The following is a conversation with Sean Carroll, part two, the second time we've spoken on the

podcast. You can get the link to the first time in the description. This time we focus on quantum

mechanics and the many worlds interpretation that he details elegantly in his new book titled

Something Deeply Hidden. I own and enjoy both the ebook and audiobook versions of it. Listening

to Sean read about entanglement, complementarity, and the emergence of space time reminds me of Bob Ross

teaching the world how to paint on his old television show. If you don't know who Bob Ross is,

you're truly missing out. Look him up. He'll make you fall in love with painting. Sean Carroll

is the Bob Ross of theoretical physics. He's the author of several popular books,

a host of a great podcast called Mindscape and is a theoretical physicist at Caltech

and the Santa Fe Institute, specializing in quantum mechanics, error of time, cosmology,

and gravitation. This is the artificial intelligence podcast. If you enjoy it,

subscribe on YouTube, give it five stars on iTunes, support it on Patreon, or simply connect

with me on Twitter at Lex Freedman, spelled F R I D M A N. And now here's my conversation

with Sean Carroll. Isaac Newton developed what we now call classical mechanics that you describe

very nicely in your new book as you do with a lot of basic concepts in physics. So with classical

mechanics, I can throw a rock and can predict the trajectory of that rock's flight. But if we could

put ourselves back into Newton's time, his theories work to predict things. But as I understand, he

himself thought that they were the interpretations of those predictions were absurd. Perhaps he

just said it for religious reasons and so on. But in particular, sort of a world of interaction

without contact. So action at a distance. It didn't make sense to him on a sort of a human

interpretation level. Does it make sense to you that things can affect other things at a distance?

It does. So that was one of Newton's worries. You're actually right in a slightly different way

about the religious worries. He was smart enough. This is off the topic, but still fascinating.

Newton almost invented chaos theory as soon as he invented classical mechanics. He realized that in

the solar system, so he was able to explain how planets move around the sun. But typically, you

would describe the orbit of the Earth ignoring the effects of Jupiter and Saturn and so forth,

just doing the Earth and the Sun. He kind of knew, even though he couldn't do the math, that if you

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

like the orbits of the planets would get out of whack. So he thought that God would intervene

occasionally to sort of move the planets back into orbit, which is how you only way you could

explain how they were there, presumably forever. But the worries about classical mechanics were

a little bit different. The worry about gravity in particular. It wasn't a worry about classical

mechanics. We're about gravity. How in the world does the Earth know that there's something called

the Sun 93 million miles away that is exerting a gravitational force on it? And he literally said,

you know, I leave that for future generations to think about because I don't know what the answer

is. And in fact, people under emphasize this, but future generations figured it out. Pierre

Simone Laplace in circa 1800 showed that you could rewrite Newtonian gravity as a field theory. So

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

field or the gravitational potential field. And then there's no action at a distance. It's exactly

the same theory empirically. It makes exactly the same predictions. But what's happening is,

instead of the Sun just reaching out across the void, there is a gravitational field in between

the Sun and the Earth that obeys an equation Laplace's equation, cleverly enough. And that tells

us exactly what the field does. So even in Newtonian gravity, you don't need action at a distance.

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

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

Sun. But also there's the speed of light as a limit in Laplace's theory, which was exactly Newton's

theory just in a different mathematical language. There could still be instantaneous action across

the universe. Whereas in general relativity, if you shake something here, its gravitational impulse

radiates out at the speed of light. And we call that a gravitational wave, and we can detect those.

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

look one way or the other. Like if it turned out that there was action at a distance in physics,

and that was the best way to describe things, then I would do it that way. It's actually a very

deep question because when we don't know what the right laws of physics are, when we're guessing at

them, when we're hypothesizing at what they might be, we are often guided by our intuitions about

what they should be. I mean, Einstein famously was very guided by his intuitions. And he did

not like the idea of action at a distance. We don't know whether he was right or not. It depends

on your interpretation of quantum mechanics. And it depends on even how you talk about quantum

mechanics within any one interpretation. So if you see every force as a field, or any other

interpretation of action at a distance, I mean, it's just stepping back to sort of cave man thinking.

Like, do you really, can you really sort of understand what it means for a force to be

a field that's everywhere? So if you look at gravity, what do you think about? I think so.

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

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

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

that you actually understand it, are you just trusting the beauty and the power of the predictive

power of science? That depends on what you mean by this idea of truly understanding something.

I mean, can I truly understand Fermat's last theorem? It's easy to state it, but do I really

appreciate what it means for incredibly large numbers? I think yes, I think I do understand it,

but like if you want to just push people on well, but your intuition doesn't go to the places where

Andrew Wiles needed to go to prove Fermat's last theorem, then I can say fine, but I still think

I understand the theorem. And likewise, I think that I do have a pretty good intuitive understanding

of fields pervading space time, whether it's the gravitational field or the electromagnetic field

or whatever, the Higgs field. Of course, one's intuition gets worse and worse as you get trickier

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

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

trained. I have different intuitions now than I had when I was a baby. That's okay. An intuition

is not necessarily intrinsic to who we are. We can train it a little bit.

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

abilities are evolved through time. And so they're biologically constrained. And so there's a clear

limit as he puts it to our cognitive abilities. And it's a very harsh limit. But you actually

kind of said something interesting in nature versus nurture thing here is we can train our

intuitions to sort of build up the cognitive muscles to be able to understand some of these

tricky concepts. So do you think there's limits to our understanding that's deeply rooted,

hardcoded into our biology that we can't overcome?

There could be limits to things like our ability to visualize. But when someone like Ed Whitten

proves a theorem about 100 dimensional mathematical spaces, he's not visualizing it. He's doing

the math. That doesn't stop him from understanding the result. I think, and I would love to understand

this better, but my rough feeling, which is not very educated, is that there's some threshold

that one crosses in abstraction when one becomes kind of like a Turing machine. One has the ability

to contain in one's brain logical, formal, symbolic structures and manipulate them. And that's a

leap that we can make as human beings that dogs and cats haven't made. And once you get there,

I'm not sure there are any limits to our ability to understand the scientific world at all. Maybe

there are. There's certainly limits on our ability to calculate things, right? People are not very

good at taking cube roots of million digit numbers in their head. But that's not an element of

understanding. It's certainly not a limited principle. So of course, as a human, you would say

there doesn't feel to be limits to our understanding. But sort of, have you thought that the universe is

actually a lot simpler than it appears to us? And we just will never be able to, like it's outside of

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

of experimental simulation devices we can put together. Is there limits to that? Is it possible

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

good reason to think that we're anywhere close to the limits is a harder question. Look, imagine

asking this question 500 years ago to the world's greatest thinkers, right? Like, are we approaching

the limits of our ability to understand the natural world? And by definition, there are questions

about the natural world that are most interesting to us that are the ones we don't quite yet

understand, right? So there's always, we're always faced with these puzzles we don't yet know. And I

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

mechanics, much less quantum mechanics. So we know that they were nowhere close to how well they could

do, right? They could do it normally better than they were doing at the time. I see no reason why

the same thing isn't true for us today. So of all the worries that keep me awake at night,

the human mind's inability to rationally comprehend the world is low on the list.

Well put. So one interesting philosophical point and quantum mechanics bring up is the,

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

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

allows us to see and the world as it is outside our mind's eye, sort of, sort of not at the

quantum mechanical level, but as just our, these particular tools we have, which is the

few senses and cognitive abilities to process those senses. Well, that last phrase, having

the cognitive abilities to process them carries a lot, right? I mean, there is our sort of

intuitive understanding of the world. You don't need to teach people about gravity for them to

know that apples fall from trees, right? That's something that we figure out pretty quickly.

