Mysteries of Modern Physics by Sean Carroll

I'm Sean Carroll I'm a theoretical physicist at the California Institute of Technology and I'll be talking about some of the mysteries of modern physics I'll say a little bit about how much physics knows which is a lot but the mysteries that we have left are extremely profound so I'll talk about the nature of quantum mechanics the nature of space and the nature of time so welcome welcome everyone to the second of the Darwin College lectures of in 2020 and as you know our theme this year is enigmas puzzles or riddles tonight we take our instructions from alexander pope the poet who called humans glory jest and riddle of the world well after that put down calling us a riddle he ordered us to go measure the earth way air and state the tides instruct the planets in what orbs to run correct old time and regulate the Sun so following that instruction tonight we turn to physics once so logical and ordered Pope declared nature and nature's laws lay hid in the night God said Let Newton be and all was light but not now as the writer JC Squire once wrote it did not last the devil shouting ho let n Stein be restored the status quo well why where do we stand now I think often modern physics can leave us more mystified than illuminated it's not just you know all of us non-experts I recall a story that was circulating in my undergraduate student days here as a student left her lecture notes in her bike basket and returned to find an old and rather ittle dressed man rifling through them through her quantum physics notes he said he just wanted to find out what people were being taught these days now since elderly tramps don't usually read advanced math she gave him what for and he apologized and shuffled off towards free school lay now that elderly man was Paul Dirac who writing about a physicist picture of nature and the differing implications of quantum physics and relativity suggested at about that time maybe slightly before that the situation could be described as though God is a mathematician of a very high order and he used very advanced mathematics in constructing the universe direct said he hoped advances in maths would better would enable better understanding of the universe so tonight to discover what puzzled Dirac what drove n Stein to the paradox about spooky action at a distance these deepest puzzles of the quantum world and multiple universities University there are multiple universes you can tell but I normally think about we have professor Sean Carroll a theoretical versus physicist at Caltech who's going to speak as you can see on the mysteries of modern physics [Applause] [Music] thank you very much it's an of course an enormous honor to be here as part of the Darwin lecture series I like the bottle of vodka that they have left for me here that's also of something you don't really get in the United States but I don't know how familiar you are with the system but when they invite you to give one of the lectures there's a theme for the year and they assign you a title you know they're suggesting what you should talk about roughly speaking but then it's understood you will change the title to be what you want it to be so this was the title I was assigned and I did change it but then I changed it back because I realized that rather than just give a list of my favorite mysteries or just talk about my one single favorite mystery I would actually like to make an interesting point about the kinds of mysteries that physics is currently faced with because we are in an interesting part of the history of physics which is always hard to know when you're in it because you don't know what's going to happen next but I think we can see some clues so I will start by this is a modern picture of course but I want to make an ancient point what is physics and how does it talk about the world you know broadly construed that's what physics is it's the world it's what the world is made of and how the world behaves and the point of this picture I mean one of the this picture to the person who took it is no doubt the cute little dog that is driving the car right there but to me I want to emphasize the difference between ancient physics and modern physics if you are Aristotle and you were thinking about physics you had a story to tell and that story was one of causes and purposes right of nature's of things to be a certain way and from the physics point of view one of the important points was in order for something to move you had to push it there had to be a reason why it was moving there was a natural state of being which was for any one thing more or less at rest until you shoved it right so here you see that a reminder that Aristotle was no dummy right in our everyday life it is generally true that if you want to get something moving you have to push it and therefore it was really quite an enormous step forward when we began to realize that that wasn't how physics worked fundamental there began to be a difference between sort of the underlying laws and the phenomenological experience that we have in our everyday life there's this turns out to be a very long story of people realizing this even sina sometimes latinized as other sena was a polymath during the Islamic Golden Age he was Persian around the Year 1000 and physicists consider him extremely annoying because his day job was he was a physician he was a doctor and he wrote many volumes on health and medicine and then like in his spare time on the weekends he invented new fundamental laws of physics right this does not endear you to the community of physicists so as far as I can tell even seeing it was the first person to say out loud that if it weren't for these messy things like friction and air resistance what would actually happen to a moving object is it would just stay moving that you don't need to keep something moving things tend to move by themselves it's just the ordinary or things would tend to move by themselves but it's just that were surrounded by dissipation and what we now call friction that that prevents them now it was a long time ago even seen his point was not that that's how the world really was he caused he thought of this as a reductio against the idea that there could be empty space because surely you don't think that things just move forever right of course now we know there are things very very close to empty space we fly spacecraft up into them and they seem to move forever there's no Rockets propelling the Voyager probe out into interstellar space it's just moving and that creates an enormous Lee different view of how the world works for one thing this would be a whole nother talk but it does help naturalize our view of the world that you know Aristotle wrote physics but then he wrote more than that and he used this idea that you needed something to do the moving to prove the existence of God right to say there must be at some point an unmoved mover if the universe can go by itself that raises a different conceptual framework again that's a whole nother talk what I want to say is that this new paradigm of not causes and goals and purposes but rather patterns the way to think about physics and this sort of codified in Galileo and Newton and Laplace is if you give me the state of the universe at any moment in time then there are equations that represent natural patterns that will tell you what happens next and they're not pushing they're not causing the universe to happen they just concisely tell you what is going on let me show you what those laws are first in cartoon form these this picture basically summarizes the laws of physics underlying what I call everyday life and I'll explain what I mean by that in a minute but you get the cartoon image of an atom you've seen things like that right there's a nucleus at the center with protons and neutrons we now know the protons and neutrons are comprised of quarks