 Good evening, everyone, and welcome to the Kelvin lecture for 2023. Many of you will be aware that Lord Kelvin was twice the president of this society, a couple of hundred years ago. Back then there's a Newtonian view of the world, cause and effect are all very clearly understood effect follows cause, nice and straightforward. Einstein came along a bit later, came up with relativity and came up with photoelectric effects and started confusing things with the birth of quantum mechanics. Tonight's speaker is a professor in optics. He's also the Kelvin chair of natural philosophy here at the University of Glasgow. He has numerous awards, including the Kelvin Medal from Royal Society of Edinburgh. And he's here tonight to give us his Kelvin lecture on titled, Does God Play Dice? Please welcome Professor Miles Padgett. So, thank you very much. I'm sorry to, of course, a few people, a little bit of panic. First of all, with my rather late arrival, I've never been the last person to arrive, and then with some AV issues, but they've all been sorted. So, we're all going to be good. So as you said, I, my name is Miles Padgett. I work here at the University of Glasgow. And it's a privilege to be speaking in front of you all this evening. The title of my talk is, Does God Play Dice? And let's, let's try and understand where that came from. And I'm going to give some bits of history. I'm going to sort of try and resolve the question as well. Let's see how that goes. The term, Does God Play Dice, is a misquote from Albert Einstein. So why did, why did Einstein say that? And what he was worried about were the fundamental laws by which the universe works. And the sort of two worldviews. The first worldview is, because I can do lots of clever calculations, not me, but people, that if I can really measure what we have now, very carefully, then I can predict the future. A little bit like weather forecasting. If I want to predict the weather tomorrow, the starting point for that is let's measure it very carefully today. And I can wind my computer program forward in time, and I can predict tomorrow, the next day, the day after that. And we can think about what the limits for that might be, and how well we can measure the here and now. And Einstein thought that you could do that. You can measure now, predict the future. Nothing is left to chance. Now he recognized, however, that there's a problem with that it's called the uncertainty principle that I'm sure many of you have heard of, which is if I measure the position of something very, very precisely, I can no longer measure its velocity. I can already, I can't measure the position. And so you can see that straight away, the Einstein model of the future comes unstuck, because quantum mechanics just tells us very fundamentally that I cannot measure everything at the same time. And why can I not do that. I'm going to use you as a prop. I'm going to let you and see where you are. Light is bouncing off you. And light has momentum. So the light will push you away. So the very act of looking at you changes you in some way. So I can't do anything about it. I'm always going to mess up things that I measure slightly. So to some extent Einstein's view was a little sort of how many angels are there on the head of a pin. Because the one hand he goes, well if I knew everything, I would be able to predict the future. On the other hand he goes I know I can't know everything. Therefore I can't predict the future. The other point of view was for Niels Bohr. And he said Einstein, you're wrong. Probably didn't say quite so bluntly. Actually, the future is inherently probabilistic. What does that mean. It means that I do the same experiment twice. Sometimes it works, sometimes it doesn't. Pretty much describes my life in the lab that, but not due to my imperfection as an experimenter. It's just that you drop the bottle and thinking of water on your hand sometimes it goes to the left sometimes it goes to the right. So Bohr believed that the future had an inherently random elements from which there was no escape. And if I worked out how to measure your position without disturbing you, I'm still buggered. I can't, I can't predict the future. And Einstein said God does not play dice, meaning God does not sit there going as it heads as it tails. Now that wasn't a religious statement he was making. It was a philosophical statement about the way the universe works. The random chance player role in the future. Bohr said yes. Einstein said no. Well that's the summary of my entire lecture, but now we'll start with you. So I'm going to try and take us through Einstein's argument and I'm going to try and take you through some of the experimental evidence and philosophical considerations that have got us where we have today. And actually what I'm in essence referring to is the Nobel Prize of last year. Okay, so the note not this year but last year, the Nobel Prize was awarded awarded to Alan Aspey and Anton Zylinger and John Clouser for their resolution of this problem. Okay, so it's in that sense it's quite current. And so I'm going to talk about effectively what the Nobel Prize was for last year. So I'm going to talk about a bit about wave particle duality. I'm going to talk about the role of the observer. What role do you and I play in quantum mechanics. I'm going to try, as I've done already to re-explain what it was that Einstein did not like about quantum mechanics. And I'm going to go for what, what the modern experiments say about this problem. In 1905, Einstein published, not one, not two, not three, but four papers, gosh, that changed the world. I'm going to say a little bit about what those four papers are and what about them in particular. The first paper he published in 1905 was his paper on relativity, special relativity. And what he said was that the speed of light is a constant. It's a phrase we've often heard. Why would that be a strange thing to say? I don't think I don't have a gun, clearly, if I had a gun, and I could shoot a bullet over there, and the bullet goes at 100 miles an hour. And then if I run along and fire the bullets again, it goes at 103 miles an hour, because I was running at three miles an hour. Okay, the velocities add. And actually, Mickelson and Morley had shown in experiments at the end of the 18 beginning of the 1900s, that actually wasn't the case, light behaved differently to that. But if I stood here still and fire the torch over there, like traveled off at the speed of light, if I ran along with the torch, thinking I could make the light go faster, I couldn't. Speed of light was a constant. And from that rather bizarre statement, all kinds of things change. And I'm not going to explain relativity tonight, but to say that there are things we sort of think we remember about relativity, which when, when things go quicker, they get shorter. It's called lens contraction. If I pick up a clock and start moving at high velocity, it ticks more slowly. In fact, we know that that's true now, because the GPS satellites, you know the satellite that allows Google Maps to tell me where I am. Those are those are actually clocks in space that go round and round the Earth's orbit. And they actually tick more slowly than clocks here on Earth. And if we didn't allow for that GPS wouldn't work. So we know that relativity is true. Moving clocks tick more slowly, moving meter rulers, no longer a meter long. And that's a consequence of that underlying truths that nothing can travel faster than speed of light. Very complicated stuff. Doesn't matter. That was one paper for which he never won the Nobel Prize. Okay, bit of a bit of a robbery that one. There was another paper he wrote, which was understanding Brownian motion. Now, does anyone know where Dr Brown worked? Just out of interest. Sorry. He was a Scott. That's a gas go unit. Yes, getting good so far. What's in the Botanic Gardens, I believe. And so