WEBVTT - How do we know the photon is real? 0:00:09.400 --> 0:00:13.040 Hey, they're listeners. You're into physics. So here's a tribua 0:00:13.160 --> 0:00:16.160 question for you. Do you know who won a Nobel 0:00:16.280 --> 0:00:19.760 Prize for relativity? That might feel like it's your question 0:00:19.840 --> 0:00:22.600 because you want to say Einstein, because they think relativity 0:00:22.840 --> 0:00:26.200 and Einstein will I'll tell you it's not Einstein. Now, 0:00:26.239 --> 0:00:28.440 maybe you're scrambling through your mind to think about the 0:00:28.520 --> 0:00:31.320 names of other physicists. You might know. How many physicists 0:00:31.360 --> 0:00:36.120 can you name? Anyway, you got Einstein, you got me. Well, 0:00:36.159 --> 0:00:39.159 I'll give you a clue. It's neither Einstein nor me. 0:00:39.760 --> 0:00:43.440 So who was it? Right? Well, some folks wanted for 0:00:43.640 --> 0:00:47.400 proving that relativity was correct. There were Nobel Prizes for 0:00:47.440 --> 0:00:51.839 gravitational waves and for binary pulsars. But the answer is 0:00:51.880 --> 0:00:55.400 that nobody wanted for relativity. Nobody who came up with 0:00:55.400 --> 0:00:58.960 this incredible earth shattering idea that now frames all of 0:00:59.040 --> 0:01:03.120 modern physics won the Nobel Prize for it. But you 0:01:03.200 --> 0:01:06.240 might be thinking, hold on, didn't Einstein win a Nobel Prize? 0:01:06.640 --> 0:01:10.240 And he did, but he wanted essentially for quantum mechanics. 0:01:26.800 --> 0:01:30.680 Hello everyone, I'm Daniel. I'm a particle physicist, and I'm 0:01:30.720 --> 0:01:34.400 the co host of this podcast together with Korge cham 0:01:34.520 --> 0:01:37.400 who can't be here this week, So you're just listening 0:01:37.400 --> 0:01:40.600 to me talking about the joys of physics and trying 0:01:40.640 --> 0:01:43.679 to simulate Jorge in my mind. Every time i'm talking, 0:01:43.720 --> 0:01:46.639 I'm thinking, here's what Jorge would say. At this moment, 0:01:46.720 --> 0:01:49.360 I'm trying to interject a little Jorge Ism for you, 0:01:49.680 --> 0:01:52.280 since we all miss him and you were listening to 0:01:52.320 --> 0:01:57.200 our podcast, Daniel and Jorge Explain the Universe, a production 0:01:57.360 --> 0:02:00.240 of I Heart Radio in which we zoom all around 0:02:00.240 --> 0:02:03.520 the universe and try to find interesting, fascinating, cool little 0:02:03.600 --> 0:02:06.640 nuggets of physics that would blow your mind, but take 0:02:06.680 --> 0:02:09.280 them apart so they don't actually explode your head and 0:02:09.360 --> 0:02:13.519 cause your brains displatter anywhere. Instead, we want to smoothly 0:02:13.600 --> 0:02:16.919 and calmly insert them into your mind so you understand them, 0:02:16.960 --> 0:02:18.680 so you can talk to your friends about them, so 0:02:18.760 --> 0:02:22.760 you can actually comprehend these amazing, wonderful facts that we 0:02:22.840 --> 0:02:26.000 have learned about the universe, and also understand all the 0:02:26.080 --> 0:02:29.200 things we don't know about the universe, which is my 0:02:29.360 --> 0:02:32.040 favorite part of physics, and that's why Jorge and I 0:02:32.120 --> 0:02:35.079 wrote the book We Have No Idea, A Guide to 0:02:35.120 --> 0:02:37.760 the Unknown universe, which takes you on an amazing tour 0:02:37.840 --> 0:02:40.960 of all the big and basic questions about the universe 0:02:41.000 --> 0:02:43.600 that we still have no idea what the answers are. 0:02:43.880 --> 0:02:46.359 And on the podcast, we've been doing something fun, which 0:02:46.400 --> 0:02:49.399 is taking a little tour of how we know what 0:02:49.480 --> 0:02:53.480 we know, and specifically how we know anything about particle physics. 