WEBVTT - Does physics have to be causal and local? 0:00:07.840 --> 0:00:09.120 The big picture task. 0:00:09.240 --> 0:00:12.680 The ultimate quest of physics is to make sense of 0:00:12.720 --> 0:00:17.079 the universe, to sort through our amazing and bonkers experience 0:00:17.120 --> 0:00:21.120 and find as compact and simple an explanation as possible 0:00:21.520 --> 0:00:25.800 for why what happens happens. We've made a lot of progress, 0:00:25.840 --> 0:00:30.320 but we've also struggled together. Do quantum particles really collapse 0:00:30.360 --> 0:00:34.199 together across space time? Why is gravity so difficult to 0:00:34.320 --> 0:00:39.000 unify with quantum theory? Why does time flow forward? Sometimes? 0:00:39.040 --> 0:00:41.440 I wonder if the reason we feel stuck is that 0:00:41.479 --> 0:00:45.640 we started from the wrong place. What if there's a simple, 0:00:45.800 --> 0:00:48.720 basic assumption that we're making that has led us down 0:00:48.720 --> 0:00:52.159 the wrong path. It's happened to us before, like when 0:00:52.200 --> 0:00:55.639 we assumed that time flows the same way across the universe, 0:00:56.120 --> 0:00:58.680 or that we were at the center of the Solar system, 0:00:58.920 --> 0:01:02.720 or that light needed medium to propagate. When we removed 0:01:02.800 --> 0:01:06.240 the mistaken assumption, the veil was pulled from our eyes 0:01:06.280 --> 0:01:11.319 in a simpler, clear if weirder explanation emerged, Could that 0:01:11.400 --> 0:01:13.560 be the cause of our current head scratching? Is there 0:01:13.640 --> 0:01:17.120 some requirement we've imposed on physics that's keeping us from 0:01:17.160 --> 0:01:20.959 seeing the simple explanation staring us in the face. Today 0:01:21.000 --> 0:01:23.160 on the podcast, will knock on the doors the most 0:01:23.200 --> 0:01:28.080 basic intuitive foundational concepts in physics and ask do we 0:01:28.160 --> 0:01:31.959 really need them? We'll question locality, do things have to 0:01:32.000 --> 0:01:34.480 be in the same place in order to interact? And 0:01:34.560 --> 0:01:38.560 its cousin causality, do causes have to come before effects? 0:01:39.319 --> 0:01:43.479 Welcome to Daniel and Kelly's extraordinary counterintuitive Universe. 0:01:56.720 --> 0:02:00.960 Hello. I'm Kelly Readersmith. I study parasites and and after 0:02:01.040 --> 0:02:04.800 today's conversation, I feel really lucky that locality and causality 0:02:04.920 --> 0:02:06.800 just make a lot more sense in biology. 0:02:07.680 --> 0:02:08.800 Hi, I'm Daniel. 0:02:08.880 --> 0:02:11.600 I'm a particle physicist, and I like to pretend sometimes 0:02:11.600 --> 0:02:12.840 to be a philosopher. 0:02:13.080 --> 0:02:15.840 Ooh yeah, I enjoy philosophy too. 0:02:16.080 --> 0:02:16.240 Well. 0:02:16.240 --> 0:02:18.240 One of the things I love about physics is that 0:02:18.320 --> 0:02:21.440 it butts right up against philosophical questions. We're asking about 0:02:21.440 --> 0:02:24.120 the deep, fundamental nature of the universe, and so the 0:02:24.160 --> 0:02:28.040 philosophical questions are like immediate and obvious. I hear a 0:02:28.040 --> 0:02:31.480 little bit less about like the philosophy of biology. You know, 0:02:31.520 --> 0:02:33.279 what does it mean to be a parasite? 0:02:33.600 --> 0:02:34.400 This sort of stuff? 0:02:35.520 --> 0:02:38.760 Well, I mean, we have plenty of arguments about that 0:02:38.800 --> 0:02:40.520 at the conference as we go to. But I feel 0:02:40.520 --> 0:02:43.839 like for us philosophy, Well, what's the difference between philosophy 0:02:43.840 --> 0:02:46.480 and ethics? Is ethics a subset of philosophy? Because we 0:02:46.520 --> 0:02:50.320 talk a lot about ethics associated with the biology stuff 0:02:50.360 --> 0:02:53.359 that we're working on. So I'm more used to tackling 0:02:53.400 --> 0:02:57.920 ethical questions than like, you know, what does causality even mean? 0:02:59.280 --> 0:03:00.880 Well, I think you can your finger on it, because 0:03:00.919 --> 0:03:04.320 anytime you get bogged down in a conversation by defining 0:03:04.320 --> 0:03:06.920 your terms, that's when you know you're doing philosophy. 