Sean Carroll: The many worlds of quantum mechanics

it's especially thrilling for me to get to talk on the subject of today's talk about quantum mechanics I love quantum mechanics I think that everyone should love quantum mechanics and know more about it it gets me so excited that I'd like to begin that talk on quantum mechanics by doing a little hop up in the air a little physical gesture to let people know how excited I am the problem is I've never been able to decide should I hop to the left or should I hop to the right fortunately I have quantum mechanics to help me out so here this is an app you can get on your iphone called universe splitter what will happen is I will push the button it will send a signal to a laboratory in Geneva Switzerland that will send a photon down a beam and then the photon will be split left and right and according to the rules of quantum mechanics there's a 50% chance that it goes left and a 50% chance it goes right according to the many-worlds interpretation of quantum mechanics both of those options actually become real in separate worlds so there will be a world in which I hop left and one in which I hop right let's see which one this will be we were splitting the universe we've contacted the lab the device is ready the photons emitted the universe has just split you are in the universe in which you should hop right thank you according to the many-worlds formulation of quantum mechanics there is another truly existent part of reality in which I just hop to the left and I'm right there and that person right there is no longer me it's a different person is gesturing over to their right saying that there's another version of me over there ok this is not science fiction this is what many of us including myself think is likely to be true a correct description of nature this many-worlds version of quantum mechanics there really is another world just like that and what I would like to do in today's talk is to you the reasons why otherwise sober-minded physicists would think something so obviously crazy alright let's get going so before we get to quantum mechanics the best way to understand it is to contrast it with classical mechanics there really been two great revolutions in the history of physics the classical physics revolution from Isaac Newton and the quantum mechanics Newton revolution that happened in the 20th century so Newtonian physics classical mechanics this is what you learned in high school in beginning of university this is what we torture our undergraduates with with inclined planes and pendulums and frictionless surfaces and billiard balls and all that stuff right even though there's a lot of math you have to do a lot of problems it's still pretty close to our intuitive view of how the world works there's stuff well there's billiard balls or swing sets or rocket ships and that stuff moves through space as time flows by there are rules about how this happens this rule is the laws of physics in particular Newton's second law F equals MA when you put a force on something it accelerates proportional to the amount of force okay Newtonian physics is a clockwork universe in the following sense if you tell me everything that's going on in the universe which is to say the position and the velocity of every piece of stuff in the universe the laws of physics can tell you exactly where it will go in the future and indeed exactly where it was in the past there is no obstacle to measuring exactly what the universe is doing in principle and practice that can be very hard that's what we were taught as youngsters and then came along quantum mechanics and I'm not gonna give you the full detailed historical sketch about quantum mechanics unlike Newtonian physics which burst from the mind of Isaac Newton there were predecessors but he really figured it out all at once quantum mechanics had a difficult birth it took many people over the course of years to figure it all out so this slide is just to let you know to remind you that the crazy ideas were going to be talking about in the rest of the talk were forced on us by trying to and the data okay they're not things that we sat around in our room late at night with a few pints going man what if there were like a lot of universes out there okay we are driven to the crazy ideas I'm going to be talking about today by trying to account for what we see in nature so for example there was moths plunk who has really launched the quantum revolution by trying to understand the radiation of hot objects we had a theoretical understanding of that but it was all wrong and plunk said well maybe when light is emitted from these hot objects it comes in discrete packets but it wasn't until Einstein five years later that he realized he said maybe just light always comes in discrete packets the difference is what plunk said was basically radiating bodies like a machine that makes you one cup of tea at a time Einstein said maybe tea only ever comes in one cup in increments that's a much more radical idea maybe what we thought was a wave is really more like a particle meanwhile Niels Bohr realized that if you believe electrons orbit in atoms they should spiral in and all atoms should collapse in a tiny fraction of a second so he said they don't for some reason it's very very mysterious why Bohr said that they shouldn't collapse but then Louis debroglie came along and said well maybe it's because the particles that we think of as electrons are really waves so you see what's going on and why I was so confusing Einstein said many maybe waves are really particles your Burleigh says maybe particles are really waves it wasn't until the late 1920s that we basically figured it all out there were two people who figured it out more or less simultaneously it looked very different but we later figured out it's the same theory Heisenberg and Schrodinger and they said you know put aside the idea of a particle with a position and a velocity invent something new which we call the wave function this Greek letter sigh there's not gonna be a lot of equations in the talk but I think I should show you this one equation this is Schrodinger's equation this is the equation that tells you how a quantum mechanical wave function evolves with time so according to this equation if that's all you have it says that there's a quantum state a wave function you ask how much energy describes and that tells you how fast it's evolving that's all you need to know Schrodinger's equation is the quantum version of Isaac Newton's second law just like if you knew the positions and velocities of all the particles in a system any new Newton's second law you could tell how it would evolve if you knew the quantum state of a system the wavefunction of a system and you have Schrodinger's equation then you can tell exactly how that wavefunction will evolve