WEBVTT - How long is a photon? 0:00:00.080 --> 0:00:02.960 If you love iPhone, you'll love Apple Card. It's the 0:00:03.040 --> 0:00:06.920 credit card designed for iPhone. It gives you unlimited daily 0:00:07.040 --> 0:00:10.000 cash back that can earn four point four zero percent 0:00:10.160 --> 0:00:12.960 annual percentage yield. When you open a high Yield savings 0:00:13.000 --> 0:00:16.280 account through Apple Card, apply for Applecard in the wallet 0:00:16.280 --> 0:00:19.520 app subject to credit approval. Savings is available to Apple 0:00:19.560 --> 0:00:22.800 Card owners subject to eligibility. Apple Card and Savings by 0:00:22.840 --> 0:00:26.120 Goldman Sachs Bank USA, Salt Lake City Branch, Member FDIC 0:00:26.520 --> 0:00:29.280 terms and more at applecard dot Com. When you pop 0:00:29.320 --> 0:00:31.240 a piece of cheese into your mouth, you're probably not 0:00:31.360 --> 0:00:34.280 thinking about the environmental impact. 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Become an Explorer and seek out unforgettable places 0:01:48.400 --> 0:01:51.840 while enjoying rewards everywhere you travel. Cards issued by JP 0:01:51.920 --> 0:01:55.920 Morgan Chase Bank NA Member FDIC subject to credit approval Offer, 0:01:56.000 --> 0:01:56.960 subject to change. 0:01:57.080 --> 0:02:08.760 Terms apply. Hey Daniel, when you think about a photon, 0:02:09.280 --> 0:02:11.080 what image comes to mind? 0:02:11.320 --> 0:02:11.760 Ooh? 0:02:11.840 --> 0:02:15.000 Depends on what what you've been smoking that day? 0:02:16.639 --> 0:02:19.440 Yes, and also on the context. Are we talking about 0:02:19.520 --> 0:02:23.760 light from distant stars or rainbows or single photon lasers 0:02:23.840 --> 0:02:24.280 or what? 0:02:24.280 --> 0:02:26.919 What aren't they all the same. Like a photon is 0:02:26.960 --> 0:02:27.760 a photon, isn't it. 0:02:28.040 --> 0:02:30.720 Nobody really knows what a photon is. There's something weird 0:02:30.760 --> 0:02:33.440 and mysterious we might never fully understand. 0:02:33.880 --> 0:02:35.680 So you're just gonna leave us in the dark. You're 0:02:35.680 --> 0:02:37.000 not gonna shed any light on it. 0:02:37.080 --> 0:02:38.760 That's as bright as I can be on the topic. 0:02:54.280 --> 0:02:56.679 Hi am horehad Mad cartoonists and the author of Oliver's 0:02:56.760 --> 0:02:57.800 Great Big Universe. 0:02:57.960 --> 0:03:00.640 Hi. I'm Daniel. I'm a particle physicist and professor at 0:03:00.680 --> 0:03:04.000 UC Irvine, and I'm still hunting for a brilliant explanation 0:03:04.160 --> 0:03:05.000 about photons. 0:03:05.200 --> 0:03:07.840 I thought brilliance was your job description. Isn't it your 0:03:07.919 --> 0:03:10.480 job to provide that brilliance. 0:03:11.680 --> 0:03:13.840 My job is to hunt for the brilliance, to try 0:03:13.880 --> 0:03:17.880 to mine the truth from the firmament of reality. We 0:03:18.000 --> 0:03:18.880 don't always find it. 0:03:19.840 --> 0:03:22.200 I guess it's hard to shine light on some of 0:03:22.240 --> 0:03:24.240 the corners of the universe that are hard to see. 0:03:24.919 --> 0:03:27.200 We just have to hope somebody out there is bright. 0:03:27.040 --> 0:03:30.560 Enough, somebody has a light bulb moment there. But what 0:03:30.560 --> 0:03:33.280 do you mean? Are you saying photons depend on where 0:03:33.320 --> 0:03:34.040 they come from? 0:03:34.160 --> 0:03:36.600 I'm saying that the language of physics we use to 0:03:36.680 --> 0:03:40.440 explain things uses as the basic mental building blocks. Things 0:03:40.480 --> 0:03:44.440 we do understand waves and bits of sand and tiny 0:03:44.480 --> 0:03:48.240 little particulate stuff, and none of those things really completely 0:03:48.280 --> 0:03:51.040 and fully describe the photon. It's those things, but also 0:03:51.240 --> 0:03:51.920 something else. 0:03:53.000 --> 0:03:55.400 I see. It's a language issue. Blame it on the 0:03:55.480 --> 0:03:59.200 linguists if we don't understand the universe. It's not the 0:03:59.240 --> 0:03:59.960 physicists fault. 