WEBVTT - Why does measuring a quantum object change it? 0:00:08.560 --> 0:00:11.520 Once upon a time, physicists seemed to have the world 0:00:11.600 --> 0:00:15.440 figured out almost There were one or two loose threads 0:00:15.480 --> 0:00:18.200 to be tidied up, but a solid idea about how 0:00:18.239 --> 0:00:21.920 the universe worked had emerged, and it made sense. At 0:00:21.920 --> 0:00:25.759 a particle level, physicist thought the universe worked basically the 0:00:25.840 --> 0:00:28.120 same way it did at the human level and at 0:00:28.160 --> 0:00:32.240 the planetary level little balls moving around, orbiting each other, 0:00:32.400 --> 0:00:36.159 occasionally even careening off of each other. How wonderful and 0:00:36.240 --> 0:00:39.880 symmetric and symbol it seemed that the same concepts worked 0:00:39.920 --> 0:00:43.040 from the very small to the very large. What a deep, 0:00:43.400 --> 0:00:47.080 satisfying truth about the universe we had revealed that explained 0:00:47.120 --> 0:00:52.159 almost everything other than a few pesky experimental results, And 0:00:52.200 --> 0:00:56.760 then those remaining loose threads unraveled the whole picture. To 0:00:56.880 --> 0:01:00.440 explain those weird experiments, we needed to break that symmetry 0:01:00.440 --> 0:01:02.600 and come up with a new vision for how the 0:01:02.720 --> 0:01:06.280 microscopic world works. At the smallest level, the world wasn't 0:01:06.319 --> 0:01:08.960 made of little spinning balls, but of something new and 0:01:09.040 --> 0:01:12.480 quite alien, quantum objects that followed a weird set of 0:01:12.560 --> 0:01:15.000 rules that gave headaches to even the geniuses of the 0:01:15.040 --> 0:01:18.960 twentieth century. So then, what is real. Is the world 0:01:19.040 --> 0:01:22.520 made of stuff that moves through space like basketballs and planets? 0:01:22.800 --> 0:01:25.880 Or does it follow some crazy new, weird quantum rules. 0:01:26.040 --> 0:01:28.440 And if the quantum rules are real, does that mean 0:01:28.480 --> 0:01:46.960 that the universe doesn't make any sense deep down? Hi? 0:01:47.160 --> 0:01:50.480 I'm Daniel. I'm a particle physicist and I'm desperate to 0:01:50.520 --> 0:01:54.120 know what is real about the universe. And with me 0:01:54.160 --> 0:01:58.400 today is a special guest astrophysicist Adam Becker, and an 0:01:58.480 --> 0:02:01.640 expert on reality, given that he's written a wonderful book 0:02:01.640 --> 0:02:05.280 about quantum mechanics titled What Is Real? Adam, Welcome to 0:02:05.280 --> 0:02:08.320 the program. Say hello, Hi, Yeah, thanks for having me Daniel. 0:02:08.360 --> 0:02:10.639 This is a lot of fun. Well, I'm especially pleased 0:02:10.680 --> 0:02:13.480 to have you on today because when listeners right in 0:02:13.520 --> 0:02:15.600 and ask me what they can read to get a 0:02:15.639 --> 0:02:18.840 better understanding of the thorny problems of quantum mechanics, I 0:02:18.880 --> 0:02:21.720 always recommend your book. So thanks for joining us and 0:02:21.760 --> 0:02:24.120 for helping us unravel a little bit about what we 0:02:24.200 --> 0:02:27.079 know and what we don't know about the universe. Well 0:02:27.120 --> 0:02:29.560 that's great. I'm really glad to hear that you're plugging 0:02:29.639 --> 0:02:34.480 my book And welcome everybody to the podcast. Daniel and 0:02:34.560 --> 0:02:36.920 Jorhigg explain in the universe where we try to do 0:02:37.080 --> 0:02:40.400 just that, embrace our knowledge and admit our ignorance. We 0:02:40.440 --> 0:02:42.640 do a deep dive into what things really mean and 0:02:42.680 --> 0:02:45.120 admit when words and science are being used to describe 0:02:45.120 --> 0:02:48.520 our lack of knowledge rather than our actual understanding. We're 0:02:48.600 --> 0:02:50.680 here to explain all of it to you, and on 0:02:50.720 --> 0:02:53.919 today's program, we're gonna attempt to do something very hard, 0:02:54.160 --> 0:02:57.919 to tackle one of the trickiest topics in modern physics, 0:02:57.960 --> 0:03:01.400 the meaning of quantum mechanics. But our goal is to 0:03:01.480 --> 0:03:04.240 find the solution today, but instead to