WEBVTT - Why is space so flat? 0:00:08.600 --> 0:00:10.920 Hey, Orge, I'm going to say a word, and I 0:00:10.960 --> 0:00:12.799 want you to tell me if you think it sounds 0:00:12.880 --> 0:00:16.000 like a positive or a negative idea. 0:00:15.720 --> 0:00:16.680 All right, go for it. 0:00:16.760 --> 0:00:18.200 The word is flat. 0:00:19.680 --> 0:00:22.120 I guess it could go either way. You know, nobody 0:00:22.200 --> 0:00:25.520 likes their soda flat or they are jokes to fall flat. 0:00:26.360 --> 0:00:28.480 But I sure do like my bed to be flat. 0:00:29.520 --> 0:00:31.360 But do you like your tires to be flat? 0:00:32.000 --> 0:00:34.560 No? But I do like my roads to be flat. 0:00:34.720 --> 0:00:36.520 Does that mean you want the earth to be flat? 0:00:38.040 --> 0:00:40.280 I like it spherical, But I also like mountains, so 0:00:40.640 --> 0:00:42.120 I guess I'm anti flat earth. 0:00:43.320 --> 0:00:45.200 Do you like falling flat from a mountain? 0:00:47.040 --> 0:00:48.520 I like landing flat on my feet. 0:00:48.640 --> 0:00:50.440 I think this discussion has run out of air. 0:01:05.800 --> 0:01:05.920 Hi. 0:01:05.959 --> 0:01:08.680 I'm Hoorhey, mccartoonist and the author of the book Oliver's 0:01:08.720 --> 0:01:09.680 Great Big Universe. 0:01:09.800 --> 0:01:12.800 Hi, I'm Daniel. I'm a particle physicist and a professor 0:01:12.880 --> 0:01:15.760 at UC Irvine, and I'm also flat footed. 0:01:15.920 --> 0:01:18.960 Are you saying metaphorically or you know, physiologically? 0:01:20.080 --> 0:01:23.280 Well, your questions often catch me flat footed, so that's metaphorical, 0:01:23.360 --> 0:01:26.520 but also literally and physiologically, I have flat feet, so yeah, 0:01:26.520 --> 0:01:27.280 I wear inserts. 0:01:27.400 --> 0:01:32.200 Does that have to make you taller you like Tom Cruise, 0:01:33.560 --> 0:01:34.680 I don't wear heels. 0:01:34.800 --> 0:01:38.920 No, I wear inserts to avoid crippling pain when running. 0:01:39.160 --> 0:01:42.320 Sounds like the solution is just not to run life 0:01:42.400 --> 0:01:43.199 flat on your back. 0:01:43.360 --> 0:01:45.560 Yeah. I'm working on a floating recliner I can live 0:01:45.560 --> 0:01:47.240 in for the rest of my life and float around. 0:01:47.880 --> 0:01:50.559 There you go. You can attach like a bicycle pedal 0:01:50.800 --> 0:01:52.520 and maybe get your workouts that way. 0:01:53.880 --> 0:01:55.400 Yeah, that's great for going upstairs. 0:01:55.480 --> 0:01:58.200 Yeah exactly. But anyways, welcome to our podcast. Daniel and 0:01:58.240 --> 0:02:01.880 Jorge Explained the Universe, a productive of iHeartRadio. 0:02:01.280 --> 0:02:04.160 Where we take all the twists and turns and curves 0:02:04.160 --> 0:02:07.160 of this crazy universe and try to flatten it all 0:02:07.200 --> 0:02:10.040 out for you. We try to untangle all of the 0:02:10.080 --> 0:02:13.360 mysteries of the nature of matter, the forces, the energy, 0:02:13.680 --> 0:02:16.640 the geometry of space time, the size and shape and 0:02:16.800 --> 0:02:19.640 history of the universe and make a nice, smooth story 0:02:19.760 --> 0:02:20.800 for you to understand. 0:02:21.120 --> 0:02:23.720 That's right, because it is an amazing universe full of 0:02:23.760 --> 0:02:29.560 stuff inflated with amazing and incredible physics and stars and 0:02:30.000 --> 0:02:33.399 galaxies and planets and particles for us to wonder at 0:02:33.480 --> 0:02:35.120 and for us to I guess poke it. 0:02:35.280 --> 0:02:38.040 There's so much that's amazing about the universe. And sometimes 0:02:38.040 --> 0:02:39.920 you can have two amazing facts that seem to be 0:02:39.960 --> 0:02:43.680 in conflict, Like, on one hand, it's amazing that we 0:02:43.760 --> 0:02:46.960 still don't really know so many basic things about the 0:02:47.040 --> 0:02:49.519 nature of the universe. How big is it? What is 