so far it seems completely parallel to classical mechanics what's the big deal why does quantum mechanics have this reputation of being so hard here's the answer there's an amazing feature of quantum mechanics that was nowhere to be found in classical mechanics which says that what you observe when you look at a system is not what you see what you see and what is really there are two different things there's a difference between what a thing is when you're looking at it and when you're not looking at it what you can possibly see is much less than what really exists this sounds weird the sounds bizarre like it's very very different than what we had in classical mechanics in classical mechanics it might be difficult to observe the position in the velocity of something but you could do it if you really tried hard enough quantum mechanics says that there is an unmistakable irreducible inability to exactly observe a quantum system you cannot observe the wavefunction what does that mean here's what I mean for a classical particle somewhere in space on a billiard table or whatever the entire state of the particle is just where it is and how fast it's moving its location and its velocity from that you can predict everything in quantum mechanics rather than having a position in a velocity you have a wave function that is spread out it is what we call a superposition of every possible position that could be in so make it very clear what I'm saying here I'm not saying that we don't know what the velocity is or what the position is I'm saying there's no such thing as their actual position or velocity of the particle there is a wave funk that is spread out all over the place it's like a cloud that says for every position the particle could possibly be seen in you assign a number and that number is the amplitude the wave function of the particle that's what reality is according to quantum mechanics and it's not just philosophy or math we can see it Niels Bohr would have told you that electrons and atoms move in circles kind of like the solar system real quantum mechanics as we now know it from Heisenberg and Schrodinger says no it's not like that there's a cloud and this cloud is an actual photograph this is data this is a very very careful physics experiment that has gone in and seen exactly where the wave function of an electron is in this particular atom so it's a completely different view of reality the question you should be asking is why does reality look normal to us at all like if if really reality is a superposition of all these different possibilities why do we see things in locations why do I look at the chair and say there it is why don't I see a probability cloud all over the place we don't know that's the short answer this is what we debate about this is the controversial question in quantum mechanics we do have a story that we tell our students our unsuspecting students who we can threaten with bad grades and so forth so they have to listen to us we called the story the Copenhagen formulation of quantum mechanics and basically it says the following thing that there is some something special about measurement that the act of observing a system plays a crucially important central role in the formulation of quantum mechanics according to the Copenhagen interpretation and what happens is you have a cloud of probability spread out all over the place the wavefunction but when you observe it you see the electron or whatever in a certain position and after you observe it the wavefunction collapses right away boom and now it's completely concentrated where you saw it so there's a dramatic instantaneous change in the state of the system from being spread out all over the place - banging in one location when you made that observation okay so what is the relationship of the to the observable outcome the relationship is that the cloud tells you the probability of getting different experimental outcomes that wavefunction which is a number it can be let's say a large number in the center where the blue is very dark a small number very far away the rules of quantum mechanics in the Copenhagen interpretation tell us you if you want to know the probability I will see the electron in a certain location tell me what the wavefunction is and I will square it let's take the quantity squared that's the probability this is known as the born rule in quantum mechanics ok this is what we teach our youngsters today this is how we teach them to use quantum mechanics the problem is it is bizarre and crazy this can't be right what we teach our students cannot possibly be how nature works at a fundamental level for one thing it should bother you that things like measurement and observations seemingly play a central role in how you formulate the laws of physics what were the laws of physics doing before there was anyone measuring things and what do you mean by a measurement anyway can does it have to be a conscious human being what if I bump into an electron by accident can a rock or a virus or a earthworm do an observation how fast does it happen how quickly when does it happen how close do you have to be what if you only look at it from the side does that count none of these questions have answers in the traditional textbook Copenhagen way of teaching quantum mechanics much less why is any of this true why is there an approximately classical world why is their probabilities etc this is what is known as the measurement problem in quantum mechanics and I'm not exaggerating when I say that story I just told you is what we teach our students and if the students have these questions we tell them to shut up I don't think this is a good policy I think we should be able to do better than that because of these questions which undergo under the rubric of the measurement problem we have a whole field of intellectual endeavor called interpretations of quantum mechanics ok there was never any field of intellectual endeavor called interpretations of classical mechanics you know what classical mechanics was about there were particles or there were waves they were mu you could predict them etc but this need this apparent need in quantum mechanics to make measurements something special is baffling to us and it's not just that it rubs us the wrong way there's plenty of examples in physics where you're surprised by what nature teaches you and you have to learn to adapt the problem is these and these questions should have answers the true once-and-for-all formulation of the best theory we have of the nature of reality should be a little bit more precise and clear and unambiguous than what we teach our students when we teach them quantum mechanics so we would like to do better you might think that in that circumstance since quantum mechanics is the foundational theory for all of modern physics you might think