0:03:06.960 --> 0:03:11.440 Ah, there we go. All right, Well, we spend a 0:03:11.480 --> 0:03:14.280 glorious amount of time talking about how to define terms today, 0:03:14.320 --> 0:03:16.120 and we've got Sean Carroll to help us do that, 0:03:16.720 --> 0:03:19.480 and I love all of the stories that he tells. 0:03:19.680 --> 0:03:22.480 But before we jump into our interview with Sean, we 0:03:22.520 --> 0:03:25.359 should hear what our audience thinks about what it means 0:03:25.520 --> 0:03:26.800 to be local and causal. 0:03:26.919 --> 0:03:28.680 That's right, as usual, I went out there to our 0:03:28.720 --> 0:03:31.320 group of volunteers to see what they thought about these 0:03:31.480 --> 0:03:35.640 fundamental concepts in physics and philosophy. So think about it 0:03:35.720 --> 0:03:37.920 for a moment before you hear these answers. Do you 0:03:38.040 --> 0:03:42.760 think physics has to be local and causal. Here's what 0:03:42.840 --> 0:03:44.240 our listeners had to say. 0:03:45.200 --> 0:03:48.119 Physics used to be local to me years ago when 0:03:48.160 --> 0:03:53.280 I dated a woman whose father was a physicist. Splash, Yes, 0:03:53.520 --> 0:03:57.120 chemist at Bell Labs. But it's not anymore. 0:03:57.640 --> 0:04:01.200 Observations and experiments at much larger scale, much smaller scales 0:04:01.800 --> 0:04:04.040 seem to show that both of those concepts break down 0:04:04.040 --> 0:04:05.240 in the right circumstances. 0:04:05.480 --> 0:04:07.440 Most physicists assume causality. 0:04:08.000 --> 0:04:12.040 I don't think physics have to be local, but I 0:04:12.080 --> 0:04:15.880 think they have to be caucal to have causality. 0:04:16.480 --> 0:04:20.440 I think classical physics is local and causal, but I 0:04:20.480 --> 0:04:26.359 think quantum mechanics, with its spontaneous random activity and its 0:04:26.839 --> 0:04:29.440 ability to have spooky action at a distance, breaks that. 0:04:29.400 --> 0:04:29.880 A little bit. 0:04:30.240 --> 0:04:32.800 Physics has to be local in causal, at least in 0:04:32.839 --> 0:04:36.680 our quarter of the universe. Otherwise Einstein's spooky action would 0:04:36.680 --> 0:04:38.560 have your lamp turned on before you hit the switch, 0:04:39.040 --> 0:04:41.400 Newton would drop his apple in reverse, and all hell 0:04:41.440 --> 0:04:42.840 would have broken loose yesterday. 0:04:43.200 --> 0:04:45.760 I would think physics has to be local in order 0:04:45.839 --> 0:04:50.880 to be testimal, so that the results are always predictable. 0:04:51.120 --> 0:04:55.120 But I'm not sure what causal means. Maybe cause and effect. 0:04:55.680 --> 0:04:57.640 I think It's really interesting to think of physics as 0:04:57.640 --> 0:05:00.760 not being local, because the implications would kind of crazy, 0:05:00.920 --> 0:05:03.240 and maybe it would explain some of the stuff that 0:05:03.279 --> 0:05:04.360 we don't really understand yet. 0:05:04.520 --> 0:05:07.799 How about it has to be at least as local 0:05:07.920 --> 0:05:12.640 as gravity and at least as causal as quantum decay. 0:05:13.160 --> 0:05:16.840 What an interesting question? Does physics have to be local 0:05:17.560 --> 0:05:20.479 and or casual? Well, if it's a dating app I 0:05:20.560 --> 0:05:24.320 was supposed to be local, and because I had a 0:05:24.320 --> 0:05:28.880 casual relationship with physics in high school, I really don't 0:05:28.920 --> 0:05:32.039 get the question. But I would say, isn't physics universal 0:05:32.320 --> 0:05:34.520 and cosmological? 0:05:34.920 --> 0:05:38.200 Maybe things change based on locality and conditions. 0:05:38.480 --> 0:05:41.360 I say, yes, physics has to be local and clusal. 0:05:41.920 --> 0:05:45.720 Love these answers both insightful and hilarious, especially the person 0:05:45.760 --> 0:05:48.039 who misinterpreted causal to be casual. 0:05:50.440 --> 0:05:52.839 Are you sure you spelled it right in your email, Daniel. 