WEBVTT - What is a light cone? 0:00:09.160 --> 0:00:11.320 Or Hey, do you ever wish that the speed of 0:00:11.440 --> 0:00:15.080 light was faster? I guess I'm not dissatisfied with the 0:00:15.080 --> 0:00:17.640 speed of light. I'm just glad it's not slower, because 0:00:17.680 --> 0:00:19.120 you know, I don't want to wait ten minutes after 0:00:19.160 --> 0:00:21.640 I flip on the light switch. But you know, if 0:00:21.680 --> 0:00:24.160 it was faster, and then we could see like further 0:00:24.239 --> 0:00:27.640 out into the universe. M m. That's true, and then 0:00:27.680 --> 0:00:30.840 we might be able to travel further to explore the stars. 0:00:30.920 --> 0:00:34.360 Also true. But there's another side to it, as always, well, 0:00:34.400 --> 0:00:36.480 how could it be bad to have a faster speed 0:00:36.479 --> 0:00:39.120 of light? Well, light speed was a million times faster 0:00:39.479 --> 0:00:42.720 than technically any alien out there in the universe could 0:00:42.760 --> 0:00:45.400 kill us instantly with a death ray. That's true. But 0:00:45.479 --> 0:00:48.840 you know, witnessing an alien death ray would be pretty cool. 0:00:49.200 --> 0:00:51.560 It would be pretty cool for about a nano second, 0:00:51.560 --> 0:00:53.440 and then you'd be dead. Also, I'm pretty sure a 0:00:53.520 --> 0:00:56.480 death ray would be hot and not cool. Might still 0:00:56.520 --> 0:00:58.520 be worth it, just for that nana second, you would 0:00:58.520 --> 0:01:09.200 know that AIDS existed. As my brain is getting FRIEDA. 0:01:16.959 --> 0:01:20.240 I am or haemade cartoonists and the creator of PhD comics. Hi, 0:01:20.480 --> 0:01:23.280 I'm Daniel, I'm a particle physicist, and I don't spend 0:01:23.319 --> 0:01:26.399 a lot of time worrying about alien death rays. You don't, 0:01:26.680 --> 0:01:28.600 do you know something I don't. I know that the 0:01:28.600 --> 0:01:30.920 speed of light protects us from most of the universe 0:01:30.959 --> 0:01:33.960 shooting us with alien death rays. Really like it forms 0:01:34.000 --> 0:01:37.120 a force field around us, protecting this like a blanket 0:01:37.319 --> 0:01:39.280 helping us sleep at night. It just makes it kind 0:01:39.319 --> 0:01:41.200 of ineffective. If you're going to shoot an alien death 0:01:41.280 --> 0:01:43.360 ray millions of light years, then you're gonna kill your 0:01:43.360 --> 0:01:47.080 target in millions of years. And like, who's geopolitical strategy 0:01:47.160 --> 0:01:49.680 really has that long and arc to it? But what 0:01:49.800 --> 0:01:53.720 if there are patient aliens? Then we're toast? Then we 0:01:53.800 --> 0:01:56.440 might literally be toast. Welcome to our podcast, Daniel and 0:01:56.520 --> 0:01:59.280 Jorge Explain the Universe, a production of I Heart Radio 0:01:59.440 --> 0:02:02.280 in which we beam our joke rays into your brain, 0:02:02.560 --> 0:02:06.000 hoping to transmit some of the incredible wonder and knowledge 0:02:06.040 --> 0:02:09.040 about the universe, all the things that science has learned 0:02:09.080 --> 0:02:12.960 about this Bonker's wondrous, beautiful, glittering cosmos, and all the 0:02:13.000 --> 0:02:16.000 things that science is still working on, all the vague 0:02:16.080 --> 0:02:18.960 questions about how the universe got here, where it's going, 0:02:19.240 --> 0:02:21.880 and who lives in it, and how to actually build 0:02:21.880 --> 0:02:25.080 a joke Ray. I mean, that's probably the next Nobel Prize. 0:02:25.320 --> 0:02:27.080 The question it doesn't make you groan or does it 0:02:27.120 --> 0:02:29.000 make you laugh? Is that where you're gonna ask when 0:02:29.000 --> 0:02:30.919 the aliens come, I'm gonna be like, tell us the 0:02:30.960 --> 0:02:33.040 secrets of spacetime and you're like, have you developed the 0:02:33.120 --> 0:02:35.240 joke Ray yet? We'll think about what it would mean 0:02:35.320 --> 0:02:37.959 for the mental health of humanity. You know, we could 0:02:38.000 --> 0:02:41.120 just have a laugh about everything. Laughter is the best medicine. 