1 00:00:08,560 --> 00:00:10,480 Speaker 1: Hey, Daniel, what do you think are the goals of 2 00:00:10,520 --> 00:00:13,520 Speaker 1: this podcast? You know, the same as they ever were, 3 00:00:13,760 --> 00:00:17,800 Speaker 1: blowing everyone's minds by revealing the incredible universe we live in. 4 00:00:18,440 --> 00:00:19,919 Speaker 1: But yeah, we've been doing this for a while. You 5 00:00:19,920 --> 00:00:22,280 Speaker 1: don't think people's minds are already blown? Can you get 6 00:00:22,320 --> 00:00:24,479 Speaker 1: your mind blown only once? Well, I mean, if you 7 00:00:24,520 --> 00:00:26,720 Speaker 1: really blow someone's mind, it's it's kind of a once 8 00:00:26,720 --> 00:00:30,280 Speaker 1: in a lifetime experience, right, or lifetime ending experience. While 9 00:00:30,320 --> 00:00:33,600 Speaker 1: not literally hoping to explode people's heads. Here, Oh, I 10 00:00:33,680 --> 00:00:38,560 Speaker 1: see there is another misleading physics metaphor. Yes, today, let's 11 00:00:38,640 --> 00:00:41,479 Speaker 1: blow up the phrase blow your mind. Well, let's just 12 00:00:41,520 --> 00:00:44,320 Speaker 1: hope we don't blow up the podcast. I actually do 13 00:00:44,400 --> 00:00:47,080 Speaker 1: hope the podcast blows up. Lets you mean blow up 14 00:00:47,120 --> 00:00:50,760 Speaker 1: like inflating? Then it just expands right exponentially? Did I 15 00:00:50,840 --> 00:01:08,119 Speaker 1: just blow up the blowing up? Hi? Am more handed 16 00:01:08,200 --> 00:01:11,360 Speaker 1: cartoonists and the co author of the book frequently asked 17 00:01:11,440 --> 00:01:14,800 Speaker 1: questions about the universe. Hi, I'm Daniel. I'm a particle 18 00:01:14,840 --> 00:01:17,759 Speaker 1: physicist and a professor at U C Irvine, and I'm 19 00:01:17,800 --> 00:01:19,920 Speaker 1: the co author of the book We Have No Idea, 20 00:01:20,000 --> 00:01:22,520 Speaker 1: A Guide to the Unknown Universe. Oh Man we're both 21 00:01:22,520 --> 00:01:26,800 Speaker 1: co authors on different books. We are exactly our fourth 22 00:01:26,840 --> 00:01:29,559 Speaker 1: friend by the two of us. By the way, let's 23 00:01:29,560 --> 00:01:33,039 Speaker 1: not mislead people here with our authored metaphors. You just 24 00:01:33,040 --> 00:01:36,360 Speaker 1: blew everybody's minds. Welcome to our podcast, Daniel and Jorge 25 00:01:36,360 --> 00:01:39,160 Speaker 1: Explain the Universe, a production of My Heart Radio in 26 00:01:39,200 --> 00:01:41,960 Speaker 1: which we hope to blow your mind by showing you 27 00:01:42,040 --> 00:01:46,080 Speaker 1: the incredible wonders of our universe, by diving deep into 28 00:01:46,120 --> 00:01:49,200 Speaker 1: the biggest questions about how the universe works, what is 29 00:01:49,320 --> 00:01:51,640 Speaker 1: at the edge of it, why can we see as 30 00:01:51,680 --> 00:01:54,240 Speaker 1: far as we can, and how is it possible to 31 00:01:54,280 --> 00:01:56,960 Speaker 1: see things that are racing away from us. We don't 32 00:01:57,000 --> 00:02:00,640 Speaker 1: shy away from asking the hardest, trickiest, deepest questions about 33 00:02:00,640 --> 00:02:04,280 Speaker 1: the nature of the universe because we are desperate to understand. 34 00:02:04,640 --> 00:02:07,960 Speaker 1: We think that our tiny little brains can somehow metaphorically 35 00:02:08,040 --> 00:02:12,120 Speaker 1: expand to understand the entire cosmos that is visible to us, 36 00:02:12,320 --> 00:02:15,120 Speaker 1: and we want to share that experience with you. That's right, 37 00:02:15,160 --> 00:02:17,799 Speaker 1: because it is a pretty amazing universe. It's a pretty 38 00:02:17,800 --> 00:02:21,520 Speaker 1: big universe full of amazing and incredible things to discover 39 00:02:21,639 --> 00:02:24,600 Speaker 1: and to puzzle about. And the extra awesome thing is 40 00:02:24,639 --> 00:02:27,720 Speaker 1: that it's actually getting bigger. The universe is expanding, and 41 00:02:27,800 --> 00:02:31,959 Speaker 1: it's not only expanding, it's expanding faster every day. That's right. 42 00:02:31,960 --> 00:02:34,040 Speaker 1: If you love the universe, you should be happy because 43 00:02:34,080 --> 00:02:36,679 Speaker 1: you're getting more of it every year for free. You 44 00:02:36,720 --> 00:02:39,760 Speaker 1: don't even have to pay extra. It just does it automatically. 45 00:02:40,040 --> 00:02:45,239 Speaker 1: It's like built in inflation, right, I mean physically and monetarily, 46 00:02:45,440 --> 00:02:47,600 Speaker 1: that's true. But the same amount of dollars buys more 47 00:02:47,720 --> 00:02:51,040 Speaker 1: universe every year. Oh? Really? Is the universe for sale? 48 00:02:52,400 --> 00:02:55,280 Speaker 1: Is it listed somewhere? You know, real estates the best investment? 49 00:02:55,840 --> 00:02:58,640 Speaker 1: And who's selling it? I can sell you andometer right 50 00:02:58,680 --> 00:03:00,639 Speaker 1: here today on the podcast aust If you cut me 51 00:03:00,680 --> 00:03:03,080 Speaker 1: a check, all right, I'll trade you for Alpha centri 52 00:03:03,960 --> 00:03:08,280 Speaker 1: done and done. Mabel Birth the co owners of both things. 53 00:03:10,080 --> 00:03:13,520 Speaker 1: That would be confusing, and humanity has long been confused 54 00:03:13,639 --> 00:03:16,239 Speaker 1: about how the universe works. We stare up into the 55 00:03:16,360 --> 00:03:19,440 Speaker 1: night sky. We try to understand what are those brilliant 56 00:03:19,480 --> 00:03:22,519 Speaker 1: little dots of light moving across our vision. We try 57 00:03:22,560 --> 00:03:24,840 Speaker 1: to build a model that can explain everything that is 58 00:03:24,880 --> 00:03:26,960 Speaker 1: happening out there. And along the way we learned some 59 00:03:27,040 --> 00:03:30,440 Speaker 1: pretty interesting things about how the universe works. But what 60 00:03:30,520 --> 00:03:33,240 Speaker 1: the rules are for things moving really really fast or 61 00:03:33,280 --> 00:03:37,160 Speaker 1: things near very heavy objects like black holes. Yeah, because 62 00:03:37,160 --> 00:03:39,120 Speaker 1: through most of human history we sort of like the 63 00:03:39,120 --> 00:03:41,120 Speaker 1: diners and thought, hey, this is a pretty big place. 64 00:03:41,320 --> 00:03:44,800 Speaker 1: But it wasn't until recently in which humans realize that 65 00:03:45,080 --> 00:03:48,280 Speaker 1: the place is actually getting bigger and getting bigger by 66 00:03:48,280 --> 00:03:50,600 Speaker 1: the day, faster and faster. And one thing we encourage 67 00:03:50,600 --> 00:03:52,560 Speaker 1: you to do on this podcast all the time is 68 00:03:52,600 --> 00:03:56,280 Speaker 1: to apply your understanding too new questions of physics. And 69 00:03:56,320 --> 00:03:58,520 Speaker 1: it's one thing to hear us blah blah blah about 70 00:03:58,760 --> 00:04:01,000 Speaker 1: some situation and ex in it to you, But if 71 00:04:01,000 --> 00:04:03,960 Speaker 1: you want to really understand a rule of physics, you 72 00:04:04,000 --> 00:04:06,280 Speaker 1: have to apply it in some new situation to see 73 00:04:06,480 --> 00:04:09,160 Speaker 1: if it really clicks together for you. Are you saying 74 00:04:09,160 --> 00:04:12,040 Speaker 1: people should jump into a black hole just for just 75 00:04:12,200 --> 00:04:15,160 Speaker 1: for their own curiosity? Yes, please do jump in a 76 00:04:15,200 --> 00:04:17,279 Speaker 1: black hole and then send me an email from within 77 00:04:17,320 --> 00:04:20,039 Speaker 1: the black hole to tell me what you see. That's impossible, 78 00:04:20,080 --> 00:04:22,720 Speaker 1: isn't it. Well figure out a way, right, Bring an 79 00:04:22,760 --> 00:04:25,320 Speaker 1: engineer with you and figure it out, and a physicist too, 80 00:04:26,440 --> 00:04:28,280 Speaker 1: so then they can be co authors on your book. 81 00:04:28,640 --> 00:04:31,479 Speaker 1: That's right. You can write multiple books and I'll be 82 00:04:31,560 --> 00:04:33,560 Speaker 1: co authors on all of the books, or you can 83 00:04:33,640 --> 00:04:36,040 Speaker 1: just stay at home and do thought experiments. A great 84 00:04:36,080 --> 00:04:39,599 Speaker 1: way to test your understanding of our ideas of physics 85 00:04:39,720 --> 00:04:42,440 Speaker 1: is to bang them against each other in extreme situations 86 00:04:42,440 --> 00:04:44,920 Speaker 1: to see you, do I really understand what's happening at 87 00:04:44,960 --> 00:04:47,080 Speaker 1: the edge of the universe or near the edge of 88 00:04:47,080 --> 00:04:49,280 Speaker 1: a black hole? Or if I fire a laser beam 89 00:04:49,320 --> 00:04:51,600 Speaker 1: into a black hole, why does it not get hot? 90 00:04:51,760 --> 00:04:53,760 Speaker 1: All these kind of questions are great ways to test 91 00:04:53,839 --> 00:04:57,039 Speaker 1: the boundaries of your own understanding. Yeah, that's right. You 92 00:04:57,040 --> 00:04:59,479 Speaker 1: can expand your own mind just sitting there and thinking 93 00:04:59,520 --> 00:05:02,640 Speaker 1: about the things, and also by listening to this podcast 94 00:05:02,680 --> 00:05:04,240 Speaker 1: and a bunch of folks right in because they are 95 00:05:04,240 --> 00:05:07,760 Speaker 1: confused about two ideas they hear us talk about. One 96 00:05:07,880 --> 00:05:10,120 Speaker 1: is that the universe is really strange in a particular 97 00:05:10,160 --> 00:05:13,640 Speaker 1: way that nothing in the universe can travel faster than 98 00:05:13,680 --> 00:05:16,440 Speaker 1: the speed of light. There's this really weird limit on 99 00:05:16,520 --> 00:05:19,400 Speaker 1: how fast things can travel. And the other is that 100 00:05:19,440 --> 00:05:22,440 Speaker 1: the universe is expanding faster and faster, and the further 101 00:05:22,520 --> 00:05:25,400 Speaker 1: you go out, the faster that expansion seems. So things 102 00:05:25,400 --> 00:05:28,000 Speaker 1: that are super far away seem to be expanding faster 103 00:05:28,200 --> 00:05:30,120 Speaker 1: than the speed of light. A lot of folks want 104 00:05:30,200 --> 00:05:33,359 Speaker 1: to understand these two ideas in their head at the 105 00:05:33,440 --> 00:05:36,000 Speaker 1: same time. So today on the podcast, we'll be asking 106 00:05:36,000 --> 00:05:44,480 Speaker 1: the question, how can the universe expand faster than the 107 00:05:44,520 --> 00:05:48,240 Speaker 1: speed of light? Pretty fascinating that the universe, as you say, 108 00:05:48,360 --> 00:05:50,840 Speaker 1: is expanding faster than the speed of light. I mean 109 00:05:50,880 --> 00:05:53,479 Speaker 1: we when we only figured this out kind of recently, right, 110 00:05:53,640 --> 00:05:56,080 Speaker 1: that's right. We've known the universe has been expanding for 111 00:05:56,120 --> 00:05:59,440 Speaker 1: about a hundred years, but it's been only twenty five 112 00:05:59,520 --> 00:06:03,040 Speaker 1: years since we discovered that that expansion is accelerating. Right. 113 00:06:03,040 --> 00:06:05,760 Speaker 1: We learned that it's accelerating, and that it's excelerating faster 114 00:06:05,880 --> 00:06:08,840 Speaker 1: and faster every year by looking at how fast the 115 00:06:08,880 --> 00:06:11,680 Speaker 1: galaxies are moving. Right, Yes, we found type one a 116 00:06:11,760 --> 00:06:14,920 Speaker 1: supernova which let us measure the distance to super duper 117 00:06:14,920 --> 00:06:18,080 Speaker 1: far away objects and look through the past and discovered 118 00:06:18,279 --> 00:06:21,240 Speaker 1: that the expansion the universe is larger than it once was, 119 00:06:21,680 --> 00:06:25,240 Speaker 1: and it's larger every year. So it's accelerating the expansion 120 00:06:25,400 --> 00:06:28,360 Speaker 1: of the universe. But even without that accelerating expansion, there's 121 00:06:28,360 --> 00:06:31,680 Speaker 1: something really fascinating about the expansion of the universe, which 122 00:06:31,720 --> 00:06:34,560 Speaker 1: is that the velocity of far away things correlates with 123 00:06:34,600 --> 00:06:37,080 Speaker 1: their distance, So it means that the further away in 124 00:06:37,160 --> 00:06:40,360 Speaker 1: space you look, the faster things are moving away from us, 125 00:06:40,480 --> 00:06:42,480 Speaker 1: which means if you look far enough, you look, if 126 00:06:42,480 --> 00:06:45,080 Speaker 1: you look deep enough into the universe, you can find 127 00:06:45,120 --> 00:06:47,640 Speaker 1: some things that are moving away from us faster than 128 00:06:47,680 --> 00:06:49,720 Speaker 1: the speed of light. It seems to be an apparent 129 00:06:49,760 --> 00:06:53,360 Speaker 1: contradiction with Einstein's relativity that says nothing can be moving 130 00:06:53,400 --> 00:06:55,560 Speaker 1: away from most faster than the speed of light. Right, 131 00:06:55,560 --> 00:06:57,320 Speaker 1: but do we know that for sure? Because it kind 132 00:06:57,360 --> 00:06:59,600 Speaker 1: of depends on how far out into the universe we 133 00:06:59,640 --> 00:07:02,360 Speaker 1: can look. Right, Well, we definitely can't see everything out 134 00:07:02,360 --> 00:07:05,760 Speaker 1: there in the universe. Is also a horizon beyond which 135 00:07:05,839 --> 00:07:09,560 Speaker 1: we just cannot see, but things within the horizon are 136 00:07:09,680 --> 00:07:12,880 Speaker 1: moving away from us faster than the speed of light. Although, 137 00:07:12,880 --> 00:07:16,200 Speaker 1: as I'm sure you won't appreciate, there are some asterisks 138 00:07:16,280 --> 00:07:19,320 Speaker 1: and caveats about exactly what it means for things far 139 00:07:19,360 --> 00:07:22,880 Speaker 1: away from us to have very higher velocities. SHO make 140 00:07:22,920 --> 00:07:26,800 Speaker 1: that the title of the podcast Asterisks and Caveats. I mean, 141 00:07:26,800 --> 00:07:30,360 Speaker 1: who doesn't love this? True crime and comedy and also 142 00:07:30,560 --> 00:07:34,280 Speaker 1: asterixtent caveats come here. Physicists form out of contradictions by 143 00:07:34,320 --> 00:07:36,960 Speaker 1: invoking cosmic loopholes. Right, But I think what you mean 144 00:07:37,080 --> 00:07:39,760 Speaker 1: is that, you know, the universe is expanding, and it's 145 00:07:39,760 --> 00:07:42,520 Speaker 1: a bit, but it's expanding like everywhere, like between me 146 00:07:42,560 --> 00:07:45,480 Speaker 1: and you, even here in southern California. The space between 147 00:07:45,560 --> 00:07:48,280 Speaker 1: us is expanding, right Like the space itself, not the 148 00:07:48,320 --> 00:07:50,240 Speaker 1: stuff in it is is expanding between you and me, 149 00:07:50,320 --> 00:07:52,160 Speaker 1: but just a little tiny, little bit. But if you 150 00:07:52,200 --> 00:07:55,720 Speaker 1: sort of added up over long distances, that expansion gets 151 00:07:55,720 --> 00:07:58,600 Speaker 1: bigger and bigger. Like between here and Jupiter, the expansion 152 00:07:58,680 --> 00:08:01,440 Speaker 1: is much bigger than between here and Orange County, and 153 00:08:01,520 --> 00:08:04,200 Speaker 1: between here and the next Galucia, it's even bigger and bigger. 