WEBVTT - How long is a photon? 0:00:08.560 --> 0:00:12.000 Hey, Daniel, When you think about a photon, what image 0:00:12.320 --> 0:00:16.079 comes to mind? Oh? Depends on what what you've been 0:00:16.079 --> 0:00:16.880 smoking that day? 0:00:18.520 --> 0:00:21.319 Yes, and also on the context. Are we talking about 0:00:21.400 --> 0:00:25.640 light from distant stars or rainbows or single photon lasers 0:00:25.720 --> 0:00:26.200 or what? 0:00:26.200 --> 0:00:28.840 What? Aren't they all the same? Like a photon is 0:00:28.840 --> 0:00:29.640 a photon, isn't it? 0:00:29.920 --> 0:00:32.600 Nobody really knows what a photon is. There's something weird 0:00:32.680 --> 0:00:35.320 and mysterious we might never fully understand. 0:00:35.800 --> 0:00:37.559 So you're just gonna leave us in the dark. You're 0:00:37.600 --> 0:00:38.879 not gonna shed any light on it. 0:00:39.000 --> 0:00:40.640 That's as bright as I can be on the topic. 0:00:56.160 --> 0:00:58.600 I am wore handmade cartoonists and the author of Oliver's 0:00:58.640 --> 0:00:59.680 Great Big Universe. 0:00:59.760 --> 0:01:02.400 Hi, I'm Daniel. I'm a particle physicist and a professor 0:01:02.400 --> 0:01:05.160 at UC Irvine, and I'm still hunting for a brilliant 0:01:05.280 --> 0:01:06.880 explanation about photons. 0:01:07.080 --> 0:01:09.760 I thought brilliance was your job description. Isn't it your 0:01:09.840 --> 0:01:12.360 job to provide that brilliance. 0:01:13.560 --> 0:01:15.720 My job is to hunt for the brilliance, to try 0:01:15.760 --> 0:01:19.759 to mine the truth from the firmament of reality. We 0:01:19.880 --> 0:01:20.800 don't always find it. 0:01:21.040 --> 0:01:23.959 M I guess it's hard to shine light on some 0:01:24.040 --> 0:01:26.120 of the corners of the universe that are hard to see. 0:01:26.800 --> 0:01:29.200 We just have to hope somebody out there is bright enough, 0:01:29.440 --> 0:01:32.480 somebody has a light bulb moment there. But what do 0:01:32.520 --> 0:01:35.360 you mean? Are you saying photons depend on where they 0:01:35.360 --> 0:01:35.880 come from? 0:01:36.040 --> 0:01:38.480 I'm saying that the language of physics we use to 0:01:38.560 --> 0:01:42.319 explain things uses as the basic mental building blocks. Things 0:01:42.360 --> 0:01:46.319 We do understand waves and bits of sand and tiny 0:01:46.360 --> 0:01:50.120 little particulate stuff, and none of those things really completely 0:01:50.160 --> 0:01:53.000 and fully describe the photon. It's those things, but also 0:01:53.120 --> 0:01:53.800 something else. 0:01:54.880 --> 0:01:57.280 I see. It's a language issue. Blame it on the 0:01:57.360 --> 0:02:01.000 linguists if we don't understand the universe. Yes, it's not 0:02:01.000 --> 0:02:01.960 the physicists fault. 0:02:02.040 --> 0:02:04.280 It also turns out to be fundamental to how we 0:02:04.360 --> 0:02:06.960 do science. We often tell different stories about the same 0:02:07.080 --> 0:02:09.960 kind of stuff depending on the question we are asking. 0:02:10.320 --> 0:02:13.200 None of our science is totally exact and complete. It's 0:02:13.200 --> 0:02:17.200 always approximate. And which approximation, which idea we use, which 0:02:17.200 --> 0:02:20.960 conceptualization is relevant, depends on the questions we're asking. 0:02:22.840 --> 0:02:26.320 Sounds like it's a big relativity problem because it's all relative. 0:02:27.240 --> 0:02:28.440 It's relatively complicated. 0:02:28.520 --> 0:02:33.239 Yeah, indeed, But anyways, welcome to our podcast. Daniel and 0:02:33.280 --> 0:02:36.920 Jorge Explain the Universe, a production of iHeartRadio. 