How do we know the photon is real?

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Speaker 1: Hey, they're listeners. You're into physics. So here's a tribua question for you. Do you know who won a Nobel Prize for relativity? That might feel like it's your question because you want to say Einstein, because they think relativity and Einstein will I'll tell you it's not Einstein. Now, maybe you're scrambling through your mind to think about the names of other physicists. You might know. How many physicists can you name? Anyway, you got Einstein, you got me. Well, I'll give you a clue. It's neither Einstein nor me. So who was it? Right? Well, some folks wanted for proving that relativity was correct. There were Nobel Prizes for gravitational waves and for binary pulsars. But the answer is that nobody wanted for relativity. Nobody who came up with this incredible earth shattering idea that now frames all of modern physics won the Nobel Prize for it. But you might be thinking, hold on, didn't Einstein win a Nobel Prize? And he did, but he wanted essentially for quantum mechanics. Hello everyone, I'm Daniel. I'm a particle physicist, and I'm the co host of this podcast together with Korge cham who can't be here this week, So you're just listening to me talking about the joys of physics and trying to simulate Jorge in my mind. Every time i'm talking, I'm thinking, here's what Jorge would say. At this moment, I'm trying to interject a little Jorge Ism for you, since we all miss him and you were listening to our podcast, Daniel and Jorge Explain the Universe, a production of I Heart Radio in which we zoom all around the universe and try to find interesting, fascinating, cool little nuggets of physics that would blow your mind, but take them apart so they don't actually explode your head and cause your brains displatter anywhere. Instead, we want to smoothly and calmly insert them into your mind so you understand them, so you can talk to your friends about them, so you can actually comprehend these amazing, wonderful facts that we have learned about the universe, and also understand all the things we don't know about the universe, which is my favorite part of physics, and that's why Jorge and I wrote the book We Have No Idea, A Guide to the Unknown universe, which takes you on an amazing tour of all the big and basic questions about the universe that we still have no idea what the answers are. And on the podcast, we've been doing something fun, which is taking a little tour of how we know what we know, and specifically how we know anything about particle physics. It's still incredible to me when I look around at the world that everything is made out of these tiny microscopic object so we can't see that we've taken thousands of years to even discover that they exist. Yet we have this really complex, really elaborate, really amazingly effective model of what's happening down there at the microscopic scale, all these tiny quantum particles interacting and zooming around. Physicists can do calculations to tell you exactly what's going to happen when this particle hits that particle. It's really incredibly complex and mature. Though of course we have lots of questions, but I think a lot of times people think of this as sort of like an idea, something people came up with a description of the universe. But it's critical that everybody understand that this isn't just an idea that came out of our heads. This is something born out of desperation. This is our attempt to grapple with the weird and bizarre and counterintuitive and frankly mind blowing experiments that have shattered our perceptions of reality. We thought the universe worked a certain way. We thought everything was smooth, You could cut objects as many times as you wanted to infinitely small pieces, but you can't. We thought the universe was deterministic, that if you did the same experiment twice you would get the same outcome. Right, that would make sense, But it's not. It's fundamentally random. And the core of that is particle physics, because it attempts to describe the entire universe in terms of these tiny, weird, nondeterministic little particles, in terms of these tiny, little, weird, nondeterministic particles that seem to follow rules that just do not describe the world that we are familiar with. So my goal is to take you on a tour of those experiments, the ones that change the way we think about the universe, that showed us that the universe is different from what we imagined, because it's not just the final idea that you want that I want you to understand. I want you to know what the evidence is. How do we know what we know? Now? Recently we talked about the discovery of the first particle, the first experiment that revealed this incredible revelation that the Uni verse is made out of time to little dots. And so today we are continuing that tour. We are talking about how do we know the photon is a thing? You're familiar with photons? To you, photon is a very normal word you hear bandied about here, talked about but how do we know that photons are there? How do we know that light is made out of photons that is chopped up into these little pieces that can't be cut down even further. What is the actual experiment that proves to us that photons are a thing, that light is not just electromagnetic waves, but it does these other weird things that you have to give it particle status to explain. So, as usual, I was wondering how many people out there know why we think the photon is a thing, Why we don't just think about light as electromagnetic waves. So I walked around the campus of U see Irvine and accosted a bunch of friendly and unsuspecting students, and I asked them, do you know how the photon was discovered? Do you have an idea of why we think the photon is a thing. So before you listen to these answers, think to yourself, or pause the podcast, or just take a moment. How do you know photons are a thing? Are you just believing physicists when they tell you, or do you know what the data says? I'm not entirely sure. I feel I shouldn't have that. I I'm sorry, I probably