help guide you 0:03:04.360 --> 0:03:08.200 through the current situation, the status of our knowledge and ignorance, 0:03:08.360 --> 0:03:10.760 to show you where the tricky bits are, why they 0:03:10.800 --> 0:03:14.240 have been so persistently tricky, and what the various ideas 0:03:14.280 --> 0:03:17.000 are for making progress. I mean, we know that quantum 0:03:17.040 --> 0:03:19.920 mechanics works right, the math is correct, it can predict 0:03:19.919 --> 0:03:22.920 the results of experiments with incredible precision. It seems like 0:03:22.919 --> 0:03:26.000 our universe is quantum mechanical. But we want to know 0:03:26.160 --> 0:03:28.600 what that means about the universe. We want to know 0:03:28.800 --> 0:03:32.200 what is real to me. That's what physics is is 0:03:32.240 --> 0:03:35.640 explaining the unknown in terms of the known. Is grappling 0:03:35.640 --> 0:03:37.920 with it and getting our intuition around it. So my 0:03:38.040 --> 0:03:40.960 first question to you, Adam, is do you think that's possible? 0:03:41.200 --> 0:03:43.760 Do we even have the tools, the mental tools to 0:03:43.920 --> 0:03:48.040 explain and understand something this alien, to understand what is real? 0:03:48.280 --> 0:03:53.080 I mean, hopefully, because that's the short answer. The longer 0:03:53.080 --> 0:03:55.400 answer is, you know, so far we seem to be 0:03:55.400 --> 0:03:58.680 doing a pretty okay job. I am certainly open to 0:03:58.720 --> 0:04:02.760 the possibility that the human mind is just not capable 0:04:02.760 --> 0:04:06.400 of comprehending some basic features of reality. On the other hand, 0:04:06.600 --> 0:04:09.240 you know, throughout the history of science, we've got a 0:04:09.240 --> 0:04:11.880 pretty good track record of you know, banging our heads 0:04:11.880 --> 0:04:14.800 against a problem and eventually coming up with a pretty 0:04:14.800 --> 0:04:17.400 good theory of what's going on in a pretty good mathematical, 0:04:17.720 --> 0:04:21.880 you know, description to go along with that theory. Quantum mechanics, 0:04:22.040 --> 0:04:26.840 of course, is deeply strange, but there's no rule against 0:04:26.839 --> 0:04:29.279 reality being strange. Right. In fact, it be kind of 0:04:29.279 --> 0:04:33.159 weird if the world we're not weird, be disappointing, yeah, exactly. Yeah, 0:04:33.160 --> 0:04:35.359 And also, come on, have you been here? Have you 0:04:35.400 --> 0:04:37.760 seen this place? Of course it's weird, but you're toutings 0:04:37.760 --> 0:04:40.440 are the history of our accomplishments, and those are wonderful, 0:04:40.600 --> 0:04:42.359 but a lot of those are limited to sort of 0:04:42.360 --> 0:04:46.040 the the narrow regime in which our intuition works, right, 0:04:46.279 --> 0:04:49.279 because we grew up experiencing at things, These are familiar 0:04:49.360 --> 0:04:52.360 questions we have. Why does the thing roll down the hill? 0:04:52.600 --> 0:04:55.640 What are these stars moving around us? Does that necessarily 0:04:55.640 --> 0:04:59.719 mean it's possible to like extrapolate to other weird realms 0:04:59.760 --> 0:05:01.640 where just have not grown up with any sort of 0:05:01.839 --> 0:05:05.080 natural experience. Yeah, I mean not necessarily. It's possible that 0:05:05.120 --> 0:05:07.480 we'll hit some sort of limit. I'm just not convinced 0:05:07.560 --> 0:05:10.640 that quantum mechanics represents that limit. I mean, first of all, 0:05:10.680 --> 0:05:15.800 we have this phenomenally accurate and precise theory in quantum 0:05:15.800 --> 0:05:18.719 mechanics and quantum physics more generally, you know, arguably the 0:05:18.720 --> 0:05:22.240 most accurate scientific theory ever in terms of, you know, 0:05:22.320 --> 0:05:26.560 how well it matches experiment. And it's not as if 0:05:27.040 --> 0:05:29.640 we've looked at the theory and thrown up our hands 0:05:29.640 --> 0:05:32.360 and said, you know, this is impossible to understand. We have, 0:05:32.520 --> 0:05:34.880 you know, mathematical tools, but there's no way that the 0:05:34.960 --> 0:05:38.320 human mind can comprehend the reality behind the math. If anything, 0:05:38.320 --> 0:05:42.279 