of parts and down quarks around that spins an electron and these different particles interact through different forces there is the strong nuclear force keeping them together inside the proton and neutron keeping the quarks together there is the electromagnetic force keeping the electron on the atom sometimes of course most everything interesting you've ever encountered in your life is due to electrons moving around between atoms and in between them and then there's of course the weak nuclear force which brings to life a fourth elementary particle called the neutrino because up and down quarks can convert into each other spitting off neutrinos and finally there's gravity pulling everything down to the earth everything gets pulled toward everything else under the force of gravity in the background of all this as we verified just a few years ago in 2012 the Large Hadron Collider in Geneva there is a Higgs field spread out all over the place as I wave my hand it's moving through the Higgs field and that affects the properties especially the masses of these particles so it's a very simple picture for particles for forces one background Higgs field you might if you've been hanging out on the wrong street corners and learning a little bit of physics think well aren't there other particles also yes there are you know the strange quark and the top quark and the muon and things like that but all these particles decay away rather quickly you are not made of any of those particles these are the particles that make up you and this table and me and this laptop and really everything that you have ever seen with your eyes touched with your fingers smelled with your nose in your life and furthermore we know how they interact with each other and even better than that the most impressive fact is that there will not be a discovery tomorrow or next century or a million years from now which says you know what there was another particle or another force that we didn't know about but now we realize plays a crucial role in our everyday life as far as our everyday life is concerned by which I really mean what you can see with your eyes touch with your hands etc we're done find me the underlying ingredients that is an enormous achievement in human history one that does not get enough credit because of course as soon as we do it we go on to the next thing physics is not done I'm not saying that physics is done but physics has understood certain things and those things include everything you encounter in your everyday life unless you're a professional experimental physicist or unless you're looking of course outside our everyday life at the universe and other places where we don't know what's going on so I know that just showing you a cartoon might not be as persuasive as it could be especially when the point is so grandiose as the laws of physics underlying everyday life are completely known some of you are sitting there thinking I'm not gonna believe that until I see an equation well here you are this is the equation this is what Nobel laureate Frank will check has dubbed the core theory of modern physics it's put in a certain mathematical language I just want to write out everything explicitly so that you're prepared for the quiz at the end of the lecture but the point of it is this is remember I told you that the paradigm of physics is if I tell you everything about the world at a moment there's a law that tells you what the world is going to be like the next moment both forward and backward in time so this is that law for the modern core theory for the quantum field theory of all those particles I just told you about it's a path integral that was worked out by richard fineman and others and all of the stuff is in there there's quantum mechanics there's space time gravity the other forces so that's the weak and strong nuclear forces and electromagnetism there's matter by which I mean the electrons and the neutrinos and the quarks and there's the Higgs boson in the background the only point of showing you this is to point out that it is a pattern it is just a relentless thing that the universe does over and over again it's not moving toward any goal nor is it being pushed from outside in any way now there's another subset of you that says sure I like equations I'm glad you showed me that but I only really like equations that can fit on a t-shirt here you go proof that this can fit on a t-shirt okay so the reason why I'm going through this even though it's not the mystery yet right this is stuff we do understand is to pinpoint this thing about the mysteries that I want to get to we do have plenty of mysteries left in physics but among those mysteries are not how things that make up this table or the earth below us or you and me or the planets and Moon and stars behave the mysteries are of another sort so there's one obvious kind of mystery that in fact in retrospect I really should've made a slide for and I feel bad now which is the collective behavior of all of these things right I understand how electrons behaved I understand how protons behave I do not understand how 10 to the 25 electrons behave especially when there are protons around they can have all sorts of emergent properties I will nod toward them later in the talk but clearly that's in fact what most working physicists actually think about and it's incredibly important there's another myth set of mysteries which is we have yet to understand what most of the universe is made of right and this is something that astronomers have taught us bless their hearts when you look out there in the sky through various ways you realize that the kinds of particles that make up the core theory the kinds of particles that we've detected in our laboratories and so forth are just not enough to account for what we see in the top left there's the famous bullet cluster where two galaxy clusters pass through each other the matter heated up because it sort of ran into the matter from one cluster ran into the matter from the other cluster that's the red but then you can use gravity and the lensing effect on light caused by gravity to way where the mass is and it's not where the ordinary visible matter is the blue is where we reconstruct the mass to be so what you interpret this picture is saying these two clusters of galaxies just went through each other and all of the mass just went through almost all of it why because almost all the mass is something called dark matter something that does not interact with the model with us particles in the core theory that we know and love and then we use evidence from the Cosmic Microwave Background and from high redshift supernovae to say that most of the energy in the universe isn't even made of matter not just it's not ordinary matter it's not even dark matter the pie chart that we've constructed in the 1990s has 70% of the universe something called dark energy which is smoothly distributed through space and most importantly does not dilute away as the universe expands there's a certain amount of energy in every cubic centimeter of space 10 to the minus 8 herbs if you want to know and it remains the same even as the total number of cubic centimeters goes up that phenomenon pushes the universe apart and that causes an acceleration that we can detect using our telescopes so 25% is therefore the dark matter leaving only 5% of the universe for the particles that were listed on the t-shirt right there okay so clearly there are mysteries in physics no one can claim otherwise but none of this stuff dark matter and dark energy it doesn't affect our everyday lives in fact there's a sense in which it's kind of every day it's kind of down to earth I mean after all dark matter is matter and we know what matter is and what it does there's probably some particle in fact it's full employment for graduate students in theoretical physics to invent their own Dark Matter particle