0:02:36.400 --> 0:02:39.200 In which we take the whole universe as the context 0:02:39.240 --> 0:02:42.359 for our goal to understand things. We want to understand 0:02:42.360 --> 0:02:46.639 how droplets form into hurricanes, how tiny little quarks make protons, 0:02:46.919 --> 0:02:51.080 how enormous masses of stuff swirl into black holes. We 0:02:51.120 --> 0:02:53.400 want to answers for everything, and we hope one day 0:02:53.400 --> 0:02:57.440 to be able to stitch those answers together into a single, comprehensive, 0:02:57.520 --> 0:03:01.320 complete understanding of the universe, even though that might actually 0:03:01.360 --> 0:03:02.200 be impossible. 0:03:02.560 --> 0:03:05.760 Yeah, we try to track the journey of humanity from 0:03:05.760 --> 0:03:09.800 the shadows into the shining light of understanding and comprehension 0:03:09.840 --> 0:03:13.280 about this amazing universe we live in. Yeah, even if 0:03:13.280 --> 0:03:17.079 it sometimes takes a few stories or different stories along 0:03:17.120 --> 0:03:17.399 the way. 0:03:17.639 --> 0:03:20.600 The history of physics is seeing stuff we don't understand 0:03:20.600 --> 0:03:23.760 and then cobbling together some sort of mathematical explanation for 0:03:24.200 --> 0:03:27.400 what might be going on. And the bigger picture is 0:03:27.440 --> 0:03:30.120 to then try to weave those explanations together into a 0:03:30.160 --> 0:03:33.760 single coherent idea. But that task is still not finished, 0:03:33.760 --> 0:03:36.160 and it leaves us sometimes in an awkward situation of 0:03:36.520 --> 0:03:40.320 not being able to answer pretty basic questions about what's 0:03:40.360 --> 0:03:41.240 going on out there? 0:03:41.320 --> 0:03:43.600 Are you saying physicists can't get their story straight. It's 0:03:43.600 --> 0:03:44.520 a little suspicious. 0:03:45.120 --> 0:03:47.320 I'm saying the universe is a little bit lack Russiamon. 0:03:47.440 --> 0:03:50.520 You know, the story you tell depends on your context. 0:03:50.680 --> 0:03:53.520 But this is not something that only physicists do. You know, 0:03:53.560 --> 0:03:56.080 if I ask you how the baseball game went yesterday, 0:03:56.160 --> 0:03:58.240 you tell me a story about the teams and who 0:03:58.280 --> 0:04:00.520 was playing well and who was struggling. You put it 0:04:00.560 --> 0:04:03.000 in context to make it exciting. You don't just give 0:04:03.040 --> 0:04:05.200 me a dry list of what happened to every single 0:04:05.240 --> 0:04:07.480 particle in the vicinity of the stadium that day. 0:04:08.200 --> 0:04:09.920 But there'd just be one story about who won and 0:04:09.960 --> 0:04:10.480 who lost. 0:04:10.640 --> 0:04:13.080 If you think that's the story, right, Maybe the story 0:04:13.160 --> 0:04:15.520 is something else, the changing of the hot dogs, how 0:04:15.520 --> 0:04:18.840 the mustard now tastes, you know, the weather. Everybody might 0:04:18.880 --> 0:04:21.479 ask different questions about the same sets of events, and 0:04:21.520 --> 0:04:24.479 then they might need to use different physical concepts, even 0:04:24.480 --> 0:04:28.360 different mathematical formalisms to get those answers, which makes a 0:04:28.480 --> 0:04:31.080 very complicated answer very basic sounding questions. 0:04:31.200 --> 0:04:33.560 Right, right, Sometimes you need hot dog particles, sometimes you 0:04:33.600 --> 0:04:36.320 need baseball particles. 0:04:36.480 --> 0:04:38.560 Yeah, exactly. You can build a whole universe. On the 0:04:38.560 --> 0:04:41.400 hot dog theory, hot dogs are the fundamental component and 0:04:41.440 --> 0:04:42.839 what's inside them doesn't really. 0:04:42.640 --> 0:04:44.800 Matter, right, right, is it the hot dog on or 0:04:44.800 --> 0:04:45.679 the hot legino? 0:04:47.839 --> 0:04:50.320 The whole brilliance of hot dogs is just enjoying them 0:04:50.320 --> 0:04:52.080 and not even caring what they're made at it. 0:04:52.839 --> 0:04:55.240 What's the shape of a hot dog on? And how 0:04:55.279 --> 0:04:55.800 long is it? 0:04:57.080 --> 0:04:58.640 That depends on which city you're in, you. 0:04:58.600 --> 0:05:01.080 Know, yeah, yeah, or which country too. 0:05:01.480 --> 0:05:04.120 That's right. And your relative velocity, because some of these 0:05:04.120 --> 0:05:05.520 things can be length contracted. 