0:02:53.800 --> 0:02:55.960 It's still incredible to me when I look around at 0:02:56.000 --> 0:02:58.799 the world that everything is made out of these tiny 0:02:58.840 --> 0:03:02.519 microscopic object so we can't see that we've taken thousands 0:03:02.520 --> 0:03:05.640 of years to even discover that they exist. Yet we 0:03:05.720 --> 0:03:11.040 have this really complex, really elaborate, really amazingly effective model 0:03:11.080 --> 0:03:14.480 of what's happening down there at the microscopic scale, all 0:03:14.480 --> 0:03:18.920 these tiny quantum particles interacting and zooming around. Physicists can 0:03:18.919 --> 0:03:21.440 do calculations to tell you exactly what's going to happen 0:03:21.440 --> 0:03:25.000 when this particle hits that particle. It's really incredibly complex 0:03:25.040 --> 0:03:28.040 and mature. Though of course we have lots of questions, 0:03:28.320 --> 0:03:30.160 but I think a lot of times people think of 0:03:30.200 --> 0:03:33.760 this as sort of like an idea, something people came 0:03:33.840 --> 0:03:37.480 up with a description of the universe. But it's critical 0:03:37.800 --> 0:03:40.600 that everybody understand that this isn't just an idea that 0:03:40.640 --> 0:03:43.600 came out of our heads. This is something born out 0:03:43.640 --> 0:03:47.360 of desperation. This is our attempt to grapple with the 0:03:47.480 --> 0:03:53.040 weird and bizarre and counterintuitive and frankly mind blowing experiments 0:03:53.080 --> 0:03:56.480 that have shattered our perceptions of reality. We thought the 0:03:56.560 --> 0:03:59.760 universe worked a certain way. We thought everything was smooth, 0:04:00.040 --> 0:04:03.000 You could cut objects as many times as you wanted 0:04:03.040 --> 0:04:06.480 to infinitely small pieces, but you can't. We thought the 0:04:06.560 --> 0:04:09.520 universe was deterministic, that if you did the same experiment 0:04:09.560 --> 0:04:12.040 twice you would get the same outcome. Right, that would 0:04:12.120 --> 0:04:15.920 make sense, But it's not. It's fundamentally random. And the 0:04:15.960 --> 0:04:18.440 core of that is particle physics, because it attempts to 0:04:18.440 --> 0:04:22.400 describe the entire universe in terms of these tiny, weird, 0:04:22.760 --> 0:04:27.240 nondeterministic little particles, in terms of these tiny, little, weird, 0:04:27.520 --> 0:04:30.880 nondeterministic particles that seem to follow rules that just do 0:04:30.960 --> 0:04:34.040 not describe the world that we are familiar with. So 0:04:34.360 --> 0:04:36.000 my goal is to take you on a tour of 0:04:36.040 --> 0:04:38.640 those experiments, the ones that change the way we think 0:04:38.680 --> 0:04:41.440 about the universe, that showed us that the universe is 0:04:41.520 --> 0:04:44.480 different from what we imagined, because it's not just the 0:04:44.520 --> 0:04:47.040 final idea that you want that I want you to understand. 0:04:47.400 --> 0:04:49.880 I want you to know what the evidence is. How 0:04:49.920 --> 0:04:53.080 do we know what we know? Now? Recently we talked 0:04:53.080 --> 0:04:56.440 about the discovery of the first particle, the first experiment 0:04:56.640 --> 0:05:00.520 that revealed this incredible revelation that the Uni verse is 0:05:00.600 --> 0:05:03.360 made out of time to little dots. And so today 0:05:03.360 --> 0:05:12.200 we are continuing that tour. We are talking about how 0:05:12.240 --> 0:05:15.760 do we know the photon is a thing? You're familiar 0:05:15.760 --> 0:05:18.440 with photons? To you, photon is a very normal word 0:05:18.480 --> 0:05:21.479 you hear bandied about here, talked about but how do 0:05:21.560 --> 0:05:24.120 we know that photons are there? How do we know 0:05:24.160 --> 0:05:26.520 that light is made out of photons that is chopped 0:05:26.600 --> 0:05:28.920 up into these little pieces that can't be cut down 0:05:29.000 --> 0:05:32.240 even further. What is the actual experiment that proves to 0:05:32.440 --> 0:05:35.760 us that photons are a thing, that light is not 0:05:35.880 --> 0:05:39.279 just electromagnetic waves, but it does these other weird things 0:05:39.279 --> 0:05:42.159 that you have to give it particle status to explain. So, 0:05:42.240 --> 0:05:44.800 as usual, I was wondering how many people out there 0:05:44.920 --> 0:05:47.479 know why we think the photon is a thing, Why 0:05:47.560 --> 0:05:50.840 we don't just think about light as electromagnetic waves. So 0:05:50.960 --> 0:05:52.960 I walked around the campus of U see Irvine and 0:05:53.040 --> 0:05:56.919 accosted a bunch of friendly and unsuspecting students, and I 0:05:57.000 --> 0:05:59.839 asked them, do you know how the photon was discovered? 