Quantum Reality: Space, Time, and Entanglement
Good. To see all of you, you. See a quote, up there by Niels Bohr one of the, founding. Figures of quantum, mechanics, anyone who thinks they can talk about quantum, mechanics without getting dizzy hasn't, yet understood, the first word of it now, why. Would that be what did Niels Bohr mean by that well basically. He meant, that we all have a good intuition. For. Classical. Physics, right, and by that I mean you know if I was to take, any little object, right, give, it a catch nice did, a one-handed. Catch right there throws a little bit further back if you go two for two no, we're still we're one for two they're. Still back in the, Dark Ages here we go you. Have that one over there good right, now each, one, of the, people, who caught so they'll be the two of you over here it's a really a really evolved. Human being no you see when we, when. We were out there in the savanna trying, to, survive right. We, needed, certain. Skills we needed to be able to know where. To throw a spear, how, to throw a rock to get the next meal we needed to dodge, some. Animal, that was running toward us and therefore, we learned. The. Basic. Physics. Of the everyday, macroscopic. So-called, classical, world we learned that intuitively, and that's, why when. I throw, an object you, don't have to go through some elaborate. Calculation to. Figure out the trajectory, of that stuffed animal you just put out your hand and catch it right, it's built into our our being. But. That's. Not the case when we go beyond, the, world of the everyday if we, explore, the world say of the very small which is what we're gonna focus on here tonight we, don't have experience. In that domain we don't have intuition. In that demand in fact were. It the case that, any of our distant. Brethren, way in the past if they did have some quantum. Mechanical. Knowledge, and, they, sat down to, think about electrons, and probability, waves and wave functions, and that sort they. Got eaten. There. Change didn't propagate, right, and. Therefore. We have to use the power of mathematics and. Experiment. And observation, to. Peer. Deeper, into, the true nature of reality, when, things are beyond. Our. Direct, sensory. Experience, and that's. What quantum mechanics is, all about it's trying to describe what happens, in the micro world in a, way that's. Both accurate, and revealing. And the, thing to bear in mind is even though our focus here, tonight will, really in some sense be in the micro, world the, world of particles we. Are all collections. Of particles, so any weirdness, that we find down there in the micro world in some sense it has an impact even, in the, macro world and maybe suppressants, will discuss. But. It's not like there's a sharp divide between the small and the big we, are big beings, made of a lot of small things so any weirdness about the small stuff really. Does apply, to, us as well and in, this journey into, the, micro world the world of quantum mechanics, we, have some of the world leading experts, to help us along to figure things out and let me now bring them on stage.
Joining. Us tonight is a professor of philosophy at, the University, of Southern California. Who, spent 22. Years at, the University, of Oxford as, a student, researcher. And faculty, member he is the author of a book on the Everett interpretation. Of quantum mechanics out of the emergent, multiverse, please welcome David. Wallace. Also. Joining us tonight is a professor. Of chemistry at, the University of California, Berkeley co-director, of the Berkeley quantum, information, and computation center and faculty. Scientists. At Lawrence Berkeley National Laboratory, she's. A fellow of the American Physical Society and, a recipient of awards from the Burgman Sloan & Alexander, von Humboldt foundations. Please, welcome Kay burr Geetha wailing. Our. Third participant. Is the professor of physics at the University, of British Columbia a, simons investigator, and a member of the Simons Foundation it, from cubed collaboration. He, was a Canada, Research Chair in, a sloan foundation, fellow and was awarded the canadian CA PCRM. Medal to theoretical, mathematical, physics in 2014, please welcome mark, my mom's dog. Our. Final, participant, is, a professor of theoretical physics at Utrecht, University in, the Netherlands, and winner of the, 1999. Nobel, Prize in Physics for work and quantum field theory that lay the foundations, for the standard model of particle physics, one. Of the greatest, minds of our era, please, welcome Gerard. At toast. All. Right so subject. Is quantum, mechanics, and part. Of the evening will, involve. Some, challenge. To the conventional, thinking about, quantum, mechanics and so. Before we get into the details, I thought I would just sort of take. Your temperature get a sense of where you stand on quantum, mechanics is, it in your mind a done deal it's finished we completely understand, it is it a provisional, theory is it something which a hundred years from now we're gonna look back on with. A quaint smile how, do they think that that's how things work so, so David your view well. I didn't think we fully understand, it yet I think it has a lot of debts left to plumb and who, knows it might turn out to a place but right at the minute I think, we don't have either empirical, or theoretical, reason to, think that anything will take its place good, Fergie, - I think, it's fair to say there. May be here. To stay you said you need to stay they may be extensions, and modifications, there may be something more complete but this will still be part of it okay, mark. Yeah. So there's a there's a frontier, in quantum mechanics that I work in and this is the frontier it's like the Wild West of theoretical, physics where we're trying to combine, quantum. Mechanics, and gravity and, we need to do that to understand black holes and hopefully eventually understand, the Big Bang and and. There there's there's a lot to do and we. Don't know if we're gonna have to modify quantum, mechanics, or it'll, all be the same quantum mechanics, but all the way down right, now. Your art you have unusual. Views. Well. Yes I could spend. The rest of the evening explaining. Them yes. But. To. My mind quantum, mechanics is a tool a very important. Mathematical. Tool to. Calculate, what happens if you have some underlying, equations. And, to. Tell telling, us how. Particles, and, other small things behave, we. Know the answer to that question the answer is quantum mechanics but you don't know the question that's still something. Good. So sort of a jeopardy issue you know the American rapper, all, right so just, a quick overview we're, gonna start, with some of the basics of quantum mechanics just to sort of make sure that all of us are more or less on the same page well, then turn to, a, section. On something, called the quantum measurement, problem something weird quantum, entanglement, as in the title of the program will. Then turn to issues of black holes space, time and quantum. Computation. Which will, take us right through to the end all, right so just to get to the the basics of quantum. Mechanics, the. Story. Of course began, more, or less in the way that I started, you, know we understood, the world using classical, physics in the in the early days way back to the 1600s.
