WEBVTT - Why does gravity slow down time? 0:00:01.280 --> 0:00:03.440 Hey, Jorhan Daniel here, and we want to tell you 0:00:03.480 --> 0:00:07.280 about our new book. It's called Frequently Asked Questions about 0:00:07.320 --> 0:00:09.880 the Universe because you have questions about the universe, and 0:00:09.960 --> 0:00:12.360 so we decided to write a book all about them. 0:00:12.440 --> 0:00:14.800 We talk about your questions, we give some answers, we 0:00:14.880 --> 0:00:17.520 make a bunch of silly jokes as usual, and we 0:00:17.600 --> 0:00:20.160 tackle all kinds of questions, including what happens if I 0:00:20.200 --> 0:00:22.880 fall into a black hole? Or is there another version 0:00:22.920 --> 0:00:25.480 of you out there that's right? 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Hey Daniel, I've noticed something pretty 0:01:10.440 --> 0:01:13.759 strange about how time works. Oh yeah, you have time 0:01:13.800 --> 0:01:17.080 to make your own theory of time. Well that's the thing, right, Like, 0:01:17.160 --> 0:01:19.920 I noticed that time seems to speed up as you 0:01:19.959 --> 0:01:23.480 get closer to a deadline. That's true. Deadlines seem to 0:01:23.480 --> 0:01:25.880 be infinitely far in the future, until all of a 0:01:25.920 --> 0:01:28.360 sudden there on top of you. Right. I think the 0:01:28.400 --> 0:01:31.400 only thing that can explain it is that time itself 0:01:31.640 --> 0:01:37.280 bends when it's closed to some procrastination. Well, time does 0:01:37.319 --> 0:01:39.920 slow down near a massive object, like a black hole. 0:01:40.040 --> 0:01:42.720 I think maybe I just need less massive deadlines maybe, 0:01:43.560 --> 0:01:45.920 or maybe your two do list is so dense it's 0:01:45.959 --> 0:02:04.840 becoming a black hole. Definitely sucks you in. Hi am Jorge. 0:02:04.840 --> 0:02:07.520 I'm a cartoonist and the creator of PhD comics. Hi 0:02:07.640 --> 0:02:10.320 I'm Daniel. I'm a particle physicist and a professor you 0:02:10.400 --> 0:02:13.400 see Irvine, and I'm just part of Jorges to do list. 0:02:14.480 --> 0:02:17.280 That sounds kind of inappropriate, Daniel, I mean I have 0:02:17.360 --> 0:02:20.840 to be like Daniel, or have to be Daniel. I 0:02:20.840 --> 0:02:23.880 mean that I just contribute elements to your to do list. Oh, 0:02:23.919 --> 0:02:26.240 I see you add to my things to do. I'm 0:02:26.320 --> 0:02:29.840 on your to do list. But anyways, welcome to our 0:02:29.880 --> 0:02:32.840 podcast Daniel and Jorge Explain the Universe, a production of 0:02:32.880 --> 0:02:35.640 My Heart Radio, in which our to do list is 0:02:35.680 --> 0:02:40.160 to explain the entire universe bit by bit, concept by concept, 0:02:40.240 --> 0:02:44.440 puzzle by puzzle to you, our wonderful listeners who deeply 0:02:44.520 --> 0:02:47.480 and desperately want to understand the nature of the universe. 0:02:47.560 --> 0:02:51.200 We find ourselves in the whole project of physics, starting 0:02:51.200 --> 0:02:53.760 with ancient man looking up at the sky and extending 0:02:53.800 --> 0:02:56.160 to the Greeks trying to understand the nature of reality. 0:02:56.480 --> 0:02:59.760 Ends here on the podcast when we try to break 0:02:59.800 --> 0:03:02.200 down on the nature of the universe and explain all 0:03:02.240 --> 0:03:05.560 of it to you and hopefully a timely matter, right, 0:03:05.600 --> 0:03:11.119 because you know, time is money and money is gravity. 0:03:11.480 --> 0:03:14.800 Money is dough and everybody needs it. Yeah, sometimes we 0:03:14.840 --> 0:03:17.000 eat too much dough. But yeah, it is a big, 0:03:17.040 --> 0:03:19.120 beautiful universe and we like to talk about all the 0:03:19.200 --> 0:03:23.359 things in it, even time itself, because time is part 0:03:23.400 --> 0:03:26.120 of the universe. Right. Time is part of the universe 0:03:26.160 --> 0:03:28.840 that we don't understand. Is it a fundamental element of 0:03:28.840 --> 0:03:31.080 the universe? Is it something that bubbles up and just 0:03:31.080 --> 0:03:34.160 sort of appears from other fundamental things like ice cream 0:03:34.240 --> 0:03:38.080 and lava and hurricanes? Or is it really deeply ingrained? 