WEBVTT - Does Quantum Mechanics make sense? 0:00:08.640 --> 0:00:11.520 If you've done some traveling, then you know the experience 0:00:11.520 --> 0:00:14.840 of feeling out of place. There are still some places 0:00:14.880 --> 0:00:18.400 on Earth where the culture is sufficiently different from ours 0:00:18.520 --> 0:00:20.600 or from yours, then you can no longer sort of 0:00:20.640 --> 0:00:24.080 trust your instincts. You don't intuitively know how to behave 0:00:24.160 --> 0:00:26.760 and how to get around. Maybe you don't know what 0:00:26.760 --> 0:00:29.160 people are saying, and you can't read the street signs, 0:00:29.200 --> 0:00:32.120 and things just seem to work differently. The street food 0:00:32.120 --> 0:00:35.880 has weird eyeballs in it where looks like it's fried scorpions. Hey, 0:00:35.920 --> 0:00:39.720 maybe it is. Well, that's the experience physicists have when 0:00:39.720 --> 0:00:43.000 we discover that the rules of the universe are totally 0:00:43.000 --> 0:00:46.519 different from what we are familiar with. That's exactly the 0:00:46.560 --> 0:00:48.919 feeling we're going for, and that's the feeling we get 0:00:49.040 --> 0:01:08.640 very often when we travel to the quantum realm. Yeah. Hi, 0:01:08.959 --> 0:01:13.160 I'm Daniel. I'm a particle physicist and I'm one half 0:01:13.200 --> 0:01:16.959 of the dynamic duo known as Daniel and Jorge, hosts 0:01:16.959 --> 0:01:21.280 of this podcast, Daniel and Jorge Explain the Universe, brought 0:01:21.360 --> 0:01:24.520 to you by I Heart Radio. My co host today, 0:01:24.640 --> 0:01:27.000 Jorge Champ can't be with us, so I'll be talking 0:01:27.040 --> 0:01:29.959 to you myself about the amazing secrets of the universe. 0:01:30.520 --> 0:01:34.319 This podcast is dedicated to revealing truths about the universe, 0:01:34.360 --> 0:01:37.880 to taking things that seem amazing and mystifying that maybe 0:01:37.880 --> 0:01:40.160 you've heard about or people talk about when they want 0:01:40.160 --> 0:01:43.160 to sound smart, but you never really understood. Well, we 0:01:43.200 --> 0:01:45.560 are here to break it down and make sure you 0:01:45.600 --> 0:01:48.280 can walk away actually understanding it. You can talk about 0:01:48.280 --> 0:01:51.640 it like an intelligent person. So if you have questions 0:01:51.680 --> 0:01:53.960 about what you heard today, please send them to us 0:01:54.240 --> 0:01:57.320 two questions at Daniel and Jorge dot com. It is 0:01:57.360 --> 0:02:00.720 our goal that you actually understand everything we are talking 0:02:00.760 --> 0:02:04.040 about and hopefully maybe even get a chuckle along the way. 0:02:04.240 --> 0:02:07.120 Since it's just me today, we'll probably have fewer jokes 0:02:07.120 --> 0:02:10.600 than usual. It's often Hooree that injects the humor into 0:02:10.639 --> 0:02:14.000 these conversations. So we're little humans. We're here on Earth, 0:02:14.080 --> 0:02:16.799 and we're used to a certain kind of experience. We 0:02:16.960 --> 0:02:19.040 used to things a certain size and moving on a 0:02:19.080 --> 0:02:22.200 certain speed. But the universe is a big place and 0:02:22.240 --> 0:02:24.320 there are lots of things out there that are not 0:02:24.440 --> 0:02:28.080 like our experience, that are really big or moving really fast. 0:02:28.600 --> 0:02:31.680 And what we've learned as humans is that most of 0:02:31.680 --> 0:02:34.440 the universe is different from what we expected, and that 0:02:34.560 --> 0:02:37.760 it follows rules that are different. And when we try 0:02:37.800 --> 0:02:39.840 to understand the rest of the universe, the things that 0:02:39.840 --> 0:02:43.040 are not balls rolling down planes or how water flows 0:02:43.080 --> 0:02:46.200 down a hill, that we need to do something mentally difficult. 