should know. But it was the slit experiments, wasn't it. And they projected a laser beam onto a single slit or double slits and it diffracted the beam and that's how they discovered it. Particle wave duality. Yeah, the photo electrical facts. You shone a light on a metal and then the metal you cross. Uh, you start for Einstein ninth. I don't remember the year. Yeah, I don't remember who did it, but I remember that you shine a light on a metal, do you give the electron enough energy to start conducting it's particles? Well, we know it's a wave because it travels through vacuum, and we know that it's a particle because you can transfer energy from it. Right, Yeah, it has it hasn't defined momentum, even though it has no mass. But the slit experiment double slip one showed that it was a wave, like a single slit showed that a particle. Well, it's not necessarily a particle. It's both a particle and a wave. And for a really long time we thought it was just a wave. But I believe the first time we figured out that it was a particle had to do um exciting metals to release photons and realized that the distributions were discreet. So I was really impressed with these answers. A lot of understanding here that photons are particles and that they're part of this larger idea of light being a wave and a particle. Even some discussion of the double slit experiment, which I'm dying to get into in a future podcast and talk all about the amazing facts of quantum mechanics. But the double slit experiment actually shows you that the photon is a wave. But there was somebody out there who talked about the photoelectric effect, and that's the key that was the experiment that showed us that photons were a thing. But before we talk about the crazy experiment to prove that quantum mechanics is our reality, that showed us that the universe is probably sliced up into little bits and not infinitely smooth. Let's set the stage. Okay, let's you remember how people thought about light, and to get the context of the story, you have to rewind all the way back to Isaac Newton. Isaac Newton, of course very famous not just for the cookies, but also for his discovery of his theory of gravity, which unified motion of objects here on Earth with motion of objects in the heavens. Really gave us access to the whole universe to imagine, Wow, maybe physics can actually describe things not just here in front of us, but out there in the universe. Those are things out there that follow laws of physics. Incredible accomplishments. But Newton also also made amazing discoveries in the field of optics. He spent a lot of time with lenses and with prisms, and he was convinced that light was a particle, and he thought a lot about how light traveled. He saw it moving in straight lines, except when he was bent by these lenses and he was convinced that light was a particle. And because he was a genius and he's a staggering influence on the field of physics, people listen to him and rightly so. And for hundreds of years people were convinced that it was a particle, even though other folks had really nice theories of light as a wave. And it wasn't until the eighteen hundreds when people started observing, like doing things that particles couldn't do, that they had to adapt their mindset. And that's the key. There you see experiment rearing its uncomfortable head again saying, oh no, no, you thought you understood the universe. You have an idea in your mind, you have a mental model of how this is working, but it can't describe what's actually happening. And that's why I'm an experimentalist. That's why I think experiment is the place to be, because experimentalists are the ones who make the discoveries. They are on the forefront of knowledge. They're out there exploring the universe, discovering things that don't make sense. Theorists, of course, do an incredible job. They tied all together they understand they predict future phenomenon. But for me, the bit about physics that's wonderful is the experimental side, is making those discoveries, is asking nature a question and demanding an answer, pinning nature in a corner so that nature has to tell you is the universe this way or that way? And so the thing that told people that photons couldn't just be a particle were wave like effects, things like interference. And you're familiar with interference, maybe you have noise canceling headphones. Now is canceling headphones work via interference? Sound is a wave. It's a shaking of air, and the air comes towards your head, and if you can create waves shake in the other direction at the same time, they basically cancel out those waves that are coming in your head. So sound canceling headphones are proof that sound is a wave because they can do this wave like thing that particles just cannot do. In the same way, people saw light behaving in a way that could only be described by a wave, and so you had interference effects, and you had all sorts of theories sort of built momentum until you get to James Clerk Maxwell his incredible genius pulled together lots of ideas about electricity and magnetism into his unified theory of electromagnetism that described light as oscillations of electromagnetic fields. And when he pulled all these equations together, he saw the equations fit together in a way to describe the oscillations of electromagnetic fields moving at a certain speed, a speed he could calculate, and that speed came out to be boom, exactly the speed of light. What a moment of epiphany that must have been for him. He pulls together all of this knowledge, he gets new insight, he looks at the world in a new way, and then it pops out this obvious, amazing prediction that light moves at this speed of light, this number that we had already known. So what amazing confirmation for him. So that was dominant, and people thought, okay, well, light's definitely a wave, right, does all these wave like things we have this beautiful theory, it's got to be a wave. Okay. So if light is a wave, right, we think about it in terms of electromagnetic radiation. It's just the waving of the field, just the