0:03:38.400 --> 0:03:40.680 And on this podcast we don't shy away from asking 0:03:40.720 --> 0:03:44.800 those really big, deep and difficult to grapple with questions, 0:03:44.840 --> 0:03:47.840 you know, like what is time anyway? How do you 0:03:47.840 --> 0:03:50.440 even define it? How do you even ask crisp and 0:03:50.520 --> 0:03:55.480 precise questions about this slipperyst but most essential of concepts. Yeah, 0:03:55.480 --> 0:03:58.400 because time always seems to slip by you no matter 0:03:58.520 --> 0:04:01.560 what do you do, and asking these questions, it's not 0:04:01.640 --> 0:04:03.240 just in ourder to do list, but it's also in 0:04:03.320 --> 0:04:05.960 our want to do list, right, Daniel, I mean, that's 0:04:06.000 --> 0:04:07.680 kind of what you're getting, kid. That is what I 0:04:07.720 --> 0:04:09.600 get paid for, not on a day to day basis, 0:04:09.800 --> 0:04:12.280 and mostly I'm cracking open particles at the large age 0:04:12.360 --> 0:04:15.480 on collider, But these are the big goals of physics, 0:04:15.560 --> 0:04:19.039 is to understand the nature of our experience. You know, 0:04:19.160 --> 0:04:21.800 some people think physics is sort of like abstract and 0:04:22.080 --> 0:04:24.440 separated from humanity. But it's taken me a long time 0:04:24.480 --> 0:04:27.120 to realize that it's the most human of sciences because 0:04:27.160 --> 0:04:29.440 it asked these questions to the very core of the 0:04:29.520 --> 0:04:32.159 context of what it's like to be alive. You know, 0:04:32.279 --> 0:04:35.599 you notice things slipping into the past through this weird 0:04:35.720 --> 0:04:39.680 instantaneous slice called the present. It's definitely something we'd like 0:04:39.720 --> 0:04:43.000 to understand. Are you saying, physics out humanities? The humanities? 0:04:43.080 --> 0:04:48.159 Daniel out philosophies the philosophers. Yeah, you know, I used 0:04:48.160 --> 0:04:51.159 to think that physics was the most interesting science because 0:04:51.200 --> 0:04:53.760 it was probably the most fundamental, the most universal, that 0:04:53.880 --> 0:04:56.760 somehow it escaped humanity, that if aliens came, they wouldn't 0:04:56.760 --> 0:05:00.000 be interested in our biological advances because they wouldn't be 0:05:00.000 --> 0:05:02.200 relevant to them, but they would be interested in what 0:05:02.240 --> 0:05:05.000 we've learned about physics. I'm not so sure about that anymore. 0:05:05.080 --> 0:05:07.280 And can hear all those mathematicians out they're laughing at 0:05:07.320 --> 0:05:10.920 you right now. You're like, you're pure, We're the purest, 0:05:11.880 --> 0:05:15.520 even mathematicians. Man, Mathematics is just codification of the logic 0:05:15.560 --> 0:05:18.560 and human brains. And I suspect that aliens might not 0:05:18.680 --> 0:05:20.479 even do math, and if they do, we might not 0:05:20.640 --> 0:05:25.039 even recognize it. Oh man, you're saying physics supersedes math 0:05:25.240 --> 0:05:27.800 from a human point of view. Oh boy, we just 0:05:27.800 --> 0:05:30.599 started in an academic warrior. You just did that. That's 0:05:30.600 --> 0:05:32.360 not what I'm saying at all. I'm saying that everything 0:05:32.360 --> 0:05:35.280 we do is based in human thought and contextualized by 0:05:35.400 --> 0:05:38.320 human questions. But that doesn't make it worthless. It means 0:05:38.360 --> 0:05:41.200 that we get to ask really interesting questions about what 0:05:41.240 --> 0:05:43.960 it's like to be alive. Yeah, and so sometimes those 0:05:44.080 --> 0:05:47.560 questions get into the idea of time itself, because I 0:05:47.560 --> 0:05:50.080 think maybe most people have a conception of the university's 0:05:50.120 --> 0:05:53.040 sort of existing outside of time, or like it's a 0:05:53.160 --> 0:05:56.760 universe moving through time, but actually physicists think of time 0:05:56.880 --> 0:05:59.880 as part of the universe, right, it's like another thing 0:06:00.040 --> 0:06:02.680 in it. Yeah, we don't really understand. Physicists like to 0:06:02.800 --> 0:06:05.640 divide elements of the universe into things that are fundamental, 0:06:06.080 --> 0:06:09.760 meaning they're like basic, they're essential, they have to be there. 