Brownian motion was so called because when people look at things in a microscope, you find that everything jiggles around a little bit. And people didn't know whether that was because the things were alive, and they were swimming around. And so Brown did the sensible thing ground up some stone put some stone flakes in look down and right now they're still moving. So nothing to do with them being alive. And people didn't really understand why that was why is it that when I look at small things in a microscope they jiggle around. And Einstein realized it was because they were constantly being bombarded by molecules that the air was not a fluid. The air was made up of lots of little red balls. Not really little red balls, but lots of atoms, and those little atoms bumped into things. And if the things were little enough themselves those other things would sort of jiggle around. So Einstein understood Brownian motion and that was one that was the second paper in 1905, for which he did not win the Nobel Prize either. Now this is another paper that he published. This is another bizarre paper. It's called understanding the photoelectric effects. So what was the photoelectric effects. So the photoelectric effects is what sort of happens in a sort of light bulby thing where I've got a big bit of metal here I've got a big bit of metal there. And it's in a vacuum. It's an old light bulb in a vacuum, and I connect to battery. So the electrons come out the battery go around the wire, get back to this bit of metal, but they can't get across because the two bits of metal don't jump up, don't meet. So it's like a broken circuits there. And people had shown. So the electrons are coming out of the battery at the bottom, and they're going round to the big orange thing that says negative, because electrons are charged negative. And then you can get a torch out. And you shine the lights onto that, it's called an electrode that that piece of metal. And then in this case the lights red lights and nothing happens. And you shine more red light really crank up the intensity. Nothing happens doesn't matter how much red light you have doesn't matter how much energy is in that red light still nothing happens. And then you shine on a little bit of blue lights, and all of a sudden, electrons start escaping, and I get a current flow. So somehow blue lights can, I'm going to say boil off electrons wrong term but you get the idea. Red light can't do it. It's not to do with the intensity. The power blue light is fundamentally different from red light. And from this Einstein deduced that light itself came in particles. We call them photons now the term photon was not invented till about the 1930s. So we call it quantum light quantum. The light was particulate. It's called the photoelectric effect. And it is for the photoelectric effect that Einstein won the Nobel Prize subsequently, not relativity, not Brownian motion photoelectric fact. And from this we deduced that light was quantized it came in lumps. Incidentally, it's the lumps aren't very big if I had a little laser pointer in my hand now. It would produce something like 10 to the 12 a million million lumps of light a second, just to give you some kind of idea as to how small these lumps are. And then you can now buy cameras that can see those individual lumps arrive one by one. So we know that light arrives in lumps of energy or quantum. But we also know that lights behaves sometimes as a way we sometimes talk about the wavelength of the lights. It's my wave. A red laser has got a wavelength of 632 nanometers. That's about half a million million, half a million of a millimeter. So small. So, one might think when talks about quantum, that the thing that makes quantum spooky is that sometimes light seems to be a particle. And at other times, light appears to be a wave. It's got a name. Of course, it has a name wave particle duality. And we sometimes think that's a bit odd, but that's certainly not what people mean when they say quantum is spooky. Both Einstein and ball went, yeah, that's right. Sometimes light looks like a particle and sometimes it looks like a wave. Sometimes I've got dark hair and sometimes I'm quite tall. I mean, you know, just, these are different things. Actually, at the end of the day, light does whatever light does. We choose to describe it as a particle or we choose to describe it as a way says more about us than it does about the light. Like just does what it does. Now, let's talk a little bit about the wave property, however, because this is important. Light is not the only way that we understand. We know that lights, I got sound waves, you're listening to me now. The wave lens of the sound, the frequency of the sound, the speed of the sound, the speed of the sound is about 300 meters second. My voice is about 300 hertz, maybe that means the wavelength is about a meter. So sound is a way. I can have water waves. I can have earthquake waves. I can have gravitational waves. I can have waves of all kinds of things. I can also have light waves. Now, so let's stick with the wave and try and understand. If I said to you, or if you said to me, Miles, you tell me lights away. How can I prove it? How can I prove it to you that light is a wave? And so if anyone said to you, is this a wave? This is what I would like. If you told me that taste was a wave, I didn't taste. I'd go really prove it to me and you'd go how do you like to prove it to you. So this is how I want. This is how I would ask you to prove it to me. One plus one equals excellent. I can see. I can see you've been listening. That's great. So waves are a bit odd because when I have two waves and they come together. Two crests come together. I get a crest, which is twice as high. Okay. On the other hand, I can have two light beams coming together. And a crest comes together with a trough. And then get darkness. So sometimes one plus one, it seems equals zero. Okay, that's fine. Now, what would I do here? I could have a source of waves here and I could have a source of waves here. And they're doing it. They're the same frequency. This is excellent. And I come here and they add up and in the middle, I get crest plus crest. I go over there now. That was coming towards you, but that one's had to travel further. So now by the time they get to you, this one's a crest and this one's a trough. Oh dear. So I get nothing. I go further. This one's a crest and this one's traveled so far. It's come back to crest again. Oh, they add up again. So if I have a light source here and a light source here. I add them together. What I get in front of me is a stripy pattern. As I move from side to side. And I can work out exactly how far apart those stripes are. Everything's fine. And I can make, you know, if it's light waves, my stripes are probably going to be a millimeter apart. I could do the same trick. It's much harder. It sounds like it should work easy. It doesn't work easy. Otherwise, I would show you. But I could put my iPhone here and it'd be going. Excellent. And I could then move my ear around. I could go, oh, they add up here. I can hear the volume. Oh, they don't add up here. Oh, they do add up here. Little experiment to try if you happen to own two iPhones. Thank you very much for charging mine up, by the way. So. So that's the important thing. Waves do something that particles never do. Sometimes. Waves add up to give me zero. It's called interference. But it's not what confused Einstein and bought. Now. There's a, there's a, there's an example of this. It's quite strange. Because now I've got my, for some reason, those two slits have come out of dark blue. They went to come out as whites. How do I do this experiment in practice? I don't have two light beams here and here. I take a piece of cards. And I cut two holes in it. And then I, I illuminate it from the back. This is how back in the, the, the middle ages, people did interference. They