0:04:00.160 --> 0:04:02.400 It also turns out to be fundamental to how we 0:04:02.480 --> 0:04:05.080 do science. We often tell different stories about the same 0:04:05.200 --> 0:04:08.080 kind of stuff depending on the question we are asking. 0:04:08.440 --> 0:04:11.280 None of our science is totally exact and complete. It's 0:04:11.320 --> 0:04:15.320 always approximate. And which approximation, which idea we use, which 0:04:15.320 --> 0:04:19.080 conceptualization is relevant, depends on the questions we're asking. 0:04:20.960 --> 0:04:24.440 Sounds like it's a big relativity problem because it's all relative. 0:04:25.360 --> 0:04:26.560 It's relatively complicated. 0:04:26.640 --> 0:04:31.360 Yeah, indeed, But anyways, welcome to our podcast, Daniel and 0:04:31.400 --> 0:04:34.680 Jorge Explain the Universe, a production of iHeartRadio, in. 0:04:34.640 --> 0:04:37.440 Which we take the whole universe as the context for 0:04:37.520 --> 0:04:40.680 our goal to understand things. We want to understand, how 0:04:40.800 --> 0:04:44.800 droplets form into hurricanes, how tiny little quarks make protons, 0:04:45.040 --> 0:04:49.159 how enormous masses of stuff swirl into black holes. We 0:04:49.200 --> 0:04:51.520 want to answers for everything, and we hope one day 0:04:51.520 --> 0:04:55.560 to be able to stitch those answers together into a single, comprehensive, 0:04:55.640 --> 0:04:59.440 complete understanding of the universe, even though that might actually 0:04:59.480 --> 0:05:00.279 be impop. 0:05:00.680 --> 0:05:03.880 Yeah, we try to track the journey of humanity from 0:05:03.880 --> 0:05:07.920 the shadows into the shining light of understanding and comprehension 0:05:07.960 --> 0:05:11.400 about this amazing universe we live in. Yeah, even if 0:05:11.400 --> 0:05:15.200 it sometimes takes a few stories or different stories along 0:05:15.240 --> 0:05:15.520 the way. 0:05:15.760 --> 0:05:18.720 The history of physics is seeing stuff we don't understand 0:05:18.720 --> 0:05:21.920 and then cobbling together some sort of mathematical explanation for 0:05:22.320 --> 0:05:25.520 what might be going on. And the bigger picture is 0:05:25.560 --> 0:05:28.240 to then try to weave those explanations together into a 0:05:28.279 --> 0:05:31.880 single coherent idea. But that task is still not finished, 0:05:31.880 --> 0:05:34.279 and it leaves us sometimes in an awkward situation of 0:05:34.640 --> 0:05:38.440 not being able to answer pretty basic questions about what's 0:05:38.480 --> 0:05:39.360 going on out there. 0:05:39.440 --> 0:05:41.720 Are you saying physicists can't get their story straight. It's 0:05:41.720 --> 0:05:42.640 a little suspicious. 0:05:43.240 --> 0:05:45.440 I'm saying the universe is a little bit lack Russiamon. 0:05:45.560 --> 0:05:48.640 You know, the story you tell depends on your context. 