Object permanence, things like that. The three dimensionality of space, even if we don't have

the mathematical language to say that, we kind of know that it's true. On the other hand, no one

opens their eyes and sees atoms, right, or molecules or cells, for that matter, forget about quantum

mechanics. So, but we got there, we got to understanding that there are atoms and cells

using the combination of our senses and our cognitive capacities. So adding the ability

of our cognitive capacities to our senses is adding an enormous amount. And I don't think

it is a hard and fast boundary. You know, if you believe in cells, if you believe that we

understand those, there's no reason you believe we can't believe in quantum mechanics just as well.

What to use the most beautiful idea in physics?

Conservation of momentum. Can you elaborate? Yeah. If you were Aristotle, when Aristotle

wrote his book on physics, he made the following very obvious point. We're on video here, right?

So people can see this. So if I push the bottle, let me cover this bottle so we do not have a

mess. But okay. So I push the bottle, it moves. And if I stop pushing, it stops moving.

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

If you don't keep pushing things, they stop moving. This is a indisputably true fact about

our everyday environment. Okay. And for Aristotle, this blew up into a whole picture of the world

in which things had natures and teleologies and they had places they wanted to be. And when

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

and stuff like that. And it took 1,000 years or 1,500 years for people to say, actually,

if it weren't for things like dissipation and air resistance and friction and so forth,

the natural thing is for things to move forever in a straight line at constant velocity, right?

Conservation of momentum. And that is the reason why I think that's the most beautiful idea

in physics is because it shifts us from a view of natures and teleology to a view of patterns

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

what's the purpose of it, what's the cause, etc., because its nature does or does not want to do

that. Whereas once you believe in conservation momentum, things just happen. They just follow

the pattern. You have Laplace's demon ultimately, right? You give me the state of the world today.

I can predict what it's going to do in the future. I can predict where it was in the past. It's

impersonal and it's also instantaneous. It's not directed toward any future goals. It's just doing

what it does, given the current state of the universe. I think even more than either classical

mechanics or quantum mechanics, that is the profound deep insight that gets modern science off

the ground. You don't need natures and purposes and goals. You just need some patterns.

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

the intuitive physical space to the space of ideas. And also the other point you said, which is

conveniently most of the interesting ideas are acting in the moment. You don't need to know

the history of time or the future. And of course, this took a long time to get there, right? The

conservation momentum itself took hundreds of years. It's weird because someone would say

something interesting and then the next interesting thing would be said like 150 or 200 years later,

right? They weren't even talking to each other. They were reading each other's books.

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

projectile would move at a constant velocity was Avicenna,

Avicenna in the Persian golden age, circa 1000. And he didn't like the idea. He used that just

like Schrodinger used Schrodinger's cat to say, surely you don't believe that, right? Avicenna

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

really vacuum, things could keep moving forever, right? But still, he got right the idea that

there was this conservation of something impetus or mile, he would call it. And that's 500 years,

600 years before classical mechanics and Isaac Newton. So Galileo played a big role in this,

but he didn't exactly get it right. And so it just takes a long time for this to sink in because

it is so against our everyday experience. Do you think it was a big leap, a brave or a difficult

leap of sort of math and science to be able to say that momentum was conserved?

I do. I think it's an example of human reason in action. Even Aristotle knew that his theory had

issues because you could fire an arrow and it would go a long way before it stopped. So if his

theory was things just automatically stop, what's going on? And he had this elaborate story. I don't

know if you've heard the story, but the arrow would push the air in front of it away and the

molecules of air would run around the back of the arrow and push it again. And anyone reading this

is going like, really, that's what you thought. But it was that kind of thought experiment that

ultimately got people to say, like, actually, no, if it weren't for the air molecules at all,

the arrow would just go on by itself. And it's always this give and take between thought and

experience back and forth, right? Theory and experiment, we would say today.

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

the difference between math and physics to you. To me, very, very roughly, math is about

the logical structure of all possible worlds and physics is about our actual world.

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

interpretations of quantum mechanics or no. I'm certainly using the word world in the

broadest sense, all of reality. So I think that reality is specific. I don't think that

there's every possible thing going on in reality. I think that there are rules,

whether it's the Schrodinger equation or whatever. So I think that there's a sensible

notion of the set of all possible worlds, and we live in one of them. The world that we're

talking about might be a multiverse, might be many worlds of quantum mechanics, might be much

bigger than the world of our everyday experience, but it's still one physically contiguous world

in some sense. But so if you look at the overlap of math and physics, it feels like when physics

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

the limit of our current understanding. What do you make of that process of sort of using math to

start maybe with intuition, or you might start with the math and then build up an intuition,

or but this kind of reaching into the darkness, into the mystery of the world with math?

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

of the physical world, which we then extrapolate and ask, what do we conclude if we take these

seriously well beyond where we've actually tested them? It is separately true that math is really,

really useful when we construct physical theories. And you know, famously Eugene Wigner asked about

the unreasonable success of mathematics and physics. I think that's a little bit wrong because

anything that could happen, any other theory of physics that wasn't the real world, but some other

world, you could always describe it mathematically. It's just that it might be a mess. The surprising

thing is not that math works, but that the math is so simple and easy that you can write it down

on a t shirt, right? I mean, that's what is amazing. That's an enormous compression of information that

seems to be valid in the real world. So that's an interesting fact about our world, which maybe we

couldn't hope to explain or just take as a brute fact, I don't know. But once you have that, there's

this indelible relationship between math and physics. But philosophically, I do want to separate

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

that doesn't apply to our world, right? We extrapolate the physical theory that we best

think explains our world. Again, an unanswerable question. Why do you think our world is so easily

compressible into beautiful equations? Yeah. I mean, like I just hinted at, I don't know if there's

an answer to that question. There could be. What would an answer look like? Well, an answer could

look like if you showed that there was something about our world that maximized something,

you know, the mean of the simplicity and the powerfulness of the laws of physics. Or, you know,

whether maybe we're just generic, maybe in a set of all possible worlds, this is what the world

would look like, right? Like, I don't really know. I tend to think not. I tend to think that there is

something specific and rock bottom about the facts of our world that don't have further explanation,

like the fact that the world exists at all. And furthermore, the specific laws of physics that

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

and that's how it is. And, you know, we can't explain anything more. I don't know how,

if we're anywhere close to that right now, but that seems plausible to me.

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

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

what's fundamental in physics. If there's anything, it feels like everything, especially with quantum

mechanics, is revealing to us is that most interesting things that I would, as a limited

human would think are fundamental can actually be explained as emergent from the more deeper

laws. I mean, we don't know, of course. You had to get that on the table. We don't know what is

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

Atoms and molecules are more fundamental than cells and organs.

Quantum fields are more fundamental than atoms and molecules. We don't know if that ever bottoms

out. I do think that there's sensible ways to think about this. If you describe something

like this table as a table, it has a height and a width, and it's made of a certain material,

and it has a certain solidity and weight and so forth. That's a very useful description as far

as it goes. There's a whole another description of this table in terms of a whole collection of

atoms strung together in certain ways. The language of the atoms is more comprehensive

than the language of the table. You could break apart the table, smash it to pieces,

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

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

as the theory gets more and more fundamental. So what do you think Newton would say,

maybe write in a book review, if you read your latest book on quantum mechanics,

something deeply hidden? It would take a long time for him to think that any of this was making

any sense. You catch him up pretty quick in the beginning. Yeah. You give him a shout out in the

beginning. That's right. I mean, he was the man. I'm happy to say that Newton was the greatest

scientists who ever lived. I mean, he invented calculus in his spare time, which would have

made him the greatest mathematician just all by himself, right? All by that one thing.