let, they let the sunlight through the curtain. The sunlight hit my piece of cards. My card had two holes in it. Some of the sunlight went through the left hole. Some of the sunlight went through the right hole. And then I would get interference. And you go, well, that's fine. But you also told me, Miles. That the, that the light is also a particle. So what happens? That's fine. I've got lots of particles. Some of the particles go through the right slit. Some of the particles go through the left slit. They all interfere. And I get my striking pattern. And I go, okay, I'll turn the light source down. I'm going to have to turn it down a long way. Because remember, even a laser pointer is a billion photons a second. So I talk about really dark stuff. And you go, well, that's going to be great. But what about when you only have one photon, Miles? When you only have one photon, it either goes through the right slit or it goes through the left slit. How can I still get an interference pattern? And I go, well, you do. And you conclude that this photon went through both slits. And you go, okay, that's what makes quantum mechanics spooky. And Einstein and Ball went, no, no, no, that's fine. And you're like, really? They said, yeah, yeah, you're just obsessing over what you mean by particle or what you mean by wave. Don't stress about it. It's because you're constrained in your thinking, Miles. That's what happens. Okay, okay, fine. Again, let's not get bogged down in that. So that wave particle duality is really strange. But it's not the problem. It's not the problem with quantum mechanics. I'm going to try and now explain what the problem is in quantum mechanics. So let's, again, I'm going to, I'm going to, I'm ashamed of, I'm English, by the way. So I'm going to use a cricket analogy because we're so good at cricket. And I'm going to think not of the slits. I'm going to think of the wickets. Okay, I've got three. I've got the, I've got the bright bit here. That's the sense of the middle stump. I've got the dark bit here. I've got a bright bit over here. That's the leg stump. And that's the off stump over there. Okay. So I've got three fringes and I could have five fringes or seven fringes or nine fringes. It doesn't matter. I'm just going to talk about three fringes. And I start off here. And my photon comes along. Passes through both slits somehow. And it's going to hit the screen where, well, I'm going to measure it somewhere. It's going to hit the screen somewhere. And the answer is going to be, it either hits the off stump, the middle stump or the leg stump. And I don't know which it's going to be. It's a probability. It's a third, a third, a third. So at that point now, it's a little bit like we're in the idea. Einstein ball space of gosh, the universe is going to have to get a three sided coin and flick it and go middle stuff. It's going to have to make some probabilistic determination. And it's that probabilistic termination, which is the problem. So let's have a look and, and see whether I can try and explain it a little bit more clearly. So that now, those are my three. Not the slits. Okay. The slits are over here. The photons have gone through both slits. They've created that interference pattern. Rest with crest in the middle. Then we got the sort of blank bits. That's crest with trough. And then at the far left, left hand, we've got pressed with crest. All the far right, we've got crest with crest. Everyone comfortable? Happy. And that's the intensity. Now then you're going to get into sort of the quantum science or the wave mechanics or anything else you want to say. That intensity is also the probability distribution of the next photon. So I'm going to send one more photon in and I do not know where it goes, but it's a 33% chance it goes into the next photon. 33% chance it goes to the middle. 33% chance it goes to the right. And you can do those experiments. You can see those experiments. And you can see those photons arriving as little flashes of light on your camera, one by one. And eventually, once you've shown enough photons into the system, you get, you know, equal numbers in the three strikes. And you can see those photons arriving as little flashes of light on your camera, you know, equal numbers in the three strikes. But make no mistake, when the first photon arrived, in that case it arrived in the center. I don't know if you saw that. It's just going to always arrive in the center because it's a PowerPoint animation, but it could have arrived at both everywhere. That sort of schematic there is exactly what you would see on a camera. It wouldn't look quite like that because it wouldn't be the stars that would be, you know, coming. So that is what one sees. And the question that Einstein and Bohr debated is when is that coin flipped? When is it flipped? So let's think of different possibilities. A long, long time ago, when we were in the galaxy far, far away, someone was designing the future universe, someone, something, some it, and it knew that at eight o'clock, on the 13th of December, is it the 13th today? 2023, a photon was going to be released and pre-programmed into that existence, it knew it was going to go to the center fringe. That would be one possibility, a bit bizarre, but you know, can't think of anything much before that. It could be, just now I'm going to make a photon, that photon comes into existence, and at that point it goes, I know what Miles is going to do. It's going to launch me at these two slips. And when he does that, I'm going to go through both slips, and I'm going to go to the center fringe. Could do that. Could be, I've released a photon, it goes through the slips, and as it goes through the slips, it goes, oh gosh, I better get diffracting, I better get interfering with myself. I know I'm going to go to the center fringe. Okay. Could be later. Photon could go through the slips, it could fringe, it could be travelling, it could go, oh good grief, I'm about to hit a screen. I'm going to have to decide where on the screen I'm going to go. Rolls a little dice. Oh, I'll go to the center fringe. Could be, that I'm going to hit the screen, I'm going to be somewhere, and I'm not going to decide until Miles decides to look at me. And when Miles looks at me, when Miles observes me, because he's a professor of physics, I better make my mind up. I can no longer be a third, a third, a third, because he's going to measure me somewhere. I have to decide. And so on the one hand, we have a sort of pre-programmed universe where everything that ever happened, and is ever going to happen, has been defined by those initial conditions of measuring the weather very well back at the moment of Big Bang. And then we predict the weather forever more. Or the very latest it could be is after I've observed it. Now, there's more subtleties there because maybe I don't observe it. Maybe I take a picture of it and then I put my phone away and I don't look at the picture until tomorrow morning. So when was it observed? Did my camera observe it? Or is the observation tomorrow morning when I look at the camera picture? Maybe I asked you to look for me. Could you look for me? But don't tell me until tomorrow, okay? Open my exam results, but please don't tell me what they're coming here. Remember that. Look at my exam results, mum, please. But only tell me if it's good news. I don't know how that was going to end. But, and then you go, are you a qualified observer? You know, what counts as an observer? Is I am I an observer? Are you an observer? Is my camera an observer? I don't know. So there's all kinds of philosophical questions there about what we regard as an observation. So lots of complicated stuff and I haven't answered it yet. But hopefully I have tried to illustrate what the problem is. The problem is not wave particle duality. That was easy. It's not which slit did the photon go through. That was easy. It's