0:05:48.800 --> 0:05:51.640 But this is not something that only physicists. Do you know, 0:05:51.680 --> 0:05:54.200 if I ask you how the baseball game went yesterday, 0:05:54.279 --> 0:05:56.360 you tell me a story about the teams and who 0:05:56.400 --> 0:05:58.640 was playing well and who was struggling. You put it 0:05:58.680 --> 0:06:01.120 in context to make it excit. You don't just give 0:06:01.160 --> 0:06:03.320 me a dry list of what happened to every single 0:06:03.360 --> 0:06:05.599 particle in the vicinity of the stadium that day. 0:06:06.320 --> 0:06:08.039 But there'd just be one story about who won and 0:06:08.080 --> 0:06:08.559 who lost. 0:06:08.720 --> 0:06:11.200 If you think that's the story, right, Maybe the story 0:06:11.279 --> 0:06:13.640 is something else, the changing of the hot dogs, how 0:06:13.640 --> 0:06:16.960 the mustard now tastes, you know, the weather. Everybody might 0:06:17.000 --> 0:06:19.599 ask different questions about the same sets of events, and 0:06:19.640 --> 0:06:22.599 then they might need to use different physical concepts, even 0:06:22.600 --> 0:06:26.480 different mathematical formalisms to get those answers, which makes a 0:06:26.600 --> 0:06:29.200 very complicated answer very basic sounding questions. 0:06:29.320 --> 0:06:31.680 Right, right, Sometimes you need hot dog particles, sometimes you 0:06:31.720 --> 0:06:34.440 need baseball particles. 0:06:34.600 --> 0:06:36.680 Yeah, exactly. You can build a whole universe on the 0:06:36.680 --> 0:06:39.520 hot dog theory. Hot dogs are the fundamental component, and 0:06:39.560 --> 0:06:41.520 what's inside them doesn't really matter, right. 0:06:41.400 --> 0:06:43.760 Right, Is it the hot dog on or the hot logino. 0:06:45.960 --> 0:06:48.440 The whole brilliance of hot dogs is just enjoying them 0:06:48.440 --> 0:06:50.200 and not even caring what they're made at it. 0:06:50.960 --> 0:06:53.360 What's the shape of a hot dog on and how 0:06:53.400 --> 0:06:53.920 long is it? 0:06:55.200 --> 0:06:56.880 That depends on which city you're in, you know. 0:06:57.120 --> 0:06:59.880 Yeah, yeah, or which country too, that's. 0:06:59.800 --> 0:07:02.480 Right, and your relative velocity, because some of these things 0:07:02.480 --> 0:07:03.599 can be length contracted. 0:07:03.760 --> 0:07:06.040 That's right. If you eat it fast, then it's a 0:07:06.120 --> 0:07:08.120 lot shorter than it is. 0:07:10.360 --> 0:07:12.720 If you're at high velocity relative to your hot dog, 0:07:12.800 --> 0:07:15.640 it will seem shorter. So, yeah, somebody shoots a hot 0:07:15.680 --> 0:07:17.920 dog into your mouth you're the speed of light, then 0:07:18.720 --> 0:07:20.240 you're gonna have an interesting experience. 0:07:20.320 --> 0:07:23.560 But then it depends on which direction it is subbody, Right, 0:07:23.600 --> 0:07:26.880 if it's shut it on the side, it's still going 0:07:26.960 --> 0:07:27.920 to be the same length. 0:07:27.960 --> 0:07:29.720 Yeah, exactly, it's just going to be thinner. 0:07:29.880 --> 0:07:31.640 Yeah, let's just spend the rest of the episode talking 0:07:31.640 --> 0:07:33.320 about hot dog physics. 0:07:34.200 --> 0:07:37.960 High velocity hot dog physics, relativistic hot dog physics, a 0:07:38.000 --> 0:07:40.760 topic nobody has ever explored. We can be the first 0:07:40.840 --> 0:07:43.720 to write a paper in the Journal of hot dog Physics. 0:07:43.800 --> 0:07:45.520 Well, I think we're definitely the first to ever talk 0:07:45.560 --> 0:07:48.760 about it in a physics podcast. I'm thinking, I don't know, 0:07:49.080 --> 0:07:51.480 I haven't done the exhaustive literature search. 