And Then. Something, happened in the early part of the 20th, century where people like, you. Know we started with Newton of course then we move on to people, like Max Planck Albert. Einstein. What. What drove the. Initial, move into. Quantum. Physics. I. Think. It was really just pushing really hard at, classical, mechanics, as it went down into the the scale of atoms and, the structure of atoms and just, finding that structure snapped and broke that. Trying. To use classical mechanics to understand, how. Hot things got or how. Electrons. Went round atoms, without collapsing, into the nucleus in. All those places we had a series, of hints, that. Something. Was amiss in, in our classical physics yes, and it. Took I guess. Most, of 30 years for those hints, to coalesce into a into a coherent theory but. That here in theory then became a not. Not really just a single physical theory but a language for writing physical, theories be being. Theories. Of particles, or fields, maybe some days and gravity and that language was more or less sort. Of solid by I guess about 1930, yep, it's, it's actually quite remarkable that it only took, that, number of years to develop a radically, new way of thinking about things and Richard, Fineman who is of course a hero of all, of us also known, to the public famously. Said that there is one, experiment. We could go through the whole history of everything that you describe with the ultraviolet, catastrophe and. Photoelectric. Effect and all these beautiful experiments, but, the double slit experiment. Luckily. For us in having a relatively, brief, conversation. Allows. Us to get to the heart of this, new idea where it came from this, actually is the the, paper on in, some sense the double slit experiment the first version, Davisson and Germer and, I'll draw, your attention to one thing you see the word accident and this. Is just, a footnote but, in, the old days people would actually describe the, blind alleys that they went down in a, scientific, paper but, as science, progressed, we were kind of taught no no don't ever say what went wrong let. Me talk about what went right but, here was an old paper and indeed this experiment, emerged from an accident, in the laboratory, of Bell Labs they, were doing a version of this experiment they turned the intensity, up too high some, glass tubes shattered. And when they redid the experiment, unwittingly, they. Had, changed the experiment to something that was actually far more interesting, than the experiment, that they're initially carrying, out so.
So Just, to talk about what this experiment is in modern, language so David again just just what's. The basic idea, of the, double, slit experiment so, you take a source. Of well, the particles, of any kind but let it be light for instance you shine that light. As a narrow beam on a screen it has two gaps in it and you, look at the pattern, of light behind the. Two. Gaps in the screen. It's. Exactly the, slits are just literally gaps in a black. Sheet of paper and prints yellow but, the lights going through if, light. Is a, particle, you'd expect one, sort of result, on the far side of the screen if light, is a wave if you might expect something different as the, light coming through one part of the slit, interferes. With the light going through the other part of the slit and the weird thing about quantum - slit experiment, is that it seems in various, ways to be doing both of those things at the same time good, so brigita if you can just take. Us through a particle. Experiment. To build up our intuition, so let's say we we carried out the experiment that David described but we don't start, with photons. Or electrons, we start with with pellet bullets or something so I think they have a little, the animation that you could take us through so what. Would we expect to happen in this experiment, so you have the source of the pellets. Here in front and it's, pitching. Out the pellets and some of them a go through the holes and the ones that go through the hole they sort of travels rectilinear. Straight, ahead as we might expect. From our classical, intuition and we get two bands, of the back indicating. The book the pellets, that went through the right slit, on. The right and the. Left band is the pellets that went through the left slit now if I was that that's completely intuitive right so this is the stuff that our forebears. Would have known even on the Savannah right now, if we took the size of the pellets, and we dial them down to a very small size before. Going to your quantum, intuition, that you have what would you expect naively. To, happen if you simply dial down the size would you expect there to be anything different if you were this. Is a leading question by the way did. You just follow me here the answer is I would. You expect anything different thing, would you expect anything good. So, they're. Not. Exactly. Right, so here's what you would naively expect. Would happen again. You've got the, particles. Going through the two slips so so, mark, tell us what actually does, happen, not that I don't. Think Brigitta, could just to give us all a little air time. So. It's oh of course it's kind of wild the place where you at least expect, to see something, on that screen is exactly behind.
That Big barrier, that's, in the middle yep and and somehow when. You actually do, the experiment you see that actually that's where most of the particles end up so. It's it's a no actually it's always exactly the opposite, and then you get this weird pattern, with other bands, going, out and. And and. So you initially. Would would stare at it and shake your head and yeah wonder, what you Doc so will will will will analyze, what that means in just a moment, but I you, know we, often I don't know probably, most, everyone in this audience has seen a still image or an animation like, this in the discussion of quantum mechanics. And I, thought it'd be kind of nice to show you that it that, this actually happened. It's not just animation. That an artist does so we're gonna actually do the. Double slit experiment, for real right, now and to. Do that I'm going to invite, a friend, of mine from Princeton, University all. Milan, can. You wheel, out if you would the, double, slit, experiment. All. Right so what. We have here is, a. Laser. On. This. Far. Side so, this is our source. So actually we're doing this in some sense opposite, to the orientation. That we saw in the animation and, we're, going to fire this laser which is photons. In essence, and the, photons, are going to go through a barrier, that has two openings, in it it's harder to see that of course mechanically, but trust me there's a barrier with two openings, and we're going to take a look at the, data that, falls, on a detector, screen which, in the modern age is a more, complicated, and somewhat. Finicky, piece of equipment, so we're all sitting here, on. Spill cos if you if, you speak any Guinness, you know exactly what I'm talking about right there but, hopefully this will this, will work out so so long let's let's why, don't we just actually see ambient, noise can we see a little bit of that first but. Can we switch over to the input. To the screen, all. Right so this is the output from. That device and now, if we actually turn, on the laser and allow us to collect, all of the photons that land. There. They're building up and. There. You see what, actually happened, so this is a result of this very device here and you see it we have a you. Can see on the very far left we, see some of the photons are landing then we get a dark region in between then, a bright a dark a bright a dark a bright a dark and a bright and dark even though this, device over here really.