you haven't worked hard enough if you haven't invented your own dark matter particle in modern physics likewise the dark energy we have a perfectly good explanation for what that could be Einstein's cosmological constant filling all of space we don't know the details but it's easy to explain these phenomena in broad terms so it's sort of normal science in Thomas Kuhns sends to figure out what these things are in addition to these kinds of mysteries we have deeper mysteries and that's why I want to do for the rest of the talk I want to bring you up to speed on some of the even deeper mysteries some of the foundational mysteries that call into question the whole paradigm that we tend to use there's a there's a secret way of doing physics implicit in this equation quantum field theory that has loopholes has things that might not be up to the task in some way and that's what I want to mention for you so I'm gonna have three mysteries mystery number one is quantum mechanics I already mentioned quantum mechanics you've heard of it how could it possibly be mysterious Paul Dirac whose name was already named tracked here one of the founders of quantum mechanics did a lot of stuff one Nobel Prizes we can use quantum mechanics to extraordinary precision to make predictions to build new technologies to understand why the table is solid why the sunshines etc nevertheless my esteemed predecessor at Caltech richard fineman said very famously I think I can safely say that nobody understands quantum mechanics this statement is still true if you interpret what he meant as I think I can safely say that nobody other than me understands quantum mechanics not because Fineman understood it or not because I understand it but there are people who think that they understand quantum mechanics but they don't think that other physicists understand quantum mechanics there's no consensus on what really is happening and this is a long story deserving of many lectures all by itself quantum mechanics are sort of put into its final form circa the 1920s the late 1920s and there's a bit of a tussle between on the one and people like Einstein and Schrodinger who said you know look so far so good I'm impressed but you can't say that we're done we need to keep going though that we but deeper understanding is required here and on the other side people like Niels Bohr and Berner Heisenberg and Wolfgang Pauli who said no no we're fine there's some mysteries in there but we'll think we don't need to figure them out it's good enough for government work the motto that is put here is shut up and calculate no one ever said that sincerely but we accuse each other of thinking that way so what this is what Fineman means it's not that we can't use quantum mechanics but when you ask what is really going on and I'll explain what I mean in a second we can't give you a significantly consensus answer and that's okay not understanding things having mysteries is great that's how you make progress trying to resolve the mysteries what's not okay is that we're not trying that the field of physics over the last 90 years has largely abandoned the project of trying to understand what goes on in quantum mechanics i analogize it to this famous fable Aesop's fable the Fox and the grapes do you know this one the Fox was walking around he sees this bunch of grapes up on the vine and the Fox says I want these grapes he jumps to get them they're just out of his reach he cannot get the grape so after a few tries the Fox says you know what I never wanted those grapes anyway they were probably sour you're not supposed to explain your parables but just so it's clear the Fox represents physicists the grapes represent understanding quantum mechanics we used to admit that we wanted that and now we're like no no we don't even want to understand quantum mechanics what's up with that let me give you a very brief reminder of why quantum mechanics came to be and what it says so here's another Cambridge icon right the Rutherford atom this is something I already showed you but we just gonna boil it down to its essentials nucleus in the center electrons orbiting around it as if they're planets in a solar system right so sorry Cambridge but I have to say this is nonsense this picture we know this picture is not right they knew it very very quickly after they invented it why because another famous Cantabrigian Isaac Newton came up with this thing called classical mechanics that was the predecessor to quantum mechanics and classical mechanics is pretty straightforward about what happens when particles and forces move around in the 19th century we put together this theory of electromagnetism Faraday and Maxwell and their friends and it said the following thing if you have a charged particle like an electron there's an electric field pointing toward it so if I move the electron the electric field adjusts to point to the new position so if I move the electron up and down the electric field responds by waving and those ripples move out at the speed of light because they are light that's what light is all of light in this room is made by electrons waving up and down giving off electromagnetic waves that's how light works these electrons should be giving off light they're spinning all around right this counts orbiting like that counts as being shaken up and down so classical mechanics together with the specific equations of classical electromagnetism predict that the electron should be losing energy and therefore spiraling in to the center of the nucleus they should not remain on constant orbits you can even ask how quickly it should happen and the answer is about 10 to the minus 11 seconds about 1/100 of a billionth of a second so I fear Attica physicists myself but it's always good to do experiments to see what happens so let's do this one it didn't happen this table you me the earth should all collapse to a point in a tiny tiny fraction of a second if classical mechanics were true now normally when you come up with a anomaly like this the physicists first move is to be conservative right can we just change the rules a little bit to save the phenomenon but eventually if you try very hard you're led to something completely dramatic in paradigm changing that's what happened in the case of the atom the idea that people came up with was electrons are not particles despite the picture that they look very particle like really the electron is spread out in a sort of a cloud that we cleverly called the wavefunction it's not really very clever so it's boring name in the world for the most interesting and important concept in the world the electron is stable with a certain size around the nucleus because it's kind of like taking a string that is attached at both ends and plucking it there's going to be a lowest frequency it can vibrate at and there's gonna be harmonics that will do come in a discrete set likewise the various shapes of the electrons wavefunction sitting in the vicinity of an atomic nucleus come in a discrete set there is a lowest energy place the electron can be and it's not a point like thing at the center of the nucleus it's a little cloud that is spread out over a certain size and then there's a rule that two electrons can't be doing exactly the same thing so if you put more and more electrons around the nucleus they get more and more Baroque shapes any of you who've taken chemistry classes have been tortured by pictures like this orbitals right you've heard of these words okay so this is a wonderful idea it explains why atoms don't decay it even explains why certain kinds of radiation from atoms come and discrete predictable frequencies because you're jumping from one of these sort of energy states to another one best of all there is an equation I love the equations here this is the Schrodinger equation okay I'm not going to go into the details of explaining this one any more than the last