0:05:05.640 --> 0:05:07.920 That's right. If you eat it fast, then it's a 0:05:08.000 --> 0:05:10.000 lot shorter than it is. 0:05:12.279 --> 0:05:14.599 If you're at high velocity relative to your hot dog, 0:05:14.680 --> 0:05:17.520 it will seem shorter. So, yeah, somebody shoots a hot 0:05:17.560 --> 0:05:19.800 dog into your mouth near the speed of light, then 0:05:20.600 --> 0:05:22.120 you're gonna have an interesting experience. 0:05:22.200 --> 0:05:25.480 But then it depends on which direction it is subndy, Right, 0:05:25.480 --> 0:05:28.880 if it's shutted on the side, it's still going to 0:05:28.920 --> 0:05:29.560 be the same length. 0:05:29.839 --> 0:05:31.640 Yeah, exactly, it's just going to be thinner. 0:05:31.760 --> 0:05:33.520 Yeah, let's just spend the rest of the episode talked 0:05:33.520 --> 0:05:35.440 about hot dog physics. 0:05:36.080 --> 0:05:39.839 High velocity hot dog physics, relativistic hot dog physics, A 0:05:39.880 --> 0:05:42.640 topic nobody has ever explored. We can be the first 0:05:42.720 --> 0:05:45.600 to write a paper in the Journal of Hot Dog Physics. 0:05:45.680 --> 0:05:47.440 Well, I think we're definitely the first to ever talk 0:05:47.440 --> 0:05:50.640 about it in a physics podcast. I'm thinking, I don't know. 0:05:50.960 --> 0:05:53.080 I haven't done the exhaustive literature search. 0:05:53.279 --> 0:05:55.279 Somebody out there let us know if we need to 0:05:55.279 --> 0:05:55.640 cite you. 0:05:56.120 --> 0:05:59.080 Yeah, somebody else do the research for us. But anyways, 0:05:59.120 --> 0:06:01.680 it is interesting to talk about how long things are, 0:06:01.960 --> 0:06:04.760 you know, basic questions like that about everyday objects we 0:06:04.880 --> 0:06:05.800 see every day. 0:06:06.040 --> 0:06:10.560 It is often really fruitful, but sometimes frustrating to ask intuitive, 0:06:10.839 --> 0:06:13.080 natural questions about the kind of things we think the 0:06:13.160 --> 0:06:15.640 universe is made out of. We think everything out there 0:06:15.640 --> 0:06:18.480 has certain properties, it has a size, a link, the mass, 0:06:18.680 --> 0:06:21.680 et cetera. And so we try to apply those concepts, 0:06:21.839 --> 0:06:24.159 these things we're familiar with from the kind of stuff 0:06:24.160 --> 0:06:27.120 we're used to interacting with, and apply that to quantum objects. 0:06:27.160 --> 0:06:28.960 But it doesn't always quite work. 0:06:29.200 --> 0:06:31.919 Yes, we've found out the quantum world is very straying, 0:06:32.040 --> 0:06:36.040 very mysterious, very uncertain, and very hard for our simple 0:06:36.160 --> 0:06:39.040 brain sometimes to understand and capture and to get an 0:06:39.040 --> 0:06:40.279 intuitive understanding of it. 0:06:40.400 --> 0:06:42.720 But that doesn't mean it's impossible, and that doesn't mean 0:06:42.760 --> 0:06:46.200 it's not useful. In fact, it's very helpful for shining 0:06:46.200 --> 0:06:48.560 a light onto what we do understand and what we 0:06:48.600 --> 0:06:51.359 don't understand, and it can help you make a better 0:06:51.440 --> 0:06:53.960 mental picture for what's going on at the quantum level. 0:06:54.400 --> 0:06:56.680 Right, But the question is can we shine a light 0:06:57.040 --> 0:06:59.640 on light itself? And so to be on the podcast 0:07:00.040 --> 0:07:09.640 jagging the question how long is a photon? Now it is? 0:07:09.760 --> 0:07:11.640 Is that a photon coming off of a hot dog? 0:07:11.760 --> 0:07:13.960 Or does it matter where it's bouncing off of. 0:07:14.640 --> 0:07:16.200 A hot dog? Colored photon? 