that the quest to understand quantum mechanics at a deep level would be recognized as one of the most important things we could possibly do in physics the people who devoted their lives to these would be academic superstars you would have different universities trying to steal them away with high-priced packages and salaries and so forth and it would be the highest prestige occupation you could have in physics sadly no that is not what we do it is the opposite of that we have adopted a strategy of denial where if you're physicists and you think hard about answering these questions you are labeled not a physicist or a physicist who is too old to do important work anymore and you're sent off to retirement for example there's a quote which is true from Richard Fineman my predecessor at Caltech might my claim to fame at Caltech is that I sit at Richard Feynman's old desk and people ask me why I got Richard Feynman's old desk and the answer is it goes to the most senior theoretical physicist at Caltech who does not deserve a brand-new desk when they move there and Fineman said many things he's very quotable physicist and one of his quotes is I think I can safely say that nobody understands quantum mechanics now this is true if what we mean by this is nobody has an understanding that all the other physicists agree is right ok some physicists think they understand quantum mechanics but their friends don't agree with them ok now this is fine as far as it goes but here's the problem rather than the rest of the community saying no one understands quantum mechanics therefore we better devote our resources to doing so we say nobody understands quantum mechanics and they never will and that's ok this is what I think is a terrible embarrassment for the field of physics I think we need to do better it's kind of like the old fable right Aesop's fable about the Fox and the grapes you know this one the Fox sees the grapes and he says though you had good juicy grapes I want to get them hops up and down tries to get the grapes but can't reach them so the Fox decides you know what I didn't want those grapes anyway they were probably sour so in case the metaphor is not clear the Fox represents physicists and the grapes represent understanding quantum mechanics ok in the first few years after Schrodinger's equation and so forth we did try hard to understand quantum mechanics the Giants of the field Einstein Bohr etc they didn't succeed to the satisfaction of anybody else and therefore for whatever reason the lure became you shouldn't try there was even a memo sent around by the editor of the Physical Review Journal saying that if anyone sends in a paper on the foundations of quantum mechanics you should reject it without reading it when I recently applied for grant money I was told that if one if I want to talk about my research in cosmology and particle physics that's good if I want to talk about my research in the foundations of quantum mechanics I shouldn't don't include it don't let anyone know you're doing that you might get in trouble I think we should do better I think we can do better let's see what doing better might look like you've seen this one right Schrodinger's cat okay there's an experiment that was invented by Erwin Schrodinger and it's important to understand why it was invented fertig er like many of the pioneers of quantum mechanics his invention went far beyond what he expected okay so he invented the wave function and furnishers equation but it had implications he didn't like Schrodinger like Einstein was convinced this whole measurement problem business was completely crazy so the implication of Schrodinger's equation that the existence of a wave function is that if we have different possible measurement outcomes before we look at the system the system is in a superposition of different of all the possible measurement outcomes so it's hurting or did was he invented this thought experiment to take that idea of a superposition of different possibilities and amplify it until it was macroscopically real so the setup is you have a Geiger counter and that's important because there's a little radiative source a little bit of radioactive material that has a certain probability of emitting a particle and that's a quantum mechanically on a certain event that's what Schrodinger's equation does it predicts what that probability is and in the thought experiment when the Geiger counter clicks a hammer Falls it breaks a vial of cyanide poison this is all in a box with a cat the cat was alive but if the vial breaks and the poison gets out then the cat dies okay personally I see no reason to kill the cat so I would like to replace the side I was sleeping gasps we can talk about an awake cat and then the sleep cat rather than a live cat or a dead cat urban Schrodinger's daughter Ruth actually said I think my father just didn't like cats we don't have to be that way about the cats so the point is that what Schrodinger was trying to illustrate is that according to classical mechanics it might very well be the case then you have a box with a cat in it and you don't know whether the cat is awake or asleep you can just have a lack of knowledge but there's still a reality there still is a single answer even if you don't know even if you would describe what's in the box as a 50% chance of a cat being awake a 50% chance of the cat being asleep there is a truth to it and the quantum mechanics says something very different quantum mechanics says that after this experimental setup has gone forward that cat itself is in a superposition of being awake and being asleep now what is the lesson we're supposed to draw from that Schrodinger says the lesson is surely you don't believe that macroscopic objects like cats can be in super positions of being awakened asleep in two different positions okay so he thought that this was a reductio Abed tsardom that something had to happen in quantum mechanics he sure what it was but like Einstein he thought that quantum mechanics was incomplete as a theory of reality despite the fact that furniture didn't like halves anymore this thought experiment is really very helpful in sharpening your quantum mechanical intuition so if the world is truly quantum mechanical we should change our view of what is obvious and what is surprising so for a classical person we might say to ourselves cats as I have ever seen them are either awake or asleep they're never in a superposition of both but and this is exactly what Schrodinger is thinking according to the laws of quantum mechanics after that experiment the cat truly is in a superposition of both awakened asleep that's weird I don't get it okay that was for two girs point of view I would like to suggest that there's a different intuition you should have if you've truly absorbed the lessons of quantum mechanics which is