Is A barrier, that has only two slits, in it so, the animation that we, showed you actually. Does hold true in real experiments. And that then forces, us to come to grips with it to try to understand, what, in the heck is actually. Going, on so thank, you all on. So. There we have it we have this, the situation in, which, we expect to get two bands and we got more, what. Does. That tell. Us where, do we go from there there's an existing, bit of mathematics. That comes, up with exactly that same pattern but. But. It has nothing to do with particles. It's, the mathematics. That you use to describe. Waves. Water waves or other kinds of waves yeah so can we see the the the the animation, has a single, so this is a warm-up. To, the problem, where we have water going through a single, opening it just tell us what we see happening, here that's, right so you've got you've, got sort of a water, wave a wave front coming along and then just. That. That slit acts as a bit of a source for for this rippling wave going out in a circular pattern and you. See it's, most wavy at the at the place behind the slit so the wall that's. Indicated, by the brightness there, yeah and then if we go on to a more relevant version. For the actual double, slit experiment yeah, so now we've got that that, same wavefront but, now there's there's two slits. And it's like there's two different sources, of waves like if you through two different pebbles, in the, pond at the same time and then, what happens is that they're. Both you're, both creating, waviness. But. Some places on the screen the way from one is doing this and the way from the other is doing this and they kind of cancel, out and, but. Right there in the middle what's happening, is that the way from the one slit is going, up right when the way from the other slit is going up and then they do this and you, get a big wave and that's the bright part but if, you work out the mathematics then. The places, that have the big waves are, exactly, these bright ones and that's just like we saw in the in. The double, slit right for the particles so, as per our Burton is Markus saying we now have a strange. Confluence. Of two, things. The data that comes out of the double slit experiment, when done with particles, and something, that seems to have nothing to do with it where we just have waves. Going, through a barrier with two, so the conclusion, then is that there's some weird connection. Between particles. And waves that's, where that connection comes from and and. Let's, push, that further so I mean let's just drive home how weird it, should be that there is any kind of connect yeah yeah so, imagine I do the two-slit experiment I cover up one of the slits the, effect completely goes away I get a bit of spreading out to the particles, but I don't, get that interference, I didn't get those bad much as we saw with water going through exactly exactly.
Much Even water going through the single slit and much as you see if your classical, intuition about particles, if I cover up the other slit. Exactly. The same result it's only if I have both slits open at the same time the effect happens so it seems to be for all the worlds if somehow, something's, going through the first slit and something. Else is going through the other slit and between, them, they're interacting, to create this strange effect and that's, why it matters so much that I can do this experiment of one particle at a time if this was just the mass of light going through no. Surprise that sunlights going through the left slit sunlights, going through the right slit the, left-hand light the right-hand light interferes, but, I can set this stuff up so only one photon goes for every hour and a half I still, see the effect it doesn't go down in this life can we see that we think we have it and then you might be thinking well maybe each individual, particle breaks in half and half the parcel goes through one slit and half the path will go the other slit but, again then you think you could look then you think you'd be seeing half strength, detection. X' yeah but that's not what you see whenever you look each time you send through the particle through if you look where it is you see the particle in one place one place only so. Trying, to reconcile, those two accounts, of what's going on makes. Your mind hurt yep exactly. So. So so we're forced, into, as they would say not just thinking that a large collection, of particles, behaves, like a wave which, maybe not wouldn't be that surprising because water waves are made of h2o molecules, particles, and therefore they're kind of wavy but, each individual. Particle somehow, has a wave-like quality. And and historically. People struggled to figure out what wave. What, kind of wave what. Is it made of and what does it represent if, you have a wave associated, with a particle, wave is spread out of particles, at a point and it. Was max born, in the. 1920s. Who came up with the, strange, idea.
Of What these waves, are so, Fergie, - what what are these waves telling us about, well the waves, what. We see is the probability which is a, square. Of the way or. Where. Modulus of the wave but so. Here's a way behind you so you said probability. Yes in essence. Amplitude. This is an amplitude which, will. Give us a probability that if we take this amplitude and look anywhere, here with some measuring device we, will find with some distinct, probability. After. Measuring many times we'll find that that's there's, a definite probability of the particle being there just as in the double slit after. Sending many particles, through we found with a certain probability they, would all appear, on the left or, all on the right so in some sense vaguely. Where. The wave is big there's. A high likelihood you're, gonna find the particle where the wave is near. Zero there's a very small probability. That you're gonna find a parent, II it so, any one particle, that would be in a place where the wave. Is very very small now, these are all just pictures, in, the. 1920s. Physicists. Were able to make this precise, so, Schrodinger. Wrote down an equation and, I think we and. Show you what the equation, looks like obviously, you don't need to know the math to follow anything that we're talking about here but uh, you, know Gerard you wanted to emphasize, that, there. Is math behind, this, because your. Experience has been that many people missed, that point so feel free to emphasize. Absolutely. Quantum mechanics, when. We talk about it. There. Is a temptation, to keep the discussion very fuzzy and so. I get very many. Letters but people who have their own ideas, about what's called mechanics is and they, are very good in reproducing, fuzzy arguments, but they, come without the equations, or the equations are equally fuzzy, and meaningless, because the. Beauty, of color mechanics is the fundamental. Mathematical. Here on certain situations you can prove that if this. Equation describes probabilities. Exactly. As you said yep before. The. And, then actually the equations, and, the probabilities, exactly, the way probabilities, are supposed, to be handled, except. Of course when, two, waves reinforce. Each other the, probabilities, become four times as big other than twice as big but, another, shot spots the waves annihilate. The probabilities, and so, for me this becomes zero, where are the waves. Vanishing. So, with, all this hangs together in a fantastically, beautiful mathematical. Math math is one thing experiment. Is another, so how do you how would you test the theory that only, gives rise to more. Probabilities. Of one outcome or another how would you go about trying. To determine if it's right or it's wrong yeah sorry it's like it's it's like if you gave me a coin and you, said you know is this is it this is a probabilistic, thing you flip it it's gonna be heads, half the time and tells half the time and, and. And I want to check that I don't I don't trust, you for I know why that would be but. I'm. Not as well so I just I. Just I. Just flip the coin, you. Know a hundred, thousand times or whatever a, lot. Of pay hands if. I want to be if I'm more, sure I want to be them where I flip it so maybe I did I do a ten times I get four and six and I maybe. I'll flip it a hundred times and then I get, forty-eight, heads and fifty-two tails. So I can, basically just, repeat the, experiment. A whole bunch of times and if I have a very precise, prediction, from those quantum. Mechanics equations, to tell me exactly. How often I should expect, to get one result versus, another though I think we have given, a little schematic what. Did we see how the luck right so we're doing there there's our wave that's describing, the the, state, of the particle the thing without a definite, location, now we're mentioned we're setting that up a whole bunch of times and measuring where the particle is each time and these, X's, are showing the results of our measurement, that's like flipping a coin and get a target so there's all these possible, locations, and what, we see is that after a while the, pattern, of how often I get one place versus, another place it's matching up to that expectation. Given. By the boy, the blue curve by this wave that's. Right so we can't predict the outcome of any given, run of the experiment, but over time building. Up the statistics we. Believe, the theory if they, align, with the probability profile, given by this wave whose, equation we showed you and that, is what works out the shape of the wave, in any given situation and just to bring this full circle if. We, look, at the double, slit experiment in this. Wave-like language. Now think of the electron, or the photon as a wave it goes through it interferes, like water waves going through the two openings, and therefore.
