Sean Carroll Explains Quantum Immortality

The J.R.O.O.G. experience. The idea that there's an enormous number of you making various choices. Yeah. And that these various choices will ultimately affect how long you exist. 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, sometimes need to mention it. Max Tegmark, who is a friend of mine, a very smart guy, popularized this idea. He said, look, what it, and it's a little bit macabre, sorry about this, a little bit 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 and if it goes one way, you don't do anything. If it goes the other way, faster than you can react, a machine is activated that kills you instantly. Okay? So you don't even know it. You don't even perceive it. You don't have any pain. You're just instantly dead. And you do this over and 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 feel like you're dead. You know, there's no 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 because in other versions you probably just died, right? I don't think that's quite right. I don't think it'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? Like if someone said, you know, you're going to die in this, in that date, might be useful information, but I'd be sad, right? That date was soon. And I think the same thing is true in the quantum immortality experiment. I don't buy the move that says, well, in all the branches where you're dead, it doesn't matter because 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 think that at the end of the day, once again, you should act in quantum mechanics just like you act in the regular world. Are there competing theories to this, that this many worlds theory that you've embraced and then discarded? Yeah. There's two big ones that are quite popular. One is more or less what Einstein had in mind, which are called hidden variable theories. So basically, you know, if you have 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's separate branching of the wave function and all that stuff, but 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 respectable version of this. 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. 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 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 I think that they're hard to make work. The other idea, which is more dramatic, 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 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 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 hundred million years. But if I have lots of electrons, like in a table, there's way more than a hundred million electrons in this table. There's 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. And this is 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 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. Adam's, the current perception by the general public of atoms is that it's mostly empty space. Yeah, that's an idea. This is not true. Well, or not correct or not. It's certainly not what many worlds says. 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 actually happens? When does that happen? That's the measurement problem. But the other problem is what I unhelpfully call the ontology problem. Because 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 completely. In many world, in 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 and has that size, right? You get the feeling 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. When you measure it. Well, yes. So many worlds says there's no such things where it is. There's only a probability of seeing it. Everyone knows that, but people kind of die. 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. They're just really thinking of the electron as a little tiny dot rather than a wave function. There is an exception to that because there is a fourth version of quantum mechanics that is somewhat popular. I said three. 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 or ignorance of the world. So your wave function is just a tool you use to make a prediction for what the experimental outcome is going to be. That's more or less what we teach our students. 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 expectations of the world. So someone like that, if you ask them, is an electron located in an atom or is it an atom mostly empty space? I think if they're honest, they would say, don't ask those questions. We don't ask reality questions. We just ask, what are you going to see? Kinds of questions. But I think that some of the less honest ones will say, sure, an atom is mostly empty space because an electron has location somewhere. We just don't know what it is. Why do they approach it in this, what you, the way you're describing it, a sloppy way? Why do you think that is so common? Well, you know, it is part of the attitude that physicists have adopted that we use quantum mechanics, but we don't try very hard to understand it. So 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 think that's terrible, but 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, it is hard. Like you said, 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 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 nother activity that a lot of physicists don't try very hard to do. Now, how was all this stuff verified or argued? Like say if you're sitting down, you're having a conversation with someone who espouses a competing theory. How are you guys working this out? Good. I think that 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 in the sense that the theoretical physicists should have since the 1930s been developing these alternatives, like many worlds, hidden variables, whatever, and using them to make predictions. But we really haven't. They were neglected. They were backwaters. There were a few people, a few plucky souls who really put their efforts into understanding these. Many of them got pushed out into philosophy departments. 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. The ball I think is in the theorist's court. The experimenters are working hard. Experimenters are doing amazing things with lasers and atoms and learning about how to manipulate quantum systems at a delicate level. But the theorists have not given them sharp experimental questions that would really illuminate the foundations of quantum mechanics. So honestly what it is is a bunch of people get around a table and talk to each other. They're like, all right, I think that what happens when the wave function branches is this. And 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 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. 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? So that's not the kind of question that you answer very easily by doing an experiment. You have to think about it. So that's the kind of thing that we argue about. How often do you guys get together? Yeah, you know, it happens. There's conferences. It's a small community. Someone asked me just the other day because the book came out, something deeply hidden last week and I've been on book tour. So I was being interviewed and someone said, how many people do you think in the world would classify themselves as working on the foundations of quantum mechanics? Maybe 100, something like that, not a very large number. Like if you say how many people would classify themselves as particle physicists, it would be tens of thousands.