the following of course cats can be in arbitrary super positions everything in the universe can be in arbitrary superpositions this idea of objects having positions and velocities and so forth is just a relic of our pre-existing classical intuition what is weird what is surprising what is hard to understand is that when you open the box you never see a cat in a superposition that's what's weird the cat being in a superposition is just the rules of quantum mechanics that makes perfect sense the weird thing is that when you observe the cat you only ever see it in one or the other either awake or asleep what can possibly be going on and people have been driven in the early days of quantum mechanics to suggest maybe there really was something special about conscious perceptions maybe the human mind really changed reality by interacting with it we don't have to think that any more this is an old out-of-date way of thinking we can do much better than that and that's what I want to tell you about so here is what the textbook Copenhagen version of quantum mechanics would say about fur dinners cat it says that there is a quantum system in this case the cat but there's a separate classical world where you and I are in according to the Copenhagen version of quantum mechanics you and I are not quantum mechanical really obey the rules of classical mechanics it's not just a mistake the tiny little microscopic system or in this case the cat might be quantum mechanical but there's also a big classical world and these two worlds can interact with each other so you might have a state of the universe where the cat's in a superposition and there's an observer who's about to open the box when the observer opens the box and looks inside the wavefunction of the cat collapses and you will either see the cat awake or the cat asleep there was a combination of both but then one of them disappears quickly and miraculously and instantaneously and we're left with the other one this is what we teach our students and then you see what the problem is right what if I just peek into the box you know like what if there was a fly that went into the box there's a million questions that this perspective does not answer it doesn't feel like the fundamental way that nature works in a very deep way how can we fix it well remember that before we added all these weird rules about measurements and observation if we just had the wave function and Schrodinger's equation quantum mechanics looked a lot like classical mechanics there was the state of the system it evolved according to an equation and it would evolve deterministically forever it was only when we added in the weird rules about measurements and observation that things got hairy so let's try the following thought experiment let's erase all of those rules let's imagine that there simply is a wavefunction and everything is quantum mechanical not only the cat and the Geiger counter and the radioactive substance but you and me let's imagine that everything obeys the rules of quantum mechanics so the rules of quantum mechanics are you obey Schrodinger's equation and that's it what would happen what's the worst that could happen let's ask ourselves if you get pretty bad is the answer but so what if there were no such thing as wave functions collapsing what if there was just the smooth deterministic evolution of the Schrodinger equation so rather than a classical world with observers in it and a quantum world with atoms and cats and so forth everything is quantum mechanical but remember I said there's a wavefunction for a particle or a cat and so forth there's a little bit of a subtlety here one of the rules of quantum mechanics is that there are not separate wave functions for every electron or every cat or every particle in the universe there is only one wave function for the entire universe so when you have different possible things different possible positions that electron could be in or different possible sleep or awake states of a cat all of those are described simultaneously by a single wave function that says what is the probability that the universe looks that way if we look at the whole universe all at once okay this is this leads to something called entanglement namely that the state of one part of the universe can be related to the state of another part so while we start in this quantum mechanical story the cat is all by itself it has its own wave function its own quantum state superposition of awake and asleep we the observers have not yet opened the box so we're just in a unique state we were wondering what's we're gonna see when we're in the box now you open the box and all you have to go on is Schrodinger's equation all you have are the laws of physics there's no mystical spooky collapse of the wavefunction so you simply interact physically with what's inside you see the light coming off you feel you put your hand in there and touch the cat whatever it is you're gonna do it's just the laws of physics and according to those laws of physics and here's where you have to hold on to your hats you become entangled with the cat so rather than the cat either being awake or asleep when you open the box and the other possibility disappearing probabilistically and instantly you smoothly evolve into a state where just as before there was a superposition of cat awake and cat asleep now there is a superposition of cat awake and you saw the cat awake with cat asleep and you saw the cat asleep that's what the Schrodinger equation predicts will happen there's a superposition of both of these possibilities at once now it should be clear why back in the 1920s the founders of quantum mechanics did not really take this possibility seriously namely dad it doesn't ever feel like we're in a superposition right when we open the box and we measured the spin of a particle or do some Geiger counter experiment the Schrodinger equation all by itself says that we move into a superposition but it doesn't feel that way this sort of bare version of quantum mechanics this most pure and austere version of quantum mechanics that just has the Schrodinger equation seems to be contradicted by our experience because it predicts things that we don't feel or experience ourselves right but there's a subtlety here's where things become kind of fun I just told you that there's only one wave function for the entire universe there's not a separate wave function for the cat and you etc that's where entanglement can come from so in this superposition on the bottom there is no possibility that the cat was awake but you think you saw it asleep right that's not part of the wave function of the universe so if it's true that there's only one wave function of the universe then we shouldn't simply include you and the cat we should include everything we should include the whole universe so we