WEBVTT - Why do moving objects look shorter? 0:00:08.840 --> 0:00:11.080 Hey, Daniel, how long does it take you to do 0:00:11.160 --> 0:00:13.240 the research for one of these episodes? 0:00:13.560 --> 0:00:15.440 You know, it used to take me quite a while, 0:00:15.520 --> 0:00:16.840 but I'm getting quicker at it. 0:00:16.960 --> 0:00:19.720 Mmmm. Is that because time passes more quickly as you 0:00:19.760 --> 0:00:22.640 get older, like a special relativity thing? 0:00:23.920 --> 0:00:26.320 Maybe? Or I guess I've just gotten more efficient at 0:00:26.360 --> 0:00:26.920 the preparation. 0:00:27.160 --> 0:00:29.840 Oh, yeah, like you can read faster, or you can 0:00:29.920 --> 0:00:31.360 process information better. 0:00:32.320 --> 0:00:35.320 No, I think I just developed a better mental whorehe 0:00:35.520 --> 0:00:38.080 simulator that can predict what you might be interested in 0:00:38.280 --> 0:00:40.040 or what you might not be interested in. 0:00:40.280 --> 0:00:43.120 Oh, it sounds like you just need like an AI 0:00:43.280 --> 0:00:45.520 version of me. Then I don't even need to be here. 0:00:46.560 --> 0:00:48.640 It's not artificial intelligence. It's natural. 0:00:48.960 --> 0:00:52.120 It's a cartoonist intelligence. Well, I guess you know. It 0:00:52.280 --> 0:00:55.480 kind of depends. You know, what's interesting is kind of relative, 0:00:55.520 --> 0:00:55.800 isn't it. 0:00:56.080 --> 0:00:59.000 Yeah, that's right. What's especially interesting is kind of relative, 0:00:59.080 --> 0:01:01.760 So I guess it kind of it is all special relativity. 0:01:02.360 --> 0:01:06.240 Yeah, I definitely have some special relatives. Did you predict 0:01:06.280 --> 0:01:07.080 I was going to say that. 0:01:07.160 --> 0:01:24.640 I predicted you were going to say something hilarious. Hi. 0:01:24.720 --> 0:01:27.560 I'm Jorge mc cartoonist and the author of Oliver's great, 0:01:27.600 --> 0:01:28.360 big Universe. 0:01:28.920 --> 0:01:32.000 Hi. I'm Daniel. I'm a particle physicist and a professor 0:01:32.080 --> 0:01:35.399 at UC Irvine, and I'm still training my brain to 0:01:35.480 --> 0:01:36.600 understand the universe. 0:01:36.800 --> 0:01:38.640 Wait, don't you need to know how the universe works 0:01:38.680 --> 0:01:40.399 before you can train your brain to understand it. 0:01:40.680 --> 0:01:42.880 We can do two things at once. We can try 0:01:42.880 --> 0:01:45.080 to figure out how the universe works, and we can 0:01:45.120 --> 0:01:46.959 try to wrap our minds around the bits that we 0:01:47.040 --> 0:01:49.960 have figured out and try to gain some intuition for 0:01:50.120 --> 0:01:51.200 all this craziness. 0:01:52.120 --> 0:01:53.880 I feel like this podcast is a little bit of 0:01:53.920 --> 0:01:56.640 training into like how to think about the universe, how 0:01:56.640 --> 0:01:59.360 to explore it, how to ask questions about it. Should 0:01:59.400 --> 0:02:00.560 we be charging tuition? 0:02:03.040 --> 0:02:05.560 We're just sharing the love, you know. But you're right 0:02:05.600 --> 0:02:08.320 because most of our modern theories or physics are expressed 0:02:08.320 --> 0:02:12.440 in a kind of inaccessible language mathematical equations that do 0:02:12.600 --> 0:02:15.720 show us the relationship between bits and pieces and allow 0:02:15.800 --> 0:02:17.840 us to make predictions and stuff. But it's not always 0:02:17.840 --> 0:02:20.000 the way our brains work. So one of the goals 0:02:20.000 --> 0:02:22.120 on this podcast is to break it down and give 0:02:22.120 --> 0:02:25.040 you an intuitive understanding, to try to make it all 0:02:25.160 --> 0:02:26.160 make sense to you. 0:02:26.400 --> 0:02:29.119 Yeah, I guess. We offer free intuition, but not at 0:02:29.200 --> 0:02:30.480 a cost of free tuition. 0:02:31.680 --> 0:02:34.079 No tuition intuition. That should have been the title of 0:02:34.080 --> 0:02:34.720 the podcast. 