0:02:46.320 --> 0:02:49.000 We need to do some sort of extrapolation. And that's 0:02:49.040 --> 0:02:50.880 the job of physics is to take us from the 0:02:50.919 --> 0:02:54.359 known into the unknown, to say, well, we understand how 0:02:54.400 --> 0:02:57.320 these things happen here on Earth. Can we also understand 0:02:57.320 --> 0:02:59.720 how the Earth moves around the sun. Can we understand 0:02:59.720 --> 0:03:02.880 the origins of the universe. Can we peel back layers 0:03:02.880 --> 0:03:05.840 of reality and see how things are built underneath? And 0:03:05.880 --> 0:03:08.240 in order to do that, we have to describe things 0:03:08.280 --> 0:03:11.080 we haven't seen in terms of things that we have seen. 0:03:11.600 --> 0:03:13.760 We talk about particles, and we like to describe them 0:03:13.800 --> 0:03:16.280 as kind of like waves and kind of like little 0:03:16.280 --> 0:03:20.000 spinning balls, because those are familiar mental constructs. Those are 0:03:20.040 --> 0:03:22.280 things we understand, so we can talk to each other 0:03:22.480 --> 0:03:25.160 about them. It's like if you drink a new wine 0:03:25.200 --> 0:03:26.800 and you try to describe it to your friends and 0:03:26.800 --> 0:03:29.840 you say, oh, it has flavors of oak and maybe BlackBerry. 0:03:30.320 --> 0:03:32.200 It's a way to describe things that you don't know 0:03:32.320 --> 0:03:34.480 in terms of things that you know. It's a basic 0:03:34.760 --> 0:03:39.160 mental strategy for understanding things. But what happens when you 0:03:39.240 --> 0:03:43.840 run into something fundamentally different, something unlike anything you've seen before, 0:03:44.280 --> 0:03:48.520 something where all of your mental constructs fail or are limited, 0:03:49.000 --> 0:03:52.240 when our experience has just not prepared us for something. 0:03:52.680 --> 0:04:04.040 That's the topic of today's podcast, Can we ever understand 0:04:04.320 --> 0:04:07.480 quantum mechanics? And before we dive in, I want to 0:04:07.480 --> 0:04:10.320 give a music shout out to Casey Hagman who sent 0:04:10.400 --> 0:04:13.000 in that alternative quest in music. Thank you very much. 0:04:13.040 --> 0:04:15.760 We loved it. And quantum mechanics is one of the 0:04:15.800 --> 0:04:18.320 most difficult things for people to grasp, one of the 0:04:18.320 --> 0:04:21.359 most intimidating topics, because people feel like it just can't 0:04:21.400 --> 0:04:26.279 make sense to them, and they hear eminent scientists, researchers, philosophers, 0:04:26.279 --> 0:04:29.640 physicists talking about quantum mechanics as if they don't understand 0:04:29.680 --> 0:04:32.000 it either. And it's true, there's a lot left to 0:04:32.000 --> 0:04:35.320 be understood about quantum mechanics. Folks like Sean Carroll talk 0:04:35.360 --> 0:04:38.440 about it very intelligently, and there's lots of lively discussion 0:04:38.440 --> 0:04:40.919 about what quantum mechanics really means, But I want to 0:04:40.920 --> 0:04:44.360 talk about something simpler. Just what does quantum mechanics say? 0:04:44.440 --> 0:04:47.160 What have we learned about the universe via quantum mechanics? 0:04:47.400 --> 0:04:51.200 Is it possible for us to develop an intuition to 0:04:51.360 --> 0:04:55.360 understand quantum mechanics? Is? Will it always forever be foreign 0:04:55.400 --> 0:04:58.600 to us? Or can we become comfortable with it? Will 0:04:58.640 --> 0:05:02.200 we by spending enough time? And it by thinking? Is 0:05:02.200 --> 0:05:05.760 it possible that by spending enough time marinating in these 0:05:05.800 --> 0:05:08.520 concepts and thinking about them in the right way, that 0:05:08.640 --> 0:05:11.800 we could eventually become familiar with them, sort of the 0:05:11.839 --> 0:05:15.679 way Chinese might seem impenetrable to a Western person at first, 0:05:15.720 --> 0:05:18.200 but you spend enough time there and it becomes part 0:05:18.200 --> 0:05:21.679 of your brain. Your brain develops sort of new ways 0:05:21.720 --> 0:05:24.880 of thinking, new patterns, new ideas flow through it, so 0:05:24.920 --> 0:05:27.640 that the new language and the topics and the strategies 0:05:27.680 --> 0:05:30.800 