WEBVTT - How fast are we moving? 0:00:07.760 --> 0:00:11.280 Picture the Solar system. Likely you have put the Sun 0:00:11.440 --> 0:00:14.640 at the center, motionless in your mind, with the planets 0:00:14.640 --> 0:00:18.080 whizzing all around it. Or maybe you've seen that viral 0:00:18.160 --> 0:00:21.480 video that breaks the common view and shows the Sun 0:00:21.680 --> 0:00:26.680 in motion through the galaxy. But isn't the galaxy moving 0:00:26.720 --> 0:00:29.960 to what's the right way to think about all this 0:00:30.080 --> 0:00:34.200 to assemble a mental universe? How fast are we going? 0:00:34.360 --> 0:00:38.600 And does that question even make sense? A new study 0:00:38.800 --> 0:00:43.080 raises some interesting puzzles. Today we're going to dig into 0:00:43.280 --> 0:00:45.479 all of that and help you make sense of it. 0:00:46.320 --> 0:00:50.240 Welcome to Daniel and Kelly's Extraordinary Universe in Motion. 0:01:03.640 --> 0:01:04.920 Hello, I'm Kelley Winer Smith. 0:01:04.959 --> 0:01:07.600 I study parasites and space, and it doesn't matter what 0:01:07.760 --> 0:01:10.880 I am moving relative to, I am always moving pretty slow. 0:01:13.640 --> 0:01:16.560 Hi. I'm Daniel. I'm a particle physicist, and I'm particularly 0:01:16.600 --> 0:01:19.000 interested in our cosmic context. 0:01:19.200 --> 0:01:21.399 So, Daniel, cosmic context? 0:01:21.560 --> 0:01:25.400 Yes, what in your mind would be the coolest spot 0:01:25.600 --> 0:01:28.960 for us to be in the universe? If we could know, like, 0:01:29.000 --> 0:01:31.119 should we be in the center, should we be at 0:01:31.120 --> 0:01:33.960 the edge, if we could be anywhere you wanted us 0:01:33.959 --> 0:01:34.840 to be? Where would we be? 0:01:36.360 --> 0:01:39.840 Wow, that's a crazy question, And I can't believe nobody's 0:01:39.880 --> 0:01:42.320 ever asked me that before, and I have never thought 0:01:42.319 --> 0:01:46.160 about that before. You know, if the universe is infinite, 0:01:46.360 --> 0:01:49.279 then there are no places. Really, They're all the same. 0:01:50.200 --> 0:01:53.320 So then the question is only interesting if the universe 0:01:53.480 --> 0:01:56.760 is not infinite. And even if the universe is finite 0:01:56.920 --> 0:02:00.480 and closed, there's still no special places. So the most 0:02:00.520 --> 0:02:03.600 interesting thing would be if the universe is finite and 0:02:03.720 --> 0:02:06.560 has an edge, in which case being near the edge 0:02:06.600 --> 0:02:08.680 would be amazing because the edge would have to be 0:02:08.800 --> 0:02:11.639 different in some way from other kinds of space, and 0:02:11.639 --> 0:02:14.440 it'll be like a different kind of lego brick that 0:02:14.480 --> 0:02:18.520 makes up the universe, and different is always fascinating because 0:02:18.560 --> 0:02:21.640 like at the edge, for example, momentum wouldn't be conserved. 0:02:22.000 --> 0:02:23.760 You know, for example, like when you throw a ball 0:02:23.880 --> 0:02:27.000 against a wall, the ball bounces back, but also the 0:02:27.040 --> 0:02:29.080 wall gets pushed back a little bit, even if the 0:02:29.080 --> 0:02:31.720 wall is attached to the Earth, right, But if you 0:02:31.760 --> 0:02:34.640 throw a ball against the edge of the universe, it 0:02:34.720 --> 0:02:38.200 bounces back and the universe doesn't get pushed it can't, 0:02:38.560 --> 0:02:41.040 and so momentum is not conserved. So that would be 0:02:41.040 --> 0:02:44.840 pretty interesting to see momentum not conservation. Hey, was that 0:02:44.880 --> 0:02:46.840 the nerdiest possible answer I could have given. 0:02:46.680 --> 0:02:47.720 You pretty close. 0:02:49.960 --> 0:02:52.239 I'm trying to max out all the metrics here today. 0:02:52.360 --> 0:02:52.760 Awesome. 0:02:52.840 --> 0:02:56.600 All right, Well, let's see what other metrics Daniel can 0:02:56.639 --> 0:03:01.400 max out today as we discuss how fast we're moving 0:03:01.440 --> 0:03:02.040 through space. 