Transcript for:
Exploring Quantum Mechanics with Professor Elitzur

I am José Luis Razo Bravo, the Executive Director of the European Institute of Science and Management and host of this channel. Today we have the great privilege of hosting a lecture by Professor Abshalam Elitzur, a theoretical physicist and the co-discoverer of the famous bomb testing experiment that Sabina Hassenfelder rightfully says is stranger than anything else in quantum mechanics. Along with a past channel guest Eliahu Cohen. Professor Elitzer is putting out some of the most important, most innovative, and most consequential work in fundamental physics. This is a bold claim, but you'll see that it's well-founded. If we think of science as a series of nested Russian dolls, then Professor Elitzer is working on the absolute innermost doll, and doing so in a very distinct way that has repercussions for all of the... other layers of science, which are the outer layers of the doll, from biology to neuroscience to eventually, we hope to demonstrate, to social science. As we listen to Professor Elitzur, I encourage you to keep this in mind, and that unlike any other approaches to interpreting or understanding quantum mechanics, such as the many worlds interpretation, what we're talking about here is empirical physics that pertains to this world. the one world where we actually live. So with that, I'll turn it over to Professor Ilitzer. Professor, thank you so much for taking the time to talk to us today. Thank you very much, my dear friends, distinguished guests, ladies and gentlemen. What I'm going to do in this talk, I shall not assume that you are well informed about quantum mechanics or even physics. And I'll take it as an advantage because I think, and I hope that I'm not being too arrogant, I think that you can use the interaction-free measurements, the subject of this talk, as an introduction to quantum mechanics. So if you don't know almost anything about quantum mechanics, what I will do, I'll begin just from the very beginning. And then I'll take, you know, there is the double-slit experiment and other experiments which demonstrate how unique quantum mechanics is. I'll take my own discovery together with Weidmann. As an introduction to quantum mechanics, I do believe with all modesty that it's a no less good introduction than the others. Let me say that this is part of a course that Yakir Aharonov and I are going to give, which will be both an academic course and a popular course. That is, you will be able to register and get credit, but you will also be able to just skip all the mathematics and enjoy the philosophy and the conceptual layer. together with producer Tzahi Tchiv from Israel, we are preparing a very interesting course. So this is the chapter actually discussing IFM, but let's begin with quantum mechanics. And let me say something about classical physics, just a very brief introduction. Imagine a cannon and a cannonball. Yeah, it's not a nice way to begin, but you know physics... Something important about physics is that every time people learn a certain physical law, they ask how I can better kill my fellow human. So think about this cannonball. You launch it. Physics tells you that once you know the initial conditions of this body, that is the cannonball, you can compute all its trajectory. You know every moment where it's going to be, what's going to be its position and momentum. What is unique about... quantum mechanics is that it forces you to give up this certainty, but it gives you something much better instead. And we should go, we begin with a question that was raised in classical physics, in Newtonian physics, actually by Newton himself. And this is about something that fills this room and our world and our lives. And Newton asked, what is light? And, you know, Newton was the founding father of all classical physics. He laid the philosophy of determinism, making physics mathematical and so on. He was specifically the father of mechanics, on which later fluids and gases, other branches of physics were based. But now he wants to be also the founder of modern optics. So he's asking the question what is light and here is his answer. Light is many little particles that animate, that come from the candle, the source, the lamp or the sun and they go straight according to the laws of mechanics, bounce on various objects and then jump to our eyes. This is how we see. And then there was Christian Huygens, a Dutch guy who took exception with Newton and he said, no, I don't think that light is particles, I think that light is waves. We know waves from, say, acoustics. When I talk now, even if you were with me, not even a single molecule would come from my mouth to your ear, but it's one molecule pushing the other and the other, so it gives rise to a wave while the molecules remain, return to their place. So here we have these two sages, it's to founders who probably went to the same barber and this guy says light is particles and he says waves. How can we decide who of them is correct? Believe it or not, this is what later would signal the fall of classical physics and the rise of quantum mechanics. Because actually the problem has been solved in classical physics. People say there is something very nice about shapes which I'm going to explain here. Very simple. although the mathematics is the calculation is a bit maybe a bit complex what happens when two waves come together they give rise to interference that is you get one wave from the two now how you get it there are two opposite extremes they can merge when they're in phase up with up down with down and as a result you are getting a wave which is just the sum of the two they strengthen one another this is what we call constructive interference And you can have destructive interference, that is, that they come in opposites, in an opposite phase, and then they obliterate one another. And of course, you can get all the things in between. Here is a simple demonstration with water waves. So here you have a wall within the wave and there are two openings. And then imagine a flat wave coming from the left and then entering through these two openings. And then you have two small waves and then you can see in red places where they... gave rise to a constructive interference. You can see it in red, so you have higher waves, and in gray no waves at all, and everything which is in between. Here you can see it even in the GIF. So again you have the wall breaker, the two openings, and you can see interference. That's nice. A man named Thomas Young found out that actually light gives rise to interference. You shed light on a partition where there are two slits, and then you get light, dark, light, dark, and everything in between. And you can even calculate it. So it turns out that actually this is, I think that this is an unequivocal answer to the question, what is light? Whether it's particles or waves, it can be particles. Particles cannot attract one another. They do not repel one another. Once you have the theory of light optics, then you can actually, you can predict given the length. the wavelength of the light, whether it's red or blue or whatever, the width of the slits and their place, you can actually predict exactly what would be the intensity of light on the screen after the partition. So Newton's vision works even when he is wrong. This is why I write Newton is right even when he is wrong. You can tell Newton, look how wrong you are. It's not particles, it's waves. You can calculate what will be the intensity. of the light at every moment, at every point on the screen, this is Newton's deterministic vision, given, remember the cannonball, given the initial conditions, given the angle and the velocity, and then you can predict, you can calculate the whole trajectory. So this actually should be a triumph of classical physics. And let me just say that if you want to make this experiment at home, then you need, you can't use the lamplight or sunlight because it's not monochromatic. You need to get a source of light. light which is monochromatic, say only red or blue or something like that. But you can see interference in even non-monochromatic light, suppose in soap bubbles. So the layer of the soap membrane is not uniform, so there are cases, there are places in which some wavelengths cancel one another or strengthen one another and then you get this beautiful plethora of colors. Okay, so this is interference and this is what you have to learn in order to go into quantum mechanics. And then comes this guy and he gets a Nobel Prize for that. And he just ruins everything. At the year 1905, when he wrote five papers, he demolished all of classical physics, he created relativity theory and quantum mechanics, and here is what he did in quantum mechanics. Actually, it tells us that we are all wrong, because there is something to light which is actually particles. And let me explain the idea. Suppose I have here a lamp. This is the lamp, and I have a jar within which there is gas. I'm going to put the light on the gas, right? So what happens to something on which you shed light? It gets heated. These are the molecules or the atoms of the light. So this is what you get. Light becomes warmer. Everything is fine, right? Then, says Einstein, and he was using an idea of Planck. There was another genius, I should have mentioned him, Planck, who proved actually that energy is like matter. It comes to small units of which there is no half, there is no third, there is either zero, one, two, nothing in between. So Einstein is taking Max Planck's idea and now brings something very revolutionary. He's making a prediction. He didn't make an experiment. He just made a prediction which stunned the world. And he said the following. I'll first simplify it and then I'll show the idea itself. So we saw that when you shed light on gas, then the gas becomes heated and the molecules or the atoms begin to move. Say Einstein, suppose that you make light dimmer and dimmer and dimmer and dimmer until it... the lamp emits that single amount of energy which Planck said cannot be divided. He called it quantum. What happens