1 00:00:09,119 --> 00:00:13,960 Speaker 1: It's not easy to hold an entire universe inside your head. 2 00:00:14,600 --> 00:00:17,520 Speaker 1: I mean, that's a whole lot of planets and stars, 3 00:00:17,560 --> 00:00:21,520 Speaker 1: and I hope aliens to squeeze into a few pounds 4 00:00:21,600 --> 00:00:25,279 Speaker 1: of brain matter. Instead, we try to whole date model 5 00:00:25,520 --> 00:00:29,120 Speaker 1: of the universe in our minds. But you know, even 6 00:00:29,160 --> 00:00:32,160 Speaker 1: that is hard because it turns out that the rules 7 00:00:32,200 --> 00:00:35,600 Speaker 1: of the universe are pretty different from the rules of 8 00:00:35,720 --> 00:00:38,800 Speaker 1: rocks and trees and water and all the things we're 9 00:00:38,800 --> 00:00:42,280 Speaker 1: familiar with. You've got relativity and the expansion of space 10 00:00:42,400 --> 00:00:45,000 Speaker 1: and things moving and light speed and all that jazz. 11 00:00:45,240 --> 00:00:47,960 Speaker 1: So it can be quite a challenge. And when we 12 00:00:48,040 --> 00:00:51,360 Speaker 1: try to bring all these crazy ideas into our minds, 13 00:00:51,440 --> 00:00:56,240 Speaker 1: sometimes things don't quite fit together. But trust me, it 14 00:00:56,440 --> 00:01:15,880 Speaker 1: actually does make sense, and it can make sense to you. Hi. 15 00:01:16,080 --> 00:01:19,200 Speaker 1: I'm Daniel. I'm a particle physicist, and I've spent a 16 00:01:19,280 --> 00:01:23,120 Speaker 1: lifetime trying to import a model of the universe into 17 00:01:23,280 --> 00:01:26,000 Speaker 1: my brain. I spent a lot of time thinking about 18 00:01:26,040 --> 00:01:29,440 Speaker 1: how the universe works and making sure that I understand 19 00:01:29,520 --> 00:01:32,080 Speaker 1: what's going on looking at it, from the tiniest little 20 00:01:32,120 --> 00:01:36,200 Speaker 1: particles to the huge cosmic scales of the universe. I 21 00:01:36,240 --> 00:01:40,399 Speaker 1: want one single idea which explains everything. I want to 22 00:01:40,440 --> 00:01:44,360 Speaker 1: have a coherent understanding of everything that's going on out there, 23 00:01:44,520 --> 00:01:48,600 Speaker 1: and I believe, without evidence, without any reason to believe this, 24 00:01:49,200 --> 00:01:52,400 Speaker 1: I do believe that the universe does make sense. I 25 00:01:52,440 --> 00:01:56,160 Speaker 1: believe that we are capable of understanding some fraction of it, 26 00:01:56,240 --> 00:02:00,280 Speaker 1: at least, that human mathematics and philosophy are able to 27 00:02:00,400 --> 00:02:04,160 Speaker 1: build constructs in our mind which can map onto that 28 00:02:04,280 --> 00:02:07,200 Speaker 1: external universe and explain it to us in a way 29 00:02:07,200 --> 00:02:11,079 Speaker 1: that we can manipulate it and understand it and examine it. 30 00:02:11,200 --> 00:02:13,720 Speaker 1: I don't know if we're capable of understanding the entire 31 00:02:13,880 --> 00:02:16,799 Speaker 1: universe at all of its incredible glory, if we are 32 00:02:16,880 --> 00:02:19,840 Speaker 1: smart enough, if we will ever develop the mathematics, indeed, 33 00:02:20,120 --> 00:02:24,840 Speaker 1: if our mathematics are even capable of describing the entire universe. 34 00:02:25,200 --> 00:02:28,360 Speaker 1: But I do know it's worthwhile, So welcome to the 35 00:02:28,360 --> 00:02:32,000 Speaker 1: podcast Daniel and Jorge Explain the Universe, in which we 36 00:02:32,080 --> 00:02:35,120 Speaker 1: attempt to take our current knowledge of the state of 37 00:02:35,160 --> 00:02:38,680 Speaker 1: the universe and download it all into your brain. We 38 00:02:38,720 --> 00:02:41,800 Speaker 1: talk about everything that we do understand, and we love 39 00:02:41,840 --> 00:02:44,880 Speaker 1: to talk about the things that we do not yet understand, 40 00:02:45,000 --> 00:02:47,680 Speaker 1: because even the smartest of human physicists do not have 41 00:02:47,760 --> 00:02:50,800 Speaker 1: a complete model of how the universe works in her brains. 42 00:02:51,000 --> 00:02:53,440 Speaker 1: We are still working on that. There are still pieces 43 00:02:53,680 --> 00:02:56,720 Speaker 1: that do not fit together. We have ideas from lots 44 00:02:56,720 --> 00:03:00,000 Speaker 1: of different physicists, and sometimes we can mail them together 45 00:03:00,000 --> 00:03:03,480 Speaker 1: into one coherent theory, and sometimes they just do not 46 00:03:03,760 --> 00:03:07,200 Speaker 1: play nicely. And we're still hard at work trying to 47 00:03:07,240 --> 00:03:10,760 Speaker 1: get an understanding of the universe. But it's those scientists 48 00:03:10,880 --> 00:03:14,840 Speaker 1: asking those questions, trying to make one theory of the universe, 49 00:03:14,880 --> 00:03:18,760 Speaker 1: trying to understand how everything out there can make sense 50 00:03:18,800 --> 00:03:22,200 Speaker 1: to our tiny little brains. That is what is driving 51 00:03:22,280 --> 00:03:25,040 Speaker 1: science forward. If you are not an active scientist, you 52 00:03:25,120 --> 00:03:27,519 Speaker 1: might have the impression that science just sort of rolls 53 00:03:27,600 --> 00:03:31,079 Speaker 1: forward on its own, results being produced year after year 54 00:03:31,240 --> 00:03:34,200 Speaker 1: as we push back the darkness and illuminate the universe 55 00:03:34,400 --> 00:03:37,120 Speaker 1: with our ideas. But in fact, science is not a 56 00:03:37,160 --> 00:03:41,880 Speaker 1: monolithic institution. It's very personal. It's all about individual people 57 00:03:42,000 --> 00:03:45,400 Speaker 1: asking questions. It's because that little girl grew up and 58 00:03:45,440 --> 00:03:49,040 Speaker 1: really needed to know how stars are formed. It's because 59 00:03:49,120 --> 00:03:52,960 Speaker 1: that little boy really wanted to understand how the universe 60 00:03:53,080 --> 00:03:56,520 Speaker 1: was expanding. It's because this person over here really wanted 61 00:03:56,600 --> 00:03:59,920 Speaker 1: to know the smallest things about the universe. It's into 62 00:04:00,000 --> 00:04:04,080 Speaker 1: a dual people asking their personal questions about the universe 63 00:04:04,320 --> 00:04:07,760 Speaker 1: and spending their life trying to find those answers. That's 64 00:04:07,840 --> 00:04:11,240 Speaker 1: how science moves forward. That is how we develop new 65 00:04:11,360 --> 00:04:15,040 Speaker 1: understandings of the universe, and that is also how you 66 00:04:15,160 --> 00:04:19,240 Speaker 1: developed new understandings of the universe by asking questions. And 67 00:04:19,240 --> 00:04:22,120 Speaker 1: one thing we love on this podcast are your questions 68 00:04:22,240 --> 00:04:26,040 Speaker 1: because we celebrate that kind of curiosity. The same curiosity 69 00:04:26,080 --> 00:04:30,000 Speaker 1: to drive signs forward also drives you to understand the universe. 70 00:04:30,400 --> 00:04:33,279 Speaker 1: And you'd be surprised how often the questions you have 71 00:04:33,400 --> 00:04:37,479 Speaker 1: in your mind are the very same questions that scientists 72 00:04:37,640 --> 00:04:40,320 Speaker 1: are asking. And so we want to bring you all 73 00:04:40,360 --> 00:04:43,159 Speaker 1: the way up to the forefront of scientific knowledge and 74 00:04:43,160 --> 00:04:46,400 Speaker 1: make sure you understand what scientists are puzzled about and 75 00:04:46,440 --> 00:04:49,640 Speaker 1: what they have figured out. So Jorge is not here today, 76 00:04:49,680 --> 00:04:52,440 Speaker 1: so I'll take the opportunity to be answering some of 77 00:04:52,480 --> 00:04:55,560 Speaker 1: your questions about the universe. And today we have a 78 00:04:55,640 --> 00:04:58,279 Speaker 1: doozy of an episode for your folks. It's all about 79 00:04:58,400 --> 00:05:01,520 Speaker 1: time and space in the expansion of the universe and 80 00:05:01,560 --> 00:05:04,120 Speaker 1: how that can all make sense, and it's going to 81 00:05:04,200 --> 00:05:07,000 Speaker 1: require you to bend your brain a little bit to 82 00:05:07,080 --> 00:05:09,719 Speaker 1: accept that the universe operates in a way that is 83 00:05:09,839 --> 00:05:15,600 Speaker 1: very unexpected and very unusual and has very weird consequences. Remember, 84 00:05:15,839 --> 00:05:18,200 Speaker 1: we live in a universe that's very different from the 85 00:05:18,240 --> 00:05:20,760 Speaker 1: one that we thought we lived in just five ten 86 00:05:20,880 --> 00:05:24,040 Speaker 1: thousand years ago, and only in the last hundred years 87 00:05:24,040 --> 00:05:27,200 Speaker 1: have we revealed these deep and fascinating truths about the universe, 88 00:05:27,400 --> 00:05:31,119 Speaker 1: which frankly upend every idea we had about how things 89 00:05:31,160 --> 00:05:33,839 Speaker 1: move and what things mean, and what distance and time 90 00:05:33,880 --> 00:05:37,760 Speaker 1: and velocity even are man And so we expect that 91 00:05:37,800 --> 00:05:40,839 Speaker 1: in the future there will be more revelations, but until then, 92 00:05:40,960 --> 00:05:43,520 Speaker 1: a lot of us are still catching up with some 93 00:05:43,600 --> 00:05:47,880 Speaker 1: of the crazy, bonkers ideas that science has so far revealed. 