0:07:16.280 --> 0:07:18.320 Wow? What is the color of a hot dog? What 0:07:18.480 --> 0:07:21.440 is the color of a hot dog? Sounds like the 0:07:21.480 --> 0:07:23.320 topic of a philosophy class here. 0:07:23.840 --> 0:07:25.840 If you eat your hot dog with eyes closed, does 0:07:25.880 --> 0:07:30.080 it have a color or not? I wonder if there's 0:07:30.080 --> 0:07:32.480 a paint shade out there that's called hot dog. 0:07:34.280 --> 0:07:37.960 I think people usually avoid having their runs painted hot dog. 0:07:39.120 --> 0:07:42.440 There's probably more adjectives to it, like bright, summer hot dog. 0:07:42.320 --> 0:07:47.880 Or something summer baseball hot dog, home run, the hot dog, 0:07:49.600 --> 0:07:55.120 hot dog, vapor, wild mountain hot dog. There's so many 0:07:55.200 --> 0:07:56.800 shades to a hot dog, isn't there? 0:07:57.960 --> 0:07:59.960 But we're not here to talk about hot dogs, though 0:08:00.040 --> 0:08:01.920 it seems like we're gonna We're here to try our 0:08:01.960 --> 0:08:04.800 best to answer a very simple but very hard question 0:08:05.280 --> 0:08:06.280 about the nature of light. 0:08:06.440 --> 0:08:09.080 Mmm, Now, how long it's a photon? Is that a 0:08:09.160 --> 0:08:12.160 question about its length or like how long it lasts? 0:08:12.240 --> 0:08:14.680 Oh? I interpreted it as a question about its length, 0:08:14.920 --> 0:08:18.240 like its physical extent. Photons can last forever, you know, 0:08:18.280 --> 0:08:21.320 their lifetime is potentially infinite. You shoot a photon into 0:08:21.360 --> 0:08:23.200 empty space, it'll just keep going forever. 0:08:23.640 --> 0:08:25.240 But you can kill a photon, can it? 0:08:25.480 --> 0:08:27.840 You can kill a photon, yes, absolutely, you can absorb it, 0:08:27.880 --> 0:08:30.240 you can interact with it. But a photon on its 0:08:30.240 --> 0:08:32.240 own will not like necessarily. 0:08:31.600 --> 0:08:34.600 Decay, can it ever, like, is there a possibility for 0:08:34.640 --> 0:08:37.760 it to, you know, have its energy convert into something else? 0:08:37.840 --> 0:08:40.920 Absolutely? A photon flying through space can just fly through space. 0:08:40.960 --> 0:08:43.679 But it can also turn into an electron and positron 0:08:43.720 --> 0:08:46.199 and then back into a photon, or into a muon 0:08:46.280 --> 0:08:48.280 or an anti muon and then back into a photon, 0:08:48.679 --> 0:08:50.719 or all sorts of other stuff. So there's lots of 0:08:50.800 --> 0:08:53.400 quantum possibilities constantly for photons. 0:08:53.440 --> 0:08:57.160 Can it turn into a hot dog technically? Like you know, 0:08:57.280 --> 0:09:00.480 in the infinity of infinities. Is there us light chance 0:09:00.480 --> 0:09:02.160 it can turn it suddenly into hot dog? 0:09:02.760 --> 0:09:05.920 Yes, there's a slight chance a very high energy photon 0:09:06.320 --> 0:09:10.120 could turn into a mutually charged hot dog momentarily. 0:09:10.240 --> 0:09:13.360 Hopefully it doesn't turn into hot dog inside your eye. 0:09:13.960 --> 0:09:15.800 And that tells me exactly what I want to paint 0:09:15.840 --> 0:09:18.319 my room next year, which is quantum hot dog. 0:09:18.640 --> 0:09:21.920 Oh boy, it's like it's different shades at the same. 0:09:21.760 --> 0:09:24.400 Time, exactly Shrewdinger's hot. 0:09:25.760 --> 0:09:29.160 It's like yellow mustard red ketchup. But it depends on 0:09:29.200 --> 0:09:31.319 how you look at it, kind of like the dress. Anyways, 0:09:31.320 --> 0:09:34.320 that's a very spicy idea. All right, let's talk about 0:09:34.640 --> 0:09:36.800 this question. But first we were wondering how many people 0:09:36.880 --> 0:09:39.440 out there had thought about the length of a photon, 0:09:39.640 --> 0:09:42.040 or even if photons have length. 