when your lamp sheds a single quantum of light into this object that is this jar with air molecules in it? Prepare yourself for a surprise and for a huge headache. It's extremely simple, very troublesome, but fascinating. What Einstein predicted, if you... shed one quantum of light, that unit of energy that cannot be divided further, you have to wait for a while because quantum mechanics is probabilistic, it's not deterministic, but then after a while one of the molecules, you don't know which, is going to get a kick and move. How much energy was absorbed in that molecule? Says Einstein, I'm just following Planck, he said that you can't have less than one quantum, not third, no half, no quarter, no third, so there must be one quantum. absorbed in this molecule. Now how much energy was emitted? One quantum. So who is laughing from the grave after 200 years? It's Newton. Waves, eh? How could a wave be, you know, absorbed only at one point, all the energy, all the momentum? It's a particle. And what Einstein said was something much worse. He said that the single quantum of light is going to rip a single electron. from its atom. Now just to give you a notion, think about the coliseum. Atom is something very small. Make it as large as the coliseum. Then the nucleus are going to be a few marbles that you hold in the middle of the coliseum. Most of the atom is just nothing, it's void. The electron, imagine just a mosquito flying very fast around this coliseum. This is how small the electron is. And still Einstein predicted that under certain circumstances very little amount of light, that is a single quantum of light, the smallest amount of energy possible of light, is going to rip an electron when light is shed, when light impinges on some metal. He made a calculation and it turns out that it depends only on the wavelength of the light, which was a proof that Planck was correct and light comes in basic units. But then, now Einstein proves that they act like particles. it acts like a marble, it hits an electron and kicks it out. Do you understand, and I'll be happy later to take questions, that we are facing here a very serious contradiction. Classical physics proved to our satisfaction that light is waves. Light gives rise to diffraction, interference, Doppler, whatever. These are waves. You can do with them just what waves do and you can control light. Now comes a guy and gets the Nobel Prize for showing that actually light is a particle. A particle does not give rise to interference, it doesn't go to all kinds of places. It is only at one point and it's smaller than an atom. So how can you reconcile? You know, imagine a wave can go millions of kilometers, be in all those places and expand over millions of kilometers at the same time. But a particle is a particle. Can you understand the contradiction? So now what I'm going to do is to... Einstein had a friend, Max... Sorry, Niels Bohr, who was also Max Born. And they had an argument about the nature of quantum mechanics. Einstein always believed that there is something classical. There is something just like in classical physics, which we just don't know. He called it hidden variables. People called it hidden variables. He believed that actually there is an objective reality to the position of the particle, to the question whether it is a particle or a wave. And eventually physics will find it. He believed that quantum mechanics is not the whole story, that there is something missing. And in order to drive his friend Niels Bohr nuts, every once in a while he used to think about a certain Gedanken experiment, that is a thought experiment. You know, this is a tradition in physics ever since Archimedeus and Galileo, that not always you have to do the experiment. You don't always have the technology or the money to do the experiment. So you run it in your head as a simulation. And if it's interesting, then later experimentalists will come and perform it. So let me stress that all these... thought experiments that Einstein presented to Bohr, today are real experiments. They were the Gedanken experiment, now there are the Gemachten experiments, experiments that every college can do, and here is one of them. Says Einstein to Bohr, you believe in quantum mechanics. I think that this is all nonsense, and I want to show it to you. Here is a source of light, and here you have two detectors, and now I place a half-silver mirror in the middle. So what happens if you shed light on this half-silver mirror? we are still in classical physics. You see a ray of light straight coming from the source to the half-silvered mirror. What happens? If it was a transparent glass, all light would go through it, would be transmitted. If it is a solid mirror, then all light will be reflected. If it is a half-silvered mirror, then light will be cut into two halves. One half is transmitted, the other is protected. That's fine. In classical physics, you don't think about single photons. Even if there are a million photons, one half goes this way and the other is being reflected. Nothing strange about it. Just to make sure that we understand it, yes. So imagine that it was a quarter silver mirror. Now the question is, what do you do when you go to quantum mechanics? Quantum mechanics tells you that a single photon cannot be split. So indeed, if you send a single photon to a half-circle mirror, it is either completely transmitted or completely reflected. It is never split. However, because there is no half-photon, now comes Max Born and says, you change intensities with probabilities. Indeed, you will never get half-photon, but repeat the experiment many, many times, and you will see that in 50% of the cases, the electron... the photon is transmitted and in the other 50% of the case the electron is reflected. Just to see that we understand it, suppose that the mirror is not half silver but quarter silver mirror. So then the statistics will change. You will have 75% being transmitted, 25% being reflected. So the idea of the Born rule is that you exchange intensities by probabilities and then everything works. That's fine. Now, says Einstein, Bohr, here's a philosophical question. You know, every time Einstein is asking a philosophical question, you should feel some anxiety because he's going to wreak havoc on physics with his silly philosophical questions. So here it is. He says, Bohr, what about the brief interval of time between the moment that the photon has hit the half-silver mirror and the time in which one of the two detectors clicked? Where is the photon at that time? Don't answer me. I'll answer and we shall not argue because they were always arguing. It is either on the right or on the left. It has been either transmitted or reflected. You can make the time longer so you have time to think about it. But it has an objective place. We just don't know it. So this is why we invented these silly statistics of 50 percent and 50 percent. OK, it is just like when I'm tossing a coin and tossing a coin. I don't know whether it is heads or tails. So I say it. 50-50%, but in reality it's not 50-50%. I raise my I lift my hand and I see that it's heads. It has been heads all the time, or tails. And here a bow says forget it. And this is another notion that we have to understand. The photon is in a superposition. It's in two places. It has both been transmitted and reflected. And it goes in this way, in superposition, into two opposite places very fast. very far from one another. Once it comes to the detectors, then a measurement occurs. So you see this dotted line means that it went both ways in some undecided way. Then there has been a measurement, so this is still a superposition. Then there is a measurement, and then there is a collapse. You can't get two photons by the conservation laws. So what you get is that one of them clicks, that means the photon went only this way and not the other way. So this is what people call collapse. And of course, in the other half of the cases, again, you have superposition. This is what Bohr, how Bohr explains the situation. It was a superposition, and then the left side detector clicks, which means that there was a collapse, normal superposition, it's only on this place. But the main thing is that before there has been a click, the photon has been in some genuine and objective way, it has been superposed. It was in two places which nobody can, which classical logic opposes very strongly. And of course, Ai-chan hated it. He said it can't be. Now here's an interesting historical anecdote. You know, as Einstein and Bohr grew older, they liked each other immensely, but they were fighting all the time. So every once in a while, Einstein felt that he can get into the mind of Bohr and think and anticipate his next move in order to anticipate him. These were two old gentlemen trying to anticipate one another and nobody understood what they were saying. So this was a case in which Einstein himself said to Bohr, here is what you are going to say. Here is your next argument. rather than placing two detectors, place two mirrors on the two sides, and then place another half-silver mirror, and then place two detectors. Don't worry, I'm going to explain it. You have two half-silver mirrors, you have two solid mirrors, and then you have two detectors, and they are arranged in such a nice diamond shape. What is going to happen now? Forget about quantum mechanics, let me say something in classical physics. this is something which is known. This is the Mach-Zehnder interferometer. Okay, as you can see, two half-circuit mirrors, two solid mirrors, two detectors. Now I'm talking about classical light, I have no problem of saying that the ray is being split into two. If you have millions of photons, so if it is half-circuit, one half-million will go this way and half-million goes this way. So now I'm thinking about, I'm turning the other, the upper half-circuit mirror, this is a technical issue, I can explain if somebody... and you can find it also in Wikipedia. But let's run the experiment. I'm sending a wave. Can you see that the wave is being split into two halves? You can also see, and this is