94 00:05:48,000 --> 00:05:56,480 Speaker 1: So today on the podcast, we'll be doing listener questions 95 00:05:56,560 --> 00:06:00,960 Speaker 1: about space and time and the whole universe Earth, And 96 00:06:01,040 --> 00:06:04,279 Speaker 1: so don't forget that you two can get your questions answered. 97 00:06:04,560 --> 00:06:07,520 Speaker 1: If you write to me two questions at Daniel and 98 00:06:07,680 --> 00:06:10,480 Speaker 1: jorgead dot com, you'll get an answer, and you might 99 00:06:10,560 --> 00:06:14,000 Speaker 1: even get your question on the podcast. So, without further ado, 100 00:06:14,120 --> 00:06:17,160 Speaker 1: let's get to the first listener question from Chris Hey. 101 00:06:17,240 --> 00:06:20,320 Speaker 1: Daniel and Jorge. I've heard that as light travels from 102 00:06:20,320 --> 00:06:23,159 Speaker 1: the edges of the universe over time, that more of 103 00:06:23,200 --> 00:06:26,800 Speaker 1: the universe is visible today than yesterday because the light 104 00:06:26,839 --> 00:06:29,880 Speaker 1: from farther away has had time to reach us. But 105 00:06:29,960 --> 00:06:33,039 Speaker 1: also that the universe expands away from us at faster 106 00:06:33,120 --> 00:06:35,239 Speaker 1: than the speed of light depending on how far away 107 00:06:35,240 --> 00:06:37,800 Speaker 1: it is. In the far future, we wouldn't be able 108 00:06:37,800 --> 00:06:41,080 Speaker 1: to see even the light from our closest neighboring galaxy. 109 00:06:41,200 --> 00:06:44,040 Speaker 1: How can both of these facts be true? Thank you? 110 00:06:44,120 --> 00:06:47,760 Speaker 1: I love the podcast. Thank you Chris for this wonderful question. 111 00:06:48,120 --> 00:06:50,159 Speaker 1: At first, I'd like to point out that I love 112 00:06:50,320 --> 00:06:53,680 Speaker 1: hearing the way Chris is doing physics. He's heard one thing, 113 00:06:53,839 --> 00:06:56,080 Speaker 1: he's heard something else, and he is trying to fit 114 00:06:56,160 --> 00:06:59,280 Speaker 1: those two ideas in his mind together. He wants to 115 00:06:59,520 --> 00:07:02,760 Speaker 1: unify understanding of the nature of the universe, not just 116 00:07:02,880 --> 00:07:06,080 Speaker 1: two cools owning ideas. He wants it all to gel 117 00:07:06,240 --> 00:07:08,479 Speaker 1: in his head. And that's what everybody should be doing. 118 00:07:08,560 --> 00:07:10,880 Speaker 1: When you hear an idea about science or read some 119 00:07:10,960 --> 00:07:14,440 Speaker 1: news article, you should ask yourself, Huh does this make 120 00:07:14,480 --> 00:07:16,760 Speaker 1: sense to me based on other things I've heard? How 121 00:07:16,760 --> 00:07:20,040 Speaker 1: does it fit together with what I understand, and it's 122 00:07:20,080 --> 00:07:23,160 Speaker 1: by doing that that you build an ever larger constellation 123 00:07:23,240 --> 00:07:25,840 Speaker 1: of ideas in your mind that are all fitting together 124 00:07:26,160 --> 00:07:30,240 Speaker 1: and giving you a unified understanding of the universe out there. 125 00:07:30,480 --> 00:07:34,160 Speaker 1: So let's talk about Chris's question. He wants to understand 126 00:07:34,520 --> 00:07:39,160 Speaker 1: two different ideas. One that our observable universe, the part 127 00:07:39,240 --> 00:07:42,160 Speaker 1: of the universe that we can see, seems to be 128 00:07:42,240 --> 00:07:45,320 Speaker 1: growing because as time goes on, light has had enough 129 00:07:45,360 --> 00:07:47,440 Speaker 1: time to get to us, so we can see deeper 130 00:07:47,480 --> 00:07:50,640 Speaker 1: and deeper into the universe. That is true. And he 131 00:07:50,760 --> 00:07:54,120 Speaker 1: also wants to understand how the fact that the universe 132 00:07:54,320 --> 00:07:58,040 Speaker 1: is expanding means that things near us will in the 133 00:07:58,120 --> 00:08:02,040 Speaker 1: future no longer be visit bowl, which is also true. 134 00:08:02,600 --> 00:08:06,720 Speaker 1: And so let's unpack these two different ideas. First, let's 135 00:08:06,720 --> 00:08:09,320 Speaker 1: make sure we understand exactly what is meant by the 136 00:08:09,360 --> 00:08:12,400 Speaker 1: observable universe, this first part of his question. So at first, 137 00:08:12,480 --> 00:08:15,160 Speaker 1: let's just simplify things and assume that we were dealing 138 00:08:15,240 --> 00:08:17,920 Speaker 1: with the universe where there is no expansion of space, 139 00:08:17,960 --> 00:08:19,640 Speaker 1: so we don't have to worry about that for now, 140 00:08:20,040 --> 00:08:23,080 Speaker 1: and just think about this question of the observable universe. 141 00:08:23,360 --> 00:08:26,160 Speaker 1: What does that mean? Well, in the end, It comes 142 00:08:26,200 --> 00:08:29,880 Speaker 1: down to the fact that light takes time to get somewhere. 143 00:08:30,040 --> 00:08:33,680 Speaker 1: Something happens on Andromeda, which is millions of light years away, 144 00:08:34,000 --> 00:08:37,199 Speaker 1: you don't see it immediately. In fact, Andromeda could get 145 00:08:37,360 --> 00:08:40,440 Speaker 1: attacked by aliens and totally blown up. We would not 146 00:08:40,559 --> 00:08:43,560 Speaker 1: see that happen for millions of years. The fact that 147 00:08:43,640 --> 00:08:46,760 Speaker 1: light does not move instantaneously means we can't see infinitely 148 00:08:46,840 --> 00:08:49,760 Speaker 1: far out there, because there could be something out there. 149 00:08:49,800 --> 00:08:53,520 Speaker 1: There almost certainly is something out there galaxy so far 150 00:08:53,600 --> 00:08:56,040 Speaker 1: away that light has not yet had time to get 151 00:08:56,080 --> 00:08:59,600 Speaker 1: to us through the whole history of the universe. Imagine 152 00:08:59,600 --> 00:09:02,520 Speaker 1: that a photon created in the very very beginning of 153 00:09:02,559 --> 00:09:06,920 Speaker 1: the universe sent towards us, traveling for billions of years 154 00:09:07,000 --> 00:09:11,360 Speaker 1: and still still not having arrived, still needing to fly 155 00:09:11,520 --> 00:09:14,160 Speaker 1: through space to get to us. And so this portion 156 00:09:14,200 --> 00:09:16,800 Speaker 1: of the universe where light has had time to get 157 00:09:16,840 --> 00:09:19,880 Speaker 1: to us, that's what we call the observable universe. It's 158 00:09:19,920 --> 00:09:23,280 Speaker 1: not really a physical thing. The edge of the observable 159 00:09:23,360 --> 00:09:26,000 Speaker 1: universe is just defined by where we are and the 160 00:09:26,040 --> 00:09:29,640 Speaker 1: age of the universe. It's centered literally around us, like 161 00:09:29,760 --> 00:09:33,680 Speaker 1: our heads. If we lived on Jupiter, the observable universe 162 00:09:33,679 --> 00:09:37,000 Speaker 1: would be centered at Jupiter. If we lived at Proximus Centauri, 163 00:09:37,120 --> 00:09:40,600 Speaker 1: the observable universe would be centered at Proximus Centauri. And 164 00:09:40,679 --> 00:09:44,600 Speaker 1: aliens living in another galaxy have a very different observable 165 00:09:44,679 --> 00:09:48,280 Speaker 1: universe because light needs time to get to them, right, 166 00:09:48,600 --> 00:09:52,720 Speaker 1: So you could have different, partially overlapping observable universes if 167 00:09:52,720 --> 00:09:55,840 Speaker 1: you're observing it from different places. And the size of 168 00:09:55,880 --> 00:09:59,840 Speaker 1: the observable universe grows every year because every year you've 169 00:09:59,840 --> 00:10:02,920 Speaker 1: a loud more time for light to get to you 170 00:10:03,160 --> 00:10:06,720 Speaker 1: from those distant galaxies. And so in a universe that's 171 00:10:06,760 --> 00:10:10,559 Speaker 1: not expanding, the observable universe grows by one light year 172 00:10:10,640 --> 00:10:14,000 Speaker 1: in radius every year, and so if you wait, the 173 00:10:14,000 --> 00:10:16,760 Speaker 1: observable universe gets bigger and bigger, and you've had time 174 00:10:16,840 --> 00:10:18,720 Speaker 1: for that light to get to you from further and 175 00:10:18,760 --> 00:10:21,480 Speaker 1: further places. And so what's at the edge of the 176 00:10:21,520 --> 00:10:25,280 Speaker 1: observable universe. The edge of the observable universe is the 177 00:10:25,320 --> 00:10:28,480 Speaker 1: oldest light that we can see. And in the universe 178 00:10:28,559 --> 00:10:31,440 Speaker 1: that's not expanding, that's not actually light from the very 179 00:10:31,559 --> 00:10:35,120 Speaker 1: very early universe. Because in the very early universe, everything 