WEBVTT - What is so beautiful about string theory? 0:00:07.040 --> 0:00:10.040 People might disagree about what kind of art they like. 0:00:10.119 --> 0:00:13.080 In fact, pretty much everybody does. But we all know 0:00:13.160 --> 0:00:16.720 what it means when we say that something is beautiful. 0:00:17.320 --> 0:00:20.040 It means that we appreciate it, that it moves us, 0:00:20.079 --> 0:00:23.160 that it strikes us, that we see an elegance in it. 0:00:23.440 --> 0:00:24.960 But what does it mean if you say a theory 0:00:25.000 --> 0:00:28.480 of physics or a little bit of math is beautiful? 0:00:28.960 --> 0:00:32.920 How can math be gorgeous? How can physics be elegant? 0:00:33.280 --> 0:00:37.120 What does that really mean? Well, string theory is the 0:00:37.120 --> 0:00:40.519 theory of physics that's most often described as a bit 0:00:40.560 --> 0:00:45.400 of twenty first century gorgeous physics that fell into our laps. 0:00:45.840 --> 0:00:49.479 What does that really mean? What is so beautiful about 0:00:49.560 --> 0:00:53.959 string theory? And just because something is beautiful, does that 0:00:54.080 --> 0:00:57.320 tell us whether it's more likely to describe our universe 0:00:57.400 --> 0:01:01.360 to actually be right? That's the question we're going to 0:01:01.400 --> 0:01:05.720 be asking today on the podcast What's so beautiful about 0:01:05.760 --> 0:01:10.200 string theory? Welcome to Daniel and Kelly's Extraordinary Universe. 0:01:24.600 --> 0:01:28.319 Hello, I'm Kelly Wiener Smith, and I know nothing about 0:01:28.400 --> 0:01:30.880 string theory. In fact, sometimes my eyes crossed and I 0:01:30.959 --> 0:01:33.640 just blankly stare at the wall. When the conversation comes up, 0:01:33.720 --> 0:01:35.560 But today I'm gonna understand it. 0:01:35.959 --> 0:01:38.760 Hi. I'm Daniel Whitson. I'm a particle physicist, which might 0:01:38.840 --> 0:01:41.039 make it sound like I should know string theory, but 0:01:41.160 --> 0:01:44.440 actually it means I just smashed particles together without understanding 0:01:44.440 --> 0:01:45.560 the nature of the universe. 0:01:45.920 --> 0:01:47.800 Oh all right, Well, so I've got a question for you. 0:01:47.840 --> 0:01:51.240 I listened to your beautiful opening about what does it 0:01:51.320 --> 0:01:54.200 mean to have a gorgeous equation? So my question for 0:01:54.280 --> 0:01:57.280 you is, what is the most beautiful equation? 0:01:59.000 --> 0:02:03.400 The most beautiful equation? Oh my gosh. To me, the 0:02:03.440 --> 0:02:06.680 most beautiful equation is actually not in physics. It's in math. 0:02:06.960 --> 0:02:11.520 Oiler's identity. It says E to the IPI plus one 0:02:11.760 --> 0:02:16.920 equals zero, And I just think it's incredible because it 0:02:16.960 --> 0:02:19.680 combines like a bunch of different stuff. You have E, 0:02:20.000 --> 0:02:24.960 pie zero, one and I all together, and it's so compact, 0:02:25.200 --> 0:02:27.360 and it's just so much encoded into it. It's like 0:02:27.440 --> 0:02:31.639 so dense with useful information. It tells you about how 0:02:31.680 --> 0:02:34.160 you can think about signs and cosigns in terms of 0:02:34.200 --> 0:02:36.760 complex numbers. To me, it's just fascinating to have so 0:02:36.840 --> 0:02:40.480 much information packed so tightly and so beautifully into a 0:02:40.520 --> 0:02:41.200 single equation. 0:02:41.520 --> 0:02:43.520 Awesome, A fine choice. 