0:05:53.480 --> 0:05:58.080 I am not sure. Maybe that was the cause. 0:05:57.920 --> 0:06:01.520 Of Oh yeah, See most things in my world there's 0:06:01.560 --> 0:06:05.479 a cause and effect relationship and there's no going back 0:06:05.520 --> 0:06:05.919 in time. 0:06:06.120 --> 0:06:08.800 Well, you know, this really is a fundamental issue in philosophy. 0:06:08.880 --> 0:06:11.200 We are just discussing on the discord this morning, Like 0:06:11.760 --> 0:06:14.040 does the universe have to make sense? Does there have 0:06:14.080 --> 0:06:17.120 to be an explanation for everything? I think as humans, 0:06:17.200 --> 0:06:20.000 we are curious and we want to know answers to 0:06:20.200 --> 0:06:22.960 questions about the universe because we assume that there are 0:06:23.080 --> 0:06:26.400 answers right, that there is a thing that is happening 0:06:26.880 --> 0:06:30.760 and we can figure it out somehow. And sometimes these 0:06:30.800 --> 0:06:33.680 discussions of like locality and causality tell me that the 0:06:33.760 --> 0:06:36.159 universe could be very, very different from the way that 0:06:36.240 --> 0:06:38.560 we assume that it is, and in a way that 0:06:38.680 --> 0:06:40.360 might never make sense to us. 0:06:40.600 --> 0:06:42.600 And you know, the more we learn about quantum mechanics, 0:06:42.640 --> 0:06:45.640 the more the more it breaks my brain, to be honest, 0:06:45.760 --> 0:06:49.599 but the more yeah interesting questions, I realize that there 0:06:49.640 --> 0:06:53.200 are left to ask like questions about locality and causality 0:06:53.200 --> 0:06:55.480 at the quantum mechanic level. That is what I will 0:06:55.520 --> 0:06:57.800 be thinking about tonight when I'm trying to fall asleep 0:06:58.360 --> 0:06:59.720 and I'm not sure if it's going to keep me 0:06:59.800 --> 0:07:03.360 up or make me fall asleep more quickly. So we'll see. 0:07:03.440 --> 0:07:06.440 And to dig deep into these topics, we invited an 0:07:06.480 --> 0:07:09.560 expert in physics and philosophy on the show and a 0:07:09.640 --> 0:07:12.320 friend of the podcast, Sean Carroll, who's not afraid to 0:07:12.320 --> 0:07:14.040 get bogged down defining terms. 0:07:14.160 --> 0:07:16.600 Yeah, things got heated between you two at a couple points, 0:07:16.600 --> 0:07:17.880 so let's jump in. 0:07:19.560 --> 0:07:21.760 All right, it's my pleasure to welcome to the podcast. 0:07:21.840 --> 0:07:23.400 Shawn Carroll Sean is a. 0:07:23.320 --> 0:07:26.320 Theoretical physicist who's done impouring work on the foundations of 0:07:26.400 --> 0:07:29.640 quantum mechanics, cosmology, and the arrow of time. He holds 0:07:29.640 --> 0:07:33.000 a joint appointment between physics and philosophy at Johns Hopkins. 0:07:33.040 --> 0:07:36.080 He's also a prolific science communicator, the host of the 0:07:36.160 --> 0:07:39.760 mind ski podcast, which I hardly recommend for its impressively 0:07:39.800 --> 0:07:43.320 deep dives, and author several books such as The Biggest 0:07:43.320 --> 0:07:46.440 Ideas in the Universe. Shawn, Welcome to the podcast. 0:07:46.520 --> 0:07:47.600 Thanks very much for having me. 0:07:47.640 --> 0:07:49.280 This is the first time I have been on a 0:07:49.320 --> 0:07:54.000 podcast hosted by two previous guests of my podcast, So 0:07:54.480 --> 0:07:56.560 I like how we are closing the triangle. 0:07:56.720 --> 0:07:57.440 This is exciting. 0:07:57.480 --> 0:08:03.760 We are accelerating towards the podcast singularity. Well, speaking of Singularity, 0:08:03.920 --> 0:08:06.600 today we're going to die really deep on something that 0:08:06.720 --> 0:08:09.440 I've always wanted to understand better in physics, and I 0:08:09.480 --> 0:08:13.040 can't imagine a better person to ask hard questions about 0:08:13.080 --> 0:08:16.400 issues on the boundary of philosophy in physics. I mean, 0:08:16.440 --> 0:08:18.000 I know a lot of people who are physicists to 0:08:18.120 --> 0:08:20.760 interest in philosophy. I know a lot of philosophers who 0:08:20.840 --> 0:08:23.160 do some physics as well, But I don't think I 0:08:23.160 --> 0:08:25.760 know anybody else who literally has an appointment in physics 0:08:25.840 --> 0:08:29.480 and in philosophy. So congratulations on straddling that barrier. 