0:06:00.080 --> 0:06:02.640 Do you have an idea of why we think the 0:06:02.640 --> 0:06:06.120 photon is a thing. So before you listen to these answers, 0:06:06.160 --> 0:06:08.839 think to yourself, or pause the podcast, or just take 0:06:08.880 --> 0:06:11.640 a moment. How do you know photons are a thing? 0:06:11.640 --> 0:06:14.479 Are you just believing physicists when they tell you, or 0:06:14.520 --> 0:06:18.119 do you know what the data says? I'm not entirely sure. 0:06:18.400 --> 0:06:21.359 I feel I shouldn't have that. I I'm sorry, I 0:06:21.440 --> 0:06:27.000 probably should know. But it was the slit experiments, wasn't it. 0:06:27.080 --> 0:06:32.200 And they projected a laser beam onto a single slit 0:06:32.320 --> 0:06:35.240 or double slits and it diffracted the beam and that's 0:06:35.240 --> 0:06:38.919 how they discovered it. Particle wave duality. Yeah, the photo 0:06:38.960 --> 0:06:41.520 electrical facts. You shone a light on a metal and 0:06:41.520 --> 0:06:46.880 then the metal you cross. Uh, you start for Einstein ninth. 0:06:46.960 --> 0:06:48.880 I don't remember the year. Yeah, I don't remember who 0:06:48.920 --> 0:06:51.120 did it, but I remember that you shine a light 0:06:51.160 --> 0:06:54.559 on a metal, do you give the electron enough energy 0:06:54.640 --> 0:06:59.479 to start conducting it's particles? Well, we know it's a 0:06:59.520 --> 0:07:01.960 wave because it travels through vacuum, and we know that 0:07:02.000 --> 0:07:07.360 it's a particle because you can transfer energy from it. Right, Yeah, 0:07:07.360 --> 0:07:11.960 it has it hasn't defined momentum, even though it has 0:07:12.000 --> 0:07:15.960 no mass. But the slit experiment double slip one showed 0:07:16.000 --> 0:07:19.239 that it was a wave, like a single slit showed 0:07:19.240 --> 0:07:24.800 that a particle. Well, it's not necessarily a particle. It's 0:07:25.320 --> 0:07:28.120 both a particle and a wave. And for a really 0:07:28.160 --> 0:07:31.040 long time we thought it was just a wave. But 0:07:31.840 --> 0:07:36.000 I believe the first time we figured out that it 0:07:36.080 --> 0:07:42.720 was a particle had to do um exciting metals to 0:07:43.800 --> 0:07:49.480 release photons and realized that the distributions were discreet. So 0:07:49.600 --> 0:07:52.080 I was really impressed with these answers. A lot of 0:07:52.160 --> 0:07:56.600 understanding here that photons are particles and that they're part 0:07:56.600 --> 0:07:59.480 of this larger idea of light being a wave and 0:07:59.520 --> 0:08:02.679 a particle. Even some discussion of the double slit experiment, 0:08:02.720 --> 0:08:05.080 which I'm dying to get into in a future podcast 0:08:05.120 --> 0:08:08.120 and talk all about the amazing facts of quantum mechanics. 0:08:08.680 --> 0:08:10.720 But the double slit experiment actually shows you that the 0:08:10.720 --> 0:08:13.600 photon is a wave. But there was somebody out there 0:08:13.600 --> 0:08:16.880 who talked about the photoelectric effect, and that's the key 0:08:17.080 --> 0:08:20.080 that was the experiment that showed us that photons were 0:08:20.120 --> 0:08:23.440 a thing. But before we talk about the crazy experiment 0:08:23.520 --> 0:08:26.920 to prove that quantum mechanics is our reality, that showed 0:08:27.040 --> 0:08:29.920 us that the universe is probably sliced up into little 0:08:29.920 --> 0:08:33.600 bits and not infinitely smooth. Let's set the stage. Okay, 0:08:33.640 --> 0:08:36.240 let's you remember how people thought about light, and to 0:08:36.280 --> 0:08:38.640 get the context of the story, you have to rewind 0:08:38.720 --> 0:08:41.400 all the way back to Isaac Newton. Isaac Newton, of 0:08:41.440 --> 0:08:44.760 course very famous not just for the cookies, but also 0:08:44.840 --> 0:08:48.120 for his discovery of his theory of gravity, which unified 0:08:48.280 --> 0:08:51.040 motion of objects here on Earth with motion of objects 0:08:51.040 --> 0:08:53.520 in the heavens. Really gave us access to the whole 0:08:53.640 --> 0:08:57.800 universe to imagine, Wow, maybe physics can actually describe things 0:08:58.120 --> 0:09:00.280 not just here in front of us, but