when do does that probability get decided? Now, I should say, so this is it. In wave particle duality, the distribution of the particles is determined by their wave-like properties. If you want to reconcile wave particle duality, whenever you observe it, it's a particle. Whenever you're trying to predict what it's going to do, it's a wave. So you use the wave mechanics to predict the outcome. That outcome sets a probabilistic distribution and then you flip the coin and it's a particle. So there's a problem. There's many problems. And this is called the Schrodinger cat paradox. So let's pretend that I've got my three fringes and the middle one has got a bottle of poison gas. But it's very fragile. So my photon comes along and it goes to the left. No problem. It goes to the right. No problem. Goes to the middle. Bad news. The poison gas is going to be released. And the cat next to the bottle is going to die. That's why it's a Schrodinger cat paradox. But I'm not going to come in until tomorrow morning. So actually the observation will take place tomorrow morning. So between now and tomorrow morning, the cat has got a two-thirds chance of being alive and a one-third chance of being dead. And so it's half cat, or in this case two-thirds life cat, one-third dead cat. And you go, that's bonkers. That's ridiculous. We're sort of happy that the photon can do its stuff. I come back tomorrow morning and go, oh, yes, the photon was in the middle frame and that's fine. I know I released it last night, but overnight it was two-thirds okay and one-third in the middle. And somehow it's okay for photons to behave that way because they're a bit odd. But the cat's not a bit odd. So that's the kind of issues people have with this idea of delayed choice or delayed outcome. Or I did the experiments, but I didn't measure what it had done until the morning. And somehow overnight, it hadn't quite decided there's a nice little cartoon here. Clearly this is not an experiment that one could actually do for various good reasons. Now, and this led Einstein with his colleagues Podolski and Rosen to write this paper which is 10 quantum mechanical description of physical reality be considered complete because there's nothing in quantum mechanics that gets you away from this probabilistic view of the world. It's a third, a third, a third. And when we observe it, we're going to roll the dice and we'll see what happens. And Einstein didn't like that. He said that quantum mechanics cannot be the whole story. There has to be something else. There are some hidden inner workings where this is all decided at some previous time or some subtle the way you did the experiments. You know, it's a little bit like Hawkeye or whatever. The thing that can predict when the bats person gets in the way and blocks the ball and then they run the video machine to go well, if the batter hadn't been there it would have hit the middle stump. You're out. That doesn't exist. So that's what this paper was about which was written in 1935 turned out one of these three people was a Soviet spy wasn't Einstein? We'll leave it there. In fact, this is got on his truth. So David, the MI5 knew they had a spy. It wasn't the MI5 then. And we knew we had a spy and we knew a code name for spy but we didn't know who the spy was. Their code name was quantum. You'd have thought they could have... at least called some people in for questioning but never mind. So let's talk about the Einstein board debate. So Einstein thought that outcome left, middle, right, life cat, dead cat, whatever was somehow encoded very subtly in the starting conditions, the initial conditions. A bit like the weather forecasting. If I measure everything very well here and I know exactly where the photon is and I know exactly direction and I know exactly the design of the slits and I know exactly where the screen is I can sit down and I can work it out that this photon is going to go to the left fringe. I know I can't because the universe is a bit fussy I've got the uncertainty principle I can't measure it that well. I didn't think you could predict the future in practice but he thought at some hidden level the future was encoded in the present. Get that? This is not I can predict the future. Okay. Now, Bohr didn't think that. So this hidden is really important. He was called a hidden variable theory. So it is predictable. The weather forecast is predictable. It's just that the starting conditions are hidden. Therefore I cannot predict the future. God does not play dice with nature. Now hopefully you understand why he said that. And Bohr said he did. Okay. Now, one never knows what people that are no longer with us so we don't weeding. I'm going to say Einstein thought this, Bohr thought that. They're not here for us to ask them. So we're going to surmise what they thought. So they disagreed fundamentally because Bohr said, nope, it's all random. It's a third, a third, a third. There's nothing you can do to predict the future. That's just the way it is. Einstein said no, that's not true. God doesn't play dice with nature. It's not predictable. It's just that we can't measure it well enough to predict it. So neither of them thought it was predictable. Okay. They were in perfect agreement that the future could not be predicted. They just disagreed on why it couldn't be predicted. And so it seems that they were thinking that they were thinking about philosophy. They were thinking about the number of angels on the head of the pin and whether they preferred the right-hand side or the left-hand side. I mean, as if one would ever know. They thought they were arguing about philosophy and it was untestable. And that then changed thanks to this man called John Bell who I did meet once. I did my PhD. He came to the laboratory and he gave us a seminar and nobody understood a single word that he understood. Okay. It was very clever. Very clever. Nice guy as well actually. Really nice guy. From Northern Ireland, as it happens. And I'm literally, I did my PhD in the Cavendish in Cambridge and Cambridge spoke to us just after the experiments to prove I'm a kid you're not. Maybe one person, Brian Kepard, who's very smart, asked a question. No one else had an absolute scoopy clue what he was on about. But we knew it was important. So hey. So now I'm going to try and explain to you so he devised this test which was called the bell inequality which allowed you to distinguish between something that was not known from something that was not knowable. If you think about that. And this is how it worked. So I'm now going to explain polarization. I'm getting a bit short of time. I've got my horizontally polarized light. It's coming up to my polarized sunglasses. And when the light's horizontally polarized it's transmitted. I've now got so my wave my light wave coming towards you either wiggles from side to side like that. Or it wiggles up and down. Like that. And light from the sun by the way has both. It wiggles up and down and it wiggles from side to side. The vertically polarized light doesn't come through. That's how your Polaroid sunglasses work. Because light that bounces off the snow is vertically polarized therefore does not go through your sunglasses. The light that comes directly from the sun some of it's horizontally polarized and does go through your sunglasses. Polaroid sunglasses cut down the reflection from the snow or the water. Now that's sort of interesting when you go fine. What about light at 45 degrees? What about light that wiggles not like this or like that but at 45 degrees? What happens then? Well, half of it gets through. That's not a surprise. But the half that gets through is now horizontally polarized. Now that seems a bit odd. Why is that? Well, it's because light that's polarized at 45 degrees is actually made up of equal amounts of light some of it's vertically polarized like that and some of it's horizontally polarized. And