1 00:00:08,480 --> 00:00:11,960 Speaker 1: Hey, Daniel, what concept in physics twist your brain more 2 00:00:12,000 --> 00:00:14,160 Speaker 1: than anything else? I think it would have to be 3 00:00:14,280 --> 00:00:18,759 Speaker 1: general relativity because it's super beautiful and gorgeous, but it's 4 00:00:18,760 --> 00:00:22,160 Speaker 1: also really hard to actually wrap your mind around. Isn't 5 00:00:22,160 --> 00:00:24,759 Speaker 1: that all relative? Though? But I guess what makes it 6 00:00:24,800 --> 00:00:27,000 Speaker 1: hard all the math. There's a lot of math, but 7 00:00:27,120 --> 00:00:29,600 Speaker 1: fundamentally it's the concepts. You know. It's just such a 8 00:00:29,640 --> 00:00:32,480 Speaker 1: different view of how to see the universe. It says 9 00:00:32,520 --> 00:00:35,720 Speaker 1: that space is like a thing and it's invisibly doing 10 00:00:35,800 --> 00:00:38,960 Speaker 1: stuff that we're blind to. Yes, pretty well, and now 11 00:00:39,000 --> 00:00:42,000 Speaker 1: we got like neutrinos, dark energy, dark matter. Seems like 12 00:00:42,040 --> 00:00:44,760 Speaker 1: most of the universe is invisible to us. We're definitely 13 00:00:44,880 --> 00:00:48,200 Speaker 1: more clueless than clued in when it comes to the universe. 14 00:00:48,840 --> 00:00:51,199 Speaker 1: It's all a giant game of clue. You know. It 15 00:00:51,400 --> 00:00:53,800 Speaker 1: was the dark energy that killed the dark matter in 16 00:00:53,840 --> 00:00:56,600 Speaker 1: the space library within lutrinos. That might be the first 17 00:00:56,640 --> 00:01:15,120 Speaker 1: time neutrinos ever killed anybody. Hi am Orhammy cartoonists and 18 00:01:15,160 --> 00:01:17,720 Speaker 1: the creator of PhD comics. Hi, I'm Daniel. I'm a 19 00:01:17,720 --> 00:01:20,360 Speaker 1: particle physicist and a professor. You see Irvine, And if 20 00:01:20,400 --> 00:01:22,319 Speaker 1: I could choose the way I go out, I'd like 21 00:01:22,360 --> 00:01:24,560 Speaker 1: it to be with neutrinos. Oh yeah, why is that? 22 00:01:24,760 --> 00:01:26,560 Speaker 1: Because it sounds unique. You know, I'd like to be 23 00:01:26,560 --> 00:01:29,440 Speaker 1: the first person ever killed by neutrinos. I don't even 24 00:01:29,440 --> 00:01:32,959 Speaker 1: know if that's possible. Like, imagine the crazy intense neutrino 25 00:01:33,040 --> 00:01:35,280 Speaker 1: beam necessary to even heat you up a little bit, 26 00:01:35,400 --> 00:01:37,360 Speaker 1: not to mention kill you. Sounds like you thought about 27 00:01:37,360 --> 00:01:42,240 Speaker 1: this a lot about how to use neutrinos to kill someone. Yeah, 28 00:01:42,280 --> 00:01:45,080 Speaker 1: for about ten seconds so far. I guess it is hard, 29 00:01:45,360 --> 00:01:47,160 Speaker 1: but I guess with enough of them that they could 30 00:01:47,200 --> 00:01:49,760 Speaker 1: be deadly, right, Yeah, if you have a powerful enough beam, 31 00:01:49,840 --> 00:01:52,040 Speaker 1: they could actually deposit enough energy and you had to 32 00:01:52,080 --> 00:01:55,600 Speaker 1: fry you just like a laser. Oh, neutrino lasers. Would 33 00:01:55,640 --> 00:01:58,480 Speaker 1: it be like a neutral tan? It'd be like a 34 00:01:58,520 --> 00:02:01,320 Speaker 1: little neutral tan. Right, because this newtrie No, maybe our 35 00:02:01,320 --> 00:02:05,520 Speaker 1: next product idea should be neutrinos blocking pree. Now there's 36 00:02:05,560 --> 00:02:08,200 Speaker 1: a science scam. Add the word quantum to it and 37 00:02:08,240 --> 00:02:11,040 Speaker 1: it'll sell cell cell good of a reading of NPF 38 00:02:11,120 --> 00:02:15,920 Speaker 1: one thousand lutrino protection factor. But anyways, welcome to our podcast. 39 00:02:16,000 --> 00:02:19,399 Speaker 1: Daniel and Jorge explain the Universe, a production of iHeartRadio, 40 00:02:19,560 --> 00:02:22,160 Speaker 1: in which we try to beam into your brain all 41 00:02:22,160 --> 00:02:25,760 Speaker 1: of the incredible mysteries and knowledge about the universe without 42 00:02:25,880 --> 00:02:28,880 Speaker 1: toasting it. We try to protect that precious little blob 43 00:02:28,960 --> 00:02:33,680 Speaker 1: of matter while also injecting ideas and questions and curiosity 44 00:02:33,760 --> 00:02:36,480 Speaker 1: into it. We hope to stimulate your brain to think 45 00:02:36,520 --> 00:02:40,079 Speaker 1: about the nature of the entire universe, what it looks like, 46 00:02:40,120 --> 00:02:43,440 Speaker 1: what it seems like, what it's actually doing behind our backs, 47 00:02:43,680 --> 00:02:47,040 Speaker 1: without toasting it aggressively, or at least neutraally toasting it. 48 00:02:47,320 --> 00:02:50,200 Speaker 1: Because it is a fascinating universe full of amazing things 49 00:02:50,240 --> 00:02:53,120 Speaker 1: out there, and it's an ever changing universe. It's a 50 00:02:53,240 --> 00:02:57,560 Speaker 1: universe that's expanding and growing and shifting and moving and 51 00:02:57,680 --> 00:03:00,640 Speaker 1: rotating space doing all kinds of things. Is doing a 52 00:03:00,680 --> 00:03:02,680 Speaker 1: whole lot of things, and as you mentioned, is doing 53 00:03:02,720 --> 00:03:05,399 Speaker 1: a lot of things that we can't see. Our senses 54 00:03:05,440 --> 00:03:08,919 Speaker 1: are like tiny little portals into the vast and complex 55 00:03:08,919 --> 00:03:11,880 Speaker 1: workings of the universe. Most of what's out there is 56 00:03:11,919 --> 00:03:14,600 Speaker 1: really invisible to us. Yeah, and that's one of the 57 00:03:14,639 --> 00:03:17,280 Speaker 1: wonderful things about the universe. So that it doesn't reveal 58 00:03:17,320 --> 00:03:19,600 Speaker 1: itself right away, we need to probe it. We need 59 00:03:19,639 --> 00:03:21,919 Speaker 1: to think about it. We need to find clever ways 60 00:03:22,440 --> 00:03:24,800 Speaker 1: to figure out what's going on out there, and so 61 00:03:24,840 --> 00:03:28,560 Speaker 1: the history of physics is filled with people noticing something weird, 62 00:03:28,720 --> 00:03:32,040 Speaker 1: something they can't quite explain, something that doesn't quite fit. 63 00:03:32,400 --> 00:03:34,800 Speaker 1: Usually that's a thread we can pull on to unravel 64 00:03:34,800 --> 00:03:38,320 Speaker 1: an entire story about something going on in the universe. 65 00:03:38,360 --> 00:03:42,080 Speaker 1: We weren't even aware of the discovery of neutrinos being 66 00:03:42,120 --> 00:03:45,160 Speaker 1: pumped out from the Sun, the discovery of vast quantities 67 00:03:45,200 --> 00:03:48,480 Speaker 1: of dark matter floating out there in space and changing 68 00:03:48,480 --> 00:03:52,280 Speaker 1: the way that galaxies spin and the entire universe is formed. 69 00:03:52,440 --> 00:03:54,880 Speaker 1: And that's not the end of the invisible things that 70 00:03:54,920 --> 00:03:58,240 Speaker 1: the universe is doing right under our noses. Yeah. Maybe 71 00:03:58,240 --> 00:04:01,160 Speaker 1: one of the most mind blowing revelations about the universe 72 00:04:01,160 --> 00:04:03,600 Speaker 1: that humans have discovered in the last hundred or so 73 00:04:03,800 --> 00:04:07,040 Speaker 1: years is this idea that space is not fixed. It's 74 00:04:07,200 --> 00:04:10,320 Speaker 1: not this kind of emptiness that we are used to 75 00:04:10,320 --> 00:04:13,160 Speaker 1: in our everyday lives. As we move around in space, 76 00:04:13,520 --> 00:04:17,440 Speaker 1: space is actually bending and curved. Yeah, space has a 77 00:04:17,440 --> 00:04:20,720 Speaker 1: lot more properties then Isaac Newton might have imagined. It 78 00:04:20,839 --> 00:04:24,040 Speaker 1: can do stuff. It's not just there. It's not just 79 00:04:24,120 --> 00:04:28,000 Speaker 1: the emptiness, the lack of stuff. It is actually a physical, 80 00:04:28,120 --> 00:04:31,720 Speaker 1: dynamical thing that has properties and can affect things in 81 00:04:31,760 --> 00:04:34,279 Speaker 1: all sorts of important ways. Yeah, and so one of 82 00:04:34,279 --> 00:04:36,920 Speaker 1: the most interesting facts about that is this idea that 83 00:04:37,040 --> 00:04:40,440 Speaker 1: space can curve, that space is not just straight up 84 00:04:40,440 --> 00:04:43,599 Speaker 1: emptiness with nothing in it. It can actually kind of bend. 85 00:04:43,920 --> 00:04:47,599 Speaker 1: This idea that space can be described geometrically is having 86 00:04:47,680 --> 00:04:50,640 Speaker 1: curvature is of course one of the great insights that 87 00:04:50,760 --> 00:04:55,000 Speaker 1: underpinn Einstein's theory of general relativity. We've had this idea 88 00:04:55,040 --> 00:04:58,159 Speaker 1: for about one hundred years and it completely reshapes the 89 00:04:58,200 --> 00:05:00,520 Speaker 1: way we think about the universe. But still going to 90 00:05:00,560 --> 00:05:03,800 Speaker 1: be pretty tricky to understand what it means. What are 91 00:05:03,800 --> 00:05:06,200 Speaker 1: we talking about here? If you have a chunk of 92 00:05:06,200 --> 00:05:08,240 Speaker 1: space in front of you, what does it mean for 93 00:05:08,320 --> 00:05:11,880 Speaker 1: it to be curved? And is it possible to actually 94 00:05:11,960 --> 00:05:16,440 Speaker 1: see it? This is literally a mind bending topic. If 95 00:05:16,480 --> 00:05:19,240 Speaker 1: your mind is part of space, then yes, bending of 96 00:05:19,320 --> 00:05:21,839 Speaker 1: space will also bend your mind. I do feel like 97 00:05:22,320 --> 00:05:24,560 Speaker 1: my head is in outer space a lot of the time, 98 00:05:25,160 --> 00:05:28,240 Speaker 1: or outer space or outer space both. We want to 99 00:05:28,240 --> 00:05:31,800 Speaker 1: blow your mind into outer space when neutrino lasers, is 100 00:05:31,800 --> 00:05:35,839 Speaker 1: that where all this is going neutrino lasers are not 101 00:05:35,880 --> 00:05:38,800 Speaker 1: really very useful for anything except for joking about how 102 00:05:38,839 --> 00:05:41,080 Speaker 1: Daniel wants to go out. Well, and technically would they 103 00:05:41,120 --> 00:05:44,920 Speaker 1: be called lasers or lasers? M yeah, good question. I 104 00:05:44,960 --> 00:05:47,320 Speaker 1: guess it depends on the frequency, right, But it'd be 105 00:05:47,360 --> 00:05:50,359 Speaker 1: pretty tricky to build an apparatus that could resonate or 106 00:05:50,400 --> 00:05:54,680 Speaker 1: focus neutrinos. Neutrino optics would be quite challenging to design. 107 00:05:55,040 --> 00:05:57,480 Speaker 1: It's even hard to make X ray lasers. So I 108 00:05:57,520 --> 00:06:00,760 Speaker 1: think neutrino lasers are pretty far from our technology capability. 109 00:06:00,920 --> 00:06:03,599 Speaker 1: So I can safely joke about using them to fry 110 00:06:03,680 --> 00:06:05,880 Speaker 1: my brain. Well, they're also pretty far from the topic. 111 00:06:05,920 --> 00:06:08,920 Speaker 1: I don't know how we just can I need degree attorney. Here. 112 00:06:09,440 --> 00:06:13,479 Speaker 1: We were almost on track there. We're talking about the 113 00:06:13,480 --> 00:06:16,920 Speaker 1: curvature space and how space is kind of bendy, and 114 00:06:17,000 --> 00:06:19,920 Speaker 1: so today on the program, we'll be asking the question 115 00:06:25,000 --> 00:06:28,400 Speaker 1: can we measure the curvature of space? Maybe we should 116 00:06:28,440 --> 00:06:30,800 Speaker 1: be measuring the curvature of the podcast, Like can we 117 00:06:30,839 --> 00:06:33,679 Speaker 1: actually keep a conversation going in a straight line or 118 00:06:33,760 --> 00:06:37,240 Speaker 1: do we constantly bend off the topic into other areas 119 00:06:37,240 --> 00:06:40,760 Speaker 1: like neutrino lasers. I think the experimental data says that 120 00:06:40,839 --> 00:06:44,839 Speaker 1: we do bend a lot, sometimes in ninety degrees, sometimes 121 00:06:45,240 --> 00:06:47,640 Speaker 1: three sixty degrees. Yeah, and sometimes those are the best 122 00:06:47,640 --> 00:06:50,159 Speaker 1: moments on the podcast when we talk about something totally 123 00:06:50,279 --> 00:06:54,559 Speaker 1: unexpected and discover a fun little corner of physics. Yeah, 124 00:06:54,600 --> 00:06:56,720 Speaker 1: but let's maybe stick to the straight and narrow here 125 00:06:56,760 --> 00:07:00,240 Speaker 1: and stay on the topic of the curvature space. This 126 00:07:00,320 --> 00:07:03,120 Speaker 1: is an interesting topic because, first of all, maybe a 127 00:07:03,120 --> 00:07:04,880 Speaker 1: lot of people out there, at least maybe in the 128 00:07:04,920 --> 00:07:08,040 Speaker 1: general public, don't know that space can curve. Yes, spatial 129 00:07:08,040 --> 00:07:11,080 Speaker 1: curature is really the foundation of general relativity. It's the 130 00:07:11,200 --> 00:07:14,640 Speaker 1: idea that gravity is not actually a force, but that 131 00:07:14,720 --> 00:07:17,440 