0:03:02.480 --> 0:03:04.359 Yeah, this is a really fun topic because we get 0:03:04.400 --> 0:03:07.160 to talk about not just like our cosmic context, where 0:03:07.200 --> 0:03:10.200 are we in the universe? How is everything slashing around? 0:03:10.600 --> 0:03:13.120 Give you like a sense for our neighborhood and the 0:03:13.120 --> 0:03:16.240 bigger picture of view of the universe. But also it 0:03:16.280 --> 0:03:19.000 touches on some really deep and basic but hard to 0:03:19.080 --> 0:03:23.520 grapple with concepts in relativity, like what does velocity even mean? 0:03:23.800 --> 0:03:24.080 Man? 0:03:27.240 --> 0:03:29.320 So get at your banana peels. We are going to 0:03:29.400 --> 0:03:30.160 smoke them today. 0:03:30.360 --> 0:03:31.840 Oh, we're getting philosophical. 0:03:31.880 --> 0:03:35.320 I guess we always get philosophical when it's physics. Episode 0:03:35.800 --> 0:03:39.800 amazing can't be avoided. So I was wondering what people 0:03:39.840 --> 0:03:42.280 thought about this question before we dive into the physics 0:03:42.320 --> 0:03:44.720 of it. So I reached out to our amazing group 0:03:44.800 --> 0:03:48.360 of volunteers who stand at the ready to offer their 0:03:48.480 --> 0:03:52.480 ungoogled opinions on questions of the day Today. I asked 0:03:52.480 --> 0:03:56.880 them how fast are we moving through space? Think about 0:03:56.880 --> 0:03:59.840 it for a moment. What would be your answer. Here's 0:04:00.120 --> 0:04:03.320 what our group of experts had to say. Should be 0:04:03.400 --> 0:04:08.440 dependent upon the relative location and velocity of the observer. 0:04:08.720 --> 0:04:11.840 We're all going back to the same location, so effectively, 0:04:11.920 --> 0:04:13.360 we haven't gone anywhere. 0:04:13.120 --> 0:04:16.599 Ninety six thousand kilometers per second. They don't know how 0:04:16.640 --> 0:04:18.880 fast the Milky Way galaxy is moving. 0:04:19.120 --> 0:04:24.640 We're moving at thousands and thousands and thousands of kilometers 0:04:25.080 --> 0:04:25.760 per second. 0:04:26.200 --> 0:04:28.800 How fast we're going depends on our frame of reference. 0:04:29.160 --> 0:04:31.480 I'm sitting down zero miles per hour. 0:04:31.880 --> 0:04:33.599 It must be thousands of miles an hour. 0:04:34.000 --> 0:04:37.240 If the universe is infinity in size, then we're going 0:04:37.279 --> 0:04:39.360 infinity speed around something. 0:04:39.360 --> 0:04:41.120 Relative to the galaxy in the universe. 0:04:41.520 --> 0:04:45.320 I don't know, man, tough question. Speed is always relative, 0:04:45.440 --> 0:04:48.480 So the question is how fast are we moving relative 0:04:48.520 --> 0:04:50.960 to what? Velocity is relative? 0:04:51.120 --> 0:04:53.640 When it comes to space time, we're moving all at 0:04:53.640 --> 0:04:54.640 the speed of light. 0:04:54.760 --> 0:04:58.359 Relative to what's not fast enough, guys, hit the nitrous 0:04:58.400 --> 0:04:58.800 are side. 0:04:58.800 --> 0:05:02.240 But on these are great answers. And having listened to 0:05:02.400 --> 0:05:06.760 you explain physics for over a year now, my guess was, 0:05:07.040 --> 0:05:08.800 it's got to be relative to something. 0:05:09.320 --> 0:05:11.320 And then I was like, and that's all I know. 0:05:11.440 --> 0:05:15.040 But I will hear a whole hour on this topic, 0:05:15.120 --> 0:05:18.600 so soon I'll be an experts And so, yeah, what 0:05:18.600 --> 0:05:19.640 did you think of these answers? 