one but the point is this wave function for the electron the Greek letter psy obeys an equation which means that we're back in the good old paradigm of physics where I tell you what the thing is doing right now this equation is what tells you what happens in the future it's once again one of these patterns that is obeyed by nature for a physicist having an equation like this is just the happiest place they can be because they can assign problems to their students solving the Schrodinger equation in all sorts of different situations this is an enormous triumph at a great cost because you changed quite dramatically what you meant you thought the electron was a particle now you're saying it's a wave there's a problem when you look at the electron which is that it doesn't look like wave so you can use the Schrodinger equation to make predictions like when a radioactive substance emits a particle how does that particle move away from the substance it was emitted by like a nucleus decaying and spitting out an electron the answer is if it looks very much like one of these shapes it will sort of spread out in a more or less spherical configuration a big puffy cloud they'll move out in all directions and get essentially attenuated over time then when you look this is what you see this is an actual uranium sample in a cloud chamber which means that when a particle moves by a charged particle will take off some electrons from the atoms nearby and you will see that as a little bit of a cloud and what you see very clearly is not one big puffy thing but a set of individual trajectories as if a particle has been emitted from the substance and is moving away from it when the radioactivity occurs this is really hard to wrap our heads around if we already said that electrons are not particles they're waves it's almost as if the electrons are waves when we're not looking at them in particles when we look at them so the greatest minds in the history of physics got together and thought about what's going on and they came up with the following paradigm electrons are waves when you're not looking at them and they're particles when you're looking at them what can I tell you this is still what we teach our students this is called the Copenhagen interpretation or the textbook interpretation of quantum mechanics it says that on the left you might have some spread-out wavefunction for an electron sitting there all by itself but when you measure it when you take a picture of it when you say where is it it suddenly collapses to a point and looks like a particle and furthermore you can't even tell me where it's going to collapse to the best you can do is calculate the probability that will it will collapse to different locations and the probability is governed by the wave function when the wave function is higher there's more probability when the wave function is very small there's less probability so I am NOT making this up when we teach Kwon to mechanics to our students whether it's Cambridge or Caltech or anywhere else there are two different sets of rules that come along with quantum mechanics there's one set of rules for how quantum systems act when nobody is looking at them and those rules are very down-to-earth and sensible there's quantum states which take the form of wave functions and they have an equation the Schrodinger equation it's very much in the classical paradigm and then when you look at them when you measure them when you observe them all hell breaks loose you can't predict exactly what's going to happen it's just a probability and the wavefunction changes suddenly and discontinuously this is what we teach our students the problem with this is that it's clearly nonsense this is clearly unacceptable as a fundamental theory of nature so here are two problems one is the measurement problem which is just if you look back at these rules what in the world do you mean by when somebody looks at it what what counts as a measurement does it have to be a human being I mean could a cat count looking at something what about a video camera would that count what if I just glanced at it at the corner of my eye how quickly does it happen why is it probabilistic etc so none of these questions is answered by the conventional Copenhagen interpretation this is why people like Einstein and furniture they didn't say quantum mechanics is wrong they said that's a good sort of halfway put-together idea we need to do better we need a more mechanistic underlying model and the other one i want to emphasize is the reality problem right I mean I said quite explicitly because I think it's true the electron is a wavefunction right that's what it is not a particle but is that exactly right there are other people who will say well reality is partly wavefunction but there's other things going on also maybe the reason why electrons act like waves sometimes in particles other times is because they're both they're both waves and particles or maybe the wave function is the tool that we use to calculate probabilities and there's no reflection of reality whatsoever the fact that we don't know ninety years after we put this theory together is an enormous embarrassed to physics and if you tried to know if you were a student in the back of the room who raised your hand and said like I don't like this thing you're telling me I would like to try to do better he gently nudge you out of the field physically or you know you go willingly or not this did not stop people from trying so here's my favorite version of how to solve these problems Hugh Everett was a graduate student in 1950s he worked with John Wheeler who in turn worked with Niels Bohr so he was in the tradition of the great people of quantum mechanics and if anything Everett's approach was just as much therapeutic as it was scientific basically what he's saying is guys you're working too hard you don't need to bend over backwards with all these crazy rules just to understand what we see in experiments you want to know what's reality it's the wave function it is precisely represented by the wave functions nothing extra there you won't know how the wave function behaves it obeys deserting your equation you know that it obeys the Schrodinger equation sometimes I claim it obeys the Schrodinger equation all the time that's all that ever happens so he says relax of course we know that there's a problem with this but let me just emphasize how simple the idea would be if it worked here's Everett's version of quantum mechanics there's not two separate kinds of rules there's just one set of rules and it's just there are quantum systems described by wave functions and the obey the Schrodinger equation and that's the end of it how much simpler the lives of students would be when we taught them quantum mechanics if that's all there were to it the problem of course is this issue I mentioned that when we look we don't seem to see wave functions we seem to see particles so how in the world can we reconcile this very beautiful pure austere rigorous framework with the evidence of our eyeballs when we actually look so that's a little subtle but it's worth explaining so I will conjure up the thought experiment stylings of urban Schrodinger who famously put a cat half way to death inside a box so Schrodinger's cat is inside a big box there is a quantum event going on like a radioactive source which is detected by a detector and if the detector Phi it lifts open a little container and releases gas throughout the box in schrödinger's way of doing things the gas was cyanide and if the gas was released the cat died killing the cat does not increase the physics understanding in any way whatsoever so in my version it's just sleeping gas and when it's release the cat goes to sleep but there's still a little smile on the cat's face okay no harm is done to any cats in these thought experiments the point of Schrodinger's thought experiment