EVERY SPOKEN WORD
150 min read · 30,069 words- 0:04 – 1:07
Rogan tackles Carroll’s quantum book: rewarding, but not breezy
- 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.
- 1:07 – 4:36
Quantum mechanics works incredibly well—yet meaning remains a “black box”
- 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.
- JRJoe Rogan
... describe RAM and processors and-
- SCSean Carroll
Probably electrons moving around-
- JRJoe Rogan
Yeah.
- SCSean Carroll
... in there, right? But, uh-
- JRJoe Rogan
Yeah.
- SCSean Carroll
... yeah, and I think, and s- in some sense, that's fine. Like, most of us don't need to know what's going on inside the smartphone to use it, but somebody (laughs) should know, right?
- JRJoe Rogan
Yes.
- SCSean Carroll
And my argument in the book is, look, th- the, if 500 years from now when historians write the history of 20th century physics, they will say two things. Uh, one is, "My God, these people were so brilliant and creative to invent quantum mechanics, and then they were so afraid to really take it seriously and try to understand it." Like, they said like, "Stop asking questions about the meaning of reality and what the world is doing." In my mind, what physics is all about is understanding reality and what the world is doing. It's not just about making predictions. Making predictions is good, but we do that, uh, you know, mostly because we're curious about what the world is doing.
- 4:36 – 7:29
Why foundations were discouraged—and why interest is returning
- JRJoe Rogan
Well, for people outside the world of academia, when I read someone like you saying that you were discouraged from pursuing this, and y- you were literally told that you should be pursuing your work in cosmology and gravitation is, that's where it's at.
- SCSean Carroll
That's serious work, yeah.
- JRJoe Rogan
Yeah. Th- that seems to me s- be so crazy. It's like, if anybody should be pursuing it, it should be people like you.
- SCSean Carroll
You know, I mean, I wanna be fair. So of course, 20th century physics was incredibly successful and, and there was... Part of the attitude was, look, we have to understand nuclear physics and particle physics. And, you know, a lot of it was the center of physics shifted from Europe to the US, and, and Europe is much more philosophical and, and, you know, willing to think about the deep ideas. And Americans are pretty pragmatic and wanna build things, right?
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
In particular, at the time, they wanted to build nuclear weapons. And so the idea of just really putting aside deep philosophical issues and putting stuff to work was attractive. And it- and the other issue is, you know, okay, let's say we do demand that we understand quantum mechanics better. H- how do you do it? Like, what experiment is it there that you can do? As, as-
- JRJoe Rogan
Right.
- SCSean Carroll
... far as we know, the cookbook that we have, even though we don't understand it, works pretty well. Like, what, what could you type into your smartphone that would help you understand what's going on inside? It's kind of hard to figure out. So I think that those attitudes were wrong, but at least, you know, they're not completely crazy. It's not just that they, uh, were afraid of the truth or anything like that. And I also think that it is finally changing now. I think that there's slowly, slowly, slowly more people are appreciating that understanding quantum mechanics is important.
- JRJoe Rogan
What, what do you attribute that to?
- SCSean Carroll
Um, a couple of things. One is (laughs) I mean, there's good news and bad news. Uh, part of the good news is technology has gotten better, so we're trying to build quantum computers, for example. And guess what? You know, some of the ad hoc rules that we had for doing quantum mechanics might not be up to the task. We need to understand the details a little bit better. Um, the other, sadder thing is that s- so much of fundamental physics is kind of stuck right now, right? We haven't (clears throat) ... we literally have not been surprised by a new experimental result in fundamental physics since the 1970s.
- JRJoe Rogan
Whoa.
- SCSean Carroll
Uh, the one... there's one exception to that, which is the universe accelerating in 1998, which was the dark energy. Um, we've had amazing accomplishments in experimental and observational physics. We found the Higgs boson. We found the top quark. We found gravitational waves, the microwave background, many, many things. But they were all predicted decades ago, right?