0:07:51.400 --> 0:07:53.400 Somebody out there let us know if we need to 0:07:53.400 --> 0:07:53.760 cite you. 0:07:54.200 --> 0:07:57.200 Yeah, somebody else do the research for us. But anyways, 0:07:57.240 --> 0:07:59.720 it is interesting to talk about how long things are, 0:08:00.120 --> 0:08:02.880 you know, basic questions like that about everyday objects we 0:08:02.960 --> 0:08:03.920 see every day. 0:08:04.160 --> 0:08:08.680 It is often really fruitful, but sometimes frustrating to ask intuitive, 0:08:08.960 --> 0:08:11.200 natural questions about the kind of things we think the 0:08:11.280 --> 0:08:13.760 universe is made out of. We think everything out there 0:08:13.760 --> 0:08:16.600 has certain properties, it has a size, a link, the mass, 0:08:16.800 --> 0:08:19.800 et cetera. And so we try to apply those concepts, 0:08:19.920 --> 0:08:22.280 these things we're familiar with from the kind of stuff 0:08:22.280 --> 0:08:25.240 we're used to interacting with, and apply that to quantum objects. 0:08:25.240 --> 0:08:27.080 But it doesn't always quite work. 0:08:27.320 --> 0:08:30.040 Yes, we've found out the quantum world is very straying, 0:08:30.160 --> 0:08:34.160 very mysterious, very uncertain, and very hard for our simple 0:08:34.280 --> 0:08:37.160 brain sometimes to understand and capture and to get an 0:08:37.160 --> 0:08:38.400 intuitive understanding of it. 0:08:38.520 --> 0:08:40.839 But that doesn't mean it's impossible, and that doesn't mean 0:08:40.880 --> 0:08:44.280 it's not useful. In fact, it's very helpful for shining 0:08:44.320 --> 0:08:46.680 a light onto what we do understand and what we 0:08:46.720 --> 0:08:49.480 don't understand, and it can help you make a better 0:08:49.559 --> 0:08:52.079 mental picture for what's going on at the quantum level. 0:08:52.520 --> 0:08:54.800 Right, But the question is can we shine a light 0:08:55.160 --> 0:08:57.840 on light itself? And so to the on the podcast, 0:08:57.840 --> 0:09:06.679 we'll be tackling the question how long is a photon? 0:09:07.480 --> 0:09:09.200 Now it is? Is that a photon coming off of 0:09:09.200 --> 0:09:11.640 a hot dog? Or does it matter where it's bouncing 0:09:11.640 --> 0:09:12.080 off of. 0:09:12.760 --> 0:09:15.400 A hot dog? Colored photon? Wow? What is the color 0:09:15.400 --> 0:09:16.000 of a hot dog? 0:09:16.360 --> 0:09:19.200 What is the color of a hot dog? Sounds like 0:09:19.440 --> 0:09:21.440 the topic of a philosophy class here. 0:09:21.960 --> 0:09:23.959 If you eat your hot dog with eyes closed, does 0:09:23.960 --> 0:09:28.200 it have a color or not? I wonder if there's 0:09:28.200 --> 0:09:30.600 a paint shade out there that's called hot dog. 0:09:32.400 --> 0:09:36.080 I think people usually avoid having the runs painted hot dog. 0:09:37.240 --> 0:09:40.480 There's probably more adjectives to it, like bright, summer hot dog. 0:09:40.440 --> 0:09:46.000 Or something summer baseball hot dog, home run, the hot dog, 0:09:47.720 --> 0:09:48.840 hot dog, vapor. 0:09:49.320 --> 0:09:50.680 Wild mountain hot dog. 0:09:52.720 --> 0:09:54.920 There's so many shades to a hot dog, isn't there? 0:09:56.080 --> 0:09:58.080 But we're not here to talk about hot dogs, though 0:09:58.080 --> 0:10:00.000 it seems like we're gonna. We're here to try our 0:10:00.080 --> 0:10:02.920 best to answer a very simple but very hard question 0:10:03.400 --> 0:10:04.400 about the nature of light. 0:10:04.559 --> 0:10:04.760 Mmm. 0:10:05.240 --> 0:10:07.559 Now, how long it's a photon? Is that a question 0:10:07.600 --> 0:10:10.240 about its length? Or like how long it lasts? 0:10:10.360 --> 0:10:12.800 Oh? I interpreted it as a question about its length, 0:10:13.040 --> 0:10:16.360 like its physical extent. Photons can last forever, you know, 0:10:16.400 --> 0:10:19.440 their lifetime is potentially infinite. You shoot a photon into 0:10:19.480 --> 0:10:21.320 empty space, it'll just keep going forever. 