You Have an interference, pattern on the screen which, is telling you where it's bright it's very likely that you'll find the particles, where it's dark it's unlikely, where it's black there's zero chance of finding the particle there and therefore, you run this experiment, with a lot of particles, and they're going to primarily, land, in the bright regions, they're gonna land somewhat, in the gray or regions and they're not going to land at all in the black regions and indeed that's, exactly what we showed in the experiment, that we, ran. With the double slit just a moment to go and that's why we believe these ideas, so, that's in some sense really, the the basics, of quantum. Mechanics, classical. Physics particle, motion is the, intuitive, one described, by trajectories. In quantum physics the particle, motion is somewhat fuzzier, it's got this probabilistic, wave-like. Character. And the. Curious, thing about a wave is sort of a wave of probability if, the wave is spread out it means there's a chance the particles, here chance, that it's here a chance that it's here and therefore, the, wave embraces. A whole distinct. Collection. Of possibilities. All at once, that in some sense is really the, weirdness of quantum mechanics. So, that's the basic structure and now, we're gonna move on to our, next chapter, where we're going to dig a little bit deeper we'll, talk about measurement. And also. Entanglement. They're. Checking the electron, microscope, and the, winner is number. Three, in our quantum finish. No, fair, you change the outcome by, measuring it. Neither. We have a very sophisticated audience, or, you just love you to Rama I'm not sure which. But-but-but-but. This is part of the issue that we now want to turn to which. Is if you have a quantum. Setup, how. Do you how do you move from. This, probabilistic. Mathematics. Saying, that the electron say could be here or here or here with, different probabilities to. The definite, reality, that mark was described when you actually do an experiment, you find the electron, here, or here, or here you, never find anything a mixture. Of results. So we want to talk about how we navigate going. From the, fuzzy probabilistic, mathematical. Description, to the single definite reality, of, everyday. Experience, and, this, is something, that many physicists, have contributed, to over the, years, again, Niels Bohr we had a quote. From him early on and he, certainly viewed, as really, one of the founding. Pioneers, of the, subject, but, let's now try to go a little bit further with. Our, understanding of, going, from the, math to, reality. And we're gonna follow in in for, this part of the program it really Anil Niels, Bohr is footsteps, and something called the Copenhagen, approach, to to quantum, physics so, David. Can you just begin to take us through what what was, you, know the ideas, of collapse of the wavefunction in, technical, language what, are those ideas all about Scylla, clear this way I've got my probability wave which is sort of humped let's say just for one particle, it's humped over here and it's humped over here so. There's kind of two ways, I could think about that you might say there's an N way, in an all way so. I could, think of it as saying that the particle is here and the. Particle is here or, you could think of it as saying or the particle is here all the particles here and the, problem is I kind of need to use both to make sense of quantum mechanics or so it seems so. If I try to explain the, two that experiment, I have, to think in the end way, to start, with I have to think the, particles, going through this slit and it's, going through this slit because, if it's just going through this slit or it's going through this slit I could close one of the slits and it wouldn't make a little difference but.
Then As soon as I look where the particle is suddenly, the and way of talking, stops making sense because. It. Doesn't seem we'll come back to this it doesn't seem as if I see the, particle here and the particle here it seems as if now I need the or way of thinking, so, what came out of the ideas of Bohr, and Heisenberg and. They're acting people in the 20s and 30s was, well. There must be some some. New bit of physics some way in which that Schrodinger, equation, we saw earlier isn't the whole story so suddenly the wavefunction, stops being peaked here, and here. And it jumps it collapses, let's see a quick picture of that, collapse, so if we have a probability. Wave here, and this. Is the and description. In your language, it could be in these variety, of different, locations and I now undertake a measurement, and I, take that measurement and it, collapses. To the or way it's only at one of those suddenly education, and the mess the wavefunction is gone and now if I turn away and, I'm stopped measuring, it, melts, back into the probabilistic, description, and, we're, back to a. Language. That feels quite unfamiliar. With, the particle, is in some sense at all of these locations simultaneously. Now. The, issue that you raised is you said look, we're. Gonna have to have some other math to make this. Happen, so so so. First if if, we just use the Schrodinger equation this beautiful, equation that is written down would that be enough to cause a wave to undergo that kind of transformation nice, and spread out and now, collapses. To one location, where, the particle is found can the Schrodinger equation do, that for us, Brigitta. No, no. No. That. Means no right it. Means yes okay, so like. I said Gerard. Has distinct. Views which are which are spectacular interesting. We're gonna come to those in just a moment but let's now follow the history of the subject, where, going to just, follow our nose and we look at the equation that we have in it and it doesn't do it so. So, what then do, we do to to, get, out of this this impasse, and to make this impasse even. A little bit more. Compelling. I'm going to take you through one, version, of this story that I hope will make the conundrum. As sharp as it can be and then we'll try to resolve, it so I'm, going to take you through a little example, over here where, we have say a particle. Somewhere, in Manhattan and. Let's. Imagine that the probability, wave makes the particle, location, peak at the, Belvedere Castle. In, Central. Park just just randomly, chosen but, that would mean as if somehow I had some measuring, device that. Could work out where, the particle, is experimentally. Observational. II indeed. It would reveal, that the particle is at that location the. Waves is, sharply. Peaked at that spot, and therefore all the probability, is focused, right, there that's quite, a straightforward, situation. Imagine. We. Do the experiment, again and, the. Probability. Wave has a different. Footprint, let's, say it's way down there at Union, Square if you, follow, the same experimental. Measurement, procedure, and you go about figuring, out through. Your observation. Where the particle is you find indeed, there it is Union Square the. Conundrum, is the issue that David. Was speaking, to where we now have a, situation. Where. We. Don't have one. Peak. But two now. It's, sort of like the particle is at, the Belvedere, Castle and, in. Union. Square and that's, puzzling. Because, if you go, about, looking. At the observation. What. Do you think will happen here. Well the naive thing is your detector, kind of doesn't know what to do it sort, of caught between the, particle, is at Belvedere, Castle and, it's, at Union Square but the thing is. Nobody. Has ever found, a detector. Well I should say nobody, who. Is sober, has ever found a detector, that, does this right, this is not what we experienced, in the real world so, this is the issue that we have to sort, out because. That naive picture. Is not borne out by experience. And I. Think many people here. And many people in the community have thought about this you. In particular. David, believe that you you, have, the. Solution, it has a long historical lineage.