0:02:35.760 --> 0:02:38.079 Yeah, hopefully it all comes to fruition. 0:02:39.880 --> 0:02:42.280 But I mean, you're welcome to pay tuition if you like, 0:02:42.520 --> 0:02:45.680 send us gold coins or dollars if the inspiration strikes you. 0:02:45.680 --> 0:02:48.160 You take tips. You have a tip jar in your classroom. 0:02:48.240 --> 0:02:49.960 I don't have a tip jar, but I did once 0:02:50.000 --> 0:02:52.600 walk into a classroom and find an envelope with thousands 0:02:52.639 --> 0:02:53.320 of dollars in it. 0:02:53.840 --> 0:02:58.000 Oh, WHOA, where is your university. 0:02:58.320 --> 0:03:00.799 It's a public university, man, So it's not like some 0:03:00.840 --> 0:03:03.480 students just like dropped a pile of cash. I think 0:03:03.480 --> 0:03:05.639 it must have been some club organizing in the room 0:03:05.680 --> 0:03:07.120 before us or something. Hmmmm. 0:03:07.840 --> 0:03:10.160 Sounds like maybe they were trying to bribe you. 0:03:10.480 --> 0:03:13.760 I don't think so. I think somebody must have panicked 0:03:13.800 --> 0:03:16.000 when they realized they left it behind. 0:03:15.919 --> 0:03:17.400 Or could have been a tip. I don't know. You 0:03:17.400 --> 0:03:18.720 could have just assumed it was a tip. 0:03:18.800 --> 0:03:19.919 I'm not so cheaply bought. 0:03:20.040 --> 0:03:22.600 But anyways, welcome to our podcast. Daniel and Jorge Explain 0:03:22.680 --> 0:03:25.480 the Universe, a production of iHeartRadio. 0:03:24.800 --> 0:03:28.160 In which we use our inspiration to improve your intuition 0:03:28.400 --> 0:03:32.000 for absolutely no tuition, whether or not the jokes come 0:03:32.000 --> 0:03:33.320 to any fruition. 0:03:33.080 --> 0:03:35.360 That's right, because it is an amazing universe for us 0:03:35.400 --> 0:03:38.320 to ask questions about and to explore and to learn 0:03:38.360 --> 0:03:39.440 about how it all works. 0:03:39.680 --> 0:03:42.320 And part of the process of doing physics is not 0:03:42.480 --> 0:03:45.040 just writing down on some sheet of paper a bunch 0:03:45.080 --> 0:03:47.760 of math and then gesturing at and saying, trust us, 0:03:47.840 --> 0:03:51.320 this works. It's about understanding. In the end, physics and 0:03:51.520 --> 0:03:54.800 all of science is motivated by our curiosity, our desire 0:03:54.920 --> 0:03:58.480 to understand, and that means translating all of this stuff, 0:03:58.520 --> 0:04:01.960 everything we've learned, back into something that makes sense to us, 0:04:02.080 --> 0:04:04.560 that clicks together in our minds, and we go, oh, well, 0:04:04.720 --> 0:04:07.640 that's kind of weird, but I guess it does make sense. Yeah. 0:04:07.640 --> 0:04:09.400 Plus, I don't know if I trust you anymore. I mean, 0:04:09.400 --> 0:04:11.040 now that I know you're open to bribery. 0:04:12.880 --> 0:04:15.280 I did say I was opening bribery. You said I 0:04:15.320 --> 0:04:16.920 was opening bribery, and I said I. 0:04:17.000 --> 0:04:21.200 Wasn't it sounds like you're open to tips quote unquote tips. 0:04:21.839 --> 0:04:24.159 People are welcome to send us donations to the podcast, 0:04:24.400 --> 0:04:26.000 but it's not going to affect how I grade in 0:04:26.040 --> 0:04:28.240 my class. Now I turned that stack of cash over 0:04:28.279 --> 0:04:29.200 to the campus police. 0:04:29.360 --> 0:04:32.279 Oh wow, are saying you tip the police. 0:04:32.720 --> 0:04:34.400 I don't know what they did with it, but I 0:04:34.400 --> 0:04:37.560 can only assume that they did something responsible. 0:04:37.839 --> 0:04:40.040 But yeah, this university needs to depend on your point 0:04:40.080 --> 0:04:42.320 of view and also how fast you're. 0:04:42.200 --> 0:04:45.000 Going, because something we've learned over the last hundred years 0:04:45.480 --> 0:04:49.440 is that our ancient intuition for how things work doesn't 0:04:49.600 --> 0:04:53.040 always work. It only works in the special cases that 0:04:53.160 --> 0:04:56.799 us and our ancestors got to see things moving pretty slowly. 