WEBVTT - Why are some things transparent? 0:00:08.520 --> 0:00:11.680 Hey, or hey, have you ever seen a glass frog? 0:00:12.360 --> 0:00:12.559 Mmm? 0:00:12.920 --> 0:00:15.680 You mean one of those transparent amphibians or is it 0:00:15.760 --> 0:00:17.759 like a it'll keep saking me ou out of class? 0:00:18.160 --> 0:00:21.000 I mean they're a real live frog. I was reading 0:00:21.000 --> 0:00:24.240 that they are native to Central America, including Panama, where 0:00:24.320 --> 0:00:25.400 you grew up. Yeah. 0:00:25.440 --> 0:00:27.680 I have heard that, but to be honest, I haven't 0:00:27.720 --> 0:00:29.960 seen one in person. You could probably find them in 0:00:30.000 --> 0:00:33.040 the jungle, but it's not like they're jumping around my house. 0:00:33.320 --> 0:00:35.159 Well did you ever wonder what it would be like 0:00:35.280 --> 0:00:37.040 to be transparent yourself? 0:00:37.400 --> 0:00:41.040 Sounds terrible? I guess you want people to see, right. 0:00:42.360 --> 0:00:43.720 Everyone wants to be seen these. 0:00:43.680 --> 0:00:45.760 Days, unless you want to sneak around the house and 0:00:45.760 --> 0:00:46.360 be invisible. 0:00:46.440 --> 0:00:48.920 I guess if you have the option of turning transparent, 0:00:49.159 --> 0:00:52.000 that's cool. Like, I think all kids dream of being 0:00:52.040 --> 0:00:52.960 invisible at some point. 0:00:53.200 --> 0:00:55.520 I always wondered what happens if you're invisible and you 0:00:55.560 --> 0:00:58.000 take a bite of an apple, Like, can everybody see 0:00:58.000 --> 0:01:00.080 that apple work its way through you? H? 0:01:00.600 --> 0:01:03.160 I guess it depends on you, know, like how the 0:01:03.200 --> 0:01:07.080 invisibility works, Like are you a space that is transparent 0:01:07.160 --> 0:01:09.400 or is it just your molecules are transparent? Or you 0:01:09.400 --> 0:01:11.320 can just eat a cookie mid out of glass frogs 0:01:11.640 --> 0:01:14.800 or eat glass. I guess that sounds less tasty. 0:01:14.880 --> 0:01:16.160 I don't think I want to take a bite out 0:01:16.160 --> 0:01:16.360 of that. 0:01:16.680 --> 0:01:34.280 Yeah, I see you, man, I see you. I am Rhem, 0:01:34.360 --> 0:01:36.720 a cartoonist and the creator of PhD comics. 0:01:36.800 --> 0:01:39.720 Hi, I'm Daniel. I'm a particle physicist and a professor. 0:01:39.880 --> 0:01:42.440 You see Irvine, And even though you can only hear 0:01:42.520 --> 0:01:44.360 my voice, I feel seen. 0:01:46.200 --> 0:01:49.120 Well, that's good. Everyone wants to be seen. I think 0:01:49.320 --> 0:01:51.440 mostly we just feel heard in this podcast. 0:01:51.520 --> 0:01:54.040 That's right. We feel heard, not hurt, because we love 0:01:54.080 --> 0:01:57.280 sharing with you our passion and curiosity about the universe. 0:01:57.440 --> 0:02:00.120 And when I hear back from listeners that's something we 0:02:00.160 --> 0:02:03.080 have said has touched on their deep seated need to 0:02:03.200 --> 0:02:06.080 understand the universe. I do feel heard and seen. 0:02:06.400 --> 0:02:09.160 Yeah, because it is a pretty amazing universe with lots 0:02:09.200 --> 0:02:11.440 to see out there and to hear. I guess if 0:02:11.440 --> 0:02:14.200 you have the right kinds of ears. Because sound doesn't 0:02:14.200 --> 0:02:15.679 travel in space, does it? 0:02:15.680 --> 0:02:18.240 It actually does, just very very slowly. 0:02:19.040 --> 0:02:21.400 Why are you kidding or are you serious? 0:02:21.600 --> 0:02:21.720 No? 0:02:21.880 --> 0:02:24.800 Space is not totally empty, and so in principle, there 0:02:24.880 --> 0:02:27.880 are sound waves that do propagate through the solar wind, 0:02:28.000 --> 0:02:30.519 for example. But in practice, if you're in outer space, 0:02:30.560 --> 0:02:32.320 you're going to freeze before you hear anything. 