154 00:08:04,200 --> 00:08:06,520 Speaker 1: And so if you go out far enough, that expansion 155 00:08:06,600 --> 00:08:09,720 Speaker 1: is huge. It's expanding, you know, thousands or millions of 156 00:08:09,760 --> 00:08:12,920 Speaker 1: kilometers per second exactly. And there's two different ways to 157 00:08:12,920 --> 00:08:14,200 Speaker 1: look at that. You can look at it as the 158 00:08:14,240 --> 00:08:16,840 Speaker 1: expansion of space, or you can look at it as 159 00:08:16,880 --> 00:08:20,680 Speaker 1: things moving through space. There are two sort of alternative 160 00:08:20,680 --> 00:08:23,720 Speaker 1: ways to view the same situation. But we'll dig into 161 00:08:23,760 --> 00:08:25,480 Speaker 1: that in a minute, all right. So then the question 162 00:08:25,560 --> 00:08:28,240 Speaker 1: is here is sort of like if the universe, parts 163 00:08:28,240 --> 00:08:30,800 Speaker 1: of the universe are expanding away from us faster than 164 00:08:30,840 --> 00:08:33,160 Speaker 1: the speed of light. How can that be right? Because 165 00:08:33,360 --> 00:08:34,960 Speaker 1: one of the rules of the universe is that you 166 00:08:35,000 --> 00:08:37,800 Speaker 1: can move faster than the speed of light. Exactly. That's 167 00:08:37,840 --> 00:08:41,160 Speaker 1: the puzzle, and it requires graduating our understanding of the 168 00:08:41,240 --> 00:08:46,280 Speaker 1: cosmic expansion from special relativity to general relativity. Sounds special, 169 00:08:46,440 --> 00:08:48,520 Speaker 1: and so, as usual, we were wondering how many people 170 00:08:48,600 --> 00:08:51,839 Speaker 1: out there had thought about this question or wondered how 171 00:08:51,880 --> 00:08:54,320 Speaker 1: the universe can expand faster than the speed of light. 172 00:08:54,559 --> 00:08:56,720 Speaker 1: So thank you very much to everybody who is willing 173 00:08:56,720 --> 00:08:59,600 Speaker 1: to answer these questions for the podcast. If this sounds 174 00:08:59,640 --> 00:09:02,000 Speaker 1: like fun to you, don't be sid Please write to 175 00:09:02,040 --> 00:09:05,560 Speaker 1: me two questions at Daniel and Jorge dot com and 176 00:09:05,640 --> 00:09:08,599 Speaker 1: I'll set you up to answer these questions for future episodes. 177 00:09:08,760 --> 00:09:10,200 Speaker 1: I think about it for a second. How do you 178 00:09:10,240 --> 00:09:12,840 Speaker 1: think the universe can expand faster than the speed of light. 179 00:09:13,440 --> 00:09:15,280 Speaker 1: Here's what people had to say. I have a heard 180 00:09:15,320 --> 00:09:19,520 Speaker 1: it said that because its expansion of space instead of 181 00:09:19,559 --> 00:09:23,720 Speaker 1: expansion in space, the universe can expand faster than the 182 00:09:23,760 --> 00:09:26,800 Speaker 1: speed of light. Nevertheless, that seems to me to imply 183 00:09:26,960 --> 00:09:32,120 Speaker 1: that things are moving at the hypervelocities that should be impossible. 184 00:09:32,600 --> 00:09:35,280 Speaker 1: I've heard it, but I don't understand it. You explained 185 00:09:35,280 --> 00:09:38,439 Speaker 1: on a previous episode how the universe can expand faster 186 00:09:38,520 --> 00:09:41,720 Speaker 1: than the speed of light. It's basically because the speed 187 00:09:41,720 --> 00:09:45,360 Speaker 1: of light in a vacuum is the speed limit for 188 00:09:45,720 --> 00:09:52,199 Speaker 1: anything to move through space. However, with the expansion of 189 00:09:52,240 --> 00:09:55,640 Speaker 1: the universe, nothing is moving through space, new spaces being 190 00:09:55,960 --> 00:10:00,440 Speaker 1: created between things. The universe expanding fashion light really makes 191 00:10:00,480 --> 00:10:03,199 Speaker 1: the eyes roll back in my head. So I'm thinking 192 00:10:03,760 --> 00:10:06,920 Speaker 1: it is light bread baking in the oven, and everything 193 00:10:07,000 --> 00:10:11,720 Speaker 1: expands together, and that expansion happens faster than the speed 194 00:10:11,760 --> 00:10:14,679 Speaker 1: of light, which is impossible, And now my eyes are 195 00:10:14,960 --> 00:10:16,720 Speaker 1: rolled back in my head. Now, I have heard this 196 00:10:16,800 --> 00:10:18,760 Speaker 1: question before and I've never really understood it. And I 197 00:10:18,840 --> 00:10:20,920 Speaker 1: understand the universe is expanding fast on the light, and 198 00:10:20,960 --> 00:10:23,520 Speaker 1: I think it's got something to do with inflation and 199 00:10:23,600 --> 00:10:27,520 Speaker 1: the fact that the overall space is expanding similar to 200 00:10:27,559 --> 00:10:30,360 Speaker 1: how a balloon expands, and that it's a different scale 201 00:10:30,440 --> 00:10:33,040 Speaker 1: or a different measuring technique to that of the speed 202 00:10:33,080 --> 00:10:35,959 Speaker 1: of light. Speed of lights finite within the universe, but 203 00:10:36,040 --> 00:10:40,680 Speaker 1: the universe is able to expand into other stuff. Perhaps 204 00:10:40,720 --> 00:10:43,720 Speaker 1: it's not the universe that is expanding faster than the 205 00:10:43,760 --> 00:10:47,600 Speaker 1: speed of light, but it's the empty spaces in between 206 00:10:47,760 --> 00:10:52,599 Speaker 1: the particles that is perhaps not limited by the universal 207 00:10:52,760 --> 00:10:57,319 Speaker 1: speed limit. Just a thought. I believe that the universe 208 00:10:57,320 --> 00:11:00,640 Speaker 1: can expand faster than light because the speed limit of 209 00:11:00,640 --> 00:11:02,760 Speaker 1: the universe, which is the speed of light, only applies 210 00:11:02,840 --> 00:11:07,320 Speaker 1: to something that is traveling through space, and that speed 211 00:11:07,320 --> 00:11:10,679 Speaker 1: limit does not apply to space itself when it expands, 212 00:11:11,280 --> 00:11:14,880 Speaker 1: because it's not expanding in space, it is space. All right, 213 00:11:15,480 --> 00:11:18,320 Speaker 1: some interesting answers here. Some people are relating it to baking. 214 00:11:20,200 --> 00:11:23,280 Speaker 1: It's a great analogy raising than a cosmic bread. But 215 00:11:23,320 --> 00:11:25,320 Speaker 1: the major point people seem to be making is that 216 00:11:25,320 --> 00:11:28,280 Speaker 1: there's no limit in the speed of expansion, even if 217 00:11:28,280 --> 00:11:31,920 Speaker 1: there's limit of motion through space. Yeah, that's kind of 218 00:11:31,960 --> 00:11:34,319 Speaker 1: the big caveat in this statement, right, like the rules 219 00:11:34,360 --> 00:11:36,920 Speaker 1: that you can move through space faster than the speed 220 00:11:36,920 --> 00:11:39,160 Speaker 1: of light, but there's no limit to what you can 221 00:11:39,200 --> 00:11:41,120 Speaker 1: do with that space. You can squish it, you can 222 00:11:41,120 --> 00:11:43,600 Speaker 1: expand it technically faster than the speed of light. Yeah, 223 00:11:43,600 --> 00:11:46,400 Speaker 1: And that's a nice shorthand for thinking about expansion, But 224 00:11:46,400 --> 00:11:47,959 Speaker 1: there are some issues with that if you want to 225 00:11:47,960 --> 00:11:50,000 Speaker 1: be precise about it, because what does it mean to 226 00:11:50,000 --> 00:11:53,360 Speaker 1: be moving through space? Space itself doesn't have a frame. 227 00:11:53,679 --> 00:11:56,559 Speaker 1: You can't measure your velocity with respect to space. You 228 00:11:56,600 --> 00:11:59,720 Speaker 1: can only measure your velocity with respect to other things 229 00:12:00,080 --> 00:12:02,880 Speaker 1: in space. And so special relativity would seem to be 230 00:12:02,920 --> 00:12:05,800 Speaker 1: in contradiction with that anyway, because it says, how can 231 00:12:05,920 --> 00:12:09,240 Speaker 1: somebody be measuring a distant galaxy moving away from us 232 00:12:09,280 --> 00:12:11,720 Speaker 1: at two times the speed of light. We're not measuring 233 00:12:11,720 --> 00:12:15,160 Speaker 1: its velocity through space, We're measuring its velocity relative to us. 234 00:12:16,440 --> 00:12:19,520 Speaker 1: Sounds confusing. Going back to thing he said earlier, he said, 235 00:12:19,559 --> 00:12:22,000 Speaker 1: there's no limit to how fast this space can expand? 236 00:12:22,080 --> 00:12:24,240 Speaker 1: Is that really true? Like can I expand space from 237 00:12:24,320 --> 00:12:27,640 Speaker 1: zero to infinity in no time? Or even the opposite, 238 00:12:27,640 --> 00:12:30,360 Speaker 1: like can I take some space and collapse it in 239 00:12:30,440 --> 00:12:33,560 Speaker 1: an instant in zero time? There's no limit to the 240 00:12:33,679 --> 00:12:35,840 Speaker 1: rate of which space can expand, but it does need 241 00:12:35,920 --> 00:12:38,880 Speaker 1: to be continuous, right, so you can put in any number, 242 00:12:39,040 --> 00:12:42,120 Speaker 1: And in fact, we've seen very dramatic expansions of space 243 00:12:42,160 --> 00:12:44,880 Speaker 1: in our history of the universe. Right. Inflation is nothing 244 00:12:44,880 --> 00:12:47,240 Speaker 1: more than expansion of the universe. And that was a 245 00:12:47,280 --> 00:12:49,840 Speaker 1: factor of like ten to the thirty in ten to 246 00:12:49,880 --> 00:12:52,600 Speaker 1: the minus thirty seconds. So as far as we understand, 247 00:12:52,600 --> 00:12:54,920 Speaker 1: there are no bounds there except that it does need 248 00:12:55,000 --> 00:12:59,880 Speaker 1: to be continuous. You can't have an instantaneous transformation of space, 249 00:13:00,040 --> 00:13:04,160 Speaker 1: so you can have any number except for infinity essentially interesting, 250 00:13:04,400 --> 00:13:08,080 Speaker 1: so literally almost instantly instantaneously, then you can collapse and 251 00:13:08,120 --> 00:13:10,560 Speaker 1: expand space. Because that's kind of what happens in the 252 00:13:10,559 --> 00:13:13,240 Speaker 1: Big Bang. We don't understand it, but general relativity can 253 00:13:13,360 --> 00:13:16,240 Speaker 1: describe it, and we've seen it happening in the universe. 254 00:13:16,280 --> 00:13:18,280 Speaker 1: We don't know what can cause it, Like we know 255 00:13:18,360 --> 00:13:21,280 Speaker 1: inflation happened and it stretched the universe dramatically. We know 256 00:13:21,360 --> 00:13:24,000 Speaker 1: dark energy is accelerating the expansion of the universe. We 257 00:13:24,040 --> 00:13:26,920 Speaker 1: don't understand the mechanism for that, like what makes that 258 00:13:27,040 --> 00:13:30,200 Speaker 1: happen and what the limits are to that mechanism. But 259 00:13:30,280 --> 00:13:33,080 Speaker 1: in principle, the framework of general relativity does allow for 260 00:13:33,200 --> 00:13:35,880 Speaker 1: very very dramatic expansions. Yes, all right, well let's dive 261 00:13:35,920 --> 00:13:38,960 Speaker 1: into it, and let's maybe start talking first about the 262 00:13:39,160 --> 00:13:41,360 Speaker 1: just the general expansion of the universe. Why do we 263 00:13:41,360 --> 00:13:43,400 Speaker 1: know about that and how do we find out? So 264 00:13:43,400 --> 00:13:48,360 Speaker 1: our understanding is that space is stretching everywhere. It's expanding. 265 00:13:48,679 --> 00:13:52,360 Speaker 1: It's getting bigger. Between me and you, space is getting larger. 