0:04:56.839 --> 0:04:58.839 And when you take those rules and try to extrapolate 0:04:58.880 --> 0:05:01.520 them up to things moving really, really quickly, you find 0:05:01.640 --> 0:05:04.039 they just don't work anymore. You need a new set 0:05:04.040 --> 0:05:07.039 of rules, and those rules give us weird new insights 0:05:07.040 --> 0:05:09.160 into the actual nature of space and time. 0:05:09.520 --> 0:05:12.480 Yeah, over eight hundred years ago, Einstein discovered something called 0:05:12.520 --> 0:05:13.719 special relativity. 0:05:13.880 --> 0:05:17.280 Einstein developed special and general relativity. That's absolutely true. I 0:05:17.320 --> 0:05:19.760 don't know if he invented it or discovered it, he 0:05:19.839 --> 0:05:22.680 certainly uses it to describe the universe that we see 0:05:22.680 --> 0:05:23.200 around us. 0:05:23.440 --> 0:05:26.200 Yeah. Over a one hundred years hegol Einstein discovered something 0:05:26.240 --> 0:05:30.039 called relativity special in general, in which he discovered that 0:05:30.080 --> 0:05:33.240 things don't always look the same in this universe. Sometimes 0:05:33.320 --> 0:05:35.560 it sort of depends on your relative point of view. 0:05:35.640 --> 0:05:39.120 Yeah, Einstein really shook the foundations of our understanding of reality. 0:05:39.440 --> 0:05:42.320 Newton gave us this idea of space and time as 0:05:42.400 --> 0:05:46.159 being absolute, like the fixed backdrop of the universe, but 0:05:46.240 --> 0:05:50.159 Einstein showed us that space is relative and so is time, 0:05:50.520 --> 0:05:54.160 and it leads to all sorts of bizarre effects. Clock sprunning, slower, 0:05:54.240 --> 0:05:58.320 meter sticks looking shorter, and information disappearing into black holes. 0:05:58.560 --> 0:06:00.760 Yeah, it kind of seems sometimes that the universe has 0:06:00.800 --> 0:06:04.400 these paradoxes and these contradictions, but actually it all sort 0:06:04.440 --> 0:06:06.680 of works if you dig into the math, right, It 0:06:06.720 --> 0:06:09.120 all does actually make sense in the end. It's just 0:06:09.240 --> 0:06:11.960 a different kind of sense. So today on the podcast, 0:06:11.960 --> 0:06:21.040 we'll be tackling the question why do moving objects look shorter? 0:06:21.520 --> 0:06:23.000 I guess moving objects look shorter. 0:06:25.680 --> 0:06:27.920 This is not like how your bank account seems to 0:06:27.960 --> 0:06:30.839 shrink when you're on a trip, you know, or on vacation. 0:06:31.120 --> 0:06:34.440 This is the effect commonly known as length contraction, that 0:06:34.520 --> 0:06:37.720 things going super duper fast relative to you seem to 0:06:37.800 --> 0:06:40.279 look shorter than they do when they're at rest. 0:06:40.720 --> 0:06:43.119 That's one of the consequences of having a speed limit 0:06:43.160 --> 0:06:44.240 in the universe, isn't it. 0:06:44.320 --> 0:06:47.520 Absolutely. It's one of the consequences of special relativity, which 0:06:47.520 --> 0:06:49.520 starts from the postulates if there's a speed limit to 0:06:49.560 --> 0:06:52.760 the universe and that light always moves at the speed 0:06:52.800 --> 0:06:55.320 of life for everybody, regardless of who they are and 0:06:55.360 --> 0:06:58.359 how fast they're going. Out of that comes all sorts 0:06:58.400 --> 0:07:01.160 of weird changes we have to make to space and 0:07:01.240 --> 0:07:05.000 time and velocity in order to make things still make sense. 