0:06:10.120 --> 0:06:12.960 They define what the universe is, and then other things 0:06:13.000 --> 0:06:14.919 that are emergent that sort of come out of the 0:06:14.960 --> 0:06:18.360 interplay of those fundamental objects. And we don't know if 0:06:18.440 --> 0:06:22.200 time is fundamental, if it's like absolutely essential it's part 0:06:22.240 --> 0:06:24.919 of the nature of the universe, or if it bubbles 0:06:24.960 --> 0:06:27.640 up from something else. We don't know if space and 0:06:27.720 --> 0:06:30.440 matter sort of sit in a framework of time which 0:06:30.480 --> 0:06:32.960 is external to them, or if the whole thing is 0:06:33.040 --> 0:06:36.240 just you know, comes up from some other weird, deep nature. 0:06:36.320 --> 0:06:39.159 We haven't even imagined, like, is it hard coded into 0:06:39.200 --> 0:06:41.880 the circuitry of the universe or is it like a 0:06:41.880 --> 0:06:44.760 program running on top of the chip of the universe 0:06:44.839 --> 0:06:47.000 that's kind of what you mean, right, Or is it 0:06:47.040 --> 0:06:49.680 just the way the humans think, you know? It might 0:06:49.720 --> 0:06:52.760 be that our tendency to think in terms of stories 0:06:52.920 --> 0:06:57.280 and narratives which have cause and effect, might bias us 0:06:57.320 --> 0:07:00.600 to see things as flowing forwards in time, when in 0:07:00.720 --> 0:07:03.919 reality the truth might be much more complex. We just 0:07:04.040 --> 0:07:07.560 an episode about how causality, cause and effect might not 0:07:07.720 --> 0:07:10.960 even be an essential element of the universe, and so 0:07:11.040 --> 0:07:13.840 it might be that time itself could be something that's 0:07:13.840 --> 0:07:16.040 just sort of very human. Yeah, it's sort of like 0:07:16.080 --> 0:07:19.120 this podcast. We have no narrative, right, We are just 0:07:19.280 --> 0:07:23.320 jumping around in time to whatever comes into our minds here. 0:07:23.440 --> 0:07:25.760 But it's these kind of basic questions that give us 0:07:25.760 --> 0:07:29.160 a launching off point for asking questions and doing studies 0:07:29.200 --> 0:07:32.120 and trying to like find a way to grapple with 0:07:32.200 --> 0:07:35.000 these things scientifically, because we could sit here and smoke 0:07:35.040 --> 0:07:37.360 man in appeals and talk about the nuture of time 0:07:37.440 --> 0:07:40.720 forever without making progress. But we want to do experiments. 0:07:40.760 --> 0:07:43.200 We want to use science to add to our body 0:07:43.280 --> 0:07:47.520 of actual knowledge about time. Yeah, so we'll be exploring 0:07:47.600 --> 0:07:50.520 a little bit about this weird concept of time and 0:07:50.640 --> 0:07:55.080 especially about how time behaves according to our theories about 0:07:55.080 --> 0:07:58.200 the universe. So today on the podcast, we'll be asking 0:07:58.240 --> 0:08:08.040 the question why does gravity slow down time? Or should 0:08:08.040 --> 0:08:12.400 I should I ask it slow down time? Or maybe 0:08:12.440 --> 0:08:15.320 you should use your gravest voice, you know, your James 0:08:15.320 --> 0:08:17.920 Earl Jones voice. You're trying to make a gravity joke. 0:08:18.120 --> 0:08:20.640 That's kind of heavy, man, don't make light of the situation. 0:08:20.840 --> 0:08:23.080 I'm a massive fan of that movie. You know. Now, 0:08:23.200 --> 0:08:25.160 this question kind has two components to it, Like you 0:08:25.240 --> 0:08:27.760 just said, it has gravity and time in it. And 0:08:27.840 --> 0:08:31.000 because we know from physics that somehow gravity affects time 0:08:31.080 --> 0:08:34.520 or is related to time, because in the time can 0:08:34.600 --> 0:08:37.559 be affected. It's not like some sort of absolute thing. 