0:10:21.760 --> 0:10:24.040 But you can kill a photon, canjin You can. 0:10:24.000 --> 0:10:26.080 Kill a photon? Yes, absolutely, you can absorb it, you 0:10:26.080 --> 0:10:28.520 can interact with it. But a photon on its own 0:10:28.520 --> 0:10:30.520 will not like necessarily decay. 0:10:30.360 --> 0:10:32.720 Can it ever? Like isn't Is there a possibility for 0:10:32.760 --> 0:10:35.880 it to, you know, have its energy convert into something else? 0:10:35.960 --> 0:10:39.040 Absolutely, a photon flying through space can just fly through space, 0:10:39.080 --> 0:10:41.800 but it can also turn into an electron and positron 0:10:41.840 --> 0:10:44.319 and then back into a photon, or into a muon 0:10:44.400 --> 0:10:46.839 or an anti muon, and then back into a photon or 0:10:46.920 --> 0:10:49.319 all sorts of other stuff. So there's lots of quantum 0:10:49.320 --> 0:10:51.520 possibilities constantly for photons. 0:10:51.559 --> 0:10:55.280 Can it turn into a hot dog technically, like you know, 0:10:55.360 --> 0:10:58.600 in the infinity of infinities? Is there a slight chance 0:10:58.600 --> 0:11:01.520 it can turn it suddenly into hot Yes. 0:11:01.480 --> 0:11:04.640 There's a slight chance a very high energy photon could 0:11:04.679 --> 0:11:08.240 turn into a mutually charged hot dog momentarily. 0:11:08.360 --> 0:11:12.320 Hopefully it doesn't turn into hot dog inside your eye and. 0:11:12.240 --> 0:11:14.000 That tells me exactly what I want to paint my 0:11:14.080 --> 0:11:16.400 room next year, which is quantum hot dog. 0:11:16.760 --> 0:11:20.040 Oh boy, it's like it's different shades at the same. 0:11:19.880 --> 0:11:22.600 Time, exactly Shrewdinger's hot. 0:11:23.840 --> 0:11:27.240 It's like yellow mustard red ketchup. But it depends on 0:11:27.320 --> 0:11:29.440 how you look at it, kind of like the dress. Anyways, 0:11:29.440 --> 0:11:32.440 that's a very spicy idea. All right, let's talk about 0:11:32.760 --> 0:11:34.959 this question. But first we were wondering how many people 0:11:35.000 --> 0:11:37.559 out there had thought about the length of a photon, 0:11:37.720 --> 0:11:40.160 or even if photons have length. 0:11:40.440 --> 0:11:42.560 Thanks very much to everybody who answered this question. I 0:11:42.600 --> 0:11:45.760 only got one response online. So I walked around campus 0:11:45.760 --> 0:11:47.680 at U see Ermine last week and I asked a 0:11:47.720 --> 0:11:51.719 bunch of psych majors and other random people about photons. 0:11:52.000 --> 0:11:54.600 All right, well, if you spot a physicist with a 0:11:54.640 --> 0:11:58.840 microphone on the Ucroine campus, make sure to I don't know, 0:11:58.920 --> 0:12:02.160 runaway or approach if you think you can answer physics 0:12:02.240 --> 0:12:02.800 questions on. 0:12:02.760 --> 0:12:05.800 The spot, or even if you don't love to hear 0:12:05.840 --> 0:12:06.280 your thoughts. 0:12:06.400 --> 0:12:08.440 All right, so think about it for a second. How 0:12:08.480 --> 0:12:12.040 long do you think a photon is? Here's what people 0:12:12.040 --> 0:12:12.480 had to say. 0:12:13.240 --> 0:12:17.440 I don't think we don't know about that yet because 0:12:17.440 --> 0:12:22.400 of the mathematics going weird, since the photon is traveling 0:12:22.440 --> 0:12:23.319 in speed of the light. 0:12:24.240 --> 0:12:25.640 So that's my guess. 0:12:26.559 --> 0:12:28.800 Oh my gosh, I don't know. 0:12:28.960 --> 0:12:34.640 I'm gonna say and like, uh like ten to the 0:12:34.679 --> 0:12:39.920 power of negative twenty centimeters. 0:12:40.120 --> 0:12:43.000 Let's say, so I guess photons. 0:12:43.360 --> 0:12:47.600 They don't have a mask or that agment, right, So 0:12:47.720 --> 0:12:51.479 it depends on the wavelength light. 