0:08:29.680 --> 0:08:29.960 Thanks. 0:08:29.960 --> 0:08:31.760 So it's not easy to make it happen, you know, 0:08:32.400 --> 0:08:35.800 as you know academia, we love our little silos, and 0:08:36.600 --> 0:08:38.800 I mean there's plenty of people in universities who are 0:08:38.800 --> 0:08:43.360 cross appointed between departments, but a humanities and a science 0:08:43.559 --> 0:08:45.719 thing getting together is really hard to pull off. 0:08:45.800 --> 0:08:47.040 Yeah, and really awesome. 0:08:47.360 --> 0:08:49.600 I think it's awesome. I'm having a good time, so 0:08:49.880 --> 0:08:51.080 you know, it makes me feel special. 0:08:51.679 --> 0:08:54.040 Well, I don't have a joint appointment philosophy. I do 0:08:54.120 --> 0:08:57.080 have a courtesy appointment in the philosophy department, which is 0:08:57.160 --> 0:09:00.079 just because I showed up to enough philosophy seminars to 0:09:00.160 --> 0:09:02.000 ask awkward questions that they were like. 0:09:02.000 --> 0:09:03.680 Who are you? 0:09:03.679 --> 0:09:05.960 And then they're like, oh, do you want to courtesy appointment? 0:09:06.000 --> 0:09:09.160 It gives you nothing. That's very yeah, exactly. 0:09:10.440 --> 0:09:12.560 It's like they're encouraging you to keep coming back though. 0:09:12.559 --> 0:09:17.000 That's nice, Yeah, exactly, yeah, yeah, although I did learn 0:09:17.040 --> 0:09:20.560 there's a big difference between the two fields. And physics seminars, 0:09:20.800 --> 0:09:24.439 it's totally normal to interrupt with questions. You don't understand something, 0:09:24.600 --> 0:09:27.200 raise your hands, speak up, start a discussion. If I 0:09:27.240 --> 0:09:29.520 give a physics seminar, I feel like it's a failure 0:09:29.520 --> 0:09:32.559 if nobody's interrupted to discuss something. First time I asked 0:09:32.600 --> 0:09:34.920 a question in the middle of a philosophy seminar, everybody 0:09:34.960 --> 0:09:37.920 in the room turned to me with horror. So it's 0:09:37.920 --> 0:09:40.680 like objecting in the middle of a wedding or something. 0:09:40.440 --> 0:09:42.560 You know, like hold your question to the end, sir. 0:09:43.360 --> 0:09:46.080 But then also at a philosophy colloquium, you know, often 0:09:46.280 --> 0:09:48.960 they will let the speaker talk for an hour, take 0:09:48.960 --> 0:09:50.880 a five minute break, and then come back for an 0:09:50.920 --> 0:09:55.120 hour of questions, which physicists would never do. Physicists be like, 0:09:55.160 --> 0:09:57.360 what you expected us to listen to? What was happening 0:09:57.920 --> 0:09:59.800 into the seminar? What's going on here? 0:10:00.600 --> 0:10:03.559 All right, Well, let's dive into the topic for today. 0:10:03.679 --> 0:10:07.880 We're talking about locality and causality, and these of course 0:10:07.960 --> 0:10:09.760 are intermingled, but we're going to take them one at 0:10:09.800 --> 0:10:12.920 a time if we can. So let's start with locality, Sean, 0:10:13.160 --> 0:10:15.720 how do you interpret the concept of locality? What does 0:10:15.760 --> 0:10:18.839 locality mean in physics? It doesn't mean like, get your 0:10:18.840 --> 0:10:20.480 donuts from the store around the corner. 0:10:20.600 --> 0:10:22.520 It kind of does. I mean, it's pretty close to that. 0:10:22.720 --> 0:10:25.079 It's the idea that the donuts aren't that far away, 0:10:25.280 --> 0:10:27.080 the donuts that you're actually going to want. You know, 0:10:27.120 --> 0:10:29.600 when I get donuts, I'm more likely to go to 0:10:29.720 --> 0:10:32.040 the place a couple blocks away than to the place 0:10:32.080 --> 0:10:36.400 three thousand miles away. And fundamentally that's because of locality. 0:10:36.440 --> 0:10:36.640 You know. 0:10:36.920 --> 0:10:40.320 Locality comes in different ways in different stretches of physics. 