0:02:32.560 --> 0:02:34.560 Also, you would be hearing it in slow motion, I 0:02:34.560 --> 0:02:37.799 guess right, if sound moves slower, Like, don't go out 0:02:37.840 --> 0:02:39.799 in space too late, you're. 0:02:39.680 --> 0:02:43.959 Dead, that's right. You should shout at your friends in space, 0:02:44.120 --> 0:02:47.120 like a year before they go space walking without a 0:02:47.160 --> 0:02:48.280 sud on so they can hear you. 0:02:49.000 --> 0:02:53.079 Yeah, oh, that's not going to help them. Also, how 0:02:53.080 --> 0:02:54.520 do you shout in space? Like you have to take 0:02:54.560 --> 0:02:57.239 off your helmet to shout, and then that's not good. 0:02:57.280 --> 0:03:00.800 Either in space nobody can hear you. Or you shouldn't 0:03:00.800 --> 0:03:02.000 be hurt or you might get hurt. 0:03:02.080 --> 0:03:04.799 But you should turn your eyeballs up to the night 0:03:04.840 --> 0:03:07.360 sky and wonder how everything works. And you should also 0:03:07.440 --> 0:03:09.799 look down on the ground beneath you and see if 0:03:09.800 --> 0:03:13.160 you can puzzle out the mysteries of everyday objects, the 0:03:13.240 --> 0:03:15.920 nature of our universe, how everything works, how it weaves 0:03:15.919 --> 0:03:19.040 itself together to make this incredible cosmos. That's one of 0:03:19.040 --> 0:03:21.680 the deep mysteries that humans want to unravel. And those 0:03:21.680 --> 0:03:23.880 are the topics we like to take a part on 0:03:23.919 --> 0:03:24.680 this podcast. 0:03:24.880 --> 0:03:27.119 Yeah, and It's amazing and lucky that we can see 0:03:27.120 --> 0:03:29.720 so many things out there in the universe and around us, 0:03:29.760 --> 0:03:31.440 so that we know where they are and we can 0:03:31.480 --> 0:03:33.880 also study them and figure out how they work, what 0:03:33.919 --> 0:03:36.640 they're made out of, and what the rules of this 0:03:36.840 --> 0:03:40.240 crazy universe are. But anyways, welcome to our podcast, Daniel 0:03:40.280 --> 0:03:44.240 and Jorge Explain the Universe, a production of iHeartRadio. 0:03:43.600 --> 0:03:45.840 In which we try to do exactly that, take apart 0:03:45.880 --> 0:03:48.960 the whole universe, see how it works, and explain it 0:03:49.160 --> 0:03:51.600 to you. Some of the questions we love to tackle 0:03:51.680 --> 0:03:54.440 involve the tiniest things in the universe, what the rules 0:03:54.480 --> 0:03:56.880 are for how they work, or the biggest things in 0:03:56.880 --> 0:04:00.000 the universe, like the whole universe itself or super massive 0:04:00.080 --> 0:04:03.040 black holes at the hearts of galaxies. But there's also 0:04:03.160 --> 0:04:06.640 a lot of fascinating physics in between. How those tiny 0:04:06.680 --> 0:04:09.440 little objects pull themselves together to behave in weird and 0:04:09.520 --> 0:04:12.800 wonderful ways. The reason ice cream is so tasty, the 0:04:12.840 --> 0:04:16.320 reason metals conduct electricity, the reason your chair holds you 0:04:16.400 --> 0:04:19.320 up is because of zillions of atoms all working together 0:04:19.760 --> 0:04:21.440 to have fascinating phenomena. 0:04:21.640 --> 0:04:24.839 Yeah, and thankfully we can see them all and hear 0:04:24.880 --> 0:04:27.080 them all so we can study them. It'd be kind 0:04:27.120 --> 0:04:29.480 of hard to know and study the universe if everything 0:04:29.520 --> 0:04:31.880 was invisible, right, or if we didn't have eyeballs. 0:04:31.880 --> 0:04:34.760 In years, that's true, and recently we've discovered the amazing 0:04:34.800 --> 0:04:38.120 fact that most of the universe is invisible. The dark 0:04:38.160 --> 0:04:40.839 matter that's out there is most of the stuff in 0:04:40.920 --> 0:04:44.000 the universe, and we didn't even know it existed until recently. 0:04:44.120 --> 0:04:47.960 Because it's invisible to us. Life passes right through it, 0:04:48.279 --> 0:04:50.200 and there's lots of things going on out there in 0:04:50.200 --> 0:04:53.279 the universe that you just cannot see because your eyeballs 0:04:53.320 --> 0:04:55.640 can't pick them up, you can't taste them, you can't 0:04:55.640 --> 0:04:59.000 smell them. So most of the universe is actually invisible. 