0:09:42.320 --> 0:09:44.440 Thanks very much to everybody who answered this question. I 0:09:44.480 --> 0:09:47.640 only got one response online. So I walked around campus 0:09:47.640 --> 0:09:49.560 at you see Irvine last week and I asked a 0:09:49.600 --> 0:09:53.640 bunch of psych majors and other random people about photons. 0:09:53.880 --> 0:09:56.480 All right, Well, if you spot a physicist with a 0:09:56.520 --> 0:10:00.720 microphone on the ucroline campus, make sure to I don't know, 0:10:00.800 --> 0:10:04.040 runaway or approach if you think you can answer physics 0:10:04.120 --> 0:10:04.720 questions on the. 0:10:04.720 --> 0:10:07.680 Spot, or even if you don't, I love to hear 0:10:07.720 --> 0:10:08.240 your thoughts. 0:10:08.280 --> 0:10:10.320 All right, So think about it for a second. How 0:10:10.360 --> 0:10:13.920 long do you think a photon is? Here's what people 0:10:13.920 --> 0:10:14.360 had to say. 0:10:15.120 --> 0:10:15.600 I don't think. 0:10:15.640 --> 0:10:20.839 We don't know about that yet because of the mathematics 0:10:20.880 --> 0:10:24.839 going weird, since the photon is traveling in speed of 0:10:24.920 --> 0:10:25.199 the light. 0:10:26.120 --> 0:10:30.719 So that's my guess. Oh my gosh, I don't know. 0:10:30.840 --> 0:10:36.520 I'm gonna say and like, uh like ten to the 0:10:36.559 --> 0:10:41.800 power of negative twenty centimeters. 0:10:42.000 --> 0:10:46.120 Let's say, so I guess photons they don't have a 0:10:46.200 --> 0:10:50.240 mask or that have amentin right an, So it depends 0:10:50.280 --> 0:10:53.360 on the wave length light. 0:10:54.360 --> 0:10:56.280 No, I wouldn't even have like a guess of like 0:10:56.440 --> 0:11:00.360 length yeah a long in science, yeah. 0:11:00.040 --> 0:11:04.640 Phot I don't even know the pot I remember from biocam, okay, 0:11:05.040 --> 0:11:06.520 like maybe in chips. 0:11:07.600 --> 0:11:10.240 Yeah, I wouldn't even know I would photons. When I 0:11:10.280 --> 0:11:13.079 think of physics, I think get like small like particles. Yeah, 0:11:13.080 --> 0:11:16.240 and then I'm thinking like centimeters and like in minute 0:11:16.280 --> 0:11:19.200 signs two millimeters. 0:11:19.280 --> 0:11:19.800 I don't. 0:11:21.360 --> 0:11:23.520 I don't even know what a photon is. I major 0:11:23.559 --> 0:11:26.839 in a criminal justice, so completely outside of my major 0:11:27.679 --> 0:11:28.920 there's a photon, a particle light. 0:11:29.080 --> 0:11:30.520 Yes, how long is it? 0:11:31.200 --> 0:11:39.280 I'm gonna say point zeros are one light years. I 0:11:39.280 --> 0:11:42.360 don't know, like some ran like maybe like zero point 0:11:42.440 --> 0:11:43.440 one microns. 0:11:43.440 --> 0:11:43.800 I don't know. 0:11:44.559 --> 0:11:47.199 All right, some pretty good answers. Some of them are 0:11:47.280 --> 0:11:52.000 very specific. Zero point one microns ten to the power 0:11:52.040 --> 0:11:54.120 of negative twenty centimeters. 0:11:55.760 --> 0:11:58.839 There's a huge, huge range of answers here. I think 0:11:58.840 --> 0:12:01.840 the biggest one is probably zero points zero zero one 0:12:02.040 --> 0:12:04.720 light years. That turns out to be a very big number. 0:12:05.480 --> 0:12:08.400 Well, I'm impressed that they even stuck to the metric system. 0:12:08.440 --> 0:12:12.240 I mean, everyone nobody switched to inches or miles. 0:12:12.360 --> 0:12:14.520 You think photons are metric? I don't know. Yeah, I 0:12:14.520 --> 0:12:15.800 believe in imperial photons. 0:12:16.000 --> 0:12:19.880 I believe photons are king, but you know, I think 0:12:19.920 --> 0:12:22.200 they should stick to the more reasonable metric system. 0:12:22.520 --> 0:12:24.720 That's why Darth Vader is all black, because there are 0:12:24.720 --> 0:12:26.559 no imperial photons. 0:12:29.200 --> 0:12:35.920 Wow, reach there took me three seconds there wait? Oh 0:12:36.000 --> 0:12:40.000 imperial Yes, Yes, that was a very dark joke. I 0:12:40.040 --> 0:12:41.960 thought you were going to go with a lightsaber response. 