0:08:37.760 --> 0:08:40.240 That's right. We do not have like a single clock 0:08:40.360 --> 0:08:43.280 for the universe. The simplest model of how time works 0:08:43.640 --> 0:08:46.439 might be imagining that the universe all over the place 0:08:46.559 --> 0:08:48.720 is in one state. You know, particles are going in 0:08:48.760 --> 0:08:51.280 some direction or they're in some location, and then things 0:08:51.320 --> 0:08:53.880 sort of tick forwards and everything takes a step forward 0:08:54.000 --> 0:08:56.960 in unison and in that picture, the whole universe has 0:08:57.040 --> 0:08:59.720 like a single clock. But we've learned over the last 0:08:59.720 --> 0:09:02.640 time of years that that conception of time is not valid. 0:09:02.679 --> 0:09:05.280 That time flows differently in different parts of the universe, 0:09:05.559 --> 0:09:09.200 and differently four different observers and moving at different speeds. 0:09:09.600 --> 0:09:13.640 So time is much weirder, more local, and less universal 0:09:13.760 --> 0:09:17.720 than we ever imagined. In addition, it's also weirdly bent 0:09:17.840 --> 0:09:21.160 by heavy objects. Yeah, things that are really heavy, like 0:09:21.200 --> 0:09:25.240 black holes or even just our planet. They have gravity, 0:09:25.360 --> 0:09:29.040 and that somehow makes time slowed down. Now, that's a 0:09:29.240 --> 0:09:31.160 that's a pretty weird concept. Bread. I guess that was 0:09:31.240 --> 0:09:34.480 in that movie Interstellar. It was exactly in that movie Interstellar. 0:09:34.520 --> 0:09:37.480 Every time they visited a really heavy planet or came 0:09:37.520 --> 0:09:39.960 near a black hole. When they left, they found that 0:09:40.000 --> 0:09:42.360 the rest of the universe had experienced a lot more 0:09:42.400 --> 0:09:44.160 time than they have. And so I think a lot 0:09:44.160 --> 0:09:47.280 of people are maybe familiar with this part of relativity, 0:09:47.280 --> 0:09:49.800 where like, if you're moving past, time slows down. But 0:09:49.920 --> 0:09:52.840 also it also happens when you're near a heavy object. 0:09:52.960 --> 0:09:55.480 That's right. These are two completely separate effects with different 0:09:55.480 --> 0:09:59.880 sources and importantly different mechanisms and different consequences. Yeah, so 0:10:00.000 --> 0:10:01.920 as usually when we were wondering how many people out 0:10:01.960 --> 0:10:06.720 there even knew that gravity slowed down time or much less, 0:10:06.840 --> 0:10:09.560 I have thought about why it slows down time. So 0:10:09.679 --> 0:10:12.160 Daniel went out there into the internet to ask people 0:10:12.480 --> 0:10:14.640 why does gravity slow down time? I like the way 0:10:14.679 --> 0:10:16.599 you say I went out there into the internet. It 0:10:16.640 --> 0:10:18.640 makes me feel like I got sucked into my computer 0:10:18.760 --> 0:10:21.920 and went and visited these people like tron like the 0:10:22.640 --> 0:10:26.240 m eighties movie plotline and you got, you know, salkt 0:10:26.280 --> 0:10:29.400 into your CRT monitor. Yeah, I'm writing those tron cycles 0:10:29.440 --> 0:10:33.320 around the Internet, gathering information from our lisitors, dodging calls 0:10:33.440 --> 0:10:36.280 and tweeting left and right. But if you out there 0:10:36.320 --> 0:10:39.160 would like me to beat myself into your inbox with 0:10:39.280 --> 0:10:42.080 crazy questions about the nature of the universe so you 0:10:42.120 --> 0:10:45.679 can hear yourself speculate about them on the podcast, Please 0:10:45.720 --> 0:10:48.480 don't be shy. Right to me two questions at Daniel 0:10:48.520 --> 0:10:51.960 and Jorge dot com. Yeah, and you actually answer every email, right, 0:10:52.200 --> 0:10:55.479 I do answer every email with every question from everybody. 