0:02:49.600 --> 0:02:51.960 its shape? How did it all come to be? We're 0:02:51.960 --> 0:02:55.360 so ignorant about the environment in which we live. And 0:02:55.480 --> 0:02:57.560 on the other hand, it's kind of amazing that we 0:02:57.639 --> 0:03:00.919 know anything about the universe, and then we've only lived 0:03:00.960 --> 0:03:03.880 on one tiny little dot and one random little corner 0:03:04.200 --> 0:03:06.400 of the universe and never really left. 0:03:06.520 --> 0:03:08.440 Yeah, it's amazing what we can learn just from this 0:03:08.480 --> 0:03:10.639 little corner of the universe. And as you said, how 0:03:10.639 --> 0:03:14.200 we can ask these big questions about how everything is 0:03:14.240 --> 0:03:15.639 the way it is and why it is the way 0:03:15.680 --> 0:03:17.840 it is, Like, for example, we don't know if the 0:03:17.919 --> 0:03:19.160 universe is flat footed or not. 0:03:20.560 --> 0:03:22.560 Does the universe even like to run? Or does just 0:03:22.560 --> 0:03:24.400 want to sit on the couch and eat snacks all day? 0:03:24.480 --> 0:03:26.040 Yeah, just sit around and spin. 0:03:27.440 --> 0:03:30.320 It has big consequences for the curvature of the universe. 0:03:30.440 --> 0:03:32.799 It is getting bigger and bigger, so you know, maybe 0:03:32.800 --> 0:03:34.920 it could use a little bit of exercise. It's getting 0:03:34.960 --> 0:03:36.920 wider and wider per second. 0:03:37.160 --> 0:03:39.760 I think we should be universe positive on this podcast. 0:03:39.800 --> 0:03:42.640 You know, universe just be whatever shape you are, We 0:03:42.800 --> 0:03:43.120 love you. 0:03:43.400 --> 0:03:45.880 Yeah, yeah, true, true, true. I guess we should love 0:03:45.880 --> 0:03:46.840 the universe the way it is. 0:03:47.480 --> 0:03:50.160 It's the only universe we got, so might as well 0:03:50.200 --> 0:03:50.640 love it. 0:03:50.840 --> 0:03:53.400 Even if we don't understand it. I guess all the 0:03:53.440 --> 0:03:56.200 time we should, you know, take it for what it is. 0:03:56.320 --> 0:03:58.400 That's kind of what science is, right, taking things for 0:03:58.480 --> 0:03:59.240 what they are. 0:03:59.080 --> 0:04:01.440 Taking things for what they are are, absolutely, but then 0:04:01.520 --> 0:04:05.120 always asking why are they this way? Why couldn't they 0:04:05.160 --> 0:04:08.760 be some other way? Why do we live in this universe? 0:04:09.000 --> 0:04:11.400 Do we live in the only universe that's possible? Or 0:04:11.400 --> 0:04:14.440 are there many possible universes and we just happen to 0:04:14.640 --> 0:04:17.440 be in this one. So often we look around as 0:04:17.440 --> 0:04:19.159 we try to tell the story of the universe, and 0:04:19.200 --> 0:04:21.799 we ask those kinds of questions like does this make sense? 0:04:22.240 --> 0:04:24.480 This seems weird? Is it random? Or is there a 0:04:24.600 --> 0:04:25.280 reason for it? 0:04:25.520 --> 0:04:28.080 Yeah, because it is pretty perplexing out there the way 0:04:28.120 --> 0:04:30.440 things are. You know, there are amazing things like black 0:04:30.480 --> 0:04:33.960 holes that seem unexplainable, and there's sort of really weird 0:04:34.000 --> 0:04:36.719 things like quantum mechanics out there that kind of keep 0:04:36.720 --> 0:04:38.680 you guessing about what the universe is going to do. 0:04:38.880 --> 0:04:41.599 And as we put together this story of physics that 0:04:41.680 --> 0:04:44.680 tells us how the universe operates, what machinery is going 0:04:44.720 --> 0:04:47.760 on behind the scenes, that controls like what happens when 0:04:47.800 --> 0:04:50.880 two particles bump into each other, or house space curves 0:04:50.880 --> 0:04:53.120 and twists in the presence of mass. We start to 0:04:53.160 --> 0:04:55.840 tell a story about the universe, we notice, like, hm, 0:04:55.960 --> 0:04:58.320 the universe seems to do this kind of thing or 0:04:58.360 --> 0:05:00.719 do that kind of thing. And sometimes the story tells 0:05:00.839 --> 0:05:03.320 is very weird. It's very surprising. It's not one that 0:05:03.360 --> 0:05:06.280 makes sense to us or seems intuitive. It makes us 0:05:06.320 --> 0:05:08.760 wonder if maybe we're missing part of the story or 0:05:08.760 --> 0:05:10.560 if we're all just very very lucky. 