266 00:13:52,400 --> 00:13:55,959 Speaker 1: Between us and other galaxies, space is getting larger. This 267 00:13:56,040 --> 00:13:59,520 Speaker 1: is an expansion that's happening everywhere at the same time, 268 00:14:00,280 --> 00:14:03,480 Speaker 1: not an explosion from a tiny dots some people might 269 00:14:03,559 --> 00:14:06,560 Speaker 1: imagine the Big Bang started. Take a very dense universe, 270 00:14:06,720 --> 00:14:09,840 Speaker 1: stretch it out to a less dense universe, a colder, 271 00:14:10,160 --> 00:14:13,240 Speaker 1: more dilute universe. Right, But when you say that space 272 00:14:13,320 --> 00:14:16,800 Speaker 1: is stretching everywhere or getting bigger everywhere, you don't mean 273 00:14:16,840 --> 00:14:20,040 Speaker 1: like space itself is getting bigger. Like you actually mean 274 00:14:20,040 --> 00:14:23,440 Speaker 1: like there's more space being created out of nothing, right, 275 00:14:23,600 --> 00:14:26,520 Speaker 1: Because it's not like a kilometer is suddenly more or 276 00:14:26,560 --> 00:14:29,080 Speaker 1: like what we call a kilometer is getting bigger. It's 277 00:14:29,080 --> 00:14:31,920 Speaker 1: like there's just more space growing all the time out 278 00:14:31,920 --> 00:14:34,920 Speaker 1: of nothingness. Right. I think both of those descriptions are accurate. 279 00:14:34,960 --> 00:14:38,040 Speaker 1: I'm not sure exactly what the distinction is between them. Like, 280 00:14:38,160 --> 00:14:40,120 Speaker 1: the universe is expanding at a rate we call the 281 00:14:40,200 --> 00:14:45,000 Speaker 1: Hubble parameter. It's seventy kilometers per second er mega parsak. 282 00:14:45,440 --> 00:14:48,640 Speaker 1: What that means is that every second a mega parsak, 283 00:14:48,800 --> 00:14:51,640 Speaker 1: which is a unit of distance and astronomical unit becomes 284 00:14:51,680 --> 00:14:55,560 Speaker 1: longer by about seventy kilometers. Right, So every second a 285 00:14:55,600 --> 00:14:58,520 Speaker 1: mega parsak does get longer, is it getting stretched or 286 00:14:58,600 --> 00:15:01,560 Speaker 1: is the universe creating new space in the middle? Doesn't 287 00:15:01,560 --> 00:15:04,520 Speaker 1: really matter either way. We just describe it as an expansion. 288 00:15:04,640 --> 00:15:06,960 Speaker 1: We say the universe is scaling up. Well, I guess 289 00:15:06,960 --> 00:15:10,200 Speaker 1: what I mean is that, like between here and Jupiter, right, 290 00:15:10,280 --> 00:15:14,360 Speaker 1: space is expanding, it's getting bigger. But between here and Jupiter, 291 00:15:14,560 --> 00:15:16,440 Speaker 1: like how long it takes light to go there and back? 292 00:15:16,480 --> 00:15:19,560 Speaker 1: It's not getting bigger every year, right, Like, that's pretty 293 00:15:19,600 --> 00:15:20,920 Speaker 1: much stay in the same. So if you take two 294 00:15:21,000 --> 00:15:23,680 Speaker 1: mirrors and you put them a megaparsec away from each other, 295 00:15:23,840 --> 00:15:26,720 Speaker 1: what's a megaparsic? So a mega parsk is about three 296 00:15:27,000 --> 00:15:30,600 Speaker 1: million light years. It's a really really big distance. So 297 00:15:30,640 --> 00:15:33,400 Speaker 1: it's how far light travels in about three million years. 298 00:15:33,640 --> 00:15:36,920 Speaker 1: So set up two mirrors three million light years apart, 299 00:15:37,000 --> 00:15:39,640 Speaker 1: one megaparsec apart from each other, it would take light 300 00:15:39,680 --> 00:15:42,000 Speaker 1: a certain time to go there and back. Right now, 301 00:15:42,080 --> 00:15:44,560 Speaker 1: wait for the universe to expand, those mirrors will be 302 00:15:44,640 --> 00:15:47,400 Speaker 1: further apart from each other, and so we will take 303 00:15:47,520 --> 00:15:50,840 Speaker 1: light longer to get there and back. The space really 304 00:15:51,000 --> 00:15:54,240 Speaker 1: is expanding, either by creating new space or by stretching 305 00:15:54,280 --> 00:15:57,600 Speaker 1: that space relative to this standard of the speed of 306 00:15:57,720 --> 00:16:00,600 Speaker 1: light between here and Jupiter. Is something else going on. 307 00:16:01,040 --> 00:16:03,840 Speaker 1: That's because the Sun is holding Jupiter in place. In 308 00:16:03,880 --> 00:16:06,240 Speaker 1: the case the two mirrors, there isn't any gravity to 309 00:16:06,320 --> 00:16:08,360 Speaker 1: keep the two mirrors at a certain distance. But the 310 00:16:08,400 --> 00:16:11,440 Speaker 1: distance between us and Jupiter isn't expanding because the Sun 311 00:16:11,560 --> 00:16:13,840 Speaker 1: is holding Jupiter in place, the same way that the 312 00:16:13,840 --> 00:16:16,320 Speaker 1: Earth is holding you in place, even though the space 313 00:16:16,360 --> 00:16:18,840 Speaker 1: between you and the Earth is expanding. Right. I guess 314 00:16:18,840 --> 00:16:21,040 Speaker 1: maybe a more concrete example would be like, let's say 315 00:16:21,040 --> 00:16:24,040 Speaker 1: I stretched the cable, a steel cable that doesn't stretch 316 00:16:24,120 --> 00:16:27,520 Speaker 1: between here and Jupiter. Right. I measure that the cable 317 00:16:27,560 --> 00:16:29,840 Speaker 1: to be a certain length, and as the space expands 318 00:16:29,920 --> 00:16:32,800 Speaker 1: between here and Jupiter, the cable is not stretching, right, 319 00:16:32,920 --> 00:16:36,360 Speaker 1: It's staying the same the length, right, Or the spa 320 00:16:36,520 --> 00:16:38,840 Speaker 1: the distance between its molecules is staying in the same. 321 00:16:38,960 --> 00:16:41,080 Speaker 1: So does that mean that the space that it's in 322 00:16:41,400 --> 00:16:45,080 Speaker 1: got thinner somehow or stretched, or they're just like more 323 00:16:45,120 --> 00:16:47,440 Speaker 1: space grew in it. That's right. The length of the 324 00:16:47,520 --> 00:16:50,800 Speaker 1: ruler is determined by the bonds of the stuff inside 325 00:16:50,840 --> 00:16:54,720 Speaker 1: the ruler, the atoms holding themselves together with those electrons, 326 00:16:55,040 --> 00:16:58,000 Speaker 1: so that doesn't change. You put that ruler out into 327 00:16:58,120 --> 00:17:00,840 Speaker 1: space and it's just floating there. The space around it 328 00:17:00,880 --> 00:17:03,480 Speaker 1: will expand you can still use that to define a distance. 329 00:17:03,520 --> 00:17:06,480 Speaker 1: You can say my ruler is one megaparsec long. But 330 00:17:06,480 --> 00:17:08,480 Speaker 1: if you started out with mirrors at either end of 331 00:17:08,520 --> 00:17:10,840 Speaker 1: your ruler, and then you waited a second, and the 332 00:17:10,920 --> 00:17:13,119 Speaker 1: mirrors would no longer be at either end of the 333 00:17:13,200 --> 00:17:16,600 Speaker 1: rulers because there's nothing holding those mirrors together. So space 334 00:17:16,680 --> 00:17:20,000 Speaker 1: is expanding between the mirrors, pushing them apart. So you 335 00:17:20,040 --> 00:17:23,000 Speaker 1: have this ruler which no longer covers the distance between 336 00:17:23,000 --> 00:17:25,359 Speaker 1: your two mirrors. Al right. Maybe an easier way to 337 00:17:25,400 --> 00:17:28,359 Speaker 1: ask my question is like, if space doubles in size 338 00:17:28,400 --> 00:17:31,359 Speaker 1: as we say it's doing, does that mean there's twice 339 00:17:31,400 --> 00:17:33,960 Speaker 1: as much space as there was before or is there 340 00:17:34,000 --> 00:17:37,080 Speaker 1: the same amount of space? It's just like stretched out dinner. 341 00:17:37,200 --> 00:17:39,560 Speaker 1: I say space doesn't get thinner, right, there's no like 342 00:17:39,640 --> 00:17:43,240 Speaker 1: density to space I would require measuring like with respect 343 00:17:43,240 --> 00:17:46,680 Speaker 1: to something else. This is an intrinsic expansion, which means 344 00:17:46,720 --> 00:17:49,600 Speaker 1: it just changes the relative distance. There's no meaning for 345 00:17:49,640 --> 00:17:52,119 Speaker 1: the density of space. So the first answer is the 346 00:17:52,119 --> 00:17:55,520 Speaker 1: more accurate description. There's more space between those two points. 347 00:17:55,680 --> 00:17:57,760 Speaker 1: Oh I see, So there's just more space popping up 348 00:17:57,800 --> 00:18:00,879 Speaker 1: everywhere in the universe, even between like the space of 349 00:18:00,960 --> 00:18:04,400 Speaker 1: my fingers. There's more space always popping up all the time. 350 00:18:04,560 --> 00:18:07,119 Speaker 1: That's right, And the expansion is actually really quite weak. 351 00:18:07,240 --> 00:18:10,920 Speaker 1: We're talking about seventy kilometers per second over three million 352 00:18:11,040 --> 00:18:14,399 Speaker 1: light years, so between your fingers that's really really tiny, 353 00:18:14,840 --> 00:18:17,800 Speaker 1: and almost anything is powerful enough to overcome it. Gravity, 354 00:18:17,960 --> 00:18:21,159 Speaker 1: the bonds in your fingers. It's also basically nothing for 355 00:18:21,200 --> 00:18:24,919 Speaker 1: our solar system, even for our galaxy, it's almost negligible. 356 00:18:25,200 --> 00:18:29,200 Speaker 1: But as distances get really really large, then it becomes overwhelming, 357 00:18:29,560 --> 00:18:33,359 Speaker 1: and between superclusters of galaxies it's the dominant thing, all right, 358 00:18:33,400 --> 00:18:34,960 Speaker 1: So maybe let's bring it down for people. Are you 359 00:18:35,000 --> 00:18:40,320 Speaker 1: saying over three hundred million kilometers over three million light years, 360 00:18:40,760 --> 00:18:44,840 Speaker 1: over three million light years Every second that distance grows 361 00:18:44,880 --> 00:18:48,520 Speaker 1: by seventy kilometers, So right now it's three million light years, 362 00:18:49,240 --> 00:18:53,679 Speaker 1: and then and now it's three million and seventy kilometers long, 363 00:18:53,760 --> 00:18:56,600 Speaker 1: and now it's three million and a hundred and forty kilometers. 364 00:18:56,600 --> 00:18:58,359 Speaker 1: Is that what kind of what you mean, yes, exactly, 365 00:18:58,520 --> 00:19:01,520 Speaker 1: So every second, each mega parts that grows fractionally by 366 00:19:01,520 --> 00:19:05,239 Speaker 1: a tiny, tiny, tiny amount. It's one point zero zero 367 00:19:05,359 --> 00:19:08,480 Speaker 1: zero zero zero zero zero zero zero zero zero zero 368 00:19:08,600 --> 00:19:12,159 Speaker 1: two megaparsex after a second. That's really tiny, because the 369 00:19:13,080 --> 00:19:15,760 Speaker 1: Milky Ways is not even a megaparsic, right, it's like 370 00:19:15,960 --> 00:19:19,000 Speaker 1: a hundred thousand light year exactly. It's a tiny fraction 371 00:19:19,119 --> 00:19:22,439 Speaker 1: of a megaparsec. Okay, but you're saying that over large 372 00:19:22,520 --> 00:19:26,879 Speaker 1: distances like businesses between galaxies and the size of these superclusters, 373 00:19:27,240 --> 00:19:29,560 Speaker 1: it starts to get pretty significant, right, It starts to 374 00:19:29,560 --> 00:19:33,000 Speaker 1: get pretty significant. And since the universe is really really big, 375 00:19:33,520 --> 00:19:36,200 Speaker 1: many many millions and billions of light years wide, and 376 00:19:36,240 --> 00:19:38,520 Speaker 1: if you go far enough away, this is really significant. 377 00:19:38,840 --> 00:19:41,000 Speaker 1: And the rate at which things are moving away from 378 00:19:41,119 --> 00:19:44,359 Speaker 1: us because of this expansion starts to get very very large. 379 00:19:44,600 --> 00:19:46,240 Speaker 1: So you can measure this velocity. We call it the 380 00:19:46,320 --> 00:19:49,760 Speaker 1: recession velocity. How fast is something moving away from us 381 00:19:49,840 --> 00:19:51,959 Speaker 1: because of this expansion and you go to the other 382 00:19:52,000 --> 00:19:54,560 Speaker 1: side of the Milky Way, the recession velocity is tiny, right, 383 00:19:54,840 --> 00:19:57,600 Speaker 1: if you go to the next galaxy, it's not tiny. 384 00:19:57,800 --> 00:20:00,359 Speaker 1: If you go really, really far away then starts to 385 00:20:00,400 --> 00:20:02,920 Speaker 1: be large, and if you go far enough away, it 386 00:20:03,000 --> 00:20:05,760 Speaker 1: actually gets to be larger than the speed of light. 387 00:20:06,960 --> 00:20:10,520 Speaker 1: So things are technically moving faster away from us than 388 00:20:10,560 --> 00:20:13,119 Speaker 1: the speed of light, which is the big question in 389 00:20:13,280 --> 00:20:17,400 Speaker 1: this episode. So let's get into that actually really means 390 00:20:17,480 --> 00:20:19,720 Speaker 1: and how we can make sense of it. But first 391 00:20:19,840 --> 00:20:35,480 Speaker 1: let's take a quick break. All right, we're talking megaparsex 392 00:20:35,760 --> 00:20:39,159 Speaker 1: and inflation and recession, which just a reminder, this is 393 00:20:39,200 --> 00:20:44,200 Speaker 1: not a financial podcast, not yet, at least not until 394 00:20:44,240 --> 00:20:48,680 Speaker 1: it blows up, until it crashes. We are talking about 395 00:20:48,720 --> 00:20:51,879 Speaker 1: bubbles though, right, Yeah, exactly. We are talking about spheres 396 00:20:52,040 --> 00:20:54,440 Speaker 1: in the universe. And there's lots of ways that we 397 00:20:54,560 --> 00:20:56,800 Speaker 1: like to think about spheres and put ourselves at the 398 00:20:56,920 --> 00:20:59,959 Speaker 1: center of them, and we tend to cosmologically to think 399 00:21:00,040 --> 00:21:02,119 Speaker 1: that's a mistake there. That's not the center of anything, 400 00:21:02,240 --> 00:21:05,399 Speaker 1: but it is at the center of what we can observe, 401 00:21:05,920 --> 00:21:08,000 Speaker 1: and so in some senses, it does make sense to 402 00:21:08,080 --> 00:21:11,120 Speaker 1: build a universe with us right in the middle of it. Yeah, 403 00:21:11,160 --> 00:21:14,560 Speaker 1: and there's also a lot of speculation, uh, in these theories. 