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 e-book 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, era of time, cosmology, and gravitation.
This is the Artificial Intelligence podcast. If you enjoy it, subscribe on YouTube,
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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 could only 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-emphasized 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, he'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, like, 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, right?
You know, 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 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 spacetime, 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, right? Like, you know, I have different intuitions now than I had
when I was a baby. That's okay. That's not 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 sort of 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, 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 Whitton
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,
you know, there's some threshold that one crosses in abstraction when one becomes kind of like a
touring machine, right? 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? You know, 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 that 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 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 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. 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 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 an 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 at the 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, et cetera, 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 give me, you have Laplace's demon ultimately, right? You
give me the state of the world today. I can predict what'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.
That 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.
And of course, this took a long time to get there, right? I mean, 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, Ibn Sina, 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?
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 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 this 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.
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, 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. 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 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
you start maybe with intuition or you might start with the math and then build up an intuition
or put 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 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. 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 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. What we extrapolate, we don't extrapolate math because there's a whole
bunch of wrong math that doesn't apply to our world. 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. 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, the mean of the simplicity and the powerfulness of the laws of physics,
or maybe we're just generic, maybe in a set of all possible worlds. This is what the world
would look 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, at some level, we're going to say, and that's how it is,
and we can't explain anything more. I don't know if we're anywhere close to that right now,
but that seems plausible to me. Speaking of rock bottom, one of the things 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 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? I think of this comprehensiveness,
the domain of validity of a theory gets broader and broader as the theory gets more and more
fundamental. What do you think Newton would say, maybe right in the 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. 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, it's funny because Newton was in some sense still a pre-modern thinker.
Rocky Kolb, 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. 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, he wrote a lot more about the Bible and alchemy than he ever
did about physics. But he was also more brilliant than anybody else and way more mathematically
astute than Galileo. So I really don't know. He might just say, give me the textbooks,
leave me alone for a few months and then be caught up. But he might have had mental blocks
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? 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 what about older philosophers like Hume or Kant, right? What would they have
thought or Aristotle? 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. 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? 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 the 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. So it dramatically changes its state right away. And 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, right? But in the textbook view, quantum mechanics, unlike any other theory of
physics, 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. And in these two different types of tin oxide, there was exactly twice as much
oxygen in one type as the other. And why is that? Why is it never 1.5 times as much?
And so 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 this, so this is a speculation, a theory, a hypothesis. But then
on the basis of that, you make other predictions. And the chemists became quickly convinced that
atoms were real. The physicists took a lot longer to catch on, but eventually they did.
And I mean, Boltzmann, who believed in atoms, 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.
And 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. So these days,
in 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 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.
But in any interpretation, what is the wave function in quantum mechanics?
Well, you know, we had this idea from Rutherford that atoms look like little solar systems.
But people very quickly realize 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 Plonk
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.
And 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. So maybe the electron can be here or 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. In 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 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 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? It looks like
empty space. It's as empty as it can be. But they're still a field. It's just,
it, 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 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, 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 the reason why it's 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. 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. 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. 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 spaces, 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 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 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. 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 that'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. If you multiply infinity by two,
you get infinity again, right? It'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, I don't want my level of
comfort to affect what I think about the world. I'm pretty open-minded about what the world could
be at the fundamental level. Yeah, but infinity is a tricky one. It's not almost a question of
comfort. It's a question of, is it an overreach of our intuition? 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. You need a lot of bits to give me what that number is, but zero and one infinity. Once
you have 300 things, you might as well have infinity things. Otherwise, you have to say when
to stop making the things. There's a sense in which infinity is a very natural number of things
to exist. That was never comfortable with infinity because it was too good to be true because in
math, it just helps make things work out. 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. I'm uncomfortable how in the average, how much we vary is lost. In that same kind of
sense, infinity seems like a convenient way to erase the details. The thing about infinity is
it seems to pop up whether we like it or not. You're trying to be a computer scientist. You
ask yourself, well, how long will it take this program to run? 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. Coming back to the textbook definition of quantum mechanics,
this idea that I don't think we talked about. One of the most interesting philosophical points,
we talked at the human level, but at the physics level, that at least the textbook definition
of quantum mechanics separates what is observed and what is real. How does that make you feel?
