EVERY SPOKEN WORD
150 min read · 30,069 words- 0:00 – 15:00
And here we go.…
- JRJoe Rogan
And here we go. Hello, Sean.
- SCSean Carroll
Hey, Joe. How's it going?
- JRJoe Rogan
Thanks for being here again, man.
- SCSean Carroll
Sure.
- JRJoe Rogan
I really appreciate it. So, uh, over the weekend, I got into your book. Whew.
- SCSean Carroll
Yes.
- JRJoe Rogan
Yes.
- SCSean Carroll
(laughs)
- JRJoe Rogan
It's great-
- SCSean Carroll
Thank you.
- JRJoe Rogan
... but, um, I mean, I really appreciate someone like you who's trying to break down quantum mechanics and quantum physics for someone like me. It's very hard to follow, and there was a lot of backing up and trying it again, and backing up and trying it again, and, like, just going over paragraphs and trying to figure out exactly what it means. But, uh, it's, it's really excellent and really perplexing at the same time.
- SCSean Carroll
Well, thank you. And you know, it, there are different styles when it comes to writing popular books, and I think there should be different styles. And my particular style is, look, it's not gonna be a breezy page-turner.
- JRJoe Rogan
Right.
- SCSean Carroll
Uh, but if you read it carefully, like, there's no prerequisites. You don't have to come into it w- as an expert. What you have to come into it is someone who's willing to sit and think about every paragraph, and then hopefully it'll be rewarding and you'll truly understand what's going on after doing that.
- JRJoe Rogan
Well, it is rewarding 'cause it is fascinating. And the history of quantum physics is also pretty fascinating 'cause I've always wondered, like, how did anybody even want to come up with this stuff? Like, these-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... and the fact that it was so long ago, it was w- the beginnings of it were in the 19th century?
- SCSean Carroll
Well, 1900 is the typical, literally that year, the turn of the century when Max Planck first, uh, got the first hints of it. And then yeah, it was, took another 27 years to put it into final shape.
- JRJoe Rogan
Now, for regular people that don't have a background of physics or that don't... This is, like, the whole idea behind it is so bizarre. It's like, why would anybody try to figure out something that... One of the things that you said that's really interesting is that you, quantum physics is used all the time.
- SCSean Carroll
Right.
- JRJoe Rogan
It's used with exact calculations, but yet we don't really understand it.
- SCSean Carroll
Yeah. Yeah, no, that's the main message of the book, really, because (clears throat) physicists, of course, do quantum mechanics every day, whether it's, you know, straightforward quantum mechanics, quantum field theory, quantum information, quantum computing. Clearly we're pretty good at it, you know? Like, transistors and lasers depend on quantum mechanics. The sun shining, figuring th- that out depends on quantum mechanics. The Higgs boson, et cetera. So to claim that we don't understand quantum mechanics is a little bit weird, but then we have quotes from people like Richard Feynman saying, "Nobody understands quantum mechanics," right?
- JRJoe Rogan
Yeah.
- SCSean Carroll
And so if he says that, then there's some authority behind it. And the reason is what we have is sci- sort of a black box, right? We, we say, you know, what, I think what I said in, uh, a New York Times article I wrote recently is physicists understand quantum mechanics in the same way that someone who owns a smartphone understands the smartphone. Like-
- JRJoe Rogan
Ah.
- SCSean Carroll
... they know how to use the apps. They can call people, they can make phone calls, they can take pictures. They don't know what's going on inside. And that's physicists with quantum mechanics. They, they use it, they can make very, very precise predictions, but if you ask them what is really going on, like, what is actually happening, what are all the details are like, "Yeah, no, that's not our job. Let's just stick to predictions."
- JRJoe Rogan
But to someone like me, that's so terrifying because, like, the, the very nature of reality is being examined by people. Like, s- if, if it is a, a f- a smartphone, it's being examined by people like me-
- SCSean Carroll
Yeah. (laughs)
- JRJoe Rogan
... who don't really understand the smartphone. I have no idea what, what's going on inside a smartphone. I've no... I know some words that have, y- used to-
- SCSean Carroll
Yep.