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

thinker. Rocky Cole, who was a cosmologist at the University of Chicago, said that Galileo,

even though he came before Newton, was a more modern thinker than Newton was. Like if you got

Galileo and brought him to the present day, you'd take him six months to catch up and then he'd

be in your office telling you why your most recent paper was wrong. Whereas Newton just

thought in this kind of more mystical way, you know, he wrote a lot more about the Bible and

alchemy than he ever did about physics. And but he was also more brilliant than anybody else and

way more mathematically astute than Galileo. So I really don't know, you know, he might have,

he might just say like, give me the textbooks, leave me alone for a few months and then

be caught up. But he or he might have had mental blocks against against seeing the world in this

way. I really don't know. Or perhaps find an interesting mystical interpretation of quantum

mechanics. Very possible. Yeah. Is there any other scientists or philosophers through history

that you would like to know their opinion of your book? That's a good question. I mean, Einstein

is the obvious one, right? You all, I mean, he was not that long ago, but I speculate at the end

of my book about what his opinion would be. I am curious as to, you know, what about older

philosophers like Hume or Kant, right? Like, what would they have thought or Aristotle, you know?

What would they have thought about modern physics? Because they do in philosophy, your

predilections end up playing a much bigger role in your ultimate conclusions because you're not

as tied down by what the data is. In physics, you know, physics is lucky because we can't stray

too far off the reservation as long as we're trying to explain the world that we actually see in our

telescopes and microscopes. But it's just not fair to play that game because the people we're

thinking about didn't know a whole bunch of things that we know, right? Like, we lived through a lot

that they didn't live through. So by the time we got them caught up, they'd be different people.

So let me ask a bunch of basic questions. I think it'll be interesting, useful for people who are

not familiar, but even for people who are extremely well familiar. Let's start with what is quantum

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

part of the 20th century that replaced classical mechanics. And it replaced classical mechanics

in a weird way that we're still coming to terms with. So in classical mechanics, you have an

object, it has a location, it has a velocity. And if you know the location and velocity of

everything in the world, you can say what everything's going to do. Quantum mechanics

has an aspect of it that is kind of on the same lines. There's something called the quantum state

or the wave function. And there's an equation governing what the quantum state does. So it's

very much like classical mechanics. The wave function is different. It's sort of a wave. It's a

vector in a huge dimensional vector space rather than a position and a velocity. But okay, that's

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

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

in our textbook formulation of quantum mechanics, in addition to saying that there's a quantum state

and it evolves in time. And all these new rules have to do with what happens when you look at

the system, when you observe it, when you measure it. In classical mechanics, there were no rules

about observing. You just look at it and you see what's going on. That was it, right? In quantum

mechanics, the way we teach it, there's something profoundly fundamental about the act of measurement

or observation and the system dramatically changes its state. Even though it has a wave

function, like the electron in an atom is not orbiting in a circle, it's sort of spread out

in a cloud. When you look at it, you don't see that cloud. When you look at it, it looks like

a particle with a location. It dramatically changes its state right away. The effects of that change

can be instantly seen and what the electron does next. Again, we need to be careful because we

don't agree on what quantum mechanics says. That's why I need to say in the textbook view,

et cetera. But in the textbook view, quantum mechanics, unlike any other theory of physics,

places gives a fundamental role to the act of measurement. So maybe even more basic,

what is an atom and what is an electron? Sure. This all came together in a few years around

the turn of the last century, right? Around the year 1900. Adams predated then, of course,

the word atom goes back to the ancient Greeks, but it was the chemists in the 1800s that really

first got experimental evidence for atoms. They realized that there were two different types

of tin oxide. In these two different types of tin oxide, there was exactly twice as much oxygen

in one type as the other. Why is that? Why is it never 1.5 times as much? Dalton said, well,

it's because there are tin atoms and oxygen atoms and one form of tin oxide is one atom of tin and

one atom of oxygen, and the other is one atom of tin and two atoms of oxygen. And on the basis of

speculation, a theory, a hypothesis, but then on the basis of that, you make other predictions.

The chemists became quickly convinced that atoms were real. The physicists took a lot longer

to catch on, but eventually they did. I mean, Boltzmann, who believed in atoms,

he had a really tough time his whole life because he worked in Germany where atoms were not popular.

They were popular in England, but not in Germany. In general, the idea of atoms is the smallest

building block of the universe for them. That was the Greek idea, but the chemists in the 1800s

jumped the gun a little bit. These days, an atom is the smallest building block of a chemical

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

than that. And that's what physicists discovered in the early 1900s, Rutherford, especially,

and his colleagues. So the atom that we think about now, the cartoon is that picture you've

always seen of a little nucleus and then electrons orbiting it like a little solar system. And we

now know the nucleus is made of protons and neutrons. So the weight of the atom, the mass,

is almost all in its nucleus. Protons and neutrons are something like 1800 times as heavy as electrons

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

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

electrons that are doing all the work. And where quantum mechanics steps in, as you mentioned,

with the position of velocity, with classical mechanics, and quantum mechanics is modeling

the behavior of the electron. I mean, you can model the behavior of anything, but the electron,

because that's where the fun is. The electron was the biggest challenge right from the start.

Yeah. So what's the wave function? You said it's an interesting detail. Yeah. But in any

interpretation, what is the wave function in quantum mechanics? Well, we had this idea from

Rutherford that atoms look like little solar systems. But people very quickly realized that

can't possibly be right. Because if an electron is orbiting in a circle, it will give off light.

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

and that's what electromagnetic waves are. And you can calculate how long would it take for the

electron just to spiral into the nucleus? And the answer is 10 to the minus 11 seconds, okay?

100 billionth of a second. So that's not right. Meanwhile, people had realized that light, which

we understood from the 1800s was a wave, had properties that were similar to that of particles,

right? This is Einstein and Planck and stuff like that. So if something that we agree was a wave

had particle like properties, then maybe something we think is a particle, the electron,

has wave like properties, right? And so a bunch of people eventually came to the conclusion,

don't think about the electron as a little point particle orbiting like a solar system.

Think of it as a wave that is spread out. They cleverly gave this the name the wave function,

which is the dopiest name in the world for one of the most profound things in the universe.

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

wave function at that point. And there's only one wave function.

Yeah, they eventually figured that out. That took longer. But when you have two electrons,

you do not have a wave function for electron one and a wave function for electron two.

You have one combined wave function for both of them. And indeed, as you say, there's only one

wave function for the entire universe at once. And that's where this beautiful dance,

can you say what is entanglement? It seems one of the most fundamental ideas of quantum mechanics.

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

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

basically the wave function, you take it and you square it, you square the number, that gives you

the probability of observing the system at that location. So if you say that for two electrons,

there's only one wave function, and that wave function gives you the probability of observing

both electrons at once doing something. Okay, so maybe the electron can be here or here or here

and the other electron can also be there. But we have a wave function set up where we don't know

where either electron is going to be seen. But we know they'll both be seen in the same place.

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

between the two of them. There's a sort of conditional statement. If we see one in one location,

then we know the other one's going to be doing a certain thing. So that's a feature of quantum

mechanics that is nowhere to be found in classical mechanics and classical mechanics.