0:02:43.600 --> 0:02:45.760 But I also have to say that in grad school, 0:02:45.800 --> 0:02:47.680 the moment I discovered I was not going to be 0:02:47.680 --> 0:02:50.480 a theoretical physicist was when I was sitting next to 0:02:50.600 --> 0:02:52.800 my office mate and I realized that he did his 0:02:52.840 --> 0:02:55.040 homework just like I did, but he did it two 0:02:55.160 --> 0:02:58.480 or three times in different fonts because he got really 0:02:58.520 --> 0:03:01.520 excited about like writing these equations. He's like, Oh, I'm 0:03:01.520 --> 0:03:04.000 all writing in italics, or I can write these symbols 0:03:04.040 --> 0:03:07.400 another way. And I realize, like, wow, this kid really 0:03:07.800 --> 0:03:11.280 jams out about like writing down the equations. It's something 0:03:11.320 --> 0:03:14.919 about being a theoretical physicist that I just didn't have. 0:03:15.360 --> 0:03:17.080 I was like happy to be done with it once. 0:03:17.440 --> 0:03:19.440 Yeah, I gotta be honest, that doesn't strike me as 0:03:19.480 --> 0:03:22.720 super efficient. I'm going to do the same thing three times, 0:03:22.840 --> 0:03:24.799 but I'm glad that he's super into it. 0:03:24.919 --> 0:03:27.519 Yeah, but there's something about the equations and the formalisms 0:03:27.520 --> 0:03:29.520 and the expressions and even the fonts. The way you're 0:03:29.520 --> 0:03:32.240 writing these mathematical symbols, then you've got to be excited 0:03:32.280 --> 0:03:34.240 about if you're going to work in the nitty gritty 0:03:34.560 --> 0:03:37.280 of figuring these things out. Because being a theoretical physicist 0:03:37.320 --> 0:03:39.760 is a lot about writing equations on paper, So if 0:03:39.760 --> 0:03:42.000 you don't like that, then probably shouldn't be one. 0:03:42.320 --> 0:03:43.240 Yeah, fair enough. 0:03:44.000 --> 0:03:48.320 Well, today we're talking about a theory that is regularly 0:03:48.360 --> 0:03:50.440 described as beautiful, and I'll tell you by the end 0:03:50.440 --> 0:03:53.800 of the episode, I'm moderately convinced that string theory is beautiful, 0:03:54.040 --> 0:03:57.480 maybe even more than moderately convinced. But I think we 0:03:57.520 --> 0:04:01.160 should see what our audience thinks about out what's so 0:04:01.200 --> 0:04:04.040 beautiful about string theory? Is this something that people know already? 0:04:04.200 --> 0:04:06.119 That's right. I reached out to our listeners to ask 0:04:06.160 --> 0:04:09.160 them what do they think is beautiful about string theory. 0:04:09.240 --> 0:04:11.560 If you'd like to contribute your voice for future episodes, 0:04:11.600 --> 0:04:15.360 please write to us two questions at Danielankelly dot org. 0:04:15.600 --> 0:04:18.200 We will sign you up. Also send us questions about anything. 0:04:18.560 --> 0:04:21.000 I got a recent question about somebody's dating life which 0:04:21.000 --> 0:04:23.640 I totally couldn't answer, but I enjoyed reading anyway, so 0:04:23.920 --> 0:04:25.120 feel free to write to us. 0:04:25.200 --> 0:04:26.200 You should have sent that to me. 0:04:26.400 --> 0:04:29.800 I was on Dan Savage's podcast, and I feel like 0:04:29.839 --> 0:04:32.120 that makes me a relationship expert, so you can just 0:04:32.160 --> 0:04:32.880 send those to me. 0:04:33.000 --> 0:04:33.800 I've got it covered. 0:04:33.960 --> 0:04:37.160 Okay, there, you go, folks, We are self proclaimed experts 0:04:37.200 --> 0:04:40.760 in anything, So think about it for a minute. What 0:04:40.800 --> 0:04:44.320 do you think is beautiful about string theory? Here's what 0:04:44.440 --> 0:04:47.400 some listeners had to say. I don't think the universe 0:04:47.440 --> 0:04:56.239 would be so elegant to be a dangled, naughty strandfield. 0:04:55.440 --> 0:05:04.560 Mess unifies general relativeivity and quantum mechanics. Physicists love elegance 0:05:04.680 --> 0:05:07.440 and symmetry. It may also provide a new baseline of 0:05:07.640 --> 0:05:09.920 what's the tiniest thing, And then we. 0:05:09.880 --> 0:05:12.359 Get to ask the question is that it? Or is 0:05:12.400 --> 0:05:15.560 there something beyond that? The amount of money that Brian 0:05:15.680 --> 0:05:20.320 Green was able to make by taking advantage of popularizing it, 0:05:20.440 --> 0:05:25.960 we're able to mathematically explain why gravity is so weak 0:05:26.120 --> 0:05:27.760 compared to the other forces. 0:05:28.080 --> 0:05:32.599 So nusks, Dodd, what is string theory? The dad says, 0:05:32.880 --> 0:05:36.040 why you ask such difficult questions? Ask me something easier. 0:05:36.040 --> 0:05:39.440 So the sun says, okay, why does mum get so angry? 