But Why, don't you tell us a little bit about the, approach that you think resolve. This okay to. Start by reminding ourselves what, what's, the problem with just saying that the wavefunction suddenly, jumps, to being in Belvedere or Union, Square and the problem is really just that we have to modify the. Equations, of physics at every level to handle that and Jake, Schrodinger equation just does not let that happen and to put it mildly we've got quite a lot of evidence for that structure of physics and for, a whole, bunch of reasons, you. Know actually. Trying, to change the, physics to make that sudden collapse of the wavefunction physical. And. Not I'm not just as Jared was putting it not just a sort of fuzzy talk is. A really really difficult technical. Problem but, you could say that we have to do that because later. I'm saying it. Doesn't seem we have a see a particle, here and here at the same time and. I think Brian's, joke. Is about right as to what our intuition, is about, what, it, will be like to see a particle here and here at the same time it would be like like being really drunk like seeing double but. Here's the thing if you want to work out what some physical process would be like and my looking at a particle is just one more physical process turns, out intuition, is not a very good way to predict what happens so. How do we ask what would it really be like to see a particle, that's here in here at the same time well, what if the physics say I'm, just one more measurement device and the physics says something like this if I saw the particle, here I'd go into a state that you might call a seeing, the particle here state if I look at the particle there and then. I go into what you call are seeing the particle there state if it's in both states at the same time then. I go, into both. States at the same time so. Being, a little loose for a minute then I now. In the state seeing the particle here and see particle there and if, I tell Brian where the particle is because I'm sure he's fascinated, Brian's. Now in the David says it's here and David says is there and I hold all the ins have to listen to me say this you're now all in, the it's. Here and it's there state at the same time and, the. Reality, is that even if I don't tell you this uncontrollable. Effects. Spread. Outward and so before, you know it the whole planet, the whole solar system is in a particle. Was seen here and particle. Was seen here at the same time stain and, if those two states don't interact, with each other they that they're that. They're way too complicated, to do the sorts of interference, exponents, we were doing with the two-slit you come to a two-slit experiment on the whole planet and so, for all intents and purposes what. The quantum theory is now describing, is two sets of goings on each, of which looks for all the world like. The. Particle being in a definite place and that's. Where the, terminology, of this way of thinking about quantum mechanics comes about the many worlds theory it. Was Co Everett who said look if you just take quantum mechanics seriously, you'll lead to this crazy sounding, idea of, there. Being many parallel, goings on at the same time every, time you make a quantum measurement, but. The thing I want to stress here is it's. Not that we say quantum, mechanics is weird but, let's bring in an even weirder idea out of the realm of science fiction to make it even stranger, it's, what, whatever it was saying and, what people have pushed his idea since then have been trying to make precise is the, idea that the quantum, theory itself, that Schrodinger, equation, itself when, you take it really seriously, tells. You that. Not. At the fundamental. Level not at the level of the microscopic, physics, but at the level that we see around us in the everyday then. Physic then the physics is describing, many. Goings on at the same time the look the the, quantum. Probability. Wave carries, on being an and wave all the way up so you're talking about many. Many, universe, many universes, so, this is where this idea of parallel, universes or many worlds come from so an example that we were looking at there, would be say, if you were undertaking, this measurement, there'd be you seeing, the particle, at Belvedere you seeing at at Union Square and as you said once, you tickle eight that we're all hearing, it and we're all going.
Along With you in one, universe and. Another. Exactly. So that's one, approach. To trying to disambiguate. A, situation. In which the quantum mechanics, has many possibilities, you're, saying no no it's not just that one of them happens they all happen, they all just happen to happen indistinct universe. Right and weirdly, that's a conservative idea I mathematically. Conservative, math, and, that's actually a vital point so so and this is an idea that's hard to communicate to, a general audience I'm sure many of you are technically, trained but those who aren't if you, stare at equations. Of quantum mechanics, and just take them at face value this, seems to be where the math takes you but. Is that convinced so it's are you guys convinced. They're. Getting you, there. Are alternative. Perspectives. But what I don't, you like this one I like. It I think it's fascinating I think it's wonderful yeah let's, bring in some information, so how much information. Are we going to keep, so. This, many-worlds hypothesis. Would say that we're, keeping every single piece of information, but. If we we, have a measuring device and, then. The measuring device is interacting, with an environment, and the environment of outside. Is also playing a role it's also affecting, the measuring. Device and all, these many many. Options. Measurements. That, can be recorded by the measuring device if the, environment, which is interact with that measurement device is. Interacting. The measuring device and, produces. Many more outcomes and yet then we throw in, producing, many much more information, but then we throw all of that information, of. The environment away then. We're left with something which reduces, to just the one one. Of these options so you're talking technical. Language of what's called decoherence, I'm, introducing, this technical, term that the coherence. Of the wave function, this the. Preservation, of these. So. Your belief is that if we don't. Focus just on a simple particle, itself but, in take into account how it talks to and interacts with the full environment, you feel that that's enough to solve the conundrum well. I'm, there's also mathematics. Yes.