0:10:55.640 --> 0:10:58.400 You take the time. It slows me down, but I 0:10:58.440 --> 0:11:01.120 love it all. Right. Well, here's what people had to say. Well, 0:11:01.200 --> 0:11:11.120 gravity is basically the same as acceleration, and when you're accelerating, 0:11:11.720 --> 0:11:15.600 the speed of light stays the same for everybody, and 0:11:15.640 --> 0:11:19.959 so it must be time that's slowing down. And when 0:11:20.000 --> 0:11:23.839 we're going at our normal speeds, you don't really notice it. Um. 0:11:24.040 --> 0:11:30.040 So gravity being equivalent to acceleration causes the same effect. 0:11:30.400 --> 0:11:33.800 Gravity slows down time because as particles with mass moved 0:11:33.800 --> 0:11:36.319 through the Higgs field, they're slowed down. The stronger the 0:11:36.400 --> 0:11:38.640 Higgs field, the gravity is the sole of the quarks 0:11:38.640 --> 0:11:41.920 and leptons move, causing quantum interactions to take place at 0:11:41.920 --> 0:11:44.160 a different rate depending on the strength of the field. 0:11:44.640 --> 0:11:47.160 Scaled up into the macro world, that's what we experienced 0:11:47.200 --> 0:11:49.480 this time. We would never notice if our particles were 0:11:49.480 --> 0:11:52.160 moving at a faster slower speed, if everything we observed 0:11:52.240 --> 0:11:54.880 was experiencing the same Higgs field along with us. I 0:11:54.920 --> 0:11:58.320 guess that the reason it's because of gravity bench down 0:11:58.360 --> 0:12:03.120 of pace time and thinks the length of space faith 0:12:03.320 --> 0:12:06.520 changes in order for the speed of life we're being constant. 0:12:07.280 --> 0:12:11.160 Was the time of flowing differently. First of all, time 0:12:11.360 --> 0:12:19.120 is like space dimensions, so the gravity can affect time. 0:12:19.760 --> 0:12:24.840 But how I'm thinking right now, like the gravity does 0:12:24.960 --> 0:12:31.600 to also the objects that contracts them. And this is 0:12:31.640 --> 0:12:36.079 what I'm thinking, That it contracts time. It squeezes it 0:12:36.480 --> 0:12:41.200 like the space and doing this it is slowing it down. 0:12:41.600 --> 0:12:45.800 I guess gravity could slow down time because gravity makes 0:12:45.840 --> 0:12:51.440 those wells and like the space time fabric, and so 0:12:51.880 --> 0:12:56.360 as that material of the universe gets stretched out, the 0:12:56.520 --> 0:13:01.680 time would slow down as it gets deeper, sort of 0:13:02.240 --> 0:13:06.360 like if you're driving over hills versus driving over flat things. 0:13:06.520 --> 0:13:09.720 I can only imagine it's got something to do with 0:13:10.480 --> 0:13:15.679 space time. Like gravity isn't just a force, but as 0:13:15.720 --> 0:13:22.319 Einstein says, it's a warping of space time. And if 0:13:22.360 --> 0:13:24.320 you've got a ton of gravity staying a black hole, 0:13:24.400 --> 0:13:27.960 warping space space is not really a thing, but space 0:13:28.040 --> 0:13:31.079 time is a thing. So you can't affect one without 0:13:31.080 --> 0:13:36.080 affecting the other, is what I'm guessing. I know time 0:13:36.160 --> 0:13:41.720 moves slower relatively speaking for someone that travels faster, So 0:13:41.840 --> 0:13:46.360 maybe gravity slows town time because it also necessarily makes 0:13:46.920 --> 0:13:50.839 an objectile person move faster. All right, some pretty good 0:13:50.920 --> 0:13:54.120 answers here. A lot of people sort of knew that 0:13:54.240 --> 0:13:57.160 time can slow down, and I guess they assume that 0:13:57.240 --> 0:14:01.480 gravity is someone related to really activity and moving fast 0:14:01.520 --> 0:14:03.880 and so why not. Yeah, a lot of really interesting 0:14:03.960 --> 0:14:06.679 and insightful and thoughtful questions here, some confusion about the 0:14:06.760 --> 0:14:08.640 role of the Higgs field, but also a lot of 0:14:08.679 --> 0:14:11.960 good concepts about the connections between space and time and 0:14:12.000 --> 0:14:14.440 how masses have to bend both of them. All right, well, 0:14:14.520 --> 0:14:17.920 let's get into this idea of time slowing down, and 0:14:17.960 --> 0:14:20.240 I guess let's recap maybe the one people are most 0:14:20.280 --> 0:14:22.480 familiar with, maybe, which is the one that when you're 0:14:22.480 --> 0:14:25.080 moving fast and especially close to the speed of light, 0:14:25.240 --> 0:14:28.800 time slows down for you. So maybe walk us through that, 0:14:28.840 --> 0:14:31.400 and then we'll get into the one about gravitation. That's right, 0:14:31.480 --> 0:14:33.600 So the one you're probably more familiar with. We call 0:14:33.800 --> 0:14:38.200 velocity time dilation. This is something that happens in special relativity. 