out there 0:09:00.280 --> 0:09:02.680 in the universe. Those are things out there that follow 0:09:02.800 --> 0:09:07.920 laws of physics. Incredible accomplishments. But Newton also also made 0:09:07.960 --> 0:09:11.040 amazing discoveries in the field of optics. He spent a 0:09:11.080 --> 0:09:14.199 lot of time with lenses and with prisms, and he 0:09:14.280 --> 0:09:17.000 was convinced that light was a particle, and he thought 0:09:17.000 --> 0:09:19.240 a lot about how light traveled. He saw it moving 0:09:19.240 --> 0:09:21.160 in straight lines, except when he was bent by these 0:09:21.240 --> 0:09:23.760 lenses and he was convinced that light was a particle. 0:09:23.800 --> 0:09:27.520 And because he was a genius and he's a staggering 0:09:27.600 --> 0:09:30.319 influence on the field of physics, people listen to him 0:09:30.320 --> 0:09:33.120 and rightly so. And for hundreds of years people were 0:09:33.160 --> 0:09:35.920 convinced that it was a particle, even though other folks 0:09:36.040 --> 0:09:39.400 had really nice theories of light as a wave. And 0:09:39.440 --> 0:09:42.720 it wasn't until the eighteen hundreds when people started observing, 0:09:43.120 --> 0:09:46.600 like doing things that particles couldn't do, that they had 0:09:46.640 --> 0:09:49.840 to adapt their mindset. And that's the key. There you 0:09:49.880 --> 0:09:54.120 see experiment rearing its uncomfortable head again saying, oh no, no, 0:09:54.559 --> 0:09:57.360 you thought you understood the universe. You have an idea 0:09:57.400 --> 0:09:59.520 in your mind, you have a mental model of how 0:09:59.559 --> 0:10:03.240 this is working, but it can't describe what's actually happening. 0:10:03.600 --> 0:10:06.760 And that's why I'm an experimentalist. That's why I think 0:10:06.760 --> 0:10:09.520 experiment is the place to be, because experimentalists are the 0:10:09.520 --> 0:10:12.320 ones who make the discoveries. They are on the forefront 0:10:12.400 --> 0:10:16.040 of knowledge. They're out there exploring the universe, discovering things 0:10:16.080 --> 0:10:19.720 that don't make sense. Theorists, of course, do an incredible job. 0:10:19.760 --> 0:10:22.600 They tied all together they understand they predict future phenomenon. 0:10:22.640 --> 0:10:25.599 But for me, the bit about physics that's wonderful is 0:10:25.720 --> 0:10:29.160 the experimental side, is making those discoveries, is asking nature 0:10:29.200 --> 0:10:32.760 a question and demanding an answer, pinning nature in a 0:10:32.800 --> 0:10:35.440 corner so that nature has to tell you is the 0:10:35.520 --> 0:10:38.760 universe this way or that way? And so the thing 0:10:38.800 --> 0:10:42.400 that told people that photons couldn't just be a particle 0:10:42.720 --> 0:10:46.840 were wave like effects, things like interference. And you're familiar 0:10:46.880 --> 0:10:50.800 with interference, maybe you have noise canceling headphones. Now is 0:10:50.840 --> 0:10:54.920 canceling headphones work via interference? Sound is a wave. It's 0:10:54.920 --> 0:10:58.040 a shaking of air, and the air comes towards your head, 0:10:58.520 --> 0:11:00.840 and if you can create waves shake in the other 0:11:00.880 --> 0:11:04.160 direction at the same time, they basically cancel out those 0:11:04.160 --> 0:11:06.920 waves that are coming in your head. So sound canceling 0:11:06.920 --> 0:11:09.080 headphones are proof that sound is a wave because they 0:11:09.120 --> 0:11:12.240 can do this wave like thing that particles just cannot do. 0:11:12.840 --> 0:11:15.000 In the same way, people saw light behaving in a 0:11:15.040 --> 0:11:18.000 way that could only be described by a wave, and 0:11:18.040 --> 0:11:20.360 so you had interference effects, and you had all sorts 0:11:20.360 --> 0:11:23.000 of theories sort of built momentum until you get to 0:11:23.120 --> 0:11:27.240 James Clerk Maxwell his incredible genius pulled together lots of 0:11:27.280 --> 0:11:31.440 ideas about electricity and magnetism into his unified theory of 0:11:31.480 --> 0:11:37.040 electromagnetism that described light as oscillations of electromagnetic fields. And 0:11:37.040 --> 0:11:39.200 when he pulled all these equations together, he saw