it's the opposite. We have too many ideas about how 0:05:42.320 --> 0:05:44.839 to understand the reality behind the math. So that doesn't 0:05:44.920 --> 0:05:49.600 sound like a failure of human intuition to me, at 0:05:49.680 --> 0:05:52.000 least not yet. You know. Yeah, I'm totally open to 0:05:52.040 --> 0:05:54.000 the idea that we'll get there at some point. All right, Well, 0:05:54.040 --> 0:05:56.360 you sound like a supporter of quantum mechanics. In fact, 0:05:56.440 --> 0:05:59.919 it sounds like you're a shill for Big Quantum. You know. 0:06:00.120 --> 0:06:02.560 I don't think anyone has ever accused me of being 0:06:02.600 --> 0:06:05.760 a shill for a big quantum before. But I can't 0:06:05.880 --> 0:06:09.200 say that you're wrong. I'm just not exactly a paid shill. 0:06:09.400 --> 0:06:11.520 There's no money in it. Oh you're not getting your checks? 0:06:11.520 --> 0:06:13.640 Oh man? Yeah, well I not from big Quantum, just 0:06:13.680 --> 0:06:17.960 from the publisher. Well, we can't tagle all of qualum 0:06:18.000 --> 0:06:20.400 mechanics in a single episode, so today I want to 0:06:20.400 --> 0:06:23.680 focus on one issue in particular, which is I think 0:06:23.720 --> 0:06:26.400 at the heart of it all and sometimes known as 0:06:26.440 --> 0:06:29.800 the measurement problem. So on today's program, we're asking the 0:06:29.920 --> 0:06:39.520 question what makes the wave function collapse? What is measurement 0:06:39.720 --> 0:06:42.920 in quantum mechanics? And Adam, before we dig into it, 0:06:43.080 --> 0:06:45.000 we do something fun on the program, which is that 0:06:45.040 --> 0:06:47.719 I ask our listeners to answer this question just to 0:06:47.720 --> 0:06:50.120 get a sense for like what out there are people thinking, 0:06:50.320 --> 0:06:52.360 How much of a grass to people have on this question. 0:06:52.400 --> 0:06:55.320 It helps orient us to know where to aim our answer. 0:06:55.680 --> 0:06:59.120 So I have for us some clips from listeners, and 0:06:59.160 --> 0:07:02.200 if you they're listening, would like to participate for future 0:07:02.240 --> 0:07:06.240 episodes to answer really hard physics questions with no preparation. 0:07:06.440 --> 0:07:09.080 Please write to me two questions at Daniel and Jorge 0:07:09.120 --> 0:07:11.400 dot com. It's a lot more fun than it's outs. 0:07:11.840 --> 0:07:15.200 Here's what our listeners had to say. Okay, I have 0:07:16.160 --> 0:07:21.640 I've been learning about wave functions and quantum physics and 0:07:21.680 --> 0:07:23.960 I think I know that's a really important part of 0:07:24.000 --> 0:07:28.160 measuring something, and I have heard about it collapsing, but 0:07:28.200 --> 0:07:29.800 I'm not sure what makes the collapse. I'm going to 0:07:29.880 --> 0:07:33.480 guess that what makes a wave function collapse is when 0:07:34.680 --> 0:07:38.800 um something's measured. I would say, once the energy in 0:07:38.920 --> 0:07:42.800 the wave is reduced to a certain point, the wave 0:07:42.880 --> 0:07:47.680 can no longer support itself and it collapses. I'm just 0:07:47.760 --> 0:07:57.320 going to guess here, probably something that intervenes from outside 0:07:57.400 --> 0:08:05.520 so on outside interaction. Alright, So those the answers from 0:08:05.560 --> 0:08:08.880 our awesome listeners Adam, what do you think about those speculations. 0:08:09.160 --> 0:08:12.160 I mean, they mostly sound like the kinds of things 0:08:12.160 --> 0:08:15.920 that you would see in a quantum mechanics textbook actually, 0:08:16.320 --> 0:08:19.960 which is to say kind of fake. Right. There's a 0:08:19.960 --> 0:08:22.120 lot of talking there about like, well, I'm not sure 0:08:22.320 --> 0:08:25.560 something about measurement, but I don't really know what that means, 0:08:25.680 --> 0:08:28.520 and like that's really the heart of the problem, right, 0:08:28.520 --> 0:08:31.320 Like nobody really knows what we mean by measurement or 0:08:31.320 --> 0:08:34.319 by observation. So we like have a word for it, 0:08:34.600 --> 0:08:36.480 we have a phrase for but we don't really know 0:08:36.640 --> 0:08:39.839 exactly what that means is