0:04:59.040 --> 0:05:01.719 Transparency turns out to be the name of the game. 0:05:02.000 --> 0:05:04.919 So the universe got its kid wish of being invisible. 0:05:05.000 --> 0:05:07.200 Is that what you're saying? What's it trying to sneak 0:05:07.200 --> 0:05:07.640 around and do? 0:05:07.800 --> 0:05:09.479 I don't know, but those dark matter kids are probably 0:05:09.480 --> 0:05:14.280 eating dark matter cookies all night long, and. 0:05:14.200 --> 0:05:17.200 They're made out of dark chocolate and taste better too. 0:05:17.520 --> 0:05:20.680 And my new wish used to be a dark matter person. 0:05:21.440 --> 0:05:24.320 They get to eat dark meat and their chicken. 0:05:24.240 --> 0:05:26.320 Well, It also answers that other question. You know what 0:05:26.360 --> 0:05:29.160 happens when a dark matter kid eats a dark matter cookie. 0:05:29.279 --> 0:05:31.960 Of course it goes through them and becomes dark matter 0:05:32.000 --> 0:05:32.599 on the way out. 0:05:32.760 --> 0:05:37.840 Darker matter, you mean, the darkest matter, because a black hole. 0:05:38.720 --> 0:05:40.040 That's where black holes come from. 0:05:40.120 --> 0:05:43.040 That's exactly right. Black holes are dark matter toilets. 0:05:43.480 --> 0:05:46.720 They're are worse through the things that go into the toilet. 0:05:47.400 --> 0:05:49.720 Here we are elevating the discourse of the nature of 0:05:49.720 --> 0:05:50.279 the universe. 0:05:50.360 --> 0:05:52.560 Yeah, I mean, listeners want to be reached into their 0:05:52.560 --> 0:05:54.800 soul and touched right and seen and hurt. 0:05:56.040 --> 0:05:58.120 I'm not sure they want to have this particular taste. 0:05:58.200 --> 0:06:02.039 However, Well, who said anything about tasting. 0:06:02.160 --> 0:06:05.000 We're talking about eating dark matter cookies. You're fantasizing about 0:06:05.080 --> 0:06:07.240 dark matter chocolate over there. It's all about taste. 0:06:07.400 --> 0:06:08.800 That's why we're all about taste here. 0:06:08.960 --> 0:06:10.360 Good taste, bad taste, you. 0:06:10.360 --> 0:06:14.359 Decide, that's right. We're we're scientists. We explore the full 0:06:14.440 --> 0:06:17.800 range of tastes available to the human experience. But it 0:06:17.880 --> 0:06:20.359 is interesting that even the things that are invisible, we 0:06:20.440 --> 0:06:23.479 can still somehow see them through other things. Right, I mean, 0:06:23.600 --> 0:06:27.080 if dark matter, dark energy was completely invisible. We wouldn't 0:06:27.120 --> 0:06:29.440 even know it was there, but somehow it has an 0:06:29.480 --> 0:06:31.800 influence on things that we can see, thankfully, and that's 0:06:31.800 --> 0:06:32.599 how we know they're there. 0:06:32.760 --> 0:06:35.880 Yes, transparency turns out to be quite a subtle issue. 0:06:36.120 --> 0:06:38.719 Some things can be transparent to one kind of light 0:06:38.760 --> 0:06:41.040 but not to another. Some kinds of light can pass 0:06:41.080 --> 0:06:44.160 through some objects but not other objects. As always, when 0:06:44.160 --> 0:06:47.000 you dig into it, you discover there's a fascinating physics 0:06:47.000 --> 0:06:49.120 story that underlines how things work. 0:06:49.360 --> 0:06:51.960 Yep, this is an interesting topic and so today on 0:06:52.000 --> 0:07:01.119 the podcast we'll be tackling the question how does transpy work? 0:07:01.400 --> 0:07:04.320 Some titled how can your kids eat cookies without you knowing? 0:07:04.560 --> 0:07:06.359 Well, I don't think they need to be transparent to 0:07:06.400 --> 0:07:10.880 do that, depending how sneaky they are and how how 0:07:10.920 --> 0:07:13.240 early are you going to sleep or wake up? Now, 0:07:13.280 --> 0:07:16.440 this is an interesting question, Daniel. I imagine it means 0:07:16.480 --> 0:07:19.760 what makes things see through? Not like how do transparencies 0:07:19.760 --> 0:07:23.320 work or how does government transparency work? Because that apparently 0:07:23.400 --> 0:07:26.880 doesn't work that well or sometimes it works too well. 0:07:26.920 --> 0:07:30.120 That's right. We want sunlight on all the operations of 0:07:30.240 --> 0:07:33.480 the universe, but in particular here we're wondering about why 0:07:33.600 --> 0:07:36.760 light can pass through some kinds of things. Why you 0:07:36.800 --> 0:07:39.520 can see through glass but you can't see through stone. 