and the tones of that language eventually become familiar. They 0:05:30.880 --> 0:05:33.760 become how you think. So the strategy there, of course, 0:05:33.880 --> 0:05:37.640 is immerging. You don't learn a new language effectively by 0:05:37.720 --> 0:05:41.080 learning vocabulary and studying in the classroom. The best way 0:05:41.160 --> 0:05:44.160 I've always found really learned to speak a new language, 0:05:44.160 --> 0:05:47.039 to think in a new language. Is to spend time 0:05:47.240 --> 0:05:49.400 doing it, is to go to that country and be 0:05:49.560 --> 0:05:52.400 part of those people and be there and use that 0:05:52.520 --> 0:05:54.800 as part of your life. And that's how your brain works. 0:05:55.240 --> 0:05:58.480 So our strategy today for helping you understand quantum mechanics 0:05:58.760 --> 0:06:01.159 is not to give you the math medical basics, but 0:06:01.279 --> 0:06:03.960 to spend some time in the quantum realm until we 0:06:04.040 --> 0:06:07.560 develop an intuition for how it works. But before we 0:06:07.600 --> 0:06:10.240 go there, I wanted to know what people understood about 0:06:10.279 --> 0:06:14.240 quantum mechanics and also whether they thought they understood quantum mechanics. 0:06:14.640 --> 0:06:17.080 Is quantum mechanics something that everybody out there feels like 0:06:17.240 --> 0:06:20.720 is impossible to understand or are most people under the 0:06:20.760 --> 0:06:23.760 impression that they have it figured out. I walked around 0:06:23.760 --> 0:06:26.160 campus that you see Irvine, and I asked people if 0:06:26.200 --> 0:06:29.360 they thought they could understand quantum mechanics. Before you hear 0:06:29.400 --> 0:06:32.440 these answers, think to yourself, how good is your grasp 0:06:32.720 --> 0:06:35.880 on the basic tenets of quantum mechanics. Do you feel fluent? 0:06:36.080 --> 0:06:38.440 Can you explain this stuff? Do you feel like you 0:06:38.480 --> 0:06:42.760 can navigate that strange quantum realm? Here's what people on 0:06:42.800 --> 0:06:44.960 the street that you see Irvine had to say. Do 0:06:45.000 --> 0:06:49.440 you feel like you understand quantum mechanics? Not anyone who 0:06:49.480 --> 0:06:52.760 answers yes to that doesn't really have an understanding of quantum. 0:06:52.880 --> 0:06:56.960 So no, very basically, I have working knowledge of quantum mechanics, 0:06:57.000 --> 0:06:59.760 but I would not say that I understand it at 0:06:59.760 --> 0:07:04.960 the a PhD physicist, I say I understand it decently. Well. 0:07:05.000 --> 0:07:09.000 Are you familiar with the double slit experiment? Yes, um 0:07:09.320 --> 0:07:13.520 show that particles. I think it was electrons that they use, 0:07:14.000 --> 0:07:16.760 and they move in waves, So if you're to fire 0:07:16.800 --> 0:07:20.880 the electrons and observe them, they'd only go on like 0:07:20.920 --> 0:07:23.520 a line. But then if you cover it you can 0:07:23.560 --> 0:07:26.080 see a wave like pattern in the back or something 0:07:26.120 --> 0:07:29.040 like that. I honestly don't even know about quantum mechanics. 0:07:29.040 --> 0:07:31.160 I'm just the goal nature of light. It's both a 0:07:31.160 --> 0:07:33.360 wave and a particle. So, as usual, we've got a 0:07:33.440 --> 0:07:36.000 very nice breath of answers from people who felt like 0:07:36.080 --> 0:07:38.880 they hardly understood anything about quantum mechanics the people who 0:07:39.000 --> 0:07:42.440 clearly had some working knowledge. So that's great, and we're 0:07:42.480 --> 0:07:45.200 hoping to bring everyone who listens to this podcast up 0:07:45.240 --> 0:07:47.120 to at least the very basic level of being able 0:07:47.200 --> 0:07:50.960 to intelligently think about and understand quantum topics, and I 0:07:51.000 --> 0:07:53.160 think the best way to start off is to hear 0:07:53.200 --> 0:07:56.840 a listener question. Here's a question from a young listener. Hi, 0:07:56.960 --> 0:07:59.480 Daniel Lynn Whole. Hey, my name is