the 0:11:39.240 --> 0:11:42.439 equations fit together in a way to describe the oscillations 0:11:42.440 --> 0:11:45.640 of electromagnetic fields moving at a certain speed, a speed 0:11:45.679 --> 0:11:48.640 he could calculate, and that speed came out to be boom, 0:11:48.840 --> 0:11:51.960 exactly the speed of light. What a moment of epiphany 0:11:52.000 --> 0:11:54.400 that must have been for him. He pulls together all 0:11:54.400 --> 0:11:56.360 of this knowledge, he gets new insight, he looks at 0:11:56.400 --> 0:11:59.080 the world in a new way, and then it pops 0:11:59.080 --> 0:12:02.719 out this obvious, amazing prediction that light moves at this 0:12:02.920 --> 0:12:05.760 speed of light, this number that we had already known. 0:12:06.320 --> 0:12:09.280 So what amazing confirmation for him. So that was dominant, 0:12:09.320 --> 0:12:12.160 and people thought, okay, well, light's definitely a wave, right, 0:12:12.280 --> 0:12:14.720 does all these wave like things we have this beautiful theory, 0:12:14.920 --> 0:12:18.000 it's got to be a wave. Okay. So if light 0:12:18.120 --> 0:12:20.280 is a wave, right, we think about it in terms 0:12:20.280 --> 0:12:23.680 of electromagnetic radiation. It's just the waving of the field, 0:12:23.960 --> 0:12:26.400 just the same way sound is waving of the air. 0:12:26.640 --> 0:12:29.560 Different kinds of waves, but that doesn't really matter. And 0:12:29.600 --> 0:12:33.640 the key thing to understand if light is just electromagnetic radiation, 0:12:33.679 --> 0:12:38.200 it's just oscillations of electromagnetic fields. That means they can 0:12:38.280 --> 0:12:42.120 have any value. You can just turn up the intensity 0:12:42.120 --> 0:12:44.000 of the light right to make the light brighter. What 0:12:44.040 --> 0:12:46.680 happens when you make light brighter in the wave theory 0:12:47.160 --> 0:12:49.720 is to just increase how much the waves are shaking, right, 0:12:49.760 --> 0:12:52.360 They're just shaking more so they have more energy. So 0:12:52.400 --> 0:12:55.600 that's sort of the classical theory of electromagnetic radiation of 0:12:55.679 --> 0:12:58.040 light as just these wiggling of the waves that can 0:12:58.080 --> 0:12:59.920 have any value at all. You can turn it up, 0:13:00.240 --> 0:13:02.480 you can turn it down, just the same way you 0:13:02.480 --> 0:13:05.280 can make music louder or softer, and you can have 0:13:05.360 --> 0:13:08.080 essentially any value to that volume. So that was the 0:13:08.080 --> 0:13:10.720 sort of prevailing thinking at the time before the photon 0:13:10.800 --> 0:13:15.160 was discovered. But then, of course, an experiment came along 0:13:15.240 --> 0:13:17.800 that couldn't be explained, and experiment came along that just 0:13:18.200 --> 0:13:21.240 had answers that did not make sense in the wave 0:13:21.440 --> 0:13:24.320 theory of the universe. So we'll dig into what that 0:13:24.400 --> 0:13:27.640 experiment was and how it worked. But first let's take 0:13:27.679 --> 0:13:42.880 a quick break. So we're back and we're talking about 0:13:43.040 --> 0:13:45.959 why we think the photon is a thing. What experiment 0:13:46.040 --> 0:13:49.240 back there in history convinced people that photons had to 0:13:49.240 --> 0:13:52.760 be a particle. And remember that in the context of 0:13:52.800 --> 0:13:55.040 this experiment, light was thought to be a wave. It 0:13:55.080 --> 0:13:58.720 was thought to be electromagnetic radiation, just this oscillation of 0:13:58.760 --> 0:14:02.760 the fields. Somebody essentially shouting in the electromagnetic spectrum, and 0:14:02.840 --> 0:14:05.840 then came along this crazy experiment. The name of the 0:14:05.840 --> 0:14:08.559 experiment is not critical, but what it studied with something 0:14:08.640 --> 0:14:12.920 called the photoelectric effect. Essentially, what you're doing here is 0:14:12.960 --> 0:14:16.959 you're shining a really powerful beam of light at some surface. 0:14:17.320 --> 0:14:19.080 And a surface, of course we know now is made 0:14:19.080 --> 0:14:21.760 out of atoms. And what they observed is that if 0:14:21.800 --> 0:14:24.800 you shone light at a surface, electrons would boil off 0:14:24.800 --> 0:14:27.160 of it. You could pull them off by putting them 0:14:27.160 --> 0:14:29.640 in an electric field, and then you can measure their energy. 