same way sound is waving of the air. Different kinds of waves, but that doesn't really matter. And the key thing to understand if light is just electromagnetic radiation, it's just oscillations of electromagnetic fields. That means they can have any value. You can just turn up the intensity of the light right to make the light brighter. What happens when you make light brighter in the wave theory is to just increase how much the waves are shaking, right, They're just shaking more so they have more energy. So that's sort of the classical theory of electromagnetic radiation of light as just these wiggling of the waves that can have any value at all. You can turn it up, you can turn it down, just the same way you can make music louder or softer, and you can have essentially any value to that volume. So that was the sort of prevailing thinking at the time before the photon was discovered. But then, of course, an experiment came along that couldn't be explained, and experiment came along that just had answers that did not make sense in the wave theory of the universe. So we'll dig into what that experiment was and how it worked. But first let's take a quick break. So we're back and we're talking about why we think the photon is a thing. What experiment back there in history convinced people that photons had to be a particle. And remember that in the context of this experiment, light was thought to be a wave. It was thought to be electromagnetic radiation, just this oscillation of the fields. Somebody essentially shouting in the electromagnetic spectrum, and then came along this crazy experiment. The name of the experiment is not critical, but what it studied with something called the photoelectric effect. Essentially, what you're doing here is you're shining a really powerful beam of light at some surface. And a surface, of course we know now is made out of atoms. And what they observed is that if you shone light at a surface, electrons would boil off of it. You could pull them off by putting them in an electric field, and then you can measure their energy. People thought, oh, that's cool, we can boil particles off of the surface by shooting light beams at it. What would a physicist do in this scenario? She would probably think, Oh, let me see what I can do and what happens that I turn it up? What happens if I turn it down? What happens? So I made the light purple? What happens if I make the light green? Right, a physicist would want to know if the results make sense under all conditions. Sure, maybe we can understand how this works in this scenario. But can we push our limits of knowledge? Can we find some wrinkle, some corner of the space in which it doesn't make sense? That's right, experimentalists are always just trying to spoil everything for theorists. That's not true at all. Actually, as Jorge would say, because every time experimentalists do something and find a result that doesn't make sense, that's an amazing clue. That's the clue the theorists need to come up with a new theory of the universe. Anyway, back to the photoelectric effect. What happens when you shine light at the surface Electrons come off. Now, if you're thinking of light as electromagnetic waves, then what should happen if you turn up the intensity. If you turn up the intensity, then electron should shoot off with more energy. Because under the classical idea, the original idea of light is a wave. Then if you turn up the intensity of the light, the strength of the light beam, then you're putting more energy. Is just electromagnetic waves oscillating with more energy, and so there should be more energy there to dump into the electrons, and so the electron should boil off with more energy, and there should be no dependence on the frequency. You can just get the energy out of the electromagnetic waves. It doesn't matter how fast they're shaking, as long as the energy is there. The energy they're depending just on the intensity. So that's the idea. They thought, if we turn up the intensity of the light, we make the light brighter, then you should get electrons coming off with more energy, and there should be no dependence on the color. All right, So that's what they thought makes perfect sense. And then because their experimentalist, because they actually want to go out and explore the universe, not just do thought experiments in their head the way the old Greeks did, they went out and they actually tried this, and what they found, of course, blew their mind. Where they found is two things that didn't make any sense at all. First of all, the energy the electrons that came off the surface didn't depend on the intensity at all. You could turn up the intensity and the energy the electrons wouldn't change. You could turn down the intensity and the energy the electrons wouldn't change. Weirdly, if you turned up the intensity got more electrons. You didn't get any electrons with more energy, but you got more electrons boiling off. And if you made the light dimmer, if you turned down the intensity again, the energy didn't change, but the number of electrons dropped. And this didn't make any sense at all in the classical idea, if light is just a wave, if it's just oscillation of the electromagnetic field, then it should depend on the intensity, but there was no dependence on the intensity at all. Instead, changing the intensity didn't change the energy the electrons coming off. It only changed the number of electrons we saw. So then they said, ah, that's weird, So let's try changing the frequency of the light. So they go from blue light down to red light and back to purple light and just to see, and they found that the energy to electrons, weirdly, did depend on the frequency of the light. At higher frequencies, the electrons had more energy, and at low enough frequencies you wouldn't get any electrons at all. So this made no sense to anybody. People who are thinking, who are confident that light with just electromagnetic radiation could not explain either of these effects. One the fact that the energy to electrons didn't depend on the intensity of the radiation, which