wave theory in classical mechanics or in optics, that when a wave is reflected its phase is being flipped. So now they have opposite faces. They meet the two halves, the two solid mirrors, so they are both reflected. Both their faces are now flipped. Now they meet the second half silver mirror. This half is now being split into two quarters and this half is being split into two quarters. It happens at the same time and just separated it. Now something interesting is happening. The two quarters that went to the left are opposite. They will cancel one another. Remember destructive interference. The two quarters that went to the right now strengthen one another. So this is constructive interference. So you have two quarters, two quarters, but rather than giving you half and half, they give you zero and one. Just by conservation laws, you begin with one photon and you end with one photon. So only one detector clicks. This is detector C. And here is the surprise. So let me just make sure that we understand it. Each ray of light that comes to this interferometer, Mach-Zehnder interferometer, from the left, then is being split and always continues to the right. It just keeps its direction. And if it came from the right, it is being split and continues to the left. Okay, right, left, right, left. This interferometer makes sure that the light emerges from it at the same direction. Everything is fine. But now, says Einstein, what happens if you run quantum? You send a single photon every once in a while. Remember that now you have probabilities rather than intensities. It turns out that 100% of the photons, all of them, go to the right detector, 0 goes to the left. You understand what it says? In some crazy way, each photon took both paths of the interparameter. Can't be, because this is quantum mechanics. Let me show you. And of course, Bohr is happy, but Einstein is unhappy. And left. and you can understand why. If you place two detectors... Sorry, my parrot does not like quantum mechanics. If you place two detectors, then you will never get half a photon here or there. But you get either one click on the left. So it was one on the left and zero on the right or the other way around. Right? Quantum mechanics. You can't have half a photon. But if you remove them, lo and behold, you get interference, which means that actually each photon went both ways. Can you see the contradiction? It drives you nuts. and he drove Einstein nuts. This is interference. Confused? Don't worry, it will soon become much worse because a guy named a prince, a French prince named Prince Louis Victor de Broglie would come and show that and say the following. He was trying to solve a problem about the orbits of electrons within the atom and he said when Einstein was young He showed that light that we all thought to be waves is particles. I think I can solve some problem about the orbits of the electrons if I show that an electron that we all think is a particle is actually a wave. And it turned out that he was correct and you can get interference with electrons. So imagine this. You send single electrons. There is no half electron, right? But you send single electrons through two slits or through this mass-Zehnder interferometer, it turns out that there was interference. The electron acted like a wave. it went both ways, cancelled itself, strengthened itself, just like a wave. So it's true for electrons, neutrons, atoms, molecules, who knows what else. So this schizophrenia of wave and particle holds for both matter and energy. This is quantum mechanics for you now, okay? You understand the contradiction? It's mind-boggling, but it's also beautiful. Luis, do we understand everything so far? You want to say something so far? Make a comment? No, perfectly. No, perfectly. Everything's perfect. We can wait on the questions on the discussion until after. If it's okay, it is perfect. Very good. Very good. So this is the Mach-Zehnder interferometer. So the idea is simple. Mach-Zehnder, he was the son of Mach with another guy, Zender. They devised this interferometer in classical physics. They were not thinking about quantum mechanics. And it works beautifully. But it turns out that when you... send single photons there, then something is very strange. If you place, let me show it again, just to show the paradox. If you place two detectors, you will never get half a photon here and half there. It's just the basic maxim of quantum mechanics that there is no half a photon. This is what Planck has discovered. But if you remove them and don't ask which side the photon was, philosophically, I would say that if you don't know where the photon is, then the photon itself doesn't know where it is. I know that sounds nuts, but it's on both sides. So this is quantum mechanics. And Einstein was really troubled about it. And he kept asking Bohr, how can you explain that? And then Bohr said, especially if you understand it, it goes also for all particles. So you have this schizophrenia that every particle that actually a wave, you can think about a single electron or single neutron going like waves in all directions over millions of kilometers. But at the end, it becomes again a particle. How can you explain that? And let me mention my dear mentor, Yakir Aharonov, who showed that actually you can take advantage of the interference of electrons, and this is how he has discovered the Aharonov-Bohm effect. I hope if my friend Jose Luis wishes, we can bring Aharonov to give a talk by himself to you. It will be a great experience. We just recently celebrated his 91st birthday, and I think it would be nice to have him. So just Let me say how we can understand this. We should not understand it, but we should understand the principle underlying this crazy thing that we cannot understand. And this is Heisenberg's uncertainty principle. And uncertainty principle says the following. You remember that I began with a cannonball. The cannonball at every moment, at every second, split second, has a definite position when you launch it. It is on its... trajectory at every moment and this is why unfortunately people keep killing one another with such devices, missiles and so on. Along its trajectory at every moment it has a definite position and definite momentum. Momentum the kind of velocity times the mass. Both of them, you can predict them with certainty at every moment. Now comes Heisenberg for which he will be later awarded the Nobel Prize and he says not anymore. You can know You can measure and know and get accurate information about the momentum of the particle, but then its position remains unknown. remember if its position remains unknown that means that the particle itself objectively does not have a precise position it's actually a wave you can see the particle is in some strange way in all these places like a wave you say no i don't like it i want to know precisely where the particle is say quantum mechanics no problem here exactly where it is but it has many momenta its momentum is is is superposed it's either going this side or this side or this side it has all those momentum at the same time, this is the price that you have to pay for having a precise position. Or you can give up the precise momentum, the precise position, and you will get again your momentum. But you can't have them both. Now, yeah, this is what people say that you should write on Heisenberg's tomb. Here is the highest probability that Heisenberg is buried. Sorry for the movie joke. So here is that. Let's now return in the light of this principle, basic principle of quantum mechanics, to try to understand what's going on within our Mach-Zehnder interferometer. I'll go back. When you place two detectors, now you know the position of the particle. Whether it was on the right and on the left, fine, but you played the price. You killed the particle. It has been absorbed. by your detectors, by one of your detectors, and you will never know its momentum, right? If you remove your detectors, then you'll remain ignorant about its position, but now you can measure its momentum. So here's how Heisenberg's principle works. Refrain from any measurement of the position and you get the momentum, or give up the momentum and you can make a position measurement. You can have them both. Said Einstein, ready? Here I have another idea. How about placing two detectors in such a way that they will not absorb the photon, they'll just get a slight kick from it and let it continue. If these are electrons or neutrons, there is no problem doing that. Now go to Heisenberg and tell him that he is wrong and he should return his Nobel Prize or whatever. I can tell you the position and the momentum of the particle. I'll wait for one of the lower detectors to click. I know its position. Then I wait for the upper detector on the right to click and give interference. I have momentum. I know both position and momentum. Say, well, ready? Let's calculate it. And here is something very nice. In half of the cases, the right-hand detector is going to click, which means that the photon has taken the right path, which means it did not take the left path. It went only on one trajectory, which clicked. That means that when it comes to the second beam splitter, it doesn't meet itself. So now it is free to go to both sides, 50-50, rather than 100% and 0%. You destroy the interference. And in the other case, the left-hand detector is going to click, which means that it went on the left. And again, you destroy the interference. You have knowledge about, you have information about its position. Now its momentum becomes blurred and you have destroyed the interference and you get 50-50. Only if you remove the O2 detectors once again, you're going to have this clean picture of interference, 100 and 0. This is Heisenberg's principle, which Einstein tried to disprove and he failed. It turns out it is working. So think. a bit about this, even when the two detectors are very delicate, so that they make a position measurement and tell you whether it went on the right and on the left, this enough suffices to destroy the interference and momentum now becomes blurred 50-50. Just