- 15:00 – 30:00
Mm-hmm. …
- SCSean Carroll
single time. We know what it is. We're gonna measure it. Measuring it, in this case, doesn't change it. It's in exactly that state. We know it, okay? Now, let's send it through a magnetic field that is oriented horizontally. So it's gonna be deflected either right or left. We know exactly what state it's in. It's spinning this way. But when you send it through that magnetic field that's oriented horizontally, it gets deflected left or right 50-50, unpredictably. There's no way we can predict it.
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
And then once it is... So, okay, now it's been spinning up, you measured its spin left, let's say, send it through another magnet that is going vertically, and now it's 50-50 again. It could be spin up or spin down (laughs) . So somehow, even though we knew exactly what state it was in, we couldn't predict what would happen next. That is part of quantum mechanics.
- JRJoe Rogan
So the act of sending it through these things where it makes it vertical or horizontal, in... What is, what is happening to it when it's going through these things?
- SCSean Carroll
So in quantum mechanics, what we say is that it's not that we don't know whether the electron is spinning clockwise or counterclockwise. Um, it can be in a super position of both. That's just the spin version of the position that the electron can be spread out in a, in a wave, right? It's r- it's truly not just that we are lacking some knowledge, it's that the knowledge really isn't there. And again, this is how we teach quantum mechanics in textbooks, and then I'm gonna correct it 'cause many worlds is much better-
- JRJoe Rogan
Okay.
- SCSean Carroll
... but this is the standard textbook version. There's a wave function, the wave function for a spin is it's either up or down or some combination, and then there's a rule that says when you measure the spin, you only get up or down. You don't see the wave function. Just like the cloud that you have for the electron's position, when you look at it, you see it at a location. So another way to get... to make the same argument is take a little piece of... I have a, a nice little, uh, image of this when I give talks, little piece of uranium, so it's a radioactive, uh, little chunk of metal, and you put it in a bubble chamber. So it is emitting radioactive particles and you detect the particles. You can see a little streak of, uh, of motion when the particle leaves the uranium, okay? Well, like I said, when you're not looking at it, this electron is supposed to obey an equation, the Schrodinger equation. And you can ask what the prediction is when a, when a radioactive nucleus decays and gives off an electron, what is its wave function gonna do? What is the wave function of the electron gonna be? And the answer is, it goes off in a spherical wave. It goes off in all directions at once.
- JRJoe Rogan
Evenly.
- SCSean Carroll
Yeah. All directions evenly. But you never see that. You always see a line.
- JRJoe Rogan
Is that roughly based on the shape of the piece of uranium? Does it vary?
- SCSean Carroll
No, because the electron gets, uh, from one individual nucleus of an atom, right? So the, the... what the ele- what the uranium is doing doesn't matter, it's just that one atom matters. And the easiest thing for the electron to do is just to go out in a sphere. It doesn't have to, it can go out in higher energy states, but the point is, it's not going out in a straight line. But when you look at it, you see a straight line, right? That's the fundamental mystery of quantum mechanics. That how we describe the thing when we're not looking at it is different than what we see when we look at it.
- JRJoe Rogan
So when you're in pursuit of an understanding, a deeper understanding of, of quantum mechanics, w- you... when you... you're thinking about people from the 1900s that are just sort of basically g-... getting the first steps going to understand this stuff. When, when you're talking about this lack of funding and the lack of encouragement for people to pursue quantum mechanics, you strongly feel like there are answers to these questions.
- SCSean Carroll
Yeah, that's right.
- JRJoe Rogan
That we just need better tools and a better understanding, better equations, more time.
- SCSean Carroll
Yeah. Me and Einstein think this, right? (laughs)
- JRJoe Rogan
(laughs)
- SCSean Carroll
So, Einstein is one of the secret heroes of the book because he has this reputation as someone who just couldn't quite accept quantum mechanics. Uh, the title, Something Deeply Hidden, is a quote from Einstein, uh, when he was talking about when he was a kid and he had a compass, right? And he was given his first magnetic compass and he could rotate it this way and that way, and it always pointed north. And you and I would go, "Huh, that's cool." But he was Einstein. He's like, "Wow, this is amazing. Why, how does it know where north is," right?