180 00:10:35,240 --> 00:10:38,240 Speaker 1: was hot and nasty and interacting with each other it 181 00:10:38,280 --> 00:10:41,320 Speaker 1: was a big, crazy plasma sort of like the inside 182 00:10:41,320 --> 00:10:44,199 Speaker 1: of our sun, and so any light that was created 183 00:10:44,240 --> 00:10:47,960 Speaker 1: then was also we absorbed almost immediately, and so it 184 00:10:48,000 --> 00:10:51,120 Speaker 1: doesn't exist any longer. The oldest light that we can 185 00:10:51,160 --> 00:10:54,559 Speaker 1: see comes from the cosmic microwave background, this moment when 186 00:10:54,559 --> 00:10:58,079 Speaker 1: the plasma cooled so that it became transparent, and then 187 00:10:58,160 --> 00:11:01,160 Speaker 1: light created in that moment. Could they fly free through 188 00:11:01,200 --> 00:11:03,920 Speaker 1: the universe. So the oldest light we can see comes 189 00:11:04,000 --> 00:11:07,600 Speaker 1: from the first moments that his cosmic microwave background radiation 190 00:11:08,000 --> 00:11:10,760 Speaker 1: was created. So we can't actually see light from the 191 00:11:10,960 --> 00:11:14,120 Speaker 1: very very earliest universe, all right. So that's the concept 192 00:11:14,200 --> 00:11:16,480 Speaker 1: of the observable universe, and it seems to make sense 193 00:11:16,640 --> 00:11:19,120 Speaker 1: that as time goes on we can see further and 194 00:11:19,120 --> 00:11:23,160 Speaker 1: further out. But there's this other thing going on at 195 00:11:23,160 --> 00:11:26,760 Speaker 1: the same time the universe is expanding. You might have 196 00:11:26,800 --> 00:11:28,559 Speaker 1: in your mind this idea of the Big Bang is 197 00:11:28,600 --> 00:11:30,880 Speaker 1: like an explosion. A lot of people think of the 198 00:11:30,920 --> 00:11:33,400 Speaker 1: Big Bang is like a tiny dot of stuff the 199 00:11:33,400 --> 00:11:36,160 Speaker 1: whole universe swished down to an atom, which then explodes 200 00:11:36,440 --> 00:11:39,120 Speaker 1: and moves through space. But the modern idea of the 201 00:11:39,120 --> 00:11:42,200 Speaker 1: Big Bang is actually quite different. It's that the universe 202 00:11:42,320 --> 00:11:45,360 Speaker 1: was infinite and always has been infinite, and that the 203 00:11:45,440 --> 00:11:48,640 Speaker 1: singularity wasn't a single point in space, but it was 204 00:11:48,679 --> 00:11:52,080 Speaker 1: a singularity in density. That is, the universe was very 205 00:11:52,120 --> 00:11:55,520 Speaker 1: hot and dense everywhere, and the Big Bang was an 206 00:11:55,520 --> 00:12:00,160 Speaker 1: expansion of that. Density is stretching of space itself, so 207 00:12:00,240 --> 00:12:03,240 Speaker 1: not stuff moving out from a little dot through space, 208 00:12:03,400 --> 00:12:07,319 Speaker 1: but an expansion of space itself. That means that new 209 00:12:07,360 --> 00:12:11,240 Speaker 1: space is being created, and that expansion is still happening. 210 00:12:11,320 --> 00:12:14,480 Speaker 1: In fact, that expansion is still accelerating. That's what we 211 00:12:14,559 --> 00:12:18,000 Speaker 1: call dark energy. So the universe is expanding, by which 212 00:12:18,040 --> 00:12:22,280 Speaker 1: we mean new space is being created between stuff all 213 00:12:22,320 --> 00:12:24,720 Speaker 1: the time. And this new space is being created between 214 00:12:24,720 --> 00:12:28,040 Speaker 1: our galaxy and other galaxies for example, So the distance 215 00:12:28,040 --> 00:12:31,520 Speaker 1: between us and other galaxies can be increasing even if 216 00:12:31,559 --> 00:12:35,240 Speaker 1: we have no relative velocity, right, just like create new 217 00:12:35,280 --> 00:12:38,760 Speaker 1: space between them. So this is called the recession of velocities, 218 00:12:38,800 --> 00:12:42,200 Speaker 1: basically the rate at which the distance is increasing even 219 00:12:42,240 --> 00:12:44,800 Speaker 1: if you don't have relative velocity. Now, a lot of 220 00:12:44,800 --> 00:12:47,200 Speaker 1: people wonder a lot of second, is this new space 221 00:12:47,240 --> 00:12:50,319 Speaker 1: that's being created only out there between galaxies or is 222 00:12:50,360 --> 00:12:53,640 Speaker 1: it also here in our solar system? Why isn't the 223 00:12:53,679 --> 00:12:56,439 Speaker 1: Sun getting expanded away from the Earth, Why is it 224 00:12:56,640 --> 00:12:59,320 Speaker 1: Mars getting blown out of the Solar System. Well, this 225 00:12:59,400 --> 00:13:02,880 Speaker 1: process dark energy that's accelerating the expansion of space is 226 00:13:02,920 --> 00:13:07,400 Speaker 1: actually a very small effect per unit space, So like 227 00:13:07,520 --> 00:13:10,760 Speaker 1: in our Solar system, it's pretty weak, and gravity is 228 00:13:10,760 --> 00:13:13,839 Speaker 1: actually much much stronger, so it's strong enough to hold 229 00:13:13,840 --> 00:13:16,959 Speaker 1: the Solar system together. But dark energy adds up as 230 00:13:17,000 --> 00:13:20,240 Speaker 1: you get more space. Over distances, it adds up, so 231 00:13:20,280 --> 00:13:22,560 Speaker 1: you get more and more dark energy, for example, over 232 00:13:22,559 --> 00:13:25,400 Speaker 1: the scale of a galaxy. But galaxies are still strong 233 00:13:25,520 --> 00:13:29,200 Speaker 1: enough to hold themselves together. Even galaxy clusters are strong 234 00:13:29,320 --> 00:13:33,040 Speaker 1: enough to hold themselves together, we think. But between clusters 235 00:13:33,040 --> 00:13:36,320 Speaker 1: of galaxies and these much much larger distances, that's where 236 00:13:36,400 --> 00:13:39,400 Speaker 1: dark energy wins. That's where the expansion of space is 237 00:13:39,480 --> 00:13:43,319 Speaker 1: stronger than the local gravity and it's tearing things apart. 238 00:13:43,600 --> 00:13:45,760 Speaker 1: What does that mean for how far we can see 239 00:13:46,040 --> 00:13:49,040 Speaker 1: for this question of the observable universe, Well, what it 240 00:13:49,120 --> 00:13:52,160 Speaker 1: means is that we can actually see much further than 241 00:13:52,240 --> 00:13:55,800 Speaker 1: thirteen point eight billion light years away, which is the 242 00:13:55,840 --> 00:13:58,680 Speaker 1: age of the universe times the speed of light. The 243 00:13:58,760 --> 00:14:01,960 Speaker 1: reason is that we can see photons from stuff that's 244 00:14:02,040 --> 00:14:06,120 Speaker 1: now much further away because space has been expanded. Right, 245 00:14:06,240 --> 00:14:09,240 Speaker 1: something sent us a photon a long time ago. It's 246 00:14:09,280 --> 00:14:13,280 Speaker 1: flying towards us, and since then, that thing, that galaxy 247 00:14:13,400 --> 00:14:16,920 Speaker 1: or whatever, the bit of the CMB has been expanded 248 00:14:16,960 --> 00:14:20,920 Speaker 1: away from us, so it's now much much further away. 249 00:14:20,960 --> 00:14:23,720 Speaker 1: So we can see things now that are forty six 250 00:14:23,760 --> 00:14:27,760 Speaker 1: and a half billion light years away, and that's because 251 00:14:27,960 --> 00:14:31,840 Speaker 1: of the expansion. So on one hand, the universe is 252 00:14:31,880 --> 00:14:34,360 Speaker 1: tearing things away from us. On the other hand, it 253 00:14:34,400 --> 00:14:36,880 Speaker 1: means that we can see things which are now much 254 00:14:36,960 --> 00:14:40,120 Speaker 1: much further away than you would expect if the universe 255 00:14:40,360 --> 00:14:42,880 Speaker 1: is not expanding all right, So that means that we 256 00:14:43,000 --> 00:14:45,960 Speaker 1: have two things going on at the same time. One 257 00:14:46,280 --> 00:14:48,920 Speaker 1: is that we have more time for photons to get here, 258 00:14:49,120 --> 00:14:52,320 Speaker 1: so the observable universe is growing. On the other hand, 259 00:14:52,560 --> 00:14:56,040 Speaker 1: the universe is expanding faster and faster. So let's dig 260 00:14:56,080 --> 00:14:58,400 Speaker 1: into how those two things play off each other. What's 261 00:14:58,440 --> 00:15:01,080 Speaker 1: happening between us and a distance galaxy is that the 262 00:15:01,120 --> 00:15:04,360 Speaker 1: space between us is being expanded, So the distance between 263 00:15:04,440 --> 00:15:06,760 Speaker 1: us and that galaxy is getting bigger, and the further 264 00:15:06,840 --> 00:15:09,960 Speaker 1: away something is, the more bits of space are being 265 00:15:10,000 --> 00:15:13,480 Speaker 1: expanded and so the faster something is moving away from us. 266 00:15:13,680 --> 00:15:16,600 Speaker 1: So the further something is away from Earth, the faster 267 00:15:16,720 --> 00:15:20,320 Speaker 1: it's moving away from us. And this was Hubble's discovery. Actually, 268 00:15:20,360 --> 00:15:24,320 Speaker 1: this is just Hubble's law that this recession velocity grows 269 00:15:24,480 --> 00:15:26,720 Speaker 1: with the distance. The further you are away from us, 270 00:15:26,920 --> 00:15:29,880 Speaker 1: the faster you're moving away from us. And this can 271 00:15:29,920 --> 00:15:32,800 Speaker 1: actually add up to a speed moving away from us 272 00:15:33,160 --> 00:15:36,600 Speaker 1: faster than the speed of light. And remember this is 273 00:15:36,640 --> 00:15:40,760 Speaker 1: not motion through space. This is the expansion of space. 