0:14:38.360 --> 0:14:41.280 And to think about this, you should imagine an empty universe, 0:14:41.320 --> 0:14:44.640 one without really massive objects that are gonna make space 0:14:44.680 --> 0:14:47.400 curvy and all sorts of weird stuff. In this universe, 0:14:47.560 --> 0:14:51.880 light travels in straight lines, and spaceships zoom around and 0:14:51.960 --> 0:14:55.800 all the time. Dilation just comes from the relative velocity 0:14:55.920 --> 0:14:59.360 of objects. And the most important thing to remember is 0:14:59.400 --> 0:15:03.280 that movie clocks run slowly, so people often make the 0:15:03.320 --> 0:15:06.280 mistake of saying, oh, I'm going fast in a spaceship, 0:15:06.320 --> 0:15:09.240 so my time should slow down, right, Well, it's only 0:15:09.320 --> 0:15:11.440 moving clocks that slow down. So if you're in a 0:15:11.520 --> 0:15:14.200 spaceship and you're holding the clock, then the clock is 0:15:14.240 --> 0:15:16.920 not moving relative to you, so you're not going to 0:15:17.000 --> 0:15:20.520 see it slow down. So you never experience velocity time 0:15:20.560 --> 0:15:24.080 dilation because you're never moving a relative to yourself. Somebody 0:15:24.080 --> 0:15:27.000 else out there on a planet that you're zipping by 0:15:27.200 --> 0:15:30.480 could see your clock running slowly because your clock is 0:15:30.600 --> 0:15:33.840 moving for them, and so moving clocks run slowly, meaning 0:15:33.880 --> 0:15:36.840 that your clock would run slow. You don't experience it, 0:15:37.000 --> 0:15:40.160 but they see your clock moving slowly. Right, Like, if 0:15:40.200 --> 0:15:42.880 your time is moving slowly, you don't notice it because 0:15:43.440 --> 0:15:46.520 you know your brain is also sort of moving slowly 0:15:46.560 --> 0:15:48.640 in a way, right, So like everything about you is 0:15:48.680 --> 0:15:51.400 moving slowly as well, and so you don't notice that 0:15:51.440 --> 0:15:53.960 you're actually moving slowly. But even that suggested some sort 0:15:53.960 --> 0:15:57.000 of like universal picture of what really happened. And in 0:15:57.080 --> 0:15:59.440 the universal picture, it makes sense for you to feel 0:15:59.440 --> 0:16:02.840 like time move normally even though it actually moved slowly. 0:16:03.040 --> 0:16:06.400 But there is no like what actually happened. Some observers 0:16:06.400 --> 0:16:10.040 seeing you go by the planet at the speed of light. 0:16:10.160 --> 0:16:13.280 They see your time as moving slowly. You see your 0:16:13.280 --> 0:16:16.320 time as moving normally. You can try to unify those 0:16:16.360 --> 0:16:18.680 into one picture of what actually happened, but there is 0:16:18.760 --> 0:16:22.880 no what actually happened. There's just what different observers observe, 0:16:23.280 --> 0:16:25.400 and what you see depends on where you are and 0:16:25.440 --> 0:16:29.320 how fast things are moving relative to you. For example, 0:16:29.440 --> 0:16:30.960 if you're on the space ship and you're looking at 0:16:30.960 --> 0:16:33.760 the clock on Earth, you see Earth moving past you 0:16:33.840 --> 0:16:37.080 at high speed, and so you see Earth clock running slowly. 