- JRJoe Rogan
Right.
- SCSean Carroll
And he said, "There must be something deeply hidden that explains why it's doing this mysterious thing." And he felt the same way about quantum mechanics, that it, we have, we gave these set of rules, which are called the Copenhagen Interpretation of Quantum Mechanics. One set of rules for when you're looking at it, one set of rules for when you're not. And Einstein was like, "Oh, come on. Clearly this is not the final answer to the nature of reality," right? He wanted to know God's thoughts. He's like, "I wanna know everything. We're not done yet. There must be more going on." And so many worlds is one of the proposed answers to what could be going on. It's not the only one. There's alternatives, but it's definitely my favorite. It's definitely the easiest one to, uh, to write down. Let's put it that way.
- JRJoe Rogan
Okay, so hit us with this many worlds theory.
- SCSean Carroll
Okay. So think about this electron. You're gonna s- you, you say that it could be either spin up or spin down. It's a combination of both. That's its wave function. You measure it, you only ever see spin up or spin down. So Copenhagen says that's because the wave function suddenly changed, snapped into place when you observed it. Don't ask me what it means to observe something. That's not what Copenhagen lets you ask.
- JRJoe Rogan
Okay.
- SCSean Carroll
Many worlds says what you're missing is two things. Number one, you are a quantum system.
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
You are obeying the rules of quantum mechanics. You're made of atoms and electrons and so forth. You have a wave function too, okay? So you are secretly treating yourself as a classical thing when you make that measurement, but you really should be treating yourself quantum mechanically, right? That's one thing. And the other thing is, uh, something that Einstein invented, namely called entanglement. When quantum mechanics says there's a wave function for a system, it doesn't say there's a separate wave function for every particle, right? It says that there's only one wave function for the whole universe. So the way I like to say it is, imagine two particles come in and bounce off of each other. Either one has a wave function and it's, you know, unpredictable exactly what angle it's gonna go off at. So both of them, both of the particles that go off, you don't know where they're going, but because momentum is conserved, if they came in at equal velocities, they'll go out at equal velocities in opposite directions. If you measure one, then you know where the other one is going, right? That's entanglement. The observed state of one system can be related to the observed state of another system. So those are the two ingredients. You're a quantum system and quantum systems can be entangled with each other. So Hugh Everett, who was a graduate student when he invented this idea in the 1950s said, "Look, when you measure that electron, what happens physically?" Like forget about you're a person, you're conscious, all of that BS. Like you're a physical system. You obey the Schrodinger equation. You, you are a quantum mechanical system. You obey the laws of physics. So you look at the electron, your wave function changes. It used to be you're just a person doing whatever you do, but then after you look at the electron, you become entangled with it. And it splits. So there is one part of the wave function that says the electron was spinning clockwise and you measured it spinning clockwise. And there's another part of the wave function that says the electron was spinning counterclockwise and you saw it spinning counterclockwise. Now everybody knows this. Like that, that far, it's not controversial at all. That's clearly the prediction of the equations of quantum mechanics. But everyone else said, "Well, that means that I'm some weird combination of, I saw it spinning one way and I saw it spinning the other way, but I've never felt that way. When I look at real electrons, I see them one way or the other. That can't be right. That can't be the final answer. The wave function must somehow collapse." And Everett said, "No, what you're missing is there's now two separate worlds." Both of those part of the wave function are real, but they're different worlds. They will never interact with each other again. What happens in one part of the wave function will not affect what happens in the other part. So now there's a version of you that saw the electron spinning clockwise and there's another version of you that saw it spinning counterclockwise. And that's just taking seriously the prediction of quantum mechanics. It's not adding any extra stuff, any extra worlds, anything like that.
- JRJoe Rogan
That is the part where my brain broke.
- SCSean Carroll
All right.
- JRJoe Rogan
The, the idea that there's a you that observes it going clockwise and a y- a you that observes it going in a different direction, like that is so hard to understand.