Robin, and I 0:07:59.480 --> 0:08:02.239 would love to know what the key differences between quantum 0:08:02.240 --> 0:08:05.040 and classical mechanics are and to what extent they agree 0:08:05.080 --> 0:08:07.080 with each other. This is a great way to start 0:08:07.440 --> 0:08:10.920 because classical physics is what we're familiar with. Classical physics 0:08:11.000 --> 0:08:14.200 is what describes how basketball moves, or how rocks roll 0:08:14.320 --> 0:08:17.400 down hills, or how things move in the atmosphere, things 0:08:17.440 --> 0:08:19.880 that we are familiar with that we have spent hundreds 0:08:19.960 --> 0:08:23.320 or thousands of years developing an intuition for things that 0:08:23.360 --> 0:08:26.760 probably our brains have evolved to be good at understanding, 0:08:26.840 --> 0:08:29.480 so that in some way they are natural objects to 0:08:29.560 --> 0:08:33.240 our mental worlds. Right, So the question really is what's 0:08:33.240 --> 0:08:37.040 the biggest difference between the classical world and the quantum world. 0:08:37.440 --> 0:08:39.200 What is it when you go to the quantum world 0:08:39.200 --> 0:08:42.559 that you can no longer assume that is obviously true 0:08:42.600 --> 0:08:46.800 in the classical world. The basic difference between classical objects 0:08:46.840 --> 0:08:50.920 and quantum mechanical objects is that quantum mechanical objects do 0:08:50.960 --> 0:08:54.640 not have a path. They don't have a trajectory through space, 0:08:54.679 --> 0:08:57.920 a well defined stead of where they were at any 0:08:58.040 --> 0:09:00.559 given time. That's the key, l Us and I want 0:09:00.600 --> 0:09:03.000 you to come away from today's podcast with. So let's 0:09:03.000 --> 0:09:06.160 talk about what that means. If you have a baseball 0:09:06.360 --> 0:09:09.840 and it's at one location now and ten seconds later 0:09:09.920 --> 0:09:13.520 it's across the baseball field, you imagine that it went 0:09:13.760 --> 0:09:17.199 from where it was to where it is now. You say, well, 0:09:17.240 --> 0:09:20.040 if it was over there before and it's over here 0:09:20.080 --> 0:09:22.160 and now, how did it get there. It must have 0:09:22.520 --> 0:09:26.079 gone between those two locations. You have two pieces of data, 0:09:26.320 --> 0:09:30.080 and you naturally interpolate because you make this assumption that 0:09:30.160 --> 0:09:33.960 a classical object has a location at every time, and 0:09:33.960 --> 0:09:37.480 that you can stitch those together into a path that 0:09:37.640 --> 0:09:41.800 traverses space time in some continuous way. And you might 0:09:41.840 --> 0:09:45.280 be thinking, well, of course everything does. Everything has a 0:09:45.280 --> 0:09:48.760 location at a given time, and only one location. But 0:09:48.840 --> 0:09:52.240 that's an intuition you've developed from experiencing the natural world, 0:09:52.280 --> 0:09:55.440 from playing baseball, or from being chased by people throwing 0:09:55.520 --> 0:09:58.040 rocks at you. You're used to things moving through the 0:09:58.080 --> 0:10:01.480 world in a responsible and understandable way. So you've made 0:10:01.480 --> 0:10:05.960 this assumption. Quantum objects do not have paths like that. 0:10:06.360 --> 0:10:10.199 They don't have locations that translate through time. For a 0:10:10.280 --> 0:10:13.439 quantum object like an electron, you can measure it at 0:10:13.480 --> 0:10:16.720 one location A and then later measure it at another 0:10:16.800 --> 0:10:20.480 location B, but it doesn't mean that it's flown through 0:10:20.520 --> 0:10:24.520 the universe from A to B in that intervening time. 0:10:24.920 --> 0:10:27.760 It can be at A later be at B, but 0:10:27.840 --> 0:10:30.880 it's not true that it's moved from A to B, 0:10:31.400 --> 0:10:34.320 meaning that if you've made a measurement halfway, you would 0:10:34.320 --> 0:10:37.760 have found it going from A to B. Now that's 0:10:37.920 --> 0:10:40.600 very confusing, that's very