Speaker 1: the reason things move as if there was gravity is 132 00:07:17,440 --> 00:07:21,080 Speaker 1: because space is invisibly doing this thing. It's bending, it's curving, 133 00:07:21,120 --> 00:07:24,720 Speaker 1: it's changing how things move through it. It really requires 134 00:07:24,720 --> 00:07:27,800 Speaker 1: complete shift in your understanding of gravity and what the 135 00:07:27,920 --> 00:07:31,120 Speaker 1: universe really is all about. Now, technically, Daniel, don't we 136 00:07:31,200 --> 00:07:33,160 Speaker 1: need to say that we're talking about the curvature of 137 00:07:33,240 --> 00:07:37,960 Speaker 1: space time? Right? Yes, because space by itself doesn't really bend. 138 00:07:38,000 --> 00:07:40,360 Speaker 1: It's really more like a bending if you include time 139 00:07:40,920 --> 00:07:43,480 Speaker 1: into it. Well, technically I think space does bend, but 140 00:07:43,480 --> 00:07:46,120 Speaker 1: you're absolutely right there. The equations and the important like 141 00:07:46,240 --> 00:07:50,360 Speaker 1: conservation laws are expressed in terms of space time because 142 00:07:50,400 --> 00:07:53,680 Speaker 1: relativity takes time and treats it as the fourth dimension 143 00:07:53,720 --> 00:07:57,040 Speaker 1: of space. So really it thinks about a four dimensional object, 144 00:07:57,120 --> 00:07:59,880 Speaker 1: not just three D space. So yes, space time is 145 00:08:00,120 --> 00:08:02,280 Speaker 1: more accurate description. Right. Wait, are you saying that three 146 00:08:02,360 --> 00:08:05,600 Speaker 1: D space bends or it's only that we should really 147 00:08:06,120 --> 00:08:08,480 Speaker 1: be using the word space time to mean the four 148 00:08:08,600 --> 00:08:12,280 Speaker 1: D concept is what bends? Well, each dimension does bend, Right, 149 00:08:12,520 --> 00:08:15,640 Speaker 1: X bends, why bends, Z bends, So space itself as x, 150 00:08:15,760 --> 00:08:18,920 Speaker 1: y Z does bend, but time also bends, and they 151 00:08:18,920 --> 00:08:21,600 Speaker 1: all sort of bend together. And that's one thing that 152 00:08:21,640 --> 00:08:23,960 Speaker 1: Einstein realized is that it makes much more sense to 153 00:08:24,000 --> 00:08:28,200 Speaker 1: think of these as four components of one larger mathematical object. 154 00:08:28,280 --> 00:08:31,239 Speaker 1: But each individual one does bend the way. For example, 155 00:08:31,360 --> 00:08:34,880 Speaker 1: time bends, right, that's time dilation. It certainly does bend, 156 00:08:34,920 --> 00:08:37,840 Speaker 1: but it also bends in conjunction with the other dimensions. 157 00:08:37,920 --> 00:08:40,640 Speaker 1: That some of the beautiful mathematics of relativities. Seeing how 158 00:08:40,640 --> 00:08:43,640 Speaker 1: all four work together cool well as usual, we were 159 00:08:43,640 --> 00:08:45,800 Speaker 1: wondering how many people out there had thought about the 160 00:08:45,920 --> 00:08:49,160 Speaker 1: question of whether and how you can measure the curvature 161 00:08:49,160 --> 00:08:51,520 Speaker 1: of space. So thanks very much to everybody who answers 162 00:08:51,520 --> 00:08:54,199 Speaker 1: these questions for us. We'd love to hear your voice 163 00:08:54,240 --> 00:08:56,920 Speaker 1: on the podcast as well. Write to me two questions 164 00:08:56,960 --> 00:08:59,800 Speaker 1: at Daniel d Jorge dot com and I'll email you 165 00:09:00,000 --> 00:09:03,480 Speaker 1: a batch of questions for future episodes. To think about 166 00:09:03,520 --> 00:09:05,840 Speaker 1: it for a second, how would you measure the curvature 167 00:09:05,840 --> 00:09:08,920 Speaker 1: of space? Here's what people had to say. Yes, we 168 00:09:09,000 --> 00:09:13,920 Speaker 1: can measure the curvature of space. I think we did 169 00:09:14,000 --> 00:09:18,720 Speaker 1: that gravitational wave detection recently, and I know at least 170 00:09:18,760 --> 00:09:22,440 Speaker 1: like the calculations work out, so I'm going to go it. Yeah, 171 00:09:22,559 --> 00:09:26,720 Speaker 1: I know we can detect distortions due to gravitational lensing 172 00:09:26,920 --> 00:09:31,199 Speaker 1: from massive objects, but I don't think that was what 173 00:09:31,240 --> 00:09:35,280 Speaker 1: you mean. I don't know how we would detect the 174 00:09:35,280 --> 00:09:40,360 Speaker 1: curvature of space well, being within the space time continuum, 175 00:09:41,080 --> 00:09:42,760 Speaker 1: I would think you'd have to be outside of it 176 00:09:42,800 --> 00:09:45,600 Speaker 1: to be able to see the curvature. Yes, I don't 177 00:09:45,640 --> 00:09:50,199 Speaker 1: know exactly how it works, but probably we can measure 178 00:09:50,280 --> 00:09:54,680 Speaker 1: it with a light, with photos something, but I don't 179 00:09:54,679 --> 00:09:58,600 Speaker 1: know exactly. Oh this might work. If we can't measure 180 00:09:58,720 --> 00:10:02,800 Speaker 1: the curvature of space, why is general relativity all about 181 00:10:03,160 --> 00:10:06,040 Speaker 1: all right. Everyone seemed pretty positive about the fact that 182 00:10:06,160 --> 00:10:08,319 Speaker 1: you can do it. Yeah, it's definitely something we know 183 00:10:08,720 --> 00:10:11,199 Speaker 1: is happening out there, right, which is sort of cool 184 00:10:11,240 --> 00:10:14,880 Speaker 1: philosophically and conceptually to accept that this thing is happening 185 00:10:14,880 --> 00:10:18,160 Speaker 1: all around you. It's sort of invisible to you, but 186 00:10:18,280 --> 00:10:21,760 Speaker 1: it's necessary to understand how things work, right, to accept 187 00:10:22,040 --> 00:10:25,200 Speaker 1: that a big fraction of the nature of the universe 188 00:10:25,240 --> 00:10:27,920 Speaker 1: itself is invisible to us. Yeah, I guess it's kind 189 00:10:27,920 --> 00:10:30,280 Speaker 1: of a weird question because like if space bent and 190 00:10:30,520 --> 00:10:32,319 Speaker 1: right here in front of me, I would probably be 191 00:10:32,360 --> 00:10:34,439 Speaker 1: able to tell, right, Well, that's the question is how 192 00:10:34,480 --> 00:10:37,480 Speaker 1: could you tell, Like, imagine it was just space, no matter, 193 00:10:37,559 --> 00:10:40,160 Speaker 1: no particles, everything was totally empty. If you had a 194 00:10:40,240 --> 00:10:42,400 Speaker 1: chunk of space in front of you, how would you 195 00:10:42,440 --> 00:10:44,959 Speaker 1: measure its curac or could you measure it without its 196 00:10:45,000 --> 00:10:48,120 Speaker 1: influence on other things? Like you can't see it bent 197 00:10:48,559 --> 00:10:50,560 Speaker 1: the way you can look at a road and say, okay, 198 00:10:50,559 --> 00:10:52,640 Speaker 1: I can see that the road is bending ahead of me. 199 00:10:52,840 --> 00:10:54,880 Speaker 1: You can't do the same thing with space because it's 200 00:10:54,880 --> 00:10:57,520 Speaker 1: bending is not directly visible. What do you mean it's 201 00:10:57,520 --> 00:10:59,480 Speaker 1: not directly visible? Like if the space in front of 202 00:10:59,480 --> 00:11:02,559 Speaker 1: me curve. Put an i'd be able to see a curve. Well, 203 00:11:02,600 --> 00:11:05,800 Speaker 1: imagine an invisible road, right, If you can't see the road, 204 00:11:05,920 --> 00:11:08,240 Speaker 1: but you can follow the cars moving along it, then 205 00:11:08,320 --> 00:11:10,680 Speaker 1: that's the way you're seeing that the road curves. So 206 00:11:10,720 --> 00:11:13,079 Speaker 1: that's a difference between a visible road where you could, 207 00:11:13,120 --> 00:11:15,480 Speaker 1: for example, see that it's bendy even when there aren't 208 00:11:15,480 --> 00:11:18,640 Speaker 1: any cars on it, and an invisible road like at night, 209 00:11:18,679 --> 00:11:21,000 Speaker 1: if you can't tell where the cars are on the mountain, 210 00:11:21,200 --> 00:11:23,360 Speaker 1: but you can follow their headlights, so you can infer 211 00:11:23,760 --> 00:11:26,320 Speaker 1: where the road must be. In the same way, space 212 00:11:26,360 --> 00:11:29,400 Speaker 1: we can't directly see its curvature unless there's matter being 213 00:11:29,440 --> 00:11:32,600 Speaker 1: influenced by that space, we can't directly tell what the 214 00:11:32,640 --> 00:11:34,959 Speaker 1: curvature is. Well, I see what you're saying. You're saying space, 215 00:11:35,040 --> 00:11:39,880 Speaker 1: but itself is invisible. You can't like see space, and 216 00:11:40,040 --> 00:11:43,760 Speaker 1: so therefore how do you know if it's bending or not? Yeah, exactly, 217 00:11:43,960 --> 00:11:46,040 Speaker 1: And you might think, oh, that's obvious, right, we're talking 218 00:11:46,080 --> 00:11:48,760 Speaker 1: about space. Space is invisible because it's space, right, But 219 00:11:48,800 --> 00:11:51,360 Speaker 1: remember that we now know that space is a thing. 220 00:11:51,440 --> 00:11:54,400 Speaker 1: It has properties. So at each point in space, it 221 00:11:54,480 --> 00:11:58,560 Speaker 1: has this property, this amount of curvature that's somehow stored 222 00:11:58,720 --> 00:12:01,319 Speaker 1: in it, and yet we can directly see it. So 223 00:12:01,360 --> 00:12:04,600 Speaker 1: even though it's invisible, there is something to it. All right, Well, 224 00:12:04,640 --> 00:12:08,040 Speaker 1: let's dig into this topic. Daniel steps through the basics 225 00:12:08,040 --> 00:12:09,839 Speaker 1: of this, like, what do you mean by curvature? What 226 00:12:09,880 --> 00:12:12,000 Speaker 1: does it mean for space to be curved? So first 227 00:12:12,040 --> 00:12:15,479 Speaker 1: let's dispense with a sort of common misunderstanding of curvature. 228 00:12:15,960 --> 00:12:18,160 Speaker 1: A lot of people have seen this example of like 229 00:12:18,200 --> 00:12:20,480 Speaker 1: a rubber sheet with a bowling ball in it, and 230 00:12:20,520 --> 00:12:22,440 Speaker 1: have been told that this is an example or an 231 00:12:22,480 --> 00:12:25,800 Speaker 1: analogy for the curvature of space. And you know, this 232 00:12:25,880 --> 00:12:28,440 Speaker 1: is helpful in some ways because it makes you think 233 00:12:28,440 --> 00:12:31,320 Speaker 1: about how space could be bendy, but it's also really 234 00:12:31,360 --> 00:12:34,440 Speaker 1: misleading in some important ways. First of all, it treats 235 00:12:34,440 --> 00:12:36,520 Speaker 1: our universe as if it was two dimensional, just like 236 00:12:36,559 --> 00:12:39,400 Speaker 1: the surface of the rubber sheet, and it suggests the 237 00:12:39,440 --> 00:12:42,960 Speaker 1: bending is happening in some third dimension up and down. 238 00:12:43,040 --> 00:12:45,520 Speaker 1: So it suggests the bending is extrinsic, that it's like 239 00:12:45,679 --> 00:12:49,600 Speaker 1: relative to some fixed external ruler. But in our universe 240 00:12:49,679 --> 00:12:52,719 Speaker 1: we think that the bending of space is intrinsic. There 241 00:12:52,840 --> 00:12:57,120 Speaker 1: is no external ruler, no fourth dimension where our space 242 00:12:57,200 --> 00:13:00,199 Speaker 1: is sort of like bending out towards the bending of 243 00:13:00,240 --> 00:13:03,280 Speaker 1: space for us, and in general, relativities intrinsic, which means 244 00:13:03,280 --> 00:13:06,800 Speaker 1: it just changes the relative distances between things, like how 245 00:13:06,800 --> 00:13:09,600 Speaker 1: far two points are apart. I guess that's weird to 246 00:13:09,679 --> 00:13:12,520 Speaker 1: me because I think what you're saying, is it curvatual 247 00:13:12,559 --> 00:13:15,920 Speaker 1: is something that happens within space, not relative to anything 248 00:13:15,960 --> 00:13:19,000 Speaker 1: outside of it. But if it's not happening to anything 249 00:13:19,040 --> 00:13:22,160 Speaker 1: relative outside of it and we're all in space, I mean, 250 00:13:22,280 --> 00:13:25,160 Speaker 1: is that still curvature or is it just that's the 251 00:13:25,160 --> 00:13:27,000 Speaker 1: way space is? You know what I mean? Like, what's 252 00:13:27,000 --> 00:13:31,240 Speaker 1: the difference between a curve space and a man curve space? Well, 253 00:13:31,280 --> 00:13:33,440 Speaker 1: you can measure it. And what does it mean? Right? Well, 254 00:13:33,440 --> 00:13:35,200 Speaker 1: we know that space can be curved and it can 255 00:13:35,240 --> 00:13:37,920 Speaker 1: also be not curved, because we have chunks of space 256 00:13:37,920 --> 00:13:39,760 Speaker 1: in our universe that are not curved that are like 257 00:13:39,800 --> 00:13:42,319 Speaker 1: far from masses and energy, and chunks of space that 258 00:13:42,360 --> 00:13:45,920 Speaker 1: are highly curved near large masses or even so curved 259 00:13:45,920 --> 00:13:48,880 Speaker 1: that they become like one directional inside a black hole. 260 00:13:49,040 --> 00:13:51,560 Speaker 1: So it's