- SCSean Carroll
(laughs)
- JRJoe Rogan
Do you, would you apply this in your regular life? Like do you think, like when you go home and you say hi to your wife and you open up-
- 30:00 – 45:00
Now when people think…
- SCSean Carroll
are separate people. So I... I... I use the analogy, it's like identical twins. They were the same zygote or whatever, and now they're different people, okay? So that's the same thing. Like you're you now, and if you hit the button and branch the wave function, there'll be two different people, both of whom used to be you, but they're not the same person anymore 'cause different things happen to them.
- JRJoe Rogan
Now when people think about the concept of quantum mechanics and the way you're talking about describing things in the micro and the macro, you think of your existence itself very similar-... in a very similar manner, the- the way you think of electrons, the way you think of things being quantum, is that you are a combination of all these quantum things.
- SCSean Carroll
Yeah.
- JRJoe Rogan
So you don't operate in some sort of static state that's very, like, here and now, and, (knocks on desk) and, and carbon, and you could put it on a scale and it'll never change. There's constant versions of you.
- SCSean Carroll
Yeah, it's kind of like a whooshing, where, you know, more and more versions of you are being created all the time. And i- it's a, it's a interesting thing because even the best-trained physicists sort of think intuitively classically. Like, "Look, here's a table, there's a bottle," right?
- JRJoe Rogan
Right. You have to.
- SCSean Carroll
And yeah, the, yeah, you have to. This is your-
- JRJoe Rogan
Red light comes, hit the brakes.
- SCSean Carroll
And this is how we evolved, right?
- JRJoe Rogan
Yes.
- SCSean Carroll
It's how our brains work, right? And th- like I said, you know, many-worlds is one respectable version of quantum mechanics. There are other respectable versions, more respectable than the textbook presentation. But they all- all the other ones somehow lean on our classical experience and the textbook version certainly does. It says, like, you're a classical person observing a quantum mechanical system and so forth. And Everett, when he was a graduate student, you know, he was- he had arguments across, uh, the ocean with people in Copenhagen, you know, who tried to push their way forward. And he's like, "W- who, why do you get to be classical and the electron has to be quantum?"
- JRJoe Rogan
Mmm.
- SCSean Carroll
Like, "Why aren't you quantum?" Like, "Why isn't everything... What's so special about you, really?"
- JRJoe Rogan
Right.
- SCSean Carroll
And he was trying to think of the quantum mechanics of the whole universe, right, where he was not a separate observer outside, because he's doing the whole universe all at once. And so everything had to be quantum. And I think that that's another thing that is pushing us to appreciate the foundations of quantum mechanics a little bit more, is that we're trying to understand quantum gravity, we're trying to understand quantum cosmology, the universe all at once obeying the rules of quantum mechanics, and the conventional Copenhagen theory is just not up to it.
- JRJoe Rogan
When I was reading it, I was thinking... A- a thought came across my mind that it's almost like the human brain is a radio that's picking up a distant signal, but getting better and better at tuning into it all the time, and that we are thinking of ourselves in this very limited, primitive, biological way, because that's how we evolved. But slowly but surely, through people like you and through work on this stuff, we're gaining this more comprehensive view of what reality is itself, and that we're experiencing these stages of comprehension. And that- that's why, I mean, uh, you know, again, going off of what you're saying about your being, uh, potentially discouraged from pursuing these things, that's why this is so important, is like for most people, like myself, who don't have a background in this at all, the- the signal is so distant. But it seems like the more you folks study it and the- the clo- the more, the Large Hadron Collider and CERN and more of these experiments get done, the closer we get to just a better signal, just a little bit better signal. And we might be talking about generations from now.
- SCSean Carroll
Right, exactly. Yeah, but no, I like the analogy very much, because the human brain did not evolve to understand quantum mechanics, right?
- JRJoe Rogan
Right.