0:10:40.840 --> 0:10:44.600 But the basic idea is that there's this thing called space, 0:10:45.160 --> 0:10:47.040 and indeed we can promote it to space time and 0:10:47.080 --> 0:10:49.880 we can talk about that too. But there are places 0:10:49.920 --> 0:10:54.360 where we're located in the universe. And that kind of sounds, 0:10:54.559 --> 0:10:56.920 you know, obvious. So yes, there are places where we're 0:10:56.920 --> 0:10:59.719 located in the universe, but it's not so obvious. It's 0:10:59.720 --> 0:11:03.160 a very strong claim that what the universe is made 0:11:03.160 --> 0:11:08.080 out of is space and things inside of space. Right, 0:11:08.200 --> 0:11:10.520 Like I have a location, you have a location. This 0:11:10.600 --> 0:11:14.320 electron has a location, et cetera. And then furthermore that 0:11:14.440 --> 0:11:17.160 when these things bump into each other. This sort of 0:11:17.280 --> 0:11:20.040 the basic ideal locality is there is space and things 0:11:20.040 --> 0:11:23.440 in it. The next level ideal locality is that when 0:11:23.559 --> 0:11:28.240 different things interact, they do so at the same point 0:11:28.320 --> 0:11:31.480 in space time, or at least at neighboring points. And 0:11:31.520 --> 0:11:33.959 you might say, well, wait a minute, Like the Sun 0:11:34.360 --> 0:11:36.959 exerts a gravitational field on the Earth, even though it's 0:11:37.040 --> 0:11:40.160 very far away, but there is a field in between 0:11:40.400 --> 0:11:42.800 the Sun and the Earth. And it's more like the 0:11:42.840 --> 0:11:46.000 Sun effects it's gravitational field right at the Sun, and 0:11:46.040 --> 0:11:48.360 that affects the gravitational field right next to that. And 0:11:48.400 --> 0:11:51.000 you work your way up to what's going on here 0:11:51.040 --> 0:11:51.520 on Earth. 0:11:51.600 --> 0:11:53.680 But let me back you up because you said something 0:11:53.679 --> 0:11:56.199 which already blew my mind, Yeah, which is that it's 0:11:56.200 --> 0:11:59.720 a strong claim essentially to say that locations exist before 0:11:59.720 --> 0:12:02.800 we get to locality requirements, that things have to interact 0:12:02.840 --> 0:12:06.079 at the same location. You're saying, it's a big idea 0:12:06.240 --> 0:12:09.360 that there is space and there are locations, right, And 0:12:09.400 --> 0:12:12.320 if that's a strong claim, like, what's the opposite, Like, 0:12:12.520 --> 0:12:15.400 are you suggesting it's possible to have a universe without 0:12:15.400 --> 0:12:16.400 locations or. 0:12:16.400 --> 0:12:18.360 A physicists just making things complicated. 0:12:19.040 --> 0:12:19.120 No. 0:12:19.320 --> 0:12:21.320 In fact, what I was just about to say is 0:12:21.400 --> 0:12:23.880 the thing about that version of locality I just said 0:12:23.960 --> 0:12:26.400 that the world is made of things located in space, 0:12:27.040 --> 0:12:31.160 is that it's false. It is clearly not true, because 0:12:31.200 --> 0:12:35.560 there's this thing called quantum mechanics, and quantum mechanics says 0:12:35.559 --> 0:12:37.840 that's not what the world is made out of. In 0:12:38.400 --> 0:12:40.920 again various different levels of precision. Even if you just 0:12:40.960 --> 0:12:44.640 have one electron, there's a wave function for the electron, 0:12:44.720 --> 0:12:47.840 and that wave function has some profile throughout space, so 0:12:47.880 --> 0:12:50.120 there's no such thing as the point at which the 0:12:50.160 --> 0:12:54.800 electron is located. But you might say, like, okay, but fine, 0:12:54.880 --> 0:12:57.600 there is something called the value of the electrons wave 0:12:57.640 --> 0:12:59.920 function at every point in space. It's kind of like 0:13:00.559 --> 0:13:03.480 the value of the electric field or the gravitational field. 