definitely something that space can do, and space 261 00:13:51,600 --> 00:13:54,960 Speaker 1: can do differently. They have different amounts of curvature. Well, 262 00:13:55,040 --> 00:13:58,880 Speaker 1: I feel like you're now defining it relative to how 263 00:13:58,920 --> 00:14:02,920 Speaker 1: it's not curved. Right, You're saying it curves relative to 264 00:14:03,120 --> 00:14:05,079 Speaker 1: how it's not curve. But isn't that also used like 265 00:14:05,120 --> 00:14:08,760 Speaker 1: an external measure of it or an extremes definition of it. Well, 266 00:14:08,800 --> 00:14:11,400 Speaker 1: I think it's still relative, and you can use that definition, 267 00:14:11,520 --> 00:14:13,880 Speaker 1: as we will talk about in this episode, to construct 268 00:14:13,880 --> 00:14:17,319 Speaker 1: like ways to measure that curvature by, for example, passing 269 00:14:17,360 --> 00:14:19,800 Speaker 1: matter through it and seeing the influence on it. Right, 270 00:14:19,920 --> 00:14:23,000 Speaker 1: in curvespace, things that are not under acceleration don't appear 271 00:14:23,040 --> 00:14:26,480 Speaker 1: to move in straight lines, whereas in flat space they do. 272 00:14:26,720 --> 00:14:29,160 Speaker 1: So there's definitely a difference in the behavior of objects 273 00:14:29,440 --> 00:14:32,120 Speaker 1: in flat space and in curved space. And it all 274 00:14:32,120 --> 00:14:35,720 Speaker 1: comes down to this definition of relative distances. Right. This metric, 275 00:14:35,760 --> 00:14:38,640 Speaker 1: which is the solution to Einstein's equations, tells you the 276 00:14:38,680 --> 00:14:41,240 Speaker 1: amount of curvature at every point, and that tells you 277 00:14:41,280 --> 00:14:43,280 Speaker 1: how things move, and that basically tells you what the 278 00:14:43,320 --> 00:14:47,400 Speaker 1: shortest path is between two points in space. And so 279 00:14:47,440 --> 00:14:50,200 Speaker 1: it's all about the relative differences, not in reference to 280 00:14:50,240 --> 00:14:53,320 Speaker 1: any external ruler, but yeah, it is relative to some 281 00:14:53,440 --> 00:14:56,920 Speaker 1: internal ruler, right, which is flat space. That's true. Right, 282 00:14:56,960 --> 00:14:59,240 Speaker 1: So you sort of comparing it to like a universe 283 00:14:59,280 --> 00:15:02,760 Speaker 1: without any masses or anything energy in it basically, right, 284 00:15:02,800 --> 00:15:05,040 Speaker 1: which is yeah, which is kind of like thinking about 285 00:15:05,040 --> 00:15:08,000 Speaker 1: it as like the exterior measure of space, like relative 286 00:15:08,040 --> 00:15:10,280 Speaker 1: to not space, which is kind of like an outside 287 00:15:10,320 --> 00:15:12,680 Speaker 1: point of view. Right. Well, I think it's a nice 288 00:15:12,720 --> 00:15:15,360 Speaker 1: way to think about it as a benchmark, compare curved 289 00:15:15,400 --> 00:15:18,000 Speaker 1: space to flat space. That's definitely a nice way to 290 00:15:18,000 --> 00:15:19,760 Speaker 1: think about it. But that flat space doesn't have to 291 00:15:19,760 --> 00:15:22,880 Speaker 1: be an external metric. It's not like our curved space 292 00:15:22,960 --> 00:15:26,760 Speaker 1: is sitting inside some larger flat space that's being used 293 00:15:26,760 --> 00:15:30,040 Speaker 1: to measure it. We can measure the curvature internally without 294 00:15:30,120 --> 00:15:33,880 Speaker 1: referencing anything outside of our universe. Maybe that's what you 295 00:15:33,880 --> 00:15:36,760 Speaker 1: mean by intrinsing, is that you can measure this curvature 296 00:15:36,880 --> 00:15:39,120 Speaker 1: without knowing what it would be like without any masses 297 00:15:39,120 --> 00:15:41,680 Speaker 1: in it. Yeah, exactly. And it's amazing that we can 298 00:15:41,760 --> 00:15:44,160 Speaker 1: that we can detect this in our universe, And in 299 00:15:44,240 --> 00:15:46,520 Speaker 1: some sense it's kind of obvious to us, like we 300 00:15:46,920 --> 00:15:50,160 Speaker 1: notice the effect of curvature all the time because we 301 00:15:50,200 --> 00:15:53,480 Speaker 1: grew up experiencing it. Our experience of gravity turns out 302 00:15:53,600 --> 00:15:57,200 Speaker 1: not to be due to some mysterious force of gravity, 303 00:15:57,240 --> 00:16:01,240 Speaker 1: as Newton described, But it's the effect of curvature changing 304 00:16:01,280 --> 00:16:04,400 Speaker 1: how things move through space. So we experienced the curvature 305 00:16:04,480 --> 00:16:06,920 Speaker 1: space all the time. It's not subtle, all right. So 306 00:16:07,240 --> 00:16:10,880 Speaker 1: curvature is kind of a property of space itself. It's 307 00:16:10,920 --> 00:16:14,720 Speaker 1: not relative to some outside space that space sits in. 308 00:16:15,240 --> 00:16:17,960 Speaker 1: And so is it related to the force of gravity? Right, 309 00:16:18,000 --> 00:16:20,920 Speaker 1: And so Newton's idea was, look, gravity is a force. 310 00:16:21,000 --> 00:16:23,360 Speaker 1: I noticed the Earth pulls on things like this apple 311 00:16:23,680 --> 00:16:26,480 Speaker 1: or that bowling ball, or my bowl of yogurt or whatever. 312 00:16:26,680 --> 00:16:30,160 Speaker 1: And so Newton explained this thing. He observed that masses 313 00:16:30,160 --> 00:16:32,440 Speaker 1: tend to attract each other in terms of some force. 314 00:16:32,680 --> 00:16:35,160 Speaker 1: And he didn't understand like a mechanism of it. He 315 00:16:35,200 --> 00:16:38,000 Speaker 1: didn't understand deep down what's happening. He just described it 316 00:16:38,040 --> 00:16:41,440 Speaker 1: and said, here's a mathematical description for what's happening. I 317 00:16:41,480 --> 00:16:44,240 Speaker 1: have an equation that describes it. It all seems to work. 318 00:16:44,680 --> 00:16:47,000 Speaker 1: So that was Newton's description. But it turns out that 319 00:16:47,040 --> 00:16:49,840 Speaker 1: gravity is not actually a force in our universe the 320 00:16:49,880 --> 00:16:53,880 Speaker 1: way for example, electromagnetism is, or the strong force or 321 00:16:53,920 --> 00:16:57,080 Speaker 1: the weak force. It turns out it's an apparent force, 322 00:16:57,320 --> 00:16:59,320 Speaker 1: something that seems to be a force but is actually 323 00:16:59,320 --> 00:17:02,160 Speaker 1: caused by something else. But wait, I feel like you're saying, 324 00:17:02,160 --> 00:17:06,080 Speaker 1: maybe the electromagnetic force is apparent. It's not a real force. 325 00:17:06,200 --> 00:17:08,320 Speaker 1: It's an apparent force. Now I was saying the opposite, 326 00:17:08,359 --> 00:17:10,960 Speaker 1: that electromagnetism and the weak force and the strong force, 327 00:17:11,000 --> 00:17:14,080 Speaker 1: those are real forces. But that gravity is different. Gravity 328 00:17:14,119 --> 00:17:16,560 Speaker 1: is an apparent force. It's not actually a force in 329 00:17:16,600 --> 00:17:19,320 Speaker 1: the universe. It's just caused by a curvature. And because 330 00:17:19,320 --> 00:17:22,400 Speaker 1: we can't see the curvature, we need to invent this 331 00:17:22,560 --> 00:17:25,520 Speaker 1: force in order to explain what we are seeing. Oh right, 332 00:17:25,560 --> 00:17:28,560 Speaker 1: I got that backwards. So then the curvature space is 333 00:17:28,800 --> 00:17:32,440 Speaker 1: Gravity is your curvaturo space. It's not gravity. So gravity 334 00:17:32,520 --> 00:17:35,600 Speaker 1: is our way to explain the effect of the curvature 335 00:17:35,640 --> 00:17:38,760 Speaker 1: of space, because we didn't realize that space was curved. 336 00:17:38,760 --> 00:17:41,080 Speaker 1: We didn't understand it was happening. Let's take a simple 337 00:17:41,119 --> 00:17:44,920 Speaker 1: example of apparent forces we sort of invent to explain things. Say, 338 00:17:44,960 --> 00:17:47,199 Speaker 1: for example, you're driving a truck and you've got a 339 00:17:47,200 --> 00:17:49,200 Speaker 1: tennis ball in the back. Now, when your truck is 340 00:17:49,240 --> 00:17:51,800 Speaker 1: not going anywhere, or it's driving a constant speed. This 341 00:17:51,960 --> 00:17:53,600 Speaker 1: tennis ball in your truck is just going to sit 342 00:17:53,640 --> 00:17:56,040 Speaker 1: in the back. It's not going to roll forwards or backwards. 343 00:17:56,160 --> 00:17:58,560 Speaker 1: But now if you hit the gas and the truck accelerates, 344 00:17:58,720 --> 00:18:01,560 Speaker 1: then what happens to the tennis ball. It suddenly rolls 345 00:18:01,560 --> 00:18:04,040 Speaker 1: to the back of the truck. Right, But in your frame, 346 00:18:04,119 --> 00:18:06,720 Speaker 1: the frame of the truck, why is the tennis ball 347 00:18:06,800 --> 00:18:10,080 Speaker 1: rolling backwards? Right? There's no force on the tennis ball. 348 00:18:10,200 --> 00:18:12,000 Speaker 1: For somebody sitting in the back of the truck, they 349 00:18:12,080 --> 00:18:14,399 Speaker 1: just see it roll backwards. Well, I guess you know, 350 00:18:14,440 --> 00:18:16,040 Speaker 1: if you were there in the truck, you would also 351 00:18:16,080 --> 00:18:18,600 Speaker 1: feel that force. Right, Yes, you're back. You're saying, maybe, 352 00:18:18,640 --> 00:18:21,040 Speaker 1: like if you had a camera inside the truck filming 353 00:18:21,040 --> 00:18:23,600 Speaker 1: this ball, someone looking at the footage, would you see 354 00:18:23,600 --> 00:18:26,320 Speaker 1: the ball suddenly start to move? Yeah, exactly. Somebody would 355 00:18:26,359 --> 00:18:28,320 Speaker 1: see the ball suddenly start to move. And so they 356 00:18:28,320 --> 00:18:31,399 Speaker 1: would say, where's this force coming from? Right, there's nothing 357 00:18:31,480 --> 00:18:34,280 Speaker 1: touching it. What is pushing on the ball. And we 358 00:18:34,359 --> 00:18:36,639 Speaker 1: know the answer is that the truck is accelerating, right, 359 00:18:36,680 --> 00:18:39,680 Speaker 1: it's actually how you measure acceleration. But in the frame 360 00:18:39,720 --> 00:18:41,600 Speaker 1: of the truck, the camera that's sitting in the back 361 00:18:41,960 --> 00:18:45,240 Speaker 1: you can't explain it without adding some external force and saying, well, 362 00:18:45,359 --> 00:18:49,119 Speaker 1: there must be some external force on this ball, right, 363 00:18:49,160 --> 00:18:51,800 Speaker 1: So you add this apparent force in order to explain 364 00:18:51,840 --> 00:18:54,040 Speaker 1: the motion you see. It's the same thing is happening 365 00:18:54,119 --> 00:18:56,040 Speaker 1: like on a merry go round. If you're on a 366 00:18:56,080 --> 00:19:00,600 Speaker 1: merry go round that's spinning, you feel this apparent force outward. Right. 367 00:19:00,640 --> 00:19:03,560 Speaker 1: There's no real force pushing you off the merrygo round. 368 00:19:03,600 --> 00:19:06,000 Speaker 1: It's just the fact that it's spinning, which again is 369 00:19:06,040 --> 00:19:10,159 Speaker 1: a kind of acceleration, creates this apparent force. So if 370 00:19:10,160 --> 00:19:11,840 Speaker 1: you want to do like F equals M and you 371 00:19:11,920 --> 00:19:14,760 Speaker 1: want to explain all acceleration in terms of forces, you 372 00:19:14,840 --> 00:19:18,560 Speaker 1: have to create this apparent force to explain what's going 373 00:19:18,600 --> 00:19:20,720 Speaker 1: on when you accelerate the truck or when you're on 374 00:19:20,760 --> 00:19:24,360 Speaker 1: the merrygoround. Those are just examples of other places where 375 00:19:24,400 --> 00:19:27,600 Speaker 1: we've had to add apparent forces in order to explain 376 00:19:27,640 --> 00:19:29,880 Speaker 1: the dynamics that we're seeing. Well, I guess maybe I'm 377 00:19:29,880 --> 00:19:32,480 Speaker 1: a little confused on this subtle point because you know, 378 00:19:32,520 --> 00:19:34,120 Speaker 1: in the case of the ball on the truck, there 379 00:19:34,200 --> 00:19:36,600 Speaker 1: is a force going on, right, like something is pushing 380 00:19:36,600 --> 00:19:38,679 Speaker 1: the truck, but nothing is pushing the ball. Yeah, but 381 00:19:38,720 --> 00:19:41,600 Speaker 1: the truck is being pushed by the engine and the 382 00:19:41,640 --> 00:19:44,800 Speaker 1: wheels and the friction with the road. What I'm seeing 383 00:19:45,119 --> 00:19:48,840 Speaker 1: is the ball not being accelerated with the truck. But 384 00:19:48,880 --> 00:19:50,760 Speaker 1: there is a force going on, right there is there 385 00:19:50,800 --> 00:19:54,160 Speaker 1: is a regular electromagnetic force pushing the truck. You're exactly right, 386 00:19:54,240 --> 00:19:56,280 Speaker 1: and that's the key insight. There is a force on 387 00:19:56,320 --> 00:19:59,320 Speaker 1: the truck and therefore the full frame of reference there 388 00:19:59,440 --> 00:20:02,199 Speaker 1: is excel rating. So what you're really seeing there is 389 00:20:02,200 --> 00:20:04,520 Speaker 1: that the frame of reference is accelerating, which makes it 390 00:20:04,560 --> 00:20:08,560 Speaker 1: a non inertial frame, and the equation F equals MA 391 00:20:08,640 --> 00:20:12,119 Speaker 1: only works in inertial frames because there's no force on 392 00:20:12,160 --> 00:20:14,119 Speaker 1: the ball. If you want to understand the acceleration of 393 00:20:14,119 --> 00:20:16,000 Speaker 1: the ball from the point of view of your camera 394 00:20:16,040 --> 00:20:17,960 Speaker 1: in the back of the truck, there is no force 395 00:20:17,960 --> 00:20:19,920 Speaker 1: in the ball, like nothing is touching it, nothing is 396 00:20:19,960 --> 00:20:23,080 Speaker 1: pushing on it. Something instead is pushing the camera, which 397 00:20:23,119 --> 00:20:26,120 Speaker 1: is attached to the truck. It's changing the frame of reference. 