0:05:21.920 --> 0:05:24.120 I thought they were great. They were very well informed 0:05:24.520 --> 0:05:26.920 and really funny. Also, but I have a question about 0:05:26.920 --> 0:05:29.520 your response, Kelly. Do you have a different relationship now 0:05:29.520 --> 0:05:31.960 to like the random physics question that might come up 0:05:32.040 --> 0:05:35.359 in conversation, Like if your kids ask you a random 0:05:35.480 --> 0:05:38.880 question about the universe, do you feel like more qualified 0:05:38.920 --> 0:05:40.760 to maybe answer it or dig into it with them 0:05:40.760 --> 0:05:41.680 than you did a year ago. 0:05:41.960 --> 0:05:45.320 I do, and I'm much more likely to like interject 0:05:45.360 --> 0:05:48.520 physics information. Like Hata said something about an electron the 0:05:48.560 --> 0:05:50.159 other day, and I was like, did you know that 0:05:50.240 --> 0:05:52.800 we don't know if the electron is made up of 0:05:52.839 --> 0:05:53.280 other bits. 0:05:53.320 --> 0:05:55.240 We've tried to figure it out, but we did. It 0:05:55.279 --> 0:05:57.120 doesn't look like it's made up of other stuff, but 0:05:57.160 --> 0:05:57.920 we just don't know. 0:05:57.839 --> 0:05:59.600 Because we haven't been able to, like, you know, if 0:05:59.600 --> 0:06:01.320 we had more money, we could dig in more. 0:06:01.600 --> 0:06:04.640 And she didn't. You know, I can't say she looked interested, but. 0:06:06.680 --> 0:06:09.919 I was excited and so so yes, I do feel 0:06:09.920 --> 0:06:12.080 like now that I know more about physics. I'm more 0:06:12.120 --> 0:06:16.920 excited about physics. It feels less opaque and I'm excited. Yeah, 0:06:16.960 --> 0:06:17.479 life is good. 0:06:17.480 --> 0:06:20.560 Okay, yeah, physician accomplished one person at a time. 0:06:20.800 --> 0:06:23.680 Woo. Yes, you want a biologist over. 0:06:24.800 --> 0:06:26.479 Well, then I'm one for two because I'm not sure 0:06:26.520 --> 0:06:28.200 Katrina feels the same way. Oh. 0:06:28.360 --> 0:06:30.360 Ouch, And how many years have you been working there? 0:06:30.440 --> 0:06:32.000 Is that like twenty years now? Yeah? 0:06:32.320 --> 0:06:35.960 Twenty six, twenty seven? Yeah? I know, right, Well, she 0:06:35.960 --> 0:06:38.919 definitely knows more particle physics than she did when she started, 0:06:39.080 --> 0:06:41.600 So I don't know if her enthusiasm has gone up, 0:06:41.600 --> 0:06:43.160 but her knowledge definitely has. 0:06:43.320 --> 0:06:46.040 Well, how do you feel about poop in the fridge? 0:06:46.160 --> 0:06:48.760 I guess maybe your knowledge of what's happening with the 0:06:48.800 --> 0:06:50.760 poop in the fridge has gone up, but your excitement 0:06:50.800 --> 0:06:51.960 about it has probably not. 0:06:52.400 --> 0:06:56.839 Yeah, so that's probably fair. Yeah, okay, comparison. All right, 0:06:57.279 --> 0:06:59.360 The day I'm excited to see poop in the fridge 0:06:59.400 --> 0:07:02.240 is the day I expect Katrina to be excited to talk. 0:07:02.120 --> 0:07:04.080 About particles, all right, urriage in a nutshell. 0:07:07.960 --> 0:07:10.480 Everybody likes to talk about the interdisciplinary work, but we're 0:07:10.520 --> 0:07:11.320 living it, baby. 0:07:11.480 --> 0:07:13.160 That's right, that's right, it's messy. 0:07:13.320 --> 0:07:15.320 All right, let's get back on track and talk about 0:07:15.360 --> 0:07:18.280 our emotion through the universe, not bowel motions that ended 0:07:18.360 --> 0:07:19.360 up in Daniel's freezer. 0:07:19.640 --> 0:07:24.280 Amazing, all right, all right, so all motion, because I've 0:07:24.280 --> 0:07:28.800 been listening, motion has to be defined relative to something else. Yeah, 0:07:29.000 --> 0:07:32.320 And is there like an obvious thing to compare your 0:07:32.400 --> 0:07:34.720 motion too for this question, or do you can just 0:07:34.800 --> 0:07:35.440 pick anything? 0:07:35.840 --> 0:07:39.120 No, the obvious thing to compare your emotion to is space. 0:07:39.440 --> 0:07:42.680 And people like to think about our motion through space. 0:07:42.720 --> 0:07:44.680 They like to think about the question how fast are 0:07:44.720 --> 0:07:47.560 we moving? How fast is the Earth moving? And they 0:07:47.840 --> 0:07:51.440 imagine that we are moving through some medium. But space 0:07:51.600 --> 0:07:55.240 is really weird. Space is not something that has a 0:07:55.280 --> 0:08:00.440 frame of reference. You cannot measure your motion relative to space. 