An Evening with SEAN CARROLL, Author of Something Deeply Hidden

good evening my name is Windsor Morgan I'm a professor of physics and astronomy and director of the planetarium at Dickinson College it's so can you hear me okay it's pretty tall it's great to see so many people here in Harrisburg for physics talk Carroll is a research professor of theoretical physics at the California's tutor technology Caltech he received his PhD in 1993 from Harvard University his research has focused on fundamental physics and cosmology especially issues of dark matter dark energy space time symmetries and the origin of the universe recently sean has worked on the foundations of quantum mechanics the emergence of space-time and the evolution of entropy and complexity Carroll is the author of something deeply hidden the big the big picture the particle at the end of the universe from eternity to here and space-time and space-time and geometry in introduction to general relativity he's been awarded many prizes and fellowships by the National Science Foundation NASA the sloan Foundation the American Institute of Physics the Royal Society of London and the Guggenheim Foundation Carroll has appeared on TV shows such as The Colbert Report PBS's Nova and through the wormhole with Morgan Freeman and it frequently serves as a science consultant for film and television he is hosted the weekly mindscape podcast and he lives in Los Angeles please join me in welcoming Sean Carroll [Applause] [Applause] thank you thank you everyone yeah so is this good if I am right here can you hear me even like I see there's like an upper level you're all very dark but all right you can hear me very good thank you so much for coming out tonight it's you know the book I wrote a book there it is something deeply hidden it's about quantum mechanics the world does not need another book about quantum mechanics necessarily there are plenty of books written about quantum mechanics but I think nevertheless there's room for something new to be said about quantum mechanics which is why I wrote this book and the empathy the impetus for writing this particular book basically comes from maybe a clicker would be a good idea if there if a clicker could magically appear that'd be awesome Richard Fineman who was my predecessor at Caltech where I'm a research professor of physics he is a famous physicist and he's once said these words I think I can safely say that nobody understands quantum mechanics now ordinarily I see that's bad all right I'm gonna just assume it's gonna work ordinarily I don't like to appeal thority in physics talks I like to try to explain things but what I'm trying to claim here is that even the people who understand quantum mechanics the best admit that they don't understand it now that's ok it's perfectly okay to admit that we don't understand things and it is perfectly okay to not understand them that's how physics makes progress there are things we don't understand we try to understand them better we invent theories we do experiments etc that's good the problem is that faced with the fact that we don't understand quantum mechanics physicists have decided that it's okay that we don't understand quantum mechanics that well under I'll explain to you a little bit about what quantum mechanics is it's the most important fundamental foundational theory in all of modern physics without quantum mechanics you cannot explain lasers and transistors and how this computer works you cannot explain why stars shine or why this table is solid quantum mechanics is absolutely crucial to how we think about modern physics and we don't understand it and we seem to think that that's okay when you what you would imagine given that our most important theory is one that we don't understand is that understanding quantum mechanics would be the highest priority goal in all of physics right people who worked on this idea trying to understand quantum mechanics better would be the superstars of physics the celebrity physicists they would get the highest salaries universities would fight to hire them and stuff like that and of course exactly the opposite is true if you are a student or a young faculty member and you let your colleagues know that you might be interested in trying to understand quantum mechanics you get gently nudged away from that if not just fired okay and this has happened in history I think it's changing a little bit but the analogy they like to use I just take this there we go the analogy I like to use is you know if we're at a bookstore I'm sure this somewhere in the store is a collection of Aesop's fables so you remember the fable of the Fox and the grapes the Fox sees these grapes up there the grapes look really juicy and yummy the Fox jumps up to try to get the grapes but they're too high the Fox cannot reach them so the Fox says you know what I never wanted those grapes anyway they were probably sour in this parable the role of physicists is being played by the Fox and the role of understanding quantum mechanics is being played by the grapes back in the 1920's 1930's physicists worked very hard to try to understand quantum mechanics they couldn't quite do it and they gave up roughly speaking and they decide they never wanted to understand it so these days you can meet physicists who will tell you to your face they don't want to understand quantum mechanics they don't want to understand reality and how nature works all they care about is making predictions for observations the moral of my story will be is we should aim higher than that we should aim to try to understand how the world works nope no clicker all right there we go I'm gonna click it myself good so now I want to give you a very very brief lightning fast history of quantum mechanics that is entirely inaccurate as history but I want to lead up to the conclusions that I want to draw because the conclusion that's conclusions I want to draw are extremely dramatic and you should not believe me just because I say them I want to give you the impression or at least let you know that I believe that these conclusions that I draw didn't come out of nowhere we were led to them by some logic and some experiments so the way I like to present quantum mechanics starts with atoms you've all seen this picture on the right the cartoon picture of an atom this is due to Ernest Rutherford a New Zealand physicist who moved to the UK and worked at Cambridge he was the one who figured out that an atom is not sort of a homogeneous blob it has a central mass called the nucleus and there are electrons orbiting around the nucleus so we called this the Rutherford atom we now know that the nucleus has protons and neutrons in it he didn't know that but that would come pretty soon thereafter this is a extremely helpful model this is why we still draw pictures of it but it's also completely wrong it can't be like this so if you think about 1909 when they were talking about these things we still believed in classical mechanics this is the theory handed down to us by Isaac Newton from the 1600s and classical mechanics like quantum mechanics will be after it classical mechanics is not a specific theory of this or that physical system it's a paradigm it's a framework in which you can do all sorts of different theories so classical mechanics had been so successful that literally every physicist afterward figured that classical mechanics was exactly right and the only thing we had to do was figure out which theory to put into that framework but if you have a system like this with a little electron orbiting an atomic nucleus classical mechanics predicts a disaster you know there's light coming from the ceiling from the projector and so forth all of these light rays are electromagnetic waves right they're little fluctuations in the electric and magnetic fields that fill this room and they are all caused by electrons jiggling up and down when you take a charged part goal so that's a charged particle like an electron has an electric field around it you shake it the electric field oscillates that's