There's no way I can say, Well, I don't know where either one of these particles is. But if I know,

if I find out where this one is, then I know where the other one is. That just never happens.

They're truly separate. And in general, it feels like if you think of a wave function like as a dance

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

So there's a closeness that's important. Well, that's another step. We have to be careful here

because in principle, if you're talking about the entanglement of two electrons, for example,

they can be totally entangled or totally unentangled no matter where they are in

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

between two electrons. But we now know that the reality of our best way of understanding

the world is through quantum fields, not through particles. So even the electron,

not just gravity and electromagnetism, but even the electron and the quarks and so forth are really

vibrations in quantum fields. So even empty space is full of vibrating quantum fields.

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

you just said. If they're nearby, if you have like two vibrating quantum fields that are nearby,

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

So what do quantum fields in a vacuum look like? Empty space?

Just like empty space. It's as empty as it can be.

But they're still a field. It's just... What is nothing...

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

which I can detect by dropping something. I don't see it, but there it is.

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

what is Hilbert space and Euclidean space?

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

Hilbert space is. But when I dig into a little bit more into quantum mechanics,

it becomes necessary. The English language was invented long before quantum mechanics,

or various forms of higher mathematics were invented. So we use the word space to mean

different things. Of course, most of us think of space as this three dimensional world in

which we live. I mean, some of us just think of it as outer space. But space around us,

it gives us the three dimensional location of things and objects. But mathematicians use any

generic abstract collection of elements as a space, a space of possibilities, momentum space,

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

or for some specific system. And it could be an infinite dimensional space, or it could be just

really, really large dimensional, but finite. We don't know, because we don't know the final

theory of everything. But this abstract Hilbert space is really, really, really big and has no

immediate connection to the three dimensional space in which we live.

What do dimensions in Hilbert space mean?

It's just a way of mathematically representing how much information is contained in the state

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

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

right? Up, down, X, Y, Z coordinates. But then I might want to give you its entire state, physical

state, which means both its position and also its velocity. The velocity also has three components.

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

of position, three dimensions of velocity. And then if it also has an orientation in space,

that's another three dimensions and so forth. So as you describe more and more information

about the system, you have an abstract mathematical space that has more and more

numbers that you need to give. And each one of those numbers corresponds to a dimension in that

space. So in terms of the amount of information, what is entropy, this mystical word that's

overused in math and physics, but has a very specific meaning in this context?

Sadly, it has more than one very specific meaning. This is this is the reason why it is hard.

Entropy means different things, even to different physicists. But one way of thinking about it is

a measure of how much we don't know about the state of the system, right? So if I have a bottle

of water molecules, and I know that, okay, there's a certain number of water molecules,

I could weigh it right and figure out, I know the volume of it, and I know the temperature and

pressure and things like that. I certainly don't know the exact position and velocity of every

water molecule, right? So there's a certain amount of information I know, a certain amount that I

don't know that is that is part of the complete state of the system. And that's what the entropy

characterizes, how much unknown information there is, the difference between what I do know about

the system and its full exact microscopic state. So when we try to describe a quantum mechanical

system, is it infinite or finite, but very large? Yeah, we don't know. That depends on the system.

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

infinite entropy, an infinite dimensional Hilbert space. So let's go back a little bit.

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

systems that will be different sizes, they could be infinite or finite. So that's the number of

numbers, the number of pieces of information you could potentially give me about the system. So

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

on what I know about it. If I don't know anything about it, then, you know, it has a huge entropy,

right? But only up to the size of its Hilbert space. So we don't know in the real physical world

whether or not, you know, this region of space that contains that water bottle has potentially

an infinite entropy or just a finite entropy. We have different arguments on different sides.

So if it's infinite, how do you think about infinity? Is this something you can,

your cognitive abilities are able to process? Or is it just a mathematical tool?

It's somewhere in between, right? I mean, we can say things about it. We can use mathematical

tools to manipulate infinity very, very accurately. We can define what we mean, you know, for any

number n, there's a number bigger than it. So there's no biggest number, right? So there's

something called the total number of all numbers, and it's infinite. But it is hard to wrap your

brain around that. And I think that gives people pause because we talk about infinity as if it's

a number, but it has plenty of properties that real numbers don't have, you know, if you multiply

infinity by two, you get infinity again, right? That's a little bit different than what we're used to.

Okay, but are you comfortable with the idea that in thinking of what the real world actually is,

that infinity could be part of that world? Are you comfortable that a world, in some

dimension, in some aspect? I'm comfortable with lots of things. I mean, you know, I don't want

my level of comfort to affect what I think about the world. You know, I'm pretty open

mind about what the world could be at the fundamental level. Yeah, but infinity is a,

is a tricky one. It's not almost a question of comfort. It's a question of,

of, is it an overreach of our intuition? Sort of, it could be a convenient, almost like when

you add a constant to an equation, just because it'll help. It just feels like it's useful to

at least be able to imagine a concept, not directly, but in some kind of way that this feels

like it's a description of the real world. Think of it this way. There's only three numbers

that are simple. There's zero, there's one, and there's infinity. A number like 318 is just bizarre,

like that, that you need a lot of bits to give me what that number is. But zero and one infinity,

like once you have 300 things, you might as well have infinity things, right? Otherwise,

you have to say when to stop, making the things, right? So there's a sense in which infinity is

a very natural number of things to exist. That was never comfortable with infinity,

because it's just such a, it was too good to be true, because in math, it just helps

make things work out. When things get very, it's, when things get very large, close to infinity,

things seem to work out nicely. It's kind of like, because my deepest passion is probably

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

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

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

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

program to run? And you realize, well, for some of them, the answer is infinitely long.

It's not because you tried to get there, you wrote a five line computer program, it doesn't halt.

So coming back to the textbook definition of quantum mechanics, this idea that we,

I don't think we talked about, can you, this, one of the most interesting philosophical points,

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

definition of quantum mechanics separates what is observed and what is real. One, how does that

make you feel? And, and two, what does it then mean to observe something? And why is it different

than what is real? Yeah, you know, I, my personal feelings, such as it is, is that things like

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

ultimate laws of physics. But my feeling that way is because so far, that's where all the evidence

has been pointing. I could be wrong. And there's certainly a sense in which it would be infinitely

cool if somehow observation or mental cogitation did play a fundamental role in the, in the nature

of reality. But I don't think so. And I don't see any evidence for it. So I'm not spending a lot of

time worrying about that possibility. So what do you do about the fact that in the textbook

interpretation of quantum mechanics, this idea of measurement or, or looking at things seems to

play an important role? Well, you, you come up with better interpretations of quantum mechanics.

And there are several alternatives. My favorite is the many worlds interpretation,

which says two things. Number one, you, the observer, are just a quantum system like anything

else. There's nothing special about you. Don't get so proud of yourself. You know, you're just a

bunch of atoms. You have a wave function. You obey the Schrodinger equation like everything else.

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

happening is you're becoming entangled with that thing. So when you think there's a wave function

for the electron, it's all spread out, but you look at it and you only see it in one location.

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

locations, but now it's entangled with the wave function of you in the following way.

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

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

think you saw it there, et cetera. So, and all of those different parts of the wave function,

once they come into being, no longer talk to each other. They no longer interact or influence

each other. It says if they are separate worlds. So this was the invention of Hugh Everett,

the third who was a graduate student at Princeton in the 1950s. And he said, basically, look,

you don't need all these extra rules about looking at things. Just listen to what the

Schrodinger equation is telling you. It's telling you that you have a wave function that you become

entangled and that the different versions of you no longer talk to each other. So just accept it.