0:12:42.080 --> 0:12:43.760 I totally set you up for that. What would be 0:12:43.800 --> 0:12:44.600 the lightsaber joke? 0:12:44.920 --> 0:12:47.440 Lightsabers only cut things in metric units. I don't know. 0:12:49.080 --> 0:12:51.840 Lightsabers are about a meter long. There you go, all right, 0:12:51.880 --> 0:12:55.040 well interesting azers. So Daniel to dig into it. For 0:12:55.120 --> 0:12:57.080 first of all, what is a photon? How do we 0:12:57.160 --> 0:13:00.920 define a photon? So, photon is like the minimum packet 0:13:01.000 --> 0:13:04.080 of light. If you take a really bright source of light, 0:13:04.160 --> 0:13:06.920 you might imagine it's just shooting out huge amounts of light. 0:13:07.120 --> 0:13:09.319 As you dial it down, it'll get dimmer and dimmer 0:13:09.360 --> 0:13:12.880 and dimmer, but it can't get infinitely dim. As you 0:13:12.960 --> 0:13:15.800 dial that light source down, eventually you'll notice that the 0:13:15.880 --> 0:13:18.240 light is actually discreete that it comes out in little 0:13:18.280 --> 0:13:21.640 packets rather than just being dimmer and dimmer waves. So 0:13:21.760 --> 0:13:25.440 photons are like the minimum unit of light. Wait are 0:13:25.480 --> 0:13:28.600 you saying that photons don't have a minimum energy. 0:13:28.640 --> 0:13:31.360 Photons do not have a minimum energy. That's true, but 0:13:31.440 --> 0:13:34.719 photons of a specific frequency have a fixed energy. And 0:13:34.760 --> 0:13:37.640 if you have, for example, a laser at his very 0:13:37.640 --> 0:13:40.199 specific wavelength and you dial it down so it's dimmer 0:13:40.200 --> 0:13:42.440 and dimmer and dimmer, eventually you're going to notice that 0:13:42.520 --> 0:13:45.040 beam gets broken up and it comes out in pieces. 0:13:46.000 --> 0:13:48.559 Like you lower the power to the laser, and eventually 0:13:48.840 --> 0:13:50.480 you'll see it go down steps. 0:13:50.760 --> 0:13:54.120 Yeah, exactly. It's just like everything else in our quantum world. 0:13:54.440 --> 0:13:57.720 Matter is not continuous. You can't zoom in forever on 0:13:57.800 --> 0:14:00.120 matter and have it always look the same way you 0:14:00.160 --> 0:14:02.280 zoom in on matter. You notice that it has a 0:14:02.320 --> 0:14:04.440 particular scale that at some point it breaks up into 0:14:04.440 --> 0:14:07.840 discrete bits out of which everything is built, just like 0:14:07.880 --> 0:14:11.560 the resolution on your screen. So light itself has a resolution. 0:14:11.679 --> 0:14:14.640 It's made out of these little quantum bits, these discrete 0:14:14.640 --> 0:14:18.400 building blocks. It's not perfectly smooth. And what is that 0:14:18.800 --> 0:14:21.400 smallest bit for light? It's a photon. That's what we 0:14:21.480 --> 0:14:24.040 call the photon. It's the smallest bit of light. 0:14:24.360 --> 0:14:26.400 Like if I if I'm shooting lighters are in frequency, 0:14:26.840 --> 0:14:28.960 the little steps that I see as I dial down 0:14:29.360 --> 0:14:31.880 the power to it, that's what you would call a photon. 0:14:32.040 --> 0:14:34.760 Exactly. Those are photons. And so if you have a 0:14:34.800 --> 0:14:37.400 bunch of light, you can always ask how many photons 0:14:37.440 --> 0:14:39.400 are there. There's a specific number. It has to be 0:14:39.440 --> 0:14:42.760 an integer number of photons. You don't usually notice this 0:14:42.800 --> 0:14:45.760 because the number of photons usually around hitting your eyeball 0:14:45.840 --> 0:14:48.760 is enormous. It doesn't really matter that they're countable. But 0:14:48.840 --> 0:14:51.040 as things get very very small. Then you can notice 0:14:51.200 --> 0:14:53.720 that you can have zero or one or two photons, 0:14:54.000 --> 0:14:55.960 you can't have one point seven photons. 