light okay but these little electrons in this picture are oscillating they're going around in an orbit very very quickly so they should be giving off light and if they give off light the electrons should lose energy which means they shouldn't orbit forever they should spiral into the nucleus and you can calculate how quickly that should happen and the answer is something like 10 to the minus 11 seconds that's very fast so what this predicts pretty unambiguously according to classical mechanics is that matter is completely unstable that you and me in this table and the earth and the Sun should all collapse to a point in 10 to the 11 seconds or something like that we can do the experiment no it didn't happen okay so there's something wrong about classical mechanics when it comes to atoms now there's a lot of effort going into what it could be we're gonna skip some steps here is what people came up with they said well we've had for a long time this puzzle about whether light is a wave or a particle maybe when we think we have a particle like an electron maybe there's something wavy about it maybe the electron is not a particle maybe we should think of the electron as a wave as sort of a cloud something to spread out and we since our imaginations are not very good we will call this wave the wave function and so instead of thinking of an electron as orbiting in some circle or an ellipse the nucleus of an atom there's a wave like thing the wave function and it bays an equation and you can sort of solve the equation and ask what could the wave be doing and the answer is there's a discrete set of things the wave could be doing if it has a definite energy there can be hops between these different energies but an electron with a fixed amount of energy is doing something specific it has a wave like structure that is one of these there's a whole there's literally an infinite number of possible shapes the wave function can have but there's they're discrete and as we go down from the top to the bottom there's more and more energy in that electron so there's a minimum energy thing the electron can do and it's not right on top of the nucleus to diffuse cloud that is spread out around the nucleus don't ask me how they thought of this there's a lot of steps involved okay but this was the idea maybe we could replace electrons little particles with waves that would explain why atoms are stable that's a good idea and then of course because we're physicists we want an equation and so I get to show you this equation because you're gonna need it for the quiz that I'm gonna hand out at the end of the talk this is the famous Schrodinger equation here's Erwin Schrodinger he wrote down this equation if we call the wavefunction by the letter sy that's the Greek capital letter sy then the wavefunction can change over time and has an extent over space and it solves this equation basically it says however much energy is described by the wave function tells you how rapidly it changes with time you actually don't need to know the details that was just a joke I know there's young people in the audience who might get worried okay there will be no quiz we're just here to learn things it doesn't matter the details of the equation what matters is there is an equation as soon as you hand physicists an equation and say this is what describes the system they're happy they say oh I can now solve this equation I can ask my students to solve this equation I can torture generations of future students by demanding that they solve this equation over and over again and that has in fact come true but the point is that this is around the time 1926 1927 when quantum mechanics reached its final form when we said okay good we got equations we have wave functions this is good we have quantum mechanics but there's a huge problem with this which is that this equation describes electrons as I said is like a cloud spread out as a wave function but when we look at electrons they don't look like puffy spread out clouds this is an actual image of a uranium sample so a little chunk of uranium which is a radioactive element in a cloud chamber so this is a particular setup where if a charged particle moves through the cloud chamber it ionizing some little bubbles they show up okay as visible streets there so in this picture every one of those little line segments that you see appearing is a charged particle usually an electron being emitted by that uranium if you look at the Schrodinger equation and you actually solve it like a physics undergraduate student would do you can ask what is the wave function of an electron that is emitted by a radioactive atom that is decaying and the answer is it comes out in a spherical cloud it goes out in all directions but when you look at it that's not what you see at all every individual electron coming out moves in a line it looks like it's a particle moving along a trajectory this is strange it seems that like the electron is doing one thing when it's just obeying the Schrodinger equation but when we look at it we see something else so clearly something is wrong right something is wrong and this is still the puzzle that we are working on right now that we don't quite agree on the answer but let me tell you the answer that they came up with in the 1920s the answer that came up with is that wave functions collapse they said look it seems as if electrons behave differently when you're not looking at them and when you are so let's resolve that by saying that electrons behave differently when you're not looking at them versus when you are let's actually take that ridiculous statement literally okay so they say that here might be a plot of a wave function for an electron that is sitting in its minimum energy state in an atom when you're not looking at it and then if you look at it so you measure somehow the position of the electron the wave function suddenly and dis continuously changes it collapses so that it's isolated it's localized on one particular location so that when you're not looking at the electrons all spread out when you look at it you see it in one particular location and all you can do is not actually say which location it will be but predict the probability for it being in different locations no I do not want to join the Internet I love the Internet don't get me wrong but I don't want to join it right now okay so this is the idea of wavefunction collapse and you know you're gonna you you think that I'm gonna say they soon realized that that was silly they didn't they enshrined that in the textbooks so if you take a quantum mechanics course right now as an undergraduate at Caltech or wherever you want to go you are taught that there are two separate rules that are obeyed by wave functions two separate ways that quantum mechanical systems operate depending on whether you are looking at them or not there's one set of rules when you're not looking which say that electron or any other quantum system has a wave function and that wave function obeys the Schrodinger equation and there's a whole other set of rules when you measure it when you observe it the wave function collapses and there's a probability for the collapse to be in different places okay this is what is called the Copenhagen interpretation although people argue about whether or not individual people in Copenhagen believed it so sometimes you just call it the textbook interpretation this is what is taught in universities when we teach people quantum mechanics so to illustrate this I mean this is a mess obviously but to illustrate it in action let's appeal to the thought experiment stylings of urban Schrodinger and he invented this little box with a cat in it right Bert angers cat you've all heard about this Schrodinger's cat is the idea that you have a box cat inside and you put up an experimental apparatus that consists of a radioactive source that decays but not very often a Geiger counter or some other kind of detector that will click when the radioactive source decays and then when it clicks it lifts open a box where it