a conversation with Sean Carroll, part 2, 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 audio book 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.

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, Arrow of Time, Cosmology, and Gravitation.

This is the Artificial Intelligence Podcast.

If you enjoy it, subscribe on YouTube, give it 5 stars on iTunes, support it on Patreon, or simply connect with me on Twitter, Alex Friedman, 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, their 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 them 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, but you know, so that was 1 of Newton's worries.

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

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 could—only way you could explain how they were there, presumably forever.

But the worries about classical mechanics were a little bit different.

It wasn't a worry about classical mechanics.

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 said, 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 underemphasize 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.

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 1 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 1 interpretation.

So if you see every force as a field or any other interpretation of action at a distance, I mean, just stepping back to sort of caveman 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?

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, the predictive power of science?

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

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

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

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.

But I think it's also okay to say that our intuitions get trained, right?

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

An intuition is not necessarily intrinsic to who we are.

So that's where I'm gonna bring in Noam Chomsky for a second who thinks that our cognitive abilities are sort of evolved through time.

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

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, hard-coded into our biology that we can't overcome?

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

But when someone like Ed Witten proves a theorem about, you know, 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 1 crosses in abstraction when 1 becomes kind of like a Turing machine, right?

1 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 that there are any limits to our ability to understand the scientific world at all.

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.

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?

Well, of course it's possible that 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 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.

They could do enormously 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.

So 1 interesting philosophical point that quantum mechanics bring up is 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 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.

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 1 opens their eyes and sees atoms, right, or molecules, or cells for that matter.

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, then there's no reason you believe we can't believe in quantum mechanics just as

What to you is the most beautiful idea in physics?

If you were Aristotle, when Aristotle wrote his book on physics, he made the following very obvious point.

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

And this kind of thing is 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.

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 a thousand years or 1500 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, the constant velocity, right?

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 nature's enteleology 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 cause, et cetera, because, you know, it's nature does or does not want to do that.

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

You give me 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 kind of 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.

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

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

It's weird, because like, 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 just reading each other's books.

And probably the first person to directly say that in outer space, in the vacuum, A projectile would move at a constant velocity was Avicenna, Ibn Sina in the Persian Golden Age circa 1000.

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

Ibn Sina 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, you know, 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 math and science to be able to say that momentum is conserved?

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?

I don't know if you've heard this story, but the arrow would push the air in front of it away and the molecules of air would run around to back of the arrow and push it again.

And anyone reading this is going like, really?

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, you know, 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.

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 1 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 1 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, so you 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, you know, 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 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.

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?

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 could 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.

We don't extrapolate math because there's a whole bunch of wrong math, you know, that doesn't apply to our world, right?

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

Why do you think our world is so easily compressible?

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

Well, an answer could look like if you showed that there was something about our world that maximized something, the mean of the simplicity and the powerfulness of the laws of physics, or maybe we're just generic, maybe in the set of all possible worlds, this is what the world would look like, right?

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 gonna, at some level, we're gonna 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, 1 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.

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.

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 has a certain solidity and weight and so forth.

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

There's a whole nother 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.

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 right in a book review, if he 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.

I'm happy to say that Newton was the greatest scientist 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?

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

Rocky Kolb, who is 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, it'd take him 6 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.

But he's also more brilliant than anybody else, and way more mathematically astute than Galileo.

You know, he might have, he might just, yeah, 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 seeing the world in this way.

Or perhaps find an interesting mystical interpretation of quantum mechanics.

Is there any other scientists or philosophers through history that you would like to know their opinion of your book?

I mean, Einstein is the obvious 1, right?

I mean, he was not that long ago, but I even speculate at the end of my book about what his opinion would be.

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Sean Carroll: Quantum Mechanics and the Many-Worlds Interpretation | Lex Fridman Podcast #47