0:05:10.880 --> 0:05:13.359 Yeah, and as you said, there are big questions about 0:05:13.360 --> 0:05:16.040 the universe that we still don't know about, like its size, 0:05:16.200 --> 0:05:19.240 its shape, and what it's going to do in the future, 0:05:19.440 --> 0:05:23.719 and also a very interesting question about its curvature. 0:05:24.160 --> 0:05:26.479 Yeah. As we develop our understanding of gravity and general 0:05:26.480 --> 0:05:30.080 relativity and we understand that space is weird and twisted 0:05:30.120 --> 0:05:32.640 and curved and that affects how things move, it also 0:05:32.680 --> 0:05:36.279 affects how the universe itself expands or contracts. So we 0:05:36.320 --> 0:05:38.599 have a lot of really fun questions to ask about 0:05:38.640 --> 0:05:42.680 why the universe looks this particular way, especially about the 0:05:42.720 --> 0:05:43.800 curvature of space. 0:05:44.040 --> 0:05:46.159 So today on the podcast, we'll be tackling the question 0:05:51.279 --> 0:05:55.160 why is space so flat? You mean as opposed to bumpy? Like, 0:05:56.000 --> 0:05:59.480 what would be the opposite of flat overinflated under pressure? 0:06:00.640 --> 0:06:03.599 Just like my arches, the opposite of flat would be curved, right, 0:06:03.680 --> 0:06:06.960 everybody likes nice curved arches for their feet, and the 0:06:07.000 --> 0:06:09.279 opposite of flat in the case of space or the 0:06:09.360 --> 0:06:10.840 universe would be curved. 0:06:11.920 --> 0:06:14.200 Well, it could also be well bumpy. I guess bumpy 0:06:14.279 --> 0:06:16.440 is also kind of curvy. It means you have a 0:06:16.480 --> 0:06:18.719 lot of little curves. You could have bumpy feet. 0:06:19.120 --> 0:06:22.240 Yeah, exactly, that's true. You got to sand those down 0:06:22.240 --> 0:06:24.080 a little bit. But in this case, yeah, we're talking 0:06:24.160 --> 0:06:26.640 about like the nature of space is space the way 0:06:26.760 --> 0:06:29.760 Euclid thought about it, You know, two parallel lines will 0:06:29.760 --> 0:06:33.680 never touch, or a space more complicated twisting and curving 0:06:33.960 --> 0:06:36.799 on a global sense. We're talking here about the curvature 0:06:36.800 --> 0:06:38.880 of the entire universe itself. 0:06:39.120 --> 0:06:41.599 Yeah, and this is I guess a pretty mind bending 0:06:41.720 --> 0:06:44.600 and space bending topic, because you know, I think we're 0:06:44.600 --> 0:06:46.640 all used to thinking of space, or at least empty 0:06:46.640 --> 0:06:50.719 space is being kind of like flat, right, not weird 0:06:51.040 --> 0:06:51.599 and curved. 0:06:51.680 --> 0:06:53.640 It's really hard to think about this in the three 0:06:53.640 --> 0:06:57.080 dimensions of our universe, and even the word flat is 0:06:57.240 --> 0:06:59.960 kind of confusing there. It comes really from thinking about 0:07:00.160 --> 0:07:03.040 a two dimensional version of the picture instead of thinking 0:07:03.120 --> 0:07:06.080 about space like I can move in three different directions, 0:07:06.440 --> 0:07:08.920 it's easier to think about it in two different directions 0:07:08.960 --> 0:07:10.600 because then we can like draw it on a piece 0:07:10.600 --> 0:07:13.080 of paper. So if you imagine like a sheet of 0:07:13.160 --> 0:07:15.880 graph paper, that's a flat sheet of paper, and two 0:07:15.920 --> 0:07:18.240 parallel lines on that piece of paper are never going 0:07:18.280 --> 0:07:20.880 to meet each other. When we talk about whether space 0:07:20.960 --> 0:07:23.320 is flat, we're asking a similar kind of question, but 0:07:23.400 --> 0:07:27.360 about three dimensional space, though it's harder to understand, like 0:07:27.440 --> 0:07:30.240 what curvature means in three dimensions than it is in 0:07:30.280 --> 0:07:31.120 two dimensions. 