Sean Carroll Explains Quantum Immortality

the gyro can 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 so I sometimes need to mention it max tegmark who is a friend of mine a very smart guy popularized this idea he said look what and it's a little bit macabre about this a little bit 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 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 you don't even know it you don't even perceive it you don't have any pain you 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 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 tag marks 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's sort of prospectively right now the idea of being dead in the future bothers me right like I if someone said you know you're going to die in this and that date might be useful information but I'd be sad if 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 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 are there competing theories to this that this many worlds theory that you've embraced and then discarded yeah yeah yeah they're a big ones that are 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 its 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 that 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 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 wavefunction is real I don't need to worry about the other ones the problem is it's very hard my particular problems 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 ok but these days we use quantum field theory in 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 they're 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 wavefunction 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 wavefunction 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 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 of 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 hmm 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 but 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 atoms the current perception by the general public of atoms is that it's mostly empty space there's no idea this is not true well we're not correct or not it's certainly not what mini-world says right 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 as 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 wavefunction it fills up the atom and the atom is not mostly empty space the atom is the wavefunction it has that size right you get the feeling that elect that atoms are mostly empty space because you think that really the electron is a point and the wavefunction is just telling you where you might see it when you measure what 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 die 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 they're just really thinking of the electron is 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 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 a reality but it opens different equations and 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 functions 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 expectations of the world and so someone like that if you ask them you know how 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 those questions those are no we don't ask reality questions we just ask what are you gonna see kinds of questions but I think that some of the less honest ones will say sure nanum 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 a sloppy way what why do you think that is so common well you know it is part of the attitude that physicists 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 we'll be making 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 if the Large Hadron Collider if we smash protons together right and those are perfectly good questions - 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 and answering them right they don't you it is hard 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 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 here come you 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 make using them to make predictions but we really haven't they were neglected they were back waters there are 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 so it's in the ball I think is in the theorists 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 they 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 wavefunction branches is this so a typical question will try to address is in ordinary quantum mechanics we say I send the electron through one way or I send a few the elect 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 in 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 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 what the kind of thing do we argue about how often you guys get together um yeah you know it happens there's conferences it's a small community someone asked me just the other day except you know the book came out something deeply hidden last week and I'm been on book tour so I was on 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 maybe a hundred something like that not a very large number like if you say how many people would classify themselves as particle physicists do be tens of [Applause]

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Id: JJwUd53-dZ8

Length: 13min 12sec (792 seconds)

Published: Mon Sep 16 2019