smashes a vial or somehow releases some gas into the box now in the original version of Furth angers cat the gas was cyanide and the cat died and this is a true quote urban Schrodinger's daughter once said I think my father just didn't like cats I do like cats so I'm changing the experiment so that it's sleeping gas in the box there's no reason to kill the cat the experiment works just as well either way and what it says is if you take literally Schrodinger's equation and you ask what is the state of the cat thought of as a quantum mechanical system okay what is the wavefunction of the cat well you you know the following you know that if the cat is perfectly alive you know what its wavefunction is if the cat is perfectly sorry awake if it's perfectly asleep you know what its wavefunction is by the rules of quantum mechanics by Schrodinger's equation the radioactive source has a wave function which says it's partly decayed and partly not that means the detector has a wave function that says it's partly clicked and partly not and the box is partly open and partly not and therefore the cat is partly awake and partly asleep okay it is not and the schrödinger's whole point was surely you don't believe this right I mean Schrodinger invented the Schrodinger equation but he didn't invent this set of rules about probabilities and collapsing and once that had come on the scene he was like I'm sorry I ever invented this he was actually worked about the whole thing so schrödinger's cat experiment is a way of amplifying the idea of a wavefunction in a superposition of two possibilities decayed atom or not decayed atom to a macroscopic situation where you have an awake cat or in a sleep cat and the rules of quantum mechanics are saying the same logic that promoted an electron from just going in an orbit to being a cloud a wave function say that it's not that we don't know it's not that the cat is either awake or asleep and we're just not sure it's literally true according to quantum mechanics that the cat is in what we call a superposition of being awake and being asleep it is both a same time so here is how we describe this classically you would say the cat is either awake or either or it's asleep maybe it's one or the other maybe we don't know that's fine maybe there's some probability when we open the box we'll get an answer just like we don't know who will win the next presidential election or so forth but there is an answer but in quantum mechanics we say it's in a superposition this is a different kind of reality the quantum mechanics opens up the classical mechanics doesn't okay it's a new kind of thing that's what a wavefunction is that's why quantum mechanics truly replaces classical mechanics as the paradigm is the framework for thinking about physics it's not just that things are awake or asleep we just don't know they're literally both at the same time and then when you open the box according to the Schrodinger according to the Copenhagen interpretation the Copenhagen interpretation insists that you treat observers as special and as in particular classical objects so in the Copenhagen interpretation there's a classical world of you and me and big macroscopic things and there's a quantum world at the microscopic level so for notational purposes here I put quantum things in parentheses and classical things in square brackets okay here I put Niels Bohr who's a big classical observer Niels Bohr is one of the godfathers of quantum mechanics one of the founders of the Copenhagen interpretation and the way that we describe the situation before he opens the box is the cat is in a superposition awake and asleep but the observer just doesn't know that observer has not yet opened the box and then you open the box you observe and then either the state afterward is the cat's awake and The Observer saw it awake or the situation is the cat's asleep and the observer saw it asleep okay that's what we teach our students this is the current state of the art among almost all physicists in terms of how well do we understand quantum mechanics okay clearly this is nonsense this is clearly a failure as a supposedly rigged theory that describes nature at its most fundamental it's clearly unacceptable not because it's wrong not because it doesn't fit the data but because it's completely imprecise okay there are questions about this theory that really need to be answered let me just highlight two of them very quickly one is what we might call the ontology problem ontology is what philosophers use the word the phosphorus use to describe what is real ontology is the study of being okay so we talked about wave functions but we also talked about what you see when you look at a wave function so what is real is the wave function a direct and complete representation of the world or is it just somehow a machine to convert our ignorant sin to predictions are there other variables in addition to the wave function people like Einstein had the idea that there were particles with positions and there was also a wave function so it's the wave function just part of it or is the wave function have nothing to do with reality at all so in quantum mechanics when you ask people who are experts questions about what is really happening they can't quite tell you because they don't agree on the answers to any of these questions and then there's what we call the measurement problem there was a whole separate set of rules in quantum mechanics for what happens to a system when you look at it what in the world does that mean what does it mean to be an observer what does it mean to observe how quickly does it happen when does it happen do you need to be conscious is like somehow human consciousness playing a role here could the cat be an observer all by itself what about a frog what if I don't observe it very carefully what do I just kind of glance at it does that count does that collapse the wave function none of these questions are answered by the Copenhagen interpretation so we would like to do better there are many ways to do better I'm gonna give you a sales pitch for my set my favorite way which was invented by this guy Hugh Everett when he was still a graduate student in the 1950s and roughly speaking you can think of whatever it did as offering the physics community a little bit of quantum mechanical therapy basically he said you know you're working too hard you guys got to chill out a little bit you're trying too hard to make things complicated when a very simple pristine austere take on the problem works perfectly well what if ever it says wavefunctions are all there is there's no extra variables there's no different way of describing reality the wave functions don't represent our ignorance they truly represent the world and what if all they ever do is obey the fruit of your equation there's not some separate set of rules for what wave functions do when you look at them that's horribly ill-defined then you should be embarrassed okay what if every wave function always and simply obeys the equation that it should when you're not looking at it in other words here is the Everitt interpretation of quantum mechanics there's only one set of rules they are obeyed all the time and they're just like classical mechanics but the system is a little bit richer so you have a wave function to describe systems rather than positions and velocities like you would in classical mechanics and you have an equation that says how they evolve the Schrodinger equation rather than Newton's laws okay he says erase all these ugly things that are giving us so much problem now there is a reason why people didn't think of this decades before right I showed you the picture of the uranium decaying and you see little trajectories even though you're predicting that it should look like a big spherical puffy wave moving away how does how do you reconcile that mr. Everett the secret is in something that is very crucial very central to quantum mechanics but again we don't emphasize it when we teach our students as undergraduates it's becoming more central to