0:08:00.840 --> 0:08:04.760 You can only measure your motion relative to other stuff 0:08:05.080 --> 0:08:08.560 in space, which you know already opens the door to 0:08:08.760 --> 0:08:12.240 deep philosophical questions like well then what is space anyway? 0:08:13.040 --> 0:08:15.400 And I think that it's really hard to hold in 0:08:15.440 --> 0:08:19.400 your mind an idea of what space is because you know, 0:08:19.440 --> 0:08:21.560 on one hand, we talk about it is like having 0:08:21.640 --> 0:08:24.960 things in it like fields and matter, which is excitation 0:08:25.120 --> 0:08:28.000 of those fields, which are a property of space. Right, 0:08:28.040 --> 0:08:31.400 So it feels like there's stuff in space. I'm talking 0:08:31.440 --> 0:08:34.320 about these fields, and they're oscillating, they're doing things. But 0:08:34.440 --> 0:08:37.120 on the other hand, I'm also telling you you can't 0:08:37.200 --> 0:08:41.040 measure your velocity relative to those fields. They somehow exist, 0:08:41.080 --> 0:08:44.120 they're out there, they're part of this medium. It's a 0:08:44.200 --> 0:08:47.040 kind of an ether theory because it's not an ether 0:08:47.160 --> 0:08:50.560 that provides a frame of reference. That's really the crucial thing. 0:08:51.200 --> 0:08:54.800 And so velocity is measured between two objects, and space 0:08:55.240 --> 0:08:58.160 is not an object. It's kind of a thing, but 0:08:58.280 --> 0:09:01.120 it doesn't have this property that you can measure your 0:09:01.200 --> 0:09:03.200 velocity relative to that thing. 0:09:03.880 --> 0:09:07.040 And if you really have smoked a lot of banana pills, 0:09:07.080 --> 0:09:09.440 you should listen to one of our early episodes What 0:09:09.920 --> 0:09:13.520 is Space, where you dig into that for a whole hour, 0:09:13.920 --> 0:09:16.440 and that is some trippy stuff. 0:09:16.960 --> 0:09:19.280 And it's amazing that we don't really have a solid 0:09:19.320 --> 0:09:22.480 answer to this question. Like we have these theories and 0:09:22.520 --> 0:09:26.360 they work, and we can pull apart the philosophical implications 0:09:26.400 --> 0:09:28.640 of them. What does it mean that you can't measure 0:09:28.640 --> 0:09:32.319 your velocity relative to the thing in which light travels, 0:09:32.320 --> 0:09:35.920 for example, But we don't still really understand what it 0:09:36.040 --> 0:09:37.719 is and how it all works, and we can't pull 0:09:37.760 --> 0:09:40.240 it all together. And like quantum mechanics, view of space 0:09:40.240 --> 0:09:42.559 and general relative to view of space are very different. 0:09:43.000 --> 0:09:45.480 And so somebody please give us a theory of quantum 0:09:45.480 --> 0:09:49.040 gravity which answers all of these questions. But you know, 0:09:49.360 --> 0:09:52.320 it means something to say that velocity is relative. It 0:09:52.400 --> 0:09:55.760 means that velocity is not a property of an object. 0:09:55.800 --> 0:09:58.360 It's a property of a pair of objects. So any 0:09:58.400 --> 0:10:01.679 physics question you ask start out saying, say I'm in 0:10:01.720 --> 0:10:03.560 a ship and I'm going ninety percent of the speed 0:10:03.600 --> 0:10:06.480 of light, I'm going to ask you with respect to 0:10:06.600 --> 0:10:09.600 what it doesn't mean anything to say I'm going ninety 0:10:09.600 --> 0:10:11.400 percent of the speed of light, because you can be 0:10:11.440 --> 0:10:13.760 going ninety percent of the speed of light relative to 0:10:13.800 --> 0:10:16.079 one observer, and ten percent of the speed of light 0:10:16.120 --> 0:10:19.880 relative to another, and zero relative to another who's standing 0:10:19.880 --> 0:10:22.480 next to you on the ship, right, Your velocity only 0:10:22.520 --> 0:10:26.280 means something if you say who's measuring it? Because again, 0:10:26.360 --> 0:10:30.800 there is no absolute frame, no preferred velocity in the universe. 