Don't Know either so this is another perspective, I'm not saying we, don't know it which is one but this is a very strong argument for saying why. We don't actually experience, many many. What's. Your view on the mini, yeah I mean I I think, it's, what. What you were describing I mean it's basically just going all in on, the Schrodinger equation saying. Okay we've got this beautiful equation, it applies, to the the atomic. World let's. Take it seriously, and, and. Just if, we, believe in it then I mean you not only. Kind. Of understand through the mathematics, there that at the local level, you. Would have effectively, get something like collapse. If. You look at just a part, of the description of the system but but then you, know the, only thing is that in the, end it's a little bit disturbing. Philosophically. That there's a maybe. A part of the wave, describing. The universe where you. Know I I'm I'm a football. Player or. Then. That question of well why why. Did what, is our experience, yes in, that picture of many worlds is there is there some way to just, to, understand, you know why does we're, just, experiencing. One thing and. Detroit. Have a no I know that you gonna take us somewhere else. When. You, ask me about this question what the wavefunction, you were nodding I was supposed to not know and, I know that yes. The. Point is this that. Quantum. Mechanics today, is the best we have to do the calculation. But, the best we have doesn't, mean that the calculations, it. Extremely, accurately correct so. According. To the equations. We see we. Get this many worlds, I agree. With that statement but I don't agree with the statement, that quantum, mechanics is correct, so we have to accept, all these, other universes, for being real no, recalculations, incomplete. There. Is much more going on that it didn't take into account and then, again you can mention. The, environment.
Other Things, that, you forgot so, we. Are so used in physics, the unimportant. Secondary. Phenomena, can be forgotten he just leaves about very dated calculation. For granted but if you do that you, don't get for certain which universe. You are in you get a superposition. Of, different universes. It, doesn't, mean that the. Real outcome. That. That what's really happening is that universe, splits into a superposition of different universes. It means, our calculations. Inaccurate, and it. Could be done better and that. Doesn't mean that our theory is wrong but, that we made simplifications. We, made lots, of simplifications. Instead, of describing, the real world we. Split. Up the real world in what I call templates, all the particles you talk about and not to view particles, they, are just mathematical. Abstractions. Of a real particle, we use that because it's the best we can do which, is perfect. It's by, far the best we can do so in. Practice. That is just fine but, you just have to be careful in interpreting, your. Result, the, result does not mean that universe. Splits in many other universes, news up means yes. This answer, is the best answer you could get now, look, at the amplitude of, the universe, these, universities, you get out the one with the biggest amplitude, is most. Likely the rightest answer but, all the other answers, could be correct I could be wrong if we, add more details. Which. We are unable to do, today. Perhaps. Also tomorrow you. Also. Will. Be unable to do, it exactly, precisely. Correctly. So, we have to do with, what we've got today and what we've got today is an incomplete, theory, we should know. Better but, unfortunately. We are not given the information that. We need to, do a more precise calculation. That precise. Calculation, does show wave functions, that do not pick a different point at the same time like you have in Manhattan. That this, address, that address, and we, are a superposition know, in the real world to be another in a superposition, because. The real world takes. Single phenomenon into, account and you cannot, ignore what, happens in the environment, and so on if, if, you ignore that then you get all this case superposition. Phenomena, if you, would do the calculation, with infinite. Precision which, nobody can do if, you calculate. Everything, that happens in this room and way beyond and take everything into account you. Would find a wave function which doesn't do that you would find one which Peaks only at the right answer, and it, gives zero at the wrong answer, now this this, view this thing, is so unstable right, that them the most minut, in, practice. In your calculation, gives you these phony. Signals. That, say, maybe universe, and this may be university that maybe it is that only, if you do it precisely correctly. Then, you get only one answer yeah but, now that that resonates obviously. With an idea that goes all the way back to Einstein yes. I. Think, I would agree this such, such, yeah I think they think that it would - maybe it's not - maybe. Yes. But anyway. It. Is to. Me it sounds like an Einsteinian. Attitude. That, no nature's absolute God doesn't gamble the gamble is in our calculation. Because we can't do any better let's, take a step back and see why Einstein came. To this conclusion that, quantum, mechanics is incomplete, which takes us to the next strangeness. Of quantum mechanics, which is something called quantum entanglement. So. This. Is an idea that has, a long history in. Physics. That would not call, entanglement. What you're about to talk about one, but rather the characteristic. Trade of quantum mechanics, the one that enforces, its entire departure, from classical, lines of thought so, so here's again one of the founding pioneers, of the theory whose thought about this notion that we're about to describe as the. Key element, that distinguishes it, from our, intuition, or a classical, way of thinking and as we'll see it quickly in the, hands of Einstein, takes, us to a viewpoint. That aligns, really with what Gerrard was saying and that, comes. Most, forcefully, in a, paper from 1935. A date that's good to keep in mind we're gonna come back to in just a little bit where, these folks, write a paper Einstein, Podolsky and Rosen and. We can just this, is actually a New. York Times article. On it you see that the. They. Call, the theory not complete, much, as as Gerard, was describing, and it's, good to get a feel for for. Why it is that they came to this conclusion and, it, involves. This. Idea of entanglement, I'm gonna like us to walk through that just. Some, of the key steps and it's good to do it in a context of an example it's not the example that Einstein and his colleagues actually use but, it's an example having to do with the quality of particles.