hard to understand. How can an 0:10:40.600 --> 0:10:43.720 object be here and later be there and never be 0:10:44.000 --> 0:10:47.000 in between? And the reason is that quantum objects are 0:10:47.040 --> 0:10:50.320 just different from the classical objects. They are not the 0:10:50.360 --> 0:10:52.240 same kind of thing. And this assumption that you have 0:10:52.320 --> 0:10:56.319 made about the universe comes from only experiencing classical objects 0:10:56.400 --> 0:11:00.000 things on our scale, but the smaller level the universe 0:11:00.240 --> 0:11:02.960 just does not work that way, and that's pretty hard 0:11:03.000 --> 0:11:05.959 to digest. But don't worry. There are things at the 0:11:06.040 --> 0:11:09.040 quantum realm which do follow your intuition in which you 0:11:09.160 --> 0:11:12.520 can use to help understand and develop an intuitive sense 0:11:12.559 --> 0:11:16.000 of the of the universe, and that's the quantum wave function. 0:11:16.600 --> 0:11:19.240 So how do the quantum object work. Well, a quantum 0:11:19.320 --> 0:11:22.600 object is here and later it's there. What determines that. 0:11:22.720 --> 0:11:25.960 It's not like quantum objects don't follow the laws of physics. 0:11:26.200 --> 0:11:28.640 And you probably know the quantum objects could also be random, 0:11:28.920 --> 0:11:30.760 that you can do the same thing to a quantum 0:11:30.800 --> 0:11:34.880 object twice and get different outcomes. And that's true. All 0:11:35.000 --> 0:11:37.840 those things are true. But the quantum object is ruled 0:11:37.880 --> 0:11:41.440 by something, and that's this quantum wave function. You don't 0:11:41.440 --> 0:11:43.560 have to know a lot of complicated math. All you 0:11:43.600 --> 0:11:45.480 need to know is that the wave function tells you 0:11:45.800 --> 0:11:48.800 where a quantum object is most likely to be. You 0:11:48.920 --> 0:11:52.160 probably have heard that electrons don't have a specific location 0:11:52.200 --> 0:11:55.559 to have a probability cloud around an atom. That's an 0:11:55.559 --> 0:11:58.040 extrapolation of the wave function. The wave function tells you 0:11:58.400 --> 0:12:01.120 where the electron is most like it to be. If 0:12:01.120 --> 0:12:03.559 it's large over here, has it's more likely to be there. 0:12:03.600 --> 0:12:06.320 If it's small somewhere else, it's less likely to be there. 0:12:06.360 --> 0:12:09.880 If it's zero somewhere, it means the particle cannot be there. 0:12:10.600 --> 0:12:14.440 Where is the particle? Actually, we don't know, and there's 0:12:14.440 --> 0:12:16.920 a lot of discussion about whether the wave function means 0:12:17.120 --> 0:12:20.800 the particle isn't actually anywhere or just we don't know 0:12:20.920 --> 0:12:24.360 where it is, but we do know that quantum mechanics 0:12:24.400 --> 0:12:28.160 says that that information does not need to exist. It's 0:12:28.240 --> 0:12:30.679 not necessarily the case that there's hidden knowledge that the 0:12:30.720 --> 0:12:34.080 electron is actually somewhere and we don't know. So the 0:12:34.080 --> 0:12:36.199 best way to think about the wave function is that 0:12:36.240 --> 0:12:39.800 it's physics saying what's allowed and what's more likely, and 0:12:39.840 --> 0:12:42.360 this wave function is what we can grab onto, what 0:12:42.480 --> 0:12:46.480 we can understand because the wave function does follow rules, 0:12:46.760 --> 0:12:50.000 and the wave function has a path that makes sense. 