404 00:21:14,720 --> 00:21:17,600 Speaker 1: So let's get into this idea then, of things moving 405 00:21:17,640 --> 00:21:20,399 Speaker 1: away from us faster than the speed of light. So 406 00:21:20,520 --> 00:21:22,959 Speaker 1: you're saying that the universe is expanding a little bit 407 00:21:23,080 --> 00:21:26,639 Speaker 1: at a time. You each spot everywhere in it, and 408 00:21:26,720 --> 00:21:31,200 Speaker 1: so over long distances things are the distance between here 409 00:21:31,240 --> 00:21:35,359 Speaker 1: and that thing that's out there is growing faster than 410 00:21:35,400 --> 00:21:37,880 Speaker 1: the speed of light. Yeah, this is Hubble's basic discovery. 411 00:21:37,960 --> 00:21:40,200 Speaker 1: He was looking at stuff that was further and further 412 00:21:40,280 --> 00:21:44,520 Speaker 1: away and measuring its velocity, and he found this linear relationship. 413 00:21:45,160 --> 00:21:47,600 Speaker 1: Things that are further away move away from us faster. 414 00:21:47,880 --> 00:21:50,760 Speaker 1: If you plot velocity versus distance, you get a straight 415 00:21:50,840 --> 00:21:53,720 Speaker 1: line and you can just keep going further and further 416 00:21:53,840 --> 00:21:57,639 Speaker 1: distances faster and faster velocities away from us. So this 417 00:21:57,760 --> 00:22:00,600 Speaker 1: was Hubble's big discovery, like more than years ago, now 418 00:22:00,840 --> 00:22:02,760 Speaker 1: that things are moving away from us at in this 419 00:22:02,960 --> 00:22:05,960 Speaker 1: linear relationship. And that's the Hubble parameter. We used to 420 00:22:06,000 --> 00:22:08,560 Speaker 1: call it the Hubble constant, except it's not really constant, 421 00:22:08,600 --> 00:22:10,440 Speaker 1: so now we call it the Hubble parameter. It's a 422 00:22:10,520 --> 00:22:14,440 Speaker 1: relationship between the distance and the speed the slope of 423 00:22:14,600 --> 00:22:17,639 Speaker 1: that line. If you go far enough away, those velocities 424 00:22:17,680 --> 00:22:20,760 Speaker 1: start to get greater than the speed of light. And 425 00:22:20,840 --> 00:22:23,080 Speaker 1: so that's the basic conundament is how can something be 426 00:22:23,240 --> 00:22:26,000 Speaker 1: moving away from us faster than the speed of light, 427 00:22:26,119 --> 00:22:28,480 Speaker 1: And I guess maybe the isn't the answer there in 428 00:22:28,560 --> 00:22:31,199 Speaker 1: that world, like, it's not technically moving away from us, right, 429 00:22:31,280 --> 00:22:34,359 Speaker 1: it's just sitting there, but there's more space growing between 430 00:22:34,480 --> 00:22:36,600 Speaker 1: us and them? Is it technically moving when you can 431 00:22:36,640 --> 00:22:40,520 Speaker 1: measure It all comes down to how you measure velocities. Right, 432 00:22:40,600 --> 00:22:42,720 Speaker 1: If you take a really big ruler and you measure 433 00:22:42,760 --> 00:22:44,879 Speaker 1: the distance from here to there, and then you do 434 00:22:44,960 --> 00:22:48,040 Speaker 1: it again in ten seconds, you can measure velocity, and 435 00:22:48,119 --> 00:22:51,280 Speaker 1: that velocity does appear to exceed the speed of light, 436 00:22:51,560 --> 00:22:54,600 Speaker 1: and so from that perspective, that does break that rule. Well, 437 00:22:54,600 --> 00:22:56,800 Speaker 1: I guess it's where it all gets relative, because I 438 00:22:56,840 --> 00:22:58,960 Speaker 1: guess the question is is that thing out there in 439 00:22:59,040 --> 00:23:02,880 Speaker 1: space actually moving within this space it's around? Like, gets 440 00:23:02,920 --> 00:23:05,680 Speaker 1: it technically moving? Or like, let's say we have something 441 00:23:05,800 --> 00:23:08,320 Speaker 1: that is maybe right next to me, and over trillions 442 00:23:08,359 --> 00:23:11,119 Speaker 1: of years, the space between me and it expands, and 443 00:23:11,280 --> 00:23:13,560 Speaker 1: so it gets further and further out, maybe to the 444 00:23:13,560 --> 00:23:15,720 Speaker 1: point where distance is growing fast in the speed of light. 445 00:23:15,760 --> 00:23:18,840 Speaker 1: But did that thing ever experience any acceleration? Right? So 446 00:23:18,920 --> 00:23:22,280 Speaker 1: let's talk about what special relativity actually allows and what 447 00:23:22,440 --> 00:23:26,800 Speaker 1: it doesn't allow. Right. Special relativity says that nobody can 448 00:23:26,880 --> 00:23:30,080 Speaker 1: measure a relative velocity to be greater than the speed 449 00:23:30,160 --> 00:23:35,040 Speaker 1: of light in their local inertial frame. Okay, and those 450 00:23:35,160 --> 00:23:38,399 Speaker 1: are the loopholes. We're going to drive this huge expanding 451 00:23:38,640 --> 00:23:43,399 Speaker 1: universe through local inertial frame. Right. What that means is 452 00:23:43,480 --> 00:23:45,440 Speaker 1: that you define a frame of reference, you define or 453 00:23:45,520 --> 00:23:47,920 Speaker 1: and you say, I'm here at the core of my X, 454 00:23:48,040 --> 00:23:50,440 Speaker 1: y Z, and I'm the one making measurements, and I'm 455 00:23:50,440 --> 00:23:53,440 Speaker 1: not accelerating with respect to this frame. I'm just sitting 456 00:23:53,520 --> 00:23:56,160 Speaker 1: here at the origin and I'm making a bunch of measurements. 457 00:23:56,240 --> 00:23:58,600 Speaker 1: The other key thing is the word local. Right. In 458 00:23:58,680 --> 00:24:01,320 Speaker 1: special relativity, we can we make measurements of things that 459 00:24:01,400 --> 00:24:03,960 Speaker 1: are nearby. We say that the speed of light never 460 00:24:04,200 --> 00:24:07,639 Speaker 1: exceeds see when it goes near you. Things that are 461 00:24:07,720 --> 00:24:10,440 Speaker 1: much further away are not in your local inertial frame, 462 00:24:10,520 --> 00:24:13,040 Speaker 1: and so the rules of special relativity don't apply. That's 463 00:24:13,040 --> 00:24:15,800 Speaker 1: when you need to apply general relativity for things that 464 00:24:15,880 --> 00:24:18,200 Speaker 1: are not in your inertial frame, like when the universe 465 00:24:18,359 --> 00:24:21,480 Speaker 1: is curved or expanding, or when things are really really 466 00:24:21,560 --> 00:24:23,719 Speaker 1: far away. What are you saying that the rules are 467 00:24:23,760 --> 00:24:26,760 Speaker 1: different depending on where you are in the universe, or 468 00:24:26,960 --> 00:24:30,440 Speaker 1: that our theories don't always work the same way in 469 00:24:30,520 --> 00:24:32,560 Speaker 1: different parts of the universe. The rules are the same 470 00:24:32,760 --> 00:24:36,440 Speaker 1: everywhere in the universe. A special relativity only applies to 471 00:24:36,520 --> 00:24:39,560 Speaker 1: the things you can see near you. It can't describe 472 00:24:39,600 --> 00:24:43,399 Speaker 1: the whole universe because there is no global inertial frame 473 00:24:43,600 --> 00:24:47,000 Speaker 1: for the universe. Like the universe has wiggles and curves 474 00:24:47,040 --> 00:24:49,800 Speaker 1: and all sorts of stuff, and that breaks special relativity. 475 00:24:49,840 --> 00:24:53,200 Speaker 1: You can't have a global inertial frame for the whole universe. 476 00:24:53,480 --> 00:24:55,960 Speaker 1: You have to use general relativity, which gives you another 477 00:24:56,000 --> 00:24:58,000 Speaker 1: way to look at the universe and make these measurements 478 00:24:58,080 --> 00:25:00,240 Speaker 1: and fitted it all together. You're saying, that's the pull. 479 00:25:00,440 --> 00:25:01,879 Speaker 1: What do you mean by that? So, one reason we 480 00:25:01,920 --> 00:25:04,080 Speaker 1: can measure these galaxies to be moving away from us 481 00:25:04,359 --> 00:25:06,399 Speaker 1: faster than the speed of light is that they are 482 00:25:06,520 --> 00:25:09,920 Speaker 1: not in our inertial frame of reference. Right, It doesn't 483 00:25:09,920 --> 00:25:13,760 Speaker 1: really make physical sense to measure their velocity relative to 484 00:25:14,080 --> 00:25:16,520 Speaker 1: us because we're not in the same frame. It's like 485 00:25:16,560 --> 00:25:18,760 Speaker 1: a frame over here near us, and there's a frame 486 00:25:18,880 --> 00:25:21,639 Speaker 1: over there where they are. But we can't really compare 487 00:25:21,760 --> 00:25:25,080 Speaker 1: velocities in hours to theirs general relativity tells us there's 488 00:25:25,119 --> 00:25:28,480 Speaker 1: no global frame. We don't know how to make those comparisons. 489 00:25:28,560 --> 00:25:32,200 Speaker 1: In fact, the definition of velocity and general relativity is 490 00:25:32,280 --> 00:25:34,600 Speaker 1: a little bit fuzzy for things that are far away 491 00:25:34,640 --> 00:25:36,120 Speaker 1: from you. Well, I feel like I'm getting a little 492 00:25:36,119 --> 00:25:39,000 Speaker 1: bit confused here because of this difference between things moving 493 00:25:39,240 --> 00:25:43,840 Speaker 1: and space expanding. Whether it's something in expanding space is moving, 494 00:25:43,960 --> 00:25:46,280 Speaker 1: and I feel like maybe you're saying it's the same thing, 495 00:25:46,359 --> 00:25:48,440 Speaker 1: but I guess it's not quite clicking in my mind. 496 00:25:48,600 --> 00:25:50,440 Speaker 1: So maybe let's go back to that example of saying, like, 497 00:25:50,520 --> 00:25:53,080 Speaker 1: if I start with something next to me, and over 498 00:25:53,200 --> 00:25:56,080 Speaker 1: trillions of years, the space between me and it expands 499 00:25:56,320 --> 00:25:58,040 Speaker 1: so that to the point where it's now that thing 500 00:25:58,160 --> 00:26:00,240 Speaker 1: is super far away from me, and maybe even that 501 00:26:00,359 --> 00:26:02,680 Speaker 1: the distance between me and it growing faster than the 502 00:26:02,680 --> 00:26:05,960 Speaker 1: speed of light. Is that thing technically moving away from me? 503 00:26:06,560 --> 00:26:09,320 Speaker 1: Or is it just sitting in space and the space 504 00:26:09,440 --> 00:26:11,240 Speaker 1: is growing around it? You know what I mean? Like, 505 00:26:11,359 --> 00:26:14,440 Speaker 1: did anything ever push that thing? Did that thing or 506 00:26:14,520 --> 00:26:17,640 Speaker 1: me experience acceleration? And if not, then maybe may it's 507 00:26:17,680 --> 00:26:19,840 Speaker 1: not moving. It's maybe it's just sitting there and space 508 00:26:19,960 --> 00:26:23,320 Speaker 1: is growing around it. Here's the technically accurate and totally unsatisfying. 509 00:26:23,320 --> 00:26:27,679 Speaker 1: Answer is that both pictures are accurate. Right in one picture, 510 00:26:28,040 --> 00:26:30,760 Speaker 1: the thing is moving away from you, it has velocity 511 00:26:30,880 --> 00:26:32,760 Speaker 1: relative to you, and the other picture, there is no 512 00:26:32,840 --> 00:26:36,080 Speaker 1: relative velocities. There's just the expansion of space. Everything is 513 00:26:36,119 --> 00:26:39,760 Speaker 1: just floating, motionless, and space between them is expanding. Those 514 00:26:39,800 --> 00:26:42,399 Speaker 1: two pictures are actually equivalent. And what I meant a 515 00:26:42,480 --> 00:26:44,639 Speaker 1: moment ago when I said that in general relativity we 516 00:26:44,680 --> 00:26:47,760 Speaker 1: don't have a well defined meaning of velocity is exactly 517 00:26:47,840 --> 00:26:50,800 Speaker 1: that that both of those pictures can describe what we see, 518 00:26:50,880 --> 00:26:54,000 Speaker 1: even though physically they sound very different. So let's step 519 00:26:54,040 --> 00:26:56,479 Speaker 1: through them one at a time. Right, In one picture, 520 00:26:56,920 --> 00:27:00,000 Speaker 1: we talk about the distance between us and some other objects, 521 00:27:00,440 --> 00:27:02,520 Speaker 1: and that means it like if we could freeze time 522 00:27:02,720 --> 00:27:06,280 Speaker 1: and somehow take a huge ruler and measure the distance 523 00:27:06,520 --> 00:27:09,040 Speaker 1: to this distant object, and we could write it down 524 00:27:09,080 --> 00:27:11,000 Speaker 1: on our piece of paper, and then we could wait 525 00:27:11,119 --> 00:27:13,520 Speaker 1: a minute or a year and do it again. We 526 00:27:13,560 --> 00:27:16,120 Speaker 1: could use that to measure the velocities. Now, no, that's 527 00:27:16,160 --> 00:27:18,720 Speaker 1: not really physical, right, because you have to freeze the 528 00:27:18,840 --> 00:27:22,000 Speaker 1: universe and like trust out your ruler and take this measurement. 