0:05:40.279 --> 0:05:40.520 Ah? 0:05:40.560 --> 0:05:43.280 Well, string theory is a theoretical framework. 0:05:43.360 --> 0:05:45.520 It's a kind of symmetrical beauty. 0:05:45.720 --> 0:05:49.680 The beauty to me is that we keep searching for 0:05:49.760 --> 0:05:53.039 the boundaries that would have to be the g string. 0:05:53.200 --> 0:05:57.760 Nothing's beautiful best string theory except that confusion is beautiful. 0:05:57.839 --> 0:06:01.280 People want to try to bring everything thing all together 0:06:01.600 --> 0:06:06.920 into one unified theory that explains everything. I think that's 0:06:06.920 --> 0:06:07.839 what makes it beautiful. 0:06:08.160 --> 0:06:11.839 And they look like worms string theory in all the theories 0:06:11.760 --> 0:06:14.760 that try to bridge this gap really show the spirit 0:06:15.240 --> 0:06:21.320 of scientists and researchers and physicists everywhere to keep on trucking. 0:06:21.200 --> 0:06:24.440 The totality of its failure in the same way that 0:06:25.040 --> 0:06:29.440 a lot of other unfalsifiable and self sealing things are 0:06:29.480 --> 0:06:30.440 in life that we. 0:06:30.400 --> 0:06:33.719 Love that the maths of it all is quite elegant. 0:06:34.000 --> 0:06:36.560 What I like about the idea of it is that 0:06:36.880 --> 0:06:40.760 rather than trying to think of the universe in discrete particles, 0:06:43.000 --> 0:06:46.239 kind of just thinking about it in like pulses of energy. 0:06:46.880 --> 0:06:47.960 I cool it confusing. 0:06:48.640 --> 0:06:51.640 Well, Daniel, it looks like we're teaching the controversy today 0:06:51.720 --> 0:06:53.400 because our. 0:06:53.120 --> 0:06:55.839 Answers range from I don't think there's anything. 0:06:55.560 --> 0:06:59.440 Beautiful about it to you know, it's pleasing esthetically, And 0:06:59.640 --> 0:07:02.080 there was a good range of answers, what do you 0:07:02.080 --> 0:07:02.960 think is it beautiful? 0:07:03.279 --> 0:07:06.599 I think it's fascinating to apply this subjective standard to 0:07:06.680 --> 0:07:09.800 something which is supposed to be objective. Right, we're talking 0:07:09.880 --> 0:07:12.000 about like the answer to the question of how the 0:07:12.160 --> 0:07:16.680 universe runs itself, the machinery of the cosmos. Why do 0:07:16.760 --> 0:07:19.720 we care about whether it's beautiful? Why should beauty be 0:07:19.880 --> 0:07:21.960 a guide? Like if we have two theories, should we 0:07:22.000 --> 0:07:24.800 pick the one that's more beautiful and follow that because 0:07:24.800 --> 0:07:27.200 we think it's more likely. I think it's this sort 0:07:27.200 --> 0:07:30.520 of like bias we have that we think nature should 0:07:30.600 --> 0:07:33.720 be beautiful because we mostly look around and we're like, oh, yeah, 0:07:33.760 --> 0:07:37.280 the world is pretty. I wonder if aliens evolving on 0:07:37.360 --> 0:07:40.360 an ugly planet, one that they find like if kind 0:07:40.360 --> 0:07:43.760 of yucky, would tend to be biased towards yucky theories 0:07:43.800 --> 0:07:47.040 of physics because their life is pretty yucky. Or maybe 0:07:47.040 --> 0:07:50.120 everybody of alves to think that their planet is beautiful 0:07:50.120 --> 0:07:52.480 and everybody tends towards beauty. I don't know. To me, 0:07:52.520 --> 0:07:55.360 it's a deep sort of philosophical question of what is 0:07:55.400 --> 0:07:57.840 beauty anyway, and why do we appreciate it in our 0:07:57.920 --> 0:08:00.000 world and why do we look for it in our physics? 0:08:00.240 --> 0:08:02.880 All right, So first an observation in my experience, it 0:08:02.920 --> 0:08:05.160 seems to me that when people say, oh, this equation 0:08:05.280 --> 0:08:07.920 is beautiful, what it usually means is it makes their 0:08:07.960 --> 0:08:12.559 life easier. It explains a lot of things. And maybe 0:08:12.560 --> 0:08:15.440 this is human laziness is the wrong answer, because most 0:08:15.440 --> 0:08:17.520 of the people working on these equations or anything but lazy. 0:08:17.520 --> 0:08:19.720 But like, oh, it's nice. It explains a lot of things. 