0:02:41.160 --> 0:02:43.160 So anytime you go to the doctor, they're just like, yep, 0:02:43.240 --> 0:02:45.760 we're prescribing you five shots with the joke Ray. And 0:02:45.800 --> 0:02:48.240 then as the aliens as they you know, enslave us 0:02:48.320 --> 0:02:51.160 and possibly eat us, we could be laughing all the way. 0:02:51.400 --> 0:02:54.960 But it is a bizarre and jokey universe because it 0:02:55.000 --> 0:02:57.400 seems like a lot of it is beyond our control 0:02:57.480 --> 0:03:01.400 or even beyond our ability to whoever interact with it. 0:03:01.600 --> 0:03:04.840 That's true, and it's also sometimes beyond our ability to 0:03:05.080 --> 0:03:08.120 really understand. We have one way of thinking about the 0:03:08.160 --> 0:03:11.240 world where things move and slide and the way time works. 0:03:11.360 --> 0:03:14.079 But about a hundred years ago we discovered that that 0:03:14.200 --> 0:03:16.360 wasn't really the way it worked, that there were different 0:03:16.440 --> 0:03:19.280 rules the things change when you move fast or when 0:03:19.360 --> 0:03:22.120 space is bent, and that we needed new rules and 0:03:22.200 --> 0:03:26.119 new ideas to understand what actually goes on in this universe. Yeah, 0:03:26.160 --> 0:03:28.399 because I guess we realized a few hundred years ago 0:03:28.480 --> 0:03:31.400 that the universe, the world doesn't really behave the way 0:03:31.440 --> 0:03:33.680 it behaves like on an everyday basis, like when you're 0:03:33.760 --> 0:03:37.320 juggling baseball or or you know, balancing a basketball. It's 0:03:37.360 --> 0:03:39.160 like that's one way that we think of the world 0:03:39.280 --> 0:03:41.520 is working. But really, like if you get down to it, 0:03:41.560 --> 0:03:44.800 in some situations, it's totally different. You know. In fact, 0:03:44.880 --> 0:03:48.160 there are really fundamentally different rules that govern how things 0:03:48.240 --> 0:03:51.520 move and how time flows. It just happens to be 0:03:51.560 --> 0:03:54.800 that in certain situations, like when you're moving really slow 0:03:55.400 --> 0:03:58.520 and when you're far from a black hole, things look 0:03:58.560 --> 0:04:01.040 a lot simpler. So we're living in a sort of 0:04:01.040 --> 0:04:05.480 a simplified corner of a very weird, very complicated universe. Right, 0:04:05.560 --> 0:04:07.800 It's almost like we're in a special case, like our 0:04:08.040 --> 0:04:11.080 perception of the universe. It's like a small example of 0:04:11.120 --> 0:04:12.720 all the ways that the universe could be. It's like 0:04:12.720 --> 0:04:14.920 you're playing the warm up levels on some video game 0:04:15.120 --> 0:04:17.160 before all the bad guys come in, just you can 0:04:17.240 --> 0:04:19.400 sort of like figure out how the controls work. Right, 0:04:19.640 --> 0:04:22.160 That's what we've been doing for physics for like thousands 0:04:22.200 --> 0:04:25.480 of years, and now we finally understand the full scope 0:04:25.480 --> 0:04:27.680 of the game. And wouldn't be great a good respond 0:04:28.880 --> 0:04:31.240 in this universe? We can just keep trying, well, we 0:04:31.279 --> 0:04:34.040 sort of do. I mean, like physicists start from where 0:04:34.200 --> 0:04:36.600 previous generations of physicists ended, right, We don't have to 0:04:36.600 --> 0:04:39.880 start from scratch on physics every single generation. So physics 0:04:39.920 --> 0:04:42.000 sort of responds in our minds, right, And we're going 0:04:42.040 --> 0:04:44.040 to run out of lives at some point. Do we 0:04:44.080 --> 0:04:46.640 need to put in more quarters into the arcade of 0:04:46.640 --> 0:04:50.599 the universe? That depends on the alien death right or 0:04:50.640 --> 0:04:53.560 the joke ray Neither one might help us or not 0:04:53.640 --> 0:04:56.320 help us. But yeah, the universe is kind of tricky, 0:04:56.520 --> 0:04:59.640 and it's tricky not just in like extreme situations like 0:04:59.680 --> 0:05:01.280 in a that hole or moving on close to the 0:05:01.279 --> 0:05:03.680 speed of light, because who knows, like we could be 0:05:03.720 --> 0:05:07.200 moving right now really fast compared