0:13:03.520 --> 0:13:06.520 Is still a kind of locality, which is fine until 0:13:06.559 --> 0:13:11.320 you have two electrons. When you have two electrons, now 0:13:11.400 --> 0:13:14.600 there's not the wave function of electron one and the 0:13:14.600 --> 0:13:18.079 wave function of electron two. There's the wave function quantum 0:13:18.120 --> 0:13:23.199 mechanically for the system of both electrons at once, and 0:13:23.280 --> 0:13:27.360 that's what the world is. And that's not things located 0:13:27.400 --> 0:13:30.480 in space. It's something much weirder than that. And so 0:13:30.960 --> 0:13:32.880 quantum mechanics, and I'm sure we'll get into it, but 0:13:32.960 --> 0:13:37.920 quantum mechanics just makes the world look really not local 0:13:38.520 --> 0:13:41.360 at all. And then the question I would argue is 0:13:41.480 --> 0:13:42.880 why does it kind of look local? 0:13:43.080 --> 0:13:43.280 Right? 0:13:43.320 --> 0:13:45.640 You know, why do we get along so well thinking 0:13:45.679 --> 0:13:48.120 that the world looks local if quantum mechanics is trying 0:13:48.120 --> 0:13:49.040 to tell us something different? 0:13:49.280 --> 0:13:53.320 Does this question about what local means? You know, does 0:13:53.360 --> 0:13:55.360 it not make sense at the quantum level, but when 0:13:55.360 --> 0:13:59.920 you go past the quantum level, it totally makes sense. 0:14:00.480 --> 0:14:00.800 Yes. 0:14:00.880 --> 0:14:03.280 But I will just like be I got to be 0:14:03.320 --> 0:14:06.199 a stickler in this whole conversation, right because where this 0:14:06.280 --> 0:14:08.720 is one of those things where we think we know 0:14:08.800 --> 0:14:11.200 what the words mean, and we're digging deeply into what 0:14:11.200 --> 0:14:15.040 they mean, and so we can't be beholden to our 0:14:15.120 --> 0:14:18.439 folk wisdom about what these words mean. So when you say, 0:14:18.480 --> 0:14:20.880 like the quantum level and not the quantum level, I 0:14:20.960 --> 0:14:24.320 just want to remind everyone everything is the quantum level. 0:14:25.120 --> 0:14:29.160 There's no non quantum level. The world is quantum. It's 0:14:29.160 --> 0:14:31.480 not like you get quantum when you look at things 0:14:31.520 --> 0:14:34.240 that are very small it's the eppisode way around. You 0:14:34.360 --> 0:14:37.760 get classical when you look at things that are very big, 0:14:37.880 --> 0:14:42.520 So classical mechanics, allah, Isaac Newton, et cetera becomes a 0:14:42.560 --> 0:14:47.440 good approximation when things are big and ponderous and macroscopic 0:14:47.520 --> 0:14:50.400 and so forth. And that's the world in which, yes, 0:14:50.440 --> 0:14:53.840 you're completely right. The world looks in the classical limit 0:14:53.880 --> 0:14:57.840 as if it's made of objects with locations bumping into 0:14:57.920 --> 0:14:59.920 other objects when they are at the same location. 0:15:00.160 --> 0:15:04.520 Okay, and before we get quantum, like Isaac Newton's theory 0:15:04.720 --> 0:15:07.840 of gravity's already non local right, like. 0:15:07.800 --> 0:15:10.840 We already got quantum, Daniel, it is too late. You've 0:15:10.880 --> 0:15:13.360 been quantum this whole time. Why are you listening? 0:15:13.440 --> 0:15:17.120 Yes, and I acknowledge that Newton is made of quantum objects. Yeah, 0:15:17.160 --> 0:15:19.000 in that sense, he is quantum, but he didn't know 0:15:19.080 --> 0:15:22.640 about quantum. And when he was developing his theory of gravity, 0:15:22.720 --> 0:15:27.240 he invented a concept which had instantaneous communication across space 0:15:27.280 --> 0:15:30.800 and time. And so if people think about non localities, 0:15:30.840 --> 0:15:34.040 this new fangled thing that emerge from quantum mechanics, isn't 0:15:34.040 --> 0:15:36.080 it sort of the old fangle thing that we sort 0:15:36.080 --> 0:15:37.520 of accepted for a while and then gave up. 0:15:37.560 --> 0:15:39.520 And now we're returning to we had to be very 0:15:39.600 --> 0:15:42.160 very careful. Once again, Sorry for being so careful, but 0:15:42.360 --> 0:15:46.360 please be careful. There are once again two different levels 0:15:46.440 --> 0:15:50.640 of locality that we have to distinguish between. One is 0:15:50.760 --> 0:15:55.640 just what I said, like, the equations of physics are local. 