274 00:15:41,080 --> 00:15:44,600 Speaker 1: And while special relativity says nothing can move through space 275 00:15:44,920 --> 00:15:47,440 Speaker 1: faster than the speed of light, there's no limit how 276 00:15:47,520 --> 00:15:51,560 Speaker 1: fast space itself can expand. And their objects out there 277 00:15:51,880 --> 00:15:55,520 Speaker 1: whose distance is growing faster than the speed of light, 278 00:15:55,680 --> 00:15:57,840 Speaker 1: and what does that mean? Can we see those things? 279 00:15:58,400 --> 00:16:01,120 Speaker 1: This area of the universe is called the Hubble volume. 280 00:16:01,760 --> 00:16:03,680 Speaker 1: Things that are moving away from us less than the 281 00:16:03,680 --> 00:16:07,040 Speaker 1: speed of light are inside this Hubble volume, this Hubble sphere, 282 00:16:07,320 --> 00:16:09,400 Speaker 1: and things that are moving away from us faster than 283 00:16:09,400 --> 00:16:12,520 Speaker 1: the speed of light are outside the Hubble sphere. And 284 00:16:12,560 --> 00:16:15,840 Speaker 1: so you might imagine, I guess things that are outside 285 00:16:15,840 --> 00:16:18,720 Speaker 1: the Hubble sphere we will never see because they're moving 286 00:16:18,760 --> 00:16:21,600 Speaker 1: away from us faster than the speed of light. And 287 00:16:21,640 --> 00:16:24,720 Speaker 1: you might be wondering, what about these things outside the 288 00:16:24,760 --> 00:16:27,440 Speaker 1: hubble sphere, these things with a distance between us and 289 00:16:27,480 --> 00:16:30,320 Speaker 1: them is growing faster than the speed of light. Can 290 00:16:30,480 --> 00:16:33,760 Speaker 1: we see those things? Well, what happens if a photon 291 00:16:34,040 --> 00:16:37,480 Speaker 1: is emitted from those things while that photon starts moving 292 00:16:37,480 --> 00:16:41,280 Speaker 1: through space, but space is expanding, And if that space 293 00:16:41,360 --> 00:16:44,080 Speaker 1: is expanding faster than the speed of light, then it's 294 00:16:44,080 --> 00:16:47,120 Speaker 1: like Usain Bolt is running towards you, but somebody's putting 295 00:16:47,160 --> 00:16:50,840 Speaker 1: down new track faster than he's running. So the distance 296 00:16:50,880 --> 00:16:54,000 Speaker 1: actually grows between us and that photon. It's sort of 297 00:16:54,040 --> 00:16:56,600 Speaker 1: like the photon is moving towards us, but it's a 298 00:16:56,720 --> 00:17:00,160 Speaker 1: distance is growing, right, It's not moving away from us, 299 00:17:00,160 --> 00:17:03,080 Speaker 1: but the distance between us and that photon sent by 300 00:17:03,120 --> 00:17:05,639 Speaker 1: something whose distance to us is growing fast than the 301 00:17:05,640 --> 00:17:09,480 Speaker 1: speed of light. Our distance that photon is also growing, 302 00:17:09,640 --> 00:17:11,360 Speaker 1: all right. So you might think, well, we're just never 303 00:17:11,359 --> 00:17:13,680 Speaker 1: going to see those things. And this is a common 304 00:17:13,720 --> 00:17:17,719 Speaker 1: misconception because this hubble sphere, this point at which the 305 00:17:17,840 --> 00:17:20,399 Speaker 1: universe is moving away from us faster than the speed 306 00:17:20,400 --> 00:17:24,280 Speaker 1: of light, this actually is expanding. Right. As the universe expands, 307 00:17:24,480 --> 00:17:27,640 Speaker 1: so does the hubble sphere. So eventually this photon can 308 00:17:27,720 --> 00:17:30,720 Speaker 1: make some progress and the hubble spear catches up to it, 309 00:17:30,880 --> 00:17:33,160 Speaker 1: and then it enters a part of the universe that's 310 00:17:33,200 --> 00:17:35,560 Speaker 1: moving away from us less than the speed of light, 311 00:17:35,720 --> 00:17:39,080 Speaker 1: and so we can actually see those things. So because 312 00:17:39,119 --> 00:17:42,480 Speaker 1: that hubble spear itself is expanding, it means we can 313 00:17:42,640 --> 00:17:45,680 Speaker 1: see light from things that we're moving away from us 314 00:17:45,720 --> 00:17:48,119 Speaker 1: faster than the speed of light. But that's because the 315 00:17:48,160 --> 00:17:52,119 Speaker 1: hubble spear was expanding. The hubble sphere was expanding. But 316 00:17:52,240 --> 00:17:55,440 Speaker 1: around seven and a half billion years into the age 317 00:17:55,440 --> 00:17:59,159 Speaker 1: of the Universe, something happened. Dark energy took over and 318 00:17:59,200 --> 00:18:02,720 Speaker 1: started to accelerate the expansion of the universe to make 319 00:18:02,800 --> 00:18:06,320 Speaker 1: this expansion happen faster and faster, and that has the 320 00:18:06,320 --> 00:18:09,760 Speaker 1: effect of actually shrinking the hubble sphere. It means that 321 00:18:09,800 --> 00:18:12,880 Speaker 1: things are moving away from us faster and faster, closer 322 00:18:12,920 --> 00:18:15,879 Speaker 1: than they were before. Right, it's speeding up this expansion, 323 00:18:16,040 --> 00:18:17,720 Speaker 1: so you don't have to be as far away from 324 00:18:17,800 --> 00:18:20,240 Speaker 1: us anymore to be moving away from us at faster 325 00:18:20,359 --> 00:18:22,479 Speaker 1: than the speed of light. So this hubble sphere is 326 00:18:22,480 --> 00:18:25,800 Speaker 1: shrinking and shrinking and shrinking, and now there are things 327 00:18:25,840 --> 00:18:28,680 Speaker 1: that are so far away from us and moving away 328 00:18:28,720 --> 00:18:31,840 Speaker 1: from us so quickly that we will never see them. 329 00:18:31,960 --> 00:18:34,560 Speaker 1: And in fact, the hubble sphere itself is shrinking and 330 00:18:34,560 --> 00:18:38,240 Speaker 1: shrinking and shrinking, and if dark energy continues, then it 331 00:18:38,320 --> 00:18:42,240 Speaker 1: will shrink basically to zero. Almost all of the universe 332 00:18:42,440 --> 00:18:44,919 Speaker 1: would be moving away from us at faster than the 333 00:18:44,960 --> 00:18:48,000 Speaker 1: speed of light, making it invisible because we can only 334 00:18:48,040 --> 00:18:50,800 Speaker 1: see photons and things moving away from us faster than 335 00:18:50,800 --> 00:18:53,520 Speaker 1: the speed of light if the hubble sphere is expanding 336 00:18:53,560 --> 00:18:56,280 Speaker 1: and catches up with those things. But the hubble sphere 337 00:18:56,320 --> 00:18:59,840 Speaker 1: is now shrinking due to dark energy and the accelerated 338 00:19:00,000 --> 00:19:04,280 Speaker 1: spansion of the universe. So what's the furthest thing that 339 00:19:04,320 --> 00:19:06,959 Speaker 1: we will ever see. Well, currently we can see things 340 00:19:07,000 --> 00:19:09,400 Speaker 1: that are about forty six and a half billion light 341 00:19:09,480 --> 00:19:12,680 Speaker 1: years away. Photons from things that are now sixty two 342 00:19:12,720 --> 00:19:16,960 Speaker 1: billion light years away will eventually make it here. They'll 343 00:19:16,960 --> 00:19:20,040 Speaker 1: make it here just as space expands away to make 344 00:19:20,080 --> 00:19:23,760 Speaker 1: it impossible. Things that are further away will never catch 345 00:19:23,840 --> 00:19:26,440 Speaker 1: up to the hubble sphere. The hubble sphere is now shrinking, 346 00:19:26,680 --> 00:19:30,159 Speaker 1: so things that are sufficiently far away can never have 347 00:19:30,400 --> 00:19:32,760 Speaker 1: their light get to us because the hubble sphere is 348 00:19:32,760 --> 00:19:35,880 Speaker 1: shrinking and they will never catch it, meaning their photons 349 00:19:35,880 --> 00:19:39,800 Speaker 1: will travel through space towards us, but will never actually 350 00:19:39,800 --> 00:19:43,200 Speaker 1: get closer to us. So this is a great question. 