529 00:27:22,040 --> 00:27:25,119 Speaker 1: That's not possible because things are expanding as you're doing it. 530 00:27:25,600 --> 00:27:28,360 Speaker 1: So in some sense, even the definition of distance there 531 00:27:28,440 --> 00:27:30,399 Speaker 1: doesn't really make sense. But that's sort of like our 532 00:27:30,480 --> 00:27:34,560 Speaker 1: intuitive idea of distance. So from that perspective, this thing 533 00:27:34,720 --> 00:27:38,040 Speaker 1: really is moving away from you faster and faster every year. 534 00:27:38,119 --> 00:27:41,520 Speaker 1: That velocity you measure is larger every year, And I 535 00:27:41,520 --> 00:27:43,560 Speaker 1: think what you're saying is that, like, maybe initially, if 536 00:27:43,560 --> 00:27:45,520 Speaker 1: it's right next to me, I'm going to measure that 537 00:27:45,640 --> 00:27:48,200 Speaker 1: it has zero velocity because it's just sitting next to me. 538 00:27:48,440 --> 00:27:50,639 Speaker 1: But maybe next year, when it's further away from me, 539 00:27:50,800 --> 00:27:52,880 Speaker 1: I'm going to start measuring, like, hey, now it's further 540 00:27:52,920 --> 00:27:55,399 Speaker 1: away from me, the distance grew, and I can divide 541 00:27:55,440 --> 00:27:57,359 Speaker 1: that by the time that passed and say, oh, now 542 00:27:57,440 --> 00:27:59,800 Speaker 1: it has a velocity. So you're saying that that's true, 543 00:27:59,840 --> 00:28:02,280 Speaker 1: even though nothing pushed it, even though it didn't feel 544 00:28:02,320 --> 00:28:06,440 Speaker 1: any acceleration, it's velocity grew between last year and today, 545 00:28:06,680 --> 00:28:09,840 Speaker 1: exactly because the universe is expanding, right, and so it's 546 00:28:10,040 --> 00:28:12,480 Speaker 1: moving these things away from each other. It's increasing the 547 00:28:12,560 --> 00:28:15,480 Speaker 1: relative velocity of these things, and you can measure that 548 00:28:15,560 --> 00:28:18,040 Speaker 1: physically also in the red shift, right, things that have 549 00:28:18,119 --> 00:28:20,760 Speaker 1: a relative velocity give you a red shift. The photons 550 00:28:20,840 --> 00:28:23,639 Speaker 1: from these things are red shifted. It starts out a 551 00:28:23,720 --> 00:28:26,399 Speaker 1: certain frequency. By the time it gets to you, it's redder, 552 00:28:26,760 --> 00:28:28,919 Speaker 1: and we use that to infer the velocity of these 553 00:28:29,000 --> 00:28:31,600 Speaker 1: things all the time. Right. So that's one picture, right, 554 00:28:31,640 --> 00:28:34,840 Speaker 1: But the thing itself didn't never felt any exceleration, right, Like, 555 00:28:34,880 --> 00:28:36,800 Speaker 1: if I was sitting in that rock that's moving away 556 00:28:36,840 --> 00:28:39,840 Speaker 1: from me, I would never feel like anything happened, right, Like, 557 00:28:39,880 --> 00:28:42,280 Speaker 1: I wouldn't feel pressed against the wall, or I wouldn't 558 00:28:42,560 --> 00:28:44,560 Speaker 1: feel like somebody pushed me. I would just be sitting there, 559 00:28:44,600 --> 00:28:48,080 Speaker 1: and my velocity the relative to me, it's just magically, magically, 560 00:28:48,080 --> 00:28:51,320 Speaker 1: it's just growing. It's like free velocity. Yes, it's free velocity. 561 00:28:51,320 --> 00:28:54,120 Speaker 1: The universe is just creating this. Right. And the other picture, 562 00:28:54,280 --> 00:28:56,440 Speaker 1: which is a bit more physical, though a little harder 563 00:28:56,520 --> 00:28:58,640 Speaker 1: to think about, is think about each of these objects 564 00:28:58,680 --> 00:29:01,240 Speaker 1: in their own frame, whether each at rest, right, and 565 00:29:01,280 --> 00:29:04,640 Speaker 1: now these frames have space expanding in between them. So 566 00:29:04,760 --> 00:29:07,240 Speaker 1: from that point of view, right, everything was at rest. 567 00:29:07,440 --> 00:29:09,680 Speaker 1: Everything is still at rest, but space is just sort 568 00:29:09,680 --> 00:29:12,960 Speaker 1: of like expanded between them. There's no velocity, it's just 569 00:29:13,120 --> 00:29:16,080 Speaker 1: that space is expanding. So this is another frame. It's 570 00:29:16,120 --> 00:29:18,600 Speaker 1: like it requires you to have a coordinate system that's 571 00:29:18,680 --> 00:29:22,920 Speaker 1: growing with the universe, like stretch the ruler as the 572 00:29:23,120 --> 00:29:27,160 Speaker 1: universe grows. These are called co moving distances, if you 573 00:29:27,200 --> 00:29:29,240 Speaker 1: want to google it and read more about it. And 574 00:29:29,320 --> 00:29:31,680 Speaker 1: so in this picture there is no velocity. It's just 575 00:29:31,880 --> 00:29:34,520 Speaker 1: that space itself is expanding. I think that's what I 576 00:29:34,640 --> 00:29:37,240 Speaker 1: was asking earlier. It's like in this other picture, and 577 00:29:37,280 --> 00:29:41,320 Speaker 1: now a kilometer is actually kind of longer, right, technically 578 00:29:41,680 --> 00:29:44,360 Speaker 1: a kilometer is now longer. And you can wonder, like 579 00:29:44,520 --> 00:29:46,200 Speaker 1: what about the red shift? The red shift is something 580 00:29:46,280 --> 00:29:48,800 Speaker 1: I see. It shouldn't depend on like what a physicist 581 00:29:48,920 --> 00:29:51,520 Speaker 1: is thinking about, whether they're thinking about two different frames 582 00:29:51,600 --> 00:29:53,720 Speaker 1: with the one big frame. It shouldn't depend on that. 583 00:29:54,160 --> 00:29:56,480 Speaker 1: And the red shift that these two pictures predict is 584 00:29:56,520 --> 00:29:58,640 Speaker 1: the same. And the first one, the red shift comes 585 00:29:58,640 --> 00:30:01,160 Speaker 1: from velocity in the set, second one, it comes from 586 00:30:01,160 --> 00:30:04,600 Speaker 1: the expansion of the universe, which also lengthens the wavelength 587 00:30:04,680 --> 00:30:06,800 Speaker 1: of the light. So in the second picture, where there 588 00:30:06,840 --> 00:30:10,240 Speaker 1: are two frames and spaces expanding between them, then the 589 00:30:10,280 --> 00:30:12,960 Speaker 1: photon that leaves one galaxy is stretched out by the 590 00:30:13,080 --> 00:30:16,640 Speaker 1: expansion of the universe, not by the relative velocity because 591 00:30:16,680 --> 00:30:18,959 Speaker 1: there is none in that second picture, and I think, 592 00:30:19,000 --> 00:30:20,920 Speaker 1: how could both of these things be true? Well, these 593 00:30:20,960 --> 00:30:23,760 Speaker 1: are two different pictures of the universe, and what general 594 00:30:23,800 --> 00:30:26,800 Speaker 1: relativity tells us is that there's no unambiguous way to 595 00:30:26,920 --> 00:30:30,320 Speaker 1: decide between these two pictures. That's why relative velocity for 596 00:30:30,480 --> 00:30:34,120 Speaker 1: very distant objects is not well defined in general relativity. Well, 597 00:30:34,160 --> 00:30:36,360 Speaker 1: I guess maybe the maybe the difference. I wonder if 598 00:30:36,400 --> 00:30:38,840 Speaker 1: the difference between the two pictures is that in one 599 00:30:38,920 --> 00:30:42,160 Speaker 1: of them, the speed of light is technically staying the same, 600 00:30:42,200 --> 00:30:43,960 Speaker 1: but in the other one, I feel like the speed 601 00:30:44,000 --> 00:30:46,920 Speaker 1: of light is kind of getting diluted, right a little bit, 602 00:30:46,960 --> 00:30:50,000 Speaker 1: because if a kilometer grows, but the time it takes 603 00:30:50,120 --> 00:30:53,440 Speaker 1: light to grow to that kilometer is no longer than 604 00:30:54,000 --> 00:30:56,880 Speaker 1: or shorter than that, somehow the speed of light went down. Well, 605 00:30:56,920 --> 00:30:58,920 Speaker 1: the speed of light is the standard, right, there's no 606 00:30:59,080 --> 00:31:01,800 Speaker 1: other metric, So the time it takes light to go 607 00:31:02,000 --> 00:31:05,080 Speaker 1: between the galaxies definitely increases. And they used to be 608 00:31:05,120 --> 00:31:07,520 Speaker 1: a kilometer apart. Now we just say that there are 609 00:31:07,600 --> 00:31:10,360 Speaker 1: five kilometers apart, and that's because it takes light longer. 610 00:31:10,560 --> 00:31:13,360 Speaker 1: Light is the way that we measure these distances, after all, 611 00:31:13,400 --> 00:31:16,640 Speaker 1: there's no other external metric, right, that we can use 612 00:31:16,800 --> 00:31:19,320 Speaker 1: to measure the distance between things other than the time 613 00:31:19,360 --> 00:31:21,440 Speaker 1: it takes light to go between them. I thought you 614 00:31:21,440 --> 00:31:24,080 Speaker 1: said that in the second scenario, a kilometer is now longer. 615 00:31:24,240 --> 00:31:27,080 Speaker 1: The kilometer space is now two kilometers of space. It's 616 00:31:27,120 --> 00:31:30,520 Speaker 1: not like the kilometer itself is getting diluted in some way. Right, 617 00:31:30,560 --> 00:31:33,000 Speaker 1: you have more space. The space and that's out there 618 00:31:33,120 --> 00:31:35,480 Speaker 1: is now longer, so it takes longer for light to 619 00:31:35,520 --> 00:31:38,280 Speaker 1: go through it. Right, So it would still be a kilometer, 620 00:31:38,400 --> 00:31:40,480 Speaker 1: It just takes light longer to go through it, which 621 00:31:40,520 --> 00:31:42,920 Speaker 1: means the speed of light technically went down. No, no, 622 00:31:43,040 --> 00:31:45,720 Speaker 1: it's not still a kilometer, right, it's more space now. 623 00:31:46,040 --> 00:31:48,000 Speaker 1: But I thought we said we stretched the kilometer like 624 00:31:48,520 --> 00:31:51,520 Speaker 1: the kilometer stretched. Oh, we don't stretch the definition of 625 00:31:51,600 --> 00:31:54,440 Speaker 1: a kilometer, right, that's comes from the speed of light. 626 00:31:54,480 --> 00:31:56,320 Speaker 1: We just stretched the space which used to be one 627 00:31:56,400 --> 00:31:59,680 Speaker 1: kilometer of space, and now it's a bigger serving of space. 628 00:32:00,000 --> 00:32:03,360 Speaker 1: We don't change the definition of the kilometer. Space itself expands, 629 00:32:03,640 --> 00:32:07,200 Speaker 1: so the kilometer stace is same now there's just more kilometers. 630 00:32:07,360 --> 00:32:09,400 Speaker 1: We define the kilometer in the same way, right, it's 631 00:32:09,520 --> 00:32:12,000 Speaker 1: how far light goes in a certain amount of time, right, 632 00:32:12,600 --> 00:32:14,520 Speaker 1: so we don't change the definition of the kilometer. I 633 00:32:14,560 --> 00:32:16,400 Speaker 1: guess we can go around the circles. But I think 634 00:32:16,440 --> 00:32:19,640 Speaker 1: the main point is that the universe is expanding and 635 00:32:20,360 --> 00:32:22,400 Speaker 1: the stuff that's really far aways moving faster than the 636 00:32:22,440 --> 00:32:24,480 Speaker 1: speed of light. The universe really is expanding, and it 637 00:32:24,600 --> 00:32:27,520 Speaker 1: really is creating space between us. And if you try 638 00:32:27,640 --> 00:32:30,440 Speaker 1: to think about relative distances in terms of like one 639 00:32:30,760 --> 00:32:33,240 Speaker 1: mega ruler where you're gonna measure these distances, then you're 640 00:32:33,240 --> 00:32:35,840 Speaker 1: sort of falling into one of the loopholes of special relativity, 641 00:32:36,080 --> 00:32:38,360 Speaker 1: which says that that's not a meaningful physical thing. You 642 00:32:38,400 --> 00:32:41,160 Speaker 1: could never actually measure that distance. Like if you're looking 643 00:32:41,200 --> 00:32:43,680 Speaker 1: at a galaxy that's billions and billions of light years away, 644 00:32:43,720 --> 00:32:45,960 Speaker 1: you can never actually make the measurement we're talking about 645 00:32:45,960 --> 00:32:48,000 Speaker 1: where you try out a ruler and measure the distance 646 00:32:48,240 --> 00:32:50,480 Speaker 1: and then wait and measure the other one. Because space 647 00:32:50,560 --> 00:32:52,440 Speaker 1: is expanding at the same time, it requires you to 648 00:32:52,520 --> 00:32:55,640 Speaker 1: like artificially freeze the universe. So those things are not 649 00:32:55,880 --> 00:32:59,360 Speaker 1: in our local inertial reference frame, so special relativity doesn't 650 00:32:59,400 --> 00:33:02,160 Speaker 1: really apply. So it's possible for these recession velocities to 651 00:33:02,240 --> 00:33:04,920 Speaker 1: be faster than the speed of light without breaking special 652 00:33:05,000 --> 00:33:07,479 Speaker 1: relativity because it doesn't really apply to things that are 653 00:33:07,520 --> 00:33:10,240 Speaker 1: not in our inertial frame, right. I think you've always 654 00:33:10,280 --> 00:33:13,360 Speaker 1: phrased it as it's not possible to move through space 655 00:33:13,480 --> 00:33:15,840 Speaker 1: faster than the speed of light, but it is possible 656 00:33:16,280 --> 00:33:19,000 Speaker 1: to make space faster than the speed of light, which 657 00:33:19,040 --> 00:33:21,240 Speaker 1: I think usually explains it. But here I feel like 658 00:33:21,320 --> 00:33:24,239 Speaker 1: we're trying to be more specific about and say there, 659 00:33:24,240 --> 00:33:26,440 Speaker 1: it's basically the same thing. Kind of, It's like it's 660 00:33:26,480 --> 00:33:28,560 Speaker 1: the same thing, and so you can go faster than 661 00:33:28,560 --> 00:33:30,360 Speaker 1: the speed of light. There are two different ways to 662 00:33:30,400 --> 00:33:33,080 Speaker 1: look at it in general relativity, yeah, and I think 663 00:33:33,120 --> 00:33:35,960 Speaker 1: the more intuitive way is the second with co moving 664 00:33:36,000 --> 00:33:38,479 Speaker 1: way to say, the universe is expanding. Everything is at 665 00:33:38,560 --> 00:33:41,440 Speaker 1: rest in its local frame, and the universe is expanding 666 00:33:41,560 --> 00:33:44,240 Speaker 1: between those two things, and there's no limit to how 667 00:33:44,320 --> 00:33:47,200 Speaker 1: fast the universe can expand. And interestingly, that means that 668 00:33:47,280 --> 00:33:49,840 Speaker 1: there are things out there that are moving away from us, 669 00:33:50,120 --> 00:33:52,960 Speaker 1: apparently faster than the speed of light. Means that a 670 00:33:53,040 --> 00:33:57,959 Speaker 1: photon leaving that galaxy is not getting closer to us, right, 671 00:33:58,040 --> 00:34:01,040 Speaker 1: It's moving through its local space, but that local space 672 00:34:01,120 --> 00:34:03,760 Speaker 1: is expanding away from us faster than the speed of light. 