happening deep down. Yeah, that's 0:08:39.880 --> 0:08:42.880 exactly right, all right, So I think maybe we should 0:08:42.920 --> 0:08:45.920 start just by orienting ourselves, just by getting sort of 0:08:45.920 --> 0:08:48.720 like using the same vocabulary and making sure everybody out 0:08:48.720 --> 0:08:51.000 there knows what we're talking about when we say the 0:08:51.040 --> 0:08:54.120 wave function, Can you give us a short definition of 0:08:54.120 --> 0:08:56.120 what the wave function is and why it's so important 0:08:56.160 --> 0:08:58.400 to this question. I mean, there's a sense in which 0:08:58.400 --> 0:09:00.680 the question of what the wave function is kind of 0:09:00.720 --> 0:09:02.720 what's at the heart of all this too. But yeah, 0:09:02.840 --> 0:09:05.000 I can take a stab of that's so in sort 0:09:05.040 --> 0:09:09.520 of classical physics, the physics of Isaac Newton, the physics 0:09:09.600 --> 0:09:12.560 of everyday life, the physics of you know, billiard balls 0:09:12.880 --> 0:09:17.640 and car crashes. The way that we describe where things are. 0:09:18.000 --> 0:09:21.199 We usually use three numbers to describe where something is, 0:09:21.280 --> 0:09:23.360 or at least where the center of mass of something is. 0:09:23.600 --> 0:09:25.240 You know, we say, okay, well, here's where it is 0:09:25.240 --> 0:09:27.240 in the X, y and z. Here's how high off 0:09:27.280 --> 0:09:29.640 the ground it is. Here's how far in front of 0:09:29.679 --> 0:09:31.440 me it is, and here's how far it is off 0:09:31.480 --> 0:09:34.000 to one side. And that's all you really need to 0:09:34.040 --> 0:09:36.640 describe where something is, because we live in a three 0:09:36.640 --> 0:09:41.400 dimensional world exactly right. But in quantum mechanics, if you 0:09:41.440 --> 0:09:43.160 want to do the same thing, if you want to 0:09:43.200 --> 0:09:46.600 take all the information we have about where something is, 0:09:46.800 --> 0:09:48.920 like if you want to talk about where an electron is, 0:09:49.240 --> 0:09:52.440 you need more than three numbers. You actually need on 0:09:52.520 --> 0:09:56.720 infinity of numbers spread out across all of space. And 0:09:57.520 --> 0:10:00.080 that's the wave function. So the numbers are higher in 0:10:00.160 --> 0:10:04.360 some spots and lower in others. And usually what we 0:10:04.520 --> 0:10:07.400 say the wave function represents is that it's to do 0:10:07.600 --> 0:10:12.280 with the probability of finding the electron in a particular 0:10:12.440 --> 0:10:16.480 spot when we look, which makes it sound like the 0:10:16.480 --> 0:10:20.320 wave function isn't really a thing in nature, and it's 0:10:20.360 --> 0:10:23.440 just a lot more about our information. Yeah, so hold 0:10:23.440 --> 0:10:25.520 on it. Let me ask you about that, because you're 0:10:25.520 --> 0:10:28.280 making an analogy to a classical particle where we're talking 0:10:28.320 --> 0:10:31.200 about like where it actually is, and you're saying that 0:10:31.360 --> 0:10:34.200 you need more information to talk about where an electron is. 0:10:34.480 --> 0:10:36.880 But now you're describing the wave function in terms not 0:10:37.200 --> 0:10:39.600 of where the object is or what it is, but 0:10:39.679 --> 0:10:42.520 about our measurements of it already, sort of like we've 0:10:42.520 --> 0:10:45.400 already sort of snuck into the electron. That's exactly right. 0:10:45.400 --> 0:10:48.880 I mean, there's there are I wasn't kidding when I 0:10:48.880 --> 0:10:50.520 said that, you know, the question of what the wave 0:10:50.559 --> 0:10:52.960 function is it's sort of part of the controversy here 0:10:53.000 --> 0:10:55.000 and part of what's at stake. We'll see if we 0:10:55.000 --> 0:10:58.040 get past this one question and the whole thing exactly. Yeah. 0:10:58.320 --> 0:11:01.679 I'm not convinced succeed, but yeah, I mean the reason 0:11:01.800 --> 0:11:06.360 I say, oh, this is sort of the quantum analog 0:11:07.160 --> 0:11:10.559 of you know, the three numbers describing where something is 0:11:10.640 --> 0:11:13.040 in Newton's physics is that, you know, it sort of 0:11:13.080 --> 