It's just he did therapy more than anything else. He said, it's okay. You don't need all these extra

rules. All you need to do is believe the Schrodinger equation. The cost is there's a whole bunch of

extra worlds out there. So the world's being created, whether there's an observer or not.

The world's created any time a quantum system that's in a superposition becomes entangled with the

outside world. What's the outside world? It depends. Let's back up. Whatever it really says,

what his theory is, is there's a wave function of the universe and it obeys the Schrodinger

equation all the time. That's it. That's the full theory right there. The question, all of the work

is how in the world do you map that theory onto reality, onto what we observe? So part of it is

carving up the wave function into these separate worlds saying, look, it describes a whole bunch

of things that don't interact with each other. Let's call them separate worlds. Another part

is distinguishing between systems and their environments. The environment is basically

all the degrees of freedom, all the things going on in the world that you don't keep

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

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

keep track of all the photons or the air molecules in this room. So that's the outside world. The

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

about the behavior of subsystem of it. So how many worlds are there?

Yeah, we don't know that one either. There could be an infinite number. There could be only a finite

number, but it's a big number one way or the other. It's just a very, very big number. In one of the

talks, somebody asked, well, if it's finite. So actually, I'm not sure exactly the logic you

used to derive this, but is there going to be overlap, a duplicate world that you return to?

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

there's some kind of equilibrium that these splitting worlds arrive at. And then maybe

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

similar to each other. Yeah. So this question of whether or not Hilbert space is finite or

infinite dimensional is actually secretly connected to gravity and cosmology. This is the

part that we're still struggling to understand right now. But we discovered back in 1998 that

our universe is accelerating. And what that means if it continues, which we think it probably will,

but we're not sure. But if it does, that means there's a horizon around us. There's because

the universe not only expanding, but expanding faster and faster, things can get so far away

from us that from our perspective, it looks like they're moving away faster than the speed of light.

We will never see them again. So there's literally a horizon around us. And that horizon

approaches some fixed distance away from us. And you can then argue that within that horizon,

there's only a finite number of things that can possibly happen, the finite dimensional Hilbert

space. In fact, we even have a guess for what the dimensionality is. It's 10 to the power of 10 to

the power of 122. That's a very large number. Yeah. Just to compare it, the age of the universe

is something like 10 to the 14 seconds, 10 to the 17 or 18 seconds, maybe the number of particles

in the universe is 10 to the 88. But the number of dimensions of Hilbert space is 10 to the 10

to the 122. So that's just crazy. If that story is right, that in our observable horizon, there's

only a finite dimensional Hilbert space, then this idea of branching of the wave function of

the universe into multiple distinct separate branches has to reach a limit at some time. Once

you branch that many times, you run out of room in Hilbert space. And roughly speaking, that corresponds

to the universe just expanding and emptying out and cooling off and entering a phase where it's

just empty space, literally forever. What's the difference between splitting and copying,

do you think? A lot of this is an interpretation that helps us model the world. So perhaps shouldn't

be thought of as philosophically or metaphysically. But even at the physics level, do you see a

difference between generating new copies of the world or splitting? I think it's better to think

of in quantum mechanics, in many worlds, the universe splits rather than new copies because

people otherwise worry about things like energy conservation. And no one who understands quantum

mechanics worries about energy conservation because the equation is perfectly clear. But if all you

know is that someone told you the universe duplicates, then you have a reasonable worry about where

all the energy for that came from. So a preexisting universe splitting into two skinnier universes

is a better way of thinking about it. And mathematically, it's just like if you draw an

x and y axis, and you draw a vector of length one at 45 degree angle, you know that you can write

that vector of length one as the sum of two vectors pointing along x and y of length one over the

square root of two. So I write one arrow as the sum of two arrows. But there's a conservation of

arrowness. There's now two arrows, but the length is the same. I'm just describing it in a different

way. And that's exactly what happens when the universe branches. The wave function of the

universe is a big old vector. So to somebody who brings up a question of saying, doesn't this violate

the conservation of energy? Can you give further elaboration?

Right. So let's just be super duper perfectly clear. There's zero question about whether or not many

worlds violates conservation of energy. It does not. And I say this definitively because there

are other questions that I think there's answers to, but they're legitimate questions about where

does probability come from and things like that. This conservation of energy question,

we know the answer to it. And the answer to it is that energy is conserved. All of the effort goes

into how best to translate what the equation unambiguously says into plain English. So this

idea that there's a universe that the universe comes equipped with a thickness and it sort of

divides up into thinner pieces, but the total amount of universe is conserved over time is a

reasonably good way of putting English words to the underlying mathematics.

So one of my favorite things about many worlds is, I mean, I love that there's something controversial

in science. And for some reason, it makes people actually not like upset, but just get excited.

Why do you think it is a controversial idea? So there's a lot of, it's actually one of the cleanest

ways to think about quantum mechanics. So why do you think there's a discomfort a little bit

among certain people? Well, I draw the distinction in my book between two different kinds of

simplicity in a physical theory. There's simplicity in the theory itself, right? How we describe what's

going on according to the theory by its own rights. But then, you know, theory is just some sort of

abstract mathematical formalism. You have to map it onto the world somehow, right? And sometimes,

like for Newtonian physics, it's pretty obvious, like, okay, here is a bottle and it has a center

of mass and things like that. Sometimes it's a little bit harder with general relativity,

curvature of space time is a little bit harder to grasp. Quantum mechanics is very hard to map

what you're the language you're talking in a wave functions and things like that onto reality.

And many worlds is the version of quantum mechanics where it is hardest to map on the

underlying formalism to reality. So that's where the lack of simplicity comes in, not in the theory,

but in how we use the theory to map onto reality. And in fact, all of the work in sort of elaborating

many worlds quantum mechanics is in this effort to map it on to the world that we see. So it's

perfectly legitimate to be bugged by that, right? To say, like, well, no, that's just too far away

from my experience. I am therefore intrinsically skeptical of it. Of course, you should give up

on that skepticism if there are no alternatives and this theory always keeps working, then

eventually you should overcome your skepticism. But right now there are alternatives that people

work to make alternatives that are by their nature closer to what we observe directly.

Can you describe the alternatives? I don't think we touched on it. So the Copenhagen interpretation

and the many worlds, maybe there's a difference between the Everettian

many worlds and many worlds as it is now, like has the idea sort of developed and so on. And just

in general, what is the space of promising contenders? We have democratic debates now,

there's a bunch of candidates, 12 candidates, 12 candidates on stage. What are the quantum

mechanical candidates on stage for the debate? So if you had a debate between quantum mechanical

contenders, there'd be no problem getting 12 people up there on stage, but there would still be

only three front runners. And right now the front runners would be Everett. Hidden variable

theories are another one. So the hidden variable theories say that the wave function is real,

but there's something in addition to the wave function. The wave function is not everything,

it's part of reality, but it's not everything. What else is there? We're not sure. But in the

simplest version of the theory, there are literally particles. So many worlds says that

quantum systems are sometimes are wave like in some ways and particle like in another because

they really, really are waves. But under certain observational circumstances, they look like

particles. Whereas hidden variable says that they look like waves and particles because there are

both waves and particles involved in the dynamics. And that's easy to do if your particles are just

non relativistic, Newtonian particles moving around, they get pushed around by the wave function

roughly. It becomes much harder when you take quantum field theory or quantum gravity into

account. The other big contender are spontaneous collapse theories. So in the conventional textbook

interpretation, we say when you look at a quantum system, its wave function collapses and you see

it in one location. A spontaneous collapse theory says that every particle has a chance per second

of having its wave function spontaneously collapse. The chance is very small for a typical particle

will take hundreds of millions of years before it happens even once. But in a table or some

macroscopic object, there are way more than 100 million particles. And they're all entangled

with each other. So when one of them collapses, it brings everything else along with it.