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
So progress is driven by being surprised, and it's been a long time since we've been surprised, so some people, including myself, say well, one of the things to do in that situation is to take a step back and reexamine the foundations. Maybe, maybe we can take a broader look and think that we're walking down the wrong path.
- 7:29 – 11:02
Quantum woo and pop distortions: the ‘What the Bleep?!’ cautionary tale
- JRJoe Rogan
Now, for people that don't have any background in physics, there, there's a, uh, a bit of an issue with, uh, public perception. And one of the things about public perception is films like What The Bleep?!
- SCSean Carroll
Yeah. (laughs)
- JRJoe Rogan
That sort of throw this-
- SCSean Carroll
(laughs)
- JRJoe Rogan
... sort of cultish monkey wrench into the, you know... W- quantum physics is weird enough as it is-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... without adding th- that movie was l- literally created by a channeler, right? I mean, wasn't-
- SCSean Carroll
A friend of mine, uh, David Albert, who is one of the leading philosophers of physics... And I should also give credit to philosophers here, because they have been taking quantum mechanics seriously longer than the physicists have, to be honest. Um, so David is one of many people who got a PhD in physics and then switched to philosophy because he cared about the foundations of quantum mechanics and no physics department would ever hire him, right?
- JRJoe Rogan
That's hilarious.
- SCSean Carroll
And, uh, yeah, I tell the story-
- JRJoe Rogan
He had to go in through the back door.
- SCSean Carroll
Yeah, I tell the story in the book, like, he wrote a bunch of influential papers as a graduate student, and then he went and said, uh, "I would like to, you know, make these papers my PhD thesis." And they said, "No, that's not really serious physics," and they punished him by making him write this incredibly technical mathematical paper on quantum field theory just to prove he could do it, and then he's like, "This... I don't wanna take this anymore. I'm switching fields." But anyway, he was in that film. He was in What The Bleep?! And they lied to him. They misrepresented themselves. They said, "We're doing a documentary about quantum mechanics," and they sat him down for three hours and asked him all these questions, you know, leading questions like, "Doesn't this mean that we're bringing reality into existence by looking at it?" And, and he's like, "No. That's not what it means. Let me explain to you." And then in the final film, there's like 30-second clips of him going, "Yes, that is a really important question." (laughs) Right?
- JRJoe Rogan
Right. Mm-hmm.
- SCSean Carroll
Like, completely misrepresenting what he said.
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
And so he went public after that and, uh, and complained about... and... uh, about the film, and he did... In a hilarious story, there was a, uh, event, some sort of convention put on in Santa Monica by supporters of the film that they thought it would be fun to get all of the people who were in the movie, In What The Bleep Do We Know?! and get them at... you know, and talk to them and charge people money to listen to them. But these people were not affiliated with the filmmakers, so they didn't know that David Albert had been completely misrepresented in the film, so they invited him. And he goes to this event in Santa Monica, and he gave a talk. You know, he, he decided... you know, he wondered, like, "Should I just go at all? But okay, I'll... why not? Let's reach a different audience." And he, he gave a talk, and he said, "Look, there's two things you can do when you are faced with fundamental puzzles of reality. One is you can face up to what the world is trying to tell you, and you can accept it and take it as what it is no matter what you like. The other is you can choose to tell a flattering story about yourself."
- JRJoe Rogan
(laughs)
- SCSean Carroll
"And the people who made this movie have decided that the mysteries of quantum mechanics are really stories about how they are powerful and have influence over reality and so forth, but it's all nonsense." And the punchline is the audience loved it.
- JRJoe Rogan
Ah.
- SCSean Carroll
They went nuts because what they wanted was a guru of some sort, and like-
- JRJoe Rogan
Right.
- SCSean Carroll
... he was just as good a guru (laughs) as anybody else.
- JRJoe Rogan
Right, right.