I've already told you that vertically polarized light doesn't come through. So all I'm left with is the half that was horizontally. Well, let's think about now at the level of single photons. My horizontally polarized photon always gets through because horizontally polarized light gets through the polarizer. My vertically polarized photon never gets through because vertically polarized light doesn't get through the polarizer. Well, what about the one at 45 degrees? Well, I can't have half the photon. So either the proton gets through or it doesn't half the time it doesn't half the time it does but when it does it's horizontally polarized. Now, why am I telling you this? So if I'm sitting here on the right hand side of the screen one of two things happens. I either measure a horizontally polarized photon or I measure nothing. If I measure a horizontally polarized photon that could be because of two different reasons. Reason one could be that the incident photon was horizontally polarized and it got through. Or it could be that the incoming photon was polarized at 45 degrees and it got lucky and got through. It rolled the dice heads to tails heads got through. I don't know. So it's quite interesting that I don't know I can measure now but I don't actually know what happened previously. Quite a powerful statement. Two different inputs had the same output. I measure the outputs I do not know which two inputs it was. Now when does the photon decide whether to get through the polarizer or not? It seems obvious doesn't it? The photon comes along at 45 degrees hits the polarizer goes I'm going to have to decide flip the coin oh heads I'm going through or tails I'm not seems perfectly logical. Sounds so obvious it has to be true turns out not to be I shall show you why. So now we're going to get on to the wonderful work of Alan Aspey, Anton Salinger, John Clouser there are special light sources which I won't describe but I'll describe what they do. If I have a light bulb the photons come out one by one okay they come out very quickly and I get millions of them billions of them every second but fundamentally the photons come out one by one you can take a different kind of light source where the photons always come out two by two a little bit like Noah's Ark in reverse I suppose but and in this case the two photons come out one of them goes to the left the other one goes to the right and I can watch that all day long and it will just carry on doing that and I go well that's interesting and then there's all kinds of conservation of energy that means they come out at the same time okay there's conservation of momentum so if one of them comes out heading upwards the other one comes out heading downwards again I would notice that I could see that it's called correlation and it happens because of conservation I notice something else about these photons they come out with opposite polarizations so if I measure one of them is polarized vertically I notice that the other one is always polarized horizontally if I notice that one of them comes out polarized 45 degrees one way the other one always comes out 45 degrees the other way it's because of conservation that always doesn't matter it's what I observe to be true these photons always come out with opposite polarizations I can't change that if one of them is slanting to the left the other one is slanting to the right now we're going to add a polarizer and we're going to play a little game which is we're going to try and predict whether the photon goes through or whether the photon doesn't go through so when the photon comes out polarized vertically it never goes through when it's horizontal it always goes through and when it's at 45 degrees either way it's at 50-50 does that make sense just watch the photons on the right-hand side I'm going to run it again when the photon comes through at 45 degrees you don't know what's going to happen it's going to go one way or the other way when it's horizontally polarized it always gets through when it's vertically polarized it never gets through and whenever it gets through the bit you see is always horizontal so let's just watch it one more time and if we understand this bit I promise you we're going to get to the end and understand everything so 50-50 no yes 50-50 yes 50-50 no no yes 50-50 yes do you get the pattern? do you get it? we know what's going to happen now guess what I'm going to do now what do you think I'm going to do now I'm going to add another polarizer on the other side incidentally that's a nice picture of Einstein and Bohr I think they're watching strictly come down to see I can't quite tell maybe it's a football match so the question is when does the photon decide what to do Einstein said it's all decided at the beginning it's all predictable it's a decided only at the point of observation not when it goes through the polarizer at the point of observation now let's think about what that's going to mean and the question I'm going to ask myself is I put these two polarizers here and the question I ask is do I ever get lucky twice because in a probabilistic world that photon comes out it's at 45 degrees it hits this polarizer and there's a 50-50 chance it gets through that's what I said so if I flip heads it goes through same things true on this side now if I have two coins I don't have any coins because post-COVID none of us carry any money with us anymore if I have two coins and I flip them sometimes I'm going to get two heads which means sometimes I'm going to get lucky and I'm going to get a photon through both polarizers this is an Einstein view of the world so let's look at that this is the Einstein view the horizontal vertical cases are really boring because one of them is going to get through the other one isn't the interesting case is when they're both polarized at 45 degrees look I got lucky oh didn't get anything 45 got one of them but not the other that's like heads tails boring boring that one don't care interesting interesting got both of them yeah don't get it all I'm doing is I'm just flipping coins and going do I ever get heads on both of them I'm wanting four times I do and that means I get a double count Einstein all of us here went that's the problem sounds pretty sensible to me but let's look at the bore bore says you don't know what the polarization is it hasn't made up its mind the only time it makes up its mind is when you observe it so when I observe that one to be horizontal the other one is always opposite yeah therefore it's vertical therefore it didn't get through what so I never get two heads because my coins are entangled they are always going to be the opposite so when I flip heads on this one I'm always going to flip tails on that one so I don't get so now this question of philosophy as to when is it that the thing makes up its mind does it make up its mind in the experiment or is it only at the point of observation is now experimentally testable and so Alan Aspey I don't want to delay us he's a fascinating lovely man I could tell a little Alan Aspey story which is quite funny I love him to pieces he cares about people he cares about careers he cares about the places in which we work I cannot think more highly of him not every Nobel Prize winner but he is a nice person but he is I'm now going to have a joke of his expense I to several things I saw him just the other week as it happened I when I was young was deciding what to do I was going to be a school teacher which was fine for me not so good for the schools probably and I was walking across the campus I was listening to the radio and I heard about this young French guy just proved Einstein wrong ok and I was like wow this is incredible so people are still doing physics are they I thought it would all be done before and so I decided at that point I was going to become a researcher this is true true true I decided I was going to become a research scientist