0:08:19.760 --> 0:08:21.640 I don't have to worry about that stuff. So you 0:08:22.080 --> 0:08:25.800 did seem earlier to think that Euler's equation was beautiful, 0:08:26.360 --> 0:08:28.840 but now you seem to be a little bit more 0:08:28.880 --> 0:08:32.680 critical of people saying talking about equations that describe the 0:08:32.800 --> 0:08:34.000 universe as beautiful. 0:08:34.000 --> 0:08:35.600 Do you feel like there's some difference there. 0:08:35.920 --> 0:08:39.200 No, I can see beauty and I can appreciate it. It's 0:08:38.880 --> 0:08:41.920 like when you see a piece of machinery and there's 0:08:41.920 --> 0:08:43.760 only a few moving parts, but it can do something 0:08:43.800 --> 0:08:46.199 really complex, or you look at a piece of code 0:08:46.200 --> 0:08:49.800 and you're like, wow, that is so simple and yet powerful. 0:08:50.120 --> 0:08:52.520 You can appreciate the beauty of that. I just don't 0:08:52.559 --> 0:08:54.360 know why the universe has to work that way, Like 0:08:54.400 --> 0:08:56.439 the universe could be a total mess. We could discover 0:08:56.480 --> 0:08:58.120 the way it works, be like, actually, I have some 0:08:58.280 --> 0:09:00.679 notes this could have been done better, you know, like 0:09:01.000 --> 0:09:05.199 more documentation please. So I can definitely appreciate beauty when 0:09:05.200 --> 0:09:07.000 I see it, and I think I can even capture what 0:09:07.040 --> 0:09:09.080 he's beautiful about something. I just don't know why we 0:09:09.160 --> 0:09:12.720 expect the universe to be beautiful. I mean, I hope 0:09:12.760 --> 0:09:14.880 that it is, but we'll see. 0:09:15.440 --> 0:09:18.000 I mean, I think the universe is beautiful, like the 0:09:18.080 --> 0:09:21.120 sunsets are beautiful, the biodiversity is beautiful. But it certainly 0:09:21.120 --> 0:09:23.480 seems to me that anytime we try to explain what's happening, 0:09:23.840 --> 0:09:25.000 there's nothing beautiful in this. 0:09:25.640 --> 0:09:26.600 But maybe that's just the. 0:09:26.559 --> 0:09:29.760 Biologist working on ecological models, where we're like, this is 0:09:29.800 --> 0:09:33.240 a mess, and cells are a mess, and everything's a mess, 0:09:33.240 --> 0:09:34.880 but we're all just muddling forward. 0:09:34.679 --> 0:09:37.160 And let's not even get into chemistry because that's a disaster. 0:09:37.240 --> 0:09:39.439 Now, we weren't going to get in a chemistry Daniel, 0:09:39.640 --> 0:09:40.480 that's not what we do. 0:09:40.720 --> 0:09:44.440 That's right, exactly. But neither of us are also string theorists. 0:09:44.440 --> 0:09:47.240 And so I reached out to somebody I know online, 0:09:47.280 --> 0:09:49.680 Thomas Van Reed, who is a string theorist and writes 0:09:49.679 --> 0:09:51.920 about this stuff. And I've seen him on social media 0:09:52.120 --> 0:09:56.080 venting gently about how string theory is not well understood 0:09:56.160 --> 0:09:59.439 and mis explained and misunderstood by the general public. So 0:09:59.480 --> 0:10:01.440 I invited him to come on the podcast and tell 0:10:01.520 --> 0:10:02.360 us all about it. 0:10:02.800 --> 0:10:03.640 Let's jump right in. 0:10:07.679 --> 0:10:09.840 So then is my great pleasure to welcome to the 0:10:09.880 --> 0:10:13.520 podcast Professor Thomas van Reid. He's a theoretical physicist at 0:10:13.559 --> 0:10:16.920 the Institute for Theoretical Physics at ku LEEUFN. Some of 0:10:16.960 --> 0:10:19.960 his recent work include papers called the Stability of axion, 0:10:20.120 --> 0:10:24.319 saxyon wormholes, and the quantum theory of gravitation, Effective field theories, 0:10:24.360 --> 0:10:27.160 and strings yesterday and today. So I thought he'd be 0:10:27.200 --> 0:10:29.720 a good person to talk to about string theory and 0:10:29.760 --> 0:10:32.679 its alleged elegance. Thomas, thank you very much for joining us. 0:10:32.920 --> 0:10:34.240 It's my pleasure to be here. 0:10:34.600 --> 0:10:37.640 So my first question for you is, what is this 0:10:37.720 --> 0:10:40.680 big problem that everybody's trying to solve. We hear a 0:10:40.720 --> 0:10:43.840 lot in popular science about how we have general relativity 0:10:44.240 --> 0:10:47.160 and we have quantum mechanics and these two theories don't 0:10:47.200 --> 0:10:50.400 work well together, and we need some theory of quantum gravity. 