- SCSean Carroll
So, you know, he had a better story to tell.
- JRJoe Rogan
A reality-based guru.
- SCSean Carroll
Reality guru, yeah. Yeah.
- JRJoe Rogan
Yeah.
- SCSean Carroll
Yeah. Yeah, so I think you're right. I mean, I think that quantum mechanics, I've said before, is, uh... of all the theories in the history of science, the most easily distorted and misrepresented in the popular mind.
- JRJoe Rogan
Well, you've done an amazing job in this book of trying to boil it down for dummies like me, but (laughs) it's hard. It is, it is a complicated and insanely nuanced subject.
- SCSean Carroll
Yeah. (laughs)
- 11:02 – 13:57
Quantum mechanics primer: wave functions and the measurement problem
- JRJoe Rogan
And it's, it's one of those things where it, it... th- like, this many-worlds theory. Uh, j- just for one example, the, the... just the possibility that there is... like, explain that.
- SCSean Carroll
Sure.
- JRJoe Rogan
Ex- explain... for, for people that don't understand what quantum mechanics even means, give them just, like, a little bit of that, and then-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... explain many-worlds theory.
- SCSean Carroll
Yeah. Uh, good. This is what I'm here to do, so-
- JRJoe Rogan
Thank you.
- SCSean Carroll
... you know, an electron. Take an electron. Um, quantum mechanics should apply to the entire universe, but it becomes unmistakable when you look at little tiny things, right? So we always are talking about electrons or atoms and so forth. An electron has a position, and... well, sorry. Let me not even say that. That... even that was wrong. It's just so hard to correctly talk about quantum mechanics, right?
- JRJoe Rogan
Right.
- SCSean Carroll
If you were Isaac Newton, before there was quantum mechanics, there was classical mechanics, and they... basically, quantum mechanics and classical mechanics are the only two big frameworks that have ever existed in physics. You know, classical mechanics was so good that everyone thought that was just right, and it was all a matter of filling in the details until quantum mechanics came along and changed things. In classical mechanics, an electron is a point. It has a position, a location in space, and it has a velocity. It's moving somewhere. And from that, you can predict what's gonna happen. Okay? Quantum mechanics says, "No, no, no."... the electron has a wave function. So i- there's a wave, you know, sometimes you hear this debate about, are, are things like electrons and photons particles or waves? The answer is that they are waves (laughs) . And the wave function has this weird property that when you're not looking at it, it's a wave, it's all spread out or it's localized somewhere, but it obeys an equation, the Schrodinger equation, so far so good, just like regular physics. There's a thing, the wave function, it obeys an equation, the Schrodinger equation, you can predict what's gonna happen next. But the weird thing about quantum mechanics is that there's a whole separate set of rules for what happens when you look at the thing, when you observe it, when you measure it.
- JRJoe Rogan
That's where things get squirrelly-
- SCSean Carroll
Yes.
- JRJoe Rogan
... with people describing it, right?
- SCSean Carroll
Yes.
- JRJoe Rogan
And that's where they wanna go woo-woo on you.
- SCSean Carroll
It's an opening to be woo-woo, right?
- JRJoe Rogan
Yes.
- SCSean Carroll
When you say, like, "What do you mean (laughs) -"
- JRJoe Rogan
Right.
- SCSean Carroll
"... observe something?" Like, does it have to be a conscious being? Could it be a video camera? You know, that's just weird, right?
- JRJoe Rogan
Right. Right. Is it the act of measuring that changes things?
- SCSean Carroll
Well, this is the puzzle, okay? This is what is called the measurement problem of quantum mechanics, that the rules we teach our students at Caltech or anywhere else when we teach them quantum mechanics in their sophomore year of college, uh, the rules say when a system is observed, when it is measured, its state, its wave function changes dramatically, suddenly, and unpredictably.