0:07:05.480 --> 0:07:08.080 And so On the podcast here we've talked about how 0:07:08.640 --> 0:07:11.240 as you're moving at close to the speed of lighter 0:07:11.360 --> 0:07:16.560 moving near really massive objects, time slows down. We've talked 0:07:16.560 --> 0:07:19.840 about that quite a bit, right, Yeah, moving clocks run slow. 0:07:19.960 --> 0:07:21.920 So if you see a clock moving past you at 0:07:21.960 --> 0:07:24.160 half the speed of light, then you will see a 0:07:24.240 --> 0:07:28.360 tick slower than a clock you are holding. That's time dilation, 0:07:28.480 --> 0:07:31.240 a concept we've talked about lots and lots of times, right. 0:07:31.280 --> 0:07:33.400 And then there's the idea that it's not only time 0:07:33.440 --> 0:07:37.800 that can sort of stretch and change, it's also distances 0:07:38.200 --> 0:07:39.440 or the lengths of things. 0:07:39.800 --> 0:07:43.200 Yeah, if you are traveling really really fast, then distances 0:07:43.280 --> 0:07:45.480 in front of you seem shorter, like if you're flying 0:07:45.520 --> 0:07:48.440 between here and Jupiter, and you're moving really really fast, 0:07:48.480 --> 0:07:52.160 then the distance between here and Jupiter seems to shrink. 0:07:52.520 --> 0:07:55.840 So like, if there was a giant meter stick between 0:07:55.880 --> 0:07:58.240 here and Jupiter, and I was moving towards Jupiter really 0:07:58.240 --> 0:08:00.880 really fast, then that meter stick would look shorter than 0:08:00.880 --> 0:08:01.880 it actually is to me. 0:08:02.080 --> 0:08:03.680 It would look shorter to you, but there is no 0:08:04.040 --> 0:08:06.559 what it actually is. There's just a lot of different 0:08:06.600 --> 0:08:08.520 viewpoints on it. It's all relative. 0:08:08.760 --> 0:08:10.360 Well, I guess what it would look like if you 0:08:10.360 --> 0:08:13.400 were not moving relative to the stick exactly. 0:08:13.880 --> 0:08:15.600 Yeah, And that's what we call the proper length. 0:08:15.800 --> 0:08:17.480 In that case, I'm moving really fast, it's not the 0:08:17.560 --> 0:08:18.920 meter stick, or is it the same thing. 0:08:19.240 --> 0:08:21.240 It's exactly the same thing. The meter stick would say 0:08:21.320 --> 0:08:23.480 you look shorter, and you would say the meter stick 0:08:23.520 --> 0:08:26.280 looks shorter. The whole effect is symmetric, same way the 0:08:26.280 --> 0:08:29.280 time valation is symmetric. If I think you're moving fast, 0:08:29.320 --> 0:08:31.880 I see your clock running slow, but you also see 0:08:31.920 --> 0:08:34.839 me moving quickly, so you see my clock running slow. 0:08:35.480 --> 0:08:38.000 But I guess, doesn't it depend on which direction this 0:08:38.160 --> 0:08:41.360 ruler is flying. Like if the ruler is flying let's 0:08:41.360 --> 0:08:44.440 say away for me really fast, it would look shorter. 0:08:44.600 --> 0:08:46.920 But what if it's moving, like from I see, moving 0:08:46.920 --> 0:08:49.240 from my left to my right, would it still look shorter? 0:08:49.400 --> 0:08:51.880 It would look thinner. In that case, the direction it's 0:08:51.920 --> 0:08:54.880 moving relative to you is the direction that gets contracted. 0:08:55.080 --> 0:08:58.000 If it's always moving along its length, then it's going 0:08:58.080 --> 0:09:00.240 to look shorter, even if that length is point. Did 0:09:00.240 --> 0:09:02.760 it along X or along y or along zo or 0:09:02.800 --> 0:09:05.400 in any direction? If it's moving along its length, it's 0:09:05.400 --> 0:09:06.120 going to look shorter. 0:09:06.559 --> 0:09:09.640 I see. So like if someone launched a ruler really 0:09:09.640 --> 0:09:11.520 really fast from my left and my right, it would 0:09:11.559 --> 0:09:13.480 seem shorter than it would if I just held it 0:09:13.520 --> 0:09:13.880 in my hand. 0:09:14.080 --> 0:09:14.920 That's exactly right. 