if you remove them and you remain oblivious, you will remain ignorant about the position, then again you will get this nice picture of interference. Do you understand it? Of course you don't. If you say that you understand it, you don't understand. If you say I don't understand it, now you're going to understand quantum mechanics. There is something very annoying here, but as you will see also very beautiful. This is the uncertainty principle that tells you that when you come to the quantum mechanical world, then realm, then you can't ask nature more than one question and you have to pay for it with the other question. And with this, and this was a Louis request, I'm coming to the bomb testing experiment. Here is how I looked many years ago. before I turn gray and this is my dear friend Weidman. So let me explain to you what led us to the idea of interaction-free measurement. Let me remind you that if there are no detectors, then always if there are no detectors in the middle, then always you're going to get a click. on the right hand detector. However, if you place two detectors in the middle, then either of them is going to click, but you lose the interference. You're going to get again 50-50. And here came the question, what happens if we... That was the question I asked myself one day, what happens if you give up one detector? Nothing genius, nothing strange, just make the experiment slightly more simple. That's it. I remember it was Thursday. and I was at the beginning of my PhD and I said, come on, no way that nobody can thought about it. Now, after so many years, you know, all these people, Bohr, Einstein, Feynman, Wheeler, all of them who played with this, I can't believe that none of them thought about the possibility of just omitting one detector. I'm saying this to the young students among you, if sometimes you encounter the thought, oh, I'm sure that somebody thought about it once. Don't be so sure. So here's what I was asking myself. I have only one detector. What is going to happen? In 50% of the cases, this single detector is going to click, which means that I know where the photon was. It took the right path, it did not take the left path, and I have to pay for it. I'm going to get 50-50% in the upper two detectors because interference was destroyed. That's fine. But in the other half, 50% of the cases, no click is going to appear. What do you know now? The detector is silent. What do you know? You know that it has taken left. Remember, as crazy as quantum mechanics is, it does not, it obeys conservation laws. So it must be somewhere. So if it's not on the right, it must be on the left. So you know that it's on the left. Now here is the question. Would you pay? the prize for this knowledge by destroying interference. Would interference be destroyed too? Let me give you a clue. If it has been not destroyed, I would get an invitation to stop. When you get the Nobel Prize for discovering the uncertainty principle, if you find a way to disprove the uncertainty principle, it's another Nobel Prize. Was I going to get it? As you probably know, I never received a Nobel Prize, and I was not expecting it this Thursday to get it, but I thought about the other possibility, which is almost as nice. Perhaps, even in this case, when there was no click, interference is destroyed, just because you know there was no interaction, single detector, there was no interaction, but you know it is taken on the left. But this is really strange. Think about it. Suppose, just to show you how strange it is, because actually something happened, the left side detector up there clicked. Just because there was no click in the lower right side detector, suppose that the single detector's battery is off. Would interference vanish? You understand that there will be no click here. Okay, the battery is off. No, there will be no click. and interference will not vanish. You will have 100% on the right and 0% on the left. Now suppose that you return the battery and you get no click. This time there will be a difference. So do you understand the problem? You have two silences. In both cases you had silence. There was a case in which you had silence because there was no battery. I know that you think that this guy is nuts. What is he talking about? But this is science. I showed you two cases. one in which there is no battery in the detector, so it can't click and it doesn't click. Interference remains intact 100% on the right, 0% on the left when the photon emerges from this interferometer. Now I'll show you another case in which the detector does not click, but it could have clicked because there is a battery. That's enough to destroy the interference when it goes out. So we have to distinguish between two kinds of nothing. right? There was no click. So the one kind of no click we should call nothing, this is the classical one, but we have to invent another word for the other kind of no click which is quantum mechanical. I called it in Yiddish gurnisht. I think that in a Lewis native tongue it would be nada. So can we say or can we now use nothing rather than nada? This is interaction-free measurement. that you have nothing, but you have nada, or gornisht, or in whatever language, another word for nothing, but that nothing is something very classical, it's very physical. And that brings us to the question of the bombs. Lev Weidman and myself decided to make it very dramatic. And we said, suppose I have, I began with the fact that human beings love physics because it helps them kill one another. So suppose that I am becoming filthy rich because I invented the kind of bomb which is super sensitive. Suffice for a single photon to go next to it to make it explode. So if somebody smokes a cigarette somewhere, it kills it. It kills this guy. And I'm selling these bombs to the army. Of course, I have to be very careful. They have to be neutralized and be in complete dark and then be placed in the dark to perform. Now, suppose that something went wrong in my factory and my worker tells me, boss, some of the bombs were damaged. They are no good. They became wet, rusty, and they will not explode. I say, how can you, which of them? He says, do you want to know which of them is good and which is not? No problem. Give me your torch, your laser. This one is no good. This one is no good. Boom. this one of course was good and so on and he tells me which of them is good and which is not. You understand that I'm not happy. What are you doing? You are destroying the remaining ones which which could still operate. Don't do it. That guy will say what do you want? You want to know which one is good or not. But the structure of the bomb, the mechanism of the bomb is such that the slightest interaction known by physics will destroy it. So what is lighter than a single photon? You can tell whether the bomb is good or not just by detonating it. And of course, I'm not happy. I want to sell the good bombs. Sorry for the morbid question. So here came Lev and myself and said it will be just like the single detector. In 50% of the cases, the bomb, if it's good, suppose that they're all good, will explode. Then we'll say it was good. But in the other 50% of the cases, if it's good, it will not explode. But then it will be just like the nada or the gornish or the kind of nothing that I explained earlier, and interference will be destroyed, which is one half of one half. Do you understand what we have here? For the first time, quantum mechanics tells you that you can do something that classical physics, even basic logic tells you that you can't do. You have the most sensitive bomb that physics can think about, and you want to know whether it's good or not without making it explode. Here is how I showed you that if you have a good bomb in one quarter of the cases, you can be sure by quantum mechanics. Just by the photon emerging to the left rather than to the right, indicating that interference is destroyed, it means that the bomb is such that had the photon go on the right side, it will explode, but it did not go on the right side, so interference is destroyed and it goes in the other way. If the bomb is not good, you will always get the photon going just like in it. in the case of interference, to the right-hand side 100%. So I showed you how I can save one quarter of the bombs. What about the other quarter? You just send another photon and hide. Once again, one half of the remaining quarter will be destroyed, the other half, one quarter would go to the other. So you just repeat the experiment. which means that now you have one over 16, then one over 64. They all converge to one-third. You understand? One-third of the bombs, if you do this experiment, you can be sure that they are good. Something that classical physics does not allow you to do. You can be sure that they are good and send them to the army to get rich. Then came Weidmann and said, why not change the first mere transmission rather than being half silver, make it quarter silver, so we raise it to one half. And then came Anton Zeilinger, who later got the Nobel Prize, and he was the first to perform this experiment, and with adding something called the Zeno effect, he brought it very close to 100. So here is the story of the bomb testing experiment. I apologize for the morbid example of taking a bomb, but it just makes it more dramatic, and it tells you something very interesting, that in quantum mechanics, an event which could have happened even if it did not happen, just by the possibility that it could have happened, this is enough to leave physical traces. That is a click in the wrong detector. We sent this paper to Nature, Science, Physical Review, Physical Review Letters, they all rejected it and later it was published in another journal, Foundations of Physics, but at that time Seilinger already performed the experiment so people understood that it's working, so it has been accepted. This is Anton Zeilinger who two years ago got the Nobel Prize. This is Roger Penrose who wrote a very beautiful book on other issues in physics and philosophy shadows of the mind and he gave a very nice description of our work