0:16:37.400 --> 0:16:40.200 So Earth sees your clock running slow. You see Earth 0:16:40.320 --> 0:16:44.040 clock running slow. What actually happened, Well, both of those 0:16:44.040 --> 0:16:47.080 things happened. Is just what happened depends on where you are. 0:16:47.280 --> 0:16:49.200 I guess maybe stem me through it. So I'm here 0:16:49.200 --> 0:16:52.040 on Earth, I'm watching you on a spaceship go by 0:16:52.600 --> 0:16:54.960 the speed of light, Like, what does it mean for 0:16:55.000 --> 0:16:57.280 me to see your time slow down? Like? I see 0:16:57.280 --> 0:17:00.520 your the clock inside of your spaceship take but it's 0:17:00.520 --> 0:17:03.400 not taking as fast as my clock exactly. So you 0:17:03.440 --> 0:17:06.240 get a telescope. It's super powerful. So you can look 0:17:06.280 --> 0:17:09.159 at a clock that's inside my ship and you watch 0:17:09.200 --> 0:17:11.480 it and you see it's ticks going and you compare 0:17:11.520 --> 0:17:13.760 it to a clock that's right in your hand, not 0:17:13.840 --> 0:17:17.359 moving relative to you. And so every time my clock 0:17:17.440 --> 0:17:19.399 ticks on the spaceship, you see the clock in your 0:17:19.480 --> 0:17:22.399 lap taking ten times. So time is moving faster for 0:17:22.480 --> 0:17:24.600 you in your lap. Then you see it moving for 0:17:24.760 --> 0:17:27.120 me on this ship. And this, of course already takes 0:17:27.119 --> 0:17:29.520 to an account the fact that it takes light time 0:17:29.600 --> 0:17:32.200 to get to you to the telescope from the ship. 0:17:32.240 --> 0:17:34.439 We sort of factor that out already. So what does 0:17:34.440 --> 0:17:36.159 it mean. So I have a telescope and I'm pointing 0:17:36.160 --> 0:17:38.879 it at you, but you're zooming by, so I have 0:17:38.960 --> 0:17:41.560 to kind of track. You have to move my telescope, 0:17:42.119 --> 0:17:44.000 and so I track you as you're going by, and 0:17:44.000 --> 0:17:47.520 I'm moving my telescope and I'm measuring your ticks and 0:17:47.560 --> 0:17:50.040 they're not taking as fast as my clock. That's right. 0:17:50.119 --> 0:17:53.040 You see my clock as running slowly. So you see 0:17:53.080 --> 0:17:56.120 me aging slowly, You see me moving slowly. You see 0:17:56.119 --> 0:17:59.240 me like waving back to you in super slow motion. 0:17:59.680 --> 0:18:02.639 So you see my clock is running slow relative to 0:18:02.680 --> 0:18:05.359 your clock. And that's the sort of a consequence of 0:18:05.440 --> 0:18:07.919 just how the universe is or or to the somehow 0:18:07.920 --> 0:18:11.040 the limitations of the speed of light. Yeah, it's really interesting. 0:18:11.160 --> 0:18:13.760 You can start from lots of different places to derive this. 0:18:13.920 --> 0:18:16.320 You can say like, well, we've seen that nothing can 0:18:16.320 --> 0:18:18.240 move faster than the speed of light. That's a hard 0:18:18.320 --> 0:18:21.520 limit on the speed of information. And from that you 0:18:21.560 --> 0:18:24.800 can derive these things, these time dilation effects, and you 0:18:24.800 --> 0:18:26.639 can walk yourself through an example. We actually have it 0:18:26.760 --> 0:18:29.720 worked out in detail in our book We have No Idea, 0:18:29.920 --> 0:18:32.320 a Guide to the Unknown Universe. And you can think 0:18:32.359 --> 0:18:36.040 about how a photon clock would tick in a spaceship 0:18:36.040 --> 0:18:38.080 as it moves up and down, and if it's moving 0:18:38.119 --> 0:18:39.879 really fast and it has to go like a little 0:18:39.880 --> 0:18:43.919 bit diagonal for example, And because light can't move faster 0:18:44.080 --> 0:18:46.400 than the speed of light, when it moves on a diagonal, 0:18:46.440 --> 0:18:48.399 it takes longer to get from one side of the 0:18:48.440 --> 0:18:51.960 clock to the other. So it's basically the constancy of 0:18:52.000 --> 0:18:54.040 the speed of light and the fact that nothing, even 0:18:54.119 --> 0:18:56.560 light can move faster than the speed of light directly 0:18:56.640 --> 0:18:59.679 lead to this consequence. The time goes slower for moving 0:18:59.680 --> 0:19:03.000 clock right. And so there's this sort of famous scenario 0:19:03.160 --> 0:19:06.159 called the twin paradox for the twin experiment where like 0:19:06.359 --> 0:19:08.280 you take a pair of twins here on Earth, and 0:19:08.320 --> 0:19:10.560 you put one of them in a spaceship that goes 0:19:10.600 --> 0:19:12.280 out into space at the speed of light and then 0:19:12.359 --> 0:19:15.919 comes back, and supposedly when they come back there are 0:19:15.960 --> 0:19:18.240 a different age than the one that stayed on Earth. 