0:07:40.120 --> 0:07:43.640 Why X rays reveal your soft tissues but not your bones. 0:07:43.680 --> 0:07:47.240 What is the underlying physics? What is the microscopic picture 0:07:47.360 --> 0:07:48.400 of transparency. 0:07:49.000 --> 0:07:51.640 We're exploring the full range of transparent behavior. 0:07:51.720 --> 0:07:53.840 We're just trying to be transparent about how the universe works. 0:07:53.880 --> 0:07:56.200 We're trying to be transparent about how the podcast works, 0:07:56.320 --> 0:07:59.720 is what I'm saying. And to be fully transparent, I 0:07:59.720 --> 0:08:03.400 have not I've read the outlete for today's episode. I 0:08:03.440 --> 0:08:05.679 am kind of winging it today, which is totally different 0:08:05.720 --> 0:08:06.680 from other days. 0:08:06.640 --> 0:08:10.400 Right exactly. If you don't notice the difference between this 0:08:10.440 --> 0:08:13.000 episode and any other episode, then folks, you learn something 0:08:13.120 --> 0:08:14.520 about how the sausage is made. 0:08:16.000 --> 0:08:19.480 You learned that Jorges could thinking done the fly and 0:08:19.520 --> 0:08:22.320 reading really quickly. But anyways, as usually, we were wondering 0:08:22.360 --> 0:08:24.680 how many people out there had thought about things that 0:08:24.720 --> 0:08:27.960 are transparent and if they know how transparency works. 0:08:28.200 --> 0:08:30.760 So thanks very much to everybody who answers these questions. 0:08:30.800 --> 0:08:33.520 It gives us a great insight into what people already 0:08:33.600 --> 0:08:35.840 know and don't know. If you'd like to help us 0:08:35.840 --> 0:08:38.880 out for a future episode of the podcast. Please don't 0:08:38.880 --> 0:08:42.560 be shy write to us two questions at Danielandjorge dot com. 0:08:42.600 --> 0:08:44.319 So think about it for a second. Do you know 0:08:44.920 --> 0:08:47.680 how transparency works? Here's what people had to say. 0:08:47.880 --> 0:08:53.680 Transparency works, in my opinion, by not refracting, or the 0:08:53.760 --> 0:08:57.640 medium that the lightest passing through does not refract in 0:08:57.679 --> 0:09:01.040 any way or change the path of the life. And 0:09:01.200 --> 0:09:05.120 that way you don't get any kind of interference and 0:09:05.240 --> 0:09:10.079 you can see through something it is transparent, so perfectly clear. 0:09:10.400 --> 0:09:13.839 I'm pretty sure that transparency works by like the light 0:09:13.920 --> 0:09:17.520 can travel through the material without being absorbed, so maybe 0:09:17.520 --> 0:09:20.520 there's a lot of spaces between it, and so yeah, 0:09:20.520 --> 0:09:23.440 that light just doesn't get absorbed, so it goes through 0:09:24.559 --> 0:09:25.440 and it's merry way. 0:09:25.600 --> 0:09:27.880 At first, I thought it was simple because the material 0:09:27.920 --> 0:09:31.040 density would lead to more or less transparency, like air 0:09:31.200 --> 0:09:34.400 or some gas and low density would be transparent to 0:09:34.440 --> 0:09:37.200 some point because of the particle density. But then again, 0:09:37.240 --> 0:09:40.760 glass is quite a high density, right, and it's still transparent, 0:09:41.240 --> 0:09:46.240 So maybe it's about reflective index. And yeah, like if 0:09:46.280 --> 0:09:48.800 you put glass in a water that comes invisible because 0:09:48.800 --> 0:09:51.560 it's fully transparent because of the reflective indices, I. 0:09:51.840 --> 0:09:55.720 Would guess transparency has to do with interactions between different 0:09:56.080 --> 0:09:59.080 forces and particles and whether they interact or not. If 0:09:59.120 --> 0:10:02.280 they interact, they are not transparent to each other, and 0:10:02.320 --> 0:10:04.800 if they don't interact, they're transparent to each other. 