0:12:52.480 --> 0:12:55.079 No, I wouldn't even have like a guess of like length. 0:12:55.000 --> 0:12:58.760 Yeah, a long in science, Yeah, I don't know. 0:12:59.360 --> 0:13:01.520 I remember if from bokam. 0:13:01.080 --> 0:13:05.920 Okay, like maybe inches Okay. 0:13:05.720 --> 0:13:08.400 Yeah, I wouldn't even know I would photons. When I 0:13:08.400 --> 0:13:11.160 think of physics, I think it's like small like particles, yes, 0:13:11.200 --> 0:13:14.360 and then I'm thinking like centimeters and like in minute 0:13:14.400 --> 0:13:19.000 signs two millimeters. 0:13:19.480 --> 0:13:21.280 I don't even know what a photon is, okay. 0:13:21.280 --> 0:13:24.520 I major in a criminal justice, so completely outside of 0:13:24.520 --> 0:13:24.959 my major. 0:13:25.800 --> 0:13:28.719 There's a photon, a particle light? Yes, how long is it? 0:13:29.280 --> 0:13:37.360 I'm gonna say point zero zero one light years. I 0:13:37.400 --> 0:13:40.520 don't know, like some random like maybe like zero point 0:13:40.559 --> 0:13:44.240 one microns. I don't know, all right, some pretty good answers, 0:13:44.760 --> 0:13:48.080 some of them are very specific. Zero point one microns 0:13:49.320 --> 0:13:52.240 ten to the power of negative twenty centimeters. 0:13:53.880 --> 0:13:56.959 There's a huge, huge range of answers here. I think 0:13:56.960 --> 0:14:00.000 the biggest one is probably zero point zero zero one 0:14:00.160 --> 0:14:02.840 light years. That turns out to be a very big number. 0:14:03.600 --> 0:14:06.520 Well, I'm impressed that they even stuck to the metric system. 0:14:06.559 --> 0:14:10.360 I mean, everyone nobody switched to inches or miles. 0:14:10.480 --> 0:14:12.640 You think photons are metric? I don't know. Yeah, I 0:14:12.640 --> 0:14:13.920 believe in imperial photons. 0:14:14.120 --> 0:14:18.000 I believe photons are king, but you know, I think 0:14:18.000 --> 0:14:20.320 they should stick to the more reasonable metric system. 0:14:20.600 --> 0:14:22.840 That's why Darth Vader is all black, because there are 0:14:22.840 --> 0:14:25.840 no Imperial photons. 0:14:27.320 --> 0:14:36.920 Wow, there took me three seconds there wait oh imperial. Yes, yes, 0:14:37.000 --> 0:14:37.960 that was a very dark joke. 0:14:38.080 --> 0:14:40.080 I thought you were going to go with a lightsaber response. 0:14:40.200 --> 0:14:41.400 I totally set you up for that. 0:14:41.440 --> 0:14:42.720 What would be the lightsaber joke? 0:14:43.040 --> 0:14:45.560 Lightsabers only cut things in metric units. I don't know. 0:14:47.200 --> 0:14:50.160 Lightsabers are about a meter long. There you go, all right, Well, 0:14:50.160 --> 0:14:53.720 interesting azers. So Daniels dig into it. For first of all, 0:14:53.720 --> 0:14:56.800 what is a photon? How do we define a photon? So, 0:14:56.840 --> 0:15:00.400 a photon is like the minimum packet of light. If 0:15:00.440 --> 0:15:02.560 you take a really bright source of light. You might 0:15:02.600 --> 0:15:05.400 imagine it's just shooting out huge amounts of light. As 0:15:05.440 --> 0:15:08.040 you dial it down, it'll get dimmer and dimmer and dimmer, 0:15:08.200 --> 0:15:11.480 but it can't get infinitely dim. As you dial that 0:15:11.600 --> 0:15:14.360 light source down, eventually you'll notice that the light is 0:15:14.400 --> 0:15:17.240 actually discreete that it comes out in little packets rather 0:15:17.280 --> 0:15:20.640 than just being dimmer and dimmer waves. So photons are 0:15:20.680 --> 0:15:24.040 like the minimum unit of light. Wait, are you saying 0:15:24.080 --> 0:15:26.720 that photons don't have a minimum energy. 