351 00:19:43,359 --> 00:19:46,560 Speaker 1: Thank you very much Chris for asking this. The observable 352 00:19:46,640 --> 00:19:51,560 Speaker 1: universe is growing, but space is expanding even faster, and 353 00:19:51,600 --> 00:19:54,159 Speaker 1: so it's pushing things out past the edge of the 354 00:19:54,200 --> 00:19:57,080 Speaker 1: observable universe. And for most things, that means we will 355 00:19:57,200 --> 00:20:00,280 Speaker 1: never see them, and those things will disappear year and 356 00:20:00,320 --> 00:20:03,520 Speaker 1: in the far far future, most of the universe will 357 00:20:03,600 --> 00:20:06,520 Speaker 1: be dark. What remains to be seen is how much 358 00:20:06,560 --> 00:20:09,800 Speaker 1: of the universe can stay local to us. Can gravity 359 00:20:09,880 --> 00:20:13,159 Speaker 1: hold our galaxy together or will it get torn apart 360 00:20:13,240 --> 00:20:16,720 Speaker 1: by dark energy? Cannot hold our solar system together or 361 00:20:16,760 --> 00:20:19,879 Speaker 1: will dark energy tear that apart as well. The problem 362 00:20:19,920 --> 00:20:22,719 Speaker 1: is we just do not understand dark energy, and we 363 00:20:22,800 --> 00:20:25,119 Speaker 1: don't know how to predict its future, and so it's 364 00:20:25,200 --> 00:20:27,919 Speaker 1: very difficult to know exactly what the future holds. So 365 00:20:28,240 --> 00:20:31,000 Speaker 1: thanks for that really wonderful question. I want to answer 366 00:20:31,000 --> 00:20:33,560 Speaker 1: a couple of more questions, but first it's time for 367 00:20:33,600 --> 00:20:49,000 Speaker 1: a break. All right. We're back and we are trying 368 00:20:49,040 --> 00:20:52,480 Speaker 1: to import the entire universe into our minds. And understand 369 00:20:52,560 --> 00:20:55,400 Speaker 1: its expansion and the speed of light and what happens 370 00:20:55,400 --> 00:20:58,680 Speaker 1: when you move through the universe nearly at the speed 371 00:20:58,720 --> 00:21:01,240 Speaker 1: of light. And so there is from Norway has a 372 00:21:01,320 --> 00:21:05,720 Speaker 1: question about just that. Hello Daniel and her Head. Thanks 373 00:21:05,800 --> 00:21:11,200 Speaker 1: very much for a great show. I have three interesting 374 00:21:11,280 --> 00:21:15,080 Speaker 1: questions for you. The first is the speed of light 375 00:21:15,920 --> 00:21:20,480 Speaker 1: and the distances. As we know, the universe is so 376 00:21:20,600 --> 00:21:24,359 Speaker 1: huge that even for the light, it takes ages to 377 00:21:24,480 --> 00:21:29,440 Speaker 1: reach such a vast distanceance. But my question is actually 378 00:21:30,000 --> 00:21:33,520 Speaker 1: we know that the faster you go, the time for 379 00:21:33,640 --> 00:21:38,000 Speaker 1: you for some who who is observing you go slower. 380 00:21:38,920 --> 00:21:43,400 Speaker 1: So my question is what's the time of photons or 381 00:21:43,480 --> 00:21:48,600 Speaker 1: of the light? Does light have time? The second question 382 00:21:48,680 --> 00:21:54,040 Speaker 1: is if we can also reach such a huge distances 383 00:21:54,080 --> 00:22:00,000 Speaker 1: by having a very strong gravitation field very close to us, 384 00:22:00,280 --> 00:22:04,720 Speaker 1: for instance, to build a space shuttle that has numerous 385 00:22:04,960 --> 00:22:11,400 Speaker 1: black holes around, which should decelerate time for those observing us. 386 00:22:11,720 --> 00:22:15,040 Speaker 1: So this is the second question, and the third question 387 00:22:15,160 --> 00:22:20,240 Speaker 1: is concerning the gravitational waves. Does gravitation have the same 388 00:22:20,680 --> 00:22:25,040 Speaker 1: speed as the speed of light. So we know that 389 00:22:25,080 --> 00:22:29,320 Speaker 1: if we remove the Sun from the middle of our 390 00:22:29,880 --> 00:22:34,119 Speaker 1: solar system, then it will take like eight minutes to 391 00:22:34,240 --> 00:22:39,760 Speaker 1: realize there is no sun. But shall we continue for 392 00:22:39,840 --> 00:22:44,880 Speaker 1: eight minutes to spin around or it will be immediate disappearance. 393 00:22:45,480 --> 00:22:48,840 Speaker 1: Thank you all right, Thank you very much Andrews for 394 00:22:48,920 --> 00:22:52,680 Speaker 1: your enthusiasm about the universe and these fine questions. I'll 395 00:22:52,680 --> 00:22:54,679 Speaker 1: answer his other questions of email, but I want to 396 00:22:54,680 --> 00:22:57,760 Speaker 1: talk about this first question here on the podcast, this 397 00:22:58,000 --> 00:23:02,320 Speaker 1: question about what's the time for a photon? Do photons 398 00:23:02,440 --> 00:23:05,800 Speaker 1: experience time or are they frozen? This is a very 399 00:23:05,800 --> 00:23:09,040 Speaker 1: common question. I think there's some misunderstandings about it, and 400 00:23:09,080 --> 00:23:12,919 Speaker 1: there's some satisfactory and also sort of unsatisfactory answers that 401 00:23:13,000 --> 00:23:15,240 Speaker 1: I can give you. First, let's review what we know 402 00:23:15,280 --> 00:23:19,280 Speaker 1: about time and clocks. The basic thing to understand is 403 00:23:19,320 --> 00:23:22,959 Speaker 1: that moving clocks run slowly. What that means is that 404 00:23:23,000 --> 00:23:25,679 Speaker 1: if you're looking at a clock and it has velocity 405 00:23:25,760 --> 00:23:29,040 Speaker 1: relative to you, it will be moving more slowly than 406 00:23:29,080 --> 00:23:31,640 Speaker 1: a clock you are holding in your hands that has 407 00:23:31,840 --> 00:23:36,160 Speaker 1: no velocity relative to you. So moving clocks run slowly. 408 00:23:36,160 --> 00:23:39,160 Speaker 1: And there's a really key concept built into that statement, 409 00:23:39,280 --> 00:23:42,000 Speaker 1: which is that somebody has to be measuring that motion, 410 00:23:42,440 --> 00:23:45,359 Speaker 1: so motion is always relative. You can't say I'm on 411 00:23:45,400 --> 00:23:47,960 Speaker 1: a spaceship. I'm going near the speed of light. My 412 00:23:48,119 --> 00:23:50,400 Speaker 1: time must be running slowly because you have to talk 413 00:23:50,400 --> 00:23:54,320 Speaker 1: about your velocity as relative to somebody else. Somebody who's 414 00:23:54,480 --> 00:23:58,040 Speaker 1: measuring your speed and your time will only be running 415 00:23:58,040 --> 00:24:01,679 Speaker 1: slowly for that person, the person who is measuring you 416 00:24:01,800 --> 00:24:04,440 Speaker 1: to be moving quickly. Because if you're on the spaceship 417 00:24:04,680 --> 00:24:07,679 Speaker 1: and you're holding a clock and you're looking down, you're thinking, 418 00:24:07,840 --> 00:24:11,040 Speaker 1: I'm going super fast. Where's the relativistic effects. I want 419 00:24:11,040 --> 00:24:14,520 Speaker 1: to feel time go slow? Right. You never feel time 420 00:24:14,560 --> 00:24:19,680 Speaker 1: going slowly because your clock is always motionless relative to you. Right. 421 00:24:19,960 --> 00:24:22,320 Speaker 1: This is the clock that tells you how life is 422 00:24:22,359 --> 00:24:25,600 Speaker 1: being experienced, and it's not moving relative to you, so 423 00:24:25,680 --> 00:24:28,720 Speaker 1: it does not slow down. Somebody else on Earth who 424 00:24:28,840 --> 00:24:32,200 Speaker 1: is measuring your velocity to be very very high, they 425 00:24:32,359 --> 00:24:36,360 Speaker 1: do see your clock moving slowly. So there's that inherent disagreement. 426 00:24:36,720 --> 00:24:38,720 Speaker 1: You look at your clock, you say it looks normal. 427 00:24:39,160 --> 00:24:40,880 Speaker 1: They look at your clock and they say, well, you're 428 00:24:40,920 --> 00:24:44,360 Speaker 1: moving quickly. I see it running slowly, right, And so 429 00:24:44,640 --> 00:24:48,560 Speaker 1: there's this contradiction between what two different observers report, and 430 00:24:48,600 --> 00:24:51,080 Speaker 1: that's at the heart of relativity. It tells you that 431 00:24:51,240 --> 00:24:56,240 Speaker 1: observations are relative, right, that time itself is relative. So 432 00:24:56,280 --> 00:24:58,439 Speaker 1: then Anders wants to take this to the extreme, and 433 00:24:58,480 --> 00:25:01,160 Speaker 1: he says, well, say I'm in my spaceship and I'm 434 00:25:01,160 --> 00:25:04,920 Speaker 1: going at the speed of light relative to Earth. When 435 00:25:05,040 --> 00:25:07,800 Speaker 1: somebody looks at my spaceship, where they see my time 436 00:25:07,840 --> 00:25:11,399 Speaker 1: slowing down so much that it's almost stopped. And the 437 00:25:11,440 --> 00:25:14,439 Speaker 1: answer that, of course is yes. As you approach the 438 00:25:14,440 --> 00:25:17,679 Speaker 1: speed of light, your time goes slower and slower and slower, 439 00:25:17,680 --> 00:25:19,480 Speaker 1: And the closer you get to the speed of light, 440 00:25:19,720 --> 00:25:22,919 Speaker 1: the slower your clock goes. Again, according to the person 441 00:25:23,160 --> 00:25:25,679 Speaker 1: for whom you have that high speed, you don't have 442 00:25:25,720 --> 00:25:28,560 Speaker 1: a high speed relative to yourself, so you don't observe 443 00:25:28,680 --> 00:25:31,119 Speaker 1: that effect. So what happens if you do go the 444 00:25:31,160 --> 00:25:35,879 Speaker 1: speed of light? Does time actually stop? Right? Is something 445 00:25:35,960 --> 00:25:38,879 Speaker 1: moving at the speed of light like frozen in time? 446 00:25:39,160 --> 00:25:41,400 Speaker 1: So unfortunately, this is not a question we can really 447 00:25:41,440 --> 00:25:45,080 Speaker 1: answer in the satisfactory way because no spaceship with a 448 00:25:45,119 --> 00:25:47,200 Speaker 1: person and a clock and it can go at the 449 00:25:47,240 --> 00:25:50,479 Speaker 1: speed of light. Those things all have mass, and nothing 450 00:25:50,480 --> 00:25:53,520 Speaker 1: that has mass can go at the speed of light. 