call that the environment literally everything other than what we care about we simply call the environment so in that box with the cat there are photons right there's light bouncing around inside the box there are atoms of air bouncing around and we don't observe them we open the box they're part of the environment so let's imagine doing a slightly more careful version of our experiment we start with the cat in a superposition we have everything else in the universe and then we have ourselves we haven't yet opened the box but this process called decoherence is simply the fact that for all intents and purposes the environment has been interacting with the cat all along right if the cat is asleep on the floor of the box or awake and climbing around the photons and the atoms in the air will interact differently with the cat so the first thing that happens even before we open the box is that the cat becomes entangled with the environment that already happens and then we open the box and we do our measuring and then we become in tango with both okay so by the time we see the cat by the time we get into the superposition that we had never really felt the cat has already become entangled with the rest of its environment now this means something very very profound once that happens once the cat has already become entangle with its environment it can never become unentangled it's very much like mixing cream and coffee together okay an increase of entropy over time it is what we call an irreversible process you notice that if you look carefully the environment states are different if they are entangled with the awake cat or a sleep cat they're very very different from each other there's no overlap there's no relationship so these two parts of the wave function the one described being awake cat the one describing in a sleep cat will never interact with each other for the rest of eternity they have split they have gone their own way okay so for all intents and purposes it is if they have become two separate worlds there is a you that sees the cat awake and there is a you that sees the cat asleep and these two versions of you can never talk to each other they can never compare notes they are the same as the version of me that hopped to the right and the version of me that hopped to the left this decoherence process this fact that the cat or whatever or you or your Geiger counter becomes entangled with the environment and that's an irreversible process branch is the wave function into two copies of the world both branches describe everything we know about in slightly different configurations and once that branching happens it never unhappen z' so the bottom line is what i described to you is pure quantum mechanics I didn't put anything in I just used the ingredients that were already there states wave functions the Schrodinger equation and so forth I simply asked the question what would happen if I left the Schrodinger equation go and watch what happened as the system evolved according to those laws of physics there's no collapse I didn't need to define what I meant by measurement because measurement was just another physical interaction I was looking at photons oh I did was obey the laws of physics what naturally happens is that the wave function of the universe branches into different parts that are not interacting with each other and describe different worlds so this is what we call the many-worlds interpretation of quantum mechanics what I've been trying to emphasize is that at no point did we put new worlds in okay the worlds were already there if you think back to what happened in the Copenhagen textbook version of the Schrodinger cat experiment we had to erase part of the wavefunction if you saw the cat awake you erase the part where the cat was asleep and vice versa the many-worlds interpretation of quantum mechanics is based on the fact that you are quantum mechanical just like everything else if an electron can be in a superposition of that place in that place then a cat can be in a superposition of awake and asleep and then you can be in a superposition of seeing the cat awakened seeing the cat asleep and the universe can be in a superposition of one where you saw the cat awake and one where you saw the cat asleep this is automatic in quantum mechanics unless you get rid of it so as one physicist said it's not that many worlds is a theory of extra worlds it's that every other interpretation of a quantum mechanics is a disappearing world's interpretation once you do quantum mechanics at all the world is already there all the different worlds are already there so this was invented by a graduate student Hugh Everett a graduate student of John wheelers in the 1950s he fought very hard against his advisor John Wheeler in writing the PhD thesis because wheelers mentor was Niels Bohr the boss of the Copenhagen interpretation of quantum mechanics and wheeler didn't really want to admit that his student had a better theory than boredom so wheeler kept pretending that they were actually compatible with each other whereas Evert knew exactly what he was talking about and he's like no the Copenhagen interpretation is a philosophical monstrosity he called it and we can do better than that all we have to believe in is what quantum mechanics is trying to tell us and what quantum mechanics is trying to tell us there's no classical realm there's no separate world of classical people making observations of quantum systems you and I are made of atoms which are made of elementary particles which obey the rules of quantum mechanics therefore you and I should obey the rules of quantum mechanics that's how it should be ever it's great insight is it taking quantum mechanics in the wavefunction seriously let's you come up with a theory that does account for the world that we observe it is not in contradiction with our experience but what it implies is that there are all these separate copies of reality these other copies of the world still exist people don't like that ok you may you may know you may have heard this people object to the idea that I could hop one way hop the other way and there's literally two copies of me okay so there are what I call silly objections to the many-worlds interpretation of quantum mechanics the first one is and this is I think really the main one that people get hung up on that's too many universes right all these universes I mean how often does it happen that the universe branches in two it's not that it branches into every time you make a decision okay so like I literally had to push the button on my iPhone to send a signal so a photon can go down a beam splitter that branched the universe into it's when a quantum mechanical system in a superposition becomes entangled with its environment and dqo here's that's when the universe branches if you don't know