- JRJoe Rogan
Now, let me ask you this. How do we know this based on if, if you're measuring it and it changes, how do we know? Because we didn't measure it before. Like, what, what observations are we making that we understand the state of it before it's measured without measuring it?
- 13:57 – 18:09
Concrete examples: spin, superposition, and why measurement changes outcomes
- SCSean Carroll
Good. There's a couple of ways. So, um, let, let, let me make things even simpler. Forget about where the electron is located and think about the electron as spinning, right?
- JRJoe Rogan
Okay.
- SCSean Carroll
The electron is spinning just like the Earth spins. It's really exactly like that. It's like a little spinning top, except when you measure the spin, you can sort of send the electron through a magnetic field and it will get deflected either up or down, depending on whether it's spinning spin up or spin down. You only ever get one of two answers. It's either going up or going down. It's nowhere in between. This is an empirical measured fact, okay? Um, so that's a part of quantum mechanics. That's, that's the quantum fact, that there's discrete set of possible answers to this question. Is it spinning clockwise or counterclockwise? Yes or no. It's just those two possibilities, nowhere in between. So if you're... if you have a magnetic field that is oriented vertically, send your electron through it, it gets deflected up. You say, "Oh, it's spin up." So now I've measured its spin. Now I know what its state is. If I send it through another magnetic field or- oriented vertically, it will always be deflected up every 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.
- 18:09 – 23:27
From Einstein’s unease to Everett’s solution: many-worlds in plain terms
- 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)
- 23:27 – 45:09
Living with many worlds: branching, identity, and the Universe Splitter app
- 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-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... the refrigerator-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... do you think of yourself as this quantum being that's e- existing in this super state?
- SCSean Carroll
So, I mean, there's a couple answers to that. One is, you know, sure. If I think about it, like I really do believe it. You know, I have a chapter in the book, which my editor resisted at, at, at first, but then he let me get away with it, which is a dialogue between, uh, a young philosopher and her father, who is a physicist. And the father is skeptical about all this philosophical nonsense, and she tries to explain how many worlds works to him. And at the end, you know, his last question is, you know, "Do you really believe this?" (laughs) He says, "You feeling taking this seriously?" And look, that's a perfectly atti- good question. It's a very respectable question because it is... Many worlds, it's not crazy or weird or bizarre, but it's certainly very, very far away from our everyday experience, right?
- JRJoe Rogan
Right.
- SCSean Carroll
So what it's asking you to do is to say, I have these equations.... they are really, really good at fitting what I do observe in the world and making predictions. You know, I can build the Large Hadron Collider, et cetera. Um, I will take them seriously even for things that I can't directly observe, 'cause they're the best equations I have, right? Until a better set of equations come along, I will believe these equations. And the implication of that is, yeah, there's a whole bunch of worlds, like a huge number, like a real, you know, gi- humongously, unimaginably big number, maybe an infinite number, maybe finite, we don't know, of different copies of you and they're being created all the time. Uh, the good news is that it doesn't really affect how you go through life. It doesn't really, um, imply that you should behave any differently than you would if you just lived in one world.
- JRJoe Rogan
But do you think of each choice that you make possibly changing everything about the world that you exist in? Do... how... are... are you... how are you looking at it?
- SCSean Carroll
(laughs) It's-
- JRJoe Rogan
You know what I'm saying?
- SCSean Carroll
So, I do.
- JRJoe Rogan
Because you... you are a guy who probably understands this as good as anybody that's alive. So as weird as this stuff sounds, to... to me, it sounds like an... I mean, it's almost impossible for me to comprehend. So I'm trying to filter it through-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... your understanding of it.
- SCSean Carroll
Well, I think that-
- JRJoe Rogan
He's taking his jacket off. We're getting serious here.
- SCSean Carroll
I know. It's getting hot in here.
