be lucky enough that every molecule of wine would hop back into the glass,

Lex Fridman (1:11:33.020)

right?

Lex Fridman (1:11:34.020)

But guess what?

Sean Carroll (1:11:35.020)

It's not going to happen in the real world.

Lex Fridman (1:11:37.100)

And the quantum wave function is exactly the same way.

Sean Carroll (1:11:40.020)

It is possible in principle to rewind everything if you start from perfect knowledge of the

Lex Fridman (1:11:44.540)

entire wave function of the universe.

Sean Carroll (1:11:46.820)

In practice, it's never going to happen.

Lex Fridman (1:11:49.320)

So time travel, not possible.

Sean Carroll (1:11:53.020)

Nope.

Lex Fridman (1:11:54.020)

At least quantum mechanics has no help.

Lex Fridman (1:11:57.760)

What about memory?

Sean Carroll (1:12:00.540)

Does the universe have a memory of itself where we could, in, in, so not time travel,

Lex Fridman (1:12:07.720)

but peek back in time and do a little like replay?

Lex Fridman (1:12:13.100)

Well, it's exactly the same in quantum mechanics as classical mechanics.

Lex Fridman (1:12:18.100)

So whatever you want to say about that, you know, the fundamental laws of physics in either

Lex Fridman (1:12:22.540)

many worlds, quantum mechanics or Newtonian physics conserve information.

Lex Fridman (1:12:28.100)

So if you have all the information about the quantum state of the world right now, your

Sean Carroll (1:12:32.620)

Laplace is demon like in your knowledge and calculational capacity, you can wind the clock

Sean Carroll (1:12:37.500)

backward.

Lex Fridman (1:12:38.500)

But none of us is.

Lex Fridman (1:12:39.500)

Right?

Lex Fridman (1:12:40.500)

And, you know, so in practice you can never do that.

Sean Carroll (1:12:42.380)

You can do experiments over and over again, starting from the same initial conditions

Lex Fridman (1:12:46.020)

for small systems.

Lex Fridman (1:12:47.860)

But once things get to be large, Avogadro's number of particles, right?

Lex Fridman (1:12:51.780)

Bigger than a cell, no chance.

Sean Carroll (1:12:54.960)

We we've talked a little bit about arrow of time last time, but in many worlds that there

Lex Fridman (1:13:00.780)

is a kind of implied arrow of time, right?

Lex Fridman (1:13:07.520)

So you've talked about the arrow of time that has to do with the second law of thermodynamics.

Lex Fridman (1:13:14.480)

That's the arrow of time that's emergent or fundamental.

Sean Carroll (1:13:18.340)

We don't know, I guess.

Lex Fridman (1:13:19.340)

No, it's emergent.

Lex Fridman (1:13:20.340)

Is that, does everyone agree on that?

Lex Fridman (1:13:23.340)

Well, nobody agrees with everything.

Sean Carroll (1:13:25.380)

They should.

Lex Fridman (1:13:26.380)

They should.

Lex Fridman (1:13:28.300)

So that arrow of time, is that different than the arrow of time that's implied by many worlds?

Lex Fridman (1:13:34.000)

It's not different, actually, no.

Sean Carroll (1:13:35.420)

In both cases, you have fundamental laws of physics that are completely reversible.

Sean Carroll (1:13:40.940)

If you give me the state of the universe at one moment in time, I can run the clock forward

Sean Carroll (1:13:45.040)

or backward equally well.

Sean Carroll (1:13:46.360)

There's no arrow of time built into the laws of physics at the most fundamental level.

Lex Fridman (1:13:51.360)

But what we do have are special initial conditions 14 billion years ago near the Big Bang.

Sean Carroll (1:13:57.920)

In thermodynamics, those special initial conditions take the form of things were low entropy and

Sean Carroll (1:14:03.520)

entropy has been increasing ever since, making the universe more disorganized and chaotic

Lex Fridman (1:14:08.480)

and that's the arrow of time.

Sean Carroll (1:14:10.540)

In quantum mechanics, the special initial conditions take the form of there was only

Sean Carroll (1:14:15.080)

one branch of the wave function and the universe has been branching more and more ever since.

Sean Carroll (1:14:20.140)

Okay, so if time is emergent, so it seems like our human cognitive capacity likes to

Lex Fridman (1:14:28.540)

take things that are emergent and assume and feel like they're fundamental.

Lex Fridman (1:14:33.200)

So what, so if time is emergent and locality, like is space emergent?

Lex Fridman (1:14:42.600)

Yes.

Sean Carroll (1:14:43.600)

Okay.

Lex Fridman (1:14:44.600)

But I didn't say time was emergent, I said the arrow of time was emergent.

Sean Carroll (1:14:46.820)

Those are different.

Lex Fridman (1:14:47.820)

What's the difference between the arrow of time and time?

Sean Carroll (1:14:52.560)

Are you using arrow of time to simply mean this, they're synonymous with the second law

Lex Fridman (1:14:56.920)

of thermodynamics?

Sean Carroll (1:14:57.920)

No, but the arrow of time is the difference between the past and future.

Lex Fridman (1:15:01.520)

So there's space, but there's no arrow of space.

Lex Fridman (1:15:04.500)

You don't feel that space has to have an arrow, right?

Sean Carroll (1:15:06.920)

You could live in thermodynamic equilibrium, there'd be no arrow of time, but there'd still

Sean Carroll (1:15:10.700)

be time.

Lex Fridman (1:15:11.700)

There'd still be a difference between now and the future or whatever.

Lex Fridman (1:15:14.740)

So if nothing changes, there's still time.

Sean Carroll (1:15:19.000)

Well things could even change, like if the whole universe consisted of the earth going

Lex Fridman (1:15:23.760)

around the sun, it would just go in circles or ellipses, right?

Lex Fridman (1:15:29.440)

Things would change, but it's not increasing entropy, there's no arrow.

Sean Carroll (1:15:32.560)

If you took a movie of that and I played you the movie backward, you would never know.

Lex Fridman (1:15:38.900)

So the arrow of time can theoretically point in the other direction for briefly.

Sean Carroll (1:15:45.680)

To the extent that it points in different directions, it's not a very good arrow.

Sean Carroll (1:15:49.000)

I mean, the arrow of time in the macroscopic world is so powerful that there's just no

Sean Carroll (1:15:53.520)

chance of going back.

Sean Carroll (1:15:55.000)

When you get down to tiny systems with only three or four moving parts, then entropy can

Sean Carroll (1:15:58.680)

fluctuate up and down.

Lex Fridman (1:16:00.480)

What does it mean for space to be an emergent phenomenon?

Sean Carroll (1:16:03.440)

It means that the fundamental description of the world does not include the word space.

Sean Carroll (1:16:07.280)

It'll be something like a vector in Hilbert space, right, and you have to say, well why

Sean Carroll (1:16:10.920)

is there a good approximate description which involves three dimensional space and stuff

Lex Fridman (1:16:16.380)

inside it?

Sean Carroll (1:16:17.380)

Okay, so time and space are emergent.

Sean Carroll (1:16:21.200)

We kind of mentioned in the beginning, can you elaborate, what do you feel hope is fundamental

Lex Fridman (1:16:28.620)

in our universe?

Lex Fridman (1:16:30.400)

A wave function living in Hilbert space.

Sean Carroll (1:16:32.160)

A wave function in Hilbert space that we can't intellectualize or visualize really.

Lex Fridman (1:16:36.960)

We can't visualize it, we can intellectualize it very easily.

Lex Fridman (1:16:40.160)

Like how do you think about?

Lex Fridman (1:16:42.240)

It's a vector in a 10 to the 10 to the 122 dimensional vector space.

Sean Carroll (1:16:45.560)

It's a complex vector, unit norm, it evolves according to the Schrodinger equation.

Lex Fridman (1:16:50.920)

Got it.

Sean Carroll (1:16:52.420)

When you put it that way.

Lex Fridman (1:16:53.880)

What's so hard, really?

Sean Carroll (1:16:56.880)

It's like, yep, quantum computers, there's some excitement, actually a lot of excitement

Lex Fridman (1:17:04.440)

with people that it will allow us to simulate quantum mechanical systems.

Lex Fridman (1:17:10.640)

What kind of questions do you about quantum mechanics, about the things we've been talking

Lex Fridman (1:17:14.680)

about, do you think, do you hope we can answer through quantum simulation?

Sean Carroll (1:17:21.720)

Well I think that there are, there's a whole fascinating frontier of things you can do

Lex Fridman (1:17:26.340)

with quantum computers.

Sean Carroll (1:17:27.640)

Both sort of practical things with cryptography or money, privacy eavesdropping, sorting things,

Lex Fridman (1:17:37.400)

simulating quantum systems, right?

Lex Fridman (1:17:40.280)

So it's a broader question maybe even outside of quantum computers.

Sean Carroll (1:17:44.800)

Some of the theories that we've been talking about, what's your hope, what's most promising

Lex Fridman (1:17:49.960)

to test these theories?

Lex Fridman (1:17:52.160)

What are kind of experiments we can conduct, whether in simulation or in the physical world

Lex Fridman (1:17:57.620)

that would validate or disprove or expand these theories?

Lex Fridman (1:18:03.160)

Well I think for, there's two parts of that question.

Sean Carroll (1:18:06.840)

One is many worlds and the other one is sort of emergent space time.

Sean Carroll (1:18:10.300)

For many worlds, you know, there are experiments ongoing to test whether or not wave functions

Sean Carroll (1:18:14.280)

spontaneously collapse.

Lex Fridman (1:18:17.480)

And if they do, then that rules out many worlds and that would be falsified.

Sean Carroll (1:18:21.660)

If there are hidden variables, there's a theorem that seems to indicate that the predictions

Lex Fridman (1:18:27.400)

will always be the same as many worlds.

Sean Carroll (1:18:29.800)

I'm a little skeptical of this theorem.

Lex Fridman (1:18:31.240)

I'm not complete.

Sean Carroll (1:18:32.240)

I haven't internalized it.

Sean Carroll (1:18:33.240)

I haven't made it in part of my intuitive view of the world yet, so there might be loopholes

Sean Carroll (1:18:36.680)

to that theorem.