0:09:14.960 --> 0:09:17.520 Yeah, so then the question for today is why does 0:09:17.559 --> 0:09:19.760 that happen? All right? Well, as usual, we were wondering 0:09:19.800 --> 0:09:21.959 how many people out there had thought about this question 0:09:22.080 --> 0:09:25.160 or wonder why things look shorter when they're moving really fast. 0:09:25.320 --> 0:09:27.959 Thanks very much to everybody who answers these questions. We 0:09:28.200 --> 0:09:31.120 love hearing your voices, and we'd love to hear more 0:09:31.120 --> 0:09:33.880 of your voices, So please don't be shy and write 0:09:33.880 --> 0:09:36.760 to us two questions at Danielandhorge dot com. 0:09:36.960 --> 0:09:38.520 So think about it for a second. Why do you 0:09:38.559 --> 0:09:42.280 think moving objects look shorter. Here's what people have to say. 0:09:42.360 --> 0:09:44.280 The Doppler effect is that what it's called the red 0:09:44.320 --> 0:09:44.920 shifting thing. 0:09:45.200 --> 0:09:48.320 So like, basically when the object, the object comes closer 0:09:48.360 --> 0:09:49.840 to you, like I guess, like, it looks like the 0:09:49.880 --> 0:09:55.359 wavelengths wavelengths shorten, right, they get shorter. So like all objects, 0:09:55.480 --> 0:09:57.720 you know, they get a little louder, the colors are a 0:09:57.720 --> 0:10:00.160 little brighter, the wavelengths shorten, and so the object look 0:10:00.200 --> 0:10:01.960 shorter because the wavelengths of the colors of the object 0:10:02.040 --> 0:10:02.360 is shorter. 0:10:02.640 --> 0:10:04.439 Yeah, that's why. 0:10:04.320 --> 0:10:06.040 I think they look shorter, just due to the fact 0:10:06.040 --> 0:10:07.800 that it's more for our brain to process, you know, 0:10:07.840 --> 0:10:10.600 as opposed to if an object was sitting still, and 0:10:10.640 --> 0:10:14.000 then obviously when there's more to interpret, that obviously creates 0:10:14.480 --> 0:10:17.040 more of a margin of error with our perception. 0:10:17.280 --> 0:10:21.640 I would assume that the reason that objects appear shorter, 0:10:22.360 --> 0:10:27.600 especially as they approach light speed, is time and space dilation. 0:10:28.880 --> 0:10:32.600 So as it approaches light speed, it would just appear 0:10:32.640 --> 0:10:34.160 to shrink. Not really sure. 0:10:34.320 --> 0:10:37.400 I think that for a human eye optics, moving objects 0:10:37.640 --> 0:10:41.720 look actually longer because of motion blur, but special relative 0:10:41.800 --> 0:10:48.760 to states that moving objects get shorter, don't exactly remember why. 0:10:48.840 --> 0:10:54.680 Moving objects look shorter because the back has gotten closer 0:10:54.720 --> 0:10:57.040 to where the front was when you saw it by 0:10:57.040 --> 0:10:58.760 the time you see the back. 0:10:58.960 --> 0:11:01.599 All right, Lots of interesting answers. 0:11:01.200 --> 0:11:05.079 Here, very creative answers today, but. 0:11:05.000 --> 0:11:09.440 They also sound very sciensy talking about Doppler effects and 0:11:10.080 --> 0:11:13.360 red shifting and space dilation. 0:11:13.720 --> 0:11:16.719 Yeah, motion blur. Some of these answers are way off 0:11:16.760 --> 0:11:18.040 and some of them are pretty close. 0:11:18.280 --> 0:11:20.400 Pretty cool. These are people that you found in your 0:11:20.400 --> 0:11:21.720 department or people on the internet. 0:11:21.800 --> 0:11:24.599 These are just folks from the internet listeners to this podcast. 0:11:25.000 --> 0:11:28.440 Nice, Well, let's dig into it. Daniel, how what is 0:11:28.480 --> 0:11:32.600 the physicist definition of length dilation? 0:11:32.920 --> 0:11:34.600 So we usually call it length contraction. 0:11:34.880 --> 0:11:37.040 Wait is there a difference? Can you also have that 0:11:37.120 --> 0:11:39.280 length dilation or is it only contract. 0:11:39.000 --> 0:11:42.040 You can only have length contraction. The longest something can 0:11:42.080 --> 0:11:44.440 be is if it's at rest relative to you. So 0:11:44.480 --> 0:11:47.120 you're holding a meter stick, that's the longest you're ever 0:11:47.160 --> 0:11:49.800 going to measure that meter stick to be that meterstick 0:11:49.880 --> 0:11:52.040 is moving past you at a certain speed, then you're 0:11:52.080 --> 0:11:54.880 going to see it shorter. And the way I like 0:11:54.920 --> 0:11:56.880 to phrase it to be very clear to make sure 0:11:56.880 --> 0:11:59.600 it's relative, is that moving objects look shorter. 