398 00:20:26,280 --> 00:20:27,920 Speaker 1: So if you want to use F equals m A, 399 00:20:28,320 --> 00:20:30,600 Speaker 1: you have to add in a force to compensate for that. 400 00:20:30,960 --> 00:20:34,200 Speaker 1: So the acceleration creates this apparent force on the ball, 401 00:20:34,280 --> 00:20:36,960 Speaker 1: even though again nothing is actually pushing the ball, but 402 00:20:37,000 --> 00:20:39,119 Speaker 1: you see it moving as if there was a force 403 00:20:39,200 --> 00:20:41,000 Speaker 1: on it. Right. But I guess that's only because you 404 00:20:41,000 --> 00:20:43,520 Speaker 1: don't know that the truck is accelerating. But if you did, 405 00:20:43,640 --> 00:20:46,439 Speaker 1: you could figure it out, you could account for it 406 00:20:46,520 --> 00:20:49,200 Speaker 1: because of the forces that are there. No exactly right, 407 00:20:49,359 --> 00:20:51,400 Speaker 1: there's two different ways to think about this. To think, oh, 408 00:20:51,560 --> 00:20:53,840 Speaker 1: I'm in an accelerating frame, so I shouldn't be using 409 00:20:53,880 --> 00:20:55,680 Speaker 1: F equals MA, I have to account for the fact 410 00:20:55,720 --> 00:20:57,679 Speaker 1: that I'm accelerating. But if you didn't know that the 411 00:20:57,680 --> 00:20:59,800 Speaker 1: camera was accelerating, then you have to add a force 412 00:21:00,000 --> 00:21:01,840 Speaker 1: to account for it. That would be an apparent force. 413 00:21:02,200 --> 00:21:05,399 Speaker 1: And that's exactly our situation when it comes to curvature 414 00:21:05,520 --> 00:21:08,760 Speaker 1: and gravity. We can't see curvature. We don't know that 415 00:21:08,800 --> 00:21:11,120 Speaker 1: it's out there. In fact, for hundreds or thousands of years, 416 00:21:11,160 --> 00:21:13,119 Speaker 1: we didn't even know it was happening, and so in 417 00:21:13,240 --> 00:21:16,400 Speaker 1: order to explain the motion of objects as we saw them, 418 00:21:16,400 --> 00:21:19,840 Speaker 1: we had to create this apparent force we call gravity 419 00:21:20,040 --> 00:21:22,879 Speaker 1: to explain the things that otherwise didn't have an explanation. 420 00:21:23,200 --> 00:21:25,800 Speaker 1: Now we know that space is curved, like knowing that 421 00:21:25,840 --> 00:21:28,960 Speaker 1: the truck was accelerating, and that can be our explanation 422 00:21:29,040 --> 00:21:33,520 Speaker 1: instead of creating this apparent fictitious force. Let's dig it 423 00:21:33,640 --> 00:21:36,960 Speaker 1: more into this idea of curvature. And then finally, how 424 00:21:36,960 --> 00:21:40,080 Speaker 1: do you even measure something that's invisible out there in space? 425 00:21:40,560 --> 00:21:54,399 Speaker 1: But first, let's take a quick break. All right, we 426 00:21:54,440 --> 00:21:56,879 Speaker 1: are bending our minds here with the curvature of space, 427 00:21:57,440 --> 00:21:59,800 Speaker 1: and I have to say I got a little bit confused. 428 00:22:00,880 --> 00:22:02,960 Speaker 1: I feel like we've talked about this for hours and 429 00:22:03,000 --> 00:22:06,120 Speaker 1: hours on this podcast, but it's still kind of hard 430 00:22:06,160 --> 00:22:09,320 Speaker 1: to process. It's kind of hard to tell the difference 431 00:22:09,400 --> 00:22:14,200 Speaker 1: between like, hey, gravity is not really a force, it's 432 00:22:14,000 --> 00:22:17,920 Speaker 1: a space that bends, and the alternative point of view, 433 00:22:17,920 --> 00:22:19,879 Speaker 1: which is like, hey, maybe if you just look at 434 00:22:19,920 --> 00:22:22,800 Speaker 1: it differently, you can see it this way, that it's 435 00:22:22,800 --> 00:22:24,720 Speaker 1: a bending us space, you know what I mean? Like, 436 00:22:24,880 --> 00:22:26,840 Speaker 1: is it that we can see it as a bending 437 00:22:26,840 --> 00:22:29,360 Speaker 1: of space or that it is a bending of space? Oh? Yeah, 438 00:22:29,520 --> 00:22:33,159 Speaker 1: great question? Right? Is it just a philosophical distinction or 439 00:22:33,240 --> 00:22:36,200 Speaker 1: is it actually a physical distinction? Doesn't matter? The answer 440 00:22:36,240 --> 00:22:37,760 Speaker 1: is that it does matter. We know that it is 441 00:22:37,760 --> 00:22:40,119 Speaker 1: the bending of space, but most of the time it 442 00:22:40,160 --> 00:22:43,000 Speaker 1: doesn't matter. Most of the time, treating it like a 443 00:22:43,080 --> 00:22:45,600 Speaker 1: force and treating it like the bending of space give 444 00:22:45,640 --> 00:22:48,680 Speaker 1: exactly the same prediction for the Earth going around the Sun, 445 00:22:48,720 --> 00:22:51,080 Speaker 1: and for all sorts of things. In some edge cases, 446 00:22:51,160 --> 00:22:55,240 Speaker 1: some corner cases, some extreme circumstances, they do give slightly 447 00:22:55,240 --> 00:22:58,320 Speaker 1: different predictions. And that's how we know that Einstein's theory 448 00:22:58,400 --> 00:23:01,000 Speaker 1: was right and then Newton's is wrong. What are some 449 00:23:01,040 --> 00:23:03,320 Speaker 1: of these extreme cases. So there are a couple of examples. 450 00:23:03,440 --> 00:23:07,120 Speaker 1: One is like spinning masses. Another is the effect on light. 451 00:23:07,400 --> 00:23:11,000 Speaker 1: So Newton's theory predicts that spinning masses have exactly the 452 00:23:11,040 --> 00:23:13,760 Speaker 1: same gravity as masses that don't spin. Like if you're 453 00:23:13,800 --> 00:23:16,080 Speaker 1: an outer space and the Earth is spinning under you, 454 00:23:16,440 --> 00:23:19,080 Speaker 1: the gravity from the Earth is not affected by the 455 00:23:19,080 --> 00:23:21,000 Speaker 1: fact that the Earth is spinning. It just depends on 456 00:23:21,040 --> 00:23:23,920 Speaker 1: the amount of mass. But Einstein says, actually there's a 457 00:23:23,920 --> 00:23:26,719 Speaker 1: little effect there. If the Earth is spinning, it's like 458 00:23:26,960 --> 00:23:30,600 Speaker 1: dragging the curvature of space with it, and this causes 459 00:23:30,640 --> 00:23:33,840 Speaker 1: a weird, little twisting effect on things out in space. 460 00:23:33,960 --> 00:23:37,040 Speaker 1: We talked about a really precise experiment called gravity Probe B, 461 00:23:37,720 --> 00:23:41,080 Speaker 1: which actually measured this and confirmed that this is happening. 462 00:23:41,280 --> 00:23:44,359 Speaker 1: The other examples the bending of light. Newton's theory says 463 00:23:44,400 --> 00:23:47,800 Speaker 1: that masses attract there's a force of gravity between objects 464 00:23:47,800 --> 00:23:51,159 Speaker 1: with mass, but photons have no mass, and yet we 465 00:23:51,200 --> 00:23:55,000 Speaker 1: see they are bent by massive objects. Einstine's theory was 466 00:23:55,040 --> 00:23:58,480 Speaker 1: famously proven when we saw light being bent around the Sun. 467 00:23:58,760 --> 00:24:03,439 Speaker 1: This is gravitational insane, and that's light moving through curved space. 468 00:24:03,600 --> 00:24:06,240 Speaker 1: So there are some differences between the view that gravity 469 00:24:06,320 --> 00:24:08,680 Speaker 1: is just a force like Newton said, and that gravity 470 00:24:08,760 --> 00:24:12,440 Speaker 1: is just an apparent force due to the bending of space, right. 471 00:24:12,600 --> 00:24:14,840 Speaker 1: I know we talked about both of those cases before 472 00:24:14,840 --> 00:24:17,080 Speaker 1: in the podcast, but I guess maybe a question I 473 00:24:17,119 --> 00:24:20,200 Speaker 1: wonder if other people have, is that whether we're used 474 00:24:20,200 --> 00:24:23,720 Speaker 1: to defining gravity only as like what happens between things 475 00:24:23,720 --> 00:24:26,919 Speaker 1: with masses, But if you know mass is also energy, 476 00:24:27,080 --> 00:24:29,800 Speaker 1: what if you just expand that definition of gravity to 477 00:24:30,119 --> 00:24:32,320 Speaker 1: be what happens between things with energy and so then 478 00:24:32,359 --> 00:24:35,560 Speaker 1: you can include things like light and that would also 479 00:24:35,600 --> 00:24:39,119 Speaker 1: explain the bending of light, wouldn't it. Or like I 480 00:24:39,160 --> 00:24:42,680 Speaker 1: wonder if if you also include energy, then the spinning 481 00:24:42,880 --> 00:24:45,520 Speaker 1: of the Earth that could also be sort of like 482 00:24:45,600 --> 00:24:48,159 Speaker 1: extra rotational energy. I don't know, I'm just making things up. 483 00:24:48,200 --> 00:24:50,920 Speaker 1: I'm just wondering. No, it's really cool. Could you add 484 00:24:50,960 --> 00:24:54,040 Speaker 1: these features, these bells and whistles to Newton's theory to 485 00:24:54,119 --> 00:24:56,040 Speaker 1: make them work? And there's a whole bunch of people 486 00:24:56,040 --> 00:24:58,560 Speaker 1: trying to think about exactly how to abridge Einstein and 487 00:24:58,600 --> 00:25:01,119 Speaker 1: Newton's theory to make Newton's theory like a special case 488 00:25:01,240 --> 00:25:03,639 Speaker 1: of Einstein's theory, to sort of like put it as 489 00:25:03,640 --> 00:25:06,359 Speaker 1: a point in a larger sort of Einsteinian space. And 490 00:25:06,480 --> 00:25:08,359 Speaker 1: you can try to do that, but it doesn't quite work. 491 00:25:08,720 --> 00:25:11,240 Speaker 1: You know, for example, treating photons as if they have 492 00:25:11,480 --> 00:25:14,919 Speaker 1: energy and therefore gravity doesn't work because photons are the 493 00:25:14,960 --> 00:25:17,800 Speaker 1: same frequency, bend through space the same amount. It depends 494 00:25:17,800 --> 00:25:20,359 Speaker 1: on the curvature of space, not on the energy of 495 00:25:20,400 --> 00:25:23,040 Speaker 1: the photon. So it doesn't quite work because I guess 496 00:25:23,080 --> 00:25:26,119 Speaker 1: photons can have different energies to them. Yeah, photons of 497 00:25:26,119 --> 00:25:29,160 Speaker 1: different frequencies have different energies, but how much they bend 498 00:25:29,200 --> 00:25:31,919 Speaker 1: depends on the curvature of space, not on the energy 499 00:25:31,920 --> 00:25:36,240 Speaker 1: of the photon. Unless there maybe there's something you're missing. Yeah, 500 00:25:36,280 --> 00:25:38,159 Speaker 1: you can add more bells and whistles if you like. 501 00:25:38,359 --> 00:25:40,320 Speaker 1: Potentially it's possible for somebody to come up with a 502 00:25:40,359 --> 00:25:44,000 Speaker 1: modification to Newtonian gravity to make it work like Einsteins 503 00:25:44,000 --> 00:25:46,600 Speaker 1: and think about it as a force. You know, in 504 00:25:46,640 --> 00:25:49,640 Speaker 1: some sense, we can never really know what's real out 505 00:25:49,640 --> 00:25:52,560 Speaker 1: there and what is just our description of the universe. 506 00:25:52,680 --> 00:25:55,439 Speaker 1: But we have a very compelling description of all of 507 00:25:55,440 --> 00:25:59,480 Speaker 1: these effects using the concept of space being curved. It's 508 00:25:59,560 --> 00:26:01,960 Speaker 1: very susful and so we'd like to think that it's real, 509 00:26:02,000 --> 00:26:05,640 Speaker 1: but ultimately we never can know what's actually happening out 510 00:26:05,640 --> 00:26:09,160 Speaker 1: there as compared to our mental image of the universe. 