- SCSean Carroll
It didn't understand, didn't evolve to understand science at all. Like, we're, uh, and, uh, some... My best friends are human beings, but we are wonderful bundles of impulses and heuristics and shortcuts and ways to rationalize our behavior and stuff like that. And the idea that we can aspire to be logical and to develop theories and reject them and to develop theories that are very, very far away from our everyday experience, uh, is a relative latecomer on the evolutionary scene, and we're still not really good at it. We're getting better at it. And th- this is part of it, you know? Quantum mechanics is the biggest challenge that we have in physics to our intuitive understanding of the world, and so there's a question, how should we try to understand it? How much of it should we lean on our intuitive understanding and how much should we just accept that the world is fundamentally super-duper different? And I think that's a perfectly good question. I'm not trying to prejudice the answer one way or the other. I mean, our experience is limited, but it's all we have, right? You know?
- JRJoe Rogan
Yes.
- SCSean Carroll
We have to, we have to be based on that. And so some people wonder, is quantum mechanics just impossible to understand? Like, is the human brain not up to the task?
- JRJoe Rogan
The current human brain, maybe.
- SCSean Carroll
The current human brain, sure. But I think that, no, I think that it, that's totally wrong. I think that, number one, quantum mechanics certainly is very understandable. And number two, I don't think that anything about nature is impossible to understand for the current human brain. I mean, maybe it is. There's no way of knowing for sure, but there's zero evidence that we will fail in our ambition to try to understand the universe. It's just hard and it takes time. Look, a hundred years ago, we didn't have quantum mechanics at all. Like, we've made enormous progress, and a hundred years is nothing, even in, uh, human life- human, uh, history, much less cosmological history. So don't be impatient.
- JRJoe Rogan
(laughs)
- SCSean Carroll
Take time.
- JRJoe Rogan
But it just seems to me that, that y- the- the human understanding of the world we live in is obviously radically changed over the last 500 years. And if we continue to exist in this current state or a slightly better state as things move on, it's going to get better. But quantum mechanics and quantum theory, to me, almost seems like an ant trying to understand the choices on Netflix. It's like those choices exist, but the- the ant really lacks all tool... I mean, without people like you-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... espici- especially describing the computations and what's been done and what, what we currently understand, for a regular person with no background or even, uh, no knowledge of it, no, no, no one's ever explained it to them at all-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... it's almost outside of the realm of our capacity for reasoning.
- 45:00 – 1:00:00
The idea that there's…
- SCSean Carroll
our integrity. And I think that how that whole story works and fits together is something that physics doesn't understand very well, but will be important going forward.
- JRJoe Rogan
The idea that there's an, an enormous number of you making various choices-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... and that these various choices will ultimately affect how long you exist.
- SCSean Carroll
In some branches. So there is a weird thing called quantum immortality, which I think is a bad idea, and I don't like to talk about it. But people hear about it, so I, I sometimes need to mention it. Max Tegmark, who is a friend of mine and a very smart guy, popularized this idea. He said, "Look, what if..." And it's a little bit macabre. Sorry about this. A little bit, uh, you know, weird, the experiment. But imagine you're doing, you're playing quantum Russian roulette. So you have your universe-splitter, okay? You have your app on your iPhone, and you split the universe.
- JRJoe Rogan
Mmm.
- SCSean Carroll
And if it goes one way, you don't do anything. If it goes the other way, uh, faster than you can react, a machine is activated that kills you instantly, okay? So you, you don't even know it. You don't even perceive it. You don't have any pain. You're just instantly dead. Um, and you do this over and over and over and over again. So in most branches of the wave function, you're dead. But in those, you're dead. You don't know anything. You don't, you don't feel like you're dead, you know. There's no ex- regret after the fact. The only version of you that survives is the one that was lucky enough to be in the branch where you didn't die every single time. So Tegmark's argument was that if you do this over and over again, and you survive, you should take that as good evidence that the many-worlds interpretation of quantum mechanics is correct (laughs) because-
- JRJoe Rogan
(laughs)
- SCSean Carroll
... in other versions, you probably just died, right? Um, I don't think that's quite right. I don't think that's a good way to go through your life. I think that the reason why we don't want to die is not just that we will experience pain, but that sort of prospectively, right now, the idea of being dead in the future bothers me, right?