0:11:16.800 plays the same role in the same sorts of physical 0:11:17.000 --> 0:11:21.760 laws in quantum physics. So the question of whether the 0:11:21.800 --> 0:11:24.480 wave function is a thing out in the world or 0:11:24.480 --> 0:11:27.680 if it's just something about our information about stuff out 0:11:27.720 --> 0:11:32.040 in the world is you know, a live question and 0:11:32.160 --> 0:11:35.480 a subject of active debate. But it's not just as 0:11:35.520 --> 0:11:39.720 simple as saying, oh, it's just our information about what's 0:11:39.720 --> 0:11:41.920 out there. It's just that in the most simple way 0:11:41.960 --> 0:11:45.959 imaginable because wave functions, I mean, like the name sort 0:11:46.000 --> 0:11:50.719 of implies the wave functions. Wave they behave in ways 0:11:50.840 --> 0:11:54.800 that we actually tend to associate with physical objects. They 0:11:54.920 --> 0:11:59.400 undulate smoothly, the numbers change smoothly from one place to another, 0:11:59.440 --> 0:12:02.120 they change over time, and they can even perform some 0:12:02.200 --> 0:12:05.000 of the same tricks that waves perform. You can send 0:12:05.320 --> 0:12:09.840 them through a narrow slit and they'll defract outward on 0:12:09.880 --> 0:12:13.120 the other side. They can interfere with themselves and cause 0:12:13.200 --> 0:12:16.200 you know, patterns of light and darkness on a photographic plate, 0:12:16.280 --> 0:12:18.079 that sort of thing. So we call it a wave 0:12:18.120 --> 0:12:21.880 function because mathematically it's described by similar equations to describe 0:12:21.880 --> 0:12:24.800 other kind of classical waves were familiar with, that's right, yeah, 0:12:24.840 --> 0:12:27.640 And it also performs some of the same tricks as 0:12:27.720 --> 0:12:30.840 classical waves that were familiar with physically, so you know, 0:12:30.920 --> 0:12:34.280 we don't tend to think of our information about a 0:12:34.400 --> 0:12:38.760 thing as doing that. And it doesn't mean that it can't. 0:12:38.800 --> 0:12:41.120 It just means that if we want an account of 0:12:41.160 --> 0:12:43.840 wave functions in terms of our information about the world, 0:12:43.920 --> 0:12:46.000 as opposed to saying, you know, the wave functions actually 0:12:46.000 --> 0:12:47.920 a wave that's out there, we need to do some 0:12:47.960 --> 0:12:50.560 extra work. So you're saying the wave function tells us 0:12:51.080 --> 0:12:53.840 how to predict the outcomes of our experiments, like if 0:12:53.840 --> 0:12:55.560 I'm going to find the electron over here, or if 0:12:55.559 --> 0:12:57.160 I'm gonna find it over there, or if I'm gonna 0:12:57.200 --> 0:12:59.319 find it's been up or going to find it's been down. 0:12:59.360 --> 0:13:01.840 It tells me what I'm likely to observe, but we're 0:13:01.840 --> 0:13:05.520 not sure whether it's like actually real and distinct separate 0:13:05.559 --> 0:13:08.040 from our ability to measure it and interact with it. 0:13:08.080 --> 0:13:10.559 We don't know sort of philosophically, whether we can take 0:13:10.600 --> 0:13:13.640 that step. Yeah, yeah, I mean, there are theorems that 0:13:13.720 --> 0:13:15.920 suggests that there's something like the way function out in 0:13:15.960 --> 0:13:18.920 the world, but it's not. You know, this is an 0:13:18.920 --> 0:13:22.400 open question and has been since the beginning of quantum 0:13:22.400 --> 0:13:24.600 physics almost addrey years ago. And I think that's sort 0:13:24.600 --> 0:13:26.880 of one of the most fascinating points. I mean, you 0:13:26.960 --> 0:13:30.839 talk about classical physics, there's no distinction there between the 0:13:30.880 --> 0:13:34.280 thing exists and we measure it to exist there. It's 0:13:34.280 --> 0:13:35.880 just like those two things are just part of the 0:13:35.920 --> 0:13:39.640 same idea. You see the ball there because it is there, right, 0:13:39.640 --> 0:13:41.440 and it would be there if you weren't here to 0:13:41.520 --> 0:13:43.560 look at it as all this kind of stuff. So 0:13:43.640 --> 0:13:46.240 I love these discoveries and physics that make us sort 0:13:46.240 --> 0:13:50.520 of crack open two ideas we thought were so naturally entangled, 0:13:50.720 --> 0:13:53.160 so like deeply connected, and now we realize there's a 0:13:53.160 --> 0:13:55.920 possibility for these two things to be actually separate and distinct. 