modern quantum mechanics it's the idea of entanglement so I said that you can think of a wave function as describing a superposition of different possibilities like the cat is awake plus the cat is asleep it's not one or the other it's truly a combination of both okay but what happens when you have more than one thing not just a cat but something else in the universe in classical McKee there be a state for the cat and a state for a dog in a state for you and separate ways of describing all the individual pieces quantum mechanics doesn't let you do that quantum mechanics says that everything is of a hole in some very direct sense so one way of talking about it is think about the Higgs boson okay the Higgs boson we discovered for the first time just in 2012 it is a elementary particle and the nice thing about the Higgs boson for my purposes today is that it doesn't spin it has zero angular momentum it just sits there quietly okay almost all elementary particles spin with a certain fixed frequency of spinning like electrons absolutely spin when you measure them you either get them spinning clockwise or counterclockwise what we call spin up or spin down those are the only choices but they're always spinning some amount you never get an electron that is not spinning so we know from data that the Higgs boson can decay into two electrons really since its neutrals decaying into an electron and an anti-electron the positron but I'm just gonna for simplicity call it two electrons okay and we also know that those electrons are spinning and the Higgs boson was not but angular momentum the total amount of spin in the universe is conserved spin doesn't pop into existence out of nowhere so we know that when the Higgs boson decays into two electrons if one of the electrons is spinning one way the other one has to be spinning the other way we don't know which way either one of them is spinning but we know they're spinning oppositely to each other okay so in other words the way to think about it in quantum terms is what the Higgs boson decays into is a superposition but it's not a random superposition of anything the two electrons could be doing it's a superposition of electron one is spin up an electron two is spin down plus electron one is spin down an electron two is spin up if you think about all the things the particles could be doing there could also be both of them are spin up or both of them are spin down but that wouldn't conserve spin so that's not allowed okay so the lesson of this is that if you asked well if I were to observe the spin of electron one what would I see well there's a 50-50 chance it's spin up or spin down if I observe the spin of electron two what do I see 50-50 chance spin up or spin down but if I observe the spin of electron one and I see spin up I instantly know that electron two is spin down that's the only way that spin can be conserved in the world so that's entanglement even though we don't know what either electron one or electron two are doing we know that if one of them is doing something the other one is doing something that we can figure out there's a connection a correlation a relationship between the different subsystems of the universe so this is so important I'm gonna say exactly the same thing again the point is in classical mechanics we can chop up the world into individual subsystems and describe each of them separately you might think that in quantum mechanics we can do the same thing but with wave functions so you might think that we can talk about the wave function of electron one and the wave function of electron two and maybe they can be a superposition of the two different spins no in quantum mechanics there is only one wave function whatever it called the universal wave function well we now usually call the wave function of the universe so the only way to describe two electrons is by entangling them well rather we're allowed to describe them by entangling them there are some states that are unentangled that's okay but entanglement is ubiquitous in nature so for the decaying electrons and the higgs-boson case the right way to describe the system is has this entangled superposition of two things going on okay ever didn't invent this this was invented by a guy named Albert Einstein right Einstein gets a bad rap when we talked about quantum mechanics you sometimes they're told that Einstein was kind of getting old and conservative at that time he couldn't handle the new hotness of quantum mechanics all of that is rubbish Einstein understood quantum mechanics better than anybody did he just wasn't be with it he wasn't satisfied with it and he shouldn't have been but he couldn't figure out a better theory so we're still a little stuck there so Everett didn't invent entanglement but he put it to use in his version of quantum mechanics there is only one wavefunction and there's no separate classical world so when you do the schrödinger's cat experiment in Eveready in quantum mechanics the secret is you have to treat the observer as a quantum system also there's nothing magical and Eveready in quantum mechanics about observations or measurements there are just different parts of the universe that interact with each other according to the laws of physics and observers are parts of the universe just like everything else so you treat both of the cat and the observer as quantum mechanical systems part of the wave function of the universe so we would say here that the cat is in a superposition there's an observer played in this role by Hugh Everett and the observer has not yet looked right the observer has not yet open the box so when the observer does open the box we call that the measurement and what happens is not a collapse of the wavefunction what happens is the observer and the cat become entangled with each other so just like the electrons the wave function of the universe is a superposition of the cats awake and the observers saw it awake plus the cats asleep and the observer saw it asleep that's a much cleaner crisper more rigorous quantitative way of thinking about things but of course there's a problem that none of us has ever felt like we were in a superposition of anything right no there's plenty of times that in undergraduate physics labs people have measured the spins of electrons I hope that they'd never put cats in boxes with poison gas but they've certainly measured other quantum mechanical systems and no one has ever said oh yes I'm in a superposition of seeing it spin clockwise and counterclockwise so how in the world this might be a very simple clean crisp theoretical model but how in the world do you reconcile it with our experience of the world the answer is remember we said that there's not separate wave functions for different parts of the world there's only one wave function for the entire universe and here I didn't give you the wave function of the entire universe did I I gave you the wave function for a cat and observer if I were more honest I should include the entire rest of the universe so let me do that I will discover a new phenomenon called decoherence the entire rest of the universe I'm gonna call the environment and represent it by a picture of leaves of grass okay so I don't know who invented the word environment but generally the environment to a quantum physicist is just the entire part of the universe that we don't keep track of when we're doing our experiment so I know what I'm doing I know the cat's doing I don't know what the rest of the universe is doing that's the environment but that means that the environment includes all the air in the room all the photons coming down from the lights and so forth I don't keep track of all that stuff but that stuff is constantly interacting with the cat in the box right so long before I open the box something called decoherence happens where the cat becomes entangled with the environment if the cat is asleep on the ground versus walking around and awake the photons in the box will hit it differently the environment responds and interacts differently with the cat