Called Spin, so just to set it up and then I'll let the panelists take it from there when. We talk about a particle say like an electron, it turns out that has a characteristic, called. Spin, you can think of it almost like a top that's spinning around and roughly. Speaking using classical, language is to get a feel for it if the spin say is counterclockwise to, say it's spinning up if it's clockwise, you say it's spinning down and weirdly, a particle, can be in a mixture of being, both up and down using. Your language, of the end and only, when you measure the particle, do you find that it snaps out of that mixture, and is that in. The in the case of the particle Manhattan it was either at one location or another here it's one spin or another it's spinning down or up but it's definite, after you do the measurement you never find it in between again, you can do a second, measurement and say it snaps out of this fuzzy haze and it's spinning up and that's, a quality. Of a single, particle that's, well-known, in in. Quantum, physics but, entanglement. Arises. When you don't have one, particle. But, rather when you have two of them and, here's the weirdness that happens, if you do a measurement in this situation, even though each particle, is 50% up or 50% down you think they're completely independent but you can set things up in such a way that when you do a measurement it's. Always the case that if, the one on the Left is up the one on the right is down. They never are both up or both down and we can go back to this story again do, another measurement and they can be as far apart as you want and you. You find je that the left one is down and the right one is up so they're kind of locked together, by. A quantum. Connection. Quantum, entanglement which, is graphically, we're representing, by this. Little, yellow line over, here now, Gerard, was talking about incompleteness. Of quantum, mechanics, what, was Einstein's, view of what was going on here, well Einstein's, view was, that really. What's going on here is if you have, particles. That the math says, they're both spinning up and spinning down at the same time if you can look deeper, to the deeper structure, that Gerard was referencing, you'd find that these particles always have a definite, spin they're not actually going up and down, that's just mathematics. They. Actually have a definite, spin and therefore if you measure them and find, that one is up in the others down they were already, like that it's not as though there was some long-distance. Connection. Or communication. Going, on and this is what's known as quantum. Entanglement. And when I describe this to general audience people often. Get the phenomenon. Yeah you measure it here it's down to measure it there it's up but then they always come back to me and say but what's really going on, you. Know like but, just tell explain to me I said just did explain to you what's, going on that that's all there is no no they say please tell me like how could this be so so, how should we interpret this. Result so Einstein says, the way you interpret it is it was like this the whole time nothing, surprising, but. Then, we try, to do. Experiments, and see if that's the case and and what happens, so there's a famous. Person that comes into the story who. John. Bell and, and, and so what does amar what does bell blows Bell do for us I. Mean I mean basically to. Put it simply. He. Finds that any, kind of simplistic. Einstein. Like description. Where. The, thing had the definite, configuration. Before we, did, that measurement it can't, explain, the, results so we. Say the results you're talking about can't observational. That's right that's right yes so he writes, this, this famous paper. What. Year is this 1964. I think this think. It's like 1964. Writes his famous paper where. He he surprisingly. Is able, to get at an experimental. Consequence. Of an Einsteinian, view.
That. Things, are definitely up or down before, you look it's just the mathematics. That's giving this weird, superposition. Quality. And then. People, go out and ultimately. Starting. Say with John clauser and, the this must be the 70s, and into 80s with Alana spay they, carry. Out the measurement, and they, find as Mark was saying that. The Einsteinian, picture. Doesn't. Describe. The actual, data. So, if, Einstein were. Here, I think, you'd have to conclude not. Necessarily. That quantum mechanics is complete, but the chink, in the armor that he thought he found isn't. Actually. Correct. So George, what's your what's, your because you're coming at it from an Einstein Ian's view how, do you deal with they, say this very experiment. May I just add. One point three first you, can think of a classical. Experiment, is very simple, but not, strange. At all, think I take. Two marbles, in a black. Box, sure one marble is ahead the other one is green. Now. I shake the mobs as much I want I put. Blindfold. Li I put one marble in one box and Alabama lamella box and I bring these boxes light-years, away from each other as, soon as somebody, who. Sits or a celestial model the earth and one of my assembly, on earth opens, his box and at, the same time the guy on Mars opens, his box. Before. They open the box didn't, know what kind of marble they had in in. There in, the box also they had one or was at the game oh you don't know the soon as the one on earth opens, box a society. They had marble, instantly. The guy and Mars knows that he has the green marble, that, information, went much faster, than light but. You also know all this is nonsense, because, they knew it in advance I did, I had one red and one green marble so what's the big issue no. Problem, right so, the. Bell experiment, is fundamentally. Different from this situation, in, the sense that yes, sir we do describe you just, probably sort of the Einsteinian, picture, in science I would say don't, get worked up up tangle, man it's just like having to read marble or blue sonic, picking would walk but perfectly, well for the box with a head Marvin Green Bible no sweat no yes faculty we understand, that situation, no, no miracle at all but, for the bell. Light experiment the spinning, particle, you are, using in fact that the particle, is a quantum. Spinning, particle, and it's. Spinning particle something very very strange because it can either spin up or spin down but. Then someone asked what about spinning, sideways if, I not locate, the particle 45, degrees of 90. Degrees and they will say yes but that's a quantum superposition. But. Now. If the one person on earth looks.
At The particle in a in spinning. Up the one of my series is spinning down but, then when the person sees, a patentable, spinning, sideways the, guy of my sees the, part, of spinning Sciences in the other direction, right and sees it I was spinning up or spinning, down we still do in the sideways direct but the well they both look in the scientist erection again see the spin opposite, knobs and that is the miracle yeah that is a thing which is very very difficult to understand. Classically, I maintain. But this is my private opinion that you can explain it but, it is because this is where I sit both have the same origin they both came eventually, form from, an atom, emitting, two, spinning, objects or two photons, or two electrons, or something like that which. Were entangled, and. So. The entanglement, can be explained in terms of correlations. So. That the initial state was, not that. The pattern could be doing just anything no, there, are correlations. All over the place this is very very difficult to explain, I would even dare to try, to go, into Ivan and have any depth but, the answer lies in in, correlations. Do you think there is a way out of this in I think there's a way out but, it's extremely non-trivial, maybe, don't do it quite right you, enter, mystified. That, being, mystified, by the situation, yeah it's also, extremely hard, to make a model, that works, that, gives, this strange-looking, phenomenon. Right so, so. Yes you have a problem but, no I think the problem has an answer but it's very difficult and you have to work very hard to make it make it or hang together properly so that'll be in the footnote to tonight's program, you'll. Receive it in your email so. David, your your your view on entanglement. Is there is there a mystery here or there's. Kind of mystery and and, it. Kind of links to our earlier mysteries I mean looking this way my. My wave might my probability wave for, the, for. The two spinning particles, you can kind of describe as something like half, is this down up and half is this up, down and again, we can ask this well do I want to think about as an and or an or do I want to fit do I want to say well. It's. This or. Is this or, do I somehow have to say it's, this and it's this now, if it's this, or this. That's. Jared's, case that's not mysterious, at all and, that's exactly what Einstein and Podolsky and Rosen hope was the case but, what Bell's results, show us is that the the. This. All this, reading of entanglement just, like in some ways the the this. Slit all this, slit reading, of the two slit experiment, would. Lead to experimental. Predictions that don't pan out we. Can't at, least straightforwardly. We can't make sense of the experiments, without seeing the, entangled system as being this and, this, and now we're right back to the mystery because understanding. How it can be this and this. Which, seems to imply. Some sort of deep connection, between the two systems. Where, somehow saying. Everything, there is about this. Side and everything, there is about this side separately, doesn't tell you everything right that weird reading seems compulsory, right hmm so. So forgive. You your view on this, should. We fret about entanglement is, it I think, I think I raised a very important, point with is that when one talks about entanglement one should not. Forget to say, how. The particles, got entangled yes and they get entangled through an interaction. And I, think to most physicists. Entanglement, is not so mysterious if we think about it in those terms because, we so even, in just, atomic or molecular terms. So. Take the two electrons in the helium atom, in. The ground state the helium atom is if, we. Were to separate the two electrons, we know we can't do that because they're they're sitting on top of each other but were you to be able to take those two electrons and pull them apart they, would be in the perfectly entangled, pair but. We know how they got there because they had an interaction that. Put them into a particular electronic, state. And. So, if. You just randomly put two particles, together they would not be entangled necessarily, yeah, to my mind though the very fact that I don't care how you set it up the fact that you can set it up still.