0:12:00.000 --> 0:12:02.559 And if they're moving like away from you or towards you, 0:12:02.679 --> 0:12:03.760 it still gets shorter. 0:12:03.960 --> 0:12:06.200 They still get shorter. Yeah, that doesn't make any difference. 0:12:06.559 --> 0:12:09.040 In the same way that like moving clocks run slow 0:12:09.240 --> 0:12:11.800 tells you that you are seeing that clock run slow 0:12:11.840 --> 0:12:14.480 because you see it moving and it sees you moving, 0:12:14.520 --> 0:12:17.200 so it sees you running slow. In the same way 0:12:17.200 --> 0:12:20.320 you see a meter stick moving, you see it looking shorter. 0:12:20.480 --> 0:12:24.480 It also sees you looking shorter because it sees you moving. 0:12:24.880 --> 0:12:26.200 It's a symmetric effect. 0:12:27.559 --> 0:12:30.839 All right, I was wrong. Well is the This is 0:12:30.880 --> 0:12:32.640 the definition of the length contraction. 0:12:32.840 --> 0:12:35.240 So it's just that moving objects look shorter. You define 0:12:35.240 --> 0:12:37.679 the proper length of something to be its length when 0:12:37.679 --> 0:12:39.839 it's not moving. So you're holding the meterstick, you call 0:12:39.880 --> 0:12:42.839 that the proper length. Then you zoom that meterstick by 0:12:42.880 --> 0:12:45.160 you at like ten percent of the speed of light, 0:12:45.280 --> 0:12:49.400 which doesn't sound impressive, but it's super duper crazy fast. 0:12:49.880 --> 0:12:51.600 Then you'll see it a little bit shorter. You'll see 0:12:51.640 --> 0:12:54.640 it like a half a percent shorter. You speed it 0:12:54.720 --> 0:12:56.800 up to like half the speed of light, and you'll 0:12:56.800 --> 0:12:59.400 see it be like only eighty five percent of its 0:12:59.440 --> 0:13:01.880 proper length length. And as you get up to like 0:13:02.240 --> 0:13:04.400 ninety nine percent of the speed of light, it'll shrink 0:13:04.440 --> 0:13:07.600 down to like fifteen percent of its proper length. If 0:13:07.600 --> 0:13:10.080 you get to like ninety nine point ninety nine percent 0:13:10.080 --> 0:13:12.160 of the speed of light, it'll be just over one 0:13:12.240 --> 0:13:15.360 percent of its proper length. So you can see things 0:13:15.400 --> 0:13:18.400 getting like super duper short as they get super duper 0:13:18.480 --> 0:13:19.600 fast relative to you. 0:13:21.000 --> 0:13:25.200 I guess I'm wondering what it means to see something 0:13:25.240 --> 0:13:26.920 like that moving that fast. You mean, like if I 0:13:27.000 --> 0:13:30.240 take a picture of it, Like, let's say a ruler 0:13:30.280 --> 0:13:33.400 is zooming by really quickly, buy me from my left 0:13:33.440 --> 0:13:35.000 to my right, and I take a picture of it 0:13:35.040 --> 0:13:36.839 as it's going in front of me. In the picture, 0:13:36.840 --> 0:13:39.520 it's going to seem shorter than it would if I 0:13:39.559 --> 0:13:40.880 held the ruler in my hand. 0:13:41.080 --> 0:13:43.080 It's great that you asked that question, because we have 0:13:43.120 --> 0:13:45.280 to be very specific about what we mean here by 0:13:45.280 --> 0:13:48.560 these measurements, because it makes a difference. And actually, if 0:13:48.600 --> 0:13:50.520 you take a picture, you would see the ruler looking 0:13:50.520 --> 0:13:53.800 weirdly longer because the picture captures all the photons that 0:13:53.840 --> 0:13:56.880 are arriving at you at one moment, and that includes 0:13:56.920 --> 0:13:59.600 another effect, which is that it takes time for the 0:13:59.679 --> 0:14:02.400 infram to get to you, and part of the ruler 0:14:02.440 --> 0:14:04.520 is further from you, and part of the rulers