There's a slight variation of this. That's a spontaneous collapse theory. There are also

induced collapse theories like Roger Penrose thinks that when the gravitational difference

between two parts of the wave function becomes too large, the wave function collapses automatically.

So those are basically, in my mind, the three big alternatives. Many worlds, which is just

there's a wave function and always obeys the Schrodinger equation. Hidden variables,

there's a wave function and always obeys the Schrodinger equation. But there are also new

variables or collapse theories, which the wave function sometimes obeys the Schrodinger equation

and sometimes it collapses. So you can see that the alternatives are more complicated in their

formalism than many worlds is, but they are closer to our experience. So just this moment of

collapse, do you think of it as a wave function fundamentally sort of a probabilistic description

of the world? And it's collapse sort of reducing that part of the world into something deterministic,

where again, you can now describe the position and the velocity in this simple classical model?

Well, there is... Is that how you think about collapse?

There is a fourth category, is a fourth contender, there's a mayor Pete of

quantum mechanical interpretations, which are called epistemic interpretations.

And what they say is, all the wave function is a way of making predictions for experimental

outcomes. It's not mapping onto an element of reality in any real sense. And in fact,

two different people might have two different wave functions for the same physical system,

because they know different things about it, right? The wave function is really just a prediction

mechanism. And then the problem with those epistemic interpretations is if you say, okay,

but it's predicting about what? Like, what is the thing that is being predicted? And I say,

no, no, no. That's not what we're here for. We're just here to tell you what the observational

outcomes are going to be. But the other interpretations kind of think that the wave

function is real. Yes. That's right. So that's an on tick interpretation of the wave function,

ontology being the study of what is real, what exists, as opposed to an epistemic

interpretation of the wave function, epistemology being the study of what we know.

I would actually just love to see that debate on stage.

There was a version of it on stage at the World Science Festival a few years ago,

that you can look up online. And on YouTube?

Yep. It's on YouTube.

Okay, awesome. I'll link it and watch it.

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right? It's like the most bare bones austere pure version of quantum mechanics. And I am someone

who is very willing to put a lot of work into mapping the formalism onto reality. I'm less

willing to complicate the formalism itself. But the other big reason is that there's something

called modern physics with quantum fields and quantum gravity and holography and space time

doing things like that. And when you take any of the other versions of quantum theory, they bring

along classical baggage, all of the other versions of quantum mechanics, prejudice or privilege,

some version of classical reality like locations in space. Okay. And I think that that's a barrier

to doing better and understanding the theory of everything and understanding quantum gravity in

the versions of space time. Whenever if you change your theory from, you know, here's a harmonic

oscillator. Oh, there's a spin. Here's an electromagnetic field in hidden variable theories

or dynamical collapse theories. You have to start from scratch. You have to say like, well,

what are the hidden variables for this theory? Or how does it collapse or whatever? Whereas

many worlds is plug and play. You tell me the theory and I can give you its many worlds version.

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

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

quantum theory that you have, which is really many worlds. So start with the quantum theory and

try to build up a model of space time, the emergence of space time. Okay, so I thought space

time was fundamental. Yeah, I know. So this sort of dream that Einstein had that everybody had

and everybody has of, you know, the theory of everything. So how do we build up from many

worlds, from quantum mechanics, a model of space time, a model of gravity? Well, yeah, I mean,

let me first mention very quickly why we think it's necessary. You know, we've had gravity in

the form that Einstein bequeathed to us for over 100 years now, like 1915 or 1916, he put general

relativity in the final form. So gravity is the curvature of space time. And there's a field that

pervades all the universe that tells us how curved space time is. And that's a fundamentally

classical. That's totally classical, right? Exactly. But we also have a formalism, an

algorithm for taking a classical theory and quantizing it. This is how we get quantum

electrodynamics, for example. And it could be tricky. I mean, you could you think you're

quantizing some things that that means taking a classical theory and promoting it to a quantum

mechanical theory. But you can run into problems. So they ran into problems and they did that with

electromagnetism, namely that certain quantities were infinity, and you don't like infinity,

right? So Feynman and Tom Monaga and Schwinger won the Nobel Prize for teaching us how to deal

with the infinities. And then Ken Wilson won another Nobel Prize for saying you shouldn't

have been worried about those infinities after all. But still, that was the it's always the

thought that that's how you will make a good quantum theory, you'll start with classical

theory and quantize it. So if we have a classical theory, general relativity, we can quantize it

or we can try to. But we run into even bigger problems with gravity than we ran into with

electromagnetism. And so far, those problems are insurmountable. We've not been able to get a

successful theory of gravity, quantum gravity by starting with classical general relativity

and quantizing it. And there's evidence that there's a good reason why this is true, that

whatever the quantum theory of gravity is, it's not a field theory. It's something that has

weird non local features built into it somehow that we don't understand. We get this idea from

black holes and Hawking radiation and information conservation and a whole bunch of other ideas

I talked about in the book. So if that's true, if the fundamental theory isn't even local in

the sense that an ordinary quantum field theory would be, then we just don't know where to start

in terms of getting a classical precursor and quantizing it. So the only sensible thing,

at least the next obvious sensible thing to me would be to say, okay, let's just start intrinsically

quantum and work backwards, see if we can find a classical limit. So the idea of locality,

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

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

stuff that's close by, interacts, stuff that's far away doesn't. So what's locality and why is

it not fundamental? And how's that even possible? Yeah, I mean, locality is the answer to the

question that Isaac Newton was worried about back in the beginning of our conversation, right? I

mean, how can the earth know what the gravitational field of the sun is? And the answer, as spelled

out by Laplace, Einstein and others, is that there's a field in between. And the way a field works is

that what's happening to the field at this point in space only depends directly on what's happening

at points right next to it. But what's happening at those points depends on what's happening right

next to those, right? And so you can build up an influence across space through only local

interactions. That's what locality means. What happens here is only affected by what's happening

right next to it. That's locality. The idea of locality is built into every field theory,

including general relativity as a classical theory. It seems to break down when we talk about black

holes. And, you know, Hawking taught us in the 1970s that black holes radiate, they give off,

they eventually evaporate away, they're not completely black once we take quantum mechanics

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

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

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

there are good arguments that can't happen while preserving locality at the same time.

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

And people should, you talk about Holography with the Leonard Susskind on your Mindscape podcast.

Oh, yes, I have a podcast. I didn't even mention that. I'm terrible.

No, I'm going to, I'm going to ask you questions about that too. And I've been

not shutting up about, it's my favorite science podcast. So, or not, it's a, it's not even a

science podcast. It's like, it's a scientist doing a podcast. That's what it is. Absolutely. Yes.

Yeah. Anyway, yeah. So, Holography is this idea when you have a black hole and black hole is a

region of space inside of which gravity is so strong that you can't escape. And there's this

weird feature of black holes that, again, is a totally thought experiment feature because we

haven't gone and probed any yet. But there seems to be one way of thinking about what happens

inside a black hole, as seen by an observer who's falling in, which is actually pretty normal. Like

everything looks pretty normal until you hit the singularity and you die. But from the point of the

view of the outside observer, it seems like all the information that fell in is actually smeared

over the horizon in a nonlocal way. And that's puzzling. And that's so Holography because

that's a two dimensional surface that is encapsulating the whole three dimensional thing

inside, right? Still trying to deal with that. Still trying to figure out how to get there.