just on the basis of that research program just on the basis of that radio program and then many years later my colleague Steve Barnard was on our last phase visiting us and we are going down to the pub tonight do you want to come down I was like shock and grief you know where is the sky blue I'm down I'm coming Steve so I walked down I went into the pub and Alan was there with he had a very impressive mustache he was French at that time almost the onion rings very typical French and I said I was an early career researcher I said oh you have a nasty and he looked at me and said your mouse patch it and I was like oh and then he said to me I just read your most recent paper I think it's absolutely fantastic imagine what that feels like when your hero says that to you so I still basically have the things on the back of my leg anyway many years later he said well you must come and give my department a seminar and it's in Paris of course he is and so I told him over there and he said I said I could take you out for you tonight but you know I like to cook and maybe you could come round to my house and my wife and I will cook for you and I was like oh fantastic so I told Alan it takes me down into his wine cellar because of course he has a wine cellar and he goes Miles I said I just want you to pick any bottle of wine that you feel that you would like and we will drink it tonight to celebrate your tour and I said Alan I said I'm tea temple I don't drink I said but thank you and he looked at me in this eye and he just said and we said you're not a vegetarian too I said no no no bring it on I'm fine but anyway so there's a picture of Alan a few years ago a big fine mustache and he's a super guy and so he did these experiments back in 1980s and proved that Einstein was indeed wrong that actually outcome was only determined at point of observation and not before now he didn't answer the question whether you count or I count or the camera counts that's still an unknown rate Nobel prize there for somebody they can look at how to do that but he disproved Einstein Einstein's view was wrong he was right it seems the universe is not predetermined you do not know what your exam results are until you open the envelope and at that point the wave function collapse and you know the answer so on that happy note I've gone on a few minutes longer than I wanted but anyway and I'm delighted I was so happy every year I predicted that he was going to win the Nobel Prize but you know eventually you get the right time and I was delighted that last year he couldn't have happened to an Einstein guy so on that happy note I will stop so if you have any questions please raise your hands for those of you who are watching online if you have questions please type them into the Q&A tab I'm sorry I overran anyone needs to go they should just go without any inhibition I can chat out the question out loudly Hello 1, 2, 3 I'm hesitating asking this question in case it's just like a really really stupid question but I'm going to ask it anyway if Einstein was so hung up on being able to predict things that depends on a kind of linear causality doesn't it but a linear causality depends on a prior course and anyone who's done any basic philosophy will realise that that's just one aspect of time because a prior course has to stop somewhere and so it's only one aspect of time so surely Einstein is basing that assumption on a partial understanding of time yes I'm not sure I know that the answer to that I think I think Einstein's saving grace on that one is that not even he thought the future was predictable because we could not predict the future the here and now that the laws of physics stopped us from measuring the here and now which meant given that we didn't know the here and now he could not predict the future and this whole dichotomy between Bohr and Einstein is not that one of them thought they could predict the future but one of them thought it was unknown and the other one thought it was unknowable if you see what I mean it's slightly more subtle and even at a very very simple level so we're not talking about anything as complicated as the weather which has lots of possible outcomes we're talking about in the case of the polarizer it's just two outcomes I mean it's an incredibly simple system it either gets through or it doesn't so I'm not disagreeing with you at all I'm not sure we should talk about it afterwards I just think that probably applies to more complicated systems than we're in the what are called two-state systems so yeah we have a question online from Alastair McDonald how confident are you about the uncertainty principle I'm sure there must be a joke there I'm actually pretty calm I mean we think the uncertainty principle is quantum it isn't actually quantum at all and at some level it's sort of okay let me I'm going to be a clock and you're going to tell me how accurate the clock is and I'm going to go tick once a second tick tick now what you're going to do you're going to sit there with your watch and you're going to go through 10 seconds and count the number of ticks I do and you know what the answer is going to be probably going to be 10 but actually if you got unlucky it would only be 9 because it's just the timing and the thing and if you got lucky you might get 11 and so this idea that there is an uncertainty in measurements and so that is I talked about I mean everyone knows the uncertainty principle in terms of if I measure the position I cannot measure the velocity it's also true that if I measure time I cannot measure energy and energy is frequency and so if I only measure my clock for 10 seconds the error in my measurement it turns out is and that's why it might be 9 might be 11 might be 10 so this thing that we call the uncertainty principle although we usually talk about it in a quantum context is just maths in that sense and the other example would be this is not entirely a joke if you want to play a flute something like a flute is a very pure tone and you go my flute might be slightly out of tune and you have perfect pitch how can I just play very quickly and if the note doesn't last long enough it's not out of tune you notice and notice out of tune the longer it goes on and so this idea of frequency and time uncertainty are hand in hand so in that sense the strange thing about about the uncertainty principle is not the uncertainty between time and frequency or between position and momentum the curious thing actually about that sort of maths is the link between wavelength and momentum so it's the or the de Broglie relationship shortest physics thesis ever written I think it was two pages for a PhD not bad going such was its profound impact on the world so the consensus is amongst most scientists that the uncertainty principle is fine where people disagree and people still do disagree is yeah this fine for photons but what about cats you know and then you go oh well photons cats I get the difference how about a molecule how about a virus how about a small microscopic creature at what point does it cease to be quantum and start to be common sense and can I really leave the cat in a half live dead state overnight or can I only do it for a fraction of a second so whether this theory is where PI people go yes this ambiguity itself collapses after a certain time depending on the mass of the system as it turns out but no one's ever done any experiments in that area that have proved these things one way or the other it's just unknown Roger Penrose who won the Nobel Prize three years ago maybe I'm trying to think for how long ago it was contemporary of Stephen Hawking just a name drop he had some very very interesting theories about that it does not need an observer to roll the dice that actually the mass of the system and mass the more massive the system the sooner the dice gets rolled and so it turns out that a photon can be undecided for