closer 0:14:04.559 --> 0:14:07.320 to you. So that combines two different effects. One is 0:14:07.520 --> 0:14:10.320 that the ruler looks shorter because it's moving faster, and 0:14:10.360 --> 0:14:12.360 the other one at the back of the ruler is 0:14:12.400 --> 0:14:14.600 further from you, so it takes light longer to get 0:14:14.640 --> 0:14:14.880 to you. 0:14:15.160 --> 0:14:16.679 No, now, but if the ruler is moving from my 0:14:16.800 --> 0:14:19.120 left to my right, like it's zooming past me like 0:14:19.160 --> 0:14:22.240 a train would like if I'm standing then next to 0:14:23.640 --> 0:14:25.760 some train tracks and I see the train going from 0:14:25.760 --> 0:14:27.680 my left to my right, and I take a picture 0:14:27.720 --> 0:14:30.200 of the train as it's passing by in front of me. 0:14:30.800 --> 0:14:33.320 Is that draining It seemed shorter in the picture or 0:14:33.480 --> 0:14:34.880 the same length or longer. 0:14:35.000 --> 0:14:37.600 Again, the picture combines two different effects. One is a 0:14:37.640 --> 0:14:40.240 special relativity effect and the other is the very normal 0:14:40.440 --> 0:14:43.720 light takes time to travel effect. And so usually what 0:14:43.760 --> 0:14:45.760 we do is not use the idea of a mental picture, 0:14:46.000 --> 0:14:48.240 but imagine that we have like a bunch of assistants 0:14:48.360 --> 0:14:50.200 all spread out to our left and to our right, 0:14:50.520 --> 0:14:52.840 and they mark like when the train front or when 0:14:52.840 --> 0:14:56.080 the train back passes them, and then we can use 0:14:56.120 --> 0:14:58.400 the distance between the train front and the train back 0:14:58.440 --> 0:14:59.720 to say how long. 0:14:59.520 --> 0:15:02.000 It is h what happens in the picture case, like 0:15:02.000 --> 0:15:03.760 what if I take a picture of the train or 0:15:03.800 --> 0:15:06.240 the ruler moving from my left and my right, Like, 0:15:06.320 --> 0:15:09.720 aren't all those photons leaving the stick at the same time, 0:15:09.880 --> 0:15:12.320 and so then it would arrive at the camera or 0:15:12.440 --> 0:15:13.560 my eye at the same time. 0:15:13.680 --> 0:15:15.800 They're leaving the train or the stick at the same time. 0:15:16.000 --> 0:15:18.440 The stick has length, so it takes longer for the 0:15:18.520 --> 0:15:21.840 back end photons to reach you than the front end photons. 0:15:22.000 --> 0:15:25.720 And I think combining photon travel time and actual length 0:15:25.800 --> 0:15:28.880 combines two different complicated things which are kind of hard 0:15:28.880 --> 0:15:30.880 to hold in your head at the same time. So 0:15:31.000 --> 0:15:33.440 usually when we talk about special relativity, we assume that 0:15:33.440 --> 0:15:36.040 we can take care of the photon travel time and 0:15:36.080 --> 0:15:37.960 say that we have a bunch of assistants all through 0:15:38.000 --> 0:15:41.280 the universe, all making measurements and take local data, and 0:15:41.280 --> 0:15:43.960 then we combine that information later to figure out what happened. 0:15:44.200 --> 0:15:46.920 So we take away this effect of photons take time 0:15:46.920 --> 0:15:49.640 to get somewhere because it adds another confusing layer. 0:15:50.600 --> 0:15:53.720 But I guess I mean we're saying that things look shorter, right, 0:15:53.760 --> 0:15:56.000 and your eye sort of look acts like a camera. 0:15:56.440 --> 0:15:58.800 Are you saying that, like, maybe what you mean by 0:15:58.880 --> 0:16:01.600 looking at something it's not quite the same, but as 0:16:01.640 --> 0:16:04.080 what everyday person might mean by looking at something. 0:16:04.200 --> 0:16:06.520 Yeah, I think that's fair. When we do physics experiments, 0:16:06.560 --> 0:16:09.680 we're very careful about how we make those measurements. So really, 0:16:09.680 --> 0:16:12.200 what we mean here is if we take a very 0:16:12.200 --> 0:16:15.160 careful measurement of the length of the object, we will 0:16:15.200 --> 0:16:17.640 measure it to be shorter than if it was stationary. 