0:10:04.960 --> 0:10:08.440 It's like a real a race. 0:10:09.120 --> 0:10:14.440 So photon gets emitted, it's absorbed by the first atom 0:10:14.840 --> 0:10:18.319 in the glass, for instance, and then it gets retransmitted 0:10:18.320 --> 0:10:21.120 to the next, and so on and so forth until 0:10:21.160 --> 0:10:23.960 it goes over the other side of the of the 0:10:23.960 --> 0:10:28.199 glass or whatever transparent material we are talking about. 0:10:28.360 --> 0:10:30.920 In order to reflect light, the photon would have to 0:10:30.960 --> 0:10:35.360 be absorbed and then re emitted, I think so. I 0:10:35.360 --> 0:10:37.800 guess in order to be transparent, it would have to 0:10:37.800 --> 0:10:43.080 be something that doesn't absorb photons for whatever reason. That'd 0:10:43.080 --> 0:10:43.880 be interesting one. 0:10:43.880 --> 0:10:47.000 That all right, some pretty intricate answers. I feel like 0:10:47.040 --> 0:10:50.720 we had some hardcore physicists here in the pool today. 0:10:50.800 --> 0:10:52.960 Did you ask people on the internet or in your department? 0:10:53.160 --> 0:10:56.520 These are all Internet answers, and they really reflect the 0:10:56.559 --> 0:11:00.720 incredible complexity of light and matter into it in a 0:11:00.760 --> 0:11:04.600 few episodes, what happens when things reflect? What color means. 0:11:04.600 --> 0:11:06.680 But there really is a whole lot going on here. 0:11:06.880 --> 0:11:09.679 Light bouncing off of matter, or passing through matter, or 0:11:09.679 --> 0:11:13.920 refracting through matter is really a very complicated phenomenon, very 0:11:13.960 --> 0:11:17.080 tricky to understand from the microphysics point of view. 0:11:17.880 --> 0:11:21.640 Yes, I like how it reflects how smarter listeners are. 0:11:21.720 --> 0:11:24.360 But so many really interesting answers, a lot of which 0:11:24.440 --> 0:11:26.520 really get at the heart of the question, which is 0:11:26.559 --> 0:11:30.640 that it's about the interactions of the photons with the material. 0:11:30.880 --> 0:11:33.960 There's also some misunderstanding there, like the idea that light 0:11:34.240 --> 0:11:36.600 goes through things if there are spaces for it to 0:11:36.600 --> 0:11:38.760 like wiggle its way through. That's not really the way 0:11:38.760 --> 0:11:40.280 that we think about it, but we'll dig into it 0:11:40.320 --> 0:11:40.880 and explain it. 0:11:40.960 --> 0:11:43.840 I guess it depends, like the difference between transparency and 0:11:43.880 --> 0:11:48.040 air and transparency in glass and transparency through different kinds 0:11:48.040 --> 0:11:49.960 of light, right, it sort of depends. 0:11:50.080 --> 0:11:52.960 There are definitely differences between air and glass, but the 0:11:52.960 --> 0:11:56.480 basic physics of transparency is quite similar in either case. 0:11:56.559 --> 0:11:59.280 Are the photons like avoiding the atoms, It's not like 0:11:59.320 --> 0:12:03.000 they're fine a pathway through. That's maybe the way transparency 0:12:03.000 --> 0:12:05.480 works for like a screen door. You can see through 0:12:05.480 --> 0:12:08.320 a screen because there are literally holes that a photon 0:12:08.400 --> 0:12:10.640 can pass through. But the reason you can see glass 0:12:10.760 --> 0:12:13.800 is not because it's like a microscreen. That glass is 0:12:13.840 --> 0:12:16.520 finding tiny little holes in the glass to wiggle its 0:12:16.559 --> 0:12:17.000 way through. 0:12:17.160 --> 0:12:19.240 I guess we'll dig into that. And so let's jump 0:12:19.320 --> 0:12:22.640 right in, Daniel, what are the basics what makes something transparent? 