0:07:31.960 --> 0:07:33.640 Are you saying we're going to use a term that 0:07:33.720 --> 0:07:37.520 doesn't quite work or describe things or is counterintuitive to 0:07:37.600 --> 0:07:39.520 actually what is actually happening. Are we going to do 0:07:39.560 --> 0:07:39.960 that again? 0:07:40.200 --> 0:07:42.000 Yeah, that's exactly right. That's the story of. 0:07:41.920 --> 0:07:43.400 Physics being inadequate. 0:07:44.360 --> 0:07:46.720 We think we understand the universe, and then it surprises 0:07:46.800 --> 0:07:49.160 us and it sort of outgrows even like our ideas 0:07:49.160 --> 0:07:52.040 and forces us to like generalize these concepts like, oh wait, 0:07:52.160 --> 0:07:55.120 flattness can apply in three dimensions, not just two. 0:07:55.320 --> 0:07:57.320 Well, as usual, we were wondering how many people out 0:07:57.360 --> 0:08:00.680 there had thought about this question whether space and why 0:08:00.720 --> 0:08:03.720 space is so flat, and so Daniel went out into 0:08:03.720 --> 0:08:05.760 the internet to ask people this question. 0:08:06.000 --> 0:08:08.560 Thanks very much to everybody who participates. If you'd like 0:08:08.600 --> 0:08:11.880 to hear your voice answering these questions for this segment 0:08:11.880 --> 0:08:14.120 of the podcast, please don't be shy. Write to me 0:08:14.200 --> 0:08:17.840 two questions at Danielandjorge dot com. You'll have a good time, 0:08:17.920 --> 0:08:18.640 I promise. 0:08:18.920 --> 0:08:20.600 So think about it for a second. Why do you 0:08:20.680 --> 0:08:24.960 think space is flat? Here's what people had to say. 0:08:25.240 --> 0:08:27.200 I think space is flat because I think at the 0:08:27.240 --> 0:08:31.440 beginning there wasn't any room for stuff to be clumpy 0:08:31.520 --> 0:08:35.560 or lumped together or have little dips or gaps. And 0:08:36.000 --> 0:08:38.800 I assume that everything was shot out from the big 0:08:38.840 --> 0:08:44.199 Bang in every direction in equal measure, and therefore it 0:08:44.360 --> 0:08:45.520 stayed flat to this day. 0:08:45.880 --> 0:08:48.120 I think that we're not one hundred percent certain that 0:08:48.200 --> 0:08:50.880 space is flat. It just seems like it's flat because 0:08:50.960 --> 0:08:54.920 space has been inflated so much from our perspective. The 0:08:54.960 --> 0:08:57.160 analogy that I've heard before is kind of like standing 0:08:57.160 --> 0:08:58.760 on the surface of the Earth, where the curvature of 0:08:58.760 --> 0:09:01.600 the Earth is so large relative to us that it 0:09:01.840 --> 0:09:02.800 seems like it's flat. 0:09:03.080 --> 0:09:05.120 I'm not sure. I thought it was three dimensional. 0:09:05.280 --> 0:09:06.599 Plant has to do with two D. 0:09:07.160 --> 0:09:13.240 I think the flat space is somehow stable equilibrium points, 0:09:13.280 --> 0:09:19.040 so the space will eventually evolve into the flat version. 0:09:19.480 --> 0:09:22.120 I was sneaking suspicion it's not so flat, but it 0:09:22.240 --> 0:09:24.800 just looks that way to us. I know there's some 0:09:24.920 --> 0:09:29.520 bits of like string theory that posit there are dimensions 0:09:29.559 --> 0:09:34.080 we simply can't perceive, and so I'm wondering if space 0:09:34.400 --> 0:09:38.120 just looks flat to us because we just don't have 0:09:38.160 --> 0:09:40.280 a capacity to see the other dimensions. 