to other aliens and 0:05:07.240 --> 0:05:09.960 other galaxies right we are. We are moving really fast 0:05:10.000 --> 0:05:12.239 compared to the Sun, and the Sun is moving really 0:05:12.240 --> 0:05:14.840 fast compared to the center of the galaxy. But all 0:05:14.880 --> 0:05:17.520 those things are kind of abstract. What really matters is 0:05:17.560 --> 0:05:20.479 your speed relative to the people who are watching you 0:05:20.640 --> 0:05:24.240 and relative to your clock. And so that's something that 0:05:24.360 --> 0:05:27.320 special relativity has taught us, is that the world looks 0:05:27.440 --> 0:05:30.839 different from different perspectives. Based on who's doing the measuring. 0:05:31.160 --> 0:05:33.359 You can get kind of a different account of what's 0:05:33.360 --> 0:05:35.680 going on and what's happened in the world. Yeah, you 0:05:35.760 --> 0:05:39.599 might not agree with your relatives about relativity, but yeah, 0:05:39.640 --> 0:05:42.160 things are kind of tricky in this universe, and so 0:05:42.720 --> 0:05:45.160 even physicists sort of struggle with it, right, I mean, 0:05:45.160 --> 0:05:48.320 it's sort of hard to rab anyone's head around these 0:05:48.440 --> 0:05:52.080 topics and how it actually works. And and so physicists 0:05:52.120 --> 0:05:54.880 have some ways to kind of think about how these 0:05:54.920 --> 0:05:57.760 things work that might help other people. Yeah, because in 0:05:57.800 --> 0:06:00.600 science we have our intuition, like give a fling about 0:06:00.640 --> 0:06:03.320 how things might work and what might happen and how 0:06:03.360 --> 0:06:06.440 things are put together. And then there's the mathematics. And 0:06:06.440 --> 0:06:09.520 the mathematics is what actually happens, what really is going on, 0:06:09.839 --> 0:06:12.919 the rules that really do control the universe. But sometimes 0:06:12.960 --> 0:06:15.159 you're looking at like a page of equations and you're like, 0:06:15.560 --> 0:06:17.760 I don't see it. I don't really get it. And 0:06:17.839 --> 0:06:20.640 for me personally, it's easier to think about these things 0:06:20.680 --> 0:06:24.640 like geometrically to understand like the relationship in space and 0:06:24.760 --> 0:06:27.360 time of these things. So sometimes you can get more 0:06:27.400 --> 0:06:30.560 of an intuition for weird things if you think about 0:06:30.560 --> 0:06:33.279 their structure and space and time sort of geometrically, like 0:06:33.360 --> 0:06:36.680 the shapes that are created by all of these ideas, right, 0:06:36.760 --> 0:06:39.159 and then you draw cartoons about it, which is where 0:06:40.440 --> 0:06:43.719 right exactly? It helps to draw cartoons, no joke. Cartoons 0:06:43.720 --> 0:06:46.000 are a great way to visualize this stuff. That's why 0:06:46.040 --> 0:06:47.919 you and I work together on a lot of these books, 0:06:48.120 --> 0:06:50.320 and folks out there should check out our book We 0:06:50.400 --> 0:06:53.200 Have No Idea, which has all sorts of really clever 0:06:53.240 --> 0:06:57.440 cartoons visualizing really tricky concepts. Yeah, and our upcoming book 0:06:57.440 --> 0:07:00.560 which is coming out in a few months frequently questions 0:07:00.600 --> 0:07:03.640 about the universe, which you can pre order right now. Man, 0:07:03.640 --> 0:07:07.080 we totally snuck in those book plugs, Daniel, totally unplanned. 0:07:07.279 --> 0:07:09.600 I keep getting emails from listeners saying, you guys should 0:07:09.600 --> 0:07:11.480 write a book. I'm like, what do we not talk 0:07:11.520 --> 0:07:13.600 about the book enough on the podcast? I guess I 0:07:13.720 --> 0:07:15.760 guess we don't. We should talk about it every time. 0:07:16.040 --> 0:07:19.080 I think we should, honestly. Anyway, back to the podcast. Yeah, 0:07:19.120 --> 0:07:20.880 back to the podcast. Let's talk about one of these 0:07:21.120 --> 0:07:23.440 tools that physicists used to kind of wrap your head 0:07:23.480 --> 0:07:26.080 around some of these tricky concepts about relativity and the 0:07:26.080 --> 0:07:28.360 speed of light. So today on the