0:16:17.960 --> 0:16:20.360 All right, well, I think we just should just be 0:16:20.560 --> 0:16:22.960 kind of clear about what we mean by things looking shorter. 0:16:23.120 --> 0:16:25.120 What we mean is that we make a careful measurement 0:16:25.160 --> 0:16:27.400 of the length of this thing involving us and a 0:16:27.440 --> 0:16:29.840 bunch of assistants all along, like the track of a 0:16:29.880 --> 0:16:32.800 train and noting when it passes by. And then later 0:16:32.840 --> 0:16:35.120 we get together and we compare our measurements and we say, 0:16:35.600 --> 0:16:37.680 you know, Juan measured the back of the train at 0:16:37.680 --> 0:16:40.080 this time, and Sally measured the front of the train 0:16:40.120 --> 0:16:42.560 at that time, and then we can use that to 0:16:42.600 --> 0:16:44.080 figure out how long the train was. 0:16:45.040 --> 0:16:47.200 All right, it sounds like we need to dig into 0:16:47.240 --> 0:16:50.720 this scenario of you and all of your assistance and 0:16:50.800 --> 0:16:54.040 how they're measuring the length of this ruler and or 0:16:54.120 --> 0:16:57.800 this train. Maybe really understand what we mean by things 0:16:57.840 --> 0:17:01.360 looking shorter or not. So let's dig into these details. 0:17:01.400 --> 0:17:16.600 But first let's take a quick break. All right, we're 0:17:16.640 --> 0:17:20.760 talking about length contraction due to special relativity, and that 0:17:20.880 --> 0:17:23.240 doesn't have to do with how long this podcast is, 0:17:23.320 --> 0:17:27.280 right or how much my questions dilate the length of 0:17:27.760 --> 0:17:29.320 the running time of this episode. 0:17:29.440 --> 0:17:31.560 That's exactly what I was going to say. Yeah, no, 0:17:31.640 --> 0:17:33.760 but it's good. It's important we explain exactly what we 0:17:33.800 --> 0:17:36.480 mean and we not use jargon. So let's hash it 0:17:36.520 --> 0:17:40.359 out because there's some really interesting, like apparent paradoxes that 0:17:40.480 --> 0:17:43.000 come up when we try to measure these lengths really carefully. 0:17:43.160 --> 0:17:45.760 Okay, so we're asking the question why the moving objects 0:17:45.760 --> 0:17:47.960 look shorter, and it seems like we're not really asking 0:17:47.960 --> 0:17:51.240 the question why they look shorter. It's more like why 0:17:51.320 --> 0:17:54.320 when a bunch of physicists try to measure something moving fast, 0:17:54.520 --> 0:17:57.000 they measure things to be shorter. Not really because the 0:17:57.359 --> 0:18:00.720 idea of looking at it you're saying, convolutes several things. 0:18:00.840 --> 0:18:02.760 Yeah, I would say, I guess that's how physicists look 0:18:02.800 --> 0:18:05.120 at something. Right, don't just take a picture. We think 0:18:05.119 --> 0:18:08.200 about what that picture is actually measuring and realizing pictures 0:18:08.320 --> 0:18:10.600 are not the best way to measure the length of 0:18:10.640 --> 0:18:13.480 something moving really really fast. And said, we hire a 0:18:13.480 --> 0:18:15.720 bunch of assistants and lay them out in a line 0:18:16.040 --> 0:18:18.040 so we can take measurements and figure out how long 0:18:18.080 --> 0:18:18.879 that thing really is. 0:18:19.000 --> 0:18:22.080 Okay, so then let's maybe paint that scenario. I'm sitting 0:18:22.160 --> 0:18:24.840 by some train tracks, and the train tracks go from 0:18:24.880 --> 0:18:27.360 my left to my right in front of me, and 0:18:27.440 --> 0:18:29.520 now there's going to be a train passing by. I 0:18:29.520 --> 0:18:32.600 want to measure how long the train is. Yeah, and 0:18:32.600 --> 0:18:34.679 you're saying, don't take a picture of it because the 0:18:34.720 --> 0:18:38.359