which was developed in correspondence with Einstein because they were on the same side in this debate it's not to say Wow look how cool quantum mechanics is it's to say surely you don't believe that because if you believe the rules of quantum mechanics it's not that the radioactive source either decays or doesn't it's that it has a wave function which is a superposition of I've decayed and I have not decayed just like the electron does not have a definite position it's a wave function spread out over all the different possibilities the quantum state of the radioactive source is not one or the other it's a superposition and when it comes to tiny microscopic things like a radioactive nucleus we think that's okay Schrodinger's thought experiment is meant to amplify that quantum weirdness to the macroscopic level so if you believe the Copenhagen interpretation that wave functions obey the Schrodinger equation until you look at them and you don't count the cat as an observer then what happens is the detector goes into a superposition of I've detected a decay and I haven't the container goes into a superposition of I have opened up and I haven't and the cat goes into a superposition of I am awake and I am asleep so that's the real off of Schrodinger's cat it's not that we don't know it's not that there's a cat in the box that is either awake or asleep and we're just ignorant of the truth it's that there's a prediction of quantum mechanics that there is a cat inside the box that is in a superposition of both awake and asleep and the story the Copenhagen interpretation would tell about this is to say here's the cat superposition of awake in asleep but the observer in square brackets is classical so I'm putting quantum things in parentheses classical things in square brackets here the role of the observer is played by Niels Bohr and the story that you're told is when you open the box there's a collapse of the wavefunction you never see the cat in a superposition of awake and asleep you either with some probability see the cat awake and the cat is awake or you saw the cat asleep and you see the cat asleep and that's what the cat was doing here is Everett's version of exactly that same story remember there's no collapse of the wavefunction but also there's no classical world you don't treat in Everett's formulation human beings any differently than you treat electrons they also have a quantum mechanical state and Everett makes use of a crucial feature of quantum mechanics that was highlighted by Einstein and collaborators in the 1930s called entanglement this is a longer story but in quantum mechanics different subsystems of the universe do not have their own wave functions there is only one wave function for the whole universe Everitt cleverly called it the universal wavefunction Stephen Hawking called it the wave function of the universe it's the same idea everything belongs to the same wave function and therefore you can't isolate the quantum state of one thing the quantum state of something else the quantum state of a third thing you have to talk about all of the universe at one time that enables this thing called entanglement where the observable features of one subsystem of the universe might be related to the observable features of another one in a way that would be impossible in classical mechanics so here's the everett version of Schrodinger's cat you start with a cat and the observer hears the cat in a superposition the observer now played by Hugh Everett is treated quantum mechanically therefore parentheses and measurement is nothing spooky anymore remember Everett just says you just evolved everything according to the Schrodinger equation there's no separate kind of evolution and here's the interesting point everyone agrees no matter what you feel about quantum mechanics everyone agrees that if you just take the observer and the cat and treat them using the rules quantum mechanics we know what they're going to evolve into what they will evolve into is a superposition of an entangled state where part of it has the cat awake and the observers saw it awake and part of it has the cat asleep and they saw it asleep everyone agrees on that part the question is what are you going to do about that and the question comes up because you would think that if that happened to you you would feel like I've kind of both seen the cat awake and seen the cat asleep but I've never felt that way no one has ever felt that way therefore this can't be right whatever it's genius was is to say that both of these parts of the wavefunction exist and are real but they go their own separate ways this part of the wave function will never interfere or otherwise affect that part of the wave function and there's math involved with this and you can argue for it etc but the point is these two parts of the wave function once that entanglement happens behave as if they are separate worlds so you can just point at you can say the me that is doing the experience of looking at the cat is not the combination of both of these things it's that there was one me and now there are two Me's there is a me that saw the cat awake and to me that's all the cat asleep so as time goes on the universe has a wave function that branches into more and more separate worlds this is why a good PR man in this case Bryce DeWitt relabeled the Eveready in quantum mechanics theory as the many-worlds interpretation of quantum mechanics but what I want to emphasize is at no point does Evert or do it or anyone else put in a bunch of extra worlds the worlds were always there all Evert ever did was remove things namely some clunky unjustified extra rules about what happens to quantum systems when you observe them if you believe that an electron can truly be in a superposition of here and there then you should believe that a cat can truly be in a superposition of awake and asleep and therefore you should truly believe that you can be in a superposition of having observed the awake cat and heavy observe you sleep cat and the universe can be in a superposition of those two things and then it's just math to show that those two parts of the superposition have a life of their own no longer affect each other our separate worlds that's not to say this is true if I had you know a 10 lecture series rather than just this one we could mention all the alternatives to this there are some very very bright people in this room who don't quite believe this picture no matter how compelling and obviously correct it is but the lesson that I want to drive home is not that many worlds is right it's not that ever it is right that's true but that's not the lesson that I'm trying to get at the less I'm trying to get at is that it matters I mean sure what the dark matter and dark energy are are perfectly good mysteries but this is a prior mystery this is a mystery has been around for longer than that and it's a deeper mystery it's not about like identifying some substance it's the basic rules by which nature operates we should not abandon the project of trying to figure this out finally maybe right that we don't understand quantum mechanics but we shouldn't be proud of it we should be embarrassed and move forward which brings us to mystery number two which will be related mystery number two is how gravity and space-time come to be so again this is a situation where there are non mysterious things this young man Albert Einstein everyone shows you a picture of when he was not a young man right but when he was young someone was combing his hair he was a sharp dressed guy he was not wearing the grumpily sweaters and he was inventing theories that changed how we thought about the universe special relativity came along in 1905 that's the theory where space and time are both part of four-dimensional space-time the speed of light is an absolute limit and so forth general relativity comes along ten years later