0:12:22.840 --> 0:12:25.720 So first let's clarify what we mean by transparent. Right, 0:12:25.760 --> 0:12:28.800 by transparent really just mean that you can see through it. 0:12:28.800 --> 0:12:31.120 It means if you shine a light from one side, 0:12:31.240 --> 0:12:34.520 the light comes out the other side with basically no scattering, 0:12:34.640 --> 0:12:36.440 Like the light is still coherent. If you have an 0:12:36.480 --> 0:12:38.760 image of an ice cream cone on one side, you're 0:12:38.800 --> 0:12:41.000 going to see the ice cream cone still on the 0:12:41.040 --> 0:12:43.439 other side. It might be refracted a little bit or 0:12:43.480 --> 0:12:47.640 bent or distorted, but it's transparent if those images are preserved. 0:12:47.760 --> 0:12:49.600 I see. So like, if light can pass through it 0:12:49.640 --> 0:12:52.360 basically is what it means to be transparent, or information 0:12:52.559 --> 0:12:56.080 from light passes through it without any distortions exactly. 0:12:56.280 --> 0:12:58.800 And even glass, which we think of as transparent, does 0:12:58.840 --> 0:13:00.400 distort it a little bit. Like if you stick your 0:13:00.400 --> 0:13:03.000 finger behind a pane of glass and only half of 0:13:03.040 --> 0:13:05.440 your finger is sticking behind it, you'll notice the finger 0:13:05.520 --> 0:13:07.800 no longer looks like a whole finger. It's like broken 0:13:07.840 --> 0:13:10.200 in half because the path of the light through the 0:13:10.200 --> 0:13:12.640 glass changes a little bit relative to the path of 0:13:12.679 --> 0:13:14.920 the light through the air. That's refraction, which is a 0:13:14.960 --> 0:13:17.920 whole other complicated topic that we dug into in another episode. 0:13:18.000 --> 0:13:19.920 But still you can see your finger right the same 0:13:19.920 --> 0:13:22.280 way you can see through water and you can see 0:13:22.280 --> 0:13:25.280 through air. You can identify things. You can recognize things 0:13:25.280 --> 0:13:27.359 even if the path of the light is slightly. 0:13:27.040 --> 0:13:29.440 Changed, all right. So that means that a light can 0:13:30.040 --> 0:13:32.720 or at least information that the light had before it 0:13:32.760 --> 0:13:35.640 went through the material makes it through unscales. That's what 0:13:35.679 --> 0:13:36.600 it means to be transparent. 0:13:36.760 --> 0:13:39.360 Yeah, And the fundamental thing that's happening there relates to 0:13:39.920 --> 0:13:45.079 how photons either interact or don't interact with the electrons 0:13:45.160 --> 0:13:47.920 in that material. And to understand that, we have to 0:13:48.000 --> 0:13:51.120 understand a little bit about the quantum mechanics of those electrons, 0:13:51.160 --> 0:13:54.120 what energy is they're allowed, and how photons decide whether 0:13:54.320 --> 0:13:56.520 or not to interact with those electrons. 0:13:56.559 --> 0:13:59.200 And you were saying, it's not like it's a screen 0:13:59.320 --> 0:14:02.520 door where like some other light is finding a pass 0:14:02.520 --> 0:14:05.240 through it, But that can also happen, Kennet, Like if 0:14:05.240 --> 0:14:10.160 you have something really thin maybe, or something really sparse 0:14:10.400 --> 0:14:12.800 or light like air, there has to be some photons 0:14:12.800 --> 0:14:15.560 that are making it through without interacting with anything. Right. 0:14:15.600 --> 0:14:18.360 Well, most materials, even like a thin sheet of paper, 0:14:18.640 --> 0:14:21.720 are dense enough that photons are not like finding holes 0:14:21.800 --> 0:14:25.120 in them. The reason that light gets through is because 0:14:25.120 --> 0:14:28.200 the photons are ignoring the atoms. It's not that they're 0:14:28.240 --> 0:14:31.600 missing them, it's that they're passing through without interacting. Because 0:14:31.600 --> 0:14:34.720 when a photon passes by an atom, it doesn't always 0:14:34.800 --> 0:14:37.080 interact with it. There are rules about whether or not 0:14:37.080 --> 0:14:39.880 photons can interact with atoms, and in a solid it's 0:14:39.880 --> 0:14:42.200 complicated because you have lots and lots of atoms. But 0:14:42.240 --> 0:14:44.080 the easiest way to understand it is to start with 0:14:44.120 --> 0:14:47.080 an individual atom. Just think about like a single hydrogen 0:14:47.080 --> 0:14:49.760 atom in space and you shoot a photon at it 0:14:49.800 --> 0:14:52.040 is that photon going to get absorbed by the atom 0:14:52.360 --> 0:14:54.880 or is the photon going to ignore the atom? And 0:14:54.920 --> 0:14:58.200 that depends on the energy of the photon, because these 0:14:58.240 --> 0:15:01.560 atoms can only absorb photon of certain energies. 