0:14:56.320 --> 0:14:59.040 Now, how do we think about light? Is it like 0:14:59.400 --> 0:15:01.640 you say, it's like a packet, Like it's a discrete 0:15:01.960 --> 0:15:02.800 little object. 0:15:03.000 --> 0:15:04.480 Yeah, So this really gets at the heart of the 0:15:04.560 --> 0:15:07.960 question because how you describe this object helps you answer 0:15:07.960 --> 0:15:10.920 the question how big is it? And the answer is 0:15:10.960 --> 0:15:14.120 that we think about light in lots of different contradictory ways, 0:15:14.160 --> 0:15:17.640 depending on the context. Sometimes we think about light as 0:15:17.680 --> 0:15:20.000 like a tiny little object, but we think about it 0:15:20.040 --> 0:15:23.240 like a particle which has no extent, just like zero 0:15:23.400 --> 0:15:27.160 volume particle. Sometimes we ignore the quantum nature of it 0:15:27.200 --> 0:15:29.680 because it doesn't matter. We're thinking about really bright sources 0:15:29.720 --> 0:15:32.040 where the quantum nature is irrelevant, So we just think 0:15:32.080 --> 0:15:35.800 about it as classical waves of electromagnetism, the way people 0:15:35.800 --> 0:15:39.120 did two hundred years ago. And sometimes we think about 0:15:39.200 --> 0:15:42.640 light interacting with quantum particles, like light hitting an electron, 0:15:42.680 --> 0:15:44.960 and then we think about it as a little quantum 0:15:45.000 --> 0:15:48.600 packet and excitation in the electromagnetic field. So we have 0:15:48.680 --> 0:15:51.600 lots of different pictures of what a photon is, and 0:15:51.640 --> 0:15:53.760 the one that we use depends kind of on the question, 0:15:53.880 --> 0:15:54.800 we're asking. 0:15:54.800 --> 0:15:57.400 Well, so do you want to then tackle each one 0:15:57.440 --> 0:15:59.200 of these different ways to look at it at a time? 0:15:59.280 --> 0:16:01.520 Yeah? Sure. I think probably the most relevant in this 0:16:01.640 --> 0:16:04.520 case is the quantum field theory one the last one 0:16:04.520 --> 0:16:06.880 we talked about, But each one gives you a different answer. 0:16:07.320 --> 0:16:09.840 All right, well, then let's maybe tackle each one of these. 0:16:10.040 --> 0:16:13.160 But what does quantum field theory say about the nature 0:16:13.200 --> 0:16:13.480 of light? 0:16:13.720 --> 0:16:17.760 Quantum field theory is an updated version of classical field theory, 0:16:17.800 --> 0:16:20.760 which sounds fancy, but it just says light is a 0:16:20.800 --> 0:16:24.640 wave in the electromagnetic field. That's what Faraday and Maxwell 0:16:24.640 --> 0:16:26.880 and those guys figured out a couple of hundred years ago, 0:16:27.360 --> 0:16:30.120 that the universe is filled with this electromagnetic field and 0:16:30.160 --> 0:16:32.880 that waves in it are what we call light, and 0:16:32.960 --> 0:16:35.400 so you can shoot light from one planet to another, 0:16:35.520 --> 0:16:38.000 and the medium for that is the electromagnetic field. Even 0:16:38.040 --> 0:16:40.400 though space is empty, it has these fields in it. 0:16:40.760 --> 0:16:42.840 So light is a ripple in those fields. And we 0:16:42.880 --> 0:16:44.720 talk about that all the time on the podcast. And 0:16:44.720 --> 0:16:47.160 you have electric fields and magnetic fields and there are 0:16:47.280 --> 0:16:49.960 ninety degrees from each other and they're oscillating, and that's 0:16:50.000 --> 0:16:52.040 what light is. From a classical point of view. That's 0:16:52.080 --> 0:16:56.080 a traditional classical field theory. The quantum field theory version 0:16:56.120 --> 0:16:58.760 of that is the same. It just says that there's 0:16:58.800 --> 0:17:01.600 a minimum to how you can oscillate, so that as 0:17:01.640 --> 0:17:03.800 you turn it down you discover that you can't turn 0:17:03.840 --> 0:17:06.879 it to any intensity. There's certain steps. So