0:09:40.920 --> 0:09:42.960 I don't think it is very flat. I think it 0:09:43.160 --> 0:09:48.240 is pretty multi dimensional. I don't know what really is 0:09:48.280 --> 0:09:53.800 meant by that. Maybe like stuff forms discs, like the 0:09:53.840 --> 0:09:58.720 Solar System or our galaxy, maybe that is meant by flat. 0:09:59.120 --> 0:10:01.640 I think that is due to so that things tend 0:10:01.720 --> 0:10:05.240 to take the shape of discs when there's rotation. 0:10:05.000 --> 0:10:08.800 Involved, all right, and not a lot of flat universes. 0:10:11.600 --> 0:10:14.400 Yeah. Here, I think you really are scoring some points 0:10:14.440 --> 0:10:17.560 because a lot of people think flat implies two dimensional. 0:10:17.800 --> 0:10:20.040 Yeah, that makes sense, right, like if something if the 0:10:20.480 --> 0:10:22.920 universe was flat, it would be the width of a 0:10:22.960 --> 0:10:24.079 sheet of paper kind of right. 0:10:24.240 --> 0:10:26.600 Mm hmm. Yeah. If somebody like bakes your birthday cake 0:10:26.760 --> 0:10:28.800 and then a truck drives over it and flattens it, 0:10:29.080 --> 0:10:31.679 then you think of it as like thinner, right, squeeze 0:10:31.720 --> 0:10:32.640 down to two. 0:10:32.400 --> 0:10:36.680 Dimensions two dimensional? Yeah, although two dimensional cake would be 0:10:36.840 --> 0:10:38.920 pretty low calorie. 0:10:40.720 --> 0:10:42.800 I don't think the calorie squeeze out of it when 0:10:42.840 --> 0:10:46.360 the truck drives over it, unless there's like fusion that happens, 0:10:46.679 --> 0:10:48.240 but that would be a pretty heavy truck. 0:10:48.520 --> 0:10:50.160 As if a truck runs over it, I don't think 0:10:50.200 --> 0:10:52.520 you want to eat it off the road. It seems 0:10:52.520 --> 0:10:53.520 like it will make you sick. 0:10:53.720 --> 0:10:55.760 I think we need to have a highly controlled experiment, 0:10:55.840 --> 0:10:58.720 one of those like roads smootheners they have in cartoons 0:10:58.760 --> 0:11:01.640 all the time that's crushing white the coyote. Squeeze a 0:11:01.679 --> 0:11:03.520 cake and see if it still makes people fat. 0:11:04.200 --> 0:11:07.199 I'm pretty sure that if you ask YouTube, somebody out 0:11:07.200 --> 0:11:09.720 there has made a video of a cake being flatted 0:11:09.720 --> 0:11:11.240 by multiple things. 0:11:11.559 --> 0:11:12.040 Thank you. 0:11:12.040 --> 0:11:14.240 YouTube sounds like the kind of thing the Internet likes. 0:11:15.559 --> 0:11:18.000 If it doesn't exist on the Internet before this podcast, 0:11:18.000 --> 0:11:19.079 it certainly will after. 0:11:19.200 --> 0:11:21.839 But yeah, it's an interesting question why is space flat? 0:11:21.880 --> 0:11:24.240 And I guess also is space flat? I guess is 0:11:24.280 --> 0:11:27.040 maybe the first question we should be asking, or maybe 0:11:27.080 --> 0:11:29.800 the question before that should be what does it mean 0:11:29.840 --> 0:11:30.720 for space to be flat? 0:11:30.840 --> 0:11:33.480 Yeah, it's a really fascinating question to even think about, 0:11:33.520 --> 0:11:35.640 like what it means for space to be flat, and 0:11:35.800 --> 0:11:37.760 to talk about how we measure it's flat and why 0:11:37.760 --> 0:11:41.000 we're surprised to find that it is flat. But you're right, 0:11:41.040 --> 0:11:43.320 first we should make sure we're clear about what we 0:11:43.480 --> 0:11:47.120 mean by flat. And there's sort of two different concepts 0:11:47.160 --> 0:11:49.520 to there, of course connected that we need to think about. 