0:14:30.520 --> 0:14:33.840 People thought, oh, that's cool, we can boil particles off 0:14:33.840 --> 0:14:36.160 of the surface by shooting light beams at it. What 0:14:36.200 --> 0:14:39.280 would a physicist do in this scenario? She would probably think, Oh, 0:14:39.400 --> 0:14:41.200 let me see what I can do and what happens 0:14:41.200 --> 0:14:42.680 that I turn it up? What happens if I turn 0:14:42.760 --> 0:14:44.640 it down? What happens? So I made the light purple? 0:14:44.760 --> 0:14:47.520 What happens if I make the light green? Right, a 0:14:47.560 --> 0:14:49.960 physicist would want to know if the results make sense 0:14:50.040 --> 0:14:53.400 under all conditions. Sure, maybe we can understand how this 0:14:53.440 --> 0:14:56.360 works in this scenario. But can we push our limits 0:14:56.360 --> 0:14:59.400 of knowledge? Can we find some wrinkle, some corner of 0:14:59.440 --> 0:15:01.920 the space in which it doesn't make sense? That's right, 0:15:02.120 --> 0:15:05.400 experimentalists are always just trying to spoil everything for theorists. 0:15:05.520 --> 0:15:08.760 That's not true at all. Actually, as Jorge would say, 0:15:08.800 --> 0:15:12.560 because every time experimentalists do something and find a result 0:15:12.640 --> 0:15:15.800 that doesn't make sense, that's an amazing clue. That's the 0:15:15.840 --> 0:15:18.240 clue the theorists need to come up with a new 0:15:18.360 --> 0:15:22.000 theory of the universe. Anyway, back to the photoelectric effect. 0:15:22.320 --> 0:15:25.720 What happens when you shine light at the surface Electrons 0:15:25.760 --> 0:15:29.680 come off. Now, if you're thinking of light as electromagnetic waves, 0:15:29.920 --> 0:15:33.280 then what should happen if you turn up the intensity. 0:15:33.600 --> 0:15:36.360 If you turn up the intensity, then electron should shoot 0:15:36.360 --> 0:15:39.320 off with more energy. Because under the classical idea, the 0:15:39.360 --> 0:15:42.200 original idea of light is a wave. Then if you 0:15:42.240 --> 0:15:44.600 turn up the intensity of the light, the strength of 0:15:44.600 --> 0:15:47.320 the light beam, then you're putting more energy. Is just 0:15:47.560 --> 0:15:51.440 electromagnetic waves oscillating with more energy, and so there should 0:15:51.440 --> 0:15:54.720 be more energy there to dump into the electrons, and 0:15:54.760 --> 0:15:57.680 so the electron should boil off with more energy, and 0:15:57.720 --> 0:16:00.480 there should be no dependence on the frequency. You can 0:16:00.560 --> 0:16:03.080 just get the energy out of the electromagnetic waves. It 0:16:03.120 --> 0:16:05.640 doesn't matter how fast they're shaking, as long as the 0:16:05.760 --> 0:16:09.320 energy is there. The energy they're depending just on the intensity. 0:16:09.440 --> 0:16:12.040 So that's the idea. They thought, if we turn up 0:16:12.080 --> 0:16:14.520 the intensity of the light, we make the light brighter, 0:16:15.000 --> 0:16:18.320 then you should get electrons coming off with more energy, 0:16:18.360 --> 0:16:21.440 and there should be no dependence on the color. All right, 0:16:21.480 --> 0:16:24.080 So that's what they thought makes perfect sense. And then 0:16:24.120 --> 0:16:26.440 because their experimentalist, because they actually want to go out 0:16:26.480 --> 0:16:29.840 and explore the universe, not just do thought experiments in 0:16:29.880 --> 0:16:32.520 their head the way the old Greeks did, they went 0:16:32.520 --> 0:16:35.120 out and they actually tried this, and what they found, 0:16:35.120 --> 0:16:38.760 of course, blew their mind. Where they found is two 0:16:38.840 --> 0:16:42.960 things that didn't make any sense at all. First of all, 0:16:43.280 --> 0:16:46.400 the energy the electrons that came off the surface didn't 0:16:46.400 --> 0:16:49.640 depend on the intensity at all. You could turn up 0:16:49.640 --> 0:16:52.680 the intensity and the energy the electrons wouldn't change. You 0:16:52.680 --> 0:16:55.920 could turn down the intensity and the energy the electrons 0:16:55.920 --> 0:17:00.400 wouldn't change. Weirdly, if you turned up the intensity got 0:17:00.440 --> 0:17:03.720 more electrons. You didn't get any electrons with more energy, 0:17:03.840 --> 0:17:07.040 but you got more electrons boiling off. And if you 0:17:07.119 --> 0:17:10.000 made the light dimmer, if you turned down the intensity again, 0:17:10.040 --> 0:17:13.600 the energy didn't change, but the number of electrons dropped. 