Lex Fridman (1:18:37.680)

I'm not sure about that.

Sean Carroll (1:18:38.680)

Part of me thinks that there should be different experimental predictions if there are hidden

Lex Fridman (1:18:42.080)

variables, but I'm not sure.

Lex Fridman (1:18:45.180)

But otherwise, it's just quantum mechanics all the way down.

Lex Fridman (1:18:47.320)

And so there's this cottage industry in science journalism of writing breathless articles

Sean Carroll (1:18:53.120)

that say, you know, quantum mechanics shown to be more astonishing than ever before thought.

Lex Fridman (1:18:57.840)

And really, it's the same quantum mechanics we've been doing since 1926.

Sean Carroll (1:19:01.840)

Whereas with the emergent space time stuff, we know a lot less about what the theory is.

Lex Fridman (1:19:06.000)

It's in a very primitive state.

Sean Carroll (1:19:07.760)

We don't even really have a safely written down, respectable, honest theory yet.

Lex Fridman (1:19:13.880)

So there could very well be experimental predictions we just don't know about yet.

Sean Carroll (1:19:17.460)

That is one of the things that we're trying to figure out.

Lex Fridman (1:19:20.200)

Yeah, for emergent space time, you need really big stuff, right?

Sean Carroll (1:19:24.320)

Well, or really fast stuff, or really energetic stuff.

Lex Fridman (1:19:27.800)

We don't know.

Sean Carroll (1:19:28.800)

That's the thing.

Sean Carroll (1:19:29.800)

You know, so there could be violations of the speed of light if you have emergent space

Sean Carroll (1:19:34.440)

time.

Sean Carroll (1:19:35.860)

Not going faster than the speed of light, but the speed of light could be different

Lex Fridman (1:19:39.240)

for light of different wavelengths, right?

Sean Carroll (1:19:42.320)

That would be a dramatic violation of physics as we know it, but it could be possible.

Sean Carroll (1:19:46.840)

Or not.

Lex Fridman (1:19:47.840)

I mean, it's not an absolute prediction.

Sean Carroll (1:19:48.840)

That's the problem.

Lex Fridman (1:19:49.840)

The theories are just not well developed enough yet to say.

Lex Fridman (1:19:54.080)

Is there anything that quantum mechanics can teach us about human nature or the human mind?

Lex Fridman (1:20:01.120)

If you think about sort of consciousness and these kinds of topics, is there...

Sean Carroll (1:20:06.820)

It's certainly excessively used, as you point out.

Lex Fridman (1:20:10.320)

The word quantum is used for everything besides quantum mechanics.

Lex Fridman (1:20:15.520)

But in more seriousness, is there something that goes to the human level and can help

Lex Fridman (1:20:23.600)

us understand our mind?

Sean Carroll (1:20:27.280)

Not really is the short answer, you know.

Lex Fridman (1:20:29.680)

Minds are pretty classical.

Sean Carroll (1:20:31.400)

I don't think.

Sean Carroll (1:20:32.920)

We don't know this for sure, but I don't think that phenomena like entanglement are crucial

Sean Carroll (1:20:36.600)

to how the human mind works.

Lex Fridman (1:20:38.440)

What about consciousness?

Lex Fridman (1:20:40.160)

So you mentioned, I think early on in the conversation, you said it would be unlikely,

Lex Fridman (1:20:48.920)

but incredible if sort of the observer is somehow a fundamental part.

Lex Fridman (1:20:54.680)

So observer, not to romanticize the notion, but seems interlinked to the idea of consciousness.

Lex Fridman (1:21:01.900)

So if consciousness, as the panpsychists believe, is fundamental to the universe, is that possible?

Sean Carroll (1:21:09.040)

Is that weight...

Lex Fridman (1:21:10.040)

I mean, every...

Sean Carroll (1:21:11.040)

Everything's possible.

Lex Fridman (1:21:12.040)

Just like Joe Rogan likes to say, it's entirely possible.

Lex Fridman (1:21:16.920)

But okay.

Lex Fridman (1:21:17.920)

But is it on a spectrum of crazy out there?

Lex Fridman (1:21:22.960)

How the statistically speaking, how often do you ponder the possibility that consciousness

Lex Fridman (1:21:28.600)

is fundamental or the observer is fundamental to...

Sean Carroll (1:21:32.120)

Personally don't at all.

Lex Fridman (1:21:33.120)

There are people who do.

Sean Carroll (1:21:34.120)

I'm a thorough physicalist when it comes to consciousness.

Lex Fridman (1:21:37.160)

I do not think that there are any separate mental states or mental properties.

Sean Carroll (1:21:41.800)

I think they're all emergent, just like space time is and space time is hard enough to understand.

Lex Fridman (1:21:46.000)

So the fact that we don't yet understand consciousness is not at all surprising to me.

Sean Carroll (1:21:51.320)

You, as we mentioned, have an amazing podcast called Mindscape.

Sean Carroll (1:21:55.280)

It's as I said, one of my favorite podcasts sort of both for your explanation of physics,

Sean Carroll (1:22:03.680)

which a lot of people love, and when you venture out into things that are beyond your expertise,

Lex Fridman (1:22:10.520)

but it's just a really smart person exploring even questions like morality, for example.

Sean Carroll (1:22:18.960)

It's very interesting.

Lex Fridman (1:22:19.960)

I think you did a solo episode and so on.

Sean Carroll (1:22:21.920)

I mean, there's a lot of really interesting conversations that you have.

Lex Fridman (1:22:28.200)

What are some from memory, amazing conversations that pop to mind that you've had?

Lex Fridman (1:22:34.680)

What did you learn from them?

Sean Carroll (1:22:36.320)

Something that maybe changed your mind or just inspired you or just what did this whole

Lex Fridman (1:22:40.360)

experience of having conversations, what stands out to you?

Lex Fridman (1:22:45.160)

It's an unfair question.

Sean Carroll (1:22:46.160)

Totally unfair.

Lex Fridman (1:22:47.160)

That's okay.

Sean Carroll (1:22:48.160)

That's all right.

Sean Carroll (1:22:49.160)

You know, it's often the ones I feel like the ones I do on physics and closely related

Sean Carroll (1:22:54.000)

science or even philosophy ones are like, I know this stuff and I'm helping people learn

Lex Fridman (1:23:01.160)

about it.

Lex Fridman (1:23:02.160)

But I learn more from the ones that have nothing to do with physics or philosophy, right?

Lex Fridman (1:23:05.900)

So talking to Wynton Marsalis about jazz or talking to a Master Sommelier about wine,

Sean Carroll (1:23:12.160)

talking to Will Wilkinson about partisan polarization and the urban rural divide, talking to psychologists

Lex Fridman (1:23:18.760)

like Carol Tavris about cognitive dissonance and how those things work.

Sean Carroll (1:23:26.320)

Scott Derrickson who is the director of the movie Dr. Strange, I had a wonderful conversation

Lex Fridman (1:23:31.160)

with him where we went through the mechanics of making a blockbuster superhero movie, right?

Lex Fridman (1:23:36.680)

And he's also not a naturalist, he's an evangelical Christian so we talked about the nature of

Lex Fridman (1:23:41.440)

reality there.

Sean Carroll (1:23:42.440)

I want to have a couple more, you know, discussions with highly educated theists who know the

Lex Fridman (1:23:51.360)

theology really well but I haven't quite arranged those yet.

Sean Carroll (1:23:56.600)

I would love to hear that.

Lex Fridman (1:23:57.600)

I mean that's, how comfortable are you venturing into questions of religion?

Sean Carroll (1:24:02.520)

Oh, I'm totally comfortable doing it.

Sean Carroll (1:24:04.960)

You know, I did talk with Alan Lightman who is also an atheist but he, you know, he is

Sean Carroll (1:24:10.320)

trying to rescue the sort of spiritual side of things for atheism and I did talk to very

Sean Carroll (1:24:18.600)

vocal atheists like Alex Rosenberg so I need to talk to some, I've talked to some religious

Sean Carroll (1:24:23.280)

believers but I need to talk to more.

Lex Fridman (1:24:26.360)

How have you changed through having all these conversations?

Sean Carroll (1:24:31.320)

You know, part of the motivation was I had a long stack of books that I hadn't read and

Sean Carroll (1:24:35.360)

I couldn't find time to read them and I figured if I interviewed their authors, forced me

Sean Carroll (1:24:38.880)

to read them, right, and that has totally worked by the way.

Lex Fridman (1:24:42.080)

Now I'm annoyed that people write such long books.

Sean Carroll (1:24:45.680)

I think I'm still very much learning how to be a good interviewer.

Sean Carroll (1:24:49.360)

I think that's a skill that, you know, I think I have good questions but, you know, there's

Sean Carroll (1:24:54.520)

the give and take that is still I think I can be better at.

Lex Fridman (1:24:58.920)

Like I want to offer something to the conversation but not too much, right?

Sean Carroll (1:25:02.480)

I've had conversations where I barely talked at all and I have conversations where I talked

Lex Fridman (1:25:06.000)

half the time and I think there's a happy medium in between there.

Lex Fridman (1:25:09.000)

So I think I remember listening to, without mentioning names, some of your conversations

Lex Fridman (1:25:13.720)

where I wish you would have disagreed more.

Sean Carroll (1:25:16.480)

As a listener, it's more fun sometimes.

Sean Carroll (1:25:20.400)

Well, that's a very good question because, you know, everyone has an attitude toward

Sean Carroll (1:25:24.960)

that.

Sean Carroll (1:25:25.960)

Like some people are really there to basically give their point of view and their guest is

Sean Carroll (1:25:32.720)

supposed to, you know, respond accordingly.

Sean Carroll (1:25:35.480)

I want to sort of get my view on the record but I don't want to dwell on it when I'm talking

Sean Carroll (1:25:41.400)

to someone like David Chalmers who I disagree with a lot.

Sean Carroll (1:25:44.120)

You know, I want to say like, here's why I disagree with you but, you know, we're here

Sean Carroll (1:25:48.860)

to listen to you.

Lex Fridman (1:25:49.980)

Like I have an episode every week and you're only on once a week, right?