0:13:56.000 --> 0:14:00.000 Observing something is there doesn't mean that it is there 0:14:00.160 --> 0:14:03.840 some deeper sense. It's like totally crazy. Yeah, no, it's 0:14:03.920 --> 0:14:09.000 it's absolutely true. And one of the strange things about 0:14:09.200 --> 0:14:12.000 wave functions and measurement is just the idea that, you know, 0:14:12.720 --> 0:14:16.079 measurement and observation are coming into what's supposed to be 0:14:16.120 --> 0:14:18.680 a fundamental theory at all, because you know, measurement is 0:14:18.679 --> 0:14:22.200 not a well defined thing. What do we mean when 0:14:22.200 --> 0:14:24.800 we say measurement? Right? And I think that a key 0:14:24.880 --> 0:14:27.520 concept we need to tackle before we get into measurement 0:14:27.880 --> 0:14:31.680 is this question of superposition, right, because quantum mechanical objects 0:14:31.680 --> 0:14:35.120 can do something that like our baseballs can't do, which 0:14:35.160 --> 0:14:37.360 is that they can for a long time, have two 0:14:37.400 --> 0:14:41.520 possible outcomes and maintain those two possibilities simultaneously. Right, So 0:14:41.600 --> 0:14:45.960 the wave function can incorporate various possible outcomes inside of itself. Yeah, 0:14:46.080 --> 0:14:49.840 now that that's exactly right. If you send uh ping 0:14:49.920 --> 0:14:53.040 pong ball into a maze, it's just gonna go down 0:14:53.080 --> 0:14:55.160 one path in that maze and you can watch it 0:14:55.280 --> 0:14:58.040 take one path. But if you send an electron or 0:14:58.040 --> 0:15:00.800 a photon or some other you know, tinique wantum object 0:15:00.960 --> 0:15:03.880 into you know, some sort of maze, like send a 0:15:03.880 --> 0:15:06.680 photon into a set of mirrors and prisms on an 0:15:06.680 --> 0:15:08.680 optical bench, you know, in the basement of a physics 0:15:08.680 --> 0:15:11.160 department somewhere. It's not going to take just one path. 0:15:11.480 --> 0:15:13.720 Quantum mechanics says, no, it's it's going to take a 0:15:13.760 --> 0:15:16.160 superposition of all of those paths. It's going to go 0:15:16.360 --> 0:15:19.240 down two or three or four different paths at the 0:15:19.280 --> 0:15:21.800 same time, or as wave function will. But then when 0:15:21.840 --> 0:15:24.000 we measure it at the end, right, we say, all right, 0:15:24.200 --> 0:15:26.120 I'm going to put a device in here. I'm going 0:15:26.160 --> 0:15:29.520 to ask the question, which one did it actually go down? 0:15:29.840 --> 0:15:33.240 I can't measure all those paths, right, I get one answer. 0:15:33.440 --> 0:15:35.200 That's right, you get one answer. If you put a 0:15:35.240 --> 0:15:37.760 detector on one of the paths, then you'll get an 0:15:37.760 --> 0:15:40.160 answer saying you know, yes it went down this path 0:15:40.360 --> 0:15:43.920 or no it didn't. But if instead you set up 0:15:44.520 --> 0:15:47.400 your optical bench, your maze, whatever you want to call it, 0:15:47.480 --> 0:15:50.520 or your double slit experiment so that there's an interference 0:15:50.520 --> 0:15:53.680 pattern on the other side, you will get interference. So 0:15:53.720 --> 0:15:58.960 when you're not asking the question of which way the 0:15:58.960 --> 0:16:02.600 photon went and when it went into your setup, it 0:16:02.960 --> 0:16:05.520 sort of goes down all of those paths and then 0:16:05.600 --> 0:16:08.400 interferes with itself. But if you ask weight which way 0:16:08.400 --> 0:16:10.680 did you go, well, then it says, oh, no, I 0:16:10.720 --> 0:16:13.520 went this way, and then it doesn't interfere with itself. 