that's the theory that says space-time has a life of its own it is curved it is warped it is dynamical and you and I feel that warping as the force of gravity Einstein says the reason the laser pointer falls down is because the earth has energy that energy bends the space-time around it and all the laser pointers trying to do is describe straight as possible line in this curved space-time background this theory as I'm sure you know has been extraordinarily successful Arthur Eddington another Cambridge guy showed that space-time the prediction that Einstein made that space-time can deflect light can be experimentally tested recently we tested it to much higher precision by looking at gravitational waves emitted by in spiraling black holes that's the good news the good news is this wonderfully amazing theory and you know I don't I feel bad zooming over it so quickly Einstein wrote down some equations in 1915 and implicit in those equations even though he didn't know it at the time were the ideas of black holes and in spiraling and gravitational waves and gravitational wave detectors and in 2016 a hundred years later we actually detected it like that the power of the combination of the human imagination and nature's stubborn insistence on obeying the laws of physics enables something scribbled down in a notebook in 1915 to show up in a billion-dollar experiment a hundred years later which I will never get tired of being amazed at the bad news is I just told you about quantum mechanics Einstein's theory is resolutely classical it says that there is something called space-time I can tell you how it evolves you can measure it etc there are no wave functions no entanglement nothing like that but in some sense in a very clear sense quantum mechanics is more fundamental than general relativity is general relativity is in the classical paradigm quantum mechanics throws away the classical paradigm how are we gonna get them together people have tried to do this without that much success so far so what I want to suggest is there's a way forward but first just remember this motto general relativity boiled down to two words geometry energy the geometry of space-time is influenced and influences in turn the energy within it so here's my suggestion even though physicists trained on quantum mechanics or very smart cookies and know a lot about what they're doing they still grow up thinking classically they're no babies there is it sorry there is a book quantum physics for babies but I don't think it really works I don't think the babies really think in terms of wave functions at the Schrodinger equation most of us have a classical intuition and because of that when we try to invent quantum theories for new phenomena we traditionally start with classical theories and quantize them this I think has the potential for holding us back because Nature doesn't do that Nature doesn't start with a classical theory in quantized nature is just quantum from the start so maybe it will work sometimes and indeed that picture I showed you with the big equation of the core theory is a series of success stories where you start with classical versions of electromagnetism and quarks and so forth and then you quantize it but when it comes to gravity that seems to have failed us if you quantize general relativity in the straightforward way it does not work what I want to suggest and other approaches like string theory are essentially proposing different classical theories that you then go quantize what I want to suggest is that maybe we should take nature seriously take quantum mechanics seriously and rather than trying to quantize gravity to try to find gravity within quantum mechanics not to start with curved space-time but to start with wave functions and entanglement and ask if we can point to curved space-time emerging out of those quantum notions so we're allowed to cheat when we play this game okay we're gonna cheat by using things that we know about the world from where physics does work forget about gravity think about quantum field theory like the core theory I showed you this is an another wonderful feature of modern physics that the world is really not made of particles it's made of fields so you know about the magnetic field here's the magnetic field in action around a magnet but in fact the electrons and protons that make up stuff that make up matter are also vibrations in quantum fields the real stuff of the world according to quantum field theory are these fields that pervade all of space and where you think that there isn't anything where you say oh there's no particles or forces or anything here it's just that the quantum fields are there but they're what we call in their vacuum state they had the least amount of energy it is possible to have this point of view changes what we mean and how we think about empty space so if you were in a particle view of nature you would think that there are particles and there's some distance between them but in between there was just space that's all there was and space was the same at every place but if you think of the quantum field view a particle is just a field that's vibrating a little bit more than it wants to and all of the other regions of space also have quantum fields in them so they're both fields so they're there and they're quantum so they can be entangled so different parts of empty space have a very interesting complicated quantum structure there are people there particle physicists whose life is devoted to understanding the vacuum to understanding empty space most physicists want to have at least a particle or two in space before they analyze it but there's a lot of richness just there in empty space and there's a feature of this entanglement so if you look at nearby regions of empty space so here I removed all the particles space is just empty but I salute a region another region nearby and there are quantum fields vibrating in those regions and guess what I can calculate the entanglement between them and the answer is if the regions are nearby they're highly entangled if the regions are far away they're not very entangled at all so when I'm in this context where I have space and geometry and so forth there's a relationship between geometry of space and entanglement what I want to suggest is we can reverse that relationship when I mention that relationship I was talking the language that space already existed but if we want to see whether space can emerge from quantum mechanics all of our initial vocabulary should be words like wave functions entanglement and so forth so instead of saying when two regions are nearby they're highly entangled let's say when two Quantum degrees of freedom are highly entangled they are nearby that is what we mean by nearby nearby is just a colorful way of talking about the amount of entanglement between abstract quantum degrees of freedom so if you give me a whole collection of abstract quantum degrees of freedom and the amount of entanglement between them there can be an emergent geometry here's a three-dimensional geometry here's Euclidian geometry here's a sphere depending on the quantum state of the system you're looking at keep that in mind and I will also mention something a bit more subtle this relationship between entanglement and energy remember when we had just empty space I could tell you how entangled different things were now let me start putting particles there let me add energy to a region of space by putting particles in it but putting particle in it in this language is actually just making the field vibrate more there and what that does is it breaks the entanglement between that region and the regions around it so when you put particles and therefore energy in a region of space the entanglement between that region and the regions around it goes down so I want to play exactly the same game let me go the other way around when I have a