0:15:26.760 --> 0:15:29.480 Photons do not have a minimum energy. That's true, but 0:15:29.520 --> 0:15:32.840 photons of a specific frequency have a fixed energy. And 0:15:32.880 --> 0:15:35.760 if you have, for example, a laser at his very 0:15:35.760 --> 0:15:38.320 specific wavelength, and you dial it down so it's dimmer 0:15:38.320 --> 0:15:40.560 and dimmer and dimmer, eventually you're going to notice that 0:15:40.640 --> 0:15:43.600 beam gets broken up and it comes out in pieces. 0:15:44.120 --> 0:15:46.680 Like you lower the power to the laser, and eventually 0:15:46.960 --> 0:15:48.600 you'll see it go down steps. 0:15:48.880 --> 0:15:52.240 Yeah, exactly. It's just like everything else in our quantum world. 0:15:52.560 --> 0:15:55.840 Matter is not continuous. You can't zoom in forever on 0:15:55.920 --> 0:15:58.120 matter and have it always look the same way. As 0:15:58.160 --> 0:16:00.320 you zoom in on matter, you notice that it has 0:16:00.320 --> 0:16:02.320 a particular scale that at some point it breaks up 0:16:02.320 --> 0:16:05.840 into discrete bits out of which everything is built, just 0:16:05.880 --> 0:16:09.080 like the resolution on your screen. So light itself has 0:16:09.120 --> 0:16:11.960 a resolution. It's made out of these little quantum bits, 0:16:12.040 --> 0:16:16.080 these discrete building blocks. It's not perfectly smooth. And what 0:16:16.240 --> 0:16:19.280 is that smallest bit for light? It's a photon. That's 0:16:19.320 --> 0:16:21.960 what we call the photon. It's the smallest bit of light. 0:16:22.480 --> 0:16:24.520 Like if I if I'm shooting lighters are in frequency, 0:16:24.960 --> 0:16:27.080 the little steps that I see as I dial down 0:16:27.440 --> 0:16:30.000 the power to it, that's what you would call a photon. 0:16:30.160 --> 0:16:32.880 Exactly. Those are photons. And so if you have a 0:16:32.880 --> 0:16:35.520 bunch of light, you can always ask how many photons 0:16:35.560 --> 0:16:37.520 are there. There's a specific number. It has to be 0:16:37.560 --> 0:16:40.840 an integer number of photons. You don't usually notice this 0:16:40.920 --> 0:16:43.880 because the number of photons usually around hitting your eyeball 0:16:43.920 --> 0:16:46.880 is enormous. It doesn't really matter that they're countable. But 0:16:46.960 --> 0:16:49.160 as things get very very small, then you can notice 0:16:49.320 --> 0:16:51.840 that you can have zero or one or two photons. 0:16:52.120 --> 0:16:54.080 You can't have one point seven photons. 0:16:54.440 --> 0:16:57.120 Now, how do we think about light? Is it like 0:16:57.560 --> 0:16:59.720 you say, it's like a packet, Like it's a discrete 0:17:00.040 --> 0:17:00.920 a little object. 0:17:01.120 --> 0:17:03.000 Yeah, So this really gets at the heart of the question, 0:17:03.360 --> 0:17:06.119 because how you describe this object helps you answer the 0:17:06.200 --> 0:17:09.119 question how big is it? And the answer is that 0:17:09.160 --> 0:17:12.240 we think about light in lots of different contradictory ways, 0:17:12.280 --> 0:17:15.760 depending on the context. Sometimes we think about light as 0:17:15.800 --> 0:17:18.119 like a tiny little object, but we think about it 0:17:18.160 --> 0:17:21.280 like a particle which has no extent, just like zero 0:17:21.520 --> 0:17:25.280 volume particle. Sometimes we ignore the quantum nature of it 0:17:25.320 --> 0:17:27.840 because it doesn't matter. We're thinking about really bright sources 0:17:27.840 --> 0:17:30.160 where the quantum nature is irrelevant, so we just think 0:17:30.200 --> 0:17:33.920 about it as classical waves of electromagnetism, the way people 0:17:33.920 --> 0:17:37.200 did two hundred