511 00:26:09,760 --> 00:26:12,240 Speaker 1: All right, well, then we have to sort of accept 512 00:26:12,280 --> 00:26:14,800 Speaker 1: them that it's a real curvature, like it really does 513 00:26:14,840 --> 00:26:17,199 Speaker 1: ben It seems to be a very accurate description of 514 00:26:17,200 --> 00:26:19,159 Speaker 1: what's happening in the universe. So it's very tempting to 515 00:26:19,200 --> 00:26:22,320 Speaker 1: say it's real. You know, philosophically, what does it mean 516 00:26:22,359 --> 00:26:24,160 Speaker 1: for it to be real? It means that, like it's 517 00:26:24,200 --> 00:26:27,000 Speaker 1: happening even if we don't look like, if humans weren't 518 00:26:27,080 --> 00:26:29,600 Speaker 1: here to measure the curvature, space would still be bent. 519 00:26:29,760 --> 00:26:32,520 Speaker 1: So that's a philosophical claim, not a scientific one. That's 520 00:26:32,520 --> 00:26:35,240 Speaker 1: not something we could ever actually test. But yeah, it's 521 00:26:35,280 --> 00:26:38,040 Speaker 1: pretty convincing when you have a theory that works this well, 522 00:26:38,119 --> 00:26:41,480 Speaker 1: it feels like you've discovered how the universe is working 523 00:26:41,800 --> 00:26:44,400 Speaker 1: rather than just describing it. Well. Maybe one thing that's 524 00:26:44,520 --> 00:26:46,760 Speaker 1: a little bit also confusing is that in terms of 525 00:26:46,800 --> 00:26:48,960 Speaker 1: the curvature space, it's sort of like you can't tell 526 00:26:48,960 --> 00:26:51,680 Speaker 1: it's curved if you're in it. You can only tell 527 00:26:51,800 --> 00:26:55,159 Speaker 1: its curved if somebody's watching you from the outside. Right, Like, 528 00:26:55,240 --> 00:26:58,480 Speaker 1: if you were writing that light being being bent through space, 529 00:26:58,800 --> 00:27:01,040 Speaker 1: you wouldn't feel any force, is right, you would think 530 00:27:01,080 --> 00:27:03,240 Speaker 1: you were going in a straight line. But to someone 531 00:27:03,320 --> 00:27:06,120 Speaker 1: outside of you, you'd be like, oh, that like rain bent. Yeah, 532 00:27:06,160 --> 00:27:11,440 Speaker 1: you're exactly right. Anything moving according to curvature feels no acceleration. 533 00:27:11,840 --> 00:27:14,320 Speaker 1: Like if you built an accelerometer, and basically that tennis 534 00:27:14,320 --> 00:27:16,280 Speaker 1: ball in the back of a truck is an accelerometer, 535 00:27:16,400 --> 00:27:19,480 Speaker 1: something that measures whether there is acceleration. Say you have 536 00:27:19,520 --> 00:27:21,720 Speaker 1: an accelerometer with you and you're in a spaceship and 537 00:27:21,760 --> 00:27:25,360 Speaker 1: you're flying through totally flat space at constant velocity. You're 538 00:27:25,359 --> 00:27:29,640 Speaker 1: watching the accelerometer, nothing happens, no surprise there. Now, say 539 00:27:29,680 --> 00:27:32,679 Speaker 1: you're flying through curved space. As you say, could you 540 00:27:32,760 --> 00:27:35,480 Speaker 1: tell that you were flying through curved space? Just by 541 00:27:35,520 --> 00:27:39,040 Speaker 1: looking at stuff inside your ship and your accelerometer. After all, 542 00:27:39,160 --> 00:27:42,520 Speaker 1: you are bending, right, you're changing your direction. Well, the 543 00:27:42,520 --> 00:27:45,719 Speaker 1: answer is no, you do not feel any acceleration inside 544 00:27:45,760 --> 00:27:49,960 Speaker 1: your spaceship. Your accelerometer does not register any acceleration because 545 00:27:49,960 --> 00:27:54,080 Speaker 1: you're moving along the curvature of space. The accelerometer only 546 00:27:54,119 --> 00:27:57,359 Speaker 1: measures sort of like forces against the curvature of space, 547 00:27:57,520 --> 00:28:00,639 Speaker 1: like resisting gravities flow. So, yeah, the only way you 548 00:28:00,640 --> 00:28:03,040 Speaker 1: can measure that curvature is by fight, for example, comparing 549 00:28:03,040 --> 00:28:07,680 Speaker 1: your position to other objects out there. You mean, as 550 00:28:07,680 --> 00:28:10,520 Speaker 1: you were writing the lightbeam. Yeah, as you're writing the lightbeam. 551 00:28:10,600 --> 00:28:13,880 Speaker 1: This concept of freefall, of moving through space without any 552 00:28:13,880 --> 00:28:16,919 Speaker 1: other forces, just letting gravity control you, it is really 553 00:28:16,960 --> 00:28:20,200 Speaker 1: important in general relativity. That's the sort of like concept 554 00:28:20,240 --> 00:28:23,440 Speaker 1: of an inertial observer. Somebody who's like skydiving, they jump 555 00:28:23,440 --> 00:28:26,960 Speaker 1: out of an airplane. They're in freefall, ignore air resistance. 556 00:28:27,119 --> 00:28:29,359 Speaker 1: Newton would say, oh, they're being pulled down by the 557 00:28:29,400 --> 00:28:32,440 Speaker 1: force of gravity, right, and Einstein would say, no, they're 558 00:28:32,480 --> 00:28:35,720 Speaker 1: just moving along the curvature of space. There's no force 559 00:28:35,840 --> 00:28:39,320 Speaker 1: on them. They're just in freefall. And Einstein's right that 560 00:28:39,360 --> 00:28:41,440 Speaker 1: if you had like an accelerometer with you after you 561 00:28:41,520 --> 00:28:45,080 Speaker 1: jumped out of the plane, it would not measure any acceleration. Right. 562 00:28:45,120 --> 00:28:46,760 Speaker 1: Maybe this is where it gets sort of tricky and 563 00:28:46,800 --> 00:28:49,320 Speaker 1: you kind of have to include the definition of time 564 00:28:49,360 --> 00:28:51,320 Speaker 1: into it, right, because I guess if you jumped out 565 00:28:51,320 --> 00:28:53,560 Speaker 1: of an airplane, you would get moved, right, like your 566 00:28:53,600 --> 00:28:57,640 Speaker 1: pocisition in space would change. Yeah, exactly, even though there's 567 00:28:57,640 --> 00:29:01,000 Speaker 1: no acceleration, right. That's because in curved space time you 568 00:29:01,080 --> 00:29:04,600 Speaker 1: have to accelerate just to remain stationary. Right. The airplane, 569 00:29:04,640 --> 00:29:07,840 Speaker 1: for example, it's applying a force to stay up. It's 570 00:29:07,880 --> 00:29:11,200 Speaker 1: actually accelerating upwards. When you jump out of the airplane, 571 00:29:11,240 --> 00:29:14,239 Speaker 1: you are no longer accelerating. You're now in freefall, so 572 00:29:14,280 --> 00:29:17,040 Speaker 1: there are no forces on you because remember gravity not 573 00:29:17,200 --> 00:29:19,920 Speaker 1: a force. You're just moving according to the curvature space, 574 00:29:20,360 --> 00:29:23,200 Speaker 1: just like that space ship out in space moving through 575 00:29:23,240 --> 00:29:25,800 Speaker 1: bent space. It's not going to notice anything. You jump 576 00:29:25,840 --> 00:29:28,160 Speaker 1: out of the airplane. You're not going to measure any acceleration. 577 00:29:28,440 --> 00:29:32,480 Speaker 1: The airplane is staying up hopefully, and so it's accelerating 578 00:29:32,560 --> 00:29:35,280 Speaker 1: up right, just like somebody who's standing on the surface 579 00:29:35,320 --> 00:29:38,840 Speaker 1: of the Earth in order to avoid moving down towards 580 00:29:38,840 --> 00:29:41,760 Speaker 1: the center of the Earth. The Earth is accelerating them upwards. 581 00:29:41,800 --> 00:29:44,320 Speaker 1: It's providing a force, the ground is pushing them up. 582 00:29:44,440 --> 00:29:48,000 Speaker 1: It's actually accelerating them all upwards. You are in freefall. 583 00:29:48,080 --> 00:29:52,680 Speaker 1: You don't measure any acceleration. You measure everybody else accelerating upwards. Right. 584 00:29:52,720 --> 00:29:54,560 Speaker 1: I think maybe this is where you kind of have 585 00:29:54,640 --> 00:29:57,280 Speaker 1: to say the word space time, right, because I mean 586 00:29:57,320 --> 00:29:59,440 Speaker 1: you can't just say like, you're following the curvature of 587 00:29:59,440 --> 00:30:03,760 Speaker 1: space because that only works if you also include time 588 00:30:03,800 --> 00:30:06,600 Speaker 1: into the word. Absolutely. Yes, time is crucial here because 589 00:30:06,600 --> 00:30:08,440 Speaker 1: we're talking about motion. So you do kind of have 590 00:30:08,480 --> 00:30:10,760 Speaker 1: to say the word really that it's a curvature of 591 00:30:10,800 --> 00:30:14,560 Speaker 1: space time, right, Yeah, curvature of space time absolutely all right. 592 00:30:14,600 --> 00:30:16,320 Speaker 1: Like you said, it's kind of hard to know that 593 00:30:16,360 --> 00:30:19,280 Speaker 1: spacetime is bending around you if you're in it, if 594 00:30:19,280 --> 00:30:23,720 Speaker 1: you're being moved along its curvature, And so I guess 595 00:30:23,720 --> 00:30:26,200 Speaker 1: the question is how do you measure then that space 596 00:30:26,320 --> 00:30:27,960 Speaker 1: is being bent? What are some of the ways that 597 00:30:28,000 --> 00:30:31,000 Speaker 1: people do that? So the most straightforward way to test 598 00:30:31,080 --> 00:30:34,880 Speaker 1: weather space is bend is to see its effect on stuff, right, 599 00:30:34,960 --> 00:30:38,200 Speaker 1: This famous description of general relativity is that matter tells 600 00:30:38,360 --> 00:30:42,080 Speaker 1: space time. How to bend. Spacetime tells matter how to move. 601 00:30:42,360 --> 00:30:44,720 Speaker 1: So if you see stuff moving in straight lines, that 602 00:30:44,760 --> 00:30:47,880 Speaker 1: tells you that spacetime in front of you is flat. 603 00:30:48,000 --> 00:30:50,720 Speaker 1: If you're an inertial observer and you see things moving 604 00:30:50,800 --> 00:30:53,600 Speaker 1: in curves, that tells you that space time in front 605 00:30:53,600 --> 00:30:56,800 Speaker 1: of you is curved somehow. So you can just watch 606 00:30:56,840 --> 00:30:59,800 Speaker 1: the motion of objects, just like looking at those cars 607 00:30:59,800 --> 00:31:02,880 Speaker 1: to sending down the mountain at night, look at their headlights, 608 00:31:02,920 --> 00:31:05,280 Speaker 1: you can tell if the road they're on is bent. 609 00:31:05,480 --> 00:31:07,960 Speaker 1: So basically you have to be kind of like an 610 00:31:08,000 --> 00:31:11,920 Speaker 1: outside or observer, or I guess you have to sit 611 00:31:11,960 --> 00:31:15,240 Speaker 1: at a distance imagine what that space in front of 612 00:31:15,280 --> 00:31:18,840 Speaker 1: you would be like if there wasn't any bending, and 613 00:31:19,000 --> 00:31:22,280 Speaker 1: if something moves through there differently than it would through 614 00:31:22,320 --> 00:31:25,360 Speaker 1: empty space, then you know it's being a bent. Yeah. Say, 615 00:31:25,400 --> 00:31:27,160 Speaker 1: for example, you didn't know the Sun was there, and 616 00:31:27,200 --> 00:31:29,640 Speaker 1: you threw a planet into the Solar System and it 617 00:31:29,720 --> 00:31:32,320 Speaker 1: didn't fly right through. Instead, it bent and it ended 618 00:31:32,400 --> 00:31:35,440 Speaker 1: up in an orbit. Right, that's definitely not the motion 619 00:31:35,480 --> 00:31:38,120 Speaker 1: you expect through flat space, And so you can tell 620 00:31:38,200 --> 00:31:41,480 Speaker 1: that space is curved because the object is not moving 621 00:31:41,480 --> 00:31:44,680 Speaker 1: in a straight line. It's following the curvature of space. 622 00:31:44,800 --> 00:31:46,880 Speaker 1: It's on what we call a geodesic them hath a 623 00:31:46,960 --> 00:31:50,800 Speaker 1: particle follows if there's no acceleration on it, And so 624 00:31:50,880 --> 00:31:53,080 Speaker 1: you can tell, for example, that the space in our 625 00:31:53,080 --> 00:31:55,960 Speaker 1: Solar System is curved because the Earth is not moving 626 00:31:56,080 --> 00:31:58,280 Speaker 1: in a straight line. Right. I think we've talked about 627 00:31:58,400 --> 00:32:00,440 Speaker 1: this before. Like if you took out the Sun and 628 00:32:00,480 --> 00:32:02,920 Speaker 1: you replace it with a black hole with the same 629 00:32:02,960 --> 00:32:05,880 Speaker 1: mass as the Sun, it would be super tiny, right, 630 00:32:06,080 --> 00:32:08,760 Speaker 1: I think maybe around the size of a bowling ball 631 00:32:08,840 --> 00:32:10,800 Speaker 1: or something like that, which you would never see from 632 00:32:10,800 --> 00:32:14,000 Speaker 1: this distance, right, because it's millions of miles away, But 633 00:32:14,400 --> 00:32:16,600 Speaker 1: the planets would still keep orbiting the same way as 634 00:32:16,600 --> 00:32:18,480 Speaker 1: they would before. Yeah, I think the Sun would actually 635 00:32:18,480 --> 00:32:21,320 Speaker 1: compressed about three kilometers. But you're absolutely right on the 636 00:32:21,360 --> 00:32:24,000 Speaker 1: point of gravity. Our Earth would move around it in 637 00:32:24,040 --> 00:32:26,960 Speaker 1: the same way, and so space was like invisibly bent 638 00:32:27,080 --> 00:32:29,480 Speaker 1: by a black hole. Then you could tell, and that's 639 00:32:29,520 --> 00:32:32,280 Speaker 1: exactly what we do. At the heart of our galaxy. 640 00:32:32,680 --> 00:32:34,760 Speaker 1: We can tell that there's a black hole there even 641 00:32:34,760 --> 00:32:38,560 Speaker 1: though it's largely invisible by the motion of the stars nearby. 