collection of quantum degrees of freedom if I lower the entanglement I interpret that as putting energy into that region therefore giving me a relationship between entanglement and energy so what I have I know this is very fast but you're gonna trust me what I have then is by using entanglement I can relate it to geometry I can also relate it to energy and therefore by the rules of arrows I can relate geometry and energy to each other but of course that's exactly what is time did and in fact when you attach the equations to this story I'm telling you you can derive in some very very limited contexts Einstein's equation for general relativity from purely quantum mechanical starting points in other words we don't know because there's a very long complicated program ahead of us but we've just started this very ambitious program of starting with pure quantum mechanics and seeing whether or not we can get curved space-time and general relativity to emerge from it rather than quantizing a classical theory so this is still a mystery right now but might be a mystery we can really make progress on by taking quantum mechanics seriously final mystery we just this is more or less understanding space why don't we try to understand time also right my favorite part of time is the arrow of time the arrow of time is just the fact that the past and the future are different in important ways so if I showed you these pictures of these young lads and said which was the younger picture and which was the older picture you would all know certain things happen to us as we age right certain things happen to society as it passes by when I give this talk in the United States that's Elvis that I show up there and this is the signing of the Declaration of Independence but I thought that'd be bad form to show here so Magna Carta is what's representing the past okay this of course everyone knows what this is I presume this is from Charles Darwin's notebook this is the first appearance of the Tree of Life going up so he didn't put a arrow saying time goes this way but it does this is him saying from one species you can evolve into many different species that's usually how it works the arrow of time at work the best thing the reason why everyone loves this picture of course is he says I think at the top that's a warning to the rest of us you know don't be don't be too confident why is the past different from the future that's not exactly the mystery that I want to highlight I think we mostly know why the past is different from the future and the answer is something called entropy entropy is a way of characterizing how disorderly random disorganized messy a physical system is and there's a law of nature the second law of thermodynamics that if you leave a system to its own devices if you don't clean it up fix it up organize it entropy will increase over time it's very natural it's a terrible terrible illustration that I chose because clearly putting in a frying pan is not a closed system but the point is that it's easy to turn an unbroken egg into scrambled eggs it's very hard to go backward same thing is true in your rooms here at college you might that they tend to get Messier left to their own devices they do not spontaneously clean themselves up law of nature behind that and this is a mystery this law of nature it was a mystery 150 years ago because the underlying laws Newton's laws or that equation I showed you for the core theory don't have any difference between past and future built into them and yet the evolution of entropy does have a difference it goes up as we go from the past to the future what's going on long story short the arrow of time has the same kind of origin as the arrow of space what do I mean by that if you're up in that a new astronaut suit in orbit there's no arrow of space all the different directions if you're floating in your astronaut suit are the same up down left right forward backward but here on earth there's clearly an arrow of space when I drop the laser pointer everyone knew it was gonna go down right there's a difference between down and up here on earth that's an arrow but you also are not under the impression that the reason why the laser pointer falls down is that as aerosol might have thought there's something fundamentally different about up-and-down we know it's a local contingent fact because where we live in the vicinity of a very influential object the earth if we're up in the astronaut suit that arrow would not exist likewise what I'm suggesting here is that the arrow of time has the same feature it's not embedded into the laws of physics the reflection of the fact that we live in the aftermath of a very influential event something you may have heard of called the Big Bang fourteen billion years ago the universe was very very orderly okay it was very very low entropy nobody knows why that's a true mystery that's a mystery worth writing books about and things like that again people in this room have done this we think that if you start with a low entropy past Boltzmann and Maxwell and Gibbs and others explained why entropy will tend to increase well we do not know is why it was so low in the past so the general fact that entropy increases I think is the correct explanation for the arrow of but there's still two very profound mysteries attached to it one is why did it start off so small this is clearly a job for cosmologists for physicists maybe theologians if you're oriented that way the other mystery is how exactly does entropy account for all of these other a symmetries of time why do the beetles look different in 1963 and 1969 is you really gonna tell me that's because of entropy yes yes I am going to tell you that so there's a lot of work to be done here this is why it's one of the mysteries let me give you enough of an argument that you can think it's reasonable why do you remember the past but not the future right why do you have photographs of the past but not the future this is a joke Mitch Hedberg joke he says why am i friends always showing me pictures of themselves saying this is what I looked like when I was younger don't all pictures show us what we look like when we were younger right we don't have records or photographs of the future why not so here's a record here's an egg this is what we use egg and cream and coffee these the only allowed options to illustrate entropy increase you come across an egg lying on the sidewalk broken you say what is the future of the egg hold maybe somebody will clean it up maybe a dog will come by maybe a rainstorm will wash away there's many possible futures of the egg what did the past of the egg probably hold most people would say I bet there used to be an unbroken egg and somebody dropped it so somehow just with that little piece of information we have much more leverage on saying what the past of the egg was than we do on the future of the egg why is that it's not because of the fundamental laws of physics the fundamental laws of physics given what you know there's a broken egg predict a number of possible futures and an exactly equal number of possible pasts the difference is we also have this belief called the past hypothesis that the universe started near the Big Bang in a low entropy state so adding that gives you an asymmetry between past and future there's no future hypothesis there's no future low entropy state at least not in the near few so knowing both the current state and the fact that it came from low entropy allows us to reconstruct that the most likely way to get a broken egg is to get an unbroken egg and break it so this is a paradigm this is a way of getting at the many different ways that the past and future are different from each other add in the fact that entropy is increasing from a low entropy past and then argue about the macroscopic visible phenomenological everyday world that's where the arrow of time is so vividly observable