like the age of the universe okay whereas a cat can only be undecided for about a nanosecond okay and it's not an observation it's a self-collapse but no one's proven these things one way or the other there's questions what happens to a bright light photon when it travels through an infrared lens and then cannot be seen at all by the naked eye well they can't be seen by the naked eye but luckily for me I have an expensive camera that does see it so I mean that whole sort of photon wave wave particle duality I mean gamma rays are extremely particulate they come along knock things to smithereens by the time you get no one's ever observed a radio frequency photon I can count I can buy a camera my iPhone camera and count photons near as damaged not quite I can certainly buy a camera for £20,000 that will count visible photons just go that was four I know that was six once you get into the infrared you can count photons it gets hard no one's ever counted a radio frequency photon because the energy is so small there's just so many of them that so lots of experimental evidence for photons everything from gamma rays x-rays UV infrared terahertz radio waves but I don't think many people would doubt their existence it's just that no one's done an experiment that ever shows the particle behaviour of a radio one can you read the t-shirt please so again the role of the observer is still really an experimental unknown and so I can't tell you whether the collapse of the wave function the rolling of the dice the left fringe middle fringe right fringe I can't tell you whether that occurs when the photon hits the screen or when I look at the screen I no experiment has been ever devised that allows me to distinguish between the moment something happens and the moment I saw it all I know is it happened after it went through the polarizer so that picture of the polarizer essentially pushes the decision point to when we saw the horizontal line appear on the screen now if you'd seen it appear on the screen but you didn't tell me to tomorrow morning I can't I have to make assumptions about you being a sentient being or I mean it gets philosophically very very difficult to go when did it become true I know when it became true to me which is the point of my observation I will take your word for it that it became true to you but maybe hey we all live in a matrix and who knows but you know I know it's ambiguous for at least the time it went through the polarizer because that's what the that's what the other aspect experiment showed us we don't know when it decided but we know it still hadn't decided when it had gone through the polarizer whether it was last night or this morning I don't know so the role of the observer and who qualifies as an observer or what camera might qualify as an observer nobody nobody has been as yet able to think of an experiment that would distinguish the two not that I want to make a difference you know I said earlier which was maybe a bit naughty of me and I apologize to the philosophers that I was sort of saying somehow as equating philosophy with something that can't be experimentally tested and I shouldn't have done that I'm sorry about that but I think we all recognize there are things that we can experimentally test and there are things that we might hypothesize about but we can't test we might think we understand how these things work but we haven't devised a test to check two thirds of the way down blue jackets thank you you've stopped at entanglement I'm sure there's more to say about that and in particular is this just a new word for action at a distance so there's lots and lots of terms that people bandy around incidentally probably the first entrance in popular culture I gave this book to Alan as a present by the way there's a series of books from a cross called The Hitchhiker's Guide to the Galaxy written by Douglas Adams and he wrote those in the mid to late 80s and in the restaurants at the end of the universe there's this line there I won't bother to read it out but you can see here talking about I can work out what's happening everywhere in the universe by looking at any one bit of it because the universe is entangled and spooky action at a distance so I think it's the first sort of reference in popular culture and I think that's one of Alan's work but they're clearly more complicated than photons and I also think Mr Kipling's little takes when it comes to these things what's entanglement I said something sort of as a little bit of a throwaway but I really meant it I've got these two coins and I can flip this one and it's heads or tails I can flip this one it's heads or tails but I've said there's something strange here that if I flip this one and it's heads this one's always tails and you think well how would I do that well I'll tell you what I'll make some little radio controlled coin this one lands down and it knows it's heads it then communicates by radio waves or something to this coin over here and this one flips itself to tails God knows how I'm going to do that experimentally impossible and you kind of think that it might be possible and you go well but Miles you didn't flip them at the same time because you flip that one and then this one had to sort of communicate and nothing can travel faster than the speed of light so that that one won't be tails until a little bit later now actually one of the things that Alan did and the thing that was really important and I didn't explain it just because of time it's really important that you make both these measurements at the same time otherwise you can imagine the whole thing gets smart and this photon goes aha I'm horizontal you do over there be vertical okay this is about this and that's now and that's like faster than light spooky action at a distance and this property we call entanglement is that these two things are always opposite one is heads the other is tails no matter how far apart I make them I am not limited by the speed of light and causality going from one to the other and so when people talk about these experiments what is important is that I have observer one over here and we're sitting here with our watch and we're going to measure that photon at exactly 12 o'clock midnight and the observer over here is going to go and I'm going to measure it at exactly the same time and because Einstein told us nothing can travel faster than the speed of light there is no way ever that my measurement here has somehow told that one what to do because these two things were entangled coming back to your magic word entanglement they were always opposite no matter how far I separated them and now you can see why Einstein had such issues with entanglement because of course he told us nothing can travel faster than the speed of light so the idea that these two things can be separated from each other and yet still be perfectly correlated caused him some concern and the get out is that there's nothing I can do to transmit information and so really when we say nothing can travel faster than the speed of light what we really mean is information can't travel faster than the speed of light what do I mean by information I mean do you want tea or coffee that's information how am I going to communicate all I've got here is if this one's tea that one's coffee but I've got no control I can't make that one coffee all I know is it's like I've got one coin I don't have one coin I shake it up and I go there's a coin in this hand I know there is there's a coin in that hand you go no of course there isn't because I've only got one coin and that'd be fine so this idea of a non-local correlation is not really spooky it's just conservation of coins the pound is not in my bank account and your bank account at the same time actually thanks to the banking system it's probably neither of our bank accounts it's in their bank accounts and they're making interest on our transaction but it certainly