673 00:34:03,920 --> 00:34:06,760 Speaker 1: So as time goes on, the photon is not getting 674 00:34:06,800 --> 00:34:09,520 Speaker 1: closer to us, even though it's moving through its local 675 00:34:09,560 --> 00:34:12,360 Speaker 1: space at the speed of light. Right, the distance between 676 00:34:12,440 --> 00:34:14,640 Speaker 1: us and that photon is not decreasing. What do you 677 00:34:14,680 --> 00:34:17,120 Speaker 1: mean the distance is not changing because it is changing, 678 00:34:17,160 --> 00:34:19,160 Speaker 1: isn't it. So the photon is emitted by that very 679 00:34:19,239 --> 00:34:21,840 Speaker 1: distant galaxy which is moving away from us faster than 680 00:34:21,880 --> 00:34:24,840 Speaker 1: the speed of light. Right or equivalently, the space between 681 00:34:25,000 --> 00:34:27,840 Speaker 1: us and it's local spaces expanding faster than the speed 682 00:34:27,880 --> 00:34:30,920 Speaker 1: of light. Right now, those photons are moving towards us 683 00:34:31,360 --> 00:34:34,279 Speaker 1: in their local space, but the distance between us and 684 00:34:34,360 --> 00:34:38,160 Speaker 1: those photons is not decreasing because space is expanding between 685 00:34:38,360 --> 00:34:40,640 Speaker 1: us and that photon. So if a photon is moving 686 00:34:40,680 --> 00:34:43,920 Speaker 1: through space which is super luminately receding, then it's not 687 00:34:44,080 --> 00:34:46,960 Speaker 1: making progress towards us. So some things that are super 688 00:34:47,080 --> 00:34:49,480 Speaker 1: far away that are moving away from us faster than 689 00:34:49,520 --> 00:34:52,319 Speaker 1: the speed of light, their photons will never reach us 690 00:34:52,440 --> 00:34:55,520 Speaker 1: no matter how long you give them, because space between 691 00:34:55,640 --> 00:34:58,839 Speaker 1: us and them is expanding faster than the speed of light. Right, Yeah, 692 00:34:58,880 --> 00:35:00,719 Speaker 1: I remember we talked about the before. It's sort of 693 00:35:00,760 --> 00:35:03,360 Speaker 1: like if someone is zooming away from you on a 694 00:35:03,520 --> 00:35:06,360 Speaker 1: spaceship and they, you know, shoot a nerf gun at you, 695 00:35:06,560 --> 00:35:08,759 Speaker 1: that nerve bullet is never going to get to you 696 00:35:08,840 --> 00:35:11,080 Speaker 1: because it's not being shot at you faster than then 697 00:35:11,160 --> 00:35:13,160 Speaker 1: the rocket is moving away from you. That's right, And 698 00:35:13,239 --> 00:35:15,960 Speaker 1: that's for like a Galilean transformation. Right, we're just linearly 699 00:35:16,000 --> 00:35:18,719 Speaker 1: adding the velocities. Light is weird and different though, and 700 00:35:18,840 --> 00:35:21,800 Speaker 1: light always moves at the speed of light. So somebody 701 00:35:21,840 --> 00:35:24,200 Speaker 1: in our local frame who's moving away from you, if 702 00:35:24,239 --> 00:35:26,560 Speaker 1: they shine a flashlight at you, that light still moves 703 00:35:26,600 --> 00:35:29,239 Speaker 1: towards you at the speed of light. Doesn't matter how 704 00:35:29,360 --> 00:35:32,480 Speaker 1: fast they are going, so it's not their relative velocity. There. 705 00:35:32,800 --> 00:35:36,600 Speaker 1: Here's the expansion of space, right, And interestingly, there's some 706 00:35:36,680 --> 00:35:38,399 Speaker 1: things that are so far away from us that their 707 00:35:38,480 --> 00:35:41,400 Speaker 1: light will never reach us. There are other things that 708 00:35:41,520 --> 00:35:43,880 Speaker 1: have always been moving away from us faster than the 709 00:35:43,880 --> 00:35:47,239 Speaker 1: speed of light, but their photons do eventually reach us. 710 00:35:47,640 --> 00:35:49,960 Speaker 1: All right, Well, let's get into that mystery and how 711 00:35:50,120 --> 00:36:06,279 Speaker 1: that's possible. But first let's take another quick break. All right, 712 00:36:06,400 --> 00:36:09,640 Speaker 1: we are expanding our minds here, trying to learn things 713 00:36:09,719 --> 00:36:12,320 Speaker 1: faster than the speed of light. And trying to understand 714 00:36:12,360 --> 00:36:14,520 Speaker 1: these concepts. I think I sort of understand what you're 715 00:36:14,520 --> 00:36:17,400 Speaker 1: saying about this contradiction between the two things. I mean, 716 00:36:17,440 --> 00:36:20,359 Speaker 1: we've always explained it as you know, nothing can move 717 00:36:20,480 --> 00:36:23,080 Speaker 1: through space faster than the speed of light, but space 718 00:36:23,239 --> 00:36:25,680 Speaker 1: can grow. You can make new space at a rate 719 00:36:25,760 --> 00:36:28,080 Speaker 1: that's faster than the speed of light. That's how things 720 00:36:28,120 --> 00:36:30,920 Speaker 1: that are really far away from us in an expanding universe, 721 00:36:31,040 --> 00:36:33,640 Speaker 1: the distance between us is growing faster than the speed 722 00:36:33,680 --> 00:36:35,920 Speaker 1: of light. But nothing in that spacing between us can 723 00:36:36,040 --> 00:36:38,279 Speaker 1: move faster than the speed of light in their sort 724 00:36:38,320 --> 00:36:40,640 Speaker 1: of local space. Right. But I think what you're saying 725 00:36:40,800 --> 00:36:43,680 Speaker 1: is that for a physicist, there's really no distinction between 726 00:36:44,280 --> 00:36:46,840 Speaker 1: space being created between us and using the word that 727 00:36:46,920 --> 00:36:49,359 Speaker 1: it's moving away from us at a certain rate. Right, 728 00:36:49,520 --> 00:36:51,279 Speaker 1: like it. It sort of makes into the sense if 729 00:36:51,280 --> 00:36:53,080 Speaker 1: you make that distinction. But I think what you're telling 730 00:36:53,160 --> 00:36:55,879 Speaker 1: us here today is that to a physicist, the fact 731 00:36:55,920 --> 00:36:58,080 Speaker 1: that there's more space growing between us is the same 732 00:36:58,160 --> 00:37:00,600 Speaker 1: thing as if it was moving away of us. Yeah, 733 00:37:00,640 --> 00:37:02,960 Speaker 1: that's exactly right, And in your local inertial frame, there 734 00:37:03,000 --> 00:37:05,840 Speaker 1: really is no difference. You try to extend that to 735 00:37:05,960 --> 00:37:08,480 Speaker 1: the whole universe, and to say I'm going to talk 736 00:37:08,560 --> 00:37:11,840 Speaker 1: about the velocity of things that are really really far away, 737 00:37:12,200 --> 00:37:14,360 Speaker 1: you end up with this weird calculation that gives you, 738 00:37:14,480 --> 00:37:16,520 Speaker 1: like the velocity of these things is two times a 739 00:37:16,560 --> 00:37:18,640 Speaker 1: speed of light or three times the speed of light. 740 00:37:18,760 --> 00:37:20,600 Speaker 1: And the reason that's strange is that you're using this 741 00:37:20,719 --> 00:37:23,640 Speaker 1: concept of a distance and velocity and how you measure velocity, 742 00:37:23,880 --> 00:37:27,279 Speaker 1: which doesn't really work over super duper long distances because 743 00:37:27,320 --> 00:37:30,399 Speaker 1: you could never actually measure those things. You can't really 744 00:37:30,520 --> 00:37:33,600 Speaker 1: measure those velocities of things. There's an apparent velocity if 745 00:37:33,600 --> 00:37:37,440 Speaker 1: you make simple assumptions and just like translate redshift into velocity, 746 00:37:37,560 --> 00:37:40,279 Speaker 1: but that velocity doesn't have a physical meaning. It makes 747 00:37:40,360 --> 00:37:42,800 Speaker 1: just much more sense to talk physically about the expansion 748 00:37:42,840 --> 00:37:45,520 Speaker 1: of space between us and them. I wonder if another 749 00:37:45,520 --> 00:37:47,120 Speaker 1: way to look at it is to say that this 750 00:37:47,280 --> 00:37:51,600 Speaker 1: expansion of the universe, this like free velocity, basically breaks 751 00:37:51,920 --> 00:37:55,280 Speaker 1: all the rules that we thought were true, right because technically, 752 00:37:55,360 --> 00:37:58,960 Speaker 1: because of this expansion, as you're saying, things are gaining 753 00:37:59,160 --> 00:38:02,560 Speaker 1: velocity are free, you know, nothing's pushing it, No energy 754 00:38:02,760 --> 00:38:05,680 Speaker 1: is actually causing this change in velocity. If it's not 755 00:38:05,760 --> 00:38:08,000 Speaker 1: equals m a in this case, right, it doesn't break 756 00:38:08,040 --> 00:38:09,839 Speaker 1: all the rules. It just breaks the rules that we're 757 00:38:10,000 --> 00:38:13,160 Speaker 1: used to applying even the things in our cosmic neighborhood. 758 00:38:13,360 --> 00:38:16,600 Speaker 1: It definitely doesn't break general relativity. General relativity can definitely 759 00:38:16,640 --> 00:38:19,160 Speaker 1: describe it, but it requires a very different picture of 760 00:38:19,200 --> 00:38:20,880 Speaker 1: the universities and when that we're not used to, and 761 00:38:21,000 --> 00:38:24,000 Speaker 1: very specifically, you can't have a whole frame of reference 762 00:38:24,040 --> 00:38:26,719 Speaker 1: for the universe and talk about like velocity of things 763 00:38:26,800 --> 00:38:29,919 Speaker 1: across the universe. General relativity says you can only really 764 00:38:30,000 --> 00:38:33,200 Speaker 1: make local observations, and the things that are really far away, 765 00:38:33,480 --> 00:38:36,160 Speaker 1: you can't even really talk about what their velocity means. 766 00:38:36,480 --> 00:38:38,560 Speaker 1: They're in their own frame over there, and we're in 767 00:38:38,680 --> 00:38:41,000 Speaker 1: our frame over here. So you a tempted to like 768 00:38:41,280 --> 00:38:43,080 Speaker 1: put your mind at the center of the universe and 769 00:38:43,200 --> 00:38:46,080 Speaker 1: expand that picture out to include everything out there. But there, 770 00:38:46,160 --> 00:38:50,000 Speaker 1: it really is no unified picture like that that makes sense, 771 00:38:50,080 --> 00:38:52,560 Speaker 1: because you can never actually make those measurements. Well, I 772 00:38:52,600 --> 00:38:54,440 Speaker 1: guess maybe I'm confused about what you mean that you 773 00:38:54,560 --> 00:38:56,239 Speaker 1: can't talk about it. I mean, we're talking about it. 774 00:38:56,280 --> 00:38:59,719 Speaker 1: We've just spent fifty minutes talking about I guess it's 775 00:38:59,760 --> 00:39:01,800 Speaker 1: really we can't talk about it. Are you saying that 776 00:39:01,880 --> 00:39:04,480 Speaker 1: the laws don't apply to the laws note work? Are 777 00:39:04,520 --> 00:39:07,040 Speaker 1: you saying that it doesn't make sense in our current 778 00:39:07,200 --> 00:39:10,000 Speaker 1: kind of way of thinking about things. It definitely works 779 00:39:10,040 --> 00:39:12,720 Speaker 1: and the laws makes sense, right, But there's no unique 780 00:39:12,800 --> 00:39:15,440 Speaker 1: way to picture it. Like in special relativity, you can 781 00:39:15,480 --> 00:39:17,799 Speaker 1: pick a unique frame and you can say, I'm gonna 782 00:39:17,840 --> 00:39:19,759 Speaker 1: put myself in the origin, I'm at rest, I'm gonna 783 00:39:19,760 --> 00:39:22,200 Speaker 1: measure everything relative to me. But you can't do that 784 00:39:22,320 --> 00:39:24,799 Speaker 1: for the universe, right. You can't say, here's how I'm 785 00:39:24,800 --> 00:39:27,120 Speaker 1: gonna define everything, and this is the right way to 786 00:39:27,200 --> 00:39:29,280 Speaker 1: do it. You can't have a single frame with everybody 787 00:39:29,320 --> 00:39:31,239 Speaker 1: in it. In order to talk about the velocities of 788 00:39:31,320 --> 00:39:34,120 Speaker 1: things that are really really far away, when space is 789 00:39:34,239 --> 00:39:37,360 Speaker 1: curved between us and them, or when the space is 790 00:39:37,440 --> 00:39:39,640 Speaker 1: expanding between us and them, you have to make some 791 00:39:39,920 --> 00:39:43,399 Speaker 1: arbitrary choices. You have to choose your coordinates, and different 792 00:39:43,440 --> 00:39:46,480 Speaker 1: people could choose different coordinates and they get different velocities. 793 00:39:46,800 --> 00:39:49,399 Speaker 1: Like we said earlier, one person could say, I'm gonna 794 00:39:49,440 --> 00:39:51,400 Speaker 1: put the Earth at the center, and I'm gonna measure 795 00:39:51,480 --> 00:39:55,040 Speaker 1: velocities using an unphysically long stick. I'm gonna get crazy 796 00:39:55,120 --> 00:39:58,080 Speaker 1: velocities two times of speed of light. Another person you 797 00:39:58,120 --> 00:40:00,759 Speaker 1: could say, now I'm gonna choose coordinate that grow with 798 00:40:00,880 --> 00:40:03,799 Speaker 1: the universe, so everything is at rest, so nothing has 799 00:40:03,840 --> 00:40:07,880 Speaker 1: any velocity. Actually, and both of those things can be true, right, 800 00:40:08,000 --> 00:40:10,920 Speaker 1: and there's no way to pick between them in general relativity. 