the quantum 0:17:06.880 --> 0:17:09.359 field theory version says the universe is filled with this 0:17:09.440 --> 0:17:13.119 electromagnetic field which has certain steps in energy that it 0:17:13.160 --> 0:17:13.560 can take. 0:17:13.760 --> 0:17:15.680 Well, I guess, first of all, I wonder if listeners, 0:17:15.720 --> 0:17:17.840 as sometimes you get confused by this like I do, 0:17:17.960 --> 0:17:20.399 which is that you say light is a wave, but 0:17:20.440 --> 0:17:22.560 like if I think that's rippling through a field, But 0:17:22.600 --> 0:17:25.159 if I think of a wave like rippling through a 0:17:25.280 --> 0:17:29.159 lake or my bathtub, it's something that ripples outwards in 0:17:29.200 --> 0:17:31.480 all directions. Or if I think of it like a 0:17:31.520 --> 0:17:34.639 wave in the ocean, it's like this broad thing that's 0:17:35.440 --> 0:17:38.560 moving and undulating across kind of a wide area. But 0:17:38.760 --> 0:17:42.040 in terms of light, it's not that right. It's not 0:17:42.040 --> 0:17:44.800 spreading in all directions, and it's not broad like that. 0:17:45.080 --> 0:17:46.960 It can be though, I mean, think about a star. 0:17:47.600 --> 0:17:50.000 A star is emitting light, and it's emitting light in 0:17:50.040 --> 0:17:53.280 all directions, and before you think about the quantum nature 0:17:53.359 --> 0:17:55.560 of it, it is in fact spreading out. And that's 0:17:55.600 --> 0:17:58.680 why stars seem more dim the further you are away 0:17:58.680 --> 0:18:01.440 from them, right, because the intense see the light drops 0:18:01.480 --> 0:18:05.120 with the distance squared, and so you have light waves 0:18:05.119 --> 0:18:07.360 which start out very intense and then they spread out 0:18:07.400 --> 0:18:10.399 and so they get dimmer and dimmer. The quantum version 0:18:10.440 --> 0:18:12.679 of that is the same, except that now you have 0:18:12.760 --> 0:18:16.040 individual photons being sent out and close to the star 0:18:16.160 --> 0:18:18.320 you have a high intensity of those photons, and further 0:18:18.400 --> 0:18:20.840 away you have a smaller intensity of those photons. And 0:18:20.840 --> 0:18:24.320 you can understand why the intensity the photon drops as 0:18:24.320 --> 0:18:27.000 you get further away because the space they're feeling is 0:18:27.040 --> 0:18:29.760 getting bigger and bigger. And so if you have like 0:18:29.800 --> 0:18:32.240 the same size eyeball and you're gonna have fewer number 0:18:32.240 --> 0:18:35.040 of photons hit your eyeball when you're further away than 0:18:35.040 --> 0:18:36.919 when you are close up to the star. 0:18:37.160 --> 0:18:38.800 Right, you can sorder think about it that way. But 0:18:38.960 --> 0:18:40.960 I guess what do you call the photon? Then? Is 0:18:41.000 --> 0:18:45.080 the photon the ripple that's shooting in all directions or 0:18:45.880 --> 0:18:49.840 just if you catch that ripple in a particular spot. 0:18:49.920 --> 0:18:51.240 You know what I mean? Like you can imagine a 0:18:51.280 --> 0:18:55.040 star and it's rippling light out. Is a photon that 0:18:55.359 --> 0:18:58.199 a ring that emanates from the star or what? 0:18:58.440 --> 0:19:01.080 Yeah, great question. Say you slow the star down so 0:19:01.160 --> 0:19:04.560 it's only emitting one photon at a time somehow, right, 0:19:04.760 --> 0:19:07.520 like a single photon star. Basically, we're just putting a 0:19:07.560 --> 0:19:09.360 laser out there in space, but it's interesting to think 0:19:09.359 --> 0:19:12.240 about how it could go in any direction. So then 0:19:12.280 --> 0:19:14.879 any individual photon has the same probability to go in 0:19:14.920 --> 0:19:18.080 any direction from the star if it's totally symmetric. And 0:19:18.160 --> 0:19:22.000 so an individual photon has a ring of probability around 0:19:22.000 --> 0:19:24.520