whether to order pizza or curry for dinner tonight and you finally make a decision that doesn't mean there's another universe in which you order the other one unless you use the phone to do a quantum random number generator okay decisions in your head are purely classical processes most of your thoughts have nothing to do with quantum mechanics but there still are a lot of worlds it's very often that a little nucleus in an atom decays okay every time a nucleus decays or doesn't decay that branch is the wave function of the universe every time particles scatter off of each other in some quantum mechanically interesting way and then become entangle with their environment that branch is the wave function of the unit so the answer is there are many branches of the wavefunction that might actually come into existence you can calculate how many were allowed to have before we run into trouble the space of all possible wave functions is called Hilbert space it's very very big just like you know a plane a 2-dimensional plane is 2-dimensional you go in two different directions space that we live in is three dimensional up down left right forward backward we think that Hilbert space for our observable universe is about 10 to the 10 to the 120 2-dimensional that's very big we're not gonna run out of room in the Hilbert space for all these different wave functions describing many many worlds our intuition is not up to the task but the math says it's there and the math says is there in every version of quantum mechanics there are other competing versions that are still much better than Copenhagen none of them say that there's no such thing as a wave function that is crucial to quantum mechanics and once you believe in wave functions you can believe that the universe can be in super positions another what I think incorrect objection is that this idea cannot be tested right it's important in science that we not just have good ideas but that we compare these important ideas to data right that we experimentally probe our ideas and people say you've invented all these new worlds how do you ever test that idea the response to that is I didn't invent any new worlds I just took quantum mechanics seriously the entirety of the assumptions that go into the many worlds theory is there are wave functions and they obey the Schrodinger equation that's it everything else is a consequence of prediction and implication of those assumptions and are those assumptions testable hell yes they are of course they are whenever we do a quantum mechanical experiment we're implicitly testing the many-worlds interpretation if you want to falsify the many-worlds interpretation remember that the prediction of many worlds is wave functions don't collapse they never do they appear to collapse because of decoherence so you and I get the appearance of a wavefunction collapsing but it never really happens if you are able to observe a wavefunction collapsing without decoherence without becoming entangle with the environment that would be evidence that many worlds is wrong and there are versions of quantum mechanics where that's exactly what happens and it's experimentally testable so many worlds has implications that can't be tested that's what really bothers people there are other worlds out there according to this theory and you can't get there you can't see them in principle you could see them just like in principle your cream and coffee could unmix from each other and practice that's never going to happen in the real world so but that's also ok every theory of physics has some predictions that can't be tested that's not what matters what matters is are there some predictions that can be tested and for many worlds that's certainly the case so I get a little frustrated by the public discussion of the many-worlds interpretation of quantum mechanics because a lot of people object to it for the wrong reasons which is frustrating because there are good reasons to object to it I think it's true I would personally say there's about a 90% chance that the many-worlds interpretation of quantum mechanics is the right one but there are perfectly rational reasons to be worried about it it's not a fully developed theory yet we haven't completely finished the task of matching many worlds on to what we see in the experimental world I would much rather see the public discussion of the theory Center on these reasonable questions from any worlds so let me just give you a very brief introduction to what physicists and philosophers are talking about at the cutting edge of this task we have this program of mapping many worlds interpretation on to the world that we see one question is probability ok I said that the rule that we teach our undergraduates and the Copenhagen version of quantum mechanics is the probability of getting a certain experimental outcome certain measurable result is given by the wave function squared and in the Copenhagen version you basically treat that as a law of nature it's a separate postulate of the theory there's something fundamentally stochastic and random about how nature works in the Everitt version and many world's there is nothing random about the world everything is completely deterministic the Schrodinger equation always applies and the Schrodinger equation simply says what will happen next with a hundred percent probability but here's the problem when you do experiments you see probabilities when you have in front of you a nucleus that you know is going to decay we have no way of predicting with certainty when that decay will happen the best we can do is probability right so there's a challenge to many worlds how do you get probabilities out of a theory that has no probabilities in it the other question which I think is very interesting is how does the classical world emerge again this is not a problem for Copenhagen because in the Copenhagen formulation of quantum mechanics the classical world is there it's part of your assumptions you're a big classical observer measuring little quantum things in Everett everything is quantum so when Schrodinger invented the Schrodinger equation he had an ambition remember de Burleigh said well maybe what we think of as particles little electrons are actually waves and Fourier invented an equation but those waves would obey but what he hoped would be that if you solved that equation for an electron moving around an atom the electron would sort of its wavefunction would squeeze into a point and you would basically see a point moving around the atom sadly for Schrodinger that's not what happens electrons spread out all throughout the atom if anyone ever tells you atoms are mostly empty space that is nonsense that is not true atoms are mostly wavefunction so the question is why when we look at the world when we look at the floor and the table and things around