Physicist Sean Carroll Explains Parallel Universes to Joe Rogan

the Joe Rogan experience you've done an amazing job in this book of trying to boil it down for dummies like me but it's hard it is it is a complicated and insanely nuanced subject yeah and it's one of those things where it like this many worlds theory did for one example the the the just the possibility that there's like explain that just explain for for people that don't understand what quantum mechanics even means give them just like a little bit of that and then explain many worlds theory yeah good this is what I'm here to do so thank you you know an electron take an electron 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 mechanics right if you were Isaac Newton before there was quantum mechanics there was classical mechanics and 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's all better filling 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 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 wavefunction so if there's a wave you know sometimes you hear this debate about are things like electrons and photons particles are waves the answer is that they are waves 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 in 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 that's where things get squirrelly with people describing it right yes and that's what it's an opening to be woowoo right we just say like what do you mean observe something like does have to be a conscious being can it be a video camera you know that's just right here right is it the act of measuring that changes things 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 the rules say when a system is observed when it is measured its state it's wavefunction changes dramatically suddenly and unpredictably 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 its measured without measuring it good there's a couple of ways so let me make things even simpler forget about where the electron is located and think about the electron is spinning right 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 deflect either up or down depending on whether it's spinning spin up or spin down you only ever get one or two answers it's either going up or going down it's nowhere in between this is an empirical measured fact okay so that's a part of quantum mechanics that 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 that's just those two possibilities no where 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 going to 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 and then once it is so okay now it's been spinning up you measured it 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 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 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 so in quantum mechanics what we say is that it's not that we don't know whether the electron is bending clockwise or counterclockwise it can be in a superposition of both that's just the spin version of the position of the electron can be spread out in a in a wave right it's it's truly not just that we are lacking some knowledge is that the knowledge really isn't there and again this is how we teach quantum mechanics and textbooks and then I'm going to correct it because many-worlds is much better 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 electrons 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 nice little image of this when I give talks a little piece of uranium so it's a radioactive 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 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 when a radioactive nucleus decays and gives off an electron what is its wavefunction going to do what is the wave function of the electron going to be and the answer is it goes off in a spherical wave if it goes off in all directions at once evenly yeah all directions evenly but you never see that is that relish ape of the piece of uranium does it very no because the electron gets from one individual nucleus of an atom right so that what they let well 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 doesn't have to we 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 so when you're in pursuit of an understanding a deeper understanding of quantum mechanics what you when you you're thinking about people from the 1900s that are just sort of basically 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 yeah that's right that we just need better tools and a better understanding better equations more time yeah me and Einstein think this right 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 the title something deeply hidden is a quote from Einstein when he was talking about when he was a kid and he had a compass right and he was giving his first magnetic compass and he could rotate it this way in 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 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 want to know everything we're not done yet there must be more going on and so many more 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 write down let's put it that way okay so hit us with this many worlds theory ok so think about this electron you're gonna you you say that it could be either spin up or spin down it's a combination of both that's its wavefunction you measure it you only ever see spin up or spin down so Copenhagen says that's because the wavefunction 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 okay many world says what you're missing is two things number one you're a quantum system you are obeying the rules of quantum mechanics you're made of atoms and electrons and so forth you have a wavefunction - okay so you're 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 something that Einstein invented namely called entanglement when quantum mechanics says there's a wavefunction for a system it doesn't say there's a separate wavefunction for every particle right it says that there's only one wavefunction 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 wavefunction and it's you know unpredictable exactly what angle is going to 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 and nups at 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 that BS like you're a physical system you obey the Schrodinger equation you you're 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 wavefunction must somehow collapse and Evert said no what you're missing is there's now two separate worlds both of those part of the wavefunction 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's all the electrons spinning clockwise and there's another version of you that's all it's spinning counterclockwise I know that's just taking seriously the prediction of quantum mechanics it's not adding any extra stuff any extra worlds anything like that that is the part where my brain broke all right the idea that there's a you that observes it going clockwise and the you that observes it going in a different direction like that is so hard to understand do 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 refrigerator do you think of yourself as this quantum being that's existing in this super state so I mean there's a couple of 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 first but then he let