0:10:31.440 --> 0:10:35.000 And for those of us who have to admit that 0:10:35.000 --> 0:10:38.199 they had a little trouble remembering the difference between velocity 0:10:38.240 --> 0:10:41.040 and acceleration and motion and all of those terms when 0:10:41.080 --> 0:10:45.680 they started physics. Let's do a real quick recap motion, velocity, acceleration. 0:10:46.120 --> 0:10:48.959 What do these terms mean? Yeah, great for the purpose 0:10:48.960 --> 0:10:49.600 of this discussion. 0:10:49.679 --> 0:10:52.360 Yeah, so let's start with location, right, because that's the 0:10:52.400 --> 0:10:56.600 basic thing. You know. Location also is relative, Like I 0:10:56.640 --> 0:10:59.320 can say, like if I live in a one dimensional universe, 0:10:59.559 --> 0:11:03.560 I can say, over here location x equal zero. Then 0:11:03.600 --> 0:11:06.360 I can measure how far I am from that location 0:11:06.600 --> 0:11:09.760 from zero. So I'm at x equals five. You're at 0:11:09.840 --> 0:11:13.000 x equal zero. I can say we're five units apart. Right, 0:11:13.360 --> 0:11:15.880 But somebody else could have put x equal zero somewhere else, 0:11:16.200 --> 0:11:19.320 So my location would be different if they're measuring it 0:11:19.520 --> 0:11:22.280 or if I'm measuring it. So even location itself is 0:11:22.360 --> 0:11:26.439 relative because there's no like glowing tick marks in space. 0:11:26.800 --> 0:11:29.800 There's no like origin where the universe says this is 0:11:29.920 --> 0:11:33.839 zero or that is zero. It's just relative. So that's 0:11:33.880 --> 0:11:37.199 the basic measurement that's your location. Velocity is how your 0:11:37.280 --> 0:11:40.760 location changes with time. I was at x equals five 0:11:41.080 --> 0:11:43.400 one second later, I'm at xequal six. Then I'm at 0:11:43.480 --> 0:11:47.720 xequal seven. That's my velocity. But that's why velocity is relative, 0:11:47.760 --> 0:11:50.080 because it's a measurement of how location is changing, and 0:11:50.200 --> 0:11:52.200 location is just relative. 0:11:52.480 --> 0:11:53.320 Got it all? 0:11:53.360 --> 0:11:55.600 That's great? And what that means, for example, is that 0:11:56.000 --> 0:12:01.000 any experiment you can do can't measure your velocity relative 0:12:01.040 --> 0:12:04.520 to space. So if I build some contraption and it 0:12:04.559 --> 0:12:08.560 does something the wiz bang experiment, and I run my 0:12:08.640 --> 0:12:11.079 experiment and does a wiz in a bang, cool, and 0:12:11.120 --> 0:12:13.160 then I put it in a box. Then I speed 0:12:13.240 --> 0:12:15.480 it up and I get it going really fast relative 0:12:15.520 --> 0:12:17.920 to Earth, and I do the same experiment, it should 0:12:17.920 --> 0:12:20.840 still whiz in bang in exactly the same way. The 0:12:20.880 --> 0:12:23.880 fact that it has some velocity now relative to Earth 0:12:24.120 --> 0:12:27.920 doesn't change the physics inside the box. And it can't 0:12:28.160 --> 0:12:30.760 because if it did, then somehow I'd be measuring the 0:12:30.840 --> 0:12:34.680 velocity like within the box without measuring my distance relative 0:12:34.720 --> 0:12:38.520 to Earth. You can only measure your velocity relative to 0:12:38.559 --> 0:12:41.600 Earth by measuring your distance relative to Earth. So if 0:12:41.600 --> 0:12:43.920 the experiment just does the wiz bang experiment, if it 0:12:43.920 --> 0:12:46.920 does not like a ruler to measure the distance to Earth, 0:12:47.520 --> 0:12:49.880 then it should get the same result regardless of its 0:12:49.920 --> 0:12:53.400 location or its velocity relative to Earth. And in fact, 0:12:53.480 --> 0:12:55.959 you can promote this to a general principle. You can 