Still, Still makes. Me in Niels, Bohr's let me dizzy but but but yes I agree that does mitigate, it to some extent but still it's so. Far outside of common experience that it's, that it's still hard to grasp but for this purposes, let's assume entanglement. Is real because now we want to move on to. Think, about how, it, manifests. Itself in some unusual, places like, in the vicinity of a black hole so. That's the the next thing that we're going. To turn to and, for. That extent, let's move on to the next, section quantum, mechanics and black holes and we'll. Also begin with a little, clip. Do. You have a stray dog down there. Two. Stray, dogs. All. Right so so black, holes I think most most, people here are quite familiar with their own but just again to get us on the, same page mark, just described what it would it's a black hole yeah so that that comes out of I'm Stein's picture, of gravity. And how the. Space that we live in is not a sort, of passive background. But it's dynamical. It, can warp and Bend and, it does that kind, of in response to the, the mass and the energy that's in the universe and them the, black hole is is the, situation. Where you take that to the extreme you have so much matter it. Could be a gigantic, star, at. The end of its life when it's burned up its fuel and then it starts to collapse and, as. Its getting denser and denser it's warping. The space more and more through, Einstein's picture and at some point you, you get a space. Of the space-time it's warped so much that. You get the the thing we call a her eyes and you get the point, of no return where, if you go past that you, can't get out you can't send signals, out light can't get out and. And. That's our basic notion, yeah black hole there many puzzles, about black hole and some of them are right, at the forefront of, research there's one in particular that I want to focus on as it will bring together, these ideas of entanglement, and ultimately the structure of space-time which is where we'll get to in the next chapter which, is simply this if something. Falls into a black hole, what. Happens to the information that it contained. Right, so to just be concrete, imagine. I was to take out my wallet and throw it into a black hole my wallet is all sorts of information credit, card information. OOP there it is they took it out of my pocket they throw it into the black hole it crosses over the horizon. The edge that, mark was referring to and at, least in the in the non quantum, in the classical, description it's, just gone right, and and, you can you, can think that the information is sort of maybe still there it's just on the other side we can't get at it unless we if, we do that their consequences, we can't come back out with the information you. Know so that's so that's sort, of the the, the classical. Story this becomes a really big, puzzle, and a bigger puzzle when, we include quantum. Mechanics. Into. The story because. Of a result that was, due to a couple of very insightful. Physicists. One who you may not have heard of one who you will have heard of so. Back. In the in the seventies, jacob, bekenstein and. Also. This. Fella over here stephen hawking they. Began. To apply quantum, ideas. To, two black holes and found. A surprising, result. Which is that black holes are actually not, completely. Black so anyone just to jump in and and what. Is it that that means or, mark oh yeah so so so hawking found when, you when. You start, to apply quantum mechanics. To. The the physics and the vicinity, of a black hole. That. There are quantum effects, that. Lead to the. Black hole seeming. To emit, particles. Out, of it as a. Graph. This. Is sort of a quantum effect where where you you have some something, happening right at the horizon of the black hole where, where, what, we would call virtual, particle, and an antiparticle that. They show, a logical is red that in oracle any particles, blue so this can happen in quantum, mechanics but. Because, of the black hole horizon the, the particles, end up going. Out and, and. So what horrors, fell in they went no see those partners, since weak and so you leaving far, away if we look at this situation. That's. Right so there we go so so the black hole looks like it's emitting stuff, and it's actually losing, some of its mass so you see it's getting smaller, Hawking.
Get A detailed, calculation, to show that it's it's behaving like an object, that's getting hotter and hotter and hotter and. And. Sort of we call it evaporating, more, and more quickly and ultimately. Disappearing. So all of the this, information. That. Might have been in the black hole it's now it's, now this this, this. Heat this thermal, radiation. Going, out into space and all, of this is happening if I understand so you got the edge of the black hole you got this quantum, process, right at the edge that we're familiar with particle, antiparticle sort, of pops out of empty space the difference is now with the black hole there it can kind of pull on one member of the pair it gets sucked in the other just rushes out and that gives rise to radiation, flying, outward, and that's, what makes this this puzzle, sharp because if the wallet, goes into the black hole and then you have this radiation, coming, out ultimately, in perhaps the black hole even disappears. Through this everything, that went in has come out but, if the radiation itself. Doesn't, have an imprint. Of the wallet doesn't, somehow embody, the information, the information would, be lost and Hawking's calculation showed. That it should not matter what. Formed the black hole you get exactly the same radiation, come with my wallet or whether it's a refrigerator. Chicken. Soup it all would sort of come out the same the information is lost now this disturbed, your, hard deeply. Yeah. Very. Much so, but, the. Statement, you just made. Was. Only about the evolution. Particle, the hard patterns, form article, a thermal, spectrum which, means that. They come out in a completely, fundamentally. Chaotic, way but. It doesn't mean that they don't know how a particle, in, what way to come out again. It's. Ponta mechanics, but again there is a theory, underlying, core mechanics which is more precise, and, which, should, provide the missing information and. Yes. There was missing, information and, yes your wallet, does. Leave an imprint on the radiation coming out so can we show across, your wallet, yes.