years ago. And sometimes we think about 0:17:37.320 --> 0:17:40.720 light interacting with quantum particles like light hitting an electron, 0:17:40.800 --> 0:17:43.080 and then we think about it as a little quantum 0:17:43.119 --> 0:17:46.720 packet and excitation in the electromagnetic field. So we have 0:17:46.760 --> 0:17:49.719 lots of different pictures of what a photon is, and 0:17:49.760 --> 0:17:51.520 the one that we use depends kind of on the 0:17:51.600 --> 0:17:52.320 question we're. 0:17:52.160 --> 0:17:55.360 Asking, well, so do you want to then tackle each 0:17:55.400 --> 0:17:57.000 one of these different ways to look at it at 0:17:57.000 --> 0:17:57.320 a time? 0:17:57.400 --> 0:17:59.600 Yeah, sure, I think probably the most relevant in this 0:17:59.720 --> 0:18:02.640 case is the quantum field theory one the last one 0:18:02.640 --> 0:18:05.000 we talked about, But each one gives you a different answer. 0:18:05.440 --> 0:18:07.960 All right, well, then let's maybe tackle each one of these. 0:18:08.320 --> 0:18:11.600 What does quantum field theory say about the nature of light. 0:18:11.840 --> 0:18:15.920 Quantum field theory is an updated version of classical field theory, 0:18:15.920 --> 0:18:18.879 which sounds fancy, but it just says light is a 0:18:18.920 --> 0:18:22.720 wave in the electromagnetic field. That's what Faraday and Maxwell 0:18:22.760 --> 0:18:25.000 and those guys figured out a couple of hundred years ago. 0:18:25.440 --> 0:18:28.239 That the universe is filled with this electromagnetic field, and 0:18:28.280 --> 0:18:31.000 that waves in it are what we call light, and 0:18:31.080 --> 0:18:33.520 so you can shoot light from one planet to another, 0:18:33.600 --> 0:18:36.120 and the medium for that is the electromagnetic field. Even 0:18:36.160 --> 0:18:38.520 though space is empty, it has these fields in it. 0:18:38.840 --> 0:18:40.960 So light is a ripple in those fields. And we 0:18:41.000 --> 0:18:42.800 talk about that all the time on the podcast. And 0:18:42.840 --> 0:18:45.280 you have electric fields and magnetic fields and there are 0:18:45.400 --> 0:18:48.080 ninety degrees from each other, and they're oscillating, and that's 0:18:48.119 --> 0:18:50.159 what light is. From a classical point of view, that's 0:18:50.200 --> 0:18:54.200 a traditional classical field theory. The quantum field theory version 0:18:54.240 --> 0:18:56.880 of that is the same. It just says that there's 0:18:56.920 --> 0:18:59.480 a minimum to how much you can oscillate, so that 0:18:59.520 --> 0:19:01.680 as you turn it down you discover that you can't 0:19:01.760 --> 0:19:04.639 turn it to any intensity there's certain steps. So the 0:19:04.720 --> 0:19:07.359 quantum field theory version says the universe is filled with 0:19:07.400 --> 0:19:11.119 this electromagnetic field which has certain steps in energy that 0:19:11.160 --> 0:19:11.680 it can take. 0:19:11.840 --> 0:19:14.000 Well, I guess, first of all, I wonder if listeners 0:19:14.000 --> 0:19:16.280 sometimes they get confused by this like I do, which 0:19:16.320 --> 0:19:18.639 is that you say light is a wave, but like 0:19:18.720 --> 0:19:20.840 if I think that's rippling through a field, But if 0:19:20.840 --> 0:19:23.640 I think of a wave like rippling through a lake 0:19:24.000 --> 0:19:28.199 or my bathtub, it's something that ripples outwards in all directions, 0:19:28.600 --> 0:19:29.920 Or if I think of it like a wave in 0:19:29.960 --> 0:19:34.080 the ocean, it's like this broad thing that's moving and 0:19:34.200 --> 0:19:37.880 undulating across kind of a wide area. But in terms 0:19:37.920 --> 0:19:40.600