642 00:32:38,760 --> 00:32:41,840 Speaker 1: They whizz around as if there was some very massive 643 00:32:41,880 --> 00:32:45,000 Speaker 1: object there. Curving space. That's what I mean. Like a 644 00:32:45,080 --> 00:32:49,520 Speaker 1: three kilometer wide bowling ball, I want to see the pins. 645 00:32:50,400 --> 00:32:53,719 Speaker 1: Yeah yeah, but you still wouldn't see that from here, probably, right, 646 00:32:53,760 --> 00:32:56,480 Speaker 1: something an object three kilometers, why you probably wouldn't see 647 00:32:56,480 --> 00:32:58,440 Speaker 1: that from here, would you, Especially if it's a black 648 00:32:58,560 --> 00:33:00,560 Speaker 1: in a black backdrop. I don't know, if it's one 649 00:33:00,600 --> 00:33:03,360 Speaker 1: of those like swirly galaxy. Bowling balls might also bend 650 00:33:03,400 --> 00:33:05,360 Speaker 1: into the background. Oh yeah, they do have those glow 651 00:33:05,400 --> 00:33:09,680 Speaker 1: in the dark bowling balls. Yeah, exactly. And so that 652 00:33:09,720 --> 00:33:12,080 Speaker 1: seems sort of obvious, and maybe that sounds like a cheat, 653 00:33:12,120 --> 00:33:15,520 Speaker 1: like we're just saying, oh, gravity is actually the curvature space. 654 00:33:15,520 --> 00:33:17,840 Speaker 1: So anywhere you see the effective gravity, you're seeing the 655 00:33:17,840 --> 00:33:20,640 Speaker 1: curvature space, and therefore you know that space is curved. 656 00:33:20,800 --> 00:33:23,320 Speaker 1: But remember we also talked about the curvature space doing 657 00:33:23,400 --> 00:33:27,000 Speaker 1: things that just Newton's gravity can't do, like bending light 658 00:33:27,080 --> 00:33:30,280 Speaker 1: around the Sun. And this was Einstein's famous test of 659 00:33:30,360 --> 00:33:33,440 Speaker 1: general relativity. He predicted that in an eclipse, we would 660 00:33:33,480 --> 00:33:37,000 Speaker 1: be able to see the bending of light from distant 661 00:33:37,040 --> 00:33:40,240 Speaker 1: stars as it goes around the Sun. Right Like, during 662 00:33:40,280 --> 00:33:42,560 Speaker 1: an eclipse, right actisse, you can see the light race 663 00:33:42,640 --> 00:33:46,600 Speaker 1: kind of bend around the eclipsing moon. Actually, the bending 664 00:33:46,760 --> 00:33:50,360 Speaker 1: is of distant stars well behind the Sun, around the Sun, 665 00:33:50,720 --> 00:33:52,400 Speaker 1: and the reason we use the eclipse it is not 666 00:33:52,400 --> 00:33:54,840 Speaker 1: because we're looking for the light being bent around the Moon, 667 00:33:55,120 --> 00:33:57,440 Speaker 1: but just that then the Moon mostly blocks out the 668 00:33:57,480 --> 00:34:00,160 Speaker 1: Sun's light, so it's easier to see these stars are 669 00:34:00,240 --> 00:34:02,880 Speaker 1: very close to the Sun. In principle, you could see 670 00:34:02,880 --> 00:34:05,320 Speaker 1: this at any time. Stars that are sort of just 671 00:34:05,440 --> 00:34:07,960 Speaker 1: behind the Sun are having their light bent by it, 672 00:34:08,160 --> 00:34:10,160 Speaker 1: but it's pretty hard to do when the Sun is on. 673 00:34:10,640 --> 00:34:12,600 Speaker 1: So basically we use the eclipse to turn the Sun 674 00:34:12,680 --> 00:34:14,640 Speaker 1: off to block it, and then we can see the 675 00:34:14,680 --> 00:34:18,600 Speaker 1: stuff around it more easily. But also technically the sun 676 00:34:18,680 --> 00:34:21,480 Speaker 1: rays are probably being bent by the Moon in front 677 00:34:21,480 --> 00:34:24,440 Speaker 1: of the Sun. Oh right. Also, yeah, absolutely a little bit, 678 00:34:24,480 --> 00:34:26,880 Speaker 1: but maybe not noticeably. Yeah, for sure, you put your 679 00:34:26,920 --> 00:34:29,640 Speaker 1: hand up to block the Sun, and your hand is 680 00:34:29,719 --> 00:34:32,200 Speaker 1: bending the light rays of the Sun, right because your 681 00:34:32,520 --> 00:34:36,160 Speaker 1: hand curves space. Everything with mass curve space. It's a 682 00:34:36,160 --> 00:34:39,160 Speaker 1: pretty subtle effect. I mean, even the Sun bends this 683 00:34:39,280 --> 00:34:42,279 Speaker 1: light by like a thousands of a degree, so it's 684 00:34:42,320 --> 00:34:44,920 Speaker 1: pretty hard to see. All right. Well, I think what 685 00:34:44,960 --> 00:34:48,040 Speaker 1: you're saying is that one way to know if curvature 686 00:34:48,080 --> 00:34:51,640 Speaker 1: of space caused by a mass is to see if 687 00:34:52,200 --> 00:34:56,200 Speaker 1: things that fly near it, including light bed Yeah, exactly. 688 00:34:56,360 --> 00:34:58,080 Speaker 1: And one time on the podcast you made a really 689 00:34:58,080 --> 00:35:00,480 Speaker 1: cool point that the best way to see this actually 690 00:35:00,560 --> 00:35:02,680 Speaker 1: use like two beams of light, you like, shine a 691 00:35:02,760 --> 00:35:06,040 Speaker 1: laser through space and see if they stay parallel, right, 692 00:35:06,080 --> 00:35:09,240 Speaker 1: because if space is flat, then they will stay parallel forever. 693 00:35:09,280 --> 00:35:11,359 Speaker 1: But if space is curved, then they will bend and 694 00:35:11,600 --> 00:35:14,400 Speaker 1: they may even cross. Okay, So that's one way to 695 00:35:14,480 --> 00:35:18,200 Speaker 1: measure how a space can bend is if you see things, 696 00:35:18,239 --> 00:35:21,640 Speaker 1: the trajectory of things, even including light bending around something. 697 00:35:21,680 --> 00:35:24,520 Speaker 1: You know that bending relative to what you're looking at. 698 00:35:24,560 --> 00:35:26,920 Speaker 1: That means that there's something there and it's bending space. 699 00:35:27,560 --> 00:35:29,160 Speaker 1: What are some other ways that we can measure the 700 00:35:29,200 --> 00:35:33,040 Speaker 1: bending of space? Time? Yes, space time exactly, very good point. 701 00:35:33,440 --> 00:35:36,399 Speaker 1: And time is also bent with space, as you've said, right, 702 00:35:36,440 --> 00:35:38,640 Speaker 1: and so the curvature is not just in space but 703 00:35:38,760 --> 00:35:41,800 Speaker 1: in space time, which means that time is also curved. 704 00:35:41,960 --> 00:35:45,759 Speaker 1: And that's this feature we call gravitational time dilation. The 705 00:35:45,880 --> 00:35:49,680 Speaker 1: curvature of space makes clocks slow down. And this is 706 00:35:49,719 --> 00:35:52,480 Speaker 1: really super fascinating and different from the kind of time 707 00:35:52,520 --> 00:35:55,879 Speaker 1: dilation we're used to thinking about from velocity. Like if 708 00:35:55,880 --> 00:35:59,000 Speaker 1: you see somebody in a spaceship traveling really really fast, 709 00:35:59,360 --> 00:36:02,120 Speaker 1: we know that your view of their clocks sees their 710 00:36:02,160 --> 00:36:06,359 Speaker 1: clocks slow down. Moving clocks run slow. That's one really 711 00:36:06,400 --> 00:36:09,439 Speaker 1: cool effect, but it's actually totally separate from this kind 712 00:36:09,480 --> 00:36:12,560 Speaker 1: of time dilation. This is time dilation just caused by 713 00:36:12,600 --> 00:36:15,439 Speaker 1: the curvature of space. So if you're in a part 714 00:36:15,440 --> 00:36:17,880 Speaker 1: of space that has a lot of curvature, your clock 715 00:36:17,960 --> 00:36:20,799 Speaker 1: will run more slowly. And so if you look out 716 00:36:20,840 --> 00:36:23,120 Speaker 1: into the universe and everybody else has clocks and you 717 00:36:23,160 --> 00:36:26,560 Speaker 1: see their clocks running faster, that means that you are 718 00:36:26,680 --> 00:36:29,279 Speaker 1: in curved space. You looked at your clock and it 719 00:36:29,320 --> 00:36:32,600 Speaker 1: seems to be running normally. Everybody else's clocks are running faster. 720 00:36:33,120 --> 00:36:36,359 Speaker 1: They see your clock as running more slowly. So that's 721 00:36:36,440 --> 00:36:39,640 Speaker 1: one way to detect the curvature of space. M I 722 00:36:39,680 --> 00:36:42,279 Speaker 1: think you mean like the example of like you know, 723 00:36:42,360 --> 00:36:44,319 Speaker 1: you can measure the curvature of space by seeing how 724 00:36:44,320 --> 00:36:47,160 Speaker 1: they bend, how their trajectory bends in space. That tells 725 00:36:47,160 --> 00:36:49,239 Speaker 1: you there's something there, But like there might be a 726 00:36:49,239 --> 00:36:53,000 Speaker 1: situation where you can't tell that space is bent even 727 00:36:53,000 --> 00:36:54,880 Speaker 1: though there's something there. Like for example, if I shoot 728 00:36:54,880 --> 00:36:58,040 Speaker 1: a laser straight at a black hole, I'm not going 729 00:36:58,120 --> 00:37:00,799 Speaker 1: to see the path of the laser move were changed, right, 730 00:37:01,080 --> 00:37:02,879 Speaker 1: just gonna keep going in a straight line, I guess, 731 00:37:03,000 --> 00:37:05,000 Speaker 1: until it hits the black hole. But before that you 732 00:37:05,040 --> 00:37:08,000 Speaker 1: wouldn't be able to tell that space with bending unless 733 00:37:08,040 --> 00:37:10,360 Speaker 1: you use something else like time. Yeah, good point. If 734 00:37:10,360 --> 00:37:12,480 Speaker 1: you shoot a laser being directly at the heart of 735 00:37:12,480 --> 00:37:15,560 Speaker 1: a black hole, like pointed bang on to the singularity, 736 00:37:15,880 --> 00:37:19,000 Speaker 1: then its path wouldn't bend right because it's sort of 737 00:37:19,040 --> 00:37:22,680 Speaker 1: moving along that curvature, but it's time would bend right, 738 00:37:23,160 --> 00:37:26,000 Speaker 1: and so anything falling towards the black hole, it's time 739 00:37:26,040 --> 00:37:29,000 Speaker 1: gets dilated. And this is something we've actually measured. But 740 00:37:29,080 --> 00:37:31,040 Speaker 1: what would that mean for a laser though, Like would 741 00:37:31,040 --> 00:37:33,359 Speaker 1: you see the lasers slow down? So this gets into 742 00:37:33,520 --> 00:37:37,080 Speaker 1: very tricky territory and general relativity about measuring velocity of 743 00:37:37,160 --> 00:37:40,160 Speaker 1: distant things. If you are near those photons as they 744 00:37:40,200 --> 00:37:43,239 Speaker 1: pass you, you measure them as having the velocity of 745 00:37:43,239 --> 00:37:45,759 Speaker 1: the speed of light. If you are far away from 746 00:37:45,800 --> 00:37:48,600 Speaker 1: them and they're moving to curved space, then the rule 747 00:37:48,640 --> 00:37:50,719 Speaker 1: that light always travels at the speed of light no 748 00:37:50,840 --> 00:37:55,879 Speaker 1: longer applies. That only applies to flat space near inertial observers, 749 00:37:56,040 --> 00:37:58,200 Speaker 1: so you could actually see light travel are all sorts 750 00:37:58,200 --> 00:38:02,160 Speaker 1: of different weird velocities. You would see it's slow down, Yes, 751 00:38:02,440 --> 00:38:04,040 Speaker 1: it is pretty weird. You sort of have to need 752 00:38:04,120 --> 00:38:06,800 Speaker 1: to like plant the clock in that light laser beam 753 00:38:06,960 --> 00:38:09,480 Speaker 1: in order to know that the space was curved. Otherwise 754 00:38:09,480 --> 00:38:12,200 Speaker 1: would you know? I suppose by measuring its velocity as 755 00:38:12,239 --> 00:38:15,439 Speaker 1: a distant observer you could measure the curvature of space there. 756 00:38:15,760 --> 00:38:17,719 Speaker 1: But your right time is a really cool way to 757 00:38:17,719 --> 00:38:20,480 Speaker 1: measure the curvature as well. And this I think is 758 00:38:20,480 --> 00:38:22,960 Speaker 1: really cool because these are experiments we have done. We 759 00:38:23,080 --> 00:38:25,080 Speaker 1: shot a laser into the heart of a black hole. 760 00:38:26,320 --> 00:38:30,360 Speaker 1: Did I miss that headline? Unfortunately, nothing's so cool because 761 00:38:30,400 --> 00:38:32,840 Speaker 1: we don't have laser beams orbiting black holes. To do 762 00:38:32,880 --> 00:38:34,960 Speaker 1: these experiments. We have to make do with measuring the 763 00:38:35,000 --> 00:38:38,200 Speaker 1: curvature of space around our Earth. And so we've done 764 00:38:38,320 --> 00:38:41,399 Speaker 1: is built really really precise clocks and see that they 765 00:38:41,480 --> 00:38:44,799 Speaker 1: run differently at different altitudes for example. And again this 766 00:38:44,880 --> 00:38:47,399 Speaker 1: is not velocity based time dilation. This has not put 767 00:38:47,400 --> 00:38:49,120 Speaker 1: a clock up in a spaceship and orbit the Earth. 