made no sense because they thought these are just classical waves and the intensity means more energy, so why aren't we getting more energy out of the electrons? And number two that the energy the electrons coming off did depend on the color of the light. But it made no sense to people because people were thinking about light as waves. Now there was somebody thinking about light in other terms, and that was Plunk. Plunk was studying a totally different problem, another unsolved question in physics, which had to do with black body radiation, which we'll talk about in another episode, and he was trying to solve that problem and he just couldn't. He was trying to explain why we didn't see in the lab what we expected to see based on the theory, and to solve his problem he had to come up with a crazy idea. He said, well, I don't know why, and I can justify this at all, But if I assume whom that light comes in little packets of energy that you can have like zero or one or two little bits of energy, but you can't have into your numbers in between. Then it solves my problem. And for him it was sort of a mathematical thing because like, I'm trying to do this calculation, it's not working. Nobody can figure it out. Oh look if I make this totally unjustified assumption that my calculation works and explains the data, and that's cool. That's a totally valid way to do theory and to do physics. And then you got to go back and say, well, what does that mean? Right? And it was Einstein who put it together. Einstein heard about Plunk's idea, he said, that's fascinating, and he heard about the photo electric effect and said, oh, interesting puzzle, and he put them together. And so Einstein, who never actually won the Nobel Prize for relativity, did win the Nobel Prize later for putting these two ideas together. And though he didn't do the experiments for the photo electric effect, and he also didn't have the original idea to break light down into little pieces, he just put the idea in the right place to solve the problem and explain this experiment. All right, So let's talk about how the idea that photons might be little particles, little packets of energy explains this experiment. But first, let's take another break. All right, we're back and we're talking about why photons are a thing. We reminded ourselves why people originally thought that photons were waves, and then we talked about the photo electric effect. This experiment with a weird result and a result that could not be explained using classical theory that could not be understood if you thought about light as a wave. So how do we explain the photo electric effect? How do we understand the weird results of this experiment just by saying that light comes in little packets? All right, Well, Einstein said, I'm gonna assume that likes in these little packets, and that the energy inside one packet is proportional to the frequency. That means that higher frequencies things like blue, have more energy than photons at lower frequencies, things like red. What that means is, if you want more energy in your photon, you need purple er photons. If you want less energy in your photons, you need redder photons. His microscopic understanding, what's happening is you have this surface of metal and it's got electrons in it, and electrons need a certain amount of energy in order to escape. They're bound to their atoms. They're happy, they're they're circling the nuclei right, They don't necessarily want to leave. In order for them to leave, they have to get a certain minimum of energy. So what happens when a photon comes and hits the surface. While photon hits the electron and either it has enough energy to kick the electron off or it doesn't. If it doesn't know, electron is kicked off. And what that means is that the frequency of the light has to be right high enough frequency to have a high enough energy to kick off any electrons. And that explains why when they turned the frequency down on the light, no matter how bright it was, if they turned the color down to deep deep red, they just didn't see any electrons coming off. And they couldn't explain that with their classical theory. With their classical theory, they thought, well, lights a wave, the color doesn't matter. We can make it red. As long as we make it really really bright, electron should still come off. But they didn't. And this theory explains why. Because the photons in little chunks, and each electron can only absorb energy from one photon at a time, and that's the critical idea. You can only interact with one photon at a time, so you if the photon doesn't have enough energy because it's too low frequency, it's too red, then it just can't get you out of your atom trap. And you have there are other photons coming down the pike if you have a really really intense beam, but those don't help because once that first photon has failed to get you out of the atom, then your back on the atom again, and the next one is also going to fail. The photons can't work together. So that's the key idea, the fact that the beam of light is not just one wave that's shaking the electrons so that if you turn it up, you're shaking them more and getting them enough energy to get out of those atoms. But it's broken up into pieces, and each piece needs enough energy on its own to get those electrons out of the atom. So the way you do it, the way you can get the electrons out of the atom is by changing the frequency because that gets more energy into each photon. And so if a purple one comes, remember purple being very high frequency, it has enough energy to get the electrons out of the atom and a little bit left over. So as you increase the frequency of the light, you're increasing the energy per photon, essentially the energy that each electron has access to, and then it has enough energy get out of the atom and to zoom off with a good amount of speed. So the higher the frequency of the light, the more energy in each photon, the more energy these electrons come out at. And that is exactly what they saw in the experiment, and that can only be explained if