podcast, we'll be 0:07:28.400 --> 0:07:37.880 asking the question, what is a light cone? Daniel, this 0:07:38.080 --> 0:07:40.400 is a light coming, I think, I guess you mean light, 0:07:40.520 --> 0:07:44.280 not like not heavy cone is exactly light as in 0:07:44.360 --> 0:07:47.280 photons is and that thing that speeds along at the 0:07:47.440 --> 0:07:50.800 maximum speed of the universe. Now, this sounds sort of 0:07:50.840 --> 0:07:53.640 like maybe a dessert, but maybe we're not a dessert 0:07:53.720 --> 0:07:55.880 you want to eat or that would taste very good. 0:07:56.160 --> 0:07:57.960 Sounds like the kind of dessert your mom would order 0:07:57.960 --> 0:08:00.800 for you when what you really want is scubavice. Where 0:08:00.840 --> 0:08:03.320 you really want is a heavy cone, the heavy cream cone. 0:08:03.560 --> 0:08:06.960 You don't want the like fat free light cone. But yeah, 0:08:07.000 --> 0:08:09.720 So this is a concept that comes up in special 0:08:09.760 --> 0:08:14.640 relativity and it has some pretty interesting consequences about causality 0:08:14.920 --> 0:08:18.600 and like what's actually real or like what's actually happening 0:08:19.080 --> 0:08:21.240 or happened in the universe. Yeah, and it helps you 0:08:21.280 --> 0:08:24.240 think about what's going on inside a black hole and 0:08:24.400 --> 0:08:27.480 even possibly maybe allows you to figure out a way 0:08:27.520 --> 0:08:31.360 to do real life time travel. Sounds like a very 0:08:31.400 --> 0:08:34.720 powerful dessert here. All right, Well, before we go on, 0:08:34.800 --> 0:08:37.080 we just want to give a quick shout out to Biorn, 0:08:37.840 --> 0:08:40.680 right he sort of helped inspire this episode. Daniel. Yeah, 0:08:40.720 --> 0:08:43.520 thanks Born from Sweden who sent me a link to 0:08:43.640 --> 0:08:47.199 his Solar System inspired music which I listened to while 0:08:47.240 --> 0:08:49.960 preparing for this podcast. You can find him on Spotify 0:08:50.080 --> 0:08:52.560 at ord cloud Services. Check them out if you like 0:08:52.600 --> 0:08:55.320 the sort of space ambiance music. Thanks a lot, Biorn 0:08:55.400 --> 0:08:57.920 for sharing music. Yeah, and again that's or Clouds. That's 0:08:57.960 --> 0:09:01.559 o RT cloud service is on Spotify, which is a 0:09:01.640 --> 0:09:05.000 pun because it's like the Orc Cloud and also cloud services. 0:09:05.040 --> 0:09:07.400 And what is the orc cloud done for us recently? Well, 0:09:07.440 --> 0:09:10.760 I guess it delivers spacey music and you can upload 0:09:10.800 --> 0:09:13.480 your pictures to it. You might never get them back, 0:09:13.640 --> 0:09:16.360 but yeah, you can upload them. It probably takes like 0:09:16.360 --> 0:09:18.960 twenty years just to get there, and then the download 0:09:19.000 --> 0:09:23.360 speed it's probably even worse. All right, Well, we were curious, 0:09:23.360 --> 0:09:25.840 as usually how many people out there are familiar with 0:09:25.920 --> 0:09:28.960 the concept we're talking about today. So Daniel went out 0:09:28.960 --> 0:09:31.960 there into the internet to ask people what is a 0:09:32.160 --> 0:09:34.280 light cone? And if you are out there on the 0:09:34.320 --> 0:09:38.040 internet and always wanted to participate in a podcast, please 0:09:38.040 --> 0:09:40.560 write to us two questions at Daniel and Jorge dot 0:09:40.559 --> 0:09:43.280 com and you can be unprepared to answer future questions 0:09:43.320 --> 0:09:46.000 for the podcast. Right, And if you're not on the Internet, 0:09:46.040 --> 0:09:48.960 please tell us your secret. I mean I would to 0:09:49.360 --> 0:09:52.200 sometimes not be on the Internet. I guess we're on 0:09:52.240 --> 0:09:55.280 the radio too, right, Daniel. I think some stations broadcasts, 0:09:55.280 --> 0:09:57.959 so they might be listening on the radio. Yeah, So 0:09:58.120 --> 0:10:00.160 thanks everybody who's out there listening to us on the 0:10:00.280 --> 0:10:02.480 radio as well. We appreciate all of your support and 0:10:02.520 --> 0:10:04.559 all of your listening. Did you know that we recently 0:10:04.600 --> 0:10:10.160 passed fifteen million episode downloads? Wow? Really, it's amazing. That's 0:10:10.160 --> 0:10:13.360 like fifty million hours that people have been listening to us. 0:10:13.480 --> 0:10:17.439 That's like fifteen million banana jokes at least at least 0:10:17.480 --> 0:10:20.720 because you know, we're a multi banana joke death Ray 0:10:20.760 --> 0:10:22.760 a joke Ray here. We try to in our in 0:10:22.760 --> 0:10:26.400 our best episodes, like a shotgun rate joke. Really, we 0:10:26.600 --> 0:10:29.079 just splatter bananappeals all over the place and now I 0:10:29.200 --> 0:10:32.400 imagining fifteen million tiny bananas coming out of the nozzle 0:10:32.400 --> 0:10:35.080 of a gun at very high to be coming out 0:10:35.080 --> 0:10:39.720 of our mouth, that's a little disturbing. But anyways, think 0:10:39.760 --> 0:10:41.760 about it for a second. If someone asked you what 0:10:41.880 --> 0:10:44.880 do you think a light cone is? What would you say? 0:10:45.040 --> 0:10:47.360 Here's what people had to say. I would say, that's 0:10:47.360 --> 0:10:50.560 what comes out of the shiny end of a flash light. Um. 0:10:50.600 --> 0:10:54.520 I believe this is a space time diagram where it 0:10:54.640 --> 0:10:59.719 shows a three dimensional object and as you go up, 0:11:01.000 --> 0:11:04.679 that's time passing, so it makes a calm like if 0:11:04.679 --> 0:11:06.760 you look at the diagram of the Big Bang and 0:11:06.760 --> 0:11:09.360 are we sure a tiny point? Then it slowly goes 0:11:09.440 --> 0:11:13.000 up into a cone as time progresses. Um. I don't 0:11:13.000 --> 0:11:16.280 know what a light cone is. I'm thinking about there 0:11:16.400 --> 0:11:21.840 was some propulsion the idea that was. I think it 0:11:21.920 --> 0:11:24.640 was largely theoretical, but it was to do with kind 0:11:24.679 --> 0:11:29.960 of using light to push spacecraft and it would capture 0:11:30.000 --> 0:11:32.320 the light using light sales. I don't know if that 0:11:32.360 --> 0:11:34.240 would be to do with it. I remember seeing a 0:11:34.280 --> 0:11:37.839 model in the Science Museum in London, but there was 0:11:37.880 --> 0:11:39.920 a cone, but that was probably just because it was 0:11:39.960 --> 0:11:47.480 made of plastic. Light corne is representation of how from 0:11:47.520 --> 0:11:52.560 the light travels from a point. For instance, if you 0:11:52.840 --> 0:11:56.360 have if I have a little flash light in my hand, 0:11:56.360 --> 0:12:02.120 then I turned it on the lightbulb trouble in space 0:12:02.200 --> 0:12:08.240 and time. Like cone, you can represent it as a cone. 0:12:09.240 --> 0:12:13.079 That is something I've heard of before. Um, it's the 0:12:13.120 --> 0:12:16.280 realm of like what you can see. So it's like 0:12:16.400 --> 0:12:20.959 ninety three billion light years across as in our observable universe, 0:12:21.000 --> 0:12:24.240 because that's like how far we could observes or something 0:12:25.000 --> 0:12:28.880 into space, or like the photons that have reached us 0:12:29.440 --> 0:12:33.640 from that distance. Although I don't understand how that's bigger 0:12:33.679 --> 0:12:37.040 than thirteen point eight billion light years since that's the 0:12:37.080 --> 0:12:40.120 age of the universe. But I'm still quite unclear on that. 0:12:40.160 --> 0:12:42.880 But I know that the light cone is basically all 0:12:42.880 --> 0:12:46.400 that we can see within our realm of light, anything 0:12:46.440 --> 0:12:49.280 that can reach us in any form. I think a 0:12:49.360 --> 0:12:52.920 light cone is an ice cream cone that's been left 0:12:52.920 --> 0:12:54.600 out in the sun too long and all the ice 0:12:54.600 --> 0:12:58.360 cream melted. A light cone is an ice cream cone 0:12:58.440 --> 0:13:01.400 that has no chocolate ice cream minute whatsoever, which is 0:13:02.120 --> 0:13:04.839 fundamentally flawed, and I think it violates one of the 0:13:04.920 --> 0:13:07.880 laws of thermodynamics, because why would you want vanilla ice cream? 0:13:07.920 --> 0:13:10.880 One chocolate is an option, so light cones make no sense. 