0:12:50.240 --> 0:12:53.079 It flows from one spot to the other. It doesn't 0:12:53.120 --> 0:12:55.880 just disappear and appear somewhere else. So you have to 0:12:56.000 --> 0:12:59.720 let go of your intuitive desire to understand the path 0:12:59.840 --> 0:13:02.800 of of an electron the path of a quantum object, 0:13:02.800 --> 0:13:05.600 because those things don't exist. But instead, when you let 0:13:05.600 --> 0:13:07.840 that go, you can grasp onto the next layer. The 0:13:07.880 --> 0:13:10.840 next layer is this wave function, the thing that determines 0:13:11.160 --> 0:13:13.760 where the electron is, the thing that determines where the 0:13:13.800 --> 0:13:17.200 electron is likely to be somewhere else. And that's the 0:13:17.280 --> 0:13:19.400 key thing, is that the wave function tells you where 0:13:19.440 --> 0:13:23.160 the electron is, and it responds to stimulation, it responds 0:13:23.200 --> 0:13:25.959 to the world, and it flows just like a wave, 0:13:26.679 --> 0:13:28.600 and you can use that you can understand how that 0:13:28.600 --> 0:13:31.760 wave flows through time to understand where the electron is 0:13:31.840 --> 0:13:34.280 likely to be in the future. Even if the electron 0:13:34.320 --> 0:13:37.240 itself doesn't have a path from A to B, it's 0:13:37.360 --> 0:13:40.640 wave function does. So the key to being comfortable with 0:13:40.760 --> 0:13:44.040 quantum mechanics is to get comfortable with the objects wave 0:13:44.080 --> 0:13:47.880 function rather than the object. All right, So the best 0:13:47.880 --> 0:13:50.520 way to learn a new language is to get practice, 0:13:50.600 --> 0:13:52.800 is to immerse yourself in it. So the next thing 0:13:52.800 --> 0:13:55.280 we're gonna do is a bunch of exercises of diving 0:13:55.320 --> 0:13:58.280 into the quantum realm and being comfortable with the wave 0:13:58.280 --> 0:14:01.079 function understanding how it flows. And to do that, we're 0:14:01.080 --> 0:14:03.960 going to explore in some detail the famous double slit 0:14:04.040 --> 0:14:07.320 experiment that shows us the crazy behavior of quantum mechanics. 0:14:07.360 --> 0:14:08.760 And at the end of it, I hope it all 0:14:08.800 --> 0:14:10.880 makes sense and the weird results of the double slit 0:14:10.920 --> 0:14:14.840 experiment feel totally natural. But first, let's take a quick break. 0:14:27.880 --> 0:14:30.400 All right, So we are talking about quantum mechanics and 0:14:30.400 --> 0:14:33.000 we're trying to get an intuitive grasp of quantum mechanics. 0:14:33.000 --> 0:14:35.400 We want not just that you say things about quantum 0:14:35.400 --> 0:14:37.880 mechanics that sound intelligent. We want to actually get an 0:14:37.960 --> 0:14:41.200 understanding how do things work at the lowest level. What 0:14:41.240 --> 0:14:43.800 intuition do we have that we can apply? And the 0:14:43.920 --> 0:14:46.920 key thing, again, remember, is the wave function. You have 0:14:47.040 --> 0:14:50.560 quantum mechanical objects like electrons and protons and photons. They 0:14:50.560 --> 0:14:55.360 are difficult to describe using our classical ideas of balls 0:14:55.400 --> 0:14:58.720 and objects that move through space because quantum objects don't 0:14:58.760 --> 0:15:02.160 have paths. And said, we're gonna focus on their wave function, 0:15:02.200 --> 0:15:04.440 the thing that says where they are likely to be, 0:15:04.640 --> 0:15:08.080 and we're gonna understand how that wave function changes through time. 0:15:08.280 --> 0:15:10.880 And we're gonna grab onto that instead of having a 0:15:10.920 --> 0:15:13.920 classical path with the object, we're gonna understand how the 0:15:13.960 --> 0:15:16.880 wave function changes through time, because that's the thing that's 0:15:16.960 --> 0:15:20.200 most like a classical path. All right, So we're gonna 0:15:20.200 --> 0:15:22.440 start very simple, and we're gonna work up to more 0:15:22.480 --> 0:15:26.520 complicated situations. Imagine that you are a photon. You are 0:15:26.560 --> 0:15:29.160 a tiny little quantum particle, and you're shot out of 0:15:29.160 --> 0:15:31.520 a laser with a bunch of your friends towards the screen. 