sir roger also received the nobel prize and we are happy for him and now what i want to do this has been the interaction free measurement and the physics behind it underlying it let me say that it has found numerous uses in physics, in computation, in imaging. Yeah, I'm proud about this imaging issue because Leo Cohen and myself, together with our friend, Ibrahim Karimi, have shown that there is a way to make imaging using x-rays, but with much smaller exposure of x-rays of the body and still get very good imaging using interaction-free measurement. People use it now in cryptography, in communication, in quantum computation. The interaction-free measurement of Lev Weidman and myself found numerous applications and ramifications, which I confess I don't know all of them, and I'm very happy, and I'm sure that it is far from being exhausted. Now, we ourselves, especially Eliyahu Cohen and myself, kept asking, kept working on this issue, trying to bring it, go even deeper. Let me again go to Yiddish. I'm sure that it exists also in other languages. In Yiddish we say if grandmother had wheels she would be a wagon, which is a silly thing. Whenever you say if I was rich I would buy this, if I could do this I would do that, but then people say if my grandmother had wheels she would be a wagon. You're not rich, you can't do that. So this is how people dismiss if statements, if I were, if I could. What I want to say is that interaction-free measurement tells you something very profound about the quantum world and that is that if your grandmother is quantum mechanical and she could have wheels, then even if she doesn't have wheels, but just by virtue of the fact that she could have wheels, she can sometimes outrace a police car. Do you understand the idea? That counterfactuals, it turns out that in quantum mechanics counterfactuals are very important. An event which could have happened, that's enough. Then even if it did not happen, think about the explosion of the bomb. leaves a physical place. This is something that you don't have in classical physics and you want to explain it. Let me say, yeah, I just mentioned that there are many applications of quantum mechanics to biology and so on. Perhaps we will do it in another talk, also to the question of consciousness. Who said, yes we can? I am now asking about the uncertainty principle. You understand that the interaction-free measurement is based on the uncertainty principle. Can you outsmart the uncertainty principle? Who said yes we can? This was Obama. And who did it? So this is Yakir Aharonov, who was my PhD advisor many years ago, and he received the White House Medal for his achievements in quantum mechanics. And I want, in the remainder of this talk, I want to say something about Aharonov's contribution to the understanding of quantum mechanics. We understand that quantum mechanics is strange. I began with the idea of the cannonball in classical physics, that if you know how it began, then you can compute all its trajectories. It's not the case in quantum mechanics. If you have a particle, you know how it has been emitted, but now you get a wave of probabilities and you don't know which place it takes. And this is strange. Aaronov said, measure the final position of the particle, which you didn't know from the beginning. because of the uncertainty principle, but now you do know it. And then compute it backwards. So you have two wave functions, one going from past to future, one going backwards from the future to the past. You're making two measurements. And it turns out that during the time between these two measurements, you know about the particle more than what the uncertainty principle allows you to do that. This is not a violation of the uncertainty principle because it's not in real time. But it tells you something very interesting. Between two measurements, actually, this kind of calculation, this kind of double computation, gives you much more information about the particle, and this information takes you to very interesting places. In order to explain what is the importance of the new idea, it is called the two-state vector formalism, or TSVF. This was the idea of Yakir Aronov, and it turns out, he says, that you can explain many oddities of quantum mechanics. just by making the following sacrifice. You know, in every scientific revolution, you have to make a sacrifice. We sacrificed absolute space and absolute time in relativity, we sacrificed certainty in quantum mechanics. Now the sacrifice is not that great. Just admit that the error of time not like in classical physics, it doesn't always go to the future from the past. At the quantum level, sometimes it can go both ways. So God plays dice. He does against what Einstein thought. You don't know where the dice is going to fall. But once it fails, make a calculation from the positions and the place on which the dice has fallen back to God's finger. So now within the two boundary conditions, it's interesting. Mathematically, you have greater knowledge about what happened in the twins, even more than what the uncertainty principle allows you. You're not violating the uncertainty principle because it's not in real time, but there is some new physics here, which I want to show. So let me say the following. We know classical physics. In classical physics, if you know where the cannonball began, you know where it's going to end, which means that if you shoot it backwards with another cannon, it will go exactly to where it began. So... Determinism and time symmetry go together in classical physics. Things are radically different in quantum mechanics. You don't know where it is going to end. Remember this wave function, so you send a particle and you don't know where it's going to fall, you have probabilities. Now that it fell in one of the places, when one of the detectors clicked, if you compute it backwards, you don't always get the lamp, the source. Sometimes it goes you to silly places where it could not have been emitted. Now something interesting is going to happen from this absurdity. So I am arguing with all modesty that there are three stages. There is classical physics, there is quantum mechanics, and there is TSVF. TSVF, which adds another layer to quantum mechanics, which is no less amazing. And what I want to talk about is an old idea of Yakir, again, which is pure mathematics. you split a particle between three boxes. You remember, you can split a photon to be in two places. So in principle, it can be in three places, four places. So you take a particle and make, just split it as you split a photon with some kind of a mirror, such that it has a probability to be either in box one or box two or box three. And then you reunite them together. If you get... an unusual result. You don't know exactly when the particles come together, what will be their value. If you get a rare pair of initial and final conditions, something very interesting is happening in the middle. And here it goes. There is certainty that the particle has been at box one. There is certainty that the particle has been at box two. Something unprecedented in quantum mechanics. In quantum mechanics, you have only probabilities. But under this kind of calculation of two times two vectors, you have something strange. The particle must have been in box one, not with probability. It must have been there. It must have been in box two. But then it must have been in a negative way in box three. What does it mean to be in a negative way? It's very profound. It's not that it has not been there. It has been there, but in a negative way. Mathematics, simple algebra gives you the minus sign. Calculation is very simple. What do you do with it? What do you do with this minus sign? Aaronov said, I'm not going to assign it to the charge. If this is an electron, it will remain an electron. I am going to assign it to the mass, something about which physics doesn't know. I would compare this achievement to what Dirac did some decades ago, when, for him, mathematics revealed the case in which an electron may have a positive charge, which is a compo. has been a complete absurdity at that time. People advised Dirac, he was very young, don't publish this paper, it's ridiculous, there is no such a thing as a positive charge electron. And he published it. Five years later, Anderson has discovered the positron. I think that this advance of the TSVF is just as far-reaching. Mathematics tells you that there is, in some way, a negative mass in one of the boxes. Just by conservation laws, if there was a certainty that it was in box two and certainty that it was in box one and certainly that it was in box two, then in order to give you one the certainty must be negative at box three. This is what mathematics is telling you. The question is can you bring some physics from it, just as Dirac did. Aronof said trust the mathematics and now can we have a rigorous proof for that. I'm going to make Ask a question that will sound to you ridiculous, even dumb, but follow me, because I promise you a very exciting physics which has been revealed. just recently by experiments. I'm going to begin with the very foundations. You remember a single particle going through a beam splitter to two detectors, and this is a half-silver mirror, say a photon. So in half of the cases, the right-hand detector is going to click, which means that nothing came to the left-hand detector. I would say that we had here, before there was a click, we had two futures, one going... This is the future and this is the future, one going to the right and one going to the left. Once there was a click, one of the futures becomes real. What do we do with the other future? Remember, there was a guy in the White House who enriched our world with a new notion of fake news. I'm going to call it fake future. Fine, right? Every time that in quantum mechanics you have two outcomes or even more. So these are different futures. One of them materializes. Then the other, you call it fake future. Now, if you use TSVF, you have to run it backwards. And then you ask, how should I compute it backwards? Or even do the experiment. Just take