depending on whether it's awake or whether it's asleep that entangles the state so even before you open the box the universe is in a state of cat's awake and the environment has interacted with an awake cat plus cats asleep and the environment is interacted with in sleep cat and you and then you open the box you measure it and then you see whether the cat is awake or asleep okay the same story I told you on the last slide except now I'm being a little bit more honest because I'm including the environment why does that matter why does it make a difference that I include the environment and the answer is because the environment in these two parts of the wavefunction is different it's a separate kind of thing and what that means is that if you ask is there any effect of this part of the wavefunction on what happens in the other part the answer is no whatever it realized was he didn't use fight these words because the phrase decoherence and the idea is quite it hadn't quite been invented yet but what he realized was that the reason why you don't feel like you're in a superposition of having seen the cat awake and having seen the cat asleep is because you're not one person anymore because these two parts of the wave function are completely independent of each other henceforth once decoherence has occurred it is as if they have become separate worlds what happened was not that the wavefunction collapse because of some mystical and unexplained measurement event what happened was at the very natural and mechanistic decoherence process has branched the wave function into two different parts which go their own way which one part can never talk to the other part no matter how much they try where there used to be a cat and a person there are now two cats and two people for all intents and purposes so ever it called this the relative state formulation of quantum mechanics because whether or not you saw the cat awake or asleep depends on where you are it's relative to where you are in the wave function no one was very fascinated by that until in the 1970s price tu-whit dubbed it the many-worlds interpretation and suddenly people got very very interested so the point is that nowhere along the line did Hugh Everett or anyone else say you know what would make quantum mechanics better is if we added a lot of worlds to it that's not what happened nothing in here is anything other than a wavefunction obeying the Schrodinger equation that is the entirety of the Everett interpretation of quantum mechanics the worlds come along for the ride the worlds were always there it's been said that every other formulation of quantum mechanics like the textbook or Copenhagen interpretation they disappearing world's version of quantum mechanics if you believe that an electron can be in a superposition of spinning clockwise and spinning counterclockwise and you believe that quantum mechanics applies to the whole universe then you should believe that the universe can be in a superposition of doing two very different things and the laws of quantum mechanics imply that those two very different things will not talk to each other they don't know that each other are there they are for all intents and purposes separate worlds that's why it makes sense to call it the many-worlds interpretation of quantum mechanics but the space of all wave functions is no bigger in many worlds than it is anywhere else we didn't add anything to it all we did it was erase some dumb and unnecessary rule the price we pay is that there are a lot of other worlds that's a prediction of the Schrodinger equation so there are many issues that people have with the many-worlds interpretation some of them are good some of them are not so good some of them are easy to answer some of them are harder to answer I would like to pay you the compliment of spending the rest of my talk talking about my favorite good question about the many-worlds interpretation so there are other questions like where do all the Worlds live how many worlds are there like how do you test this theory there are answers to all those and there in my book you can buy the book ok but what I want to answer what I want to talk about is the question that I think is most interesting which is hard to answer in fact there are two of them but I only really have time to talk about one so there are two questions about many worlds that in my mind are the most reasonable and important right now one is the idea of probabilities remember that in the Copenhagen interpretation we said you couldn't predict the probability of getting any particular answer all you could do sorry you couldn't predict what answer you're going to get all you could do is predict that you could get different answers with different probabilities now many-worlds is an entirely deterministic theory the Schrodinger equation has no probabilities in it at all you have a wavefunction and it evolves and you can evolve it forward and backward in time there's no loss of information there's no irreversible there's no stochastic jumps or anything like that everything is perfectly smooth so how in the world do you come to say that for some reason there's a probability of getting certain answers so there is a an answer to that the very short version is let me give you the very short version here over here there's this moment the wave function of the universe has already branched right because decoherence is what branches the wave function so this person who thinks he's only one person is actually - there's a version of the observer that is on the branch for the cats awake and there's a version of the observer that it's on the branch where the cat's asleep but neither one of those knows which branch there on the branching of the wave function always happens faster than you learn about it so even if you know the entire wave function of the universe at this moment in time you don't know where you are in it you don't know what branch you're on and you can ask is there any way to credibly assign a probability to being on different branches and the answer is yes and it's exactly the rule that we teach our students in undergraduate quantum mechanics that it's the wave function squared all right there's a longer story there but that is the basic idea the other question which I think is fascinating is how does the classical world emerge if everything is wave functions and they just obey the Schrodinger equation if there's no separate macroscopic classical reality why does the world look so darn classical Albert Einstein teased a friend of his once by asking do you really think the moon isn't there when no one is looking at it well it is of course it is there in fact we can predict where the moon will be millions of years in the future and we don't use the Schrodinger equation to do that we use Newtonian gravity or even better Einstein's theory of gravity so why is that why is the world looks solid why does it seem to be pretty well approximated by classical physics that's what I really want to get through so the issue here when I'm talking at least to my fellow physicist there's there's a problem that I need to convince them first that there is a problem and then that I might be able to solve it so no one thinks there's a problem because you see the classical world there's the podium here it is why do I have to find it in a wave function somewhere right so we are so trained by our experience in the world to think classically that when we do physics we start with classical stuff and then we build a quantum mechanical theory of it so we start with electromagnetism or quarks or whatever we start with a classical version of these objects and then we what we call quantize that description so we start with Katz leaves people and we do some mathematical tricks to turn it into a wave function and the mathematical description of that wave function is it's a vector living in a big vector space you don't need to know those details I do a three dimensional vector space here the real vector space has a lot more than three dimensions it's not space it's