wasn't in yours and my account at the same time now that's not spooky or quantum it's just conservation it's just there's not information there okay on the left hand side towards the back it seems to me that wouldn't it be simpler if the properties were defined at the point of creation and they were just defined as opposite at that point but in which case if it were defined as opposite at that point I could get lucky there's going to be 45 and minus 45 this one comes along there's a 50-50 chance it gets through the polarizer this one comes along there's a 50-50 chance it gets through a polarizer so there's a one in four accounts there would be some hidden variable that would make them have the same decision so what okay so so that would okay so the hidden variable theory that I've just I don't know whether I quite agree with you there I mean I agree that the experiments that people actually do are a little bit more subtle incidentally just as a little aside brilliant best question I've ever been asked by anybody I went like isn't this interesting you never get photons at the same time and I there was this kid young school kid had his hand up he said how do you know your experiments just not broken such a good question and well there's an old test to do maybe I should have mentioned that if I was 45 degrees I'm 45 degrees I would have a 50-50 chance now what you're saying is but how would I if I pre-program I say that I pre-program the outcome of the coin toss no the yeah well yes in a way yeah but it was they were the they were the same for each or the opposite for each each yeah so what I think is for you from your light bulb how it creates two photons going in opposite direction so but whatever that mechanism is there must be some way then that they were it sounds like they were reciprocal products of something so yeah undoubtedly these correlations exist because of conservation laws whether it be energy momentum or angular momentum I still don't quite get what you're saying if I I go along here I've got a 50-50 chance here and a 50-50 chance there no but you don't because the one the decision has already been made by the hidden variable and they are reciprocal to each other so you're conserving their choices I will need to think about that for a while so email me email me we'll chat another question halfway down on the left please I guess it's kind of linked to that question but if you're observing the 45 degree but you can't really observe it because by the time you've observed it it's horizontal how do you know that there are 45 degree photons A existing or B coming out of the light bulb in order to say that they've decided thankfully I can answer that question so the experiment I've described it to you is not quite the experiment that is done the reasons that you say but what do you actually do you should go I'm going to have my polarisers like this and my experiment runs and then I run exactly the same experiment but I go actually I'm not going to have my polarisers like this I'm going to redefine what I mean by vertical and horizontal I'm going to have my polarisers like this I'm going to have my polarisers like that the important thing is no matter how I orient these polarisers I never you know if you might say well maybe your thingy only gives you out horizontally and vertically polarised photons it never gives you out 45 ones I'll go fine I'll just rotate the polarisers by 45 degrees and we'll go again and I find exactly the same results so the the experimental result is rotationally invariant in that sense and the key and this is incidentally what Alan did which basically differentiated so John Clouser did the first experiments back in the 70s Alan in the 80s had his polarisers and then kept on changing them very quickly and he changed them at a rate which was quick compared to the time it took for the photons to get from the source to the polarisers because the argument had been that this polarisers is telling the photons what to do at the source that somehow there was a back communication from the setting of the experiment that was telling the source what to do. No one knew how that would be or understood a physical mechanism or any law of physics that would allow it to happen but there could potentially be a back communication to go just do horizontal and vertical photons okay don't do these stupid 45 degree ones because they might fail and so you go well actually I'm going to trick it into it thinks it's doing these but now I'm going to make it do those and it doesn't matter what orientation you never get the simultaneous count so that and it was that rapid switching of the polarisers which was Alan's inventive step for which he ultimately was awarded the Nobel Prize beyond the work of Clouser who had done it with fixed polarisers and then Anton Zeilinger's contribution if we're going through that why did he get a share turns out there's also a loophole if your experiment only works some of the time you don't necessarily have a fair sample and so you just got lucky effectively again in some very bizarre way and so Anton Zeilinger did it with detectors which are effectively 100% efficient he measured all of the photons Alan didn't measure all of the photons most of his photons came out and the detector didn't work perfectly and so Clouser did the first experiments which we would do now in the lab in our undergraduate labs and just go yeah yeah that's it that's shown and everyone accept it now rightly or wrongly is a demonstration of the bell inequality Alan removed one loophole which is the which is the non-local the non-local by being very quick with his polarisers and Anton then removed the second loophole which was the the fair sampling loophole sorry a bit specialist but that was the citation on the Nobel Prize thank you very much I think I'd now like to invite our president Pat to give the voice of thanks and the world to Kelvin Middle what a pleasant surprise you should have been able to predict it Miles I shouldn't I shouldn't I've been watching I've been watching Lord Kelvin and I could see him frown down again and shake his head and you can't tell me that I didn't Miles has told us about something that's very complex and he's done a very good job I think in simplifying simplifying these complex things for us not necessarily that it made it crystal clear to us what the explanation is because he also had to tell us the difference between unknown and unknowable and that's quite hard in a way to get your head round and while I was listening to the questions I was thinking about science and being a biologist I tend to think of science as something that is exact and precise and that what we're looking for is the truth and certainty but when it comes to physics it seems to be a bit different and it made me think that when Lord Kelvin was doing physics it was called natural philosophy and then it turned into physics which made it sound a bit more precise but I think it's coming back to being maybe not natural philosophy maybe unnatural philosophy because sometimes miles use the word spooky and quantum as being one or the other but sometimes quantum seems a bit spooky so miles I want to really thank you for making us all think about philosophy about nature and how nature is dealing with this complexity it made me think how surprising it is the animals can see because they don't even know whether it's a wave or a photon that's coming at them but nonetheless we manage to make sense of our world we tend to think there is causality that's how we have to live our lives and yet maybe there isn't maybe it is all uncertainty probably that's the case so miles on behalf of the audience on behalf of Lord Kelvin who's probably listening somewhere I'd like to award you the Royal Philosophical Society of Glasgow's Kelvin Medal thank you very much and of course the paperweight thank you very much please have a drink outside