0:15:56.040 --> 0:15:59.400 Means that you write down the things the world is 0:15:59.400 --> 0:16:04.040 made of, and the whole equation is a function of position. Right, 0:16:04.120 --> 0:16:08.840 so in Newtonian gravity, you're right, that's not quite true. 0:16:09.080 --> 0:16:12.120 Like when Isaac Newton literally wrote down the inverse square 0:16:12.200 --> 0:16:15.640 law for gravity, the rule that says that the gravitational 0:16:15.680 --> 0:16:19.840 force is weaker when you're further away, stronger when you're 0:16:19.880 --> 0:16:22.680 close by a factor of one over the distance squared. 0:16:23.360 --> 0:16:26.920 And he worried that this seemed non local, that here's 0:16:27.000 --> 0:16:30.000 the Earth. The Earth is being pulled on by the 0:16:30.040 --> 0:16:34.320 Sun and the Moon and everything in the universe instantaneously. 0:16:34.400 --> 0:16:36.520 Because he's Isac Newton, he doesn't know about relativity. It 0:16:36.520 --> 0:16:40.800 doesn't know about special relativity anyway. So there is absolute space, 0:16:40.880 --> 0:16:44.880 absolute time. And Newton in the principia said like, yeah, 0:16:44.920 --> 0:16:48.120 this bothers me, the fact that somehow the information about 0:16:48.120 --> 0:16:50.920 the gravitational field of everything in the universe is conveyed 0:16:50.960 --> 0:16:54.200 to the Earth immediately, and he literally said, I'm going 0:16:54.240 --> 0:16:57.280 to leave this for future generations to sort out. I 0:16:57.320 --> 0:17:00.000 don't like it. It bugs me. I'm not smart enough. 0:17:00.040 --> 0:17:03.480 I'mder Isaac Newton. You know, eventually we will figure it out. Now, 0:17:03.680 --> 0:17:06.960 there's a sort of little known thing that happened, but 0:17:07.040 --> 0:17:10.159 I think is really crucial. In a circa eighteen hundred, 0:17:11.000 --> 0:17:14.640 Pierre Simone Laplace solved that problem. And no one ever 0:17:14.640 --> 0:17:18.280 gives them credit for this, but Laplace pointed out that 0:17:18.359 --> 0:17:22.679 you can write down Newtonian gravity as a field theory. 0:17:23.119 --> 0:17:27.040 He invented the idea of the gravitational potential. And by 0:17:27.080 --> 0:17:29.320 a field theory, we just mean rather than just saying 0:17:29.600 --> 0:17:32.320 there's a force acting on the Earth, and to know 0:17:32.359 --> 0:17:34.119 what the force is, you have to know what everything 0:17:34.119 --> 0:17:37.240 in the universe is doing. He says, there is something 0:17:37.280 --> 0:17:39.919 called the gravitational potential field, and it's a field, so 0:17:39.920 --> 0:17:42.720 it has a value at every point in space, and 0:17:42.800 --> 0:17:47.520 the equation that that field obeys is entirely local. What 0:17:47.680 --> 0:17:50.199 is happening through the field right here at some point 0:17:50.720 --> 0:17:53.280 only depends on what's happening at right next door points. 0:17:53.600 --> 0:17:55.720 And it's exactly like we said at the beginning, that 0:17:56.080 --> 0:17:59.400 the Sun creates a dimple in the gravitational field at 0:17:59.440 --> 0:18:03.400 where thus is, and that pulls on the field next 0:18:03.400 --> 0:18:05.119 to it, and that pulls on the field next to it, 0:18:05.200 --> 0:18:07.600 and it works its way out to the Earth. So, 0:18:07.840 --> 0:18:11.240 in that sense, because there is a local field theory 0:18:11.280 --> 0:18:15.520 description of Newtonian gravity, Lutonian gravity is entirely local. 0:18:15.600 --> 0:18:17.040 But is it still instantaneous? 