Lex Fridman (1:25:53.240)

So I have an upcoming podcast episode with Philip Goff who is a much more dedicated pan

Lex Fridman (1:26:00.760)

psychist and so there we really get into it.

Sean Carroll (1:26:02.640)

I think that I probably have disagreed with him more on that episode than I ever have

Lex Fridman (1:26:06.680)

with another podcast guest but that's what he wanted so it worked very well.

Sean Carroll (1:26:10.240)

Yeah, yeah.

Lex Fridman (1:26:11.240)

That kind of debate structure is beautiful when it's done right.

Sean Carroll (1:26:15.880)

Like when you're, when you can detect that the intent is that you have fundamental respect

Lex Fridman (1:26:21.400)

for the person.

Sean Carroll (1:26:22.400)

Yeah.

Sean Carroll (1:26:23.400)

That, and that's, for some reason, it's super fun to listen to when two really smart people

Sean Carroll (1:26:29.040)

are just arguing and sometimes lose their shit a little bit if I may say so.

Sean Carroll (1:26:32.920)

Well, there's a fine line because I have zero interest in bringing, I mean, like, I mean,

Sean Carroll (1:26:39.160)

maybe you implied this, I have zero interest in bringing on people for whom I don't have

Lex Fridman (1:26:43.280)

any intellectual respect.

Sean Carroll (1:26:44.600)

Like I constantly get requests like, you know, bring on a flat earther or whatever and really

Lex Fridman (1:26:49.120)

slap them down or a creationist, like I have zero interest.

Sean Carroll (1:26:52.800)

I'm happy to bring on, you know, a religious person, a believer, but I want someone who's

Lex Fridman (1:26:56.800)

smart and can act in good faith and can talk, not a charlatan or a lunatic, right?

Lex Fridman (1:27:02.160)

So I will only, I will happily bring on people with whom I disagree, but only people from

Lex Fridman (1:27:08.160)

whom I think the audience can learn something interesting.

Lex Fridman (1:27:10.320)

So let me ask, the idea of charlatan is an interesting idea.

Sean Carroll (1:27:15.740)

You might be more educated on this topic than me, but there's, there's folks, for example,

Sean Carroll (1:27:22.920)

who argue various aspects of evolution sort of try to approach and say that evolution

Lex Fridman (1:27:31.280)

sort of our current theory of evolution has many holes in it, has many flaws.

Lex Fridman (1:27:37.440)

And they argue that I think like Cambridge, Cambrian explosion, which is like a huge added

Sean Carroll (1:27:46.820)

variability of species, doesn't make sense under our current description of evolution

Lex Fridman (1:27:52.160)

and theory of evolution sort of, if you had to, were to have the conversation with people

Sean Carroll (1:27:57.520)

like that, how do you know that they're the difference in outside the box thinkers and

Lex Fridman (1:28:05.720)

people who are fundamentally unscientific and even bordering on charlatans?

Lex Fridman (1:28:13.080)

That's a great question.

Lex Fridman (1:28:15.080)

And you know, the further you get away from my expertise, the harder it is for me to really

Lex Fridman (1:28:19.200)

judge exactly those things.

Sean Carroll (1:28:21.040)

And, you know, yeah, I don't have a satisfying answer for that one because I think the example

Sean Carroll (1:28:25.520)

you use of someone who, you know, believes in the basic structure of natural selection,

Lex Fridman (1:28:30.840)

but thinks that, you know, this particular thing cannot be understood in the terms of

Lex Fridman (1:28:35.240)

our current understanding of Darwinism.

Lex Fridman (1:28:38.360)

That's a perfect edge case where it's hard to tell, right?

Lex Fridman (1:28:41.560)

And I would have, I would try to talk to people who I do respect and who do know things and

Sean Carroll (1:28:45.680)

I would have to, you know, given that I'm a physicist, I know that physicists will sometimes

Lex Fridman (1:28:50.920)

be too dismissive of alternative points of view.

Sean Carroll (1:28:53.480)

I have to take into account that biologists can also be too dismissive of alternative points

Lex Fridman (1:28:57.320)

of view.

Sean Carroll (1:28:58.320)

So, yeah, that's a tricky one.

Lex Fridman (1:29:00.760)

Have you gotten heat yet?

Sean Carroll (1:29:02.040)

I get heat all the time.

Sean Carroll (1:29:03.040)

Like there's always something, I mean, it's hilarious because I do have, I try very hard

Sean Carroll (1:29:09.000)

not to like have the same topic several times in a row.

Sean Carroll (1:29:12.080)

I did have like two climate change episodes, but they were from very different perspectives,

Lex Fridman (1:29:16.320)

but I like to mix it up.

Lex Fridman (1:29:17.320)

That's the whole, that's why I'm having fun.

Lex Fridman (1:29:19.040)

And every time I do an episode, someone says, oh, the person you should really get on to

Lex Fridman (1:29:22.720)

talk about exactly that is this other person.

Sean Carroll (1:29:24.600)

I'm like, well, I don't, but I did that now.

Lex Fridman (1:29:26.080)

I don't want to do that anymore.

Sean Carroll (1:29:28.080)

Well, I hope you keep doing it.

Lex Fridman (1:29:30.880)

You're inspiring millions of people, your books, your podcasts.

Sean Carroll (1:29:34.240)

Sean, it's an honor to talk to you.

Lex Fridman (1:29:36.240)

Thank you so much.

Sean Carroll (1:29:37.240)

Thank you very much, Lex.

Lex Fridman (20:03.740)

math, you know, that doesn't apply to our world, right?

Sean Carroll (20:06.060)

We extrapolate the physical theory that we best think explains our world.

Lex Fridman (20:09.980)

Again, an unanswerable question.

Lex Fridman (20:12.700)

Why do you think our world is so easily compressible into beautiful equations?

Lex Fridman (20:19.780)

Yeah.

Sean Carroll (20:20.780)

I mean, like I just hinted at, I don't know if there's an answer to that question.

Lex Fridman (20:23.940)

There could be.

Lex Fridman (20:24.940)

What would an answer look like?

Sean Carroll (20:26.460)

Well, an answer could look like if you showed that there was something about our world that

Sean Carroll (20:31.380)

maximizes something.

Sean Carroll (20:33.260)

You know, the mean of the simplicity and the powerfulness of the laws of physics or, you

Sean Carroll (20:40.100)

know, maybe we're just generic.

Lex Fridman (20:41.920)

Maybe in the set of all possible worlds, this is what the world would look like, right?

Sean Carroll (20:44.980)

Like I don't really know.

Lex Fridman (20:46.500)

I tend to think not.

Sean Carroll (20:48.340)

I tend to think that there is something specific and rock bottom about the facts of our world

Lex Fridman (20:54.220)

that don't have further explanation.

Sean Carroll (20:56.060)

Like the fact of the world exists at all.

Lex Fridman (20:58.260)

And furthermore, the specific laws of physics that we have.

Sean Carroll (21:00.780)

I think that in some sense, we're just going to, at some level, we're going to say, and

Lex Fridman (21:04.660)

that's how it is.

Sean Carroll (21:05.660)

And, you know, we can't explain anything more.

Sean Carroll (21:07.340)

I don't know how, if we're anywhere close to that right now, but that seems plausible

Sean Carroll (21:11.580)

to me.

Lex Fridman (21:12.580)

And speaking of rock bottom, one of the things sort of your book kind of reminded me or revealed

Sean Carroll (21:16.780)

to me is that what's fundamental and what's emergent, it just feels like I don't even

Lex Fridman (21:23.340)

know anymore what's fundamental in physics, if there's anything.

Sean Carroll (21:28.420)

It feels like everything, especially with quantum mechanics, is revealing to us is that

Sean Carroll (21:33.980)

most interesting things that I would, as a limited human would think are fundamental

Sean Carroll (21:41.460)

can actually be explained as emergent from the more deeper laws.

Lex Fridman (21:48.980)

I mean, we don't know, of course.

Sean Carroll (21:51.140)

You had to get that on the table.

Lex Fridman (21:52.480)

We don't know what is fundamental.

Lex Fridman (21:54.620)

We do have reasons to say that certain things are more fundamental than others, right?

Lex Fridman (22:00.480)

Atoms and molecules are more fundamental than cells and organs.

Sean Carroll (22:03.980)

Quantum fields are more fundamental than atoms and molecules.

Lex Fridman (22:07.600)

We don't know if that ever bottoms out.

Sean Carroll (22:09.780)

I do think that there's sensible ways to think about this.

Sean Carroll (22:13.860)

If you describe something like this table as a table, it has a height and a width and

Sean Carroll (22:18.140)

it's made of a certain material and it has a certain solidity and weight and so forth.

Lex Fridman (22:22.580)

That's a very useful description as far as it goes.

Sean Carroll (22:24.940)

There's a whole other description of this table in terms of a whole collection of atoms

Lex Fridman (22:29.380)

strung together in certain ways.

Sean Carroll (22:31.640)

The language of the atoms is more comprehensive than the language of the table.

Sean Carroll (22:36.340)

You could break apart the table, smash it to pieces, still talk about it as atoms, but

Lex Fridman (22:41.100)

you could no longer talk about it as a table, right?

Lex Fridman (22:43.940)

So I think that this comprehensiveness, the domain of validity of a theory gets broader

Lex Fridman (22:48.380)

and broader as the theory gets more and more fundamental.

Lex Fridman (22:52.900)

So what do you think Newton would say?

Sean Carroll (22:57.620)

Maybe right in the book review, if you read your latest book on quantum mechanics, something

Lex Fridman (23:02.140)

deeply hidden.

Sean Carroll (23:03.140)

It would take a long time for him to think that any of this was making any sense.

Lex Fridman (23:08.140)

You catch him up pretty quick in the beginning.

Sean Carroll (23:10.260)

Yeah.

Lex Fridman (23:11.260)

You give him a shout out in the beginning.

Sean Carroll (23:13.220)

That's right.

Lex Fridman (23:14.220)

He is the man.

Sean Carroll (23:15.220)

I'm happy to say that Newton was the greatest scientist who ever lived.

Sean Carroll (23:19.460)

He invented calculus in his spare time, which would have made him the greatest mathematician

Sean Carroll (23:22.500)

just all by himself, all by that one thing.