0:10:50.720 --> 0:10:53.160 Why do we need a theory of quantum gravity? What 0:10:53.320 --> 0:10:56.280 is this big issue? Why can't we just have gr 0:10:56.440 --> 0:10:58.920 and quantum mechanics and be happy with those. 0:10:59.280 --> 0:11:03.280 So in everyday life, gravity is a classical force and 0:11:03.320 --> 0:11:07.439 there's no problem in understanding gravity. Sometimes it's a bit complicated, 0:11:07.840 --> 0:11:11.600 especially you know when you're looking at say, black hole mergers, 0:11:11.679 --> 0:11:15.959 you need full blown relativity. But it's still a classical theory. 0:11:16.160 --> 0:11:18.760 You can put it on a computer, you can do 0:11:18.840 --> 0:11:21.400 advanced calculations and you can understand what's going on. 0:11:21.559 --> 0:11:23.680 And what do you mean when you say a classical theory? 0:11:24.160 --> 0:11:26.280 What does classical mean? It sounds like a technical word 0:11:26.320 --> 0:11:26.760 you're using. 0:11:26.960 --> 0:11:30.360 Classical can mean two things. In physics, it's very confusing. 0:11:31.200 --> 0:11:36.520 So classical can mean that you do newtont mechanics and 0:11:36.559 --> 0:11:40.720 you don't do relativity. Relativity is a correction to Newton mechanics, 0:11:41.240 --> 0:11:44.120 and the correction takes into account that the speed of 0:11:44.200 --> 0:11:48.640 light is finite. So physics theories are always corrected by 0:11:48.640 --> 0:11:52.920 some numbers. And you can think of the difference between 0:11:52.960 --> 0:11:56.360 relativity and Newton mechanics to be that one theory is 0:11:56.360 --> 0:11:59.720 corrected by the other by numbers which go like one 0:12:00.000 --> 0:12:01.760 added by the speed of light, which is a very 0:12:01.800 --> 0:12:04.800 tiny number. So that's the first sense of classical. I 0:12:04.880 --> 0:12:08.200 actually meant the second sense of classical, and that's where 0:12:08.240 --> 0:12:11.760 you say I take say it doesn't matter whether it's 0:12:11.760 --> 0:12:16.200 a Newtonian or a relativistic theory, but I add quantum mechanics. 0:12:17.720 --> 0:12:20.160 They're in quantum mechanics. We also have a small number 0:12:20.480 --> 0:12:25.920 called Plank's constant, right, So very informally speaking, you could 0:12:25.960 --> 0:12:31.920 say that quantum mechanics correct classical mechanics by terms in 0:12:32.000 --> 0:12:35.080 equations that are powers of this small number. 0:12:35.320 --> 0:12:38.360 So classical is a fuzzy word that basically means old fashioned, 0:12:38.800 --> 0:12:41.680 the same way like you might call classical music Mozart, 0:12:41.920 --> 0:12:43.640 but the kind of music I like to listen to 0:12:43.760 --> 0:12:46.280 is called classic rock on the radio, even though it's 0:12:46.320 --> 0:12:48.840 not that old. And so you're talking about two different 0:12:48.840 --> 0:12:52.080 senses in which physics has evolved from Newton to Einstein, 0:12:52.200 --> 0:12:55.160 and then from Einstein to like Schrodinger and Heisenberg and stuff. 0:12:55.400 --> 0:12:57.120 And so in this sense, when you say a classical 0:12:57.120 --> 0:12:59.160 theory physics, you mean without quantum mechanics. 0:12:59.160 --> 0:13:00.640 And you know, some of that must is pretty old. 0:13:00.679 --> 0:13:03.640 By that man, we are getting a little bit old, 0:13:03.640 --> 0:13:04.160 I'm sorry. 0:13:04.280 --> 0:13:06.079 And for the record, notes are totally rocks. 0:13:06.080 --> 0:13:09.800 Ok Okay, no, I'm not disagreeing there. So the biologist 0:13:09.880 --> 0:13:12.480 who's trying to keep up with the physicists here, all right, 0:13:12.520 --> 0:13:15.240 So it sounded to me like you were saying, quantum 0:13:15.240 --> 0:13:20.240 mechanics and general relativity can be reconciled if you just. 0:13:20.240 --> 0:13:21.880 Divide by the right terms. 