Two, what does it then mean to observe something, and why is it different than what is real?
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. 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 nature of reality. But I don't think so,
and again, 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 looking at things seems to play an important
role? Well, 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'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, etc. 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 worlds are created at 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. And 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. 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 88th, 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've branched that many times, you've 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 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 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 a 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 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 are that, you know,
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 variables says 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 in 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 this Schrodinger equation. Hidden variables, there's a wave function that always obeys this
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 its 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? Is that how you think about collapse?
There is a fourth category. There's 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, 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, 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.
For one. I won.
I don't know. There was no vote. There was no vote. But those, there's Brian Green was the
moderator and David Albert stood up for a spontaneous collapse and Shelley Goldstein
was there for hidden variables. And Rutiger Shock was there for epistemic approaches.
Why do you, I think you mentioned it, but just elaborate, why do you find many worlds so compelling?
Well, there's two reasons actually. One is, like I said, it is the simplest, 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 spacetime 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. And I think that that's a barrier to doing better and
understanding the theory of everything and understanding quantum gravity and the emergence
of spacetime. Whenever, if you change your theory from, 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, 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 as many worlds version. So when we
have a situation like we have with gravity and spacetime, 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 spacetime, the emergence of spacetime. Okay, so I thought spacetime 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 spacetime 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 spacetime. And there's a field that pervades
all the universe that tells us how curved spacetime 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 something. So 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 Tomonaga
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's always the thought that that's how you will make a good quantum
theory. You'll start with a 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. And 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, 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 is that even possible?
Yeah, I mean, locality is the answer to the question that Isaac Newton was worried about
back at 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 and 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 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 non-locally 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. This is 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 right. 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 non-local 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? We're 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. I think the weird things happen with spacetime. I mean,
that's what it is. Quantum mechanics is still just quantum mechanics. But our ordinary notions
of spacetime 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. 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 a rewind back. Do you see this as something that could be perfectly
rewind back, winded back? Yeah. You know, 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, okay, 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
in not time travel, but peak 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, you know, 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 in 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 error of time last
time, but in many worlds that there is a kind of implied error of time, right? So you've talked
about the error of time that has to do with the second law of thermodynamics. That's the error
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 error of time,
is that different than the error 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 arrow of time. In quantum mechanics,
these 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 assume 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 the 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. If the whole universe consisted of the earth going
around the sun, it would just go in circles or ellipses, right? That's the thing.
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 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, 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?
Yep. 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 you 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. 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 part of my intuitive
view of the world yet. 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. Otherwise, it's just quantum mechanics all the way down. 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. 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. 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. 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, it would be unlikely, but incredible
if sort of the observer is somehow a fundamental part of. So if observer, not to romanticize the
notion, but seems interlinked to the idea of consciousness. So if consciousness is, as the
panps like us believe is fundamental to the universe, is that possible? Is that weight? I mean, every.
It'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? How, how the statistics speaking, how often do you
ponder the possibility that consciousness is fundamental or the observer is fundamental to.
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 you know, 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, 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 what did this whole experience of having conversations of what stands out to you?
It's an unfair question. It's totally unfair. 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 Wynton Marsalis
about jazz or talking to a master sommelier about wine, talking to Will Wilkinson about partisan
polarization and the urban rural 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 when 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. You know, I did talk with Alan Lightman,
who is also an atheist, but he, you know, 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, 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, force me to read them, right? And that's, that has 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 can 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've had 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, 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, 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 beautiful when 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, has 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 to 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, you know, the further you get away
from my expertise, the harder it is for me to really judge exactly those things. And, you know,
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, right? And I would try to talk to people who I do respect and
who do know things, and I would have to, you know, 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. Like,
there's always something, I mean, it's hilarious because I do have, I try very hard not to like
have the same topic several times in a row. I did have like two climate change episodes,
but they're from very different perspectives. But I like to mix it up. That's the whole,
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 don't,
but 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.