Lex Fridman (23:25.620)

But of course, it's funny because Newton was in some sense still a pre modern thinker.

Sean Carroll (23:33.540)

Rocky Kolb, who is a cosmologist at the University of Chicago said that Galileo, even though

Lex Fridman (23:39.480)

he came before Newton, was a more modern thinker than Newton was.

Sean Carroll (23:43.740)

If you got Galileo and brought him to the present day, it would take him six months

Sean Carroll (23:46.660)

to catch up and then he'd be in your office telling you why your most recent paper was

Sean Carroll (23:49.580)

wrong.

Lex Fridman (23:50.740)

Whereas Newton just thought in this kind of more mystical way.

Sean Carroll (23:55.440)

He wrote a lot more about the Bible and alchemy than he ever did about physics, but he was

Sean Carroll (24:01.920)

also more brilliant than anybody else and way more mathematically astute than Galileo.

Lex Fridman (24:06.260)

So I really don't know.

Sean Carroll (24:08.420)

He might have, he might just, yeah, say like, give me the textbooks, leave me alone for

Sean Carroll (24:12.060)

a few months and then be caught up.

Lex Fridman (24:15.200)

But he might have had mental blocks against seeing the world in this way.

Sean Carroll (24:19.900)

I really don't know.

Lex Fridman (24:20.900)

Or perhaps find an interesting mystical interpretation of quantum mechanics.

Sean Carroll (24:24.780)

Very possible.

Lex Fridman (24:25.780)

Yeah.

Sean Carroll (24:26.780)

Is there any other scientists or philosophers through history that you would like to know

Lex Fridman (24:31.500)

their opinion of your book?

Sean Carroll (24:33.580)

That's a, that's a good question.

Lex Fridman (24:36.420)

I mean, Einstein is the obvious one, right?

Sean Carroll (24:38.500)

We all, I mean, he was not that long ago, but I even speculated at the end of my book

Lex Fridman (24:42.100)

about what his opinion would be.

Lex Fridman (24:44.140)

I am curious as to, you know, what about older philosophers like Hume or Kant, right?

Lex Fridman (24:50.140)

Like what would they have thought?

Lex Fridman (24:51.140)

Or Aristotle, you know, what would they have thought about modern physics?

Sean Carroll (24:55.840)

Because they do in philosophy, your predilections end up playing a much bigger role in your

Sean Carroll (25:01.980)

ultimate conclusions because you're not as tied down by what the data is in physics.

Sean Carroll (25:07.180)

You know, physics is lucky because we can't stray too far off the reservation as long

Sean Carroll (25:11.060)

as we're trying to explain the world that we actually see in our telescopes and microscopes.

Lex Fridman (25:15.820)

But it's just not fair to play that game because the people we're thinking about didn't know

Lex Fridman (25:20.900)

a whole bunch of things that we know, right?

Lex Fridman (25:23.580)

Like we lived through a lot that they didn't live through.

Lex Fridman (25:26.380)

So by the time we got them caught up, they'd be different people.

Lex Fridman (25:32.700)

So let me ask a bunch of basic questions.

Sean Carroll (25:35.940)

I think it would be interesting, useful for people who are not familiar, but even for

Lex Fridman (25:40.220)

people who are extremely well familiar.

Lex Fridman (25:42.940)

Let's start with what is quantum mechanics?

Sean Carroll (25:47.260)

Quantum mechanics is the paradigm of physics that came into being in the early part of

Sean Carroll (25:51.820)

the 20th century that replaced classical mechanics, and it replaced classical mechanics in a weird

Lex Fridman (25:58.380)

way that we're still coming to terms with.

Lex Fridman (26:00.380)

So in classical mechanics, you have an object, it has a location, it has a velocity, and

Sean Carroll (26:05.580)

if you know the location and velocity of everything in the world, you can say what everything's

Sean Carroll (26:08.660)

going to do.

Lex Fridman (26:10.420)

Quantum mechanics has an aspect of it that is kind of on the same lines.

Sean Carroll (26:15.660)

There's something called the quantum state or the wave function.

Lex Fridman (26:19.340)

And there's an equation governing what the quantum state does.

Lex Fridman (26:22.260)

So it's very much like classical mechanics.

Lex Fridman (26:23.980)

The wave function is different.

Sean Carroll (26:25.660)

It's sort of a wave.

Sean Carroll (26:26.980)

It's a vector in a huge dimensional vector space rather than a position and a velocity,

Lex Fridman (26:31.380)

but okay, that's a detail.

Sean Carroll (26:33.060)

The equation is the Schrodinger equation, not Newton's laws, but okay, again, a detail.

Sean Carroll (26:37.700)

Where quantum mechanics really becomes weird and different is that there's a whole other

Sean Carroll (26:41.380)

set of rules in our textbook formulation of quantum mechanics in addition to saying that

Sean Carroll (26:46.820)

there's a quantum state and it evolves in time.

Lex Fridman (26:49.320)

And all these new rules have to do with what happens when you look at the system, when

Sean Carroll (26:52.820)

you observe it, when you measure it.

Lex Fridman (26:55.100)

In classical mechanics, there were no rules about observing.

Sean Carroll (26:58.420)

You just look at it and you see what's going on.

Lex Fridman (27:00.260)

That was it, right?

Sean Carroll (27:01.580)

In quantum mechanics, the way we teach it, there's something profoundly fundamental about

Sean Carroll (27:06.940)

the act of measurement or observation, and the system dramatically changes its state.

Sean Carroll (27:12.260)

Even though it has a wave function, like the electron in an atom is not orbiting in a circle,

Sean Carroll (27:16.780)

it's sort of spread out in a cloud, when you look at it, you don't see that cloud.

Sean Carroll (27:21.460)

When you look at it, it looks like a particle with a location.

Lex Fridman (27:24.840)

So it dramatically changes its state right away, and the effects of that change can be

Sean Carroll (27:29.380)

instantly seen in what the electron does next.

Lex Fridman (27:32.120)

So again, we need to be careful because we don't agree on what quantum mechanics says.

Lex Fridman (27:39.060)

That's why I need to say like in the textbook view, et cetera, right?

Lex Fridman (27:41.660)

But in the textbook view, quantum mechanics, unlike any other theory of physics, gives

Sean Carroll (27:48.540)

a fundamental role to the act of measurement.

Lex Fridman (27:51.060)

So maybe even more basic, what is an atom and what is an electron?

Sean Carroll (27:56.980)

Sure.

Lex Fridman (27:57.980)

This all came together in a few years around the turn of the last century, right?

Sean Carroll (28:01.780)

Around the year 1900.

Sean Carroll (28:05.180)

Atoms predated then, of course, the word atom goes back to the ancient Greeks, but it was

Sean Carroll (28:09.220)

the chemists in the 1800s that really first got experimental evidence for atoms.

Lex Fridman (28:15.300)

They realized that there were two different types of tin oxide.

Lex Fridman (28:21.020)

And in these two different types of tin oxide, there was exactly twice as much oxygen in

Lex Fridman (28:25.300)

one type as the other.

Lex Fridman (28:27.180)

And like, why is that?

Lex Fridman (28:28.940)

Why is it never 1.5 times as much, right?

Lex Fridman (28:31.700)

And so Dalton said, well, it's because there are tin atoms and oxygen atoms, and one form

Sean Carroll (28:38.280)

of tin oxide is one atom of tin and one atom of oxygen, and the other is one atom of tin

Lex Fridman (28:42.940)

and two atoms of oxygen.

Lex Fridman (28:44.820)

And on the basis of this, you know, a speculation, a theory, right, a hypothesis, but then on

Sean Carroll (28:49.220)

the basis of that, you make other predictions, and the chemists became quickly convinced

Lex Fridman (28:52.900)

that atoms were real.

Sean Carroll (28:54.540)

The physicists took a lot longer to catch on, but eventually they did.

Lex Fridman (28:58.880)

And I mean, Boltzmann, who believed in atoms, had a really tough time his whole life because

Sean Carroll (29:04.380)

he worked in Germany where atoms were not popular.

Lex Fridman (29:07.220)

They were popular in England, but not in Germany.

Lex Fridman (29:09.800)

And there, in general, the idea of atoms is, it's the most, the smallest building block

Lex Fridman (29:15.160)

of the universe for them.

Sean Carroll (29:17.080)

That's the kind of how they thought it was.

Sean Carroll (29:18.080)

That was the Greek idea, but the chemists in the 1800s jumped the gun a little bit.

Lex Fridman (29:22.760)

So these days, an atom is the smallest building block of a chemical element, right?

Sean Carroll (29:27.860)

Hydrogen, tin, oxygen, carbon, whatever, but we know that atoms can be broken up further

Sean Carroll (29:33.180)

than that.

Sean Carroll (29:34.180)

That's what physicists discovered in the early 1900s, Rutherford, especially, and his colleagues.

Lex Fridman (29:40.620)

So the atom that we think about now, the cartoon, is that picture you've always seen of a little

Lex Fridman (29:46.740)

nucleus and then electrons orbiting it like a little solar system.

Lex Fridman (29:50.260)

And we now know the nucleus is made of protons and neutrons.

Lex Fridman (29:53.180)

So the weight of the atom, the mass, is almost all in its nucleus.

Sean Carroll (29:58.640)

Protons and neutrons are something like 1800 times as heavy as electrons are.

Sean Carroll (30:03.880)

Protons are much lighter, but because they're lighter, they give all the life to the atoms.

Lex Fridman (30:09.020)

So when atoms get together, combine chemically, when electricity flows through a system, it's

Lex Fridman (30:13.740)

all the electrons that are doing all the work.

Lex Fridman (30:16.900)

And where quantum mechanics steps in, as you mentioned, with the position of velocity with

Sean Carroll (30:21.300)

classical mechanics and quantum mechanics is modeling the behavior of the electron.

Sean Carroll (30:26.900)

I mean, you can model the behavior of anything, but the electron, because that's where the

Lex Fridman (30:30.620)

fun is.

Sean Carroll (30:31.900)

The electron was the biggest challenge right from the start.

Lex Fridman (30:34.580)

Yeah.