801 00:40:11,040 --> 00:40:12,880 Speaker 1: So that's what we mean when we say it's not 802 00:40:13,080 --> 00:40:15,759 Speaker 1: meaningful to talk about those velocities because they're a little 803 00:40:15,760 --> 00:40:18,879 Speaker 1: bit arbitrary. They depend on the coordinates that you pick, 804 00:40:18,960 --> 00:40:20,960 Speaker 1: and you're free to pick any coordinates you like. In 805 00:40:21,040 --> 00:40:23,800 Speaker 1: general relativity, maybe what you're saying, it's a general relativity 806 00:40:23,840 --> 00:40:26,640 Speaker 1: allows you to use non inertial frames kind of or 807 00:40:26,719 --> 00:40:29,879 Speaker 1: frames that are growing. Absolutely, general relativity can describe non 808 00:40:29,960 --> 00:40:32,920 Speaker 1: inertial motion, things that are accelerating, things that are expanding, 809 00:40:33,000 --> 00:40:35,640 Speaker 1: even if that expansion is accelerating, and you can use 810 00:40:35,760 --> 00:40:39,440 Speaker 1: non inertial frames, and in general relativity, absolutely, yes, general 811 00:40:39,440 --> 00:40:41,520 Speaker 1: relativity you can describe all sorts of things like that. 812 00:40:41,800 --> 00:40:43,879 Speaker 1: All right, Well, let's get into that mystery. You said 813 00:40:43,880 --> 00:40:47,000 Speaker 1: earlier that there are things that are growing away from 814 00:40:47,080 --> 00:40:49,319 Speaker 1: us faster than the speed of light, but that maybe 815 00:40:49,560 --> 00:40:51,239 Speaker 1: and you would think you can never see them because 816 00:40:51,400 --> 00:40:53,239 Speaker 1: if they're moving away from as fast in the speed 817 00:40:53,280 --> 00:40:54,680 Speaker 1: of light, we will never see their light. But you're 818 00:40:54,719 --> 00:40:56,759 Speaker 1: saying that it is possible for us to see their 819 00:40:56,840 --> 00:40:59,799 Speaker 1: light on some things. Yes, it's possible to see things 820 00:41:00,040 --> 00:41:03,240 Speaker 1: that are moving away from us faster than the speed 821 00:41:03,280 --> 00:41:05,760 Speaker 1: of light, which seems a little strange, but it's because 822 00:41:05,840 --> 00:41:09,000 Speaker 1: this recession velocity we're talking about is not constant. The 823 00:41:09,080 --> 00:41:12,279 Speaker 1: expansion of the universe is not happening at a constant rate. 824 00:41:12,320 --> 00:41:16,160 Speaker 1: It's changing with time, So that changes over time which 825 00:41:16,239 --> 00:41:18,200 Speaker 1: parts of the universe are moving away from us fast 826 00:41:18,239 --> 00:41:20,400 Speaker 1: in the speed of light and slower than the speed 827 00:41:20,440 --> 00:41:22,480 Speaker 1: of light. So some things were moving away from us 828 00:41:22,520 --> 00:41:25,440 Speaker 1: faster than the speed of light. Always other things were 829 00:41:25,560 --> 00:41:27,359 Speaker 1: moving away from us faster than the speed of light, 830 00:41:27,480 --> 00:41:29,560 Speaker 1: but then no longer were, and then might be again 831 00:41:29,600 --> 00:41:32,160 Speaker 1: in the future because of the acceleration. There's a whole 832 00:41:32,160 --> 00:41:34,759 Speaker 1: sort of different outcomes based on how far away from 833 00:41:34,880 --> 00:41:38,439 Speaker 1: us you are. Wait, I thought that the universe has 834 00:41:39,400 --> 00:41:41,680 Speaker 1: I mean, the universe started expanding at the Big Bang, 835 00:41:41,760 --> 00:41:44,160 Speaker 1: and it's always been expanding. That hasn't been contracting in 836 00:41:44,280 --> 00:41:46,920 Speaker 1: any point, right, It hasn't been contracting, but it was 837 00:41:47,080 --> 00:41:50,320 Speaker 1: decelerating right. Expansion the universe has a few different phases. 838 00:41:50,400 --> 00:41:52,920 Speaker 1: There's inflation, which is a very very rapid expansion in 839 00:41:52,920 --> 00:41:54,680 Speaker 1: the very beginning, and then this sort of a quiet 840 00:41:54,760 --> 00:41:58,960 Speaker 1: period where things are expanding slower and it's actually decelerating. 841 00:41:59,239 --> 00:42:01,640 Speaker 1: The expansion are is just driven by like the density 842 00:42:01,719 --> 00:42:03,880 Speaker 1: of the universe. Solve the Einstein equations, you get a 843 00:42:03,920 --> 00:42:06,400 Speaker 1: certain expansion based on how much stuff there is in 844 00:42:06,480 --> 00:42:09,759 Speaker 1: the universe. But because the density of the universe decreases 845 00:42:10,000 --> 00:42:13,640 Speaker 1: as it expands, then that expansion changes. So the expansion 846 00:42:13,760 --> 00:42:17,080 Speaker 1: was actually slow in the universe, was decelerating until recently 847 00:42:17,400 --> 00:42:19,359 Speaker 1: when dark energy took over and it kicked it back 848 00:42:19,440 --> 00:42:21,560 Speaker 1: up in the gear again, and the expansion has been 849 00:42:21,680 --> 00:42:24,480 Speaker 1: accelerating since. So you have these sort of three different 850 00:42:24,520 --> 00:42:27,080 Speaker 1: phases in the history of the universe, and that change 851 00:42:27,160 --> 00:42:29,279 Speaker 1: is like sort of what's been moving away from us 852 00:42:29,320 --> 00:42:31,160 Speaker 1: fast in the speed of light and the fate of 853 00:42:31,200 --> 00:42:34,319 Speaker 1: all of those photons. Okay, you're saying that before dark 854 00:42:34,480 --> 00:42:37,840 Speaker 1: energy kind of pressed the accelerator and made the universe 855 00:42:37,920 --> 00:42:40,840 Speaker 1: really start expanding faster and faster as we see it today. 856 00:42:41,040 --> 00:42:44,680 Speaker 1: Before that happened, maybe there are things that we're moving 857 00:42:45,160 --> 00:42:48,360 Speaker 1: the universe is sort of decelerating at some point, right, 858 00:42:48,360 --> 00:42:50,719 Speaker 1: because it wasn't growing as fast, which means that there's 859 00:42:50,800 --> 00:42:52,560 Speaker 1: kind of a window there for us to see things 860 00:42:52,600 --> 00:42:54,480 Speaker 1: that we're once moving faster than the speed of light, 861 00:42:54,560 --> 00:42:56,160 Speaker 1: but then at some point we're not. So maybe we 862 00:42:56,160 --> 00:42:57,680 Speaker 1: can see those photos for a little bit, but then 863 00:42:57,719 --> 00:42:59,880 Speaker 1: eventually we won't be able to see them again exactly. 864 00:43:00,040 --> 00:43:02,200 Speaker 1: And the key thing to understand is this concept of 865 00:43:02,280 --> 00:43:05,479 Speaker 1: the hubble sphere. Inside the hubble sphere, things are moving 866 00:43:05,520 --> 00:43:08,600 Speaker 1: away from us less than the speed of light. Outside 867 00:43:08,600 --> 00:43:11,719 Speaker 1: the hubble sphere, the apparent recession velocity is greater than 868 00:43:11,760 --> 00:43:13,640 Speaker 1: the speed of light. So it's just a definition. It says, 869 00:43:13,719 --> 00:43:16,120 Speaker 1: let's draw boundary where things are moving away from us 870 00:43:16,400 --> 00:43:18,960 Speaker 1: at the speed of light, and everything further away from 871 00:43:19,000 --> 00:43:21,520 Speaker 1: that is outside the hubble sphere. That's what the hubble 872 00:43:21,880 --> 00:43:23,800 Speaker 1: sphere is. That's how you define it. Yeah, that's the 873 00:43:23,840 --> 00:43:26,439 Speaker 1: definition of the hubble sphere, the bubble of space where 874 00:43:26,560 --> 00:43:29,400 Speaker 1: things are expanding away from us the less than the 875 00:43:29,440 --> 00:43:32,040 Speaker 1: speed of light. And that's different than the observable universe, right, 876 00:43:32,320 --> 00:43:35,160 Speaker 1: It's very different from the observable universe. And this hubble 877 00:43:35,200 --> 00:43:37,760 Speaker 1: sphere depends on the Hubble constant. Right, The Hubble constant 878 00:43:37,760 --> 00:43:40,120 Speaker 1: tells you how fast things are moving away based on 879 00:43:40,239 --> 00:43:43,040 Speaker 1: how far away they are. So for a given Hubble constant, 880 00:43:43,120 --> 00:43:46,080 Speaker 1: that defines a Hubble sphere. But the Hubble constant is changing, 881 00:43:46,120 --> 00:43:48,560 Speaker 1: it's not really a constant. As we said earlier, the 882 00:43:48,760 --> 00:43:52,080 Speaker 1: universe was decelerating for a long time because it was expanding. 883 00:43:52,200 --> 00:43:55,360 Speaker 1: That changes the energy density, which then changes the expansion. 884 00:43:55,480 --> 00:43:58,960 Speaker 1: So the Hubble constant was actually decreasing, right, which grows 885 00:43:59,160 --> 00:44:02,000 Speaker 1: the hubble sphere. Means you have to go further away 886 00:44:02,160 --> 00:44:04,239 Speaker 1: to find objects that are now moving away from you 887 00:44:04,400 --> 00:44:07,080 Speaker 1: at the speed of light. So this hubble sphere, the 888 00:44:07,200 --> 00:44:09,680 Speaker 1: portion of the universe inside of it, which is moving 889 00:44:09,719 --> 00:44:11,440 Speaker 1: away from us at less than the speed of light, 890 00:44:11,800 --> 00:44:15,000 Speaker 1: is expanding as time goes on. And is it is 891 00:44:15,040 --> 00:44:18,000 Speaker 1: that growing with the expansion of the universe or is 892 00:44:18,040 --> 00:44:19,719 Speaker 1: it that it does it have its own kind of 893 00:44:19,880 --> 00:44:22,200 Speaker 1: like growth rate as its own growth rate, that it 894 00:44:22,320 --> 00:44:24,120 Speaker 1: is linked to the expansion of the universe because it's 895 00:44:24,120 --> 00:44:26,759 Speaker 1: the expansion of the universe that changes a density, which 896 00:44:26,840 --> 00:44:29,120 Speaker 1: then changes the Hubble parameter. So it's a bit of 897 00:44:29,160 --> 00:44:33,000 Speaker 1: a complicated relationships like nasty differential equations in there, but 898 00:44:33,080 --> 00:44:35,040 Speaker 1: they don't exactly track each other, which is why the 899 00:44:35,080 --> 00:44:37,920 Speaker 1: hubble sphere can grow to encompass things that we're once 900 00:44:38,040 --> 00:44:40,399 Speaker 1: moving away from us faster than the speed of light, 901 00:44:40,800 --> 00:44:44,399 Speaker 1: including and here's the key. You can find photons which 902 00:44:44,440 --> 00:44:48,320 Speaker 1: were once in superluminately receding regions and put them inside 903 00:44:48,360 --> 00:44:50,839 Speaker 1: the hubble sphere so that we can now see them. Right. 904 00:44:50,920 --> 00:44:52,680 Speaker 1: I think what you're saying is that if the universe 905 00:44:52,840 --> 00:44:56,920 Speaker 1: is expanding faster and faster, the hubble sphere shrinks. And 906 00:44:57,040 --> 00:45:01,120 Speaker 1: if the universe slows down and and starts to not 907 00:45:01,280 --> 00:45:04,560 Speaker 1: grows fast, and the hubble sphere grows. So it depends 908 00:45:04,560 --> 00:45:07,040 Speaker 1: a little bit on the coordinates. Right in physical units 909 00:45:07,080 --> 00:45:09,879 Speaker 1: where you're like trotting out a ruler to measure things, 910 00:45:10,040 --> 00:45:12,880 Speaker 1: the hubble sphere always just grows. It just grows faster 911 00:45:13,320 --> 00:45:16,440 Speaker 1: or slower. It's growing because the universe is expanding. It's 912 00:45:16,480 --> 00:45:19,520 Speaker 1: growing because the Hubble parameter is decreasing right as the 913 00:45:19,640 --> 00:45:23,759 Speaker 1: universe expands and the density drops because the universe is expanding. Yes, 914 00:45:23,840 --> 00:45:26,359 Speaker 1: the universe expansion is driving it. But what I say 915 00:45:26,440 --> 00:45:28,680 Speaker 1: was still true right Well, in physical units, the hubble 916 00:45:28,719 --> 00:45:31,680 Speaker 1: sphere never shrinks. It only grows. It does shrink in 917 00:45:31,760 --> 00:45:34,920 Speaker 1: the co moving units, so it's always growing. The whole sphere. 