0:13:21.960 --> 0:13:25.040 I was under the impression that they describe completely different 0:13:25.080 --> 0:13:27.400 phenomena and kind of don't work together at all. 0:13:27.520 --> 0:13:29.920 It's too quick to say that they can easily be combined, 0:13:29.920 --> 0:13:32.800 but it's also too quick to say that they cannot 0:13:32.800 --> 0:13:37.559 be combined. So, first of all, indeed, are there regimes 0:13:37.600 --> 0:13:42.280 of interest where the two theories should be combined. Because 0:13:42.360 --> 0:13:45.480 usually gravity we think of very large things. Gravity is 0:13:45.480 --> 0:13:47.480 so weak that in order to see it you need 0:13:47.480 --> 0:13:51.160 to have large objects, massive objects. You know, the Earth 0:13:51.240 --> 0:13:54.720 is pretty big, and I can still lift my glass 0:13:54.720 --> 0:13:58.000 of water from my table, meaning that, you know, the 0:13:58.040 --> 0:14:01.120 electromagnetic forces in my body are stronger than the gravity 0:14:01.120 --> 0:14:03.720 of the full Earth. So gravity is weak and things 0:14:03.720 --> 0:14:05.199 need to be big to be able to see it. 0:14:06.160 --> 0:14:09.000 There's another option, if things are dense enough, you know, 0:14:09.160 --> 0:14:12.199 imagine taking the Earth and compressing it into the size 0:14:12.240 --> 0:14:17.080 of my water cup, Okay, and then even compressing it 0:14:17.120 --> 0:14:19.880 more so, then of course I will get into a 0:14:19.920 --> 0:14:23.640 regime where you say, well, you know, it becomes very small, 0:14:23.880 --> 0:14:27.360 and then the theory of econom mechanics becomes important. Yet 0:14:27.440 --> 0:14:32.040 also gravity becomes strong. And you can ask do we 0:14:32.160 --> 0:14:34.760 know of such regimes? And we do. I would say 0:14:34.760 --> 0:14:38.400 it's the most important regime for all of physics. It's 0:14:38.400 --> 0:14:41.400 the early universe. So if we go back in time 0:14:41.560 --> 0:14:43.640 and we look with our telescopes, so looking with the 0:14:43.680 --> 0:14:46.120 telescope means that you look back into the you look 0:14:46.120 --> 0:14:49.320 into the past, you see that the universe was denser. 0:14:49.560 --> 0:14:53.440 And if we just follow our classical equations, it actually 0:14:53.520 --> 0:14:57.760 tells us that the density will go to infinity, which 0:14:57.760 --> 0:14:59.960 is of course not true, but it isn't in the 0:15:00.120 --> 0:15:02.880 cation that in the very early universe, you know, everything 0:15:02.960 --> 0:15:07.680 was very tiny, so quantum mechanics was absolutely important and 0:15:07.760 --> 0:15:11.440 gravity was huge, so we need a theory of quantum gravity. 0:15:11.600 --> 0:15:16.160 One other example that we have already measured are black holes. 0:15:16.520 --> 0:15:18.720 Some black holes have always been a sort of a 0:15:18.800 --> 0:15:23.120 theoretical invention, but they're not anymore. We have seen them. 0:15:23.200 --> 0:15:28.080 They're out there, okay. And what you sometimes wrongly hear 0:15:28.520 --> 0:15:32.040 when people you know, talk about signs in a for 0:15:32.080 --> 0:15:34.840 the for the bigger public, they would tell you that 0:15:35.880 --> 0:15:38.440 black holes, for sure are objects were Quantum gravity is 0:15:38.440 --> 0:15:41.240 important because gravity is strong near a black hole. That's 0:15:41.280 --> 0:15:43.440 actually not entirely correct. If you look at a big 0:15:43.480 --> 0:15:46.920 black hole, for instance, a black hole in the in 0:15:46.960 --> 0:15:49.640 the middle of our galaxy, the gravitational parts that the 0:15:49.640 --> 0:15:51.680 horizon of that black hole is big, but it's not 0:15:51.800 --> 0:15:55.120 ridiculously big, okay, And the bigger a black hole, the 0:15:55.160 --> 0:15:59.480 weaker gravity is at the horizon of a black holes. 