Lex Fridman (30:35.580)

So what's a wave function?

Sean Carroll (30:36.580)

You said it's an interesting detail, but in any interpretation, what is the wave function

Lex Fridman (30:43.540)

in quantum mechanics?

Sean Carroll (30:44.540)

Well, you know, we had this idea from Rutherford that atoms look like little solar systems,

Lex Fridman (30:50.260)

but people very quickly realize that can't possibly be right because if an electron is

Lex Fridman (30:54.760)

orbiting in a circle, it will give off light.

Sean Carroll (30:57.640)

All the light that we have in this room comes from electrons zooming up and down and wiggling.

Lex Fridman (31:01.540)

That's what electromagnetic waves are.

Lex Fridman (31:03.260)

And you can calculate how long would it take for the electron just to spiral into the nucleus?

Lex Fridman (31:07.660)

And the answer is 10 to the minus 11 seconds, okay, 100 billionth of a second.

Lex Fridman (31:12.180)

So that's not right.

Sean Carroll (31:14.380)

Meanwhile, people had realized that light, which we understood from the 1800s was a wave,

Lex Fridman (31:21.300)

had properties that were similar to that of particles, right?

Lex Fridman (31:23.940)

This is Einstein and Planck and stuff like that.

Lex Fridman (31:26.740)

So if something that we agree was a wave had particle like properties, then maybe something

Lex Fridman (31:34.020)

we think is a particle, the electron has wave like properties, right?

Lex Fridman (31:38.540)

And so a bunch of people eventually came to the conclusion, don't think about the electron

Lex Fridman (31:42.900)

as a little point particle orbiting like a solar system.

Sean Carroll (31:47.180)

Think of it as a wave that is spread out.

Sean Carroll (31:49.780)

They cleverly gave this the name the wave function, which is the dopiest name in the

Sean Carroll (31:53.100)

world for one of the most profound things in the universe.

Sean Carroll (31:57.420)

There's literally a number at every point in space, which is the value of the electron's

Sean Carroll (32:03.100)

wave function at that point.

Lex Fridman (32:04.980)

And there's only one wave function.

Sean Carroll (32:07.580)

Yeah, they eventually figured that out.

Lex Fridman (32:09.600)

That took longer.

Lex Fridman (32:10.960)

But when you have two electrons, you do not have a wave function for electron one and

Lex Fridman (32:15.220)

a wave function for electron two.

Sean Carroll (32:17.140)

You have one combined wave function for both of them.

Lex Fridman (32:20.420)

And indeed, as you say, there's only one wave function for the entire universe at once.

Lex Fridman (32:26.980)

And that's where this beautiful dance, can you say what is entanglement?

Lex Fridman (32:32.620)

It seems one of the most fundamental ideas of quantum mechanics.

Sean Carroll (32:35.580)

Well, let's temporarily buy into the textbook interpretation of quantum mechanics.

Lex Fridman (32:39.580)

And what that says is that this wave function, so it's very small outside the atom, very

Sean Carroll (32:43.820)

big in the atom, basically the wave function, you take it and you square it, you square

Sean Carroll (32:48.940)

the number that gives you the probability of observing the system at that location.

Lex Fridman (32:54.220)

So if you say that for two electrons, there's only one wave function, and that wave function

Lex Fridman (32:58.700)

gives you the probability of observing both electrons at once doing something, okay?

Lex Fridman (33:03.060)

So maybe the electron can be here or here, here, here, and the other electron can also

Lex Fridman (33:07.220)

be there.

Lex Fridman (33:08.300)

But we have a wave function set up where we don't know where either electron is going

Lex Fridman (33:12.600)

to be seen.

Lex Fridman (33:14.100)

But we know they'll both be seen in the same place, okay?

Lex Fridman (33:17.800)

So we don't know exactly what we're going to see for either electron, but there's entanglement

Sean Carroll (33:22.100)

between the two of them.

Lex Fridman (33:23.740)

There's a sort of conditional statement.

Sean Carroll (33:25.460)

If we see one in one location, then we know the other one's going to be doing a certain

Lex Fridman (33:29.440)

thing.

Lex Fridman (33:30.440)

So that's a feature of quantum mechanics that is nowhere to be found in classical mechanics.

Sean Carroll (33:34.220)

In classical mechanics, there's no way I can say, well, I don't know where either one of

Sean Carroll (33:37.660)

these particles is, but if I know, if I find out where this one is, then I know where the

Lex Fridman (33:40.700)

other one is.

Sean Carroll (33:41.700)

That just never happens.

Lex Fridman (33:42.700)

They're truly separate.

Sean Carroll (33:43.700)

I don't know, it feels like, if you think of a wave function like as a dance floor,

Sean Carroll (33:47.980)

it seems like entanglement is strongest between things that are dancing together closest.

Lex Fridman (33:53.860)

So there's a closeness that's important.

Lex Fridman (33:56.420)

Well, that's another step.

Sean Carroll (33:58.340)

We have to be careful here because in principle, if you're talking about the entanglement of

Sean Carroll (34:02.660)

two electrons, for example, they can be totally entangled or totally unentangled no matter

Sean Carroll (34:08.860)

where they are in the universe.

Sean Carroll (34:10.180)

There's no relationship between the amount of entanglement and the distance between two

Sean Carroll (34:15.180)

electrons.

Lex Fridman (34:16.180)

But we now know that the reality of our best way of understanding the world is through

Sean Carroll (34:21.120)

quantum fields, not through particles.

Lex Fridman (34:23.860)

So even the electron, not just gravity and electromagnetism, but even the electron and

Sean Carroll (34:28.540)

the quarks and so forth are really vibrations in quantum fields.

Lex Fridman (34:33.160)

So even empty space is full of vibrating quantum fields.

Lex Fridman (34:38.340)

And those quantum fields in empty space are entangled with each other in exactly the way

Lex Fridman (34:42.840)

you just said.

Sean Carroll (34:43.840)

If they're nearby, if you have like two vibrating quantum fields that are nearby, then they'll

Lex Fridman (34:47.020)

be highly entangled.

Sean Carroll (34:48.580)

If they're far away, they will not be entangled.

Lex Fridman (34:50.540)

So what do quantum fields in a vacuum look like?

Lex Fridman (34:52.880)

Empty space?

Lex Fridman (34:53.880)

Just like empty space.

Sean Carroll (34:54.880)

It's as empty as it can be.

Lex Fridman (34:56.600)

But there's still a field.

Lex Fridman (34:57.980)

It's just, what does nothing look like?

Sean Carroll (35:02.420)

Just like right here, this location in space, there's a gravitational field, which I can

Sean Carroll (35:06.340)

detect by dropping something.

Lex Fridman (35:07.980)

Yes.

Sean Carroll (35:08.980)

I don't see it, but there it is.

Lex Fridman (35:12.860)

So we got a little bit of an idea of entanglement.

Lex Fridman (35:17.860)

Now, what is Hilbert space and Euclidean space?

Sean Carroll (35:23.980)

Yeah, you know, I think that people are very welcome to go through their lives not knowing

Lex Fridman (35:28.180)

what Hilbert space is.

Lex Fridman (35:29.360)

But if you dig into a little bit more into quantum mechanics, it becomes necessary.

Sean Carroll (35:33.580)

You know, the English language was invented long before quantum mechanics, or various

Lex Fridman (35:39.420)

forms of higher mathematics were invented.

Lex Fridman (35:41.260)

So we use the word space to mean different things.

Sean Carroll (35:45.020)

Of course, most of us think of space as this three dimensional world in which we live,

Lex Fridman (35:48.700)

right?

Lex Fridman (35:49.700)

I mean, some of us just think of it as outer space.

Sean Carroll (35:51.420)

Okay, but space around us gives us the three dimensional location of things and objects.

Lex Fridman (35:56.420)

But mathematicians use any generic abstract collection of elements as a space, okay?

Sean Carroll (36:05.820)

A space of possibilities, you know, momentum space, etc.

Lex Fridman (36:09.940)

So Hilbert space is the space of all possible quantum wave functions, either for the universe

Sean Carroll (36:14.480)

or for some specific system.

Lex Fridman (36:16.740)

And it could be an infinite dimensional space, or it could be just really, really large dimensional

Lex Fridman (36:21.300)

but finite.

Lex Fridman (36:22.300)

We don't know because we don't know the final theory of everything.

Lex Fridman (36:24.960)

But this abstract Hilbert space is really, really, really big and has no immediate connection

Lex Fridman (36:29.540)

to the three dimensional space in which we live.

Lex Fridman (36:31.780)

What do dimensions in Hilbert space mean?

Sean Carroll (36:35.220)

You know, it's just a way of mathematically representing how much information is contained

Sean Carroll (36:40.020)

in the state of the system.

Lex Fridman (36:41.660)

How many numbers do you have to give me to specify what the thing is doing?

Lex Fridman (36:45.720)

So in classical mechanics, I give you the location of something by giving you three

Lex Fridman (36:51.020)

numbers, right?

Sean Carroll (36:52.020)

Up, down, left, X, Y, Z coordinates.

Lex Fridman (36:54.340)

But then I might want to give you its entire state, physical state, which means both its

Sean Carroll (37:00.100)

position and also its velocity.

Lex Fridman (37:02.680)

The velocity also has three components.

Lex Fridman (37:04.520)

So its state lives in something called phase space, which is six dimensional, three dimensions

Lex Fridman (37:09.860)

of position, three dimensions of velocity.

Lex Fridman (37:12.540)

And then if it also has an orientation in space, that's another three dimensions and

Lex Fridman (37:16.200)

so forth.

Lex Fridman (37:17.200)

So as you describe more and more information about the system, you have an abstract mathematical

Lex Fridman (37:22.620)

space that has more and more numbers that you need to give.

Lex Fridman (37:26.460)

And each one of those numbers corresponds to a dimension in that space.

Lex Fridman (37:29.840)

So in terms of the amount of information, what is entropy?

Sean Carroll (37:34.760)

This mystical word that's overused in math and physics, but has a very specific meaning

Lex Fridman (37:40.740)

in this context.

Sean Carroll (37:41.940)

Sadly, it has more than one very specific meeting.