0:11:49.640 --> 0:11:52.400 We can think about locally being flat, like is the 0:11:52.520 --> 0:11:55.760 universe bumpy? And we can think about globally like is 0:11:55.760 --> 0:11:59.199 the universe curved? On some big scale. Thinking about locally 0:11:59.280 --> 0:12:01.360 is a little bit easy, though of course still twists 0:12:01.360 --> 0:12:03.400 your brain a little bit. This is just the idea 0:12:03.440 --> 0:12:06.720 that matter bends space, that the reason things don't seem 0:12:06.760 --> 0:12:08.800 to move in straight lines but seem to be bent 0:12:09.080 --> 0:12:12.080 by gravity. Is not because gravity is a force, but 0:12:12.120 --> 0:12:15.120 that space itself is curved. So things are moving through 0:12:15.160 --> 0:12:18.600 that curved space. The Sun bends the space around it, 0:12:18.640 --> 0:12:21.160 so the Earth moves in that circle, which is the 0:12:21.240 --> 0:12:25.000 natural inertial motion for an object in that curved space. 0:12:25.120 --> 0:12:27.160 That's sort of local curvature. 0:12:26.920 --> 0:12:28.800 Right, right, although I think we always have to give 0:12:28.800 --> 0:12:31.640 the caveat that. You mean space time, right, Like space 0:12:31.720 --> 0:12:33.400 time is what's curved. 0:12:33.559 --> 0:12:36.160 Well, space is a part of space time, and the 0:12:36.200 --> 0:12:38.120 curvature of space time is a little bit different from 0:12:38.160 --> 0:12:40.120 the curvature of space. But in this case, space is 0:12:40.160 --> 0:12:40.959 also curved. 0:12:41.280 --> 0:12:43.720 But I guess I mean like curved space makes me 0:12:43.760 --> 0:12:45.959 think of like a road. Like a road is curved, 0:12:47.000 --> 0:12:50.240 and so anything that tries to follow a straight line 0:12:50.280 --> 0:12:52.200 on that road is going to follow the same path. 0:12:52.280 --> 0:12:54.840 But maybe in real life it would kind of depend, right, 0:12:54.840 --> 0:12:56.679 I don't know how fast you're going or what your 0:12:56.760 --> 0:12:58.600 mass is or right, isn't it? 0:12:58.720 --> 0:13:01.760 Yeah, A curved road is a helpful analogy. Things that 0:13:01.800 --> 0:13:05.800 are moving through space are basically following an invisible road 0:13:05.840 --> 0:13:08.440 that we can't see. You know, space is curved, but 0:13:08.480 --> 0:13:10.679 in this way that we can't directly observe it. Like 0:13:10.720 --> 0:13:12.000 you can look at a road and say, oh, there's 0:13:12.000 --> 0:13:13.560 a curve coming up ahead, but you can't look at 0:13:13.559 --> 0:13:16.600 space and see the curvature directly. But it does affect 0:13:16.600 --> 0:13:18.760 the way things move through it. So you try to 0:13:18.840 --> 0:13:21.400 drive your car on the curve road of space, and 0:13:21.440 --> 0:13:23.960 space moves your car forward. You sort of like guides 0:13:24.000 --> 0:13:27.439 it along the curvature of space and fundamensional space time. 0:13:27.440 --> 0:13:31.240 It's really fascinating because time and space bend together to 0:13:31.280 --> 0:13:34.640 make space. Time itself have these invariants, these things that 0:13:34.720 --> 0:13:39.200 don't actually change, but space curves and time curves due 0:13:39.200 --> 0:13:41.240 to the presence of mass. Right. 0:13:41.320 --> 0:13:43.360 But like if I throw a bowling ball at a 0:13:43.400 --> 0:13:44.920 low speed and a high speed, and I threw a 0:13:44.920 --> 0:13:46.920 feather at a low speed and as high speed, they 0:13:46.920 --> 0:13:50.080 would sort of curve through space, or I would see 0:13:50.080 --> 0:13:52.760 them curve in a different way, or would they all 0:13:52.800 --> 0:13:53.720 curve the same way. 0:13:53.840 --> 0:13:55.840 We know that because, for example, you throw a baseball 0:13:55.880 --> 0:13:59.000 at different speeds, it's going to go a different path, right, 0:13:59.080 --> 0:14:01.800 So it definitely depends on the velocity of the object 0:14:01.840 --> 0:14:04.960 as it moves through curve space. So that's all inertial motion. 0:14:05.040 --> 0:14:08.959 That's motion under no forces, just the curvature of space. 0:14:09.480 --> 0:14:11.160 So I wonder if it's more accurate to say that 0:14:11.240 --> 0:14:14.600 space has curvature and not that space is curved. 