0:17:13.920 --> 0:17:15.679 And this didn't make any sense at all in the 0:17:15.720 --> 0:17:18.640 classical idea, if light is just a wave, if it's 0:17:18.640 --> 0:17:22.520 just oscillation of the electromagnetic field, then it should depend 0:17:22.520 --> 0:17:24.840 on the intensity, but there was no dependence on the 0:17:24.880 --> 0:17:28.800 intensity at all. Instead, changing the intensity didn't change the 0:17:28.920 --> 0:17:31.679 energy the electrons coming off. It only changed the number 0:17:31.720 --> 0:17:35.080 of electrons we saw. So then they said, ah, that's weird, 0:17:35.600 --> 0:17:38.239 So let's try changing the frequency of the light. So 0:17:38.280 --> 0:17:40.760 they go from blue light down to red light and 0:17:40.800 --> 0:17:42.880 back to purple light and just to see, and they 0:17:42.880 --> 0:17:45.960 found that the energy to electrons, weirdly, did depend on 0:17:46.000 --> 0:17:49.399 the frequency of the light. At higher frequencies, the electrons 0:17:49.440 --> 0:17:52.959 had more energy, and at low enough frequencies you wouldn't 0:17:53.000 --> 0:17:56.280 get any electrons at all. So this made no sense 0:17:56.320 --> 0:17:59.520 to anybody. People who are thinking, who are confident that 0:17:59.640 --> 0:18:03.240 light with just electromagnetic radiation could not explain either of 0:18:03.240 --> 0:18:06.880 these effects. One the fact that the energy to electrons 0:18:07.000 --> 0:18:10.160 didn't depend on the intensity of the radiation, which made 0:18:10.160 --> 0:18:13.240 no sense because they thought these are just classical waves 0:18:13.240 --> 0:18:15.560 and the intensity means more energy, so why aren't we 0:18:15.560 --> 0:18:18.480 getting more energy out of the electrons? And number two 0:18:18.920 --> 0:18:21.800 that the energy the electrons coming off did depend on 0:18:21.880 --> 0:18:24.120 the color of the light. But it made no sense 0:18:24.119 --> 0:18:27.800 to people because people were thinking about light as waves. 0:18:28.359 --> 0:18:31.399 Now there was somebody thinking about light in other terms, 0:18:31.640 --> 0:18:35.440 and that was Plunk. Plunk was studying a totally different problem, 0:18:35.520 --> 0:18:38.840 another unsolved question in physics, which had to do with 0:18:38.920 --> 0:18:42.160 black body radiation, which we'll talk about in another episode, 0:18:42.520 --> 0:18:44.080 and he was trying to solve that problem and he 0:18:44.160 --> 0:18:46.639 just couldn't. He was trying to explain why we didn't 0:18:46.640 --> 0:18:49.200 see in the lab what we expected to see based 0:18:49.200 --> 0:18:52.320 on the theory, and to solve his problem he had 0:18:52.359 --> 0:18:54.560 to come up with a crazy idea. He said, well, 0:18:54.880 --> 0:18:57.880 I don't know why, and I can justify this at all, 0:18:58.320 --> 0:19:03.160 But if I assume whom that light comes in little 0:19:03.240 --> 0:19:06.000 packets of energy that you can have like zero or 0:19:06.080 --> 0:19:09.240 one or two little bits of energy, but you can't 0:19:09.280 --> 0:19:12.440 have into your numbers in between. Then it solves my problem. 0:19:12.520 --> 0:19:14.320 And for him it was sort of a mathematical thing 0:19:14.720 --> 0:19:17.600 because like, I'm trying to do this calculation, it's not working. 0:19:17.840 --> 0:19:20.000 Nobody can figure it out. Oh look if I make 0:19:20.040 --> 0:19:24.280 this totally unjustified assumption that my calculation works and explains 0:19:24.280 --> 0:19:26.920 the data, and that's cool. That's a totally valid way 0:19:26.960 --> 0:19:28.960 to do theory and to do physics. And then you 0:19:29.000 --> 0:19:31.520 got to go back and say, well, what does that mean? Right? 