451 00:25:53,760 --> 00:25:57,600 Speaker 1: That's a fundamental rule. Light always moves at the speed 452 00:25:57,640 --> 00:25:59,920 Speaker 1: of light, no matter who is measuring it. But thing 453 00:26:00,119 --> 00:26:04,560 Speaker 1: that have mass, particles, people, lava, hamsters, nothing that has 454 00:26:04,680 --> 00:26:07,400 Speaker 1: mass to it can move at the speed of light. 455 00:26:07,600 --> 00:26:09,959 Speaker 1: So that means that you can't do that experiment and 456 00:26:10,080 --> 00:26:12,960 Speaker 1: see time stop for the ship moving at the speed 457 00:26:12,960 --> 00:26:15,280 Speaker 1: of light because the ship cannot move at the speed 458 00:26:15,320 --> 00:26:18,280 Speaker 1: of light. That's fine because and it's actually asked a 459 00:26:18,320 --> 00:26:22,680 Speaker 1: slightly different question. He said, what is time for a photon? 460 00:26:23,080 --> 00:26:26,440 Speaker 1: Because photons do move at the speed of light. Can 461 00:26:26,440 --> 00:26:28,680 Speaker 1: we think about what it's like to be a photon 462 00:26:29,119 --> 00:26:32,320 Speaker 1: or can we think about what time is like for 463 00:26:32,440 --> 00:26:36,080 Speaker 1: a photon? Unfortunately, this isn't a question that really has 464 00:26:36,119 --> 00:26:39,359 Speaker 1: a satisfactory answer because it's not something we really even 465 00:26:39,440 --> 00:26:42,720 Speaker 1: know how to measure. I mean, think about the measurement 466 00:26:42,720 --> 00:26:45,760 Speaker 1: we did where we had somebody on the spaceship. We 467 00:26:45,800 --> 00:26:47,919 Speaker 1: had their clock and we had our clock, and we 468 00:26:47,920 --> 00:26:50,960 Speaker 1: were comparing our clock and their clock. If we want 469 00:26:51,000 --> 00:26:54,159 Speaker 1: to think about a photons version of time, where's the 470 00:26:54,200 --> 00:26:58,399 Speaker 1: photons clock? Right? How do we compare our clock to 471 00:26:58,640 --> 00:27:02,760 Speaker 1: their clock? Basically we need in both frames to have 472 00:27:02,920 --> 00:27:06,919 Speaker 1: a clock that's stationary with respect to the object. So 473 00:27:06,960 --> 00:27:08,840 Speaker 1: when you were on the spaceship, you had a clock 474 00:27:08,920 --> 00:27:11,280 Speaker 1: that was not moving relative to you, And I had 475 00:27:11,280 --> 00:27:13,480 Speaker 1: a clock that was not moving relative to me, and 476 00:27:13,520 --> 00:27:16,560 Speaker 1: I was comparing your clock to my clock, and you 477 00:27:16,600 --> 00:27:19,080 Speaker 1: were moving quickly relative to me, So I saw your 478 00:27:19,080 --> 00:27:23,520 Speaker 1: clock running slowly. But a photon doesn't have a clock, right, 479 00:27:23,840 --> 00:27:27,440 Speaker 1: You can't put a clock in the photons frame. Another 480 00:27:27,480 --> 00:27:30,320 Speaker 1: way to think about this is that we could measure 481 00:27:30,400 --> 00:27:33,720 Speaker 1: time passing in the photons frame by having it passed 482 00:27:33,760 --> 00:27:36,600 Speaker 1: two stationary clock like put a clock on one side 483 00:27:36,600 --> 00:27:38,680 Speaker 1: of a track and on the other side of the track, 484 00:27:38,920 --> 00:27:41,800 Speaker 1: and fire a laser beam. And the way we measure 485 00:27:42,320 --> 00:27:44,560 Speaker 1: time on something that's going from one side of the 486 00:27:44,600 --> 00:27:47,479 Speaker 1: track to the other. As we look at the clock 487 00:27:47,560 --> 00:27:50,119 Speaker 1: that thing is holding, we asked how much time is 488 00:27:50,160 --> 00:27:53,320 Speaker 1: elapsed from the beginning to the end, And typically we 489 00:27:53,400 --> 00:27:55,679 Speaker 1: put two synchronized clocks, one of the beginning of the 490 00:27:55,840 --> 00:27:57,639 Speaker 1: race and one of the end of the race, so 491 00:27:57,720 --> 00:27:59,680 Speaker 1: we can compare it to the one that the thing 492 00:27:59,720 --> 00:28:03,600 Speaker 1: that's moving fast is carrying. But again, a photon cannot 493 00:28:03,720 --> 00:28:07,960 Speaker 1: carry a clock, right, I cannot hold anything. And in fact, 494 00:28:08,040 --> 00:28:12,680 Speaker 1: special relativity only deals with inertial frames, frames of reference 495 00:28:12,680 --> 00:28:14,679 Speaker 1: where you can measure things some way. You can have 496 00:28:14,760 --> 00:28:17,960 Speaker 1: things at rest or things in motion, but a photon 497 00:28:18,160 --> 00:28:21,920 Speaker 1: cannot have a frame. There is no frame of reference 498 00:28:22,000 --> 00:28:25,280 Speaker 1: for the photon. That's because a photon can't be at 499 00:28:25,320 --> 00:28:28,760 Speaker 1: rest with respect to anything. Anybody who measures a speed 500 00:28:28,760 --> 00:28:31,520 Speaker 1: of light will always find it to be moving at 501 00:28:31,560 --> 00:28:34,240 Speaker 1: the speed of light. There's no way you can like 502 00:28:34,440 --> 00:28:37,879 Speaker 1: catch up to a photon. I remember, a photon is 503 00:28:37,960 --> 00:28:41,200 Speaker 1: nothing but motion. It has no mass to it, so 504 00:28:41,360 --> 00:28:43,680 Speaker 1: if you caught up to a photon, it wouldn't be 505 00:28:43,800 --> 00:28:47,360 Speaker 1: anything anymore. So there is no frame of reference. You 506 00:28:47,360 --> 00:28:51,960 Speaker 1: can't do these special relativity calculations to transform yourself from 507 00:28:51,960 --> 00:28:55,160 Speaker 1: our frame to the photons frame and calculate how much 508 00:28:55,200 --> 00:28:58,160 Speaker 1: time has passed. It's a bit of an unsatisfactory answer 509 00:28:58,240 --> 00:29:01,840 Speaker 1: because while the limit special relativity as you approach the 510 00:29:01,880 --> 00:29:05,960 Speaker 1: speed of light seems to suggest that time stops at 511 00:29:06,040 --> 00:29:08,680 Speaker 1: the speed of light, actually doing these calculations at the 512 00:29:08,680 --> 00:29:12,120 Speaker 1: speed of light gives you nonsense answers, right, And what 513 00:29:12,160 --> 00:29:16,320 Speaker 1: that means is that essentially photons can't be observers. They 514 00:29:16,360 --> 00:29:19,959 Speaker 1: can't like make measurements about the universe. You can't calculate 515 00:29:20,280 --> 00:29:23,360 Speaker 1: what it's like to be a photon using special relativity, 516 00:29:23,520 --> 00:29:27,040 Speaker 1: because special relativity requires these frames of reference to have 517 00:29:27,160 --> 00:29:30,720 Speaker 1: an observer who's capable of making these measurements, and photons 518 00:29:30,760 --> 00:29:33,920 Speaker 1: just are not. And I totally understand that it's an 519 00:29:33,960 --> 00:29:37,080 Speaker 1: unsatisfactory answer. We'd like to say something awesome and cool, 520 00:29:37,120 --> 00:29:41,000 Speaker 1: like photons don't experience any time, and when they're born 521 00:29:41,040 --> 00:29:44,280 Speaker 1: in the Drameda, they're here the next moment, But really 522 00:29:44,280 --> 00:29:46,960 Speaker 1: that's not something that we can strictly say. We can 523 00:29:46,960 --> 00:29:49,320 Speaker 1: write science fiction novel about it, we can think it's 524 00:29:49,360 --> 00:29:51,920 Speaker 1: really cool, but we can't actually know what it's like 525 00:29:52,080 --> 00:29:54,800 Speaker 1: to be a photon. And if you're a science fiction 526 00:29:54,880 --> 00:29:57,320 Speaker 1: enthusiastic might be imagining, like, well, what if we had 527 00:29:57,320 --> 00:30:00,400 Speaker 1: a being who has made only out of photo potons? 528 00:30:00,760 --> 00:30:04,760 Speaker 1: Could you have a conscious observer made out of photons? Well, 529 00:30:04,800 --> 00:30:07,360 Speaker 1: these photons needs to be moving at the speed of light, 530 00:30:07,440 --> 00:30:10,080 Speaker 1: and photons don't interact with each other, so it seems 531 00:30:10,120 --> 00:30:14,480 Speaker 1: pretty incomprehensible to like build an intelligence or consciousness or 532 00:30:14,480 --> 00:30:17,520 Speaker 1: an observer out of these photons. And even if you 533 00:30:17,560 --> 00:30:20,480 Speaker 1: could remember, it would still have no frame of reference. 