0:15:01.720 --> 0:15:03.880 Well, I guess it also maybe depends on where you 0:15:03.920 --> 0:15:06.800 shoot the photon, right, Like, if I have an hydrogen atom, 0:15:06.840 --> 0:15:09.360 floating out there in the air and I shoot a 0:15:09.480 --> 0:15:12.400 laser beam, you know, a mile to one side of it. 0:15:12.400 --> 0:15:14.280 It's not going to interact with the hydrogen, is it. 0:15:14.400 --> 0:15:16.960 No, that's right. But if you have a huge wall 0:15:17.080 --> 0:15:19.480 of hydrogen atoms and you shoot the laser beam at it, 0:15:19.560 --> 0:15:22.640 then it's not going to be able to avoid the hydrogen, right, 0:15:23.000 --> 0:15:24.720 It's going to have to get through the other side. 0:15:24.760 --> 0:15:27.480 It's going to have to pass through the hydrogen. And 0:15:27.520 --> 0:15:30.080 that's what makes things transparent. I mean, one thing is 0:15:30.120 --> 0:15:31.640 to have holes in it. Sure, where you can see 0:15:31.680 --> 0:15:34.200 through a screen door, even if it's made of metal 0:15:34.360 --> 0:15:36.480 or stone, as long as there are holes in it. 0:15:36.800 --> 0:15:39.760 But I think the physics of transparency is more interesting 0:15:39.760 --> 0:15:43.000 if we think about what happens when light ignores the material, 0:15:43.000 --> 0:15:47.000 if it passes through the material, rather than finding holes 0:15:47.080 --> 0:15:48.000 or ways around it. 0:15:48.120 --> 0:15:51.760 Well, I know it's more interesting to you as a physicist, 0:15:52.120 --> 0:15:54.760 but I guess for me, I'm just curious about which 0:15:54.840 --> 0:15:57.040 has more of an effect to make something transparent, right, 0:15:57.080 --> 0:16:00.480 Because you know, people always talk about like material like 0:16:00.480 --> 0:16:02.720 our skin, our bodies, they're made out of atoms. But 0:16:02.760 --> 0:16:04.680 the atoms are really sort of far apart and the 0:16:04.760 --> 0:16:07.040 nucleus is really far apart from the electrons, So there's 0:16:07.080 --> 0:16:10.000 a lot of sort of like supposedly empty space, even 0:16:10.040 --> 0:16:13.960 within solid matter like ourselves. I'm just wondering, like, you know, 0:16:14.160 --> 0:16:16.160 there's it seems like there's a lot of space for 0:16:16.520 --> 0:16:20.040 a light to squeeze through or to pass by without 0:16:20.160 --> 0:16:22.160 even knowing there are other things there. 0:16:22.240 --> 0:16:24.040 That's an interesting point, and I think a lot of 0:16:24.040 --> 0:16:26.440 people have maybe the wrong mental picture of what an 0:16:26.440 --> 0:16:29.000 atom sort of looks like to a photon. You know, 0:16:29.040 --> 0:16:30.520 even if you have a grid of atoms, like a 0:16:30.520 --> 0:16:33.200 sheet of atoms making them material, you might imagine that 0:16:33.200 --> 0:16:35.640 it's mostly empty space, as you said. And it's true 0:16:35.640 --> 0:16:38.920 that the nucleus is small compared to the distances between 0:16:38.960 --> 0:16:41.720 the atoms, right, So you have this like grid of atoms, 0:16:41.720 --> 0:16:44.040 and there is a lot of space between the nuclei. 