us why do they seem to have positions why do they seem to be localized in space how do you extract that classical reality out of the quantum mechanical wave function so both of these questions are really good ones ok we don't know the once-and-for-all final answer we're working on it I think that we do have really good suggestions as to where the final answers will come from so let me tell you about that this is a little bit of a an intro to the cutting edge of research in this field so for probabilities here's the problem remember we have a version of the laws of physics which is just the Schrodinger equation it's completely deterministic wave functions smoothly evolved we know exactly what the future wave function is going to be but we also know that those wave functions describe branching of the universe into multiple copies so the way the probability comes in is that among other things you branch right I branched when I did the hopping left and right there will necessarily be a short period of time when the branching has already happened there are two copies of me to future selves okay and neither one of them knows which branch they're on this is called self locating uncertainty you know everything there is to know about the wave function of the universe about the entire state of reality but you don't know where you are within it we can see this happening remember when we started with the cat the environment and the observer the observer by the way you now know is Hugh Everett who also by the way left physics after he got his PhD because he didn't want to take the guff that he was getting I mean Wheeler actually sent him to Copenhagen to try to talk to meals Bohr and the other copenhagen people and it was a disaster it did not go well so the story that we told was one where before you the observer know the answer to your question which which branch of the wavefunction am I on the branching has already happened this process of decoherence in which the cat or the quantum system interacts with this environment is incredibly fast numbers like 10 to the minus 20 seconds get thrown around okay so no matter how quickly you open the box and look at it decoherence has happened much before your conscious mind can process the outcome of that particular experiment and therefore you always reach a stage that is similar to what we've pictured here where the wave function is already branch there are now two copies of you but those two copies are identical so you could imagine talking to those two copies and saying well which branch of the wave function do you think you're on it turns out to be there's a process there's a correct way of rationally assigning the probability that you're on one branch of the wave function versus another one and what it works out to be is that it's the wave function squared if the amplitude of the wave function for the cap being asleep was the square root of 30% then you should give yourself a 30% chance of being on the branch of the wave function where the cat is asleep and vice versa so it works out the fact that the actual probability rule in quantum mechanics is the probability is the wave function squared is more or less exactly what you would expect if the many-worlds interpretation were true it is not put in as a separate assumption this is one of the reasons why the Everitt interpretation or the many-worlds interpretation is actually simpler and more compact than the Copenhagen interpretation or anything else you don't put in things like the probability rule as extra assumptions you derive them from the formalism now the other one the classical world this is where I really get excited I wish I could talk to you about this for longer but this is very interesting because we're beginning to understand a way of thinking about the emergence of space-time itself from the rules of quantum mechanics you may have heard that one of the problems with quantum mechanics is that we can't yet make it compatible with gravity right Einstein invented his general theory of relativity back in 1915 he says that what we think of as gravity is really the curvature of space-time Isaac Newton would have told you that space and time are fixed in absolute Einstein tells you that space and time themselves are dynamical they can change they can warp they can move around this is only one of the four forces of nature that we know about right particle physicists know about electromagnetism the weak nuclear force the strong nuclear force and gravity the other three forces electromagnetism strong and weak all have perfectly good quantum mechanical theories behind them we do not have a perfectly good quantum mechanical theory for gravity quite yet okay so when you dig into it however what do you realize is that when we say we don't have a good quantum mechanical theory of gravity what we really mean is we don't have a complete quantum mechanical theory of gravity when you get to the extremes where gravity is extremely strong at the center of a black hole or at the beginning of the universe with the big bang that's what we don't know how to describe quantum mechanically when we have a relatively benign situation like here in this room where things fall down if you drop them because of gravity there we can describe that particular weak force of gravity in perfectly quantum mechanical terms so here is a way that we can try to make sense of how to do that remember we're gonna use remember I told you about this phenomena of entanglement right that quantum state of two different pieces of reality can be entangled with each other so if you know something about one you know something about the other there's a typical way of thinking about entanglement that goes back to Einstein of all people in 1935 he wrote a paper called the EPR paper Einstein Podolsky and Rosen and he tried to make he was just like Schrodinger with his cat Einstein was trying to make you worry about quantum mechanics he said I can have two particles they can both be spinning right electrons have an intrinsic spin so would you measure them there either spinning clockwise or spinning counterclockwise and Einstein said if I have two electrons because of entanglement they can be in a state where either they're both spinning clockwise or they're both spending counterclockwise there's no possibility that one is spinning clockwise and the other spinning counterclockwise that's what entanglement says but if I ask you what will I observe when I look at one particle the answer is I have no idea it's a 50-50 chance it could be clockwise or counterclockwise all I know is that the other one is the same so Einstein says take one of those particles and send them to Alpha Centauri put them in a rocketship send them light-years away okay and then I observe my particle here I see oh it's clockwise instantly according to the