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
Like I, yeah, if someone said, you know, "You're going to die in this and that date," might be useful information, but, um, I would be sad, (laughs) right? If that date was soon. And I think the same thing is true in the quantum immortality experiment. It, I, I don't buy the move that says, "Well, in all the branches where you're dead, it doesn't matter 'cause you're dead. You don't feel anything." Like, I think that right now, it's okay for me to be bothered by the prospect that in many future worlds, I will not be there. So I, you know, I think that at the end of the day, once again, you should act in quantum mechanics just like you act in the regular world.
- JRJoe Rogan
Are there competing theories to this, that, this many-worlds theory that you've embraced and then discarded?
- SCSean Carroll
Yeah.
- JRJoe Rogan
Or, yeah?
- SCSean Carroll
Yeah, there are. So there's two big ones that are, that are quite popular. Um, one is more or less what Einstein had in mind, which are called hidden variable theories. So basically, you know, if you have, uh, an electron and you say, "Look, when I'm not looking at it, it's wave-like. When I look at it, it's like particle-like." Maybe it's both. Maybe there is a wave, and there is a particle. So in a hidden variable theory, there's a wave function just like there is in many-worlds. But there's also another set of variables saying there's really a location of the electron, right? Maybe I don't know where it is, but there really is an electron located somewhere. And that location of the electron is pushed around by the wave function, but it's a whole new part of reality. So there's not... So there, there's separate branching of the wave function and all that stuff, but that, none of that is reality. Where reality is, is where the particles are. And this is now called Bohmian mechanics. David Bohm, in the 1950s, developed the most, uh, respectable version of this. Um, it's sort of therapeutic if you don't like all the other worlds. It's basically, you know, the equations are the same as many-worlds, except there's new equations and new stuff. So it complicates the theory by adding new variables. Uh, but the good news is it says only one of the branches of the wave function is real. I don't need to worry about the other ones. The problem is it's very hard... My, my particular problem is it's very hard to-... reconcile these ideas with modern physics. Like, if you thought the world was made of individual particles, it would be, do okay. But these days, we use quantum field theory and quantum gravity and things like that, and those more modern ideas are harder to attach hidden variables to. So, hidden variables are, you know, an old idea, but, uh, they're, I think that they're hard to make work. The other idea, which is more dramatic and a little bit more fun is, every single electron has a wave function, and it seems to you that when you observe it, it collapses. But, but maybe what's really going on is the following, that there's a random probability every second that every electron will just spontaneously collapse. So it's all spread out, but its wave function just randomly loc- localizes to some particular region of space. Very, very rarely, like, if you have one electron and you wait for it to happen, it will happen like, once every 100 million years, okay? But if I have lots of electrons, like in a table, there's way more than 100 million electrons in this table. There's, there's, you know, billions and billions and billions of electrons. So somewhere in the table, all the time, an electron is localizing at one particular position, and because that electron is entangled with all the other electrons, the table maintains a location in space.
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
And this is, uh, called spontaneous collapse, or GRW theory, after the initials of the people who invented the theory. And the great thing about GRW theory is that it's experimentally distinguishable from many-worlds, because it says that if I have a collection of atoms, even if I'm not observing it, even if I'm not entangling it, one of the wave functions should c- spontaneously localize occasionally, and that will heat it up. Energy is not conserved in this theory. So people are doing experiments to test this, so it's really, you know, legit experimental science.
- JRJoe Rogan
D- m- atoms. C- uh, the current perception by the general public of atoms is that it's mostly empty space.
- SCSean Carroll
Yeah. That's a bad idea.
- JRJoe Rogan
This is not, this is not true.
- SCSean Carroll
(laughs) Well, it-
- JRJoe Rogan
Or not correct, or not-
- SCSean Carroll
It's certainly not what many-worlds says.
- JRJoe Rogan
Right.