not where we live it's a mathematical object it could be infinite dimensional for all we know we don't really know that the point is nature doesn't do this reality doesn't start with some classical stuff and then quantize it reality is this quantum from the beginning so if we were to follow nature what we should do is start with a wave function and ask why does that wave function appear to us to describe classical things like cats and grass and people that's a harder question than you might have thought and we should have been studying this question for the last half a century but we've been completely ignoring it okay and there's many things to say about this question but I want to focus on one because it is the most speculative and ill understood but it's also possibly the most profound because as much as we say you know we don't understand quantum mechanics there's another problem that we don't know the answer to which is gravity right Einstein became famous not for his work on quantum mechanics although he won the Nobel Prize for quantum mechanics Einstein never won a Nobel Prize for relativity in one of the great tragedies of the Nobel Prize he invented the fact that liked is quantized into what we call photons that's what he won the Nobel Prize for but what he became famous for was the theory of relativity in particular general relativity the idea that we live not just in space evolving with time but in space time in a single four dimensional manifold and that space-time you and I think of as gravity as the reason why apples fall from trees or the earth goes around the Sun it's because all this stuff is moving in a curved space-time so we would like to take that idea space-time is curved and gives us gravity and reconcile it with quantum mechanics general relativity is an entirely classical theory it's very much at home in the Newtonian paradigm for all of the other forces of nature electricity magnetism the nuclear forces and for all the particles that we know about quantum mechanics works great quantum mechanics describes everything that's going on gravity is the one exception and maybe that's because gravity is somehow deeper than these other things gravity is not a field or a particle living on space time gravity is space-time itself but when we've tried to take Einstein's theory and quantize that we failed so maybe these problems are related to each other maybe the fact that we don't understand quantum mechanics is holding us back from understanding quantum gravity in a fundamental way so let's try to do this let's try to get classical general relativity out of quantum mechanics and this is the part where no one's going to understand anything no you will understand some things but this is definitely the more challenging part so don't feel bad if your level of comprehension is only 98% as opposed to the hundred percent it was for the first part of the talk this is cutting-edge stuff this is stuff that we've professional physicists don't ourselves understand we're trying to figure it out right now well we can do is we can take hints from what we do understand so we do understand the other forces of nature the non-gravitational forces I said electricity magnetism the nuclear forces etc and the answer for those forces is that forces and matter are all described not by particles but by fields so this is separate from the wave function the wave function is a quantum object and when we were just doing little electrons moving around atoms we took a particle we promoted it to a wave function what quantum field theory does is take a field and promote it to a wave function so it's field enos on top of field enos okay and this kind of makes sense for things like the forces you know about electricity and magnetism you know about the electric field the magnetic field but even for particles this is how modern physics describes things there are fields stretching through space and then if you start them vibrating that looks to us like a particle okay so the difference between quantum particles and quantum fields is that in a particle theory empty space is boring empty space is just empty there's a particle here a particle there in between there's literally nothing but space but in quantum field theory empty space is an exciting place even when there's no particles there there's still fields even if the electric field is zero at a particular location in space there is still a thing they're called the electric field it just has a value of zero if you know what I mean and likewise for the gravitational field likewise for all the other fields so in quantum field theory we can talk about vibrating jiggling modes as we say of all the quantum fields making up all of nature at every point in space and guess what these modes are entangled with each other and how much they're entangled depends on how far away they are so if you pick two regions of space that are nearby the modes that are vibrating an empty space will be highly entangled with each other if you pick two regions that are far away the modes vibrating empty space will be almost completely unentangled that is our understanding on the basis of conventional well on quantum field theory without gravity but what I'm saying is we should stop quantizing classical things like fields and just start with a wavefunction and get all that stuff out so that includes space itself in a wavefunction there's nothing that is singled out as space versus matter versus energy or anything like that part of our task in extracting the classical world from a quantum wave function is saying what is space itself what do you mean by the distance between two different things so here is our guess and right now it's just a guess we don't know in ordinary physics as we understand it if there are two regions that are nearby they become highly entangled if there are two regions that are far away they're not very entangled let's turn that around let's play that game backwards if two parts of the quantum wave function are highly entangled let's define that to be what we mean by nearby if they're not very entangled let's define that what to be what we mean by far away in other words let's define the distance between different things in space by how entangled they are in the gigantic wave function of the universe the more entanglement this the shorter the distance okay and what that does is depending on how things are entangled with each other it gives us a geometry the geometry need not be the flat tabletop geometry of Euclid depending on how all the different vibrating fields are entangled with each other you get whatever geometry you want with this emergent notion of space it's perfectly natural for geometry to be curved if that's what it wants to be so that's fact number one geometry is related to entanglement fact number two is that entanglement is related to entropy entropy you might know is how it was what we use to describe systems where we don't completely know what's going on we know for example for the air in this room it's made of molecules it's made of atoms but we don't know where exactly every molecule and atom is right so the less we know the more entropy something has entropy is a measure of our ignorant this guy Chand phenomen pointed out that in quantum mechanics you can know everything and there are still things you don't know there can still be entropy even if you know the entire wave function of the universe because look here are two systems a and B that are entangled with each other let's say we know their wavefunction but remember the spins right spin up spin down the electrons I can know the total wave function and I still don't know the wave function of the first electron because it's entangled with the other one so if anointment says if a is entangled with B then a has an entropy there's something we don't know about it because it's unknowable some of its knowledge is that leaked out into that entanglement so he defined a formula for how much entropy there is given by the amount of entanglement so you can measure entanglement by