0:12:56.000 --> 0:12:59.160 say you can't measure your location or your velocity. If 0:12:59.160 --> 0:13:01.520 you're like trapped in inside a box with no access 0:13:01.520 --> 0:13:04.080 to the outside universe, there's nothing you can do to 0:13:04.120 --> 0:13:07.400 measure your velocity relative to stuff in the outside universe 0:13:07.640 --> 0:13:11.200 because your velocity only has meaning relative to that stuff, 0:13:11.200 --> 0:13:13.280 and if you don't have access to that stuff, you 0:13:13.320 --> 0:13:15.679 can't measure your velocity. Does that all make sense? 0:13:16.160 --> 0:13:19.240 So you could still measure your velocity with respect to 0:13:19.280 --> 0:13:22.000 stuff in the box, right, but not? Okay, got it. 0:13:22.040 --> 0:13:27.120 But the amazing thing is that that's not true for acceleration. Right. 0:13:27.440 --> 0:13:29.960 So we talked about location, and we talked about how 0:13:30.440 --> 0:13:34.280 change in location is velocity. You can also talk about 0:13:34.280 --> 0:13:38.280 the change in velocity. So for those mathematically inclined, these 0:13:38.320 --> 0:13:42.760 are derivatives. Right. Velocity is the first derivative of location. 0:13:43.320 --> 0:13:48.119 Acceleration is the second derivative of location. It's how velocity 0:13:48.160 --> 0:13:52.480 is changing with time. Now, acceleration is something that's absolute. 0:13:52.960 --> 0:13:55.640 If you are in a box, you can measure whether 0:13:55.679 --> 0:13:59.000 that box is accelerating or not. It's very easy. You 0:13:59.160 --> 0:14:02.760 just like draw a ball. If the box is not accelerating, 0:14:02.960 --> 0:14:05.320 the ball will float in front of you. If the 0:14:05.360 --> 0:14:08.480 box is accelerating, the ball will move to the back 0:14:08.520 --> 0:14:11.240 of the box. If the box has a negative acceleration, 0:14:11.360 --> 0:14:13.719 like somebody's putting on the brakes, the ball will move 0:14:13.720 --> 0:14:15.760 to the front of the box. It's just like having 0:14:15.760 --> 0:14:17.840 a bowling ball in the back of a truck. You 0:14:17.840 --> 0:14:20.680 can use that bowling ball to tell if you're accelerating 0:14:20.800 --> 0:14:23.640 or decelerating. You can't use it to measure the velocity 0:14:23.680 --> 0:14:25.760 of the truck, but you can use it to measure 0:14:25.840 --> 0:14:30.760 the acceleration. So acceleration is absolute, but velocity is not. 0:14:31.520 --> 0:14:32.800 Okay, I get that. 0:14:32.880 --> 0:14:35.440 It still feels kind of counterintuitive because if it's the 0:14:35.480 --> 0:14:38.840 second derivative of location, it still feels like location should matter. 0:14:38.880 --> 0:14:42.600 But your box example, I understand how that works. 0:14:42.920 --> 0:14:45.520 And you know, for those philosophically inclined, you might wonder, like, well, 0:14:45.560 --> 0:14:48.480 why is that Why it's taking one more derivative make 0:14:48.520 --> 0:14:52.760 it absolute instead of relative. And that's a whole digression. 0:14:53.200 --> 0:14:59.440 But very briefly, acceleration is philosophically very similar to curvature. Right. 0:14:59.520 --> 0:15:02.200 The effect acceleration is almost exactly the same as the 0:15:02.200 --> 0:15:06.400 effect of space time curvature. In fact, acceleration can create 0:15:06.440 --> 0:15:09.480 event horizons. There are scenarios for example, where if you 0:15:09.480 --> 0:15:14.200 are constantly accelerating, you can outrun photons, effectively making like 0:15:14.240 --> 0:15:17.440 an event horizon relative to photons. That's a whole other 0:15:17.480 --> 0:15:18.720 episode we should dig into. 0:15:19.040 --> 0:15:21.760 Wait wait, wait, whoa, whoa. Did you say something can 0:15:21.840 --> 0:15:22.720 go faster than light? 