Lex Fridman (37:44.300)

This is the reason why it is hard.

Sean Carroll (37:46.660)

Entropy means different things even to different physicists.

Lex Fridman (37:49.200)

But one way of thinking about it is a measure of how much we don't know about the state

Sean Carroll (37:54.120)

of a system.

Lex Fridman (37:55.160)

So if I have a bottle of water molecules, and I know that, OK, there's a certain number

Sean Carroll (38:00.540)

of water molecules.

Lex Fridman (38:01.540)

I could weigh it and figure out.

Sean Carroll (38:02.940)

I know the volume of it, and I know the temperature and pressure and things like that.

Lex Fridman (38:06.740)

I certainly don't know the exact position and velocity of every water molecule.

Lex Fridman (38:12.280)

So there's a certain amount of information I know, a certain amount that I don't know

Lex Fridman (38:15.600)

that is part of the complete state of the system.

Lex Fridman (38:18.540)

And that's what the entropy characterizes, how much unknown information there is, the

Sean Carroll (38:23.340)

difference between what I do know about the system and its full exact microscopic state.

Lex Fridman (38:28.300)

So when we try to describe a quantum mechanical system, is it infinite or finite but very

Lex Fridman (38:36.660)

large?

Sean Carroll (38:37.660)

Yeah, we don't know.

Lex Fridman (38:38.660)

That depends on the system.

Sean Carroll (38:39.660)

You know, it's easy to mathematically write down a system that would have a potentially

Lex Fridman (38:44.300)

infinite entropy, an infinite dimensional Hilbert space.

Lex Fridman (38:47.480)

So let's go back a little bit.

Sean Carroll (38:50.100)

We said that the Hilbert space was the space in which quantum wave functions lived for

Sean Carroll (38:54.340)

different systems that will be different sizes.

Lex Fridman (38:57.220)

They could be infinite or finite.

Lex Fridman (38:58.420)

So that's the number of numbers, the number of pieces of information you could potentially

Lex Fridman (39:03.380)

give me about the system.

Lex Fridman (39:04.620)

So the bigger Hilbert space is, the bigger the entropy of that system could be, depending

Lex Fridman (39:11.100)

on what I know about it.

Sean Carroll (39:12.100)

If I don't know anything about it, then it has a huge entropy, right, but only up to

Lex Fridman (39:16.260)

the size of its Hilbert space.

Lex Fridman (39:18.160)

So we don't know in the real physical world whether or not, you know, this region of space

Sean Carroll (39:23.860)

that contains that water bottle has potentially an infinite entropy or just a finite entropy.

Sean Carroll (39:29.140)

We have different arguments on different sides.

Lex Fridman (39:31.580)

So if it's infinite, how do you think about infinity?

Sean Carroll (39:35.640)

Is this something you can, your cognitive abilities are able to process or is it just

Lex Fridman (39:42.020)

a mathematical tool?

Lex Fridman (39:44.060)

It's somewhere in between, right?

Lex Fridman (39:45.060)

I mean, we can say things about it.

Sean Carroll (39:46.420)

We can use mathematical tools to manipulate infinity very, very accurately.

Lex Fridman (39:51.220)

We can define what we mean.

Sean Carroll (39:52.980)

You know, for any number n, there's a number bigger than it.

Lex Fridman (39:56.180)

So there's no biggest number, right?

Lex Fridman (39:58.180)

So there's something called the total number of all numbers.

Lex Fridman (40:00.580)

It's infinite.

Lex Fridman (40:01.640)

But it is hard to wrap your brain around that, and I think that gives people pause because

Sean Carroll (40:07.180)

we talk about infinity as if it's a number, but it has plenty of properties that real

Sean Carroll (40:11.380)

numbers don't have.

Lex Fridman (40:12.380)

You know, if you multiply infinity by two, you get infinity again, right?

Sean Carroll (40:15.820)

That's a little bit different than what we're used to.

Lex Fridman (40:18.500)

Okay.

Lex Fridman (40:19.500)

But are you comfortable with the idea that in thinking of what the real world actually

Lex Fridman (40:25.780)

is that infinity could be part of that world?

Lex Fridman (40:28.780)

Are you comfortable that a world in some dimension, in some aspect?

Lex Fridman (40:31.380)

I'm comfortable with lots of things.

Sean Carroll (40:33.020)

I mean, you know, I don't want my level of comfort to affect what I think about the world.

Sean Carroll (40:40.740)

You know, I'm pretty open minded about what the world could be at the fundamental level.

Sean Carroll (40:44.020)

Yeah, but infinity is a tricky one.

Lex Fridman (40:47.900)

It's not almost a question of comfort.

Lex Fridman (40:50.460)

It's a question of, is it an overreach of our intuition?

Sean Carroll (40:56.420)

Sort of, it could be a convenient, almost like when you add a constant to an equation

Sean Carroll (41:01.180)

just because it'll help, it just feels like it's useful to at least be able to imagine

Sean Carroll (41:06.860)

a concept, not directly, but in some kind of way that this feels like it's a description

Sean Carroll (41:13.380)

of the real world.

Lex Fridman (41:14.380)

Think of it this way.

Sean Carroll (41:15.940)

There's only three numbers that are simple.

Lex Fridman (41:19.420)

There's zero, there's one, and there's infinity.

Sean Carroll (41:24.380)

A number like 318 is just bizarre.

Lex Fridman (41:29.580)

You need a lot of bits to give me what that number is.

Lex Fridman (41:33.220)

But zero and one and infinity, like once you have 300 things, you might as well have infinity

Lex Fridman (41:36.660)

things, right?

Lex Fridman (41:37.660)

Otherwise, you have to say when to stop making the things, right?

Lex Fridman (41:40.060)

So there's a sense in which infinity is a very natural number of things to exist.

Sean Carroll (41:44.860)

I was never comfortable with infinity because it's just such a, it was too good to be true.

Lex Fridman (41:50.420)

Because in math, it just helps make things work out.

Sean Carroll (41:55.780)

When things get very large, close to infinity, things seem to work out nicely.

Lex Fridman (42:02.800)

It's kind of like, because my deepest passion is probably psychology.

Lex Fridman (42:07.740)

And I'm uncomfortable how in the average, the beauty of how much we vary is lost.

Sean Carroll (42:18.660)

In that same kind of sense, infinity seems like a convenient way to erase the details.

Lex Fridman (42:24.340)

But the thing about infinity is it seems to pop up whether we like it or not, right?

Sean Carroll (42:29.320)

Like you're trying to be a computer scientist, you ask yourself, well, how long will it take

Lex Fridman (42:33.020)

this program to run?

Lex Fridman (42:34.540)

And you realize, well, for some of them, the answer is infinitely long.

Sean Carroll (42:37.440)

It's not because you tried to get there.

Lex Fridman (42:39.540)

You wrote a five line computer program, it doesn't halt.

Lex Fridman (42:43.420)

So coming back to the textbook definition of quantum mechanics, this idea that I don't

Sean Carroll (42:48.220)

think we talked about, can you, this one of the most interesting philosophical points,

Sean Carroll (42:55.300)

we talked at the human level, but at the physics level, that at least the textbook definition

Lex Fridman (43:02.100)

of quantum mechanics separates what is observed and what is real.

Lex Fridman (43:07.660)

One, how does that make you feel?

Lex Fridman (43:13.100)

And two, what does it then mean to observe something and why is it different than what

Lex Fridman (43:19.020)

is real?

Sean Carroll (43:20.020)

Yeah, you know, my personal feeling, such as it is, is that things like measurement

Lex Fridman (43:27.700)

and observers and stuff like that are not going to play a fundamental role in the ultimate

Lex Fridman (43:32.100)

laws of physics.

Lex Fridman (43:33.100)

But my feeling that way is because so far, that's where all the evidence has been pointing.

Lex Fridman (43:38.500)

I could be wrong.

Lex Fridman (43:39.960)

And there's certainly a sense in which it would be infinitely cool if somehow observation

Lex Fridman (43:45.920)

or mental cogitation did play a fundamental role in the nature of reality.

Lex Fridman (43:52.180)

But I don't think so.

Lex Fridman (43:53.180)

And again, I don't see any evidence for it.

Lex Fridman (43:54.580)

So I'm not spending a lot of time worrying about that possibility.

Lex Fridman (43:58.280)

So what do you do about the fact that in the textbook interpretation of quantum mechanics,

Lex Fridman (44:02.020)

this idea of measurement or looking at things seems to play an important role?

Sean Carroll (44:07.860)

Well, you come up with better interpretations of quantum mechanics and there are several

Sean Carroll (44:11.780)

alternatives.

Lex Fridman (44:12.780)

My favorite is the many worlds interpretation, which says two things.

Sean Carroll (44:17.180)

Number one, you, the observer, are just a quantum system like anything else.

Lex Fridman (44:22.180)

There's nothing special about you.

Sean Carroll (44:23.660)

Don't get so proud of yourself, you know, you're just a bunch of atoms.

Sean Carroll (44:27.260)

You have a wave function, you obey the Schrodinger equation like everything else.

Lex Fridman (44:31.300)

And number two, when you think you're measuring something or observing something, what's really

Lex Fridman (44:35.940)

happening is you're becoming entangled with that thing.

Lex Fridman (44:40.480)

So when you think there's a wave function for the electron, it's all spread out.

Lex Fridman (44:43.820)

But you look at it and you only see it in one location.

Sean Carroll (44:46.660)

What's really happening is that there's still the wave function for the electron in all

Lex Fridman (44:50.060)

those locations.

Lex Fridman (44:51.060)

But now it's entangled with the wave function of you in the following way.

Sean Carroll (44:55.960)

There's part of the wave function that says the electron was here and you think you saw

Sean Carroll (44:59.340)

it there.

Lex Fridman (45:00.340)

The electron was there and you think you saw it there.

Sean Carroll (45:02.380)

The electron was over there and you think you saw it there, etc.

Lex Fridman (45:05.620)

So in all of those different parts of the wave function, once they come into being,

Sean Carroll (45:10.260)

no longer talk to each other.

Lex Fridman (45:11.600)

They no longer interact or influence each other.

Sean Carroll (45:13.820)

It's as if they are separate worlds.