calculating the entropy the more entangled the higher the entropy fact number two fact number three entropy is related to energy so I showed you a picture of empty space where there were no particles in and I said everything is entangle with everything else if I break a little bit of the entanglement if I take a little part of space and cut that off so it's not entangled with the region around it and it's vibrating faster I call that a particle so that means that the entanglement between that region and the rest of the world has gone down and in the process I've created energy in the form of a particle there so there's another equation and I'm not going to show you but if there is an equation which relates the entropy in a region to the amount of energy in that region there's a minus sign so that as the entropy goes down the energy goes up but still they're proportional to each other so look at what we have we very naturally have and again we didn't put anything in this is what naturally comes out geometry is related to entanglement entanglement is related to entropy and entropy is related to energy therefore geometry is related to energy the geometry of this emerge space that we are extracting from the quantum mechanical wave function very naturally is responding to the amount of energy in the region of space that we're talking about but this fact that the geometry of space responds to the amount of energy in it is exactly stop that talk about a dramatic moment all right is exactly I'm Stein's general relativity this is what general relativity is just like in Newtonian gravity the gravitational force field depends on how much mass and object has in general relativity the curvature of space depends on how much energy there is in that region of space this is another equation you don't have to worry about the details the point is just that there is an equation and the equation says the curvature of space-time is related to the amount of energy in it so what is the lesson that we've learned the last few slides are sketching out a very speculative program for trying to extract the classical world from the quantum mechanical wave function in a way that secretly makes intimate use of the many-worlds interpretation of quantum mechanics because in every other interpretation of quantum mechanics they put the classical world in by hand one way or another Copenhagen does it hidden variables theory do it all the different alternatives to many worlds many worlds is the most quantum of all the different ways you can be quantum mechanical and so it is perfectly suited to this program of forgetting that we know that there's a classical world starting with a truly quantum description and extracting space and time and the rest of the classical world from it the good news seems to be that maybe the reason we have not succeeded in quantizing gravity is because that's not what we should have been doing at all we shouldn't been starting with gravity and quantizing and we should have been starting with quantum mechanics and finding gravity within it so far so good is what I would say for the prospects of this program but it's still very early and I'm the first one to say it might crash and burn if my little introduction to it didn't make perfect sense the first time my strong advice is there's a book that has more details I wrote it you can read it you can buy it right here thank you very much for coming [Applause] [Music] [Applause] [Music] we're gonna open it up to an audience Q&A just for a few minutes here so if you have a question please raise your hand and I'll try to get to as many people as possible starting in the front row couple of weeks ago on your podcast you discussed intellectual vices with Professor Qasim in this world that you're in how prone might people be to wishful thinking and confirmation bias and particularly older people who would like to wrap it all up before they're gone this is an excellent question yes so I was briefly mentioning in the introduction I do have a podcast called mindscape you're welcome to subscribe to that and I don't only talk about physics I also talk to a whole bunch of people doing different things including philosophers and neuroscientists and so forth so I talked to Kassim kasam who is in epistemologists a philosopher of knowledge and learning and we talked about intellectual vices including wishful thinking and stubbornness and things like that things that get in the way of finding the truth and you know what physicists are exactly as prone as anyone else to these kinds of intellectual vices no more or no less you might think that older physicists are more likely to be desperately rushing around chasing every idea hoping that they finally hit the right one because their time is running out but the truth is the opposite maybe this is not so surprising the older physicists are much more likely to be stick-in-the-muds that stick with whatever Theory they've been working on for the last thirty years if it's not really very promising there is a joke it's not a great joke but it's you know the only way the science progresses is through the death of old scientists right especially when you're in a field where there's not a lot of data gravity is hard to collect data with quantum gravity it's next to impossible just because gravity is a really weak force so progress is slow exactly because of that and that's where intellectual vices can become a real problem so all that we can do is try really really hard to not succumb to those vices there's no magic bullet for avoiding them individually question in the fourth row yeah to most people a awfully lot of what you just said sounds even weirder than Harry Potter and Deepak Chopra and so how do you separate your highly speculative positions from their highly speculative positions sure I mean that's actually a very good question because I completely get that it does sound that way here's the answer I have that and they don't there's two things that science has that and it that's only half the half of a joke because there's two things that science has going for it that other crazy sounding things don't one is it is rigorous and quantitative right there's no wiggle room here they'll be very very easy to do an experiment which show that the Schrodinger equation was false right and then you'd be stuck you'd have to change your theory somehow the second is we have experiments right we are constantly trying to make ourselves famous by showing that our favorite theories are false like there's this feeling out there among non scientists sometimes that scientists are defensive and want to you know come to the rescue of general relativity or the Big Bang Theory or quantum mechanics when it's under siege but really the way you become a famous scientist is to show that Einstein was wrong not to show that he was right right so the great thing about science is full of people who are trying to tear down the established wisdom that's really what separates it from other crazy things the other thing I should say is scientists aren't trying to be crazy they're not trying to sound very very unrealistic they're trying to fit the data and what happens is since the time of Copernicus as we've learned more and more about the universe our scientific theories have become less and less like our everyday experience and that should be completely unsurprising because their scientific theories are encompassing greater and greater realms that are bigger and bigger compared to our everyday experience the realm of quantum mechanics is so far away from what we see in our everyday lives it should be completely unsurprising that it seems counterintuitive to us question the third row yeah hi when you were talking about entanglement sometimes you spoke of it as though a were they were discreet it was kind of this binary on/off yes/no kind of thing other times you spoke of it as a very continuous curve and I'm wondering is is so is there like some kind of a threshold where things become experienced as discreet on an otherwise continuous curve I don't