electrons can only interact with one particle of light at a time, and the light is in fact a particle. It also explains why the energy of electrons does not depend on the intensity of the beam. You can have a really powerful red beam, but it's too low frequency. All those photons are wasted because none of them have enough energy to get the electrons out. It doesn't matter how high you turn it up. And even if you're turn it up to green and you have enough energy to get the electrons out of there, you don't get more energetic electrons by increasing the intensity. Again, you have to change the energy in each photon. That's hitting the electron. You can only do that by changing the frequency. And this assumes again that electrons can only interact with one photon at a time, which is pretty solid assumption. So the amazing thing is that this idea, which really came from Plank, explain these experiments which really were done by other people. But the unification of it, the bringing together the idea, the moment of inside the explanation of this weird experiment, was done by Einstein. And that's what Einstein won the Nobel Prize for, not for doing the experiment, not for having the idea, but for being sort of in the right place at the right time to bring that idea to solve this open problem. Now, the photon was not named as a particle for decades later. All of this happened just around the turn of the nineteenth century, and Einstein won the Nobel Prize later for it, but it wasn't until nineteen twenty six that people started calling these things photons. And it comes from the Greek word for light. But it also touches on something I think is really interesting, which is the sort of concept of a particle. I like to imagine what we're physicists thinking back then, what did they think that the universe looked like at a microscopic scale, Because to us, the notion of a particle is kind of familiar. I mean, they're weird, they do things that we don't understand. They follow rules and make no sense to us. But we're comfortable with the idea that the universe is atomic, meaning that's made up of little bit it and all we have to do is sort of figure out what those bids do. But at the time, this whole concept of a particle was kind of new. Remember where they had discovered the electron. That was only recently. That was the first piece of evidence that there was something as a particle. Sort of the invention of the concept of a particle was the discovery of the electron. And all he really did there was identify something tiny that had both mass and charge, and so he said, oh, look there's a thing there as these two attributes. I'm going to call it a particle. Actually he called it a corpus skule. But the concept of intellectual groundwork was laid then for a particle. So then you get to the photon. Now the photon has energy, it has direction, but it doesn't have mass. It's not a thing in that sense, there's no stuff to it, So that immediately sort of bends your mind around what is this concept of a particle. Anyway, we've created this idea to accommodate the discovery the electron. We hope, oh, maybe there are other particles, And later on the podcasts will take a tour of the discoveries of other particles, which have hilarious and amazing and dramatic stories to them. But very early in the history of particles we had to already bend the rules and say, oh, well, we were talking about particles is a little bits of stuff. But they can also be not stuff, right, they can also just be energy. And so to me, it's amazing that this field of particle physics was founded on such crazy discoveries. So to me, it's wonderful that the field of particle physics is founded on such crazy discoveries. And you've got to give a lot of credit to the theorists, of course, who put these ideas together and helped us understand what we were seeing. But to me, the most exciting moments are those moments of experimental surprise when the universe does something that we don't understand when the unit, when we predict the universe will do a and instead it does be because that's the universe talking to us, or that's the universe answering our questions, that's the universe being the subject of our interrogation when we say we want to know how this works, prove it to us, or reveal to us the underlying mechanism. And that's what experimental physics is about. Is about cornering the universe and forcing it to reveal something new to you. And a lot of times that revelation happens when you didn't expect that. You thought, oh, we're just double checking this over here. We're pretty sure we understand it. Just dotting the eyes and crossing the teas and all of a sudden, oops, you get something totally surprising. But those are the moments that we learned something new about the universe, And those are the moments I'm striving for my own personal research. When I'm smashing particles together at the LHC. We think we understand what's going to happen, but I'm always secretly hoping that a student will come to me and say, hey, Daniel, what's this. I found this weird thing in our data that just doesn't make any sense, and that's only happened once or twice in my entire career, and I look forward to it happening again. So maybe one day we'll be hearing about a crazy discovery we made at the Large Adeon Collider. Until then, thanks for listening to this description of how we know the photon is a thing, and please, if you're interested in learning more about the history of physics or understanding how we know how the universe works and what we don't know, please send me a suggestion to feedback at Daniel and Jorge dot com. Thanks for tuning in. If you still have a question after listening to all these explanations, please drop us a line. We'd love to hear from you. You can find us at Facebook, Twitter, and Instagram at Daniel and Jorge That's one word, or email us at Feedback at Daniel and Jorge dot com. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of I Heart Radio. For more podcast from my Heart Radio, visit the I heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.