the particle which has been absorbed, if you can rescue it, and emit it backwards. But do it even just mathematically. You get something very strange. You get 50% in the probabilities that it will go back to the source which has emitted the photon. But in the other 50% of the cases, it's going to another place, to the wall, to a refrigerator. which is a complete absurdity. But mathematics tells you that when you compute probabilities not only forward but backwards, we have been accustomed now more than 100 years that we get all kinds of fake futures, that is things that did not materialize. Do you remember the bomb, the bomb testing experiment? The bomb did not explode, so this is a fake future. We knew that for 100 years. Now it turns out that if you take the two-state vector formalism of Aharonov seriously you get fake past which is a complete nonsense thank you very much for bearing up with me so far because here is a statement that i'm going to make and show you that it works here it goes at every quantum experiment when you have which is a bit complex you have various fake futures and various fake paths if there is a place where these two fakes overlap so i'll make the fake future blue it's very fast red and the overlap in purple. Every time that the overlap fits, tights to your, fasten your silk belts, you're going to get a real particle. Here is an experiment and this is due to Lev Weidmann. What you see here in white, I hope that, can you see Luis the white trajectories on the slide? Yes, they're visible. Very good, they are visible. So here is an interferometer. You remember the interferometer that I showed you earlier, very simple. two half-silver mirrors, two solid mirrors, and then two detectors. So let's make it a bit more complex. A small interferometer within the large interferometer. With somewhat slightly different mirrors, which are not half silver, but one third silver and so on. So what do you have? Here is the possible trajectory of your photon. In two thirds of the cases, it will go this side. In one third of the cases, it will go this side. If it goes on the right side, there is a smaller interferometer into which it will go, either go to this detector, But if it goes on the left, it will go either to detector 2 or detector 3. So you have three detectors. It looks complicated and boring, right? Prepare yourself for a surprise. Here is the evolution. Suppose that you made the experiment and detector D did not click. That means that the photon never went on the right side. Neither did detector D three click, so it didn't end up there. Detector D two clicked. Here is the evolution. This is your fake future. It's very simple. The photon never went this way, never went into the smaller nested in performater, never ended up in D1. It went, it's a solid line, only on the left, came to the second beam splitter and went to D2, it never went on D3. Fine, simple, you get it in one third of the cases. Now the surprise, run it backwards. This is fake future, this is fake future, now run it backwards. In red, you get this is the past. We know that this is the only path it could have taken. But in this kind of calculation, you get a fake past. It goes to a place where it has never been, right? Absolute nonsense. Here's what I promised you. Place the two histories one over the other with their nonsense. These fake futures, these fake pasts, and you are going to get from the blue and the red, you're going to get some purple. Do you see that? Here you have a real particle. you are sure that the particle has taken only the left-hand path. It never went into the nested interferometer. Never went there! And still, mathematics tells you that there has been a particle there. For a very brief time, out of nothingness, there was a particle, just because of this overlap of the two histories. I know that it sounds very crazy and you need to make some experiments. Here we collaborated with two Japanese guys, Ryo Okamoto and Shigeki Takahuchi, me and Eliyahu Cohen. We did some of the experiments. They showed it actually, that there is a new physics here and that for a very brief time there is a particle there. Here, Scientific American celebrated it by proclaiming quantum physics may be even spookier than you think. And this is what I think is happening in interaction-free measurement. What happens with this bomb that did not explode? Rather than going into philosophy and saying that quantum mechanics is about our knowledge, or going to a hidden many-words interpretation, or invoking all kinds of hidden variables that you cannot think about, actually what the TSVF, the two-state vector formalism, is showing is that there is a very subtle physics going on there, very likely some mass of the photon. has been exchanged between the source and the bomb. But then there has been a negative exchange of the photon, so nothing, apparently nothing went there. What was the genius of Lev Weidman was to show that in one of the nothings you can split it into a particle and a minus particle. It's not an anti-particle, it's something more radical. It's a minus particle with a minus mass, so they don't annihilate, they give rise to an absolute nothing, but sometimes you can show that beneath this nothing there's been something going on. As I say, there are various implications of interaction-free measurement. There is reason to believe that even in quantum biology, organisms are taking advantage of quantum mechanics and even of IFM. Is it, as Roger Pembroke believes, also happening in our brain, explaining consciousness, creativity, free will? Who knows? This is what I think is what the bomb after 30 years leads us to. And it's... who knows, you know, the sky is the limit, what else is still waiting for us. So I would say that God plays dice, but this is an act of mercy of God by insisting against Einstein that he should play dice with the universe in order to endow us with surprises, with creativity, with novelty in this universe. Here are some papers of Eliyahu Cohen and myself and of Ronan and myself. I feel very privileged of being together with my mentor, who, as I said, is now 91. And as we say in Hebrew, may he live long with us and keep enriching us. And I have the privilege of working with younger people whom I initiated into physics. And I wish to thank you very much, Luis, for hosting me in this meeting. And I will be very happy to take questions. Professor, thank you so much for that wonderful, wonderful presentation. Let me only add that I will send you the presentation itself to make it available to everybody who wants it. It's where it's copy left on this presentation, so everybody can use it. So I'll send it to you, make it available to all your to all your audience. OK, fantastic. I appreciate that. I think there were some moments where maybe the. The video that showed your face covered up some part of the presentation, so that's a great idea. Perfect idea. Okay, excellent. Well, I began the initial introduction by talking about nested dolls, nested Russian dolls, and how, in some sense, what you're working on, and perhaps quantum physics as a whole, pertains to the innermost doll. In some sense, you've taken that innermost doll and you're going even deeper. with a new kind of physics. I want to come back to that concept of a nested doll a little bit later, but I'd like you to elaborate, please, and make clear, maybe not make clear, it's already clear, but emphasize the interesting claim that physics or the history of physics has gone through three historical developments. classical physics, quantum physics, and now the two-state vector formalism. Can you elaborate on this and where your... interaction-free measurement fits into that picture? Yes. I think to take a visual pictorial example, let's take the cannonball. So this is classical physics and it's easy. You know, everybody can do that. You know how it starts when you launch a cannonball, you know the angle, you know the velocity. And then given that, you can now calculate where it's going to be next second, two seconds, three seconds, you get the whole trajectory. This is how Newton some 200 years ago was capable of calculating the orbits of planets very easily. Just take a certain point, the location, precise location, astronomical location, and velocity, and then you get the whole ellipse or circle or whatever. So it's not... different from the ellipse for the parabol or hyperbol that the other trajectory is. Then came quantum mechanics and said forget it, you don't have this determinism anymore. When you emit a particle, so it's not a cannonball, you shoot a particle from the source, what you get is a wave and the wave is spreading in all directions but still it's a single particle so at the end you're going to get a single particle but you can prove. with the double slit experiment with all kinds of interference experiments that even a single particle gives rise to interference it has been through this slit it has been through that slit seven slits 700 slits it doesn't matter it's still although you get a single particle at the end the place where it is going to be depends on the positions of the slits which of them is open which of them is closed which of them is wide which of them is narrow and so on so It looks like every particle is making this amazing calculation, but it has to be in all these places. So you lose one thing. You lose certainty, which helps you so much in classical physics. But you gain with this interference. You can make transistors. You can make technology and so on and so on. Then came the two-state vector formalism due to Haronov and said the following. It is frustrating. that you don't know when you have the initial condition, you don't know what the final condition will be. Unlike the cannonball. Cannonball, you know the initial condition, you know the final condition, you can derive them from one another, run it forward, backwards, it's redundant, they complete one another. It's not so in the particle. You have the initial condition, you emitted it, but you don't know where it is going to be detected. However, once it is detected, people