0:15:59.480 --> 0:16:01.640 It's kind of entry intuitive a bit is that because 0:16:01.640 --> 0:16:02.760 you're further from the. 0:16:02.720 --> 0:16:05.680 Center exactly, and it's also because the density of the 0:16:05.720 --> 0:16:09.200 black hole goes down as the black hole grows like 0:16:09.240 --> 0:16:12.600 the black hole. I think, if I'm not mistaken, you 0:16:12.640 --> 0:16:15.440 can always fact check this. But I think the black 0:16:15.440 --> 0:16:17.640 hole in the center of our galaxy as a density 0:16:17.680 --> 0:16:20.600 compared to water. So it's wrong to say that for 0:16:20.640 --> 0:16:23.600 sure quantum mechanics will be important near the horizon of 0:16:23.600 --> 0:16:27.640 a black hole. But what we are pretty convinced of 0:16:27.840 --> 0:16:29.400 is that if you would jump in a black hole, 0:16:29.440 --> 0:16:32.440 we don't know what's there. But it cannot be classical 0:16:32.480 --> 0:16:36.400 physics anymore because at some point the classical equations tell 0:16:36.440 --> 0:16:38.880 you rubbish. They tell you things which are impossible, so 0:16:38.960 --> 0:16:41.760 we know the classical theory has to break down. So 0:16:41.880 --> 0:16:44.880 the assumption is that just as in the early universe, 0:16:45.520 --> 0:16:48.040 in the very center of a black hole, there's also 0:16:48.360 --> 0:16:52.040 quantum mechanics and gravity at play at the same time. 0:16:52.360 --> 0:16:53.840 So I want to get back to what you said 0:16:53.840 --> 0:16:56.200 about things breaking down, but in a minute. First, I 0:16:56.240 --> 0:16:58.560 want to focus on this question of quantum mechanics and 0:16:58.600 --> 0:17:01.000 gravity at play at the same time. So you told 0:17:01.080 --> 0:17:04.720 us earlier that general relativity, or we can call it gravity, 0:17:04.800 --> 0:17:09.240 describes usually big things, and quantum mechanics usually describe small things. 0:17:09.480 --> 0:17:10.920 And now you're saying that at the beginning of the 0:17:11.000 --> 0:17:14.680 universe and inside black holes, we think both of those 0:17:14.720 --> 0:17:17.520 are relevant. And that's why we need a unified theory, 0:17:17.600 --> 0:17:20.320 because we need some way to describe that and to 0:17:20.720 --> 0:17:24.119 disagree to conflict. Why is there a conflict and their predictions. 0:17:24.600 --> 0:17:26.800 Couldn't it just be that they make the same prediction 0:17:26.960 --> 0:17:29.600 for what happens in that scenario. Couldn't it just be beautiful, 0:17:29.840 --> 0:17:32.000 a fortunate harmony among the theories. 0:17:32.240 --> 0:17:34.520 How what's the quickest way to explain? So let me 0:17:34.600 --> 0:17:37.800 give you an example that I hope more people know, 0:17:38.560 --> 0:17:43.240 maybe even from high school or first year of university. Say, okay, 0:17:44.560 --> 0:17:47.160 think of an electric field. So you have a charge particle, 0:17:48.440 --> 0:17:52.080 and a charge particle is surrounded by an electric field 0:17:52.280 --> 0:17:55.080 that it sources itself. So when you look at it classically, 0:17:56.320 --> 0:17:58.720 and when you think of a particle classically, it means 0:17:58.720 --> 0:18:01.159 that a particle is a point. And maybe if people 0:18:01.200 --> 0:18:03.760 remember this still, there was this formula that it said 0:18:03.800 --> 0:18:07.560 that the strength of this electric field went like a 0:18:07.800 --> 0:18:12.479 negative power of the distance from the particle. Say, you know, 0:18:13.000 --> 0:18:16.560 take it's one over our squared. That's actually that the force. 0:18:18.119 --> 0:18:21.880 Then you find that this force becomes infinitely big as 0:18:21.880 --> 0:18:26.280 you approach the particle. Okay, it's the same for the energy. 0:18:26.640 --> 0:18:29.840 The total energy carried by that particle would be infinite. 