768 00:38:49,280 --> 00:38:51,640 Speaker 1: Really really fast. They have, for example, a super precise 769 00:38:51,640 --> 00:38:54,319 Speaker 1: atomic clock that they can raise and lower by like 770 00:38:54,400 --> 00:38:57,879 Speaker 1: a foot, and they can see a difference in how 771 00:38:57,960 --> 00:39:00,440 Speaker 1: fast it runs if they raise and low or just 772 00:39:00,520 --> 00:39:03,120 Speaker 1: buy one foot, because the curvature is different as you 773 00:39:03,160 --> 00:39:05,520 Speaker 1: move further away from the surface of the Earth. All right, well, 774 00:39:05,560 --> 00:39:07,160 Speaker 1: let's get into some of the other ways that you 775 00:39:07,200 --> 00:39:10,680 Speaker 1: can measure the curvature of space, and then let's talk 776 00:39:10,719 --> 00:39:13,640 Speaker 1: about what this all means. Man, But first let's take 777 00:39:13,640 --> 00:39:27,480 Speaker 1: another a quick break. All right, we're talking about the 778 00:39:27,560 --> 00:39:31,120 Speaker 1: curvature of space. It's bending my mind a bit. And 779 00:39:31,120 --> 00:39:33,319 Speaker 1: how you might measure this bending of space If you 780 00:39:33,360 --> 00:39:35,440 Speaker 1: didn't know, I guess if space was being bent, I 781 00:39:35,480 --> 00:39:38,480 Speaker 1: guess if it's not obvious, the space is invisible technically, yeah, 782 00:39:38,480 --> 00:39:41,239 Speaker 1: if you can't see the curvature directly, how can you 783 00:39:41,280 --> 00:39:44,160 Speaker 1: tell that it's there? And I love this philosophical question 784 00:39:44,200 --> 00:39:45,839 Speaker 1: of even if you measure it, do you really know 785 00:39:45,880 --> 00:39:48,000 Speaker 1: that it's there? Or if it's just some weird effect, 786 00:39:48,080 --> 00:39:51,320 Speaker 1: Like could somebody come up with another theory of physics 787 00:39:51,320 --> 00:39:54,239 Speaker 1: it doesn't require the curvature of space, but requires some 788 00:39:54,320 --> 00:39:57,880 Speaker 1: other weird change in our understanding of reality. That can 789 00:39:57,920 --> 00:40:01,040 Speaker 1: also explain everything we see, possibly, and then you might 790 00:40:01,080 --> 00:40:04,120 Speaker 1: be forced to believe that that's how reality actually is, 791 00:40:04,160 --> 00:40:07,680 Speaker 1: That space doesn't curve, it's actually this other thing that's happening. Yeah, 792 00:40:07,719 --> 00:40:10,120 Speaker 1: so we're sort of along from the ride as physics 793 00:40:10,239 --> 00:40:12,719 Speaker 1: is figuring it out. Is this shifting of the clocks 794 00:40:12,880 --> 00:40:16,200 Speaker 1: in relativity, like this idea that time slows down maybe 795 00:40:16,800 --> 00:40:20,160 Speaker 1: also another way to show that gravity is not like 796 00:40:20,239 --> 00:40:22,960 Speaker 1: Newton imagined it, right, because there's nothing in Newton's laws 797 00:40:23,040 --> 00:40:26,879 Speaker 1: that account for like time slowing down. Right, Yeah, great, 798 00:40:26,880 --> 00:40:28,680 Speaker 1: point is there? I don't know, No, there is not. 799 00:40:29,040 --> 00:40:33,120 Speaker 1: Newton thinks of space and time as absolute and universal, right, 800 00:40:33,200 --> 00:40:36,000 Speaker 1: So this is another feature of relativities, connecting space and 801 00:40:36,040 --> 00:40:38,840 Speaker 1: time together, tying it all up into one four D 802 00:40:39,080 --> 00:40:42,600 Speaker 1: mathematical object, and accepting that they're related to each other 803 00:40:42,640 --> 00:40:45,120 Speaker 1: and in special relativity, space and time are all twisted 804 00:40:45,200 --> 00:40:47,799 Speaker 1: up together. How you measure clocks depends on where you 805 00:40:47,840 --> 00:40:50,640 Speaker 1: are and how fast you're going, so they're definitely tied 806 00:40:50,760 --> 00:40:53,640 Speaker 1: together in a way that Newton never anticipated. Right. All right, Well, 807 00:40:53,680 --> 00:40:56,480 Speaker 1: we're talking about how to measure this invisible thing of 808 00:40:56,520 --> 00:40:58,440 Speaker 1: an invisible thing, and so what are some of the 809 00:40:58,440 --> 00:41:00,480 Speaker 1: other ways that you might measure the bend of space. 810 00:41:00,640 --> 00:41:02,840 Speaker 1: One really cool way to measure the bending of space 811 00:41:03,120 --> 00:41:06,200 Speaker 1: is to look at the geometry of objects. Like you 812 00:41:06,239 --> 00:41:09,880 Speaker 1: can tell if space is curved by building triangles or 813 00:41:09,880 --> 00:41:13,120 Speaker 1: by measuring the circumference of circles. And this is easiest 814 00:41:13,120 --> 00:41:15,200 Speaker 1: to understand if you think about like what happens on 815 00:41:15,239 --> 00:41:17,920 Speaker 1: the surface of a sphere versus like what happens out 816 00:41:18,000 --> 00:41:19,960 Speaker 1: in space. If you're like out in space and you 817 00:41:19,960 --> 00:41:21,879 Speaker 1: build a triangle and you measure its angles, you get 818 00:41:21,880 --> 00:41:23,719 Speaker 1: one hundred and eighty degrees. Now, if you're on the 819 00:41:23,719 --> 00:41:25,680 Speaker 1: surface of the Earth and you build like a really 820 00:41:25,719 --> 00:41:28,440 Speaker 1: big triangle, then you measure all of its angles and 821 00:41:28,480 --> 00:41:31,120 Speaker 1: add them up, you'll discover that they don't actually add 822 00:41:31,200 --> 00:41:33,279 Speaker 1: up to one hundred and eighty degrees. To add up 823 00:41:33,320 --> 00:41:35,560 Speaker 1: to a little bit more, because the angles of a 824 00:41:35,560 --> 00:41:37,600 Speaker 1: triangle add up to one hundred and eighty degrees only 825 00:41:37,640 --> 00:41:40,960 Speaker 1: on a flat surface, not on a curved surface. Well, 826 00:41:40,960 --> 00:41:44,320 Speaker 1: I guess only if you kind of project that shape 827 00:41:44,360 --> 00:41:47,439 Speaker 1: onto the surface of the Earth, right, you can still 828 00:41:47,440 --> 00:41:50,239 Speaker 1: have a perfect triangle. It's just sitting on top of 829 00:41:50,239 --> 00:41:53,200 Speaker 1: the Earth in a weird way, isn't it. Yes, if 830 00:41:53,239 --> 00:41:56,400 Speaker 1: you follow the curvature of the Earth, then that triangle 831 00:41:56,400 --> 00:41:58,400 Speaker 1: has an angle greater than one hundred and eighty degrees. 832 00:41:58,520 --> 00:42:00,040 Speaker 1: If you don't follow the curvature of the Earth in 833 00:42:00,080 --> 00:42:02,880 Speaker 1: your triangles like sort of awkwardly sitting on top of 834 00:42:02,920 --> 00:42:05,719 Speaker 1: the Earth, then yeah, it can still be flat, but 835 00:42:05,800 --> 00:42:08,040 Speaker 1: if it follows the curvature, it won't have angles that 836 00:42:08,120 --> 00:42:10,239 Speaker 1: add up to one hundred and eighty m. That's the 837 00:42:10,360 --> 00:42:12,800 Speaker 1: one way that you can tell if that the surface 838 00:42:12,840 --> 00:42:15,719 Speaker 1: of the Earth is curved, right, Yeah, exactly. You can 839 00:42:15,719 --> 00:42:17,440 Speaker 1: measure the curvature of the Earth. And that's sort of 840 00:42:17,440 --> 00:42:20,319 Speaker 1: a famous example, but it applies to other objects too, 841 00:42:20,360 --> 00:42:22,400 Speaker 1: and I think maybe people haven't heard about this, and 842 00:42:22,440 --> 00:42:24,560 Speaker 1: I think it's really cool. You can also measure the 843 00:42:24,560 --> 00:42:27,520 Speaker 1: curvature by measuring pie right like draw circle, measure the 844 00:42:27,560 --> 00:42:31,400 Speaker 1: diameter and the circumference. The ratio is pie, and on 845 00:42:31,440 --> 00:42:35,000 Speaker 1: a flat surface, pie is three point one, four, one, five, nine, etc. Etc. 846 00:42:35,360 --> 00:42:38,520 Speaker 1: But on a curved surface it's not right. On a 847 00:42:38,600 --> 00:42:42,200 Speaker 1: curved surface, the diameter gets longer. Instead of just being 848 00:42:42,239 --> 00:42:45,240 Speaker 1: like straight across the circle, it's now like rising above 849 00:42:45,360 --> 00:42:49,080 Speaker 1: the circle and coming back down. So pie changes as 850 00:42:49,280 --> 00:42:51,920 Speaker 1: space gets curved. I think what you mean is that 851 00:42:52,000 --> 00:42:54,520 Speaker 1: if you drew a circle across the equator, or if 852 00:42:54,560 --> 00:42:57,640 Speaker 1: you thought of the equator as a circle, and then 853 00:42:57,680 --> 00:43:00,440 Speaker 1: you try to measure its radius from the north pole, 854 00:43:00,719 --> 00:43:02,640 Speaker 1: you wouldn't get pie. You would get something else because 855 00:43:02,640 --> 00:43:06,520 Speaker 1: you're measuring the radius along the surface of the Earth. Right, 856 00:43:07,080 --> 00:43:10,719 Speaker 1: You're measuring this basically the curvature, the longitude basically line 857 00:43:10,760 --> 00:43:12,960 Speaker 1: which are longer than if you just do a line 858 00:43:13,000 --> 00:43:15,600 Speaker 1: through the center of the Earth on that circle exactly. 859 00:43:15,600 --> 00:43:17,600 Speaker 1: And so if you were like not aware that the 860 00:43:17,640 --> 00:43:20,600 Speaker 1: Earth was curved, you try this huge circle and you 861 00:43:20,640 --> 00:43:23,600 Speaker 1: walk from one part of the equator across the north 862 00:43:23,680 --> 00:43:26,360 Speaker 1: bold and then back down to your circle and measure that. 863 00:43:26,600 --> 00:43:28,920 Speaker 1: Then you would get an answer that's much longer than 864 00:43:29,000 --> 00:43:30,879 Speaker 1: just drilling a hole through the center of the Earth. 865 00:43:31,000 --> 00:43:33,000 Speaker 1: And so basically that's a measurement of pie. And so 866 00:43:33,200 --> 00:43:36,120 Speaker 1: pie is a measurement of curvature. And so if you 867 00:43:36,160 --> 00:43:38,720 Speaker 1: go out in space and make a really big circle 868 00:43:38,760 --> 00:43:42,080 Speaker 1: and then measure its diameter using a laser beam, you 869 00:43:42,080 --> 00:43:45,800 Speaker 1: can measure the curvature of that space by comparing what 870 00:43:45,960 --> 00:43:48,279 Speaker 1: you get to pie. But what if you make a 871 00:43:48,360 --> 00:43:51,680 Speaker 1: giant pie the size of the equator, wouldn't you still 872 00:43:51,719 --> 00:43:54,560 Speaker 1: be measuring pie and fill it with neutrinos? I can't 873 00:43:54,560 --> 00:44:00,280 Speaker 1: tell if you're joking. Are you talking to giant pie pie? Yeah, 874 00:44:00,360 --> 00:44:02,799 Speaker 1: I mean a giant apple pie to say, ride with 875 00:44:02,840 --> 00:44:07,640 Speaker 1: the diameter of the equator. But then you still measure pie. Well, 876 00:44:07,640 --> 00:44:10,319 Speaker 1: that's a good question. If you build a flat pie 877 00:44:10,320 --> 00:44:12,920 Speaker 1: out in space that's ignoring the curvature of space, it's 878 00:44:12,920 --> 00:44:15,920 Speaker 1: being held together by electromagnetic bonds which are really strong, 879 00:44:16,320 --> 00:44:18,759 Speaker 1: then it could still be flat, right, But if you're 880 00:44:18,800 --> 00:44:21,240 Speaker 1: following the curvature of space, you're using like a laser 881 00:44:21,280 --> 00:44:23,959 Speaker 1: beam to follow the curvature of space, then you would 882 00:44:23,960 --> 00:44:27,080 Speaker 1: not get pie measured across your giant apple pie, which 883 00:44:27,080 --> 00:44:29,120 Speaker 1: would have to cut I guess, with a neutrino laser 884 00:44:29,120 --> 00:44:33,600 Speaker 1: beam because it'd be so big. What kind of ice 885 00:44:33,600 --> 00:44:36,239 Speaker 1: cream do you serve with neutrino apple pie? I don't 886 00:44:36,239 --> 00:44:39,640 Speaker 1: even know. Obviously a dark ice all right. So that's 887 00:44:39,680 --> 00:44:43,120 Speaker 1: another way to measure the curvature space is using geometry, 888 00:44:43,160 --> 00:44:47,000 Speaker 1: which is like a sixth grade subject just measuring angles 889 00:44:47,040 --> 00:44:50,279 Speaker 1: between things, and they don't come out to be what 890 00:44:50,360 --> 00:44:52,359 Speaker 1: you would expect in flat space, then you know your 891 00:44:52,360 --> 00:44:54,719 Speaker 1: space is what are some other ways that we can 892 00:44:54,760 --> 00:44:57,160 Speaker 1: measure the vending space? So I think one of the 893 00:44:57,160 --> 00:44:59,960 Speaker 1: coolest ways is this experiment we talked about much earlier, 894 00:45:00,160 --> 00:45:03,600 Speaker 1: which measures frame dragging. This is an effect that doesn't 895 00:45:03,640 --> 00:45:07,480 Speaker 1: happen in Newtonian gravity at all. So, as we said before, 896 00:45:07,520 --> 00:45:10,080 Speaker 1: the Earth is spinning, and as it spins, it's sort 897 00:45:10,080 --> 00:45:13,160 Speaker 1: of like drags space time with it a little bit. 898 00:45:13,320 --> 00:45:15,360 Speaker 1: And so if you're an object out orbiting the Earth, 899 00:45:15,800 --> 00:45:18,440 Speaker 1: you feel a different force because the Earth is spinning 900 00:45:18,640 --> 00:45:21,239 Speaker 1: than you would if it wasn't spinning. Would you feel 901 00:45:21,320 --> 00:45:23,880 Speaker 1: is a little torque, like a little twist, not just 902 00:45:23,920 --> 00:45:26,400 Speaker 1: a force inwards towards the center of the Earth, but 903 00:45:26,520 --> 00:45:28,799 Speaker 1: like a little twist that spins you a little bit, 904 00:45:28,880 --> 00:45:31,480 Speaker 1: because how space is sort of flowing over you. So 905 00:45:31,480 --> 00:45:34,439 Speaker 1: there's these awesome experiments called gravity Probe B that