Lex Fridman (45:16.380)

So this was the invention of Hugh Everett III, who was a graduate student at Princeton

Sean Carroll (45:21.620)

in the 1950s.

Lex Fridman (45:22.620)

And he said, basically, look, you don't need all these extra rules about looking at things.

Sean Carroll (45:28.600)

Just listen to what the Schrodinger equation is telling you.

Sean Carroll (45:31.020)

It's telling you that you have a wave function, that you become entangled, and that the different

Sean Carroll (45:35.300)

versions of you no longer talk to each other.

Lex Fridman (45:37.820)

So just accept it.

Sean Carroll (45:39.420)

It's just he did therapy more than anything else.

Lex Fridman (45:41.540)

He said, like, it's okay.

Sean Carroll (45:42.980)

You don't need all these extra rules.

Lex Fridman (45:45.060)

All you need to do is believe the Schrodinger equation.

Sean Carroll (45:47.200)

The cost is there's a whole bunch of extra worlds out there.

Lex Fridman (45:50.140)

So are the worlds being created whether there's an observer or not?

Sean Carroll (45:57.340)

The worlds are created any time a quantum system that's in a superposition becomes entangled

Lex Fridman (46:01.520)

with the outside world.

Lex Fridman (46:04.020)

What's the outside world?

Lex Fridman (46:06.140)

It depends.

Sean Carroll (46:07.140)

Let's back up.

Sean Carroll (46:08.700)

Whatever it really says, what his theory is, is there's a wave function of the universe

Lex Fridman (46:13.980)

and it obeys the Schrodinger equation all the time.

Lex Fridman (46:17.240)

That's it.

Sean Carroll (46:18.240)

That's the full theory right there.

Sean Carroll (46:20.580)

The question, all of the work is how in the world do you map that theory onto reality,

Lex Fridman (46:27.140)

onto what we observe?

Lex Fridman (46:29.280)

So part of it is carving up the wave function into these separate worlds, saying, look,

Sean Carroll (46:33.740)

it describes a whole bunch of things that don't interact with each other.

Lex Fridman (46:36.180)

Let's call them separate worlds.

Sean Carroll (46:38.120)

Another part is distinguishing between systems and their environments.

Sean Carroll (46:41.860)

The environment is basically all the degrees of freedom, all the things going on in the

Sean Carroll (46:45.660)

world that you don't keep track of.

Lex Fridman (46:48.100)

So again, in the bottle of water, I might keep track of the total amount of water and

Sean Carroll (46:52.980)

the volume.

Lex Fridman (46:53.980)

I don't keep track of the individual positions and velocities.

Sean Carroll (46:57.140)

I don't keep track of all the photons or the air molecules in this room.

Lex Fridman (47:00.700)

So that's the outside world.

Sean Carroll (47:02.120)

The outside world is all the parts of the universe that you're not keeping track of

Lex Fridman (47:06.180)

when you're asking about the behavior of subsystem of it.

Lex Fridman (47:10.840)

So how many worlds are there?

Lex Fridman (47:14.220)

Yeah, we don't know that one either.

Sean Carroll (47:17.100)

There could be an infinite number.

Sean Carroll (47:18.860)

There could be only a finite number, but it's a big number one way or the other.

Sean Carroll (47:21.660)

It's just a very, very big number.

Lex Fridman (47:23.860)

In one of the talks, somebody asked, well, if it's finite.

Lex Fridman (47:32.020)

So actually I'm not sure exactly the logic you used to derive this, but is there going

Lex Fridman (47:38.700)

to be overlap, a duplicate world that you return to?

Lex Fridman (47:47.540)

So you've mentioned, and I'd love if you can elaborate on sort of idea that it's possible

Sean Carroll (47:52.020)

that there's some kind of equilibrium that these splitting worlds arrive at and then

Sean Carroll (47:56.780)

maybe over time, maybe somehow connected to entropy, you get a large number of worlds

Lex Fridman (48:03.700)

that are very similar to each other.

Sean Carroll (48:05.260)

Yeah.

Lex Fridman (48:06.260)

So this question of whether or not Hilbert space is finite or infinite dimensional is

Sean Carroll (48:11.860)

actually secretly connected to gravity and cosmology.

Sean Carroll (48:16.420)

This is the part that we're still struggling to understand right now, but we discovered

Sean Carroll (48:19.460)

back in 1998 that our universe is accelerating and what that means if it continues, which

Lex Fridman (48:25.220)

we think it probably will, but we're not sure.

Lex Fridman (48:26.940)

But if it does, that means there's a horizon around us.

Sean Carroll (48:31.340)

Because the universe is not only expanding, but expanding faster and faster, things can

Sean Carroll (48:34.820)

get so far away from us that from our perspective, it looks like they're moving away faster in

Lex Fridman (48:40.220)

the speed of light.

Sean Carroll (48:41.220)

We will never see them again.

Lex Fridman (48:42.580)

So there's literally a horizon around us and that horizon approaches some fixed distance

Sean Carroll (48:47.400)

away from us.

Lex Fridman (48:48.780)

And you can then argue that within that horizon, there's only a finite number of things that

Sean Carroll (48:53.260)

can possibly happen, the finite dimensional Hilbert space.

Lex Fridman (48:55.980)

In fact, we even have a guess for what the dimensionality is.

Sean Carroll (48:59.500)

It's 10 to the power of 10 to the power of 122.

Lex Fridman (49:04.540)

That's a very large number.

Sean Carroll (49:05.540)

Yes.

Sean Carroll (49:06.540)

Just to compare, the age of the universe is something like 10 to the 14 seconds, 10 to

Sean Carroll (49:11.340)

the 17 or 18 seconds maybe.

Lex Fridman (49:13.340)

The number of particles in the universe is 10 to the 88th.

Lex Fridman (49:16.300)

But the number of dimensions of Hilbert space is 10 to the 10 to the 122.

Lex Fridman (49:21.320)

So that's just crazy big.

Sean Carroll (49:23.340)

If that story is right, that in our observable horizon, there's only a finite dimensional

Sean Carroll (49:28.020)

Hilbert space, then this idea of branching of the wave function of the universe into

Sean Carroll (49:32.620)

multiple distinct separate branches has to reach a limit at some time.

Lex Fridman (49:37.780)

Once you branch that many times, you've run out of room in Hilbert space.

Lex Fridman (49:41.780)

And roughly speaking, that corresponds to the universe just expanding and emptying out

Lex Fridman (49:46.620)

and cooling off and entering a phase where it's just empty space, literally forever.

Lex Fridman (49:53.380)

What's the difference between splitting and copying, do you think?

Sean Carroll (49:58.900)

In terms of, a lot of this is an interpretation that helps us sort of model the world.

Lex Fridman (50:09.220)

So perhaps shouldn't be thought of as like, you know, philosophically or metaphysically.

Lex Fridman (50:16.860)

But in even at the physics level, do you see a difference between generating new copies

Lex Fridman (50:24.180)

of the world or splitting?

Sean Carroll (50:26.900)

I think it's better to think of in quantum mechanics in many worlds, the universe splits

Sean Carroll (50:31.700)

rather than new copies, because people otherwise worry about things like energy conservation.

Lex Fridman (50:36.540)

And no one who understands quantum mechanics worries about energy conservation, because

Sean Carroll (50:40.340)

the equation is perfectly clear.

Lex Fridman (50:42.340)

But if all you know is that someone told you the universe duplicates, then you have a reasonable

Sean Carroll (50:45.720)

worry about where all the energy for that came from.

Lex Fridman (50:48.660)

So a pre existing universe splitting into two skinnier universes is a better way of

Sean Carroll (50:53.620)

thinking about it.

Lex Fridman (50:54.620)

And mathematically, it's just like, you know, if you draw an x and y axis, and you draw

Sean Carroll (50:58.380)

a vector of length one, 45 degree angle, you know that you can write that vector of length

Sean Carroll (51:04.540)

one as the sum of two vectors pointing along x and y of length one over the square root

Sean Carroll (51:10.420)

of two.

Lex Fridman (51:11.420)

Okay, so I write one arrow as the sum of two arrows.

Lex Fridman (51:14.940)

But there's a conservation of arrowness, right?

Sean Carroll (51:17.020)

Like there's now two arrows, but the length is the same, I just I'm describing it in a

Sean Carroll (51:20.940)

different way.

Lex Fridman (51:21.940)

And that's exactly what happens when the universe branches, the the wave function of the universe

Sean Carroll (51:25.700)

is a big old vector.

Lex Fridman (51:27.380)

So to somebody who brings up a question of saying, doesn't this violate the conservation

Lex Fridman (51:34.020)

of energy?

Lex Fridman (51:35.420)

Can you give further elaboration?

Lex Fridman (51:38.060)

Right?

Lex Fridman (51:39.060)

So let's just be super duper perfectly clear.

Sean Carroll (51:42.100)

There's zero question about whether or not many worlds violates conservation of energy.

Lex Fridman (51:46.700)

Yes, it does not.

Sean Carroll (51:47.700)

Great.

Lex Fridman (51:48.700)

And I say this definitively, because there are other questions that I think there's answers

Sean Carroll (51:51.980)

to, but they're legitimate questions, right about, you know, where does probability come

Sean Carroll (51:55.620)

from and things like that, this conservation of energy question, we know the answer to

Sean Carroll (51:59.900)

it.

Lex Fridman (52:00.900)

And the answer to it is that energy is conserved.

Sean Carroll (52:03.020)

All of the effort goes into how best to translate what the equation unambiguously says into

Lex Fridman (52:09.220)

plain English, right?

Lex Fridman (52:11.040)

So this idea that there's a universe that has that that the universe comes equipped

Sean Carroll (52:14.480)

with a thickness, and it sort of divides up into thinner pieces, but the total amount

Sean Carroll (52:18.700)

of universe is is conserved over time, is a reasonably good way of putting English words

Lex Fridman (52:25.360)

to the underlying mathematics.

Lex Fridman (52:27.380)

So one of my favorite things about many worlds is, I mean, I love that there's something

Lex Fridman (52:33.220)

controversial in science.

Lex Fridman (52:35.400)

And for some reason, it makes people actually not like upset, but just get excited.