- JRJoe Rogan
(laughs)
- SCSean Carroll
Physics is heating us up. Um, yeah. I'm not... I'm not exactly sure how to say it the best. You know, um, it... it doesn't... it... it doesn't change who you are. It's certainly not true that you making a decision is what branches the wave function of the universe. I guess that's the right thing to say.
- JRJoe Rogan
Good, 'cause I wanna stop all woo-
- SCSean Carroll
Yeah, you can't do that.
- JRJoe Rogan
... before it happens.
- SCSean Carroll
Everyone, you know, believe me-
- JRJoe Rogan
Hit the brakes.
- SCSean Carroll
The joke about how certain political choices imply that we're living in the wrong branch of the wave function has been made many, many times, right?
- JRJoe Rogan
Oh, okay.
- SCSean Carroll
But, uh, you... the... it's not that your choices create different universes. Different universes get created, and maybe you're different in them by a little bit. In fact, I... you know, I like to point out there is an app you can download if you have an iPhone called Universe Splitter, which will, uh, branch the wave function of the universe for you. And then if you agree ahead of time to do one thing in one branch and another thing in another branch, then there will be multiple copies of you who are living different lives. Then you can deal with that and your therapist however you like. But, uh-
- JRJoe Rogan
But what is the application exactly doing?
- SCSean Carroll
What it's doing is, uh, basically a version of measuring the spin of-
- 45:09 – 47:25
Extreme implication (and why to reject it): ‘quantum immortality’
- 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.
- 47:25 – 50:35
Competing interpretations: Bohmian hidden variables, GRW collapse, and epistemic views
- 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.
- 50:35 – 55:30
What is ‘real’ in quantum theory: atoms, empty space, and physicists’ sloppiness
- 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.
- SCSean Carroll
But I think that some of the less honest ones will say, "Sure, an atom is mostly empty space 'cause an electron has a location somewhere, we just don't know what it is."
- JRJoe Rogan
Why do they approach it in this, what you, uh, the way you're describing it, a sloppy way? Why, why do you think that is so common?
- SCSean Carroll
Well, you know, it is part of, um, the attitude that physicis- physicists have adopted, that we use quantum mechanics but we don't try very hard to understand it. So y- you know, you can talk to plenty of physicists on the street and they will tell you to your face that understanding reality is not their job, and I, I think that's terrible, but it, but-
- JRJoe Rogan
Right.
- SCSean Carroll
... they will say it. And so when you press them too much on questions like, you know, "Is the atom mostly empty space? You know, what happens when you make an observation?" They just kind of get uncomfortable and say, "No, you're asking the wrong questions. Let's ask questions about-"... what will we see at the Large Hadron Collider if we smash protons together, right? And those are perfectly good questions too, but I think that the what's really going on questions are also interesting. So because they don't care about these questions, they will often be sloppy in answering them, right? They don't... y- It is hard. Like, like, like you said, it's, it's hard when you read the book, it's hard when you write the book, it's hard when you think about these things as a professional physicist. It's, it's not natural, it's not easy, it's not intuitive. So even if you're a super-duper expert at solving the equations and making predictions, understanding what's going on is a whole another activity that a lot of physicists don't try very hard to do.
- 55:30 – 1:14:20
How disagreements get resolved: probability, Born rule, Bohr’s influence, and physics being stuck
- JRJoe Rogan
Now, how was all this stuff v-verified or argued? Like say, uh, here come... y- you're sitting down, you're having a conversation with someone who, uh, espouses a competing theory, how are you guys working this out?