534 00:30:20,480 --> 00:30:23,920 Speaker 1: It couldn't measure anything in the universe. All right, Andrews, 535 00:30:23,920 --> 00:30:26,560 Speaker 1: thanks very much for that question. Unfortunately, we do not 536 00:30:26,760 --> 00:30:29,640 Speaker 1: know what it's like to be a photon. We cannot 537 00:30:29,720 --> 00:30:32,360 Speaker 1: strictly answer that question. We know that as you approach 538 00:30:32,400 --> 00:30:35,200 Speaker 1: a speed of light, your time seems to slow down 539 00:30:35,280 --> 00:30:37,440 Speaker 1: from the point of view of other people, but we 540 00:30:37,480 --> 00:30:40,160 Speaker 1: don't actually know what it's like to go at the 541 00:30:40,160 --> 00:30:42,120 Speaker 1: speed of light, and we don't even know if it's 542 00:30:42,160 --> 00:30:45,560 Speaker 1: like anything. Well, with that in mind, let's take another 543 00:30:45,560 --> 00:30:48,280 Speaker 1: break and I'm gonna dig into one more big question 544 00:30:48,440 --> 00:31:04,520 Speaker 1: about the universe. Alright, we're back and we're tackling the 545 00:31:04,560 --> 00:31:07,840 Speaker 1: biggest of questions today about the expansion of the universe 546 00:31:07,880 --> 00:31:10,000 Speaker 1: and the speed of light. And here's one about the 547 00:31:10,120 --> 00:31:13,719 Speaker 1: very age of the universe. Hi, then, Jorge, I may 548 00:31:13,800 --> 00:31:16,680 Speaker 1: do from Argentina, I have a question for you. So, 549 00:31:16,800 --> 00:31:19,880 Speaker 1: general relativity tells us that there is no absolute frame 550 00:31:19,920 --> 00:31:23,560 Speaker 1: of reference by which you can measure time. It always 551 00:31:23,600 --> 00:31:28,400 Speaker 1: depends on the observer. So if that's the case, how 552 00:31:28,480 --> 00:31:30,880 Speaker 1: can we mark the passage of time in such a 553 00:31:30,920 --> 00:31:34,440 Speaker 1: way that allows us to define the edge of the 554 00:31:34,600 --> 00:31:39,280 Speaker 1: universe as being thirteen point eight billion years old? What 555 00:31:39,480 --> 00:31:41,360 Speaker 1: is the frame of reference used to arrive to that 556 00:31:41,560 --> 00:31:44,160 Speaker 1: number and how can it change? If you choose another frame. 557 00:31:44,760 --> 00:31:49,040 Speaker 1: Thanks a lot, guys. All right, Eduardo, excellent question again. 558 00:31:49,200 --> 00:31:52,760 Speaker 1: Eduardo is trying to understand how two things can't fit 559 00:31:52,800 --> 00:31:56,280 Speaker 1: together in his mind. On one hand, we say all 560 00:31:56,280 --> 00:31:58,120 Speaker 1: the time, and I just said a few minutes ago, 561 00:31:58,680 --> 00:32:02,320 Speaker 1: that the passage of time depends on who is observing 562 00:32:02,760 --> 00:32:06,040 Speaker 1: the clock and what velocity is being measured, and who 563 00:32:06,120 --> 00:32:09,720 Speaker 1: is measuring that velocity. There is no absolute frame of reference. 564 00:32:09,960 --> 00:32:13,480 Speaker 1: On the other hand, we also say the universe is 565 00:32:13,560 --> 00:32:15,760 Speaker 1: thirteen four and eight billion years old, and there's been 566 00:32:15,760 --> 00:32:17,720 Speaker 1: a certain amount of time for a light to get 567 00:32:17,760 --> 00:32:19,960 Speaker 1: to us. For example, how can we talk about the 568 00:32:20,000 --> 00:32:24,280 Speaker 1: universe as having an age like one single number if 569 00:32:24,360 --> 00:32:28,640 Speaker 1: everybody's clocks are different, if there is no universal sense 570 00:32:28,680 --> 00:32:32,360 Speaker 1: of time. And really interesting consequence of relativity is that 571 00:32:32,440 --> 00:32:36,200 Speaker 1: you can't really talk about simultaneity of events if they 572 00:32:36,200 --> 00:32:39,440 Speaker 1: are separated in space. That the order of events of 573 00:32:39,480 --> 00:32:42,800 Speaker 1: things that are far apart can be different according to 574 00:32:42,840 --> 00:32:46,400 Speaker 1: your perspective. If you're moving fast or if you're moving slow, 575 00:32:46,440 --> 00:32:48,560 Speaker 1: if you're over here, if you're over there, you might 576 00:32:48,600 --> 00:32:51,600 Speaker 1: see A happen before B, and somebody else might see 577 00:32:51,640 --> 00:32:55,520 Speaker 1: B happen before A, and you can both be correct, 578 00:32:56,000 --> 00:32:58,959 Speaker 1: because in this universe, the order of things is not 579 00:32:59,160 --> 00:33:02,920 Speaker 1: necessarily nailed down if you're far enough apart. We dug 580 00:33:02,960 --> 00:33:05,560 Speaker 1: into this in a whole episode about a foot race 581 00:33:05,760 --> 00:33:08,440 Speaker 1: running really really fast near the speed of light, and 582 00:33:08,520 --> 00:33:11,600 Speaker 1: who wins the race depends on who is watching and 583 00:33:11,640 --> 00:33:15,280 Speaker 1: their velocity relative to the race, And so all of 584 00:33:15,320 --> 00:33:18,040 Speaker 1: this up ends the whole concept of having a single 585 00:33:18,120 --> 00:33:23,000 Speaker 1: age of the universe, which suggests a simultaneous event happening 586 00:33:23,120 --> 00:33:27,120 Speaker 1: all across the universe, all at once. And so Eduardo 587 00:33:27,280 --> 00:33:30,480 Speaker 1: is asking, is that possible? Does that make any sense? 588 00:33:30,800 --> 00:33:33,200 Speaker 1: And if it doesn't, then how do you define the 589 00:33:33,280 --> 00:33:36,520 Speaker 1: age of the universe. So the short answer is, Eduardo, 590 00:33:36,640 --> 00:33:39,800 Speaker 1: you're absolutely correct. The age of the universe is not 591 00:33:40,040 --> 00:33:43,880 Speaker 1: a fixed number. It does indeed depend on your frame 592 00:33:43,880 --> 00:33:47,640 Speaker 1: of reference or who is asking the question to think about, like, 593 00:33:47,680 --> 00:33:50,840 Speaker 1: the length of time that our universe has existed is 594 00:33:50,880 --> 00:33:54,160 Speaker 1: not even a number, right that it depends on who 595 00:33:54,280 --> 00:33:57,040 Speaker 1: is asking the question. And that's because the passage of 596 00:33:57,120 --> 00:34:01,280 Speaker 1: time does seem to depend on your beat. Right, Your 597 00:34:01,360 --> 00:34:04,680 Speaker 1: observation of how clocks are taking depends on how fast 598 00:34:04,960 --> 00:34:08,400 Speaker 1: those clocks are taking relative to You can't go all 599 00:34:08,440 --> 00:34:09,879 Speaker 1: the way to the speed of light, like we talked 600 00:34:09,880 --> 00:34:12,280 Speaker 1: about a minute ago. But you can get pretty dramatic 601 00:34:12,360 --> 00:34:17,319 Speaker 1: effects by going or of the speed of light. So 602 00:34:17,360 --> 00:34:20,040 Speaker 1: what does that mean. It means that like, something moving 603 00:34:20,480 --> 00:34:23,719 Speaker 1: really really fast relative to most of the rest of 604 00:34:23,760 --> 00:34:27,360 Speaker 1: the stuff in the universe will have its clock running 605 00:34:27,400 --> 00:34:30,680 Speaker 1: more slowly than our clocks, and if it ever slows 606 00:34:30,760 --> 00:34:34,080 Speaker 1: down and comes back into our frame of reference and 607 00:34:34,160 --> 00:34:38,200 Speaker 1: stops moving relative to us, it will seem younger than 608 00:34:38,280 --> 00:34:40,880 Speaker 1: everything else in the universe. And this is not something weird. 609 00:34:40,920 --> 00:34:43,800 Speaker 1: We observed this all the time. Take for example, a muan. 610 00:34:44,120 --> 00:34:47,080 Speaker 1: A muon is a sub atomic particle and it only 611 00:34:47,200 --> 00:34:50,520 Speaker 1: lasts for a few micro seconds before it decays into 612 00:34:50,560 --> 00:34:53,279 Speaker 1: electrons and other stuff. But we have muans which are 613 00:34:53,400 --> 00:34:55,960 Speaker 1: created in the upper atmosphere and make it all the 614 00:34:56,000 --> 00:34:59,120 Speaker 1: way down here to the ground. How is that possible 615 00:34:59,360 --> 00:35:02,200 Speaker 1: if mu wants only live for a few microseconds. Well, 616 00:35:02,200 --> 00:35:05,120 Speaker 1: the answer is that they're moving relativistically, and so their 617 00:35:05,239 --> 00:35:08,879 Speaker 1: clocks aren't running slower, and so when they get here 618 00:35:09,120 --> 00:35:11,839 Speaker 1: there are only a few microseconds old in their frame 619 00:35:11,880 --> 00:35:14,799 Speaker 1: of reference, right, and so they're like younger than they 620 00:35:14,880 --> 00:35:17,239 Speaker 1: otherwise would have been. They haven't aged as much as 621 00:35:17,239 --> 00:35:20,520 Speaker 1: they otherwise would have because relativity has kept them young. 