0:13:11.440 --> 0:13:14.720 All right. A lot of people went with my interpretation, 0:13:14.800 --> 0:13:18.160 which is that it sounds like a dessert possibly and 0:13:18.240 --> 0:13:20.400 that it's light. Yeah, and we also heard some people 0:13:20.440 --> 0:13:23.360 throwing shade on vanilla ice cream. Wow, that was rough. 0:13:23.600 --> 0:13:26.079 I know, how can you be down in vanilla ice cream? 0:13:26.400 --> 0:13:30.199 It's like the nicest flavor gets along with everybody. That's true. 0:13:30.200 --> 0:13:32.880 I mean I would always pick chocolate over vanilla. But 0:13:32.960 --> 0:13:35.000 you know, if all you got is vanilla, I'm happy 0:13:35.000 --> 0:13:37.560 to get. But if you take vanilla and sprinkle chocolate 0:13:37.600 --> 0:13:41.040 on it, see that's the versatility of vanilla. Or take 0:13:41.120 --> 0:13:44.280 chocolate and sprinkled chocolate on it, it's still better. But 0:13:44.320 --> 0:13:46.680 you can't take chocolate and sprinkle vanilla on it. That's 0:13:46.760 --> 0:13:49.360 when you do. That's vanilla doesn't add to anything. It's 0:13:49.400 --> 0:13:53.160 just sort of like the substrate he lean. It makes 0:13:53.200 --> 0:13:55.920 most cookies taste good. It's just sort of vanilla. That's 0:13:55.960 --> 0:13:58.360 the reason why they call it vanilla vanilla, you know, 0:13:58.960 --> 0:14:02.720 because it's so dependent and solid and she just always 0:14:02.760 --> 0:14:05.920 there for you. Right. If somebody reads you're writing and 0:14:05.960 --> 0:14:07.720 they describe it as a vanilla. Would you take that 0:14:07.760 --> 0:14:11.080 as a compliment? Depends am I writing the bag of 0:14:11.120 --> 0:14:13.840 an ice cream carton? Then it would meet its cool 0:14:14.440 --> 0:14:17.160 But yeah, so a light cone, Daniel tell me, this 0:14:17.240 --> 0:14:19.840 is something I was surprised to see in our topic 0:14:19.960 --> 0:14:23.480 because it is it's like a graphical interpretation of physics, right, 0:14:23.760 --> 0:14:26.320 And so it's a little tricky to describe on a podcast. 0:14:26.400 --> 0:14:28.360 It's a little tricky to describe on a podcast. We'll 0:14:28.400 --> 0:14:30.920 see how we do. But I thought it was useful 0:14:30.920 --> 0:14:33.800 because I find that every time we talk about special relativity, 0:14:33.920 --> 0:14:36.440 I have light cones in my mind. I got these 0:14:36.440 --> 0:14:39.440 things in my brain. They really helped me think about 0:14:39.560 --> 0:14:42.400 sort of the structure of space and time, and they 0:14:42.480 --> 0:14:44.520 lead to a lot of interesting consequences. And so I 0:14:44.560 --> 0:14:47.400 hope if we can somehow download the concept of a 0:14:47.480 --> 0:14:49.640 light cone into the minds of our listeners, it will 0:14:49.680 --> 0:14:52.120 help them think about special relativity. We need like a 0:14:52.200 --> 0:14:55.160 light cone ray to shoot listen to people. Maybe, Yeah, 0:14:55.200 --> 0:14:57.200 I think it's a podcast starring me and you're talking 0:14:57.200 --> 0:15:01.840 about throwing out vananative. All right, well, m step us 0:15:01.840 --> 0:15:05.000 through it. What is technically a light cone? So a 0:15:05.120 --> 0:15:08.840 light cone defines the part of the universe that you 0:15:08.880 --> 0:15:12.240 can influence, the part of the future that you can 0:15:12.440 --> 0:15:16.120 visit geometrically. And you can think about it like if 0:15:16.160 --> 0:15:18.840 you turn on a flashlight for a moment, so you 0:15:19.160 --> 0:15:21.880 shine photons in every direction, they leave you in sort 0:15:21.920 --> 0:15:24.280 of a spear. After one second, they are one light 0:15:24.360 --> 0:15:27.840 second away from you. After one year, they are one 0:15:28.000 --> 0:15:30.560 light year away from you. So imagine a sphere and 0:15:30.600 --> 0:15:34.240 then that sphere sort of growing as time goes on, right, 0:15:34.520 --> 0:15:38.360 So that set of spears versus time as those spears grow, 0:15:38.640 --> 0:15:41.080 this is your future light cone. This is the part 0:15:41.160 --> 0:15:45.120 of the universe that you can influence where your choices, 0:15:45.160 --> 0:15:47.360 the things that you do as a person could have 0:15:47.520 --> 0:15:51.600 sort of an effect downstream. It's very easy to understand why. 0:15:52.000 --> 0:15:55.040 The reason is that everything that's outside of that growing 0:15:55.080 --> 0:15:58.760 set of spears you can't influence because information would have 0:15:58.800 --> 0:16:01.840 to travel faster and the speed of light to get there. 