0:16:44.200 --> 0:16:46.640 But that space is not empty, right. That space is 0:16:46.680 --> 0:16:51.320 filled with electrons and electronic fields and forces that are 0:16:51.320 --> 0:16:54.680 holding the nucleus and the electrons together. So the electrons 0:16:54.680 --> 0:16:57.760 are in these buzzy clouds all around the nucleus, and 0:16:57.800 --> 0:17:00.800 the electrons weave the atoms together, right, So it's not 0:17:00.840 --> 0:17:04.439 like there's space between the atoms. The atoms are held together, 0:17:04.480 --> 0:17:07.280 they're in these bonds that tie them together into a 0:17:07.320 --> 0:17:10.159 big grid, into a big lattice, and the electrons can 0:17:10.200 --> 0:17:13.040 sometimes slide back and forth and move between the atoms, 0:17:13.400 --> 0:17:15.480 so there isn't a lot of space between the atoms. 0:17:15.720 --> 0:17:18.040 And then inside the atom, all that space you imagine 0:17:18.080 --> 0:17:21.480 might be empty is really filled with electrons. So from 0:17:21.480 --> 0:17:23.800 the point of view of a photon hitting like a 0:17:23.840 --> 0:17:27.000 sheet of material, a sheet of paper or a sheet 0:17:27.040 --> 0:17:29.840 of glass or whatever, it really is hitting a wall 0:17:29.960 --> 0:17:33.320 of electrons that it can't find a way around unless 0:17:33.359 --> 0:17:35.240 you like physically punch holes in it. 0:17:35.480 --> 0:17:37.680 You just made me wonder, like when you have a material, 0:17:37.960 --> 0:17:42.959 how close together are the electrons shells between atoms, Like 0:17:43.080 --> 0:17:45.280 are are they bumping up against each other or is 0:17:45.320 --> 0:17:48.240 there a certain amount of space between. 0:17:48.160 --> 0:17:49.639 It depends a little bit on the kind of atom 0:17:49.680 --> 0:17:52.920 you have. But atoms tend to bond with their outermost electrons, 0:17:53.400 --> 0:17:55.760 and we'll dig into that if you let me talk 0:17:55.800 --> 0:17:59.119 about how these electrons move between the atoms. But you 0:17:59.160 --> 0:18:02.720 really are tying the outer levels of the electron orbitals 0:18:02.760 --> 0:18:06.479 together and then informing these common electron energy levels. So 0:18:06.520 --> 0:18:09.880 these atoms are sharing electrons that really are woven together. 0:18:10.160 --> 0:18:11.879 I see. I think what you're saying is that to 0:18:11.920 --> 0:18:15.240 a photon a grid of electrons, it's like a solid 0:18:15.280 --> 0:18:18.840 wall almost because it's full of electron fields, and photons 0:18:19.200 --> 0:18:22.200 interact with electrons. But like, if you were something else 0:18:22.240 --> 0:18:24.359 that was not an electron, you would maybe see a 0:18:24.359 --> 0:18:26.480 lot of empty space. But because you are a photon, 0:18:26.560 --> 0:18:30.720 then you're walking into a solid wall of electron fuzziness. 0:18:30.800 --> 0:18:34.080 Yeah, if you're a neutrino, for example, and you don't 0:18:34.080 --> 0:18:37.680 feel electric charge, then the fact that there are electromagnetic 0:18:37.720 --> 0:18:40.600 fields all through these materials is relevant to you, and 0:18:40.680 --> 0:18:44.000 you pass right through it. So like even a block 0:18:44.119 --> 0:18:47.960 of lead is basically transparent to a neutrino because it 0:18:47.960 --> 0:18:51.480 doesn't interact with the stuff. So transparency comes down to 0:18:51.520 --> 0:18:54.600 whether you interact with the material there, not really whether 0:18:54.640 --> 0:18:57.480 there are holes there. So photons they can either pass 0:18:57.520 --> 0:19:00.359 through a material or they can interact with it, and 0:19:00.400 --> 0:19:04.000 that depends on the atomic structure and the energy levels 0:19:04.000 --> 0:19:06.560 that the electrons have and whether or not they interact 0:19:06.560 --> 0:19:09.080 with that photon. Even though a photon sees a wall 0:19:09.119 --> 0:19:12.000 of material, a buzzing blob of electrons in front of it, 0:19:12.000 --> 0:19:16.119 it can still sometimes pass right through without interacting, and 0:19:16.160 --> 0:19:17.439 that's what transparency is. 0:19:17.640 --> 0:19:19.880 Yeah, I guess it can be transparent not just to light, 0:19:19.920 --> 0:19:22.600 but to other things. Right, Like, isn't there a famous 0:19:22.640 --> 0:19:26.359 like gold foil experiment that kind of help people figure 0:19:26.359 --> 0:19:27.560