0:19:32.080 --> 0:19:34.960 And it was Einstein who put it together. Einstein heard 0:19:35.000 --> 0:19:38.280 about Plunk's idea, he said, that's fascinating, and he heard 0:19:38.280 --> 0:19:41.480 about the photo electric effect and said, oh, interesting puzzle, 0:19:41.760 --> 0:19:45.399 and he put them together. And so Einstein, who never 0:19:45.560 --> 0:19:48.919 actually won the Nobel Prize for relativity, did win the 0:19:48.960 --> 0:19:52.439 Nobel Prize later for putting these two ideas together. And 0:19:52.480 --> 0:19:55.600 though he didn't do the experiments for the photo electric effect, 0:19:55.880 --> 0:19:58.560 and he also didn't have the original idea to break 0:19:58.640 --> 0:20:01.520 light down into little pieces, he just put the idea 0:20:01.720 --> 0:20:03.959 in the right place to solve the problem and explain 0:20:04.040 --> 0:20:07.520 this experiment. All right, So let's talk about how the 0:20:07.560 --> 0:20:10.919 idea that photons might be little particles, little packets of 0:20:11.000 --> 0:20:15.119 energy explains this experiment. But first, let's take another break. 0:20:28.240 --> 0:20:30.840 All right, we're back and we're talking about why photons 0:20:30.920 --> 0:20:35.000 are a thing. We reminded ourselves why people originally thought 0:20:35.000 --> 0:20:37.639 that photons were waves, and then we talked about the 0:20:37.640 --> 0:20:40.720 photo electric effect. This experiment with a weird result and 0:20:40.760 --> 0:20:44.840 a result that could not be explained using classical theory 0:20:44.880 --> 0:20:47.880 that could not be understood if you thought about light 0:20:48.000 --> 0:20:50.560 as a wave. So how do we explain the photo 0:20:50.560 --> 0:20:53.440 electric effect? How do we understand the weird results of 0:20:53.520 --> 0:20:56.680 this experiment just by saying that light comes in little packets? 0:20:56.960 --> 0:21:00.080 All right, Well, Einstein said, I'm gonna assume that likes 0:21:00.080 --> 0:21:03.800 in these little packets, and that the energy inside one 0:21:03.840 --> 0:21:07.520 packet is proportional to the frequency. That means that higher 0:21:07.560 --> 0:21:11.919 frequencies things like blue, have more energy than photons at 0:21:11.960 --> 0:21:15.760 lower frequencies, things like red. What that means is, if 0:21:15.760 --> 0:21:18.200 you want more energy in your photon, you need purple 0:21:18.240 --> 0:21:20.440 er photons. If you want less energy in your photons, 0:21:20.680 --> 0:21:24.800 you need redder photons. His microscopic understanding, what's happening is 0:21:24.800 --> 0:21:27.280 you have this surface of metal and it's got electrons 0:21:27.320 --> 0:21:30.600 in it, and electrons need a certain amount of energy 0:21:30.680 --> 0:21:33.680 in order to escape. They're bound to their atoms. They're happy, 0:21:33.720 --> 0:21:36.800 they're they're circling the nuclei right, They don't necessarily want 0:21:36.800 --> 0:21:38.480 to leave. In order for them to leave, they have 0:21:38.520 --> 0:21:41.919 to get a certain minimum of energy. So what happens 0:21:41.920 --> 0:21:45.000 when a photon comes and hits the surface. While photon 0:21:45.160 --> 0:21:48.919 hits the electron and either it has enough energy to 0:21:49.080 --> 0:21:51.879 kick the electron off or it doesn't. If it doesn't know, 0:21:51.960 --> 0:21:54.600 electron is kicked off. And what that means is that 0:21:54.680 --> 0:21:57.280 the frequency of the light has to be right high 0:21:57.400 --> 0:21:59.840 enough frequency to have a high enough energy to kick 0:22:00.000 --> 0:22:03.040 off any electrons. And that explains why when they turned 0:22:03.080 --> 0:22:06.160 the frequency down on the light, no matter how bright 0:22:06.200 --> 0:22:08.520 it was, if they turned the color down to deep 0:22:08.560 --> 0:22:11.560 deep red, they just didn't see any electrons coming off. 0:22:12.359 --> 0:22:14.520 And they couldn't explain that with their classical theory. With 0:22:14.520 --> 0:22:16.919 their classical theory, they thought, well, lights a wave, the 0:22:16.920 --> 0:22:19.600 color doesn't matter. We can make it red. As long 0:22:19.640 --> 0:22:22.680 as we make it really really bright, electron should still 0:22:22.720 --> 0:22:26.159