0:16:13.640 --> 0:16:16.480 And so we know that the wave function does this 0:16:16.600 --> 0:16:20.680 superposition thing having two possible outcomes at the same time 0:16:20.760 --> 0:16:24.560 because we see the interference, that's proof of superposition. But 0:16:24.600 --> 0:16:27.600 then when we try to measure the photon, the superposition 0:16:27.720 --> 0:16:30.560 sneaks away. And for you, is that like the clearest 0:16:30.600 --> 0:16:34.200 encapsulation of this question, this measurement problem, like how the 0:16:34.200 --> 0:16:38.240 wave function goes from having like various quantum mechanical possibilities 0:16:38.280 --> 0:16:41.240 for the paths of a photon to basically picking one. 0:16:41.480 --> 0:16:44.400 How the universe like ends up rolling that die choosing 0:16:44.600 --> 0:16:46.640 this path or that path or the other path. I 0:16:46.640 --> 0:16:48.720 think it's a very clear example of that. The way 0:16:48.720 --> 0:16:51.120 that I usually talk about it is a little different, 0:16:51.360 --> 0:16:53.400 you know, I I say, Okay, this is one example 0:16:53.560 --> 0:16:57.680 of the measurement problem, But in general, the measurement problem 0:16:57.800 --> 0:17:01.280 is more about that sort of undulating property of the 0:17:01.320 --> 0:17:03.880 wave functions that I was talking about before. Right, we 0:17:04.040 --> 0:17:08.280 have this very nice equation, the shroding Er equation. It's 0:17:08.440 --> 0:17:10.399 a law of physics. Right, we think of it as 0:17:10.400 --> 0:17:15.680 a law of nature, and that describes how wave functions wave. Right, 0:17:15.720 --> 0:17:20.880 it says that they always wave sort of smoothly and evenly, 0:17:21.000 --> 0:17:23.960 and you know, they don't make any abrupt transitions. But 0:17:25.320 --> 0:17:30.399 sometimes the shroding Er equation is temporarily suspended. What you 0:17:30.480 --> 0:17:33.200 can't do that you can just pause the laws of physics. 0:17:33.200 --> 0:17:35.840 Sometimes it says, if someone just hits pause on the 0:17:35.840 --> 0:17:39.639 shrodding Er equation and instead you have to use this 0:17:39.720 --> 0:17:42.960 other thing called the Born rule, which says, okay, take 0:17:43.000 --> 0:17:47.000 the wave function and look at all the different possibilities. 0:17:47.040 --> 0:17:50.840 It gives you for the way that your experiment is 0:17:50.880 --> 0:17:54.399 going to turn out, and that gives you the probabilities 0:17:54.400 --> 0:17:56.400 for the different outcomes. That's what the wave function does. 0:17:56.760 --> 0:17:59.359 When you use the Born rule, And then whichever one 0:17:59.400 --> 0:18:01.960 you actually serve, cut out all the other parts of 0:18:02.000 --> 0:18:04.640 the wave function, that's the only one that's left. All 0:18:04.640 --> 0:18:08.000 the others just instantly go to zero and then violates 0:18:08.000 --> 0:18:11.000 the shorting your equation, and that violates the shroting your equation. 0:18:11.000 --> 0:18:13.160 The shrodding your equation doesn't have anything in it like that. 0:18:13.600 --> 0:18:16.959 And then when you stop looking, that's what happens when 0:18:17.000 --> 0:18:18.959 you make a measurement. And then when you stop looking, 0:18:19.680 --> 0:18:22.879 the shorting your equation applies again. And so then the 0:18:23.000 --> 0:18:25.639 question is, okay, well, so if you have these two 0:18:25.680 --> 0:18:30.040 different rules and they contradict each other, then first of all, 0:18:30.119 --> 0:18:36.119 why And second we better be really, really really clear 0:18:36.320 --> 0:18:38.600 about when to use one and when to use the other. 0:18:39.920 --> 0:18:42.240 And this is the collapse of the wave function. And 0:18:42.280 --> 0:18:44.720 so the usual answer to the question when do we 0:18:44.840 --> 0:18:46.400 use one and when do we use the other? When 0:18:46.440 --> 0:18:50.400 does the wave function collapse. The usual answer that is, 0:18:50.800 --> 0:18:53.919 when we make a measurement. And so now the fact 0:18:53.960 --> 0:18:58.240 that the word measurement is kind of vague goes from 0:18:59.000 --> 0:19:02.840 you know, a little troublesome or annoying to a really 0:19:03.040 --> 0:19:07.399 serious problem. Right, So that's a great encapsulation of the 0:19:07.520 --> 0:19:10.720 question we're facing. Like, we have this beautiful equation, the 0:19:10.760 --> 0:19:13.920 shortening your equation that tells us how this wave function 0:19:14.040 --> 0:19:17.199 moves