0:18:17.480 --> 0:18:20.760 Well, you've skipped ahead to the world of relativity, where 0:18:20.800 --> 0:18:24.400 there's a speed limit. So when relativity comes along, which 0:18:24.440 --> 0:18:28.159 is long after either Newton or laplace, now we have 0:18:28.240 --> 0:18:31.439 an idea that even if the laws of physics are 0:18:31.480 --> 0:18:35.919 local in space time, which they are, like Einstein's equations 0:18:35.960 --> 0:18:41.040 and Maxwell's equations and whatever, still, if something happens at 0:18:41.080 --> 0:18:44.119 a particular point in space time, the effects of that 0:18:44.160 --> 0:18:47.040 thing only ripple out slower than or at the speed 0:18:47.080 --> 0:18:51.320 of light. Right, You do not affect things instantaneously far away. 0:18:52.119 --> 0:18:55.560 And what that means is that we can have a better, stronger, 0:18:55.720 --> 0:19:00.439 more satisfying version of locality, which is that not only 0:19:00.680 --> 0:19:05.040 do the equations simply exist at every point in space 0:19:05.320 --> 0:19:08.399 or every point in space time, but that you're not 0:19:08.560 --> 0:19:13.800 being affected by things infinitely far away instantaneously. So we 0:19:13.920 --> 0:19:18.159 have ex post facto retrofitted our notion of locality to 0:19:18.320 --> 0:19:23.439 demand not instantaneous communication between two different points of space. 0:19:23.480 --> 0:19:25.919 For Newton, that would have been fine. For in the 0:19:25.960 --> 0:19:28.520 post Einstein world, that's no longer okay, because the speed 0:19:28.520 --> 0:19:29.840 of light is a fundamental limit. 0:19:30.040 --> 0:19:32.000 Okay, So the biologist who lives in the world where 0:19:32.080 --> 0:19:36.160 locality works, fine, thank you very much. Is so if 0:19:36.160 --> 0:19:40.159 I can sort of summarize what's happening. So locality is 0:19:40.200 --> 0:19:44.000 a problem because at the quantum level it doesn't really 0:19:44.000 --> 0:19:46.359 work and you scale up. But it sounds like you 0:19:46.400 --> 0:19:48.879 were just saying that locality is fine when you're talking 0:19:48.880 --> 0:19:52.560 about things like the Sun impacting the Earth because you've 0:19:52.560 --> 0:19:54.840 got a field and you're connected every point along the way, 0:19:55.240 --> 0:19:59.240 And so is our conversation then mostly about the quantum level. 0:19:59.680 --> 0:20:03.960 Now am I following, Well, there's lots of conversations. Okay, 0:20:03.960 --> 0:20:06.919 you know, there's definitely that. The distinction we were just 0:20:07.040 --> 0:20:11.639 drawing is between pre relativity classical physics and post relativity 0:20:11.640 --> 0:20:15.800 classical physics, where we have glommed onto the notion of locality. 0:20:15.920 --> 0:20:19.159 The idea that signals don't travel faster than light, so 0:20:19.240 --> 0:20:24.399 there cannot be any instantaneous communication across distances. You know, 0:20:24.480 --> 0:20:28.760 in Isaac Newton's world, I could send a signal to 0:20:29.200 --> 0:20:33.480 the Andromeda Galaxy instantaneously by taking a planet and shaking 0:20:33.560 --> 0:20:36.720 it a little bit right. Its gravitational force in principle, 0:20:36.840 --> 0:20:41.160 although not in practice, would change instantaneously in the Andromeda Galaxy. 0:20:41.200 --> 0:20:44.440 And so is it local, You know, kind of it's 0:20:44.480 --> 0:20:47.440 fading away, but still strictly speaking, it's hard to wrap 0:20:47.440 --> 0:20:50.000 your brain around. But now, in Einstein's universe, if you 0:20:50.080 --> 0:20:52.080 shake a planet, it's going to take you a million 0:20:52.160 --> 0:20:54.760 light years for that signal to get to the Andromeda Galaxy. 0:20:55.440 --> 0:20:59.080 But still that's all within the limit that we called 0:20:59.080 --> 0:21:03.760 the classical limit of quantum mechanics. And so one place 0:21:04.000 --> 0:21:07.320 where our notions of locality are challenged and need to 0:21:07.359 --> 0:21:10.840 be updated is in the quantum realm. But by the way, 0:21:10.880 --> 0:21:13.919 another place that their challenge need to be updated is 0:21:13.960 --> 0:21:17.600 in things like biology. Oh no, yes, because you know 0:21:17.640 --> 0:21:22.800 in biology what happens is most of the biological organisms 0:21:22.800 --> 0:21:25.080 that you and I know and love are moving very 0:21:25.119 --> 0:21:28.679 slow compared to the speed of light. Yeap so even 0:21:28.760 --> 0:21:31.720