built 906 00:45:34,480 --> 00:45:37,880 Speaker 1: like the most spherical objects known to man, these super 907 00:45:37,960 --> 00:45:42,000 Speaker 1: precise gyroscopes out in space that were like mind in 908 00:45:42,040 --> 00:45:45,120 Speaker 1: Brazil and then polished by like German grandmothers for years 909 00:45:45,120 --> 00:45:47,440 Speaker 1: and years and years, and these were able to measure 910 00:45:47,480 --> 00:45:50,000 Speaker 1: this very very small effect. But it's real, you know, 911 00:45:50,040 --> 00:45:51,600 Speaker 1: we have a whole episode about this. But this sort 912 00:45:51,640 --> 00:45:54,000 Speaker 1: of related to tidal forces too, right, Like, if you 913 00:45:54,040 --> 00:45:57,040 Speaker 1: have an object down in space near Earth spinning, some 914 00:45:57,080 --> 00:45:59,319 Speaker 1: things are sort of closer to it than others, and 915 00:45:59,360 --> 00:46:02,880 Speaker 1: so there's some delay in how the gravity kind of 916 00:46:03,000 --> 00:46:05,000 Speaker 1: goes from one end to the other. Yeah, exactly, this 917 00:46:05,200 --> 00:46:08,120 Speaker 1: only happens on objects that are not points, that objects 918 00:46:08,120 --> 00:46:10,520 Speaker 1: that have an extent, because as you say, they're experiencing 919 00:46:10,560 --> 00:46:13,920 Speaker 1: space differently, and so it's that relative effect across the 920 00:46:14,040 --> 00:46:17,480 Speaker 1: object that ends up causing the torque. So you're right. Conceptually, 921 00:46:17,520 --> 00:46:20,360 Speaker 1: it is similar to tidal forces in that way. The 922 00:46:20,400 --> 00:46:23,759 Speaker 1: effect is larger for bigger objects and zero for point 923 00:46:23,800 --> 00:46:26,760 Speaker 1: like objects. But this frame dragging is a way to 924 00:46:26,800 --> 00:46:30,880 Speaker 1: confirm that space can curve, but you sort of need 925 00:46:30,920 --> 00:46:33,919 Speaker 1: as guidance spinning object to cause that kind of effect. Yeah, 926 00:46:33,920 --> 00:46:36,720 Speaker 1: you don't get frame dragging around objects that are not spinning. 927 00:46:36,840 --> 00:46:39,000 Speaker 1: So in one way, it's really a test of Einstein's 928 00:46:39,040 --> 00:46:42,279 Speaker 1: theory to say, is this effect that Einstein predicts but 929 00:46:42,360 --> 00:46:44,920 Speaker 1: Newton doesn't, is it real in our universe? And the 930 00:46:44,920 --> 00:46:47,840 Speaker 1: answer is yes, And that's a consequence of all of 931 00:46:47,880 --> 00:46:51,600 Speaker 1: Einstein's math and his concept that space is curved. This 932 00:46:51,760 --> 00:46:55,000 Speaker 1: is a direct result of space being curved and how 933 00:46:55,040 --> 00:46:58,800 Speaker 1: space reacts and how that curvature reacts to mass, especially 934 00:46:58,840 --> 00:47:02,000 Speaker 1: spinning masses, and so in that sense, it's an indirect 935 00:47:02,080 --> 00:47:06,560 Speaker 1: confirmation that space is actually curved. The scientists who work 936 00:47:06,600 --> 00:47:08,200 Speaker 1: on this project, they think of this is like one 937 00:47:08,239 --> 00:47:11,000 Speaker 1: of the most direct measurements of the curvature of space. 938 00:47:11,000 --> 00:47:14,360 Speaker 1: Because so many other measurements could be explained by Newton's theory, 939 00:47:14,560 --> 00:47:17,680 Speaker 1: this one only can be explained by Einstein's theory. So 940 00:47:17,719 --> 00:47:20,719 Speaker 1: they take it as really proof that Einstein was right 941 00:47:20,800 --> 00:47:24,919 Speaker 1: and therefore space is curved or space can curve right, Yes, 942 00:47:25,200 --> 00:47:29,960 Speaker 1: can curve, are more likely space time can curry. Spacetime 943 00:47:30,000 --> 00:47:33,400 Speaker 1: has the ability to have curvature, which is really still 944 00:47:33,440 --> 00:47:36,359 Speaker 1: bongles my mind. Yeah, so I guess maybe to wrap 945 00:47:36,400 --> 00:47:38,520 Speaker 1: it all up, like, let's say I wanted to tell 946 00:47:39,200 --> 00:47:42,480 Speaker 1: if the space around our Solar System or even the 947 00:47:42,520 --> 00:47:47,080 Speaker 1: space around our galaxy was curved, maybe not due to 948 00:47:47,120 --> 00:47:50,279 Speaker 1: the mass of the galaxy, but just like overall, like 949 00:47:50,400 --> 00:47:53,080 Speaker 1: are we living in a spherical universe or are we 950 00:47:53,120 --> 00:47:55,560 Speaker 1: living in a cube universe? Or are we living in 951 00:47:55,560 --> 00:47:59,000 Speaker 1: a donut universe? Like how do you tell that you're 952 00:47:59,080 --> 00:48:02,480 Speaker 1: space around is curve Would you be able to tell 953 00:48:02,520 --> 00:48:04,360 Speaker 1: with any of these methods, or do would some of 954 00:48:04,360 --> 00:48:06,920 Speaker 1: these methods work, and other thoughts. So, so, to measure 955 00:48:06,960 --> 00:48:10,440 Speaker 1: the curvature is space on like a cosmological scale, you 956 00:48:10,480 --> 00:48:14,239 Speaker 1: could use this if you could construct a giant triangle 957 00:48:14,760 --> 00:48:18,640 Speaker 1: bigger than galaxies, or shoot laser beams between galaxies in 958 00:48:18,680 --> 00:48:21,200 Speaker 1: a big triangle, then you could use these methods to 959 00:48:21,239 --> 00:48:24,120 Speaker 1: measure them. But that's not really practical, right, But what 960 00:48:24,160 --> 00:48:27,120 Speaker 1: we can do instead is see the effect of space 961 00:48:27,280 --> 00:48:30,080 Speaker 1: on things that are already out there. For example, we've 962 00:48:30,120 --> 00:48:34,120 Speaker 1: looked at the cosmic microwave background radiation, this light left 963 00:48:34,239 --> 00:48:37,160 Speaker 1: over from just after the Big Bang, and that light 964 00:48:37,200 --> 00:48:39,319 Speaker 1: has like wiggles in it has like hot spots and 965 00:48:39,360 --> 00:48:42,520 Speaker 1: cold spots, and we know something about how big those 966 00:48:42,560 --> 00:48:46,000 Speaker 1: hot spots should be because of how much time things 967 00:48:46,000 --> 00:48:48,279 Speaker 1: had to like even out and cool off. And then 968 00:48:48,280 --> 00:48:51,439 Speaker 1: the curvature of space affects the size of those hot 969 00:48:51,440 --> 00:48:54,319 Speaker 1: spots as we see them. If space is curved in 970 00:48:54,360 --> 00:48:56,319 Speaker 1: one way, then the spots get bigger, they get like 971 00:48:56,400 --> 00:48:59,480 Speaker 1: blown up by lensing. If space is curved in another way, 972 00:48:59,520 --> 00:49:02,040 Speaker 1: they get shrunk. So we can actually measure like the 973 00:49:02,160 --> 00:49:06,319 Speaker 1: overall curvature of space near us by looking at the 974 00:49:06,400 --> 00:49:09,960 Speaker 1: size of hot spots in the cosmic microwave background radiation, 975 00:49:10,120 --> 00:49:13,120 Speaker 1: because I guess that light comes from really really really 976 00:49:13,120 --> 00:49:15,480 Speaker 1: far away kind of and all around us. Yeah, and 977 00:49:15,600 --> 00:49:18,000 Speaker 1: all around us. It's been traveling from billions and billions 978 00:49:18,000 --> 00:49:20,880 Speaker 1: of years, and so it's probing a really really big space. 979 00:49:20,920 --> 00:49:22,799 Speaker 1: It's like the oldest light that we can see. So 980 00:49:22,840 --> 00:49:26,440 Speaker 1: it comes from like the very edge of the observable universe. 981 00:49:27,040 --> 00:49:29,680 Speaker 1: What does that light say? Is the universe bending right 982 00:49:29,760 --> 00:49:33,080 Speaker 1: or left? That light says that, to within our accuracy 983 00:49:33,160 --> 00:49:36,319 Speaker 1: to measure it, the universe is flat. Like there are 984 00:49:36,400 --> 00:49:39,279 Speaker 1: little bendy spots here and there near galaxies, but that 985 00:49:39,480 --> 00:49:42,680 Speaker 1: overall there is no curvature to the universe. That the 986 00:49:42,760 --> 00:49:45,640 Speaker 1: universe seems to be mostly flat. That's sort of a 987 00:49:45,680 --> 00:49:49,520 Speaker 1: weird result, though, I would maybe say maybe you're just wrong, 988 00:49:50,200 --> 00:49:54,040 Speaker 1: And it's sort of like when you're trying to see 989 00:49:54,040 --> 00:49:56,200 Speaker 1: if something is right or let and you say, oh, 990 00:49:56,200 --> 00:49:59,520 Speaker 1: it's neither. I'm like, well, how do you know you 991 00:49:59,560 --> 00:50:01,600 Speaker 1: got it right? Yeah, it's a good question. And lots 992 00:50:01,640 --> 00:50:03,840 Speaker 1: of people have done these experiments with lots of different 993 00:50:03,840 --> 00:50:06,719 Speaker 1: technologies and look at lots of different aspects of it. 994 00:50:06,960 --> 00:50:10,080 Speaker 1: So we're pretty confident. But you know, there's also statistical 995 00:50:10,160 --> 00:50:13,800 Speaker 1: uncertainty there. It's like flat to within about one percent, 996 00:50:14,200 --> 00:50:16,680 Speaker 1: and so there's a possibility that space is a little 997 00:50:16,680 --> 00:50:19,799 Speaker 1: bit bent one way or the other sort of overall, 998 00:50:20,280 --> 00:50:22,560 Speaker 1: but we know that space is bent near us, right 999 00:50:22,560 --> 00:50:24,480 Speaker 1: effect of all, the mass of the galaxy in the 1000 00:50:24,480 --> 00:50:27,680 Speaker 1: Solar System is definitely a curving space in our neighborhood. 1001 00:50:28,040 --> 00:50:31,840 Speaker 1: What if we build a giant pie the size of 1002 00:50:31,920 --> 00:50:34,320 Speaker 1: the galaxy. I'm in, I don't need to hear anymore. 1003 00:50:36,360 --> 00:50:38,279 Speaker 1: I mean, well, technically you could do also do that 1004 00:50:38,400 --> 00:50:41,720 Speaker 1: right that thing. That's what you meant earlier by giant lasers. 1005 00:50:41,840 --> 00:50:45,360 Speaker 1: Like if you build a giant circle the size of 1006 00:50:45,360 --> 00:50:49,080 Speaker 1: the galaxy and measure pie by measuring the conference and radius, 1007 00:50:49,080 --> 00:50:51,520 Speaker 1: then you would be able to tell right that things 1008 00:50:51,520 --> 00:50:55,080 Speaker 1: are bent. Hm. Yes, galaxy sized pie is a good 1009 00:50:55,120 --> 00:50:59,200 Speaker 1: experiment in any case, because even if it turns out 1010 00:50:59,239 --> 00:51:01,960 Speaker 1: to be inconclusive, it might still be delicious fun fun 1011 00:51:02,080 --> 00:51:05,560 Speaker 1: fund all right, Well, it sounds like there are many 1012 00:51:05,560 --> 00:51:09,040 Speaker 1: ways to measure the curvature of space, some of them 1013 00:51:09,080 --> 00:51:12,440 Speaker 1: more delicious and or dangerous than others. But the amazing 1014 00:51:12,440 --> 00:51:16,759 Speaker 1: fact is that is space time does bend. It's been confirmed, 1015 00:51:17,200 --> 00:51:19,560 Speaker 1: and there are experiments you can do to measure it. 1016 00:51:19,840 --> 00:51:22,480 Speaker 1: That's right, and it is possible to get a grasp 1017 00:51:22,560 --> 00:51:26,560 Speaker 1: on what's going on with our universe understanding it's otherwise 1018 00:51:26,600 --> 00:51:29,440 Speaker 1: invisible doings if we are clever enough, and if we 1019 00:51:29,600 --> 00:51:32,200 Speaker 1: follow the threads of all those weird little things we 1020 00:51:32,239 --> 00:51:34,719 Speaker 1: can't otherwise explain. I wonder what people who think that 1021 00:51:34,760 --> 00:51:37,319 Speaker 1: the Earth is flat think about this concept, like it's 1022 00:51:37,600 --> 00:51:39,759 Speaker 1: it's just too much for them, or do you think 1023 00:51:39,760 --> 00:51:41,959 Speaker 1: there are people out there who say, yes, the Earth 1024 00:51:42,040 --> 00:51:45,840 Speaker 1: is flat, but the universe is cur I don't know. 1025 00:51:45,920 --> 00:51:48,879 Speaker 1: I'm a flat universe er. All right. Another exploration of 1026 00:51:48,880 --> 00:51:53,040 Speaker 1: how mind bending and also space time bending the universe is, 1027 00:51:53,400 --> 00:51:56,040 Speaker 1: I guess a good reminder that sometimes we think the 1028 00:51:56,160 --> 00:51:58,839 Speaker 1: universe is one way from our local experience, but if 1029 00:51:58,880 --> 00:52:01,800 Speaker 1: you go out there into the universe or to explore 1030 00:52:01,880 --> 00:52:05,240 Speaker 1: extreme situations, it turns out that the universe is different. 1031 00:52:06,040 --> 00:52:08,520 Speaker 1: Hope you enjoyed that. Thanks for joining us, see you 1032 00:52:08,600 --> 00:52:18,880 Speaker 1: next time. Thanks for listening, and remember that Daniel and 1033 00:52:18,960 --> 00:52:22,560 Speaker 1: Jorge explain the universe is a production of iHeartRadio. For 1034 00:52:22,800 --> 00:52:26,400 Speaker 1: more podcast from my heart Radio, visit the iHeartRadio app, 1035 00:52:26,680 --> 00:52:30,160 Speaker 1: Apple Podcasts, or wherever you listen to your favorite shows.