and changes and squishes through the world, and we 0:19:17.400 --> 0:19:20.920 verified it to zillions of degrees of accuracy, except that, 0:19:21.000 --> 0:19:24.000 as you say, sometimes it seems to be ignored and 0:19:24.080 --> 0:19:26.960 sometimes it doesn't seem to apply. And so we have 0:19:27.040 --> 0:19:29.520 this question of like how a wave function goes from 0:19:29.520 --> 0:19:32.480 this smooth under lending property to like collapsing into just 0:19:32.640 --> 0:19:35.560 a point when we make a measurement, what that measurement 0:19:35.680 --> 0:19:37.600 even means. So I want to dig into that in 0:19:37.720 --> 0:19:40.240 much more detail, but first let's take a quick break. 0:19:53.080 --> 0:19:55.680 All right, we're back and we're talking to ask ify 0:19:55.680 --> 0:19:58.560 is this Adam Becker, author of What is real about 0:19:58.640 --> 0:20:01.600 the nature of the quantum a function and what happens 0:20:01.640 --> 0:20:03.960 to it when we look at it? And I have 0:20:04.119 --> 0:20:07.000 this quote that I love from Bell who says either 0:20:07.080 --> 0:20:09.600 the way function as given by the Shorting your equation 0:20:09.880 --> 0:20:13.199 is everything, or it's not right. What do you make 0:20:13.240 --> 0:20:15.160 of that? I see what he's getting that there, and 0:20:15.280 --> 0:20:17.920 I think he's kind of right. I mean, there's got 0:20:17.920 --> 0:20:22.000 to be something more that tells us, Okay, when do 0:20:22.119 --> 0:20:24.679 we apply the shorting our equation and when don't we 0:20:25.080 --> 0:20:28.120 because otherwise, you know, uh, And I think Bell said 0:20:28.200 --> 0:20:30.159 something along these lines as well. We're stuck in this 0:20:30.240 --> 0:20:33.000 situation where these laws of physics that are supposed to 0:20:33.000 --> 0:20:36.040 be fundamental or just kind of hopelessly vague. So we 0:20:36.080 --> 0:20:38.359 have to take the Shortinger equation and the wave function, 0:20:38.400 --> 0:20:40.399 all of which seems wonderful and perfect and beautiful, and 0:20:40.680 --> 0:20:42.960 we love it, and we need to add something else. 0:20:43.040 --> 0:20:46.400 We need to say. Plus, there's this other mechanism that 0:20:46.560 --> 0:20:49.399 makes things collapse. And this, I think is where people 0:20:49.400 --> 0:20:54.080 disagree how to describe this weird combination of ideas and 0:20:54.200 --> 0:20:57.440 sort of a holistic concept. And so first let's talk 0:20:57.440 --> 0:21:00.359 about maybe the most mainstream way to a hack, this 0:21:00.600 --> 0:21:03.720 the most common, the one that people most read about 0:21:03.840 --> 0:21:05.879 in their textbooks. And then I think a lot of 0:21:05.880 --> 0:21:09.280 our listeners referred to which is basically the Copenhagen interpretation, 0:21:09.520 --> 0:21:12.200 where you add something to the shorting your equation that 0:21:12.240 --> 0:21:14.639 makes the wave function collapse when you look at it. 0:21:14.800 --> 0:21:16.919 So how does that actually happen? Like, what do you 0:21:16.960 --> 0:21:20.320 add to your system, you know, mathematically to make that happen. 0:21:20.359 --> 0:21:21.960 You can't just have like a thing that says and 0:21:22.160 --> 0:21:24.119 by the way, if there's a measurement, I mean, doesn't 0:21:24.160 --> 0:21:26.760 everything have to be like you know, written down in 0:21:26.840 --> 0:21:28.960 some sort of equation. The thing is it is just 0:21:29.080 --> 0:21:32.200 kind of added on. They do basically just say, yeah, 0:21:32.480 --> 0:21:35.120 use the shorting your equation, except when a measurement occurs, 0:21:35.600 --> 0:21:38.320 and what a measurement occurs, use this other thing instead. 0:21:38.440 --> 0:21:41.160 Use this collapse roule called the Born rule, named after 0:21:41.240 --> 0:21:44.280 Backs Born, you know, one of the architects of quantum physics. 0:21:44.320 --> 0:21:46.480 And it's just got a completely different mathematical form and 0:21:46.520 --> 0:21:49.479 it's said, you know, it says, okay, you know, like 0:21:49.520 --> 0:21:51.760 I said before, it lets you use the wave function 0:21:51.760 --> 0:21:56.400