continue and say, well, we didn't know. No, now you do know. go backwards because you have two points in history, the beginning and the end, and make the two computations. People would think that it is redundant. In classical physics, if you make calculations twice, then they complement one another, but they don't add anything to one another. What is beautiful is that because quantum physics tells you that you don't know everything from the beginning, then you don't know everything from the end, you run it backwards and in the middle you get a clear history. So that was a nice achievement. In other words, you can know both the position and momentum of the particle at the same time, even though Heisenberg and the uncertainty principle says that you cannot, you can, but it's only in the past, you can't, you don't know it in real time. Then later came these amazing results, purely mathematical. that say that under special conditions this double calculation gives you exotic physical variables too large mass too small mass even negative mass strange numbers what do you do with it you can dismiss it as people often do when mathematics is is strange but you can take it seriously i said again you know when our runoff was 23 then the haronov bomb effect this quantum potential was just a mathematical creature. Nobody took it seriously, but he insisted that if it is there in the equations, it must be there also in nature. And this is how he discovered the effect. I think that here we have such a breakthrough. If it turns out that between two measurements, there are very, very brief moments in which very exotic physics is happening, then first of all, we have to validate it in experiment. And I believe I've shown it. shown one experiment, there are a few more that actually validate it. And perhaps Aronov doesn't completely agree with me, but I think that what he is doing is a completely new physics, which is still within quantum mechanics, but it's new. It's not, it doesn't take you to all kinds of metaphysical ideas like many worlds or guide waves and things like that. It shows you that for a brief time, there is a very exotic physics going on. Why I like it? Well, now that quantum mechanics is incomplete. It's strange. I mean, you say, is the electron a particle or a wave? And the usual answer is shut up and calculate. look don't ask these questions this is philosophy go to philosophy in physics we have learned that we shouldn't ask such questions shut up and calculate it works it gives you technology it brings you money it gives you grants whatever that's it don't ask too much questions and this is not a good way to teach students students are asking questions young people are asking questions children uh asking questions when i teach children physics and the questions are good. What we should say is that we don't know. There is some new physics here. It's not the end, by no means. Quantum mechanics is incomplete. We don't know how to complete it, but we begin to do that by experiments which I showed, which tell you that mass can be sometimes negative, very briefly, that time runs backwards at the quantum world. Let me give you one example. Think about the EPR experiment. What do you have in the APR? You have two particles coming from the same source, going very far away, and then Alice and Bob are measuring them, and with Bell's theorem you prove that whatever Alice did to her particle affected Bob's distant particle. We know that. But it's strange. They are very far away. It is not so strange if you believe that the act of measurement goes backwards, because they have a common origin in the past. The two particles have been emitted by the same atom. So Alice and Bob may be measuring them on Monday, but at Sunday they came from the same atom. How about some zigzag in four dimensions in Einstein's four-dimensional world, space and time? You have a zigzag going backwards to the past, just like in science fiction movies. How come that something here is here? If you go out of space and time, and probably this is a story, we have more than... three spatial dimensions you should also take time and perhaps there is even more so it looks like questions of non-locality are better explained by by this time symmetric formalism and as i said a counterfactual event uh an event that could happen but did not happen but just because it could happen it leaves a trace as in the bond non-explosion case this i think can be best explained by this new approach. So this is why I think there is no physics here. I believe that there will be many future experiments in which people will show that very strange forms of matter and energy exist very briefly within the wave function. And it is these physical phenomena that give rise to the odd quantum mechanical phenomena that we see. Basic principle, perhaps basic laws of nature, who knows what is. waiting for us there. Fantastic. Well, you mentioned this attitude of shut up and calculate. That appears to be changing little by little. There are more and more well-respected physicists and philosophers who are taking up these questions, people like Sean Carroll and, of course, philosopher of science Tim Modlin, who we've had on the channel in the past. As a guest, of course, he's very popular and influential. I'm going to meet him, by the way, in two weeks in London. We are going to have a discussion about the nature of time with Michio Kaku and Jimena Canales. Excellent. I look forward to watching that, if possible. Is it going to be anything online? Do you know? Fantastic. Fantastic. Well, thank you. Tim Modlin in particular has expressed skepticism, as others have, to this interpretation of quantum mechanics, which I find surprising because one of the things that I think most people find appealing about Tim is his no-nonsense approach. He just goes directly and is very logical and very rational and reasonable. But he talks about... ontology, and he's big on ontology. If we want to talk about physics, we have to say what's there. And in this case, you're actually... Unlike the Bohmian interpretation or the many worlds or the Copenhagen, you're actually saying what's there. There's a new physics there and you're demonstrating it through experiments, through technology that's giving rise. And yet, Tim expresses concern about the ontology here. And so you talk about in Bohmian mechanics, there's in Bohmian mechanics and the many worlds interpretation. interpretation, there's an excess of ontology. And in the Copenhagen, there's insufficient ontology. Can you explain this? And let's talk about why you think Tim Modlin has problems with this new ontology that's being proposed that's empirical. One would think that Tim has, well, this is empirical. This is very interesting. But his approach is different. So let's talk about, first, can you tell us about the... the excess of ontology and the the um not enough ontology and excess ontology yes uh my colleague lev ideman i believe it's now we celebrated 30 years to the interaction free measurement paper in chapman university i believe he even confessed it that he collaborated with me i was then just a young beginner beginner a phd student beginner and I told him about the initial idea, and I believe that the reason why he collaborated with me, this is what he said, is that it fits well within his, shall I say, obsession with the many-worlds interpretation. Weizmann is a very, very strong supporter of the many-worlds interpretation. He believes that, actually he says that physics is done. There is nothing more to find out. The many-worlds interpretation has solved all these problems. Why? This is how... This is how Sean Carroll also speaks. So yeah, that's very interesting, very hard to understand. But I'm sorry, continue. Yeah, they say every time you have a problem, who told you that this is the only universe? There are many universes. In each of them, there is another Absalom, another Lewis, and with a slight difference. And we split and we split. But from the beginning of this talk a few minutes ago, now we have the universe that splits into zillions of universes. In each of them, there is another Absalom and Lewis. Now, you can't probe it. you can do anything with it you can show it with an experiment you ask them okay does it give me any physical prediction zero to me it looks like metaphysics i should mention my grandfather my scientific grandfather david bond david bond dedicated most of his life to show that every time that the particle gives rise to interference actually it's not just a particle there is a guide wave on which the particle is riding So when it goes to two slits, the wave is going both ways, but the particle is riding on one of them. So it is somehow affected by the interference. Beautiful idea. But Bohm and De Broglie, and then later Vigier and all the people who follow this had to admit, you will never find out this wave. This wave has no momentum, has no energy. Actually, they were very careful about that because if you find the particle in one place and some traces of the wave, in another place, this empty wave, you're going against the uncertainty principle. You can't do that. So just like the many worlds, the hidden variables is a theory that tells you actually that hidden variables will forever remain hidden. If you ever can see a hidden variable, you can violate relativity. I can show this to you. You can find the preferred frame of preference. So it looks like kind of religion, you know, as God says in the Bible, you can never see me. you can see only my back but no person will see me and remain alive. So this is religion. Copenhagen were even worse rather than you know invoking all kinds of metaphysics. They said don't ask this question. Every time you ask a question they solve it by deeming the question meaningless. It's terrible. I mean many students lose their enthusiasm for physics once you tell them that this question is meaningless and this question is meaningless. Actually, I don't know who said it, but I'm sure that it's true, that Bell had to wait until Bohr died in order to publish his Bell's Inequality. I mean, thinking about