0:15:23.240 --> 0:15:25.600 No, you can never go faster than light, but you 0:15:25.720 --> 0:15:31.360 can outrun a photon. So, for example, if you take 0:15:31.400 --> 0:15:35.320 off in your spaceship and you're moving slowly but you're 0:15:35.320 --> 0:15:40.440 accelerating constantly, if I then try to shoot a laser 0:15:40.480 --> 0:15:42.840 beam to catch you, it will never catch you. If 0:15:42.880 --> 0:15:48.240 you're accelerating constantly, you can outrun that photon. So it's 0:15:48.240 --> 0:15:51.280 not like if we have a race Kelly versus the photon, 0:15:51.720 --> 0:15:54.320 that you'll go faster than the photon. You'll never go 0:15:54.400 --> 0:15:56.880 faster than light, but I can't catch you with a 0:15:56.920 --> 0:16:00.520 photon if you have left earlier and are constantly exciting. 0:16:01.400 --> 0:16:04.080 Okay, all right, Yeah. 0:16:03.840 --> 0:16:06.720 So acceleration is weird. It's kind of like curvature. It's 0:16:06.760 --> 0:16:09.800 a whole thing in general relativity. But the point is 0:16:10.280 --> 0:16:13.320 that velocity is relative, and this leads to some sort 0:16:13.320 --> 0:16:16.240 of weird things like, for example, what if it's just 0:16:16.360 --> 0:16:19.680 you in the universe and nothing else, imagine an empty 0:16:19.800 --> 0:16:24.640 universe with just you. What's your velocity? There is no velocity, 0:16:25.200 --> 0:16:28.800 not that your velocity is zero. Velocity has no meaning 0:16:29.120 --> 0:16:31.720 because it's a property of pairs of objects. In an 0:16:31.760 --> 0:16:34.400 empty universe, there are no pairs of objects. 0:16:35.520 --> 0:16:36.560 It sounds very lonely. 0:16:39.000 --> 0:16:40.560 It's like being married to yourself. 0:16:40.720 --> 0:16:43.680 Oh, I can't imagine to be around for very long either, 0:16:43.960 --> 0:16:46.000 in the vastness of space. If it was just you, 0:16:46.120 --> 0:16:48.800 it would be a short, lonely existence. 0:16:49.960 --> 0:16:53.800 Always thinking about the practical aspects of weird philosophical hypotheticals, 0:16:53.840 --> 0:16:54.240 love it. 0:16:54.280 --> 0:16:57.080 Always thinking of death is pretty much where it goes to. 0:16:58.880 --> 0:17:02.880 And conversely, you can never imagine a scenario where you 0:17:02.920 --> 0:17:06.560 are at rest relative to a photon because photons always 0:17:06.640 --> 0:17:10.040 have velocity of the speed of light relative to everything. 0:17:10.320 --> 0:17:13.520 So you can never pull up alongside of photon and say, oh, look, 0:17:13.560 --> 0:17:15.800 this is what a photon looks like when it's at rest, 0:17:15.960 --> 0:17:18.200 because the photon is pure motion. 0:17:18.640 --> 0:17:20.359 If you pulled up alongside a photon, what kind of 0:17:20.400 --> 0:17:21.240 music do you think it would be? 0:17:21.240 --> 0:17:21.639 Listening to? 0:17:25.240 --> 0:17:26.280 Classic rocks? 0:17:26.520 --> 0:17:26.720 Ah? 0:17:26.920 --> 0:17:28.760 Nice, Yeah, all right, let's go with that. 0:17:29.320 --> 0:17:32.280 Yeah, or quantum punk. I'm not sure actually, so What 0:17:32.320 --> 0:17:35.120 this means is, you know, space has no texture, there's 0:17:35.119 --> 0:17:37.840 no reference frame. It feels like space is a thing, 0:17:38.119 --> 0:17:41.959 but it's not, and it's the same thing everywhere. We 0:17:42.040 --> 0:17:44.640 mentioned earlier the idea of space having a boundary and 0:17:44.800 --> 0:17:47.520 its connection to conservation and momentum, and this tells you 0:17:47.600 --> 0:17:49.960 that the idea that space has no reference frame has 0:17:50.040 --> 0:17:53.920 really important consequences. Like Nother's theorem is what tells us 0:17:54.200 --> 0:17:58.560 that space being the same everywhere leads to conservation of momentum, 0:17:58.880 --> 0:18:01.480 which is why space have an edge to it would 0:18:01.560 --> 0:18:04.600 lead to a violation of the conservation of momentum. So 0:18:04.640 --> 0:18:07.280 the fact that we never see violations of the conservation 0:18:07.320 --> 0:18:10.840 of momentum tells us space really is the same everywhere, 0:18:11.400 --> 0:18:13.159 and we've never noticed that. Like if you do an 0:18:13.200 --> 0:18:15.880 experiment here and you do an experiment there, you get 0:18:15.920 --> 0:18:19.560 different answers. Your whiz bang experiment always whizzes and bangs 0:18:19.600 --> 0:18:23.760 the same way no matter where you are. So velocity 0:18:23.880 --> 0:18:28.480 and location are purely relative, right, And this means that, 0:18:28.520 --> 0:18:32.480 for example, it doesn't mean anything to ask are you motionless? 0:18:32.520 --> 0:18:36.240 Can we be motionless with respect to space? Right? Because 0:18:36.320 --> 0:18:38.920 you can't measure your velocity relative to space. 0:18:38.800 --> 0:18:41.360 At all, because there's no space, man. 0:18:41.200 --> 0:18:44.040 Yeah, exactly. No space is a thing. It just doesn't 0:18:44.080 --> 0:18:47.240 have a velocity. So in one sense, it's like trivial 0:18:47.280 --> 0:18:49.480 to be motionless. You just say, well, I'm gonna choose 0:18:49.920 --> 0:18:52.160 my frame of reference to be me, and I'm gonna 0:18:52.160 --> 0:18:55.520 measure my velocity relative to myself. Okay, look, I'm going zero. 0:18:55.960 --> 0:18:56.160 Yay. 0:18:56.680 --> 0:18:58.760 It's also kind of trivial to have a velocity near 0:18:58.760 --> 0:19:01.720 the speed of light. Choose any of the zillions of 0:19:01.760 --> 0:19:04.359 cosmic rays that are approaching the Earth at nearly the 0:19:04.400 --> 0:19:06.720 speed of light and say that's my reference frame. 0:19:06.960 --> 0:19:07.960 I do go fast. 0:19:08.320 --> 0:19:10.920 Yes, And from the point of view of those cosmic rays, Kelly, 0:19:11.040 --> 0:19:13.360 you are moving towards them at nearly the speed of light, 0:19:13.680 --> 0:19:14.800 So buckle. 0:19:14.520 --> 0:19:17.919 Up, way to go me. I was also fast that 0:19:17.960 --> 0:19:19.960 time I jumped out of a plane that. 0:19:19.880 --> 0:19:23.120 Was fast too ooh fast, and then slow I hope. 0:19:23.200 --> 0:19:27.320 Yeah, yeah, yeah yeah yeah, and then not split not 0:19:28.240 --> 0:19:30.679 very slow, not fast and slow than very slow. 0:19:30.800 --> 0:19:32.520 But anyway, all right. 0:19:32.640 --> 0:19:36.720 So we have learned that space is confusing and you 0:19:36.840 --> 0:19:39.520 need to be careful what you're talking about your speed 0:19:39.560 --> 0:19:42.440 relative to And when we get back from the break, 0:19:42.720 --> 0:19:45.320 we're going to talk about our speed relative to lots 0:19:45.320 --> 0:19:46.280 of stuff in the universe. 0:20:06.200 --> 0:20:07.160 All right, we're back. 0:20:07.280 --> 0:20:11.520 Let's start by talking about our speed relative to let's say, 0:20:11.680 --> 0:20:14.119 my favorite planet in the Solar System. 0:20:14.320 --> 0:20:14.600 Earth. 0:20:18.320 --> 0:20:21.520 Yeah. Earth is already kind of complicated. You might think, well, 0:20:21.720 --> 0:20:25.119 I'm standing on Earth. My velocity is zero, right, but 0:20:25.200 --> 0:20:28.240 the Earth itself is spinning, right, So which part of 0:20:28.280 --> 0:20:30.720 the Earth are we talking about? Yes, if you say 0:20:30.720 --> 0:20:32.760 the part of the Earth that's under your feet, your 0:20:32.800 --> 0:20:36.280 velocity is zero, that's kind of boring. But if you 0:20:36.320 --> 0:20:39.160 say the center of the Earth, then at the surface 0:20:39.160 --> 0:20:42.359 of the Earth you're already moving quite fast. Now, at 0:20:42.359 --> 0:20:44.440 the north pole you're not moving at all due to 0:20:44.480 --> 0:20:47.119 the spin, But at the equator, the Earth is spinning 0:20:47.160 --> 0:20:51.359 at sixteen hundred kilometers per hour. Woa, Like, that's not 0:20:51.440 --> 0:20:52.560 a small amount of motion. 0:20:52.800 --> 0:20:56.159 No, you know, I realize that we don't feel that, 0:20:56.720 --> 0:20:58.760