0:15:32.080 --> 0:15:36.240 What happens while you leave the laser right later you 0:15:36.360 --> 0:15:40.400 hit the screen. Does that mean that you flew through 0:15:40.520 --> 0:15:45.080 the room from the laser to the screen. Not necessarily, right, 0:15:45.200 --> 0:15:48.760 You do not have a classical path. Just because you 0:15:48.880 --> 0:15:52.120 left the laser and later hit the screen doesn't mean 0:15:52.240 --> 0:15:55.240 you have a trajectory. Doesn't mean that you necessarily can 0:15:55.280 --> 0:16:00.640 be found in between things in the quantum realm. Don't go, however, 0:16:01.000 --> 0:16:05.360 your wave function does. When you are created inside the laser, 0:16:05.440 --> 0:16:07.880 just at the aperture of the laser, your wave function 0:16:07.960 --> 0:16:10.520 says where you're likely to be, And then your wave 0:16:10.600 --> 0:16:15.080 function flows through the room. If it isn't interacting with everything, 0:16:15.360 --> 0:16:18.280 it just flows nicely through the room, showing the most 0:16:18.320 --> 0:16:21.160 likely place to find your location. This is not the 0:16:21.200 --> 0:16:23.080 same as having a classical path. This is sort of 0:16:23.080 --> 0:16:26.560 the path of your wave function. Then it hits the screen, 0:16:26.880 --> 0:16:29.640 and the screen essentially measures it. It says, okay, wave function, 0:16:30.000 --> 0:16:32.720 where is this photon? And that's the moment when the 0:16:32.800 --> 0:16:36.359 universe has to decide. Instead of having a probability distribution 0:16:36.360 --> 0:16:40.080 about where you are, it says where is this actual photon? 0:16:40.600 --> 0:16:42.880 So you have two measurements. When you leave the laser 0:16:43.240 --> 0:16:45.840 and when you hit the screen. What happens in between 0:16:46.240 --> 0:16:49.320 we don't know, but the wave function tells us, given 0:16:49.320 --> 0:16:52.240 that you've left the laser, what's the likely place to 0:16:52.320 --> 0:16:54.000 land on the screen. And then when you land on 0:16:54.000 --> 0:16:56.480 the screen, the universe rolls a die and says where 0:16:56.480 --> 0:17:00.440 you actually are. Okay, that's easy. Now let's do the 0:17:00.440 --> 0:17:03.000 experiment again. A bunch of photons flying out from a 0:17:03.080 --> 0:17:06.120 laser towards the wall. Let's bring in some black walls 0:17:06.200 --> 0:17:08.560 so that instead of having a totally clear path of 0:17:08.640 --> 0:17:11.000 the screen, you have sort of a narrow gap, not 0:17:11.119 --> 0:17:13.600 too small, you know, a few inches. So what's going 0:17:13.640 --> 0:17:17.240 to happen, Well, some of the photons that leave the 0:17:17.320 --> 0:17:20.480 laser are going to go right through that gap and 0:17:20.560 --> 0:17:22.639 hit the screen, just like before. Some of them are 0:17:22.640 --> 0:17:24.760 gonna hit the walls that block a fraction of the 0:17:24.840 --> 0:17:28.080 laser and so on the screen behind. What you'll get 0:17:28.119 --> 0:17:31.080 is sort of a geometric shadow places where the photon 0:17:31.160 --> 0:17:33.000 made it through the gap. You're gonna hit the screen 0:17:33.040 --> 0:17:35.359 at the back, places where they hit the walls that 0:17:35.400 --> 0:17:37.720 we introduced to make this a little gap. They're going 0:17:37.760 --> 0:17:39.320 to hit those walls and they're not going to make 0:17:39.359 --> 0:17:42.520 it to the screen. All right, So instead of having 0:17:42.600 --> 0:17:45.200 just the laser splash on the back screen, now we 0:17:45.240 --> 0:17:48.520 have something called a geometric shadow. Geometric because it has 0:17:48.600 --> 0:17:51.600 crisp edges and it follows the shape of the of 0:17:51.680 --> 0:17:55.760 the thing that the laser went through. No big deal. Now, 0:17:56.480 --> 0:18:00.520 let's narrow the gap. Let's squeeze those two walls that 0:18:00.560 --> 0:18:03.199 can find the laser. Two very very small, something like 0:18:03.280 --> 0:18:06.920 hundreds of nanometers approximately the wavelength of light that's going 0:18:06.920 --> 0:18:09.800 through it. Now what we see on the back wall changes. 0:18:10.040 --> 0:18:12.399 Instead of seeing a geometric shadow, what we see is 0:18:12.440 --> 0:18:15.000 sort of a spray of light. The light is spreading 0:18:15.000 --> 0:18:18.