how to prove that something non-local is going on in the EPR experiment could not have been done as long as Bohr was a very good man and he was a genius. But his influence was so... depressing on the thinking so oppressive on the thinking of the community that people did not dare to ask certain questions uh it's a bad way to do science you know the exercise that i give i say Suppose that young Einstein was a positivist when he thought about the contradiction between Maxwell's equations and Galilean invariance when he was 26. Relativity theory would never be invented. You can just be a Copenhagenist and tell Einstein this is complementary. You can think about waves, Maxwell's equations, and you can think about Galilean invariance. These are different pictures of the world. They don't contradict. They contradict only in your mind. Forget about it. Nobody would invent relativity theory. What I think should be done is to say that for 100 years, there is a very serious problem in physics. There is a very strong contradiction between the wave and the particle, but there must be some ontology behind it. And with all due respect. with all modesty. Here you have a new ontology, something new about time, something that even Einstein didn't know. When in our world, say if I throw a rock on this window, the window will break. So there is cause, there is an effect, it goes one way. Second law of thermodynamics, entropy increase, you will never see a window breaking and then pumping a stone in it. So there is a direction. It's not so. We know that it's not so at the practical level, at the quantum level. Things are time symmetric. And it turns out that sometimes you can affect your past. There is another paper of ours. It is called the Quantum Liar Experiment by Aronov, Lee Smolin, Elia Okoye and myself. And there we show that sometimes you can in quantum mechanics, if you leave the past open. Then you come later and you can affect it backwards. Actually, I can give you an example. Think about Schrodinger's cat. You know, this old problem, you place a cat and then you open it, find it dead or alive. Has it been superposed? Many years ago, a student of mine asked me, why are you physicists making so much fuss whether the cat was superposed or not? As long as you did not open the box. Here's what you should do. You should make the experiment long. Leave the cat three days closed in the box. The little event either occurs or does not occur at the first day. Then when you open the box, you will find not only a dead or alive cat. You will find either a dead cat, which is already decomposing, or an alive cat, which is very lean and angry at you, and all scratches and remains in the box. So you have the whole history. So you know that the cat has not been superposed. It's a very good question. She said, I can prove to you that there was no superposition. If you see a live cat and it's clean and there is urine there and all whatever within the box, it tells you that it has not been superposed. It has been alive all the time. Good question. It turns out, actually, by the formalism that you have a superposition not only of states, but of histories. There are two histories within this box. A cat. either dropping dead because the event has occurred and then decomposing and so on. A cat surviving and then waiting. When you open the box one of the histories materialize which means that you are actually affecting history backwards. You're creating it just by your observation. There is another famous experiment, the delay choice experiment by Wheeler. He says that if you get a single photon coming from a very distant galaxy actually the very form of measurement that you choose to make on it determines what kind of history it has. So the revolution is that we should think about time in a new way, even far beyond what Einstein believed that we should ascribe to time. Absolutely. Very good. So in the case of Tim Modlin, his focus on ontology, one would think... that this would raise interest rather than skepticism. Have you been able to understand more why the skepticism, specifically from his point of view, or maybe from skeptics in general? What are some of the, in your eyes, the more powerful objections? I should say, you know, Aharonov for many years said he can prove all the predictions of... TSVF by making weak measurements. Weak measurement is an interesting form of measurement. Remember the problem that we have. You say that between two measurements there is a very interesting physics going on. but how can you validate it? You can't make measurements because then it will not be between measurements, it will be upon measurement. So you can't make measurements. So what do you do? Leave it to philosophy or to mathematics and say I'm sure about it? No, in physics you have to validate it. Aharonov devised the weak measurement which is a genius idea. You make a measurement which such that the coupling to the particle is very weak such that you get a lot of noise. inaccurate measurement. But if you make a large ensemble of particles and you make the measurement on all of them, then the disturbance to each of them is negligibly small and you still get a signal. I think that it's a genius idea. I think that people, I don't know whether Modlin is one of them, objected to this and said, look, extraordinary claims need extraordinary evidence. If you are suggesting such a bizarre physics going on, we have to see it's not by just you know many measurements which are inaccurate and having noise and so on later yakiya found out a way yakiya and weidman how you can validate claims of the tsbs by ordinary measurements just like our bomb testing experiment so this is a revolution now frankly i should have followed morgan i don't know and i'll ask him thank you for telling me i'm gonna meet him in two weeks I don't know why Modlin, who is a great philosopher, very sober and very brave and clear thinking, why he objects to this kind of ontology. I'll ask him and I'll show him. Perhaps he's unaware of the recent advances. Say the experiments done by Okamoto and his colleagues and Cohen was published only three months ago. So it's very new. What I'm sharing with your viewers, with you and with your viewers, is something very fresh that many professors in the university still do not know. So this is something that I'm going to show. I'm going to ask Modlin why he is weary of this. You know, there have been attempts in quantum mechanics, of revolutions which failed. There was this bold idea of Girardi, Brimini and Weber, the GRW, they tried to add something new to quantum mechanics. didn't work much so so i can understand why modeling is cautious you should be cautious i'll talk to him and you can bring him or we can we can have a forum here oh i'd love that i'd love that i'll ask him i'll ask him that would be very very interesting i think another aspect of it that sean carroll talks about is that once a person goes down a particular line of research it's often a matter of of time and effort that it takes to really learn you another system. And I think that maybe that's a factor. I'm not sure. But I was struck by the case of Modlin because he just seems so rational. And this is exactly what he's asking for. He says, if you want to propose a theory or model of physics, you have to tell the ontology. You have to reveal the ontology. And there's a fascinating new ontology being proposed here. So that's very interesting. And I'd like to emphasize for our listeners that All of this is not philosophy. It's physics. Unlike the many worlds, unlike the Bohmian, what's being proposed is physics, and it leads to actual advances in technology, so it has technological implications. So as a last thought, you've been very, very generous with your time, Professor. I don't want to abuse that. I hope this is the first of a... of a series of visits that we can have from you. We're super interested in your work, as you know from our collaboration with many other eminent scientists, which you've been generous enough to join that collaboration. There's interest in this on many, many different levels, in biology, in neuroscience. Our own interest is in managing it in political theory, but there's some political philosophers and some social choice theorists. who are very, very interested in this general framework. So this idea that there's nested dolls, the work that you're doing is so fundamentally important. It has implications for every layer of that doll up to, if this is actually the way nature works, then the implications are necessarily all the way up through cosmology. But our interest is especially in social sciences, in some of the... biggest open questions about ecology and the planet and how we're going to survive as a species. Last question, Professor, can you tell us where people can follow you or know more about your activity? I'm in Chapman University. I'm affiliated to Chapman University. So you can see some video. There have been two events, one 30 years to the interaction free measurement and now celebrating Yacir's 90th birthday, which was actually his 91st birthday because it took some time to organize it. There will be a course, I'll let you know, an online course. That course will be both popular and academic. It depends. It will be either on the on the YouTube for people who just want to have the concepts and the science and the history, but also a course academic that students will be accredited for. I will let you know. I have my own website. Eli has his website. You can look for us. As you know, most of my stuff is copy left which means that there is no copyright and everybody is welcome to to use it uh so people are most welcome to follow me uh also on um on youtube facebook whatever i'm and write to me on mail i'm i'm always available great thank you thank you very much professor you've been very generous and i should let uh listeners know that uh your generosity is is knows no bounds you've done this of course pro bono You're very open with your work and what you're contributing. And so we're very, very grateful for that. Thank you very much, Professor. All the best.