Lex Fridman (52:41.780)

Why do you think it is a controversial idea?

Lex Fridman (52:45.960)

So there's a lot of, it's actually one of the cleanest ways to think about quantum mechanics.

Lex Fridman (52:52.380)

So why do you think there's a discomfort a little bit among certain people?

Sean Carroll (52:57.380)

Well, I draw the distinction in my book between two different kinds of simplicity in a physical

Lex Fridman (53:02.740)

theory.

Lex Fridman (53:03.740)

There's simplicity in the theory itself, right?

Lex Fridman (53:06.300)

How we describe what's going on according to the theory by its own rights.

Lex Fridman (53:10.220)

But then, you know, theory is just some sort of abstract mathematical formalism, you have

Lex Fridman (53:13.480)

to map it onto the world somehow, right?

Lex Fridman (53:16.860)

And sometimes, like for Newtonian physics, it's pretty obvious, like, okay, here is a

Lex Fridman (53:23.000)

bottle and has a center of mass and things like that.

Sean Carroll (53:26.260)

Sometimes it's a little bit harder with general relativity, curvature of space time is a little

Lex Fridman (53:30.660)

bit harder to grasp.

Sean Carroll (53:33.180)

quantum mechanics is very hard to map what you're the language you're talking in a wave

Lex Fridman (53:37.620)

functions and things like that on to reality.

Lex Fridman (53:40.460)

And many worlds is the version of quantum mechanics where it is hardest to map on the

Lex Fridman (53:45.260)

underlying formalism to reality.

Lex Fridman (53:47.860)

So that's where the lack of simplicity comes in, not in the theory, but in how we use the

Lex Fridman (53:53.260)

theory to map on to reality.

Sean Carroll (53:54.660)

In fact, all of the work in sort of elaborating many worlds quantum mechanics is in the this

Lex Fridman (54:01.740)

effort to map it on to the world that we see.

Lex Fridman (54:04.420)

So it's perfectly legitimate to be bugged by that, right?

Sean Carroll (54:08.340)

To say like, well, no, that's just too far away from my experience, I am therefore intrinsically

Sean Carroll (54:15.320)

skeptical of it.

Lex Fridman (54:16.580)

Of course, you should give up on that skepticism if there are no alternatives.

Lex Fridman (54:19.620)

And this theory always keeps working, then eventually you should overcome your skepticism.

Lex Fridman (54:23.280)

But right now there are alternatives that are that, you know, people work to make alternatives

Sean Carroll (54:28.060)

that are by their nature closer to what we observe directly.

Lex Fridman (54:31.780)

Can you describe the alternatives?

Sean Carroll (54:33.180)

I don't think we touched on it, sort of the Copenhagen interpretation and the many worlds.

Sean Carroll (54:40.480)

Maybe there's a difference between the Everettian many worlds and many worlds as it is now,

Sean Carroll (54:47.260)

like has the idea sort of developed and so on.

Lex Fridman (54:50.020)

And just in general, what is the space of promising contenders?

Sean Carroll (54:54.060)

We have democratic debates now, there's a bunch of candidates.

Lex Fridman (54:57.060)

12 candidates on stage.

Lex Fridman (54:59.540)

What are the quantum mechanical candidates on stage for the debate?

Lex Fridman (55:02.480)

So if you had a debate between quantum mechanical contenders, there'd be no problem getting

Sean Carroll (55:08.540)

12 people up there on stage, but there would still be only three front runners.

Lex Fridman (55:14.220)

And right now the front runners would be Everett, hidden variable theories are another one.

Lex Fridman (55:19.460)

So the hidden variable theories say that the wave function is real, but there's something

Lex Fridman (55:24.180)

in addition to the wave function.

Sean Carroll (55:25.820)

The wave function is not everything, it's part of reality, but it's not everything.

Lex Fridman (55:29.440)

What else is there?

Sean Carroll (55:30.880)

We're not sure, but in the simplest version of the theory, there are literally particles.

Lex Fridman (55:36.020)

So many worlds says that quantum systems are sometimes are wave like in some ways and particle

Sean Carroll (55:43.100)

like in another because they really, really are waves, but under certain observational

Lex Fridman (55:48.060)

circumstances they look like particles.

Sean Carroll (55:50.460)

Whereas hidden variable says they look like waves and particles because there are both

Lex Fridman (55:54.980)

waves and particles involved in the dynamics.

Lex Fridman (55:58.760)

And that's easy to do if your particles are just non relativistic Newtonian particles

Lex Fridman (56:03.780)

moving around.

Sean Carroll (56:04.780)

They get pushed around by the wave function roughly.

Sean Carroll (56:07.740)

It becomes much harder when you take quantum field theory or quantum gravity into account.

Sean Carroll (56:13.300)

The other big contender are spontaneous collapse theories.

Lex Fridman (56:17.820)

So in the conventional textbook interpretation, we say when you look at a quantum system,

Sean Carroll (56:22.580)

its wave function collapses and you see it in one location, a spontaneous collapse theory

Sean Carroll (56:27.260)

says that every particle has a chance per second of having its wave function spontaneously

Sean Carroll (56:35.340)

collapse.

Sean Carroll (56:36.460)

The chance is very small for a typical particle, it will take hundreds of millions of years

Sean Carroll (56:39.980)

before it happens even once, but in a table or some macroscopic object, there are way

Sean Carroll (56:44.380)

more than a hundred million particles and they're all entangled with each other.

Lex Fridman (56:48.140)

So when one of them collapses, it brings everything else along with it.

Lex Fridman (56:52.860)

There's a slight variation of this.

Sean Carroll (56:54.260)

That's a spontaneous collapse theory.

Sean Carroll (56:55.620)

There are also induced collapse theories like Roger Penrose thinks that when the gravitational

Sean Carroll (57:00.260)

difference between two parts of the wave function becomes too large, the wave function collapses

Lex Fridman (57:05.500)

automatically.

Lex Fridman (57:06.860)

So those are basically in my mind, the three big alternatives, many worlds, which is just

Sean Carroll (57:11.700)

there's a wave function and always obeys the Schrodinger equation, hidden variables.

Sean Carroll (57:16.060)

There's a wave function that always obeys the Schrodinger equation, but there are also

Sean Carroll (57:18.820)

new variables or collapse theories, which the wave function sometimes obeys the Schrodinger

Sean Carroll (57:24.420)

equation and sometimes it collapses.

Lex Fridman (57:26.220)

So you can see that the alternatives are more complicated in their formalism than many worlds

Sean Carroll (57:31.380)

is, but they are closer to our experience.

Lex Fridman (57:34.380)

So just this moment of collapse, do you think of it as a wave function, fundamentally sort

Sean Carroll (57:42.220)

of a probabilistic description of the world and this collapse sort of reducing that part

Sean Carroll (57:49.140)

of the world into something deterministic, where again, you can now describe the position

Lex Fridman (57:53.740)

and the velocity in this simple classical model?

Lex Fridman (57:56.980)

Well there is...

Lex Fridman (57:57.980)

Is that how you think about collapse?

Sean Carroll (57:58.980)

There is a fourth category, there's a fourth contender, there's a mayor Pete of quantum

Sean Carroll (58:03.140)

mechanical interpretations, which are called epistemic interpretations.

Lex Fridman (58:08.140)

And what they say is all the wave function is, is a way of making predictions for experimental

Sean Carroll (58:13.220)

outcomes.

Lex Fridman (58:14.220)

It's not mapping onto an element of reality in any real sense.

Lex Fridman (58:18.780)

And in fact, two different people might have two different wave functions for the same

Lex Fridman (58:22.300)

physical system because they know different things about it, right?

Sean Carroll (58:25.340)

The wave function is really just a prediction mechanism.

Lex Fridman (58:28.100)

And then the problem with those epistemic interpretations is if you say, okay, but it's

Lex Fridman (58:32.980)

predicting about what, like what is the thing that is being predicted?

Lex Fridman (58:37.980)

And they say, no, no, no, that's not what we're here for.

Sean Carroll (58:41.460)

We're just here to tell you what the observational outcomes are going to be.

Lex Fridman (58:44.500)

But the other, the other interpretations kind of think that the wave function is real.

Sean Carroll (58:49.100)

Yes, that's right.

Lex Fridman (58:50.760)

So that's an ontic interpretation of the wave function, ontology being the study of what

Sean Carroll (58:55.980)

is real, what exists, as opposed to an epistemic interpretation of the wave function, epistemology

Lex Fridman (59:01.060)

being the study of what we know.

Sean Carroll (59:02.060)

That would actually just love to see that debate on stage.

Sean Carroll (59:06.580)

There was a version of it on stage at the world science festival a few years ago that

Sean Carroll (59:10.500)

you can look up online.

Lex Fridman (59:11.500)

On YouTube?

Sean Carroll (59:12.500)

Yep.

Lex Fridman (59:13.500)

It's on YouTube.

Sean Carroll (59:14.500)

Okay, awesome.

Lex Fridman (59:15.500)

I'll link it and watch it.

Lex Fridman (59:16.500)

Who won?

Lex Fridman (59:17.500)

I won.

Sean Carroll (59:18.500)

I don't know, there was no vote, there was no vote, but those there's Brian Green was

Sean Carroll (59:24.700)

the moderator and David Albert stood up for a spontaneous collapse and Shelley Goldstein

Sean Carroll (59:29.420)

was there for hidden variables and Rüdiger Schock was there for epistemic approaches.

Lex Fridman (59:34.460)

Why do you, I think you mentioned it, but just to elaborate, why do you find many worlds

Lex Fridman (59:38.980)

so compelling?

Lex Fridman (59:39.980)

Well, there's two reasons actually.

Lex Fridman (59:43.460)

One is, like I said, it is the simplest, right?

Lex Fridman (59:45.540)

It's like the most bare bones, austere, pure version of quantum mechanics.

Lex Fridman (59:49.960)

And I am someone who is very willing to put a lot of work into mapping the formalism onto

Lex Fridman (59:55.380)

reality.

Sean Carroll (59:56.380)

I'm less willing to complicate the formalism itself.

Lex Fridman (59:59.100)

But the other big reason is that there's something called modern physics with quantum fields