me get away with it which is a dialogue between a young philosopher and her father who was 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 I said you feel like taking this seriously and look that's a perfectly at a 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 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 etc I will take them seriously even for things that I can't directly observe because it 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 joy 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 the good news is that it doesn't really affect how you go through your life it doesn't really imply that you should behave any differently than you would if you just lived in one world but do you think of each choice that you make possibly changing everything about the world that you exist in how are you looking at it if you don't know saying because you know you are a guy who probably understands it as good as anybody that's alive so as weird as this stuff sounds to me it sounds like an I mean it's almost impossible for me to comprehend so I'm trying to filter it through yeah your understanding of it well I think that he's jacking off we're gonna see I know it's getting hot in here physics is heating us up yeah I'm not exactly sure how to say it the best you know it doesn't it doesn't change who you are it's certainly not true that you making a decision is what branch is the wavefunction in the universe I guess that's the right thing to say because I want to stop all whoo yeah you can't happen everyone you know believe the breaks the joke about how certain political choices imply that we're living in the wrong branch of the wavefunction has been made many many times but you that it's not that your choices create different universe different universes get created and maybe you're different in them by a little bit in fact I you know I'd like to point out there is an app you can download if you have an iPhone called universe splitter which will branch the wave function the universe for you and then if you agree ahead of time to do one thing in one branch and other thing in another branch then there will be multiple copies of you or living different lives then you can deal with that in your therapist however you like but what is the application exactly doing what it's doing is a basically a version of measuring the spin of it's called universe glitter universe splitters only for iPhones sorry people yeah sorry it's not even a webpage is only a only an app there's the equivalent web pages but okay I'm pulling it up right now yeah and what so what you can do it basically it sends a signal to a lab that coincidentally is located in Geneva Switzerland but there's nothing to do with the Higgs boson or anything like that they send a single photon down a pipe to what's called a beam splitter so the wave function of the photon goes 50/50 it gets sent left it gets sent right and if you agree and so then it sends back whether you ended up in the branch of the wavefunction where it went left or where it went there you go 199 come on yeah that's colors of changing the universe is in your hand down loaded I paid for it I got it right here yeah right so if you have any tough choices you can type in like you know I want to have pizza or I want to have Chinese food for dinner tonight well it says in one universe I will take a chance in the other one I will play it yeah but you can you can correct those you can fill in whatever you want going yeah that's the good part but what is what is happening what I will ask her to marry me I will not ask her to marry me I will accept this job I will go somewhere else equivalent of a quantum fortune cookie yeah but except that all possible fortunes in different universes the bad news is you can't ever find out how things went in the other universe I don't Cox is a different universe that's the problem right so the problem a paralyzed by analysis that's why it's you should act the same as if you just lived in one universe because you can never talk to the people and the other ones hmm so but now let's hit hit the brakes on the whoo again yeah because people would like to believe that there are I mean are there an infinite number of views existing at the exact same time making various choices which send you off in a different directions so number one we don't know if it's infinite number or just really big but there's certainly a really really big number it's big enough to be you know big enough for whatever you want but it's not everything it's not the theory does not say everything happens somewhere right the theory says the Schrodinger equation is obeyed there's an equation that is obeyed so electrons will never convert into protons because electrons are negatively charged and protons are positively charged and nowhere in the Schrodinger equation can you violate the conservation of charge right so there's plenty of things that don't happen but then there are plenty of things that do happen and some things are more likely than others for you to experience so again it's it's sort of a you know it's a mind-bending thing but it's a straightforward prediction of the equations and it doesn't affect our lives it there's no rule that says you know to be a moral person to be a good utilitarian and make the world happy knowing that the world the wavefunction is branching into multiple copies I should actually differently somehow it's it's exactly the same as it would be in the ordinary world so you and you are the ordinary world no matter how copies how many copies of you there are how many versions of you there's nothing so when all these copies are being made there's no essence of you that is traveling through one of the copies right like all these people are separate people so 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 wavefunction there'll be two different people both of whom used to be you but not the same person anymore because different things happen to them 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 that 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 yeah so you don't operate in some sort of static state that's very like here and now and and and carbon and you could put it on a scale and it will never change there's constant versions of you yeah it's kind of like a whooshing where you know more and more versions of you are being created all the time and it's a it's an interesting thing because even the best trained physicists sort of think intuitively classically like look here's a table there's a bottle right yeah you had like some to the breaks and this is how we evolved right that's how our brains work right and he's 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 the ocean with people in Copenhagen you know who tried to push their way forward and he's like why do you get to be classical and the electron has to be quantum like why aren't you quantum like why isn't everything what's so special about you really right and he was trying to think of the quantum mechanics of the whole universe right where he is 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 [Applause]

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Id: 1_ZsDBMPQMI

Length: 21min 43sec (1303 seconds)

Published: Mon Sep 16 2019