- SCSean Carroll
So this is, you know, there are two enormous problems with our current way of presenting quantum mechanics. One is the measurement problem, which is this question like, what do you mean look at it? What do you mean observe? Like, what, what, what actually happens? When does that happen? That's the measurement problem. But the other problem is what I unhelpfully call the ontology problem. (laughs) 'Cause ontology is the philosophy of being, of what is real, what is actually existing. So we just talked about hidden variable theories. So in Everett, what's real is the wave function. The wave function of the universe describes the universe exactly and, and completely. In many-world, in, uh, hidden variable theories, there's a wave function and there's also particles, so there's extra ontology, extra pieces of reality. So the question of, is the atom mostly empty space, depends on what you think is real. So the wave function of the electron fills the atom. So if you're a many-worlds person like me, you think what is real is the wave function. It fills up the atom, and the atom is not mostly empty space. The atom is the wave function, it has that size, right? You get the feeling that elec- that atoms are mostly empty space because you think that really, the electron is a point, and the wave function is just telling you where you might see it.
- JRJoe Rogan
When you measure it.
- SCSean Carroll
When, well, yes. So many-worlds says there's no such thing as where it is, there's only a probability of seeing it. Everyone knows that, but people kind of deny it. They talk as if there really is a location of the electron, even if they should know better. So people who, generally people who say that atoms are mostly empty space are just being sloppy.
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
They're just really thinking of the electron as a little tiny dot rather than a wave function. There is a, an exception to that, because there is a fourth version of quantum mechanics that is somewhat popular. I said three, I said many-worlds, hidden variables, and spontaneous collapse. There's another version that just says, "Look, the wave function has nothing to do with reality." In many-worlds, it's all of reality. In spontaneous collapse, it's all of reality, but it obeys different equations. In hidden variables, the wave function is part of reality, but there's also particles. In the other approach, which is called an epistemic approach to quantum mechanics, the wave function is just a way of talking about your personal knowledge of the world, your knowledge or lack of knowledge, your ignorance of the world. So your wave function's just a tool you use to make a prediction for what the experimental outcome is gonna be, right? And that's more or less what we teach our students, and this approach says, "Don't bother about reality." What we should concern ourselves with is the experiences of agents who make predictions and update their probability ex- expectations of the world. And so someone like that, if you ask them, you know, "How i- is an electron located in an atom?" Or, "How, is it, an atom mostly empty space?" I think if they're honest, they would say, "Don't ask th- those questions. Those are, no, we, we don't ask reality questions. We just ask what are you gonna see kinds of questions."
- JRJoe Rogan
Mm-hmm.
- 1:00:00 – 1:14:20
And a lot of-…
- SCSean Carroll
a couple other people in the philosophy department, uh, who care about this-
- JRJoe Rogan
And a lot of-
- SCSean Carroll
... but small.
- JRJoe Rogan
... folks, you were saying, get pushed into philosophy. And, uh-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... why, why is that? I mean, is it just because it's so complex that it's so esoteric, there's so many people that just... they don't... the, the, the support for it's not there but the support for philosophy is more common in mainstream?
- SCSean Carroll
Yeah. You know, there's different kinds of support. One kind of support in academia is who do you hire, right? W- what, what areas do you want? Like, a physics department will generally say, "Yeah, we should have some people doing particle physics, some people doing astrophysics, some people doing condensed matter and solid state physics." And, and then... and then it becomes hard. Do we need people doing biophysics? Do we need people doing this? And by the time they get to the foundations of quantum mechanics, there's, there's usually very little support. Um, philosophers, their, their job is being patient and clarifying difficult conceptual questions. And so they get that quantum mechanics is fertile territory for philosophy. Like, it... You know, one of the big problems in philosophy compared to science is that many of the questions that they're asking cannot be tested experimentally, right?
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
What is infinity-Well, you know, okay, (laughs) it's hard to do an experiment there.
- JRJoe Rogan
Right.
- SCSean Carroll
But it's an important question, right?
- JRJoe Rogan
Sure.