0:16:02.000 --> 0:16:03.680 So it's a direct outcome of the fact that we 0:16:03.720 --> 0:16:06.760 have a maximum speed limit to the universe. It divides 0:16:06.880 --> 0:16:09.920 the universe into parts that you can influence and parts 0:16:10.000 --> 0:16:12.440 you cannot. So the light cone is the part of 0:16:12.480 --> 0:16:15.640 these universe, including the future. Right, we're talking space and 0:16:15.760 --> 0:16:18.680 time here, the part of the future universe that you 0:16:18.680 --> 0:16:21.000 can have any sort of influence over. So I turn 0:16:21.080 --> 0:16:24.040 on a light like a light bulb, and that light 0:16:24.240 --> 0:16:27.080 is growing comes out of the light bulb of growth 0:16:27.160 --> 0:16:30.000 in a sphere, and you're saying, that's the light cone. 0:16:30.000 --> 0:16:31.840 So it really should be called like a light sphere. 0:16:31.880 --> 0:16:34.480 Maybe it'd be a better name, like the light sphere 0:16:34.640 --> 0:16:40.280 is the sphere around you which you can reach with light. 0:16:40.520 --> 0:16:42.360 The reason it's often called the light cone is that 0:16:42.400 --> 0:16:45.480 it's hard to draw four dimensions on a sheet of paper. 0:16:45.800 --> 0:16:47.160 The most we can really draw on a sheet of 0:16:47.200 --> 0:16:50.560 papers three dimensions, you know, by having like some perspective, 0:16:50.920 --> 0:16:52.880 and so you need to have time on there. So 0:16:52.960 --> 0:16:55.520 you can really only have two other dimensions of space 0:16:55.800 --> 0:16:57.560 on your sheet of paper. And so when you're drawing 0:16:57.600 --> 0:17:00.880 these diagrams, people typically treats base as if it was 0:17:00.920 --> 0:17:03.600 a two D surface instead of like our three D world, 0:17:03.880 --> 0:17:06.440 just to sort of simplify it. So in two dimensions 0:17:06.960 --> 0:17:10.280 the sphere we were just talking about, it becomes a circle. Right. Imagine, 0:17:10.320 --> 0:17:13.600 for example, taking a smooth sheet of water and hitting 0:17:13.640 --> 0:17:16.560 it somewhere. What happens you have a wave that leaves 0:17:16.560 --> 0:17:19.040 where you hit it and it travels outwards. So now 0:17:19.080 --> 0:17:21.240 you get a set of circles. Now, if you take 0:17:21.280 --> 0:17:23.800 those set of circles and you extend them like vertically 0:17:23.840 --> 0:17:27.680 above the surface in time and make time that vertical direction, 0:17:27.960 --> 0:17:30.160 then what you get is a cone. Right. It starts 0:17:30.160 --> 0:17:32.440 from a point when you hit the surface of the water, 0:17:32.680 --> 0:17:35.320 and then as time goes on, that circle gets larger 0:17:35.320 --> 0:17:38.040 and larger, and that defines a cone. So it's a 0:17:38.080 --> 0:17:41.040 cone only in a sort of universe that has two 0:17:41.080 --> 0:17:44.320 spatial dimensions and one time dimension. Right, It's kind of 0:17:44.320 --> 0:17:46.440 like if you take a slice of those light spheres 0:17:46.520 --> 0:17:48.600 we were talking about, Like if you take a slice 0:17:48.640 --> 0:17:50.920 of it and you see those that circle on that 0:17:51.119 --> 0:17:56.720 slice growing, and then you imagine that circle growing but 0:17:56.800 --> 0:17:59.520 growing and also moving up maybe at the same time, 0:18:00.040 --> 0:18:02.639 it kind of makes like a cone, yeah, exactly, and 0:18:02.680 --> 0:18:05.080 it moves up because now we're imagining time to be 0:18:05.119 --> 0:18:07.399 one of our axes. Right, we have two dimensions of 0:18:07.440 --> 0:18:10.760 space and we have another direction, which is time. And 0:18:10.800 --> 0:18:13.760 