622 00:35:21,120 --> 00:35:23,120 Speaker 1: And when they come down here to the service of 623 00:35:23,120 --> 00:35:25,680 Speaker 1: the Earth and they slow down and they stop, they 624 00:35:25,719 --> 00:35:28,239 Speaker 1: see the rest of the universe is sort of older 625 00:35:28,400 --> 00:35:32,280 Speaker 1: than they expect. So if you've been flying around, moving 626 00:35:32,320 --> 00:35:34,960 Speaker 1: really really fast relative to the rest of the stuff 627 00:35:35,000 --> 00:35:37,880 Speaker 1: in the universe for most of the life of the universe, 628 00:35:38,160 --> 00:35:39,960 Speaker 1: and then you slow down and you sort of hang 629 00:35:39,960 --> 00:35:41,680 Speaker 1: out with the rest of the stuff in the universe 630 00:35:41,920 --> 00:35:44,400 Speaker 1: and stop your motion, you will see the rest of 631 00:35:44,400 --> 00:35:47,239 Speaker 1: the universe is sort of surprisingly old. You will have 632 00:35:47,320 --> 00:35:49,799 Speaker 1: only lived for a billion years, for example, and you'll 633 00:35:49,800 --> 00:35:52,400 Speaker 1: be surprised to find the rest of the universe seeming 634 00:35:52,440 --> 00:35:55,480 Speaker 1: to be like fourteen billion years old. So there's an 635 00:35:55,480 --> 00:35:57,719 Speaker 1: idea and here that's been implicit in my answer that 636 00:35:57,760 --> 00:36:00,680 Speaker 1: I want to make explicit, which is the stuff in 637 00:36:00,719 --> 00:36:05,839 Speaker 1: the universe. Eduardo asks, if time is actually relative, then 638 00:36:05,880 --> 00:36:08,920 Speaker 1: how do we measure the age of the universe. Because 639 00:36:08,960 --> 00:36:12,319 Speaker 1: malogists have this idea of a proper time, they say, well, 640 00:36:12,440 --> 00:36:15,520 Speaker 1: everybody measures time differently, so let's just agree on a 641 00:36:15,600 --> 00:36:19,520 Speaker 1: definition of time as being according to a clock that's 642 00:36:19,560 --> 00:36:22,399 Speaker 1: stationary with respect to an object. So I can talk 643 00:36:22,440 --> 00:36:25,040 Speaker 1: about my proper time. It's the time measured by a 644 00:36:25,040 --> 00:36:28,400 Speaker 1: clock that I'm I'm holding. Somebody else moving fast past 645 00:36:28,520 --> 00:36:31,600 Speaker 1: me might see my clock moving slowly, but they can 646 00:36:31,640 --> 00:36:34,960 Speaker 1: agree that I measure my time and that's my proper time. 647 00:36:35,120 --> 00:36:37,200 Speaker 1: So this is a long way of saying that we 648 00:36:37,320 --> 00:36:40,800 Speaker 1: define one specific frame of reference when we talk about 649 00:36:40,800 --> 00:36:42,960 Speaker 1: the age of the universe, and that's the frame of 650 00:36:43,000 --> 00:36:46,400 Speaker 1: reference that's at rest with respect to most of the 651 00:36:46,560 --> 00:36:49,279 Speaker 1: stuff in the universe. Because when we're talking about the 652 00:36:49,280 --> 00:36:51,240 Speaker 1: age of the universe, we want to know, like how 653 00:36:51,239 --> 00:36:53,600 Speaker 1: old is the stuff around us? How long has this 654 00:36:53,760 --> 00:36:56,000 Speaker 1: stuff been here? I mean, I know there's also a 655 00:36:56,000 --> 00:36:57,960 Speaker 1: deeper level on which we want to ask the question 656 00:36:58,000 --> 00:37:01,880 Speaker 1: of like the whole existence of the universe, separate from 657 00:37:01,920 --> 00:37:04,160 Speaker 1: the question of like the stuff in it. But we 658 00:37:04,280 --> 00:37:06,399 Speaker 1: have to pick one frame of reference when we talk 659 00:37:06,480 --> 00:37:09,080 Speaker 1: about this. So we pick the stuff in the universe, 660 00:37:09,200 --> 00:37:11,960 Speaker 1: and most specifically, we picked the frame of reference in 661 00:37:12,080 --> 00:37:17,120 Speaker 1: which the cosmic microwave background radiation is at rest. That 662 00:37:17,320 --> 00:37:21,600 Speaker 1: early universe plasma that was created three hundred eighty thousand 663 00:37:21,680 --> 00:37:24,359 Speaker 1: years after the birth of the universe, that had a 664 00:37:24,400 --> 00:37:26,719 Speaker 1: frame of reference. There's a frame in which that is, 665 00:37:26,800 --> 00:37:30,680 Speaker 1: on average, not in motion, and that's not an absolute 666 00:37:30,719 --> 00:37:35,000 Speaker 1: reference frame, right, Like, the universe doesn't prefer that reference 667 00:37:35,040 --> 00:37:37,840 Speaker 1: frame for any reason. It just happens to be the 668 00:37:37,880 --> 00:37:40,479 Speaker 1: one in which that stuff is at rest. The laws 669 00:37:40,520 --> 00:37:43,239 Speaker 1: of physics still work in other reference frames. That's what 670 00:37:43,320 --> 00:37:46,240 Speaker 1: we mean when we say there's no absolute reference frame. 671 00:37:46,320 --> 00:37:48,640 Speaker 1: But there is a reference frame at which most of 672 00:37:48,640 --> 00:37:51,120 Speaker 1: the stuff in the universe is at rest, and that's 673 00:37:51,160 --> 00:37:54,040 Speaker 1: the one where the CMB is at rest. And so 674 00:37:54,239 --> 00:37:56,880 Speaker 1: that's how we define the age of the universe. The 675 00:37:56,960 --> 00:38:00,000 Speaker 1: universe is thirteen point eight billion years old according to 676 00:38:00,120 --> 00:38:03,400 Speaker 1: clocks that are not in motion relative to the cosmic 677 00:38:03,480 --> 00:38:07,800 Speaker 1: microwave background radiation. Now is the Earth in motion relative 678 00:38:07,840 --> 00:38:12,120 Speaker 1: to the cosmic microwave background radiation? Actually, yes, we are 679 00:38:12,200 --> 00:38:15,200 Speaker 1: moving through the CMB because you know, our motion is 680 00:38:15,239 --> 00:38:18,040 Speaker 1: actually quite complicated when moving around the Sun. The Sun 681 00:38:18,120 --> 00:38:21,080 Speaker 1: is moving around the center of the galaxy. Our galaxy 682 00:38:21,120 --> 00:38:23,240 Speaker 1: is moving around the center of mass of the local 683 00:38:23,320 --> 00:38:26,759 Speaker 1: galactic cluster, So our motion is quite complicated. And we're 684 00:38:26,760 --> 00:38:32,000 Speaker 1: actually moving through the CMB. It's several hundred kilometers per second. 685 00:38:32,680 --> 00:38:34,880 Speaker 1: So what that means is that the age of the 686 00:38:34,960 --> 00:38:38,680 Speaker 1: universe according to Earth clocks is a tiny little bit 687 00:38:38,719 --> 00:38:41,520 Speaker 1: different than it would be if our clocks were not 688 00:38:41,640 --> 00:38:45,320 Speaker 1: at motion relative to the CMB, But even several hundred 689 00:38:45,400 --> 00:38:48,960 Speaker 1: kilometers per second is pretty small compared to the speed 690 00:38:49,000 --> 00:38:51,839 Speaker 1: of light, and so it's a very small effect. In fact, 691 00:38:51,920 --> 00:38:55,760 Speaker 1: it's much much smaller than our uncertainty on the age 692 00:38:55,760 --> 00:38:58,640 Speaker 1: of the universe, so it's not a dominant effect at all, 693 00:38:58,840 --> 00:39:01,879 Speaker 1: and velocity is actually not the only effect. There are 694 00:39:01,880 --> 00:39:05,359 Speaker 1: other things that can slow down your clocks. If you 695 00:39:05,360 --> 00:39:08,240 Speaker 1: are near an object with mass, like a black hole 696 00:39:08,480 --> 00:39:11,919 Speaker 1: or a son, that also slows down your clock. That's 697 00:39:11,960 --> 00:39:15,640 Speaker 1: called gravitational time dilation, and we'll dig into that in 698 00:39:15,719 --> 00:39:18,640 Speaker 1: another episode the reasons for that. But for example, if 699 00:39:18,640 --> 00:39:21,800 Speaker 1: you get near a black hole, then your clock slows down. 700 00:39:22,080 --> 00:39:24,680 Speaker 1: So if you've spent a good fraction of your life 701 00:39:24,840 --> 00:39:27,960 Speaker 1: near a black hole, then you will not have aged 702 00:39:28,000 --> 00:39:30,960 Speaker 1: as much and you would think the universe is surprisingly old. 703 00:39:31,200 --> 00:39:33,960 Speaker 1: So the short answer to Edwardo's question is that we 704 00:39:34,040 --> 00:39:37,239 Speaker 1: do pick a frame of reference. It's the CMB, and 705 00:39:37,280 --> 00:39:39,840 Speaker 1: the age of the universe is quoted as relative to 706 00:39:39,880 --> 00:39:43,880 Speaker 1: the CMB. For observers that have not been near very 707 00:39:43,920 --> 00:39:47,360 Speaker 1: massive objects, So thanks very much for that question, Edwardo. 708 00:39:47,560 --> 00:39:50,400 Speaker 1: Thanks for thinking hard about whether these ideas really fit together. 709 00:39:50,600 --> 00:39:53,319 Speaker 1: I hope I help those click together in your mind. 710 00:39:53,560 --> 00:39:56,480 Speaker 1: And thanks to everybody who has sent in questions, who 711 00:39:56,480 --> 00:40:00,239 Speaker 1: continues to support the podcast, who shares with us your curiosity, 712 00:40:00,400 --> 00:40:03,440 Speaker 1: and it's been along for this super fun ride exploring 713 00:40:03,520 --> 00:40:07,160 Speaker 1: our bonkers universe. If you have questions you like answers to, 714 00:40:07,320 --> 00:40:10,240 Speaker 1: please don't be shy send them to me to questions 715 00:40:10,320 --> 00:40:13,480 Speaker 1: at Daniel and Jorge dot com. I know you're out there. 716 00:40:13,719 --> 00:40:15,959 Speaker 1: I know you have a question. I know you haven't 717 00:40:16,000 --> 00:40:26,560 Speaker 1: sent it to me. I just don't know why. Thanks 718 00:40:26,600 --> 00:40:29,600 Speaker 1: for listening, and remember that Daniel and Jorge explained the universe. 719 00:40:29,640 --> 00:40:33,080 Speaker 1: It's a production of I Heart Radio. For more podcast 720 00:40:33,200 --> 00:40:35,880 Speaker 1: from my Heart Radio, visit the I Heart Radio Apple 721 00:40:36,120 --> 00:40:39,600 Speaker 1: Apple Podcasts, or wherever you listen to your favorite shows.