know yeah no actually this is a wonderful question in the sense that it's really hard to answer the amount of entanglement between two things is not discrete it can go from zero to some maximum number and it often does the the very word quantum in quantum mechanics is something of a misnomer like I said quantum field theory is like waviness on top of waviness the quantumness comes about just because when you solve Schrodinger's equation you get these solutions that look different from each other and they sort of form a discrete set so it's a set of discrete things but the discrete things are waves okay quantum mechanics is really a theory of waves and that means it's a not a pixilated theory of a discrete universe in any sense so you should think of everything is sort of being smooth and there's no thresholds but we live in a world that is very very close to a classical world where it's a very very good approximation to say that things either are or not entangled with each other thank you very much for this nice talk and I appreciate the courage to go against the classical so it does Schrodinger equation explains the difference between a dead body and a live body what are the parameters in the Schrodinger equation that is associated a dead body and a live body and does it Schrodinger equation explains this phenomena or we need something else or a field field theory is the body needs a field to be alive that's my question right the Schrodinger equation does a perfectly good job in distinguishing between alive bodies and dead bodies because the difference between a live body and a dead body is whether or not there is activity in the brain an activity in the brain is electrochemical and electricity and chemicals are governed by the Schrodinger equation that would be my answer and I'm up here we have time for two more questions so fourth row and then front row hi you mentioned the idea of not feeling it we live in a superposition do you think that free will and choice whatever that means is a way of for observers to choose their wavefunction or is it do you think it's maybe the other way around it's the other way around so the question is is there any somehow freewill being involved in choosing your wavefunction there's no choice in the wave function the wave function very very rigidly obeys this equation in my version of quantum mechanics now it can be the case that on different branches you know the electrons in your brain did slightly different things so it manifests itself at the macroscopic level is you making different choices but it's the electrons doing different things on different branches of the wavefunction that made you make a different choice not you making a choice that made the electrons do something different causality always goes up in levels never down we'll go to questions up here ok here's my question hi hey Robin hey here's my question it's when you say the universe is a wavefunction my difficult one of my difficulties with this whole interpretation is the meaning that's attached to being a wavefunction I mean in classical mechanics when we talk we give x and y vary position and momentum for instance to a particle we already have a concept of a particle like a billiard ball from our everyday experience but one of the unique things about quantum mechanics Lisa when it started it had it defined what a wave function is in terms of observation otherwise it's just a meaning it's just a mathematical symbol has no meanings like saying oh the universe is x and y or the universe is a matrix well what is that you know mathematical matrix what does that mean so I'm wondering when you say it's just a wave function what how do you attach a meaning to the idea of the wave function yeah I think it's exactly the same thing personally in classical mechanics accessing quantum mechanics in classical mechanics there's a journey you take when you do science from these data of your senses of your everyday experience to some mathematical formalization of the theory in classical mechanics yeah we start with like objects and particles and things like that and then if you're Isaac Newton you say I assign positions and velocities to these and then if you're William Rowan Hamilton you say it's actually better to think of it as positions and momenta and then when you become a math e20 20th century physicist you say the world is a point in a symplectic manifold but it's still classical mechanics it's still the same world and the same thing is true in quantum mechanics you start with tracks in bubble chambers and so forth and you say okay I have a probability distribution I'm gonna call that a wavefunction it's complex numbers and then you say really it's an element in a normed complete vector space that I call hilbert space but to me it's exactly the same kind of process so sometimes I'm sloppy and I say the world is a wavefunction what I really mean is there's a world and the thing it is is the world and the best theory I have of the world is that is completely isomorphic to a complex vector obeying the Schrodinger equation hi thanks for coming out I know you said you were gonna talk about the interesting questions about quantum mechanics but I've already bought the book so yeah my money all right and I think most people I don't have one please you know no but I am interested in how one might go about experimentally testing something like in many worlds I think this is maybe as I read your book I'll find out that you don't believe that there's many worlds splitting and that there aren't in different locations or no so then so then I mean I assume then that there's a lot of pushback that this could be some unfalsifiable claim if you don't have some kind of way to test it yes but in that case I can once again appeal to Authority one of the biggest fans of the many-worlds interpretation was Karl Popper who invented the idea of falsifiability the reason is for those of you who know just tiny bit of philosophy of science you might have come across the idea that a good scientific theory should be falsifiable and people debate about whether not that's the best way of thinking about things I think it's not but popper was on to something right and certainly if a theory is falsifiable which says there could in principle be an experiment you could do such that if you got a certain result of that experiment you would say my theory is wrong okay if that's true it is scientific I think that's sufficient if not necessarily necessary but it's absolutely satisfied by the many worlds theory because this is the many worlds theory systems are described by wave functions and wave functions of a the Schrodinger equation both of those statements are eminently falsifiable find other variables other physical things in addition to wave functions or see a wave function not obeying the Schrodinger equation and in fact there are alternatives to many worlds which take advantage of both of those possibilities and there are experimental tests going on right now to test them so if you make a so there's one theory called the grw theory where wave functions really collapse but not when you look at them they just collapse spontaneously all by themselves with a tiny probability per particle but if you have a whole lot of particles it will happen before too long so if you have a really cold collection of atoms in everybody in quantum mechanics it will just sit there doing nothing near absolute zero in grw occasionally one of the atoms will collapse its wavefunction and the thing will heat up a little bit so you can test that and they're testing it if they find that things that are very cold spontaneously heat up ever it will have been falsified can we give a huge round of applause for professor strong Carol you you

Channel: Midtown Scholar Bookstore

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Keywords: books, bookstore, Sean Carroll, Something Deeply Hidden, Midtown Scholar, author, book, author talk, science, quantum physics

Id: MScOpMCkNQM

Length: 69min 20sec (4160 seconds)

Published: Sun Sep 22 2019