- SCSean Carroll
And so, you need patience, but also it's harder to make progress because it's easy to be trapped by your intuition, right? Like when it's just you thinking and trying to think hard and be rational and so forth, it's easy to fall into a trap of, "Well, this looks reasonable to me." And quantum mechanics doesn't look reasonable to anybody, so it's a wonderful corrective. It's a wonderful reality check when you think, "Well, reality has to be this way." And then someone can say, "Well, look at quantum mechanics, that's different than what you said."
- JRJoe Rogan
So philosophy and quantum mechanics, they sort of, they- they- they share some sort of a border?
- SCSean Carroll
Yeah. Oh, yeah, absolutely. I mean, the things... So I- I- I was always, uh, a big fan of philosophy ever since I was an undergraduate and I discovered it for the first time. But when I was an undergraduate and my favorite philosophy classes were like the philosophy of morality or p- political philosophy, right? I took philosophy of science classes, but they seemed to be kind of dry to me because they were all about how scientific theories are constructed and chosen. You know, The Structure of Scientific Revolutions is the famous book that everyone reads. Uh, people like Thomas Kuhn and Paul Feyerabend and so forth. And, okay, that's interesting, but it's sort of meta science, right? It's like how science is done, not how the world works. And it wasn't until, you know, circa 2000 that I discovered that there are philosophers of physics who are kind of really doing physics. You know, they're not asking how physics works, they're asking how the world works, but they're asking in a way that is comfortably, uh, located in philosophy departments and right now not so much in physics departments.
- JRJoe Rogan
There was a part of the book that- that shocked me because I had a ridiculous idea once, and this idea was not my idea. Apparently Laplace had a very similar idea as a thought experiment. I had an idea once that if one day there was a computer that was so powerful that it could accurately describe every single object on Earth that we would be able to figure out the past.
- SCSean Carroll
Yeah.
- JRJoe Rogan
And Laplace was saying that not only that, but he proposed for the entire universe.
- SCSean Carroll
Right. (laughs)
- JRJoe Rogan
Like every single object, electron, everything in the- an atom in the entire universe, that you would not only be able to show the past, but also predict the future.
- SCSean Carroll
That's right. So this is called Laplace's demon, although he never called it that. Pierre-Simon Laplace, uh, was a brilliant guy. He, he deserves to be much more well-known. So I base- I think I've mentioned him- his name in every book that I've ever written for totally different reasons. Uh, he helped invent probability as we currently understand it-
- JRJoe Rogan
Hmm.
- SCSean Carroll
... for example. Um, but yeah, so Isaac Newton came up with the rules of classical mechanics in the 1600s, but it wasn't until Laplace around the year 1800 that this implication of classical mechanics was realized. It's a clockwork universe, that the way classical mechanics works is if you tell me the state of a system right now at one moment, by which in classical mechanics you would mean the position and the velocity of every part, and you knew the laws of physics, and you had arbitrarily large computational capacity, Laplace said a vast intelligence, okay? Then to that vast intelligence, the past and future would be as determined and known as the present was because that's the clockwork universe. It's deterministic. Everything is fixed once you know the present moment. Now, quantum mechanics comes along and throws a spanner into the works a little bit. If you're a many worlds person, Laplace's demon is still possible. So if you know the wave function of the universe exactly and you have infinite calculational capacity, you could predict the past and the future with perfect accuracy, but what you're predicting is all of the branches of the wave function. So any individual person inside the wave function still experiences apparently random events, right? So you can't predict what will happen to you even if you can predict what will happen to the entire universe.
- JRJoe Rogan
Whoo!
- SCSean Carroll
Yeah.
- JRJoe Rogan
Sean Carroll.
- SCSean Carroll
(laughs)
- JRJoe Rogan
My goodness, there's a lot of people pausing this podcast right now just shaking their head like...
- SCSean Carroll
You know, I wrote a little article that just appeared in Quanta Magazine, which by the way, if anyone here is a- is a science fan, Quanta Magazine is the best, um, online magazine for science these days. They have really, really good high level ar- um, articles about all sorts of things. And so I wrote an article called What is Probability 'cause, you know, again, this is a philosopher's kind of question.
- JRJoe Rogan
Yeah.
Episode duration: 1:30:55
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