but that there's a lot of work remaining to be done there you know there's no equations in this picture okay there are many ways in which the past is different from the future not just memory not just aging but again biological evolution what about cause and effect if I swing my arm and I knock the glass off the table you're very naturally going to say something like the glass fell because I swung my arm no one has ever tempted to say I swung my arm because the glass was going to fall there's a temporal asymmetry here I think it's because of entropy but I haven't actually proven that mathematically yet I'm literally working on a paper about that I think it's provable but this is an incredibly rich area to think about for all the young physicists for proto physicists out there the fact that entropy is increasing is enormous ly important literally to our existence here's a picture by a famous artist named Roger Penrose who's also dabbled in mathematics and physics the point of this picture is when you think about the Sun the Sun is a bright orange thing that we see in California that you may never have seen here okay but what do we get from the Sun well you might say we get energy from the Sun true but it's not quite the point the earth modulo global warming the earth radiates back into the universe just as much energy as it gets from the Sun the difference is for every one photon of light that we receive from the Sun we radiate away twenty photons back to the universe we get visible light from the Sun and we radiate infrared light back to the universe with on average one twentieth of the N each again go to the math what that means is we have increased the entropy of that energy we get from the Sun by a factor of 20 the reason why life is possible is because the Sun is a hot spot in a cold sky reflecting the fact that we live at very very far from equilibrium existence if the Sun was not there and the whole sky was the temperature of the night sky we would all freeze and life would cease but if the Sun if the whole sky was the temperature of the Sun we would get a lot more energy and we would all burn and all life would cease what keeps us going what keeps the fish rolling around in the fishbowl is the fact that there's all this low entropy energy which we can then use to live and give back to the universe in a highly degraded form more stuff we would like to understand better we have a sketch in our brain of what's going on but nowhere near a full quantitative understanding finally sometimes people say when they see things like cows and people and fish and colleges and countries they say you know all this looks very organized to me some countries more organized than others we can bond over the lack of sensible organization our countries of late but where did that come from if the natural law of nature is for entropy and disordered just to increase how would how in the world does increase of disorder lead to such exquisitely ordered things like people and societies so the answer again we don't know the full answer but we have the hint the hint is entropy can increase here's cream and coffee going from low entropy to high entropy all separated low entropy all mixed high entropy but you notice that this is very simple all the creams on the top all the coffee's on the bottom this is also very simple they're all mixed together it's in between when the cream and coffee are just beginning to mix together where it's a medium entropy state that's when things look complicated that's when there are swirls and tendrils and fractal patterns inside what's going on literally if you took a photograph of this and put it on your computer this image and this image would take up less bytes on your computer than this one because there's more information there to be stored in the image that seems to be not an absolute rule but a very common feature in the universe entropy just goes up but complexity first goes up and then goes away as you approach equilibrium so the point is it's not despite the fact that entropy is increasing that we can get orderly things like you and me it's because of the fact that entropy is increasing if entropy were not increasing we would not be able to use this resource that we have to live and think and do interesting things and this idea that complexity first comes and then goes is not just cups of coffee the universe shows exactly the same behavior at very early times it was very very simple and low entropy at late times it will be very very simple again and high entropy it's in between it's right now cosmically speaking that the universe is at its most complex and most interesting we live in the fun Friday night let's go crazy part of the history of the universe something to be grateful for so just to conclude my very last slide is there are so many ways in which these mysteries could possibly be resolved yet or remain to be resolved I'm trying to highlight there's sort of the famous mysteries that everybody talks about what I've tried to do in the talk is to highlight some of the lesser-known but in some sense more profound mysteries that we have and I'm very much of the belief that concentrating on these deep mysteries opens the possibility to understand a whole bunch of things there's no quantum gravity of course here's a black hole artist's conception this much less interesting picture is an attempt to apply quantum mechanics to the universe as a whole to think of the whole universe as a quantum computer in some sense in a very different vein this is a potential beginning place for life itself life is not something that is anti-entropic life is something that uses increasing entropy to come into existence filling in the details will take time and then once life starts how life works is something which also relies on increasing entropy we have some crazy ideas me and others about why the early universe had low entropy that might take us back to the starting point trying to understand quantum mechanics and the emergence of space-time so I think that you know the lesson is mysteries are the good part of science right mysteries exist on all levels of complexity and solvability physics is nice enough to be in a situation where some questions have been answered and yet the ones we are still facing are very very deep indeed thank you very much [Applause] [Music] thank you [Applause] thank you very much indeed John thank you for taking us from you know this guided journey from everyday core physics which seemed pretty straight forward the way you put it to the mysteries of quantum entanglement many worlds space time and gravity time entropy and so on I mean in some ways you know for all of us non physicists it's really quite encouraging that even physicists have no common understanding of these hard matters and I just note that Max Planck once commented that science cannot solve the ultimate mystery of nature I wonder and I'm sure it's a question for hours and hours of discussion later tonight you know could there be some day a common understanding how far can we get in these mysteries so next week we move back in time to ancient Greek technology and astronomy when dr. Joe Marchant journalist and also intellectual on decoding the heavens the Antikythera mechanism and we hope to see you there then you'll hear then rather so finally Sean thank you very much indeed for coming to Cambridge to talk to us this evening fascinating lecture thank you [Applause] you

Channel: Darwin College Lecture Series

Views: 522,628

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Keywords: Darwin College, Darwin College Lectures, Darwin College lecture series, Enigma, mysteries of physics, Sean Carroll, Professor Sean Carroll, Caltech, University of Cambridge, Quantum physics, quantum entanglement, gravity, neutrinos, Higgs field, Higgs boson

Id: rBpR0LBsUfM

Length: 66min 38sec (3998 seconds)

Published: Wed Jan 29 2020