conventional rules of quantum mechanics the other particle is clockwise also so Weinstein said this is spooky this is the origin of the phrase spooky action at a distance how does the particle faraway know that the universe is that we got the result we did for this particle a many-worlds person says well when you observe this particle the universe branched its wavefunction and it branches it all over and that's not very mysterious but the point is that this amount of entanglement between the two particles doesn't depend on how far away the particles are okay I'm telling you this because if you've heard a little bit about quantum mechanics you may have heard that but there's an additional fact that these days our best theories of the universe are not theories of particles they're theories of fields there's the electric field the magnetic field the gravitational field even the particles that you know about neutrinos electrons quarks etc these are all based on fields and what a particle is is a vibration in the field there's a field there's many different fields all throughout this room they're all gently vibrating if it's vibrating very softly we don't see anything it's vibrating enough you see a particle there those are the rules of quantum field theory so while it is true that two particles can be separated by an enormous distance and still be just as entangled as they ever were even in empty space even in between the particles there are still quantum mechanical field degrees of freedom as we call them there's still little vibrating quantum fields even in empty space and these are also entangled and they're entangled in a way that it really does matter how far away they are namely if they're nearby if you take a region right here and right there the two vibrating fields are very highly entangled with each other whereas if you take two spots very very far away they're very unentangled they're not really related to each other so that makes sense to us at least this is how we conventionally think about quantum field theory fields are highly entangled when they're nearby not that entangled when they're far away so here's the fun suggestion that I could make what if we invert that the statement I just made assumes that you know what you mean by nearby and far away right because we're working in a classical space-time where we have rods and cloth we can measure things what if we didn't have that what if we really were truly quantum nothing but quantum nothing but a wavefunction what we have are different quantum mechanical degrees of freedom that are entangled or not so what if you say to yourself when the degrees of freedom are highly entangled we will define that to be nearby and when they are unentangled we will define that to be far away in other words you get an emergent notion of geometry of distances and times out of the quantum mechanical properties of entanglement so in other words well it's a long story but here's the punchline it works it seems to work best we can say right now rather than starting with space-time there's a new exciting perspective on the problem of quantum gravity whenever we try in the conventional way of doing things to come up with a quantum mechanical theory of something let's say electromagnetism we start with the classical theory and we quantize it the classical theory of electromagnetism was given to us by Maxwell and Faraday and others in the 1800s there are rules for if you have a classical theory converting it into a quantum mechanical theory quantization but presumably nature doesn't work that way Nature doesn't start with a classical theory then quantize it nature just as quantum from the start so maybe the reason why it's been so difficult to quantize gravity is that we shouldn't be quantizing gravity we shouldn't be starting with the classical theory of general relativity and applying rules to turn it into a quantum theory maybe we should be starting with quantum mechanics maybe all we should be doing is not quantizing gravity but finding gravity within quantum mechanics and the first very tentative very crude steps in this direction have been taken and what we find what we seem to find under assumptions that seem reasonable is that this emergent geometry that we define from quantum entanglement obeys at an equation just like anything should in a good well-defined physical theory and the equation it obeys is Einsteins equation for general relativity Einstein's equation is the other equation I want to show you besides furnitures it's just as much fun just as impenetrable to understand if you're not an expert but it's not that conceptually hard on the left-hand side there's a expression which means how much curvature is there in space-time and on the right-hand side there's an expression which means how much stuff is there in the universe how much energy heat momentum and so forth so this is Einsteins version of the gravitational field between two bodies depends on how far away they are Newton's law of gravity okay this rule governs how the curvature of space-time responds to energy and momentum and we're able to see that rule emerge from a theory that doesn't even have space-time in it that has nothing but quantum entanglement so it seems like maybe for optimistic cross our fingers by taking the interpretational problems of quantum mechanics seriously by thinking deeply about what it means to be a quantum state how it evolves branching decoherence etc and asking questions about the emergence of the classical world in that theory we not only get an answer that explains cats and electrons but maybe the universe itself again this is very tentative new stuff it might go away I think it's a good lesson for how new research directions can be driven by thinking deeply about the hardest problems so to finish let me do it give you a quote from Oxford physicist David Deutsch he's a big promoter of have ready and or many worlds quantum mechanics he says despite the unrivaled empirical success of quantum theory the very suggestion that it may be literally true as a description of nature is still greeted with cynicism incomprehension than even anger what he means of course by quantum theory is too many world's version of quantum theory quantum theory without anything else anything besides wave functions and the Schrodinger equation that theory leads you to believe in a lot of stuff including multiple branches of the wavefunction with as many copies of you and you might not want to believe that but after my talk I think I hope at least the there is less in comprehension in the room the cynicism and the anger are up to you thank you very much [Applause]

Channel: New Scientist

Views: 63,986

Rating: 4.884017 out of 5

Keywords: Physics, quantum, quantum physics

Id: p7XIdFbCQyY

Length: 55min 48sec (3348 seconds)

Published: Wed Jun 24 2020