0:14:14.840 --> 0:14:16.120 What's the distinction in your mind? 0:14:16.160 --> 0:14:18.439 Well, space like if you say that space is curved, 0:14:18.520 --> 0:14:20.400 make me think of it like a tunnel. Like, if 0:14:20.440 --> 0:14:23.160 a tunnel is curved, then no matter how fast you're 0:14:23.200 --> 0:14:25.520 going or what you throw, you're going to bend the 0:14:25.520 --> 0:14:28.320 same way. You can follow the same path. But that's 0:14:28.360 --> 0:14:31.800 not sort of how space really works, right, Not everything 0:14:31.840 --> 0:14:33.360 is stuck in the same kind of path. 0:14:33.480 --> 0:14:36.280 Yeah, you're right, there aren't rigid tunnels that things have 0:14:36.360 --> 0:14:37.960 to go through. It's not like if you enter a 0:14:37.960 --> 0:14:40.040 pipe and you get flushed out the bottom or something. 0:14:40.120 --> 0:14:43.040 The curvature of space does affect how you move, So yeah, 0:14:43.080 --> 0:14:44.200 that distinction makes sense to me. 0:14:44.400 --> 0:14:47.440 Okay, So then there's local curvature of space. What you're 0:14:47.440 --> 0:14:49.560 talking about is sort of like how things go around 0:14:49.560 --> 0:14:52.479 the Sun, for example, or how the moon goes around. 0:14:52.200 --> 0:14:54.160 The Earth m or how we stay on the Earth, 0:14:54.320 --> 0:14:57.600 all that kind of stuff. Things being gravitationally bound, even 0:14:57.600 --> 0:15:01.280 like the galaxy holding itself together. That's all local curvature 0:15:01.320 --> 0:15:04.120 in comparison to the global curvature, which is a question 0:15:04.200 --> 0:15:07.160 about the whole universe and its shape. 0:15:07.600 --> 0:15:10.480 So there's local and global, and I guess what's the difference, 0:15:10.640 --> 0:15:13.400 just like the size of it the scale, Like are 0:15:13.400 --> 0:15:16.080 you saying that? Like if I'm orbiting around the center 0:15:16.080 --> 0:15:18.440 of the galaxy, it's a different kind of curvature of 0:15:18.640 --> 0:15:21.040 doue to gravity than the Moon orpening around the Earth. 0:15:21.120 --> 0:15:24.800 It's not fundamentally different. It's all described by general relativity 0:15:24.840 --> 0:15:27.600 in Einstein's equations. It's sort of like the difference between 0:15:27.640 --> 0:15:30.160 the Earth being a sphere and the Earth having mountains. 0:15:30.640 --> 0:15:32.320 Like the Earth could be flat and it could be 0:15:32.360 --> 0:15:34.960 a sphere, but it could still have mountains in either case. Right, 0:15:35.000 --> 0:15:37.880 the Earth could be bumpy locally, it could have valleys 0:15:37.920 --> 0:15:40.760 and mountains even if it's flat, or even if it's 0:15:40.800 --> 0:15:43.480 a sphere. So global curvature is more about the question 0:15:43.560 --> 0:15:45.480 of like is the Earth a sphere or is the 0:15:45.520 --> 0:15:48.520 Earth flat? Local curvature is about like is the Earth 0:15:48.600 --> 0:15:52.320 bumpy or is it all perfectly smooth everywhere you go? 0:15:52.440 --> 0:15:54.320 So we are asking if the universe is bumping. 0:15:54.920 --> 0:15:57.080 We're asking if the universe is sort of bent Right, 0:15:57.200 --> 0:15:59.520 is three D space more like the surface of a 0:15:59.560 --> 0:16:03.840 three sphere? Right? Or is it flat? And morn analogy 0:16:03.880 --> 0:16:06.240 to like a sheet of paper. So the global curvature 0:16:06.320 --> 0:16:09.160 space is asking about like the big picture, whereas the 0:16:09.200 --> 0:16:12.320 local curvature is asking about the little picture right right. 0:16:12.360 --> 0:16:14.920 But I guess my question was, or is you know 0:16:14.960 --> 0:16:16.880 where do you draw when do you draw this distinction 0:16:16.920 --> 0:16:20.440 between local and global, like at the galaxy level, at 0:16:20.440 --> 0:16:23.520 the galaxy cluster level, or it's only global if it's everything. 