obviously things just flow forward in time at one second 0:18:13.760 --> 0:18:17.400 per second, and so this circle just sort of grows 0:18:17.680 --> 0:18:20.680 in time. If you instead had something which wasn't moving. Right, 0:18:20.800 --> 0:18:23.560 if you just like drew a circle around you, then 0:18:23.640 --> 0:18:27.199 that circle would slide through time and make a cylinder, right, 0:18:27.240 --> 0:18:29.840 because it wouldn't be growing. But if it is growing, 0:18:29.880 --> 0:18:31.640 then it makes a cone. It starts from the point 0:18:31.640 --> 0:18:34.040 where you've made it, and it makes a cone. And 0:18:34.160 --> 0:18:36.000 I guess if you're having trail visual license, you can 0:18:36.000 --> 0:18:38.520 always just pop on Google images or something and look 0:18:38.560 --> 0:18:41.439 for light cone. I imagine you'll get a picture of this, right. 0:18:41.880 --> 0:18:44.520 How do people do podcast before Google image search anymore? 0:18:45.240 --> 0:18:51.640 They probably avoided topics that involved geometry and and multiple dimensions. 0:18:51.720 --> 0:18:53.960 But it's kind of an interesting concept, and I think 0:18:54.040 --> 0:18:57.119 you as a physics you like it because it defines 0:18:57.400 --> 0:19:00.240 kind of like a volume, right. It defines kind of 0:19:00.280 --> 0:19:03.440 like a set of space time that is either inside 0:19:03.480 --> 0:19:05.600 the cone or outside the cone. That's kind of the 0:19:05.680 --> 0:19:08.679 power of this kind of visualization. Yeah, and it's not 0:19:08.760 --> 0:19:10.919 just arbitrary, like you can make whatever shape you want 0:19:10.920 --> 0:19:13.600 in spacetime. But this one is special. This one is 0:19:13.680 --> 0:19:17.480 really important because everything that's inside your future light cone 0:19:17.880 --> 0:19:21.160 are things that you can influence. Right. You can't send 0:19:21.160 --> 0:19:24.600 the message to anything outside your future light cone, like 0:19:24.640 --> 0:19:27.880 anything outside your future. Light cone by definition, is too 0:19:27.920 --> 0:19:30.880 far away for any message to reach because you would 0:19:30.920 --> 0:19:33.400 have to travel faster than the speed of light. And 0:19:33.680 --> 0:19:35.520 if you can send a message, it means you can't 0:19:35.600 --> 0:19:38.840 send any information. That means you can't influence it at all. 0:19:39.119 --> 0:19:41.639 And one of the really tricky, complicated things about special 0:19:41.640 --> 0:19:44.560 relativity is that it plays sometimes with the order of 0:19:44.600 --> 0:19:47.439 events and it seems to have consequences for causality. So 0:19:47.600 --> 0:19:49.720 this helps you sort of keep track of that the 0:19:50.000 --> 0:19:52.600 part of the universe that you can influence, and also 0:19:53.040 --> 0:19:56.679 looking backwards, the part of the universe that can influence you. Right, 0:19:56.720 --> 0:19:58.560 all right, Well, it's get into some of these weird 0:19:58.640 --> 0:20:02.520 consequences for because reality and simultaneity and alien death rays. 0:20:03.440 --> 0:20:18.640 But first let's take a quick break. Alright. We're talking 0:20:18.640 --> 0:20:22.800 about the speed of light, causality, the universe, and light cones, 0:20:23.200 --> 0:20:25.960 which are sort of like a visualization in physics that 0:20:26.080 --> 0:20:29.480 help you kind of figure out what you can't affect 0:20:29.520 --> 0:20:33.040 and what you cannot affect because I guess that's something 0:20:33.119 --> 0:20:36.680 is far enough away you can't affect it right now, 0:20:36.760 --> 0:20:38.359 right because you have to wait for the speed of 0:20:38.440 --> 0:20:41.359 light to get from here there or there to hear 0:20:41.440 --> 0:20:44.000 it doesn't even have to be far away. Something that's 0:20:44.080 --> 0:20:47.040 one foot away from you, like literally one foot away 0:20:47.040 --> 0:20:49.919 from you right now. You can have no influence on 0:20:49.960 --> 0:20:53.199 what's happening right there, right now, because even if you 0:20:53.240 --> 0:20:55.800