- SCSean Carroll
Good. I think that, um, i- if everything were going along really, really well, we would be making experimental predictions and testing them. But I think the theorists have sort of dropped the ball here, um, in the sense that the theoretical physicists should have, since the 1930s, been developing these alternatives like many-worlds hidden variables, whatever, and make... using them to make predictions. Um, but we really haven't. They were neglected. They were backwater. There are few people, a few plucky souls, who, uh, really put their efforts into understanding these, many of them got pushed out into philosophy departments, um, but that's what we need to do. We need to, like, catch up on the last 70 years of lost time and work out what the implications are of these ideas. So it's in... The ball, I think, is in theorist's court. The experimenters are working hard. Experimenters are doing amazing things with lasers and, um, atoms and, and learning about how to manipulate quantum systems at a delicate level, but the theorists have not given them sharp experimental questions that they... that would really illuminate the foundations of quantum mechanics. So honestly what it is is a bunch of people get around the table and talk to each other. (laughs) And they're like, "Oh, I think that what happens when the wave function branches is this." So a typical question we'll try to address is... In ordinary quantum mechanics, we say, if I send the electron through one way or I send it through the elec- the other way, there's a 50/50 chance that I will see it go left or go right. And someone says, "What do you mean 50/50 chance?" Especially in many-worlds where there's a 100% chance there'll be a world where it goes left and a world where it goes right. How, what, what is the meaning of the phrase there's a 50/50 chance? What is the nature of probability in this game where everything is perfectly deterministic, right?
- JRJoe Rogan
Mm-hmm.
- SCSean Carroll
So that's not the kinda question that you a-answer very easily by doing an experiment. You have to think about it.
- JRJoe Rogan
Right.
- SCSean Carroll
So that's what... the kinda thing that we argue about.
- JRJoe Rogan
Now how often do you guys get together?
- SCSean Carroll
Um, yeah, you know, it happens. Uh, there's conferences. It's a small community. Someone asked me just the other day, 'cause that... you know, the book came out, Something Deeply Hidden, last week and I'm... uh, been on book tour. So I was on a... I was being interviewed and someone said, "How many people do you think in the world are... would classify themselves as working on the foundations of quantum mechanics?" Um, maybe 100, something like that, not a very large number. Like, if you say how many people would classify themselves as particle physicists, there'll be tens of thousands of people.
- JRJoe Rogan
I remember there's a woman who came to The Comedy Store the... after the last podcast that we did and she apparently is also working on it, and she was trying to explain it to me, her version of it, you know, after hearing your version of it. It was very similar, but I believe she was from Romania so she was struggling a little bit with English.
- SCSean Carroll
Ah, okay.
- JRJoe Rogan
But she was so excited to discuss it. It's so fascinating when you see someone who's... Like, for the limited number of how many of you guys there are, and gals, there are out there, I mean, whatever the number is, when that spark gets ignited and other people start tuning into it. She was so excited that this was being discussed on a podcast and she wanted to talk to me about it-
- SCSean Carroll
Yeah.
- JRJoe Rogan
... to say, you know, "Please have more people on. Please talk about this more." You know, "We need support. We need, you know..." It's...
- SCSean Carroll
It, it is... It's... It does baffle me a little bit how difficult it is swimming uphill, um, to get more support for this kind of thing because it is just an enormous privilege to be able to call your job thinking about the fundamental nature of reality, right?
- JRJoe Rogan
Yeah.
- SCSean Carroll
Like, you know, I gave... You know, my first book tour talk was last Tuesday and I had dinner the night before with, you know, several philosophers of physics in the New York area, if, you know... from Columbia and NYU, and every... And, and, you know, we're all friends and we could talk about, you know, our cats and our cars but every single word discussed at the table all night long was about the philosophy of physics. (laughs)
- JRJoe Rogan
Is it because you guys work in isolation essentially and then when you get together you just... you're so pumped up to be discussing these things-
- SCSean Carroll
In part, yeah.
- JRJoe Rogan
... with like-minded souls?
- SCSean Carroll
You know, in, in part, yeah. I, I mean, I... there's no one else a- at... in the physics department at Caltech who cares about these issues. I mean, some of them care in the sense that they are happy that I'm doing it, but no one does it really-
- JRJoe Rogan
But it's just you?
- SCSean Carroll
... themselves. Yeah. Well, there's 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.
Episode duration: 1:30:55
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