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

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

looking at the two dimensional projection of it, it means that locality is not necessary.

Well, it means that somehow it's only a good approximation. It's not really what's going on.

How are we supposed to feel about that? It's supposed to feel liberated.

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

started quantizing gravity. So, we're just beginning now to face up to the dramatic

implications of quantizing gravity. Is there other weird stuff that happens

to quantum mechanics in black hole? I don't think that anything weirds happen

with quantum mechanics. Weird things happen with space time. I mean, that's what it is.

Like quantum mechanics is still just quantum mechanics. But our ordinary notions of space

time don't really quite work. And there's a principle that goes hand in hand with

holography called complementarity, which says that there's no one unique way to describe

what's going on inside a black hole. Different observers will have different descriptions,

both of which are accurate, but sound completely incompatible with each other. So,

it depends on how you look at it. You know, the word complementarity in this context is

borrowed from Niels Bohr, who points out you can measure the position or you can measure the

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

So, a couple of questions on many worlds. How does many worlds help us understand our particular

branch of reality? So, okay, that's fine and good that is everything is splitting,

but we're just traveling down a single branch of it. So, how does it help us understand our

little unique branch? Yeah, I mean, that's a great question. But that's the point is that

we didn't invent many worlds because we thought it was cool to have a whole bunch of worlds,

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

And what we observe here in our world are wave functions collapsing, okay? We do have a situation

where the electron seems to be spread out, but then when we look at it, we don't see it spread

out. We see it located somewhere. So, what's going on? That's the measurement problem of

quantum mechanics. That's what we have to face up to. So, many worlds is just a proposed solution

to that problem. And the answer is nothing special is happening. It's still just the

Schrodinger equation, but you have a wave function too. And that's a different answer than would

be given in hidden variables or dynamical collapse theories or whatever. So, the entire point of

many worlds is to explain what we observe. But it tries to explain what we already have

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

something other than quantum mechanics. But the idea that there's worlds that we didn't observe

that keep branching off is kind of stimulating to the imagination. So, is it possible to hop

from, you mentioned the branches are independent. Is it possible to hop from one to the other?

No. So, it's a physical limit. The theory says it's impossible.

There's already a copy of you in the other world, don't worry. Yes. Then leave them alone.

No, but there's a fear of missing out, FOMO. Yes. That I feel like immediately start to wonder if

that other copy is having more or less fun. Yeah. Well, the downside to many worlds is that you're

missing out on an enormous amount. And that's always what it's going to be like. And I mean,

there's a certain stage of acceptance in that. Yes. In terms of rewinding, do you think we can

rewind the system back? Sort of the nice thing about many worlds, I guess, is it really emphasizes

the, maybe you can correct me, but the deterministic nature of a branch. And it feels like it could

be rewind back. Do you see this as something that could be perfectly rewinded back? Yeah.

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

glass of Bordeaux and you knock it over and the wine spills across the tablecloth. If the world

were classical, it would be possible that if you just lifted the wine glass up, you'd be lucky enough

that every molecule of wine would hop back into the glass, right? But guess what? It's not going

to happen in the real world. And the quantum wave function is exactly the same way. It is possible

in principle to rewind everything if you start from perfect knowledge of the entire wave function

of the universe. In practice, it's never going to happen. So time travel, not possible. Nope.

At least quantum mechanics has no help. What about memory? Does the universe have a memory

of itself where we could not time travel, but peek back in time and do a little like replay?

Well, it's exactly the same in quantum mechanics as classical mechanics. So whatever you want to

say about that, the fundamental laws of physics in either many worlds, quantum mechanics or Newtonian

physics, conserve information. So if you have all the information about the quantum state of the

world right now, your Laplace's demon like and your knowledge and calculational capacity,

you can wind the clock backward. But none of us is, right? And so in practice, you can never do

that. You can do experiments over and over again starting from the same initial conditions for

small systems. But once things get to be large, Avogadro's number of particles, right, bigger

than a cell, no chance. We talked a little bit about era of time last time, but in many worlds

that there is a kind of implied era of time, right? So you've talked about the era of time that has

to do with the second law of thermodynamics. That's the era of time that's emergent or fundamental.

We don't know, I guess. No, it's emergent. Is everyone agree on that? Well, nobody agrees with

everything. They should. So that era of time, is that different than the era of time that's implied

by many worlds? It's not different actually, no. In both cases, you have fundamental laws of physics

that are completely reversible. If you give me the state of the universe at one moment in time,

I can run the clock forward or backward equally well. There's no arrow of time built into the

laws of physics at the most fundamental level. But what we do have are special initial conditions

14 billion years ago near the Big Bang. In thermodynamics, those special initial conditions

take the form of things where low entropy and entropy has been increasing ever since,

making the universe more disorganized and chaotic, and that's the era of time.

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

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

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

that are emergent and feel like they're fundamental. So if time is emergent, and locality is space

emergent? Yes. Okay. But I didn't say time was emergent. I said the arrow of time was emergent.

Those are different. What's the difference between the arrow of time and time? Are you using

arrow of time to simply mean that they're synonymous with the second law of thermodynamics?

No, but the arrow of time is the difference between the past and future. So there's space,

but there's no arrow of space. You don't feel that space has to have an arrow, right? You could

live in thermodynamic equilibrium. There'd be no arrow of time, but there'd still be time. There'd

still be a difference between now and the future or whatever. Okay, so if nothing changes, there's

still time? Well, things could even change. Like if the whole universe consisted of the

earth going around the sun, okay? It would just go in circles or ellipses, right?

Things would change, but it's not increasing entropy. There's no arrow. If you took a movie

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

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

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

the arrow of time in the macroscopic world is so powerful that there's just no chance of going back.

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

up and down. What does it mean for space to be an emergent phenomena? It means that the fundamental

description of the world does not include the word space. It'll be something like a vector in

Hilbert space, right? And you have to say, well, why is there a good approximate description

in which involves three dimensional space and stuff inside it? Okay, so time and space are

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

fundamental in our universe? A wave function living in Hilbert space?

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

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

Like, well, how do you think about? It's a vector in a 10 to the 10 to the 122 dimensional vector

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

Got it. When you put it that way. What's so hard, really?

Yeah, quantum computers, there's some excitement, actually a lot of excitement with people

that it will allow us to simulate quantum mechanical systems. What kind of questions do

about quantum mechanics, about the things we've been talking about? Do you think,

do you hope we can answer through quantum simulation?

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

computers, both sort of practical things with cryptography or money, privacy eavesdropping,

sorting things, simulating quantum systems. It's a broader question, maybe even outside

of quantum computers. Some of the theories that we've been talking about, what's your hope?

What's most promising to test these theories? What are experiments we can conduct,

whether in simulation or in the physical world, that would validate or disprove or expand these

theories? Well, I think there's two parts of that question. One is many worlds and the other one is

sort of emergent space time. For many worlds, there are experiments ongoing to test whether

or not wave functions spontaneously collapse. And if they do, then that rules out many worlds

and that would be falsified. If there are hidden variables, there's a theorem that seems to indicate

that the predictions will always be the same as many worlds. I'm a little skeptical of this

theorem. I haven't internalized it. I haven't made it in part of my intuitive view of the world yet.