0:16:23.680 --> 0:16:25.800 It's only global if it's everything. And you know, when 0:16:25.840 --> 0:16:28.520 we solve these equations in general relativity, and by we, 0:16:28.800 --> 0:16:32.920 I mean those guys who knows all equations in general relativity. 0:16:32.760 --> 0:16:33.120 Not me. 0:16:33.760 --> 0:16:36.760 They can only solve those in certain situations, in situations 0:16:36.800 --> 0:16:39.440 where they assume, like the universe is empty, or the 0:16:39.520 --> 0:16:43.960 universe is filled with matter, but that matter is perfectly smooth, like. 0:16:44.040 --> 0:16:47.240 Nobody can solve the general relativity equations for our universe, 0:16:47.280 --> 0:16:49.680 which has like clumps of matter in it. So when 0:16:49.720 --> 0:16:52.680 we talk about the whole universe and its curvature, basically 0:16:52.680 --> 0:16:55.360 we're talking about a simplification of our universe where all 0:16:55.360 --> 0:16:58.360 the matter is spread out evenly. There are no lumps 0:16:58.400 --> 0:17:01.320 at all, because that's all they can. And in that case, 0:17:01.360 --> 0:17:05.159 there's still this question of the global curvature. Is the 0:17:05.280 --> 0:17:08.639 universe curved on some large scale and how is it curved? 0:17:08.920 --> 0:17:11.680 Is it flat? Is it open? Is it closed? These 0:17:11.680 --> 0:17:14.360 are the questions of the global curvature of space time, 0:17:14.359 --> 0:17:17.600 which are irrelevant to the little details because they still 0:17:17.640 --> 0:17:20.040 exist even if you smooth all the matter and energy 0:17:20.080 --> 0:17:21.320 out everywhere like peanut butter. 0:17:21.520 --> 0:17:24.119 Mmm, but I guess, you know, if it's everything, we 0:17:24.160 --> 0:17:27.560 don't really know what everything is, right, Like the whole 0:17:27.680 --> 0:17:30.320 entire observable universe might be just a little bump in 0:17:30.400 --> 0:17:32.080 a ginormous infinite universe. 0:17:32.119 --> 0:17:34.720 Well, you're totally right, But the curvature around here is 0:17:34.760 --> 0:17:38.640 dependent on the energy density, doesn't depend on what's happening 0:17:38.680 --> 0:17:41.440 out there. And then we assume that what's happening here 0:17:41.520 --> 0:17:44.439 is what's happening everywhere else, And that's an assumption we 0:17:44.480 --> 0:17:46.600 don't know. It could be that what we're calling global 0:17:46.600 --> 0:17:49.920 curvature is actually local on a much larger scale, that 0:17:49.960 --> 0:17:52.199 if you zoom out from the observable universe to the 0:17:52.240 --> 0:17:55.240 actual full universe, that we could never see that the 0:17:55.240 --> 0:17:57.280 curvature is different, right, and that what we were talking 0:17:57.320 --> 0:18:00.320 about the whole time is quote unquote global curvature is 0:18:00.320 --> 0:18:03.160 actually the local curvature of the observable universe. 0:18:03.640 --> 0:18:05.840 I guess it's sort of like how before we used 0:18:05.880 --> 0:18:07.920 to think that the Earth was flat because we only 0:18:08.000 --> 0:18:10.840 knew sort of the local area here around this and 0:18:11.000 --> 0:18:13.400 looks pretty flat. But actually if you sort of keep 0:18:13.440 --> 0:18:16.000 going or you're taken to account the whole planet, then 0:18:16.040 --> 0:18:18.200 you see that the whole planet is curved and round. 0:18:18.400 --> 0:18:20.520 Yeah, exactly. And if you lived on a part of 0:18:20.560 --> 0:18:23.040 the Earth that was literally flat, like maybe you were 0:18:23.080 --> 0:18:24.920 really precise about it, and you took out a bunch 0:18:24.960 --> 0:18:26.680 of tools and you try to measure the curvature of 0:18:26.680 --> 0:18:29.280 the Earth. Imagine you lived on like a truly flat 0:18:29.480 --> 0:18:32.120 part of the Earth and you measured it to be flat, 0:18:32.280 --> 0:18:34.280 and then you left that and you realized, oh, actually 0:18:34.280 --> 0:18:36.239