918 00:45:35,120 --> 00:45:37,760 Speaker 1: It's always growing, but sometimes it grows faster and sometimes 919 00:45:37,800 --> 00:45:40,880 Speaker 1: it grows slower. Okay, And meantime the universe is expanding, 920 00:45:40,920 --> 00:45:43,520 Speaker 1: but sometimes it grows faster or slower. And so you're saying, 921 00:45:43,560 --> 00:45:46,120 Speaker 1: there's this kind of like this relationship between the two spheres, 922 00:45:46,600 --> 00:45:49,839 Speaker 1: the observable universe fear and the hubble sphere, and there 923 00:45:49,960 --> 00:45:52,040 Speaker 1: they have this kind of dance like sometimes one is 924 00:45:52,040 --> 00:45:54,120 Speaker 1: bigger than the other and sometimes when it's smaller than 925 00:45:54,120 --> 00:45:55,960 Speaker 1: the other. So if you think about what happens for 926 00:45:56,120 --> 00:45:58,320 Speaker 1: objects at various distances, you can get an idea for 927 00:45:58,360 --> 00:46:01,640 Speaker 1: how these interplays determine the fate of a photon which 928 00:46:01,719 --> 00:46:05,759 Speaker 1: is shot at us from those distances. Right, So I 929 00:46:05,840 --> 00:46:08,040 Speaker 1: think the end result then is that there are photons 930 00:46:08,160 --> 00:46:10,279 Speaker 1: coming towards us, which at some point we're like, oh, 931 00:46:10,280 --> 00:46:12,200 Speaker 1: I'm never gonna get to horrid. He's never going to 932 00:46:12,280 --> 00:46:14,040 Speaker 1: see me. But now then then the universe kind of 933 00:46:14,080 --> 00:46:16,200 Speaker 1: shifted gears and it's like, oh, oh, now I'm gonna 934 00:46:16,280 --> 00:46:18,680 Speaker 1: make it too hori after all. Kind of exactly because 935 00:46:18,719 --> 00:46:22,080 Speaker 1: the radius of the hubble sphere is increasing. Some photons 936 00:46:22,239 --> 00:46:25,640 Speaker 1: that were initially in a super illuminally receding region and 937 00:46:25,680 --> 00:46:27,759 Speaker 1: a portion of space that was expanding away from us 938 00:46:27,800 --> 00:46:30,719 Speaker 1: faster than the speed of light, can now find themselves 939 00:46:30,840 --> 00:46:34,200 Speaker 1: in a sub speed of light receding region, right, And 940 00:46:34,280 --> 00:46:36,200 Speaker 1: not because they make it there, but because the hubble 941 00:46:36,239 --> 00:46:39,280 Speaker 1: sphere expands to sort of engulf them, and the objects 942 00:46:39,360 --> 00:46:43,000 Speaker 1: that admitted those photons have now moved to larger distances, 943 00:46:43,080 --> 00:46:47,160 Speaker 1: and so they're still receding superluminally. So something that has 944 00:46:47,239 --> 00:46:49,680 Speaker 1: always been moving away from us faster than the speed 945 00:46:49,719 --> 00:46:51,680 Speaker 1: of light, we do have a window to see it, 946 00:46:51,920 --> 00:46:55,279 Speaker 1: because it's photons cross into the hubble sphere and make 947 00:46:55,320 --> 00:46:57,279 Speaker 1: it into this portion of the universe that they can 948 00:46:57,320 --> 00:47:00,359 Speaker 1: then zip through and actually make progress towards us. Right. 949 00:47:00,440 --> 00:47:03,080 Speaker 1: I guess maybe the part that it's hard to grasp 950 00:47:03,160 --> 00:47:05,520 Speaker 1: because we don't have the math in front of us 951 00:47:05,640 --> 00:47:07,960 Speaker 1: is that, you know, the hubble sphere is not a thing, right, 952 00:47:08,040 --> 00:47:10,000 Speaker 1: It's just something you were using to describe it. I 953 00:47:10,040 --> 00:47:12,240 Speaker 1: think the point is that there are regions of space 954 00:47:12,440 --> 00:47:15,600 Speaker 1: that are stretching faster than other regions, and so sometimes 955 00:47:15,640 --> 00:47:17,960 Speaker 1: the photon might be in a region that is stretching 956 00:47:18,080 --> 00:47:19,920 Speaker 1: faster than the speed of light. But if it gets 957 00:47:19,960 --> 00:47:22,239 Speaker 1: into a region where it's threatened not stretching as much, 958 00:47:22,320 --> 00:47:24,360 Speaker 1: then maybe it has a chance to catch up to 959 00:47:24,520 --> 00:47:26,720 Speaker 1: that stretching and get to us. Yeah. Well, the stretching 960 00:47:26,840 --> 00:47:29,759 Speaker 1: is having at the same rate everywhere in space. It's 961 00:47:29,800 --> 00:47:32,200 Speaker 1: just not the same rate everywhere in time. And then 962 00:47:32,280 --> 00:47:35,359 Speaker 1: remember the recession velocity is a function of distance from us. 963 00:47:35,760 --> 00:47:37,759 Speaker 1: So as the phoneton is moving to the universe and 964 00:47:37,920 --> 00:47:40,400 Speaker 1: time is passing, this hubble sphere can sort of sweep 965 00:47:40,440 --> 00:47:42,600 Speaker 1: across it. You say, the hubble sphere is not a thing. 966 00:47:42,680 --> 00:47:45,200 Speaker 1: It's just like where things are traveling away from us 967 00:47:45,239 --> 00:47:47,759 Speaker 1: faster or slower than the speed of light. And so 968 00:47:47,840 --> 00:47:49,840 Speaker 1: then you can find itself in a portion of space 969 00:47:49,880 --> 00:47:52,120 Speaker 1: that's within our hubble sphere, then you can make it 970 00:47:52,200 --> 00:47:54,160 Speaker 1: to us because space between us and it is no 971 00:47:54,280 --> 00:47:56,880 Speaker 1: longer expanding faster than the speed of light. Well, you 972 00:47:56,960 --> 00:47:59,400 Speaker 1: just said something that raised something with me here. He 973 00:47:59,440 --> 00:48:01,359 Speaker 1: said that the expansion of the universe is the same 974 00:48:01,480 --> 00:48:03,600 Speaker 1: everywhere in space. Is that really true? I mean, aren't 975 00:48:03,600 --> 00:48:06,480 Speaker 1: there spots in space that are denser than others wouldn't 976 00:48:06,480 --> 00:48:08,839 Speaker 1: the maybe the expansion be a little uneven. Well, that's 977 00:48:08,880 --> 00:48:11,480 Speaker 1: a great point. We think that the contribution to the 978 00:48:11,560 --> 00:48:14,400 Speaker 1: expansion is the same everywhere, and that's accurate only in 979 00:48:14,440 --> 00:48:16,880 Speaker 1: the picture of the universe is like totally homogeneous right 980 00:48:16,920 --> 00:48:19,799 Speaker 1: where there are no like lumpy bits. Of course, once 981 00:48:19,840 --> 00:48:22,080 Speaker 1: you add in lumpy bits, things are different, and we 982 00:48:22,160 --> 00:48:25,359 Speaker 1: mostly ignore those when we talk about like big cosmological things. 983 00:48:25,440 --> 00:48:27,880 Speaker 1: We don't think about like the gravitational impact of Jupiter 984 00:48:28,400 --> 00:48:31,560 Speaker 1: or even a galaxy or a black hole, and most 985 00:48:31,600 --> 00:48:34,400 Speaker 1: of those things we can only solve in flat space, 986 00:48:34,480 --> 00:48:37,480 Speaker 1: assuming that the universe is not expanding, So we mostly 987 00:48:37,520 --> 00:48:39,759 Speaker 1: just ignore those little bits when we talk about like 988 00:48:39,920 --> 00:48:43,600 Speaker 1: really big cosmological questions. Right, but that there are clusters 989 00:48:43,640 --> 00:48:46,600 Speaker 1: and walls and bubbles of superclusters. Right, are you saying 990 00:48:46,640 --> 00:48:48,320 Speaker 1: the expansion of the universe is the same in the 991 00:48:48,480 --> 00:48:51,680 Speaker 1: big empty spaces between clusters of galaxies and it is 992 00:48:51,880 --> 00:48:54,080 Speaker 1: in the within the clusters. That's our theory, and we 993 00:48:54,200 --> 00:48:57,040 Speaker 1: think that it's the same everywhere. We think it's totally homogeneous. 994 00:48:57,080 --> 00:49:00,759 Speaker 1: It doesn't vary with space, does vary with time, because 995 00:49:00,800 --> 00:49:03,520 Speaker 1: it depends on the overall density of matter and radiation 996 00:49:03,640 --> 00:49:06,600 Speaker 1: in the universe, and that does evolve, but we don't 997 00:49:06,640 --> 00:49:08,960 Speaker 1: think it varies with space. So the Hubble constant, we 998 00:49:09,000 --> 00:49:12,320 Speaker 1: think is constant across space, but not across time, or 999 00:49:12,360 --> 00:49:14,960 Speaker 1: at least you assume it's clossant across space. Yes, we're 1000 00:49:15,000 --> 00:49:18,040 Speaker 1: assuming that, and all of our observations are consistent with that, 1001 00:49:18,280 --> 00:49:19,840 Speaker 1: but you know, we don't really know, and again we 1002 00:49:19,920 --> 00:49:23,160 Speaker 1: don't really understand the expansion of space or its acceleration. 1003 00:49:23,440 --> 00:49:26,200 Speaker 1: So certainly possible that dark energy could be doing different 1004 00:49:26,239 --> 00:49:28,359 Speaker 1: things in different parts of the universe. We don't see 1005 00:49:28,400 --> 00:49:30,440 Speaker 1: any evidence of that, but it's certainly possible. Well, it's 1006 00:49:30,480 --> 00:49:32,760 Speaker 1: kind of hard to measure what's going on in empty space, 1007 00:49:32,960 --> 00:49:35,120 Speaker 1: isn't it. Well, we can measure what happens in some 1008 00:49:35,239 --> 00:49:38,000 Speaker 1: of those voids by looking at photons that passed through them. 1009 00:49:38,160 --> 00:49:40,000 Speaker 1: We see photons from the very early universe, and we 1010 00:49:40,000 --> 00:49:42,440 Speaker 1: can sort of measure the density of space between us 1011 00:49:42,920 --> 00:49:45,880 Speaker 1: and where they were generated and the expansion of that 1012 00:49:46,080 --> 00:49:48,720 Speaker 1: space by seeing how they're red shifted and blue shifted 1013 00:49:48,719 --> 00:49:51,120 Speaker 1: as they go through like denser and less dense regions. 1014 00:49:51,400 --> 00:49:53,840 Speaker 1: So we have one way to probe that space because 1015 00:49:53,840 --> 00:49:56,479 Speaker 1: photons do pass through them, and we talked once about 1016 00:49:56,520 --> 00:49:59,200 Speaker 1: really cosmic voids. If you remember, we talked once about 1017 00:49:59,200 --> 00:50:01,720 Speaker 1: hotspots and could spots in the universe as those photons 1018 00:50:01,760 --> 00:50:04,120 Speaker 1: passed through those big voids, so we do have one 1019 00:50:04,360 --> 00:50:07,520 Speaker 1: handle on them. Cool. Well, my mind is definitely expanded 1020 00:50:07,760 --> 00:50:10,239 Speaker 1: and um, but I'm not quite sure my understanding. You're 1021 00:50:10,280 --> 00:50:12,920 Speaker 1: caught up to the speed of light here. But I 1022 00:50:13,000 --> 00:50:15,720 Speaker 1: think the main interesting picture here is that the universe 1023 00:50:15,960 --> 00:50:17,960 Speaker 1: is almost like a living thing, like it's growing. It 1024 00:50:18,080 --> 00:50:20,800 Speaker 1: has phases of fast growth and phases of slow growth, 1025 00:50:20,960 --> 00:50:23,040 Speaker 1: and you know, and we don't know how what's going 1026 00:50:23,080 --> 00:50:25,200 Speaker 1: to happen in the future, which means that it's sort 1027 00:50:25,200 --> 00:50:27,800 Speaker 1: of unpredictable how much of the universe will be able 1028 00:50:27,840 --> 00:50:29,960 Speaker 1: to see right, It is a little bit unpredictable. In 1029 00:50:30,040 --> 00:50:32,879 Speaker 1: our current models. We have some ideas for what those 1030 00:50:32,960 --> 00:50:35,200 Speaker 1: numbers are, and they're sort of mind boggling. We think 1031 00:50:35,239 --> 00:50:38,200 Speaker 1: the things that are now forty six billion light years away, 1032 00:50:38,640 --> 00:50:40,960 Speaker 1: if they emitted light at the very very beginning of 1033 00:50:41,000 --> 00:50:43,960 Speaker 1: the universe, that light is just reaching us now. And 1034 00:50:44,040 --> 00:50:47,280 Speaker 1: we think that things sixty two billion light years away 1035 00:50:47,520 --> 00:50:49,000 Speaker 1: if it admitted light at the very beginning of the 1036 00:50:49,080 --> 00:50:51,640 Speaker 1: universe it will eventually reach us at the very very 1037 00:50:51,880 --> 00:50:55,799 Speaker 1: end of the universe, but anything further than that will 1038 00:50:55,960 --> 00:50:59,120 Speaker 1: never enter the hubble sphere, will never reach us. Will 1039 00:50:59,200 --> 00:51:03,120 Speaker 1: always be moving through space that's expanding faster than the 1040 00:51:03,160 --> 00:51:06,080 Speaker 1: speed of light. So even though locally those photons are 1041 00:51:06,120 --> 00:51:09,560 Speaker 1: pumping away at a crazy speed, they're not actually making 1042 00:51:09,640 --> 00:51:13,200 Speaker 1: any progress towards us. Right, although never say never, right? 1043 00:51:13,360 --> 00:51:16,279 Speaker 1: Didn't you say earlier in another episode, And haven't you 1044 00:51:16,360 --> 00:51:18,920 Speaker 1: always said that dark energy is kind of unpredictable. It 1045 00:51:19,040 --> 00:51:21,799 Speaker 1: might you know, reverse course or shift gears or something 1046 00:51:21,880 --> 00:51:24,279 Speaker 1: like that might make the universe crunched down again. Yeah, 1047 00:51:24,440 --> 00:51:28,120 Speaker 1: great point. This is all assuming our current understanding of 1048 00:51:28,200 --> 00:51:30,560 Speaker 1: the universe and the blend of dark energy and dark 1049 00:51:30,640 --> 00:51:32,759 Speaker 1: matter is correct. We could learn one day that dark 1050 00:51:32,840 --> 00:51:35,279 Speaker 1: energy is something totally different, and you could flip around 1051 00:51:35,320 --> 00:51:37,359 Speaker 1: and compress the universe, and then we could see much 1052 00:51:37,440 --> 00:51:39,920 Speaker 1: much more of it one day before we eventually get 1053 00:51:40,000 --> 00:51:43,600 Speaker 1: squished too much of it, seem Yeah, the universe crunches 1054 00:51:43,640 --> 00:51:45,759 Speaker 1: down into a tiny dot again. Will all be very 1055 00:51:45,840 --> 00:51:49,200 Speaker 1: familiar with every light that's ever been admitted, right exactly 1056 00:51:49,320 --> 00:51:52,120 Speaker 1: just before it fries us to a crisp All right, Well, um, 1057 00:51:52,480 --> 00:51:54,400 Speaker 1: stay tuned. I guess as we learned more about the 1058 00:51:54,560 --> 00:51:58,040 Speaker 1: universe and what dark energy is, and we'll know more 1059 00:51:58,080 --> 00:52:01,200 Speaker 1: about how the universe is changing. Well, thanks for joining us. 1060 00:52:01,520 --> 00:52:12,600 Speaker 1: Hope you enjoyed that. See you next time. Thanks for listening, 1061 00:52:12,640 --> 00:52:15,360 Speaker 1: and remember that Daniel and Jorge explained the universe is 1062 00:52:15,400 --> 00:52:18,800 Speaker 1: a production of I Heart Radio. For more podcast for 1063 00:52:18,920 --> 00:52:22,640 Speaker 1: my heart Radio, visit the i heart Radio app, Apple Podcasts, 1064 00:52:22,800 --> 00:52:25,120 Speaker 1: or wherever you listen to your favorite shows.