So there might be loopholes to that theorem. I'm not sure about that. Part of me thinks that

there should be different experimental predictions if there are hidden variables, but I'm not sure.

But otherwise, it's just quantum mechanics all the way down. And so there's this cottage industry

in science journalism of writing breathless articles that say quantum mechanics shown to be

more astonishing than ever before thought. And really, it's the same quantum mechanics we've

been doing since 1926. Whereas with the emergent space time stuff, we know a lot less about what

the theory is. It's in a very primitive state. We don't even really have a safely written down

respectable honest theory yet. So there could very well be experimental predictions we just

don't know about yet. That is one of the things that we're trying to figure out.

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

Well, or really fast stuff or really energetic stuff, we don't know. That's the thing. So

there could be violations of the speed of light if you have emergent space time.

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

for light of different wavelengths. That would be a dramatic violation of physics as we know it,

but it could be possible. Or not. I mean, it's not an absolute prediction. That's the problem.

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

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

Do you think about sort of consciousness and these kinds of topics? Is there?

It's certainly excessively used as you point out. The word quantum is used for everything

besides quantum mechanics. But in more seriousness, is there something that goes to the human level

and can help us understand our mind? Not really. It's the short answer.

Mines are pretty classical. I don't think. We don't know this for sure, but I don't think

that phenomena like entanglement are crucial to how the human mind works.

What about consciousness? You mentioned, I think early on in the conversation, you said it would be

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

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

So if consciousness as the panpsychist believes is fundamental to the universe, is that possible?

Is that weight? I mean, everything's possible. Just like Joe Rogan likes to say it's entirely

possible. But okay, but is it on a spectrum of crazy out there? Statistically speaking,

how often do you ponder the possibility that consciousness is fundamental or the observer

is fundamental? I personally don't at all. There are people who do. I'm a thorough physicalist

when it comes to consciousness. I do not think that there are any separate mental states or

mental properties. I think they're all emergent, just like space time is. And space time is hard

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

to me. You, as we mentioned, have an amazing podcast called Minescape. It's, as I said,

one of my favorite podcasts, sort of both for your explanation of physics, which a lot of people love.

And when you venture out into things that are beyond your expertise, but it's just a really

smart person exploring even questions like, you know, morality, for example, was very interesting.

I think you did a solo episode and so on. I mean, there's a lot of really interesting

conversations that you have. What are some from memory, amazing conversations that pop to mind

that you've had? What did you learn from them? Something that maybe changed your mind or just

inspired you or just did this whole experience of having conversations? What stands out to you?

It's an unfair question. It's totally unfair, but that's okay. That's all right. You know,

it's often the ones, I feel like the ones I do on physics and closely related science or even

philosophy ones are like, I know this stuff and I'm helping people learn about it. But I learn

more from the ones that have nothing to do with physics or philosophy, right? So talking to Winton

Marsalis about jazz or talking to a master sommelier about wine, talking to Will Wilkinson

about partisan polarization and the urban world divide, talking to psychologists like Carol

Tavres about cognitive dissonance and how those things work. Scott Derrickson, who is the director

of the movie Dr. Strange, I had a wonderful conversation with him where we went through

the mechanics of making a blockbuster superhero movie, right? And he's also not a naturalist.

He's an evangelical Christian. So we talked about the nature of reality there. I want to have a

couple more discussions with highly educated theists who know the theology really well,

but I haven't quite arranged those yet. I would love to hear that. I mean, that's

how comfortable are you venturing into questions of religion? Oh, I'm totally comfortable doing it.

I did talk with Alan Lightman, who is also an atheist, but he is trying to rescue the sort

of spiritual side of things for atheism. And I did talk to very vocal atheists like Alex Rosenberg.

So I need to talk to some, I've talked to some religious believers, but I need to talk to more.

How have you changed through having all these conversations?

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

couldn't find time to read them. And I figured if I interviewed their authors for me to read them,

right? And that's that is totally worked, by the way. Now I'm annoyed that people write such long

books. I think I'm still very much learning how to be a good interviewer. I think that's a skill

that, you know, I think I have good questions. But, you know, there's the the give and take

that is still, I think I could be better at like, I want to offer something to the conversation,

but not too much, right? I've had conversations where I barely talked at all, and I have

conversations where I talked half the time, and I think there's a happy medium in between there.

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

where I wish you would have disagreed more. Yeah. As a listener, it's more fun sometimes.

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

toward that. Like, some people are really there to basically give their point of view, and their

guest is supposed to, you know, respond accordingly. I want to sort of get my view on the record,

but I don't want to dwell on it when I'm talking to someone like David Chalmers,

who I disagree with a lot. You know, I want to say, like, here's why I disagree with you.

But, you know, I want, we're here to listen to you. Like, I have an episode every week,

and you're only on once a week, right? So I have an upcoming podcast episode with Philip Goff,

who is a much more dedicated panpsychist. And so there we really get into it. I think that

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

guest. But that's what he wanted. So it worked very well. Yeah. Yeah. That kind of debate structure

is a beautiful one. It's done right. Like, when you're, when you can detect that the intent

is that you have fundamental respect for the person that, and that's, for some reason,

it's super fun to listen to when two really smart people are just arguing and sometimes

lose their shit a little bit if I may say so. Well, there's a fine line because I have zero

interest in bringing, I mean, like, I mean, maybe you implied this, I have zero interest in

bringing on people for whom I don't have any intellectual respect. Like, I constantly get

requests to like, you know, bring on a flat earth or whatever and really slap them down,

or a creation is like, I'm at zero interest. I'm happy to bring on, you know, a religious

person, a believer, but I want someone who's smart and can act in good faith and can talk,

not a charlatan or a lunatic, right? So I will only, I will happily bring on people with whom I

disagree, but only people from whom I think the audience can learn something interesting.

So let me ask, the idea of charlatan is an interesting idea. You might be more educated

on this topic than me, but there's folks, for example, who argue various aspects of evolution

sort of try to approach and say that evolution, sort of our current theory of evolution,

has many holes in it, as many flaws. And they argue that I think like Cambridge,

Cambrian explosion, which is like a huge added variability of species, doesn't make sense under

our current description of evolution, theory of evolution, sort of, if you were to have the

conversation with people like that, how do you know that there, the difference between outside

the box thinkers and people who are fundamentally unscientific and even bordering on charlatans?

That's a great question. And the further you get away from my expertise, the harder it is for me

to really judge exactly those things. Yeah, I don't have a satisfying answer for that one,

because I think the example you use of someone who believes in the basic structure of natural

selection, but thinks that this particular thing cannot be understood in the terms of our current

understanding of Darwinism, that's a perfect edge case where it's hard to tell. And I would

try to talk to people who I do respect and who do know things, and I would have to... Given that

I'm a physicist, I know that physicists will sometimes be too dismissive of alternative

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

alternative points of view. So yeah, that's a tricky one. Have you gotten heat yet? Yeah,

heat all the time. There's always something... I mean, it's hilarious because I try very hard

not to have the same topic several times in a row. I did have two climate change episodes,

but they were from very different perspectives, but I like to mix it up. That's the whole point,

that's why I'm having fun. And every time I do an episode, someone says, oh, the person you

should really get on to talk about exactly that is this other person. I'm like, well, I did that

now. I don't want to do that anymore. Well, I hope you keep doing it. You're inspiring millions of

people, your books, your podcasts. Sean, it's an honor to talk to you. Thank you so much.

Thanks very much, Lex.