0:18:31.080 --> 0:18:34.479 And we know that can that cannot be correct. And 0:18:34.520 --> 0:18:38.520 then we have learned later on one hundred years ago, 0:18:38.640 --> 0:18:42.440 when people understood the quantum theory of charged particles, there 0:18:42.520 --> 0:18:46.880 was nothing going infinite. Things were just super well behaved, 0:18:47.040 --> 0:18:51.200 numbers were finite. Nothing weird was happening. And gravity is 0:18:51.359 --> 0:18:55.000 that sounds completely analogous, right. Even the formula for the 0:18:55.000 --> 0:18:57.360 force of gravity is almost the same as a formula 0:18:57.400 --> 0:19:01.960 for the Kulan force. Both can give you infinities. And 0:19:02.000 --> 0:19:05.080 for the cool on force, we learned that that infinity 0:19:05.160 --> 0:19:06.960 is gone when you treat it quantum mechanically. 0:19:07.560 --> 0:19:11.520 Both general relativity and quantum mechanics at some point start 0:19:11.560 --> 0:19:13.960 giving you infinities that make no sense when you push 0:19:13.960 --> 0:19:14.879 them to their extremes. 0:19:15.119 --> 0:19:18.720 So quantum mechanics doesn't give you infinities, but the classical 0:19:18.760 --> 0:19:22.479 theory does, so relativity does. Yeah. So then the question is, 0:19:22.480 --> 0:19:25.399 if you treat relativity in a quantum mechanical way, would 0:19:25.400 --> 0:19:30.320 you get sensible numbers? And do you Well, that's a 0:19:30.359 --> 0:19:34.280 good question, so yeah, to be able to answer it, 0:19:34.400 --> 0:19:36.560 I should I should tell you this is the theory 0:19:36.760 --> 0:19:40.040 that of quantum gravity. And to say that something is 0:19:40.040 --> 0:19:42.280 the theory of quantum gravity, I mean you can write 0:19:42.320 --> 0:19:44.879 down a theory which is extremely hard, but imagine you 0:19:44.880 --> 0:19:48.600 succeed and people succeeded. You don't know whether it's the 0:19:48.600 --> 0:19:51.160 only option, right, so you need to Normally you test 0:19:51.200 --> 0:19:53.760 the theory. And the problem is that to go out 0:19:53.800 --> 0:19:57.280 and test it requires you to look for these, you know, 0:19:57.480 --> 0:20:00.600 places where gravity is so strong, and I guess none 0:20:00.640 --> 0:20:03.359 of us wants to jump in a black hole. You could, 0:20:03.840 --> 0:20:05.879 but you know, you could test it, but unfortunately you 0:20:05.880 --> 0:20:08.040 would not be able to tell anymore to anybody else 0:20:08.040 --> 0:20:11.040 because you cannot escape from the black hole. Right, So 0:20:11.160 --> 0:20:15.080 nature is playing a very mean trick on us. It 0:20:15.119 --> 0:20:19.119 seems that humanity, in order to test the theory of 0:20:19.200 --> 0:20:24.800 quantum bravity, it is forced to do something that kills you. 0:20:24.800 --> 0:20:26.960 You know, this actually goes under the name of censorship, 0:20:27.000 --> 0:20:30.320 cosmic censorship. It's actually something quite serious in the physics community. 0:20:30.680 --> 0:20:35.160 Gravity works such that you could actually just see quantum bravity. Unfortunately, 0:20:35.200 --> 0:20:37.879 there's always what we call a cosmic horizon preventing you 0:20:37.920 --> 0:20:40.960 to see it. Either it's the horizon of a black hole, 0:20:41.840 --> 0:20:43.520 or you have to go back in time. But if 0:20:43.560 --> 0:20:46.960 you take your telescope, it's actually impossible to directly look 0:20:47.000 --> 0:20:49.920 at the Big Bang. So that's kind of mean. Otherwise 0:20:49.960 --> 0:20:51.080 you could just observe it. 0:20:51.280 --> 0:20:52.680 Yeah, it's frustrating. 0:20:53.280 --> 0:20:56.080 Religious, I would say God is playing you know, is 0:20:56.600 --> 0:20:58.280 an evil person, so to speak. 0:20:58.640 --> 0:21:00.919 Yeah, I also believe in cause free speech. I think 0:21:00.960 --> 0:21:03.119 the universe should be free to tell us how it works. 0:21:03.320 --> 0:21:06.600 And I'm bummed about all this censorship. Okay, I have 0:21:06.640 --> 0:21:09.680 a lot more questions, but first let's take a quick 0:21:09.720 --> 0:21:29.480 break and let our brains rest a moment. Okay, we're 0:21:29.480 --> 0:21:32.080 back and we are talking to Thomas Van Riet, a 0:21:32.119 --> 0:21:35.479