0:19:18.480 --> 0:19:20.639 This is a wonderful paradox because it gets to the 0:19:20.680 --> 0:19:23.960 heart of like what actually happened. Because in the example 0:19:24.000 --> 0:19:27.399 we're talking about, it feels awfully symmetric, right like I'm 0:19:27.440 --> 0:19:29.720 looking at you through my telescope and I'm seeing your 0:19:29.920 --> 0:19:33.120 clock take slowly. But maybe on the ship you're also 0:19:33.240 --> 0:19:36.040 looking at me on Earth and you see my clock 0:19:36.080 --> 0:19:38.040 taking slowly. So you want to feel like, well, what 0:19:38.160 --> 0:19:40.399 actually happens? And the way to bring that to a 0:19:40.440 --> 0:19:43.120 point is to like bring those two people back together. 0:19:43.560 --> 0:19:46.240 So if one twin goes off on their spaceship twin 0:19:46.320 --> 0:19:49.159 on Earth sees the spaceship twin aging slowly, and the 0:19:49.280 --> 0:19:52.560 spaceship twin sees the Earth twin aging slowly, and so 0:19:52.600 --> 0:19:54.879 you want to feel like, well, which one is actually younger? 0:19:55.359 --> 0:19:57.840 And so you turn the spaceship twin around bring it 0:19:57.880 --> 0:20:00.080 back to Earth, and you ask like, well, which and 0:20:00.320 --> 0:20:02.720 is younger? And what you discover is that the spaceship 0:20:02.760 --> 0:20:06.600 twin is younger, and so that sort of breaks this symmetry. 0:20:06.880 --> 0:20:08.560 And you wonder, like, hold on a second, if this 0:20:08.600 --> 0:20:10.680 is supposed to be symmetric, if it just depends on 0:20:11.040 --> 0:20:14.240 relative velocities, why is it that one of them is 0:20:14.280 --> 0:20:16.439 now younger than the other. So that means it's not 0:20:16.480 --> 0:20:19.800 just like a perception thing, it's like time actually slowed 0:20:19.800 --> 0:20:22.280 down for the space twin. In that case, it is 0:20:22.400 --> 0:20:25.359 because they've broken special relativity. One of the rules of 0:20:25.400 --> 0:20:29.120 special relativity is no acceleration. You can fly at high 0:20:29.200 --> 0:20:31.320 velocity and then you can make all these measurements and 0:20:31.359 --> 0:20:34.080 do these calculations, and things are just as we described. 0:20:34.359 --> 0:20:37.399 But as soon as you accelerate, then you're out of 0:20:37.440 --> 0:20:40.000 the bounds of special relativity. It's something you can do. 0:20:40.200 --> 0:20:41.840 We can you can do it, we can calculate it. 0:20:41.840 --> 0:20:44.680 But it makes things more complicated, and the simple rules 0:20:44.680 --> 0:20:48.400 we described earlier of moving clocks run slow get much 0:20:48.400 --> 0:20:51.200 more tricky. So here, for example, when the spaceship twin 0:20:51.359 --> 0:20:54.280 turns around to come back to Earth so that he 0:20:54.359 --> 0:20:57.800 or she can compare her age with her twin, then 0:20:58.119 --> 0:21:02.439 she's accelerating because changing your direction means accelerating, And what 0:21:02.480 --> 0:21:04.920 happens when you accelerate is you break the symmetry. Now 0:21:05.000 --> 0:21:07.840 one of the twins is accelerating, the other one is not, 0:21:08.400 --> 0:21:12.080 And when you accelerate, weird things happen to time. Specifically, 0:21:12.080 --> 0:21:14.680 when the twin in the spaceship turns around and accelerates, 0:21:14.920 --> 0:21:18.000 time jumps forward for the rest of the universe. So 0:21:18.119 --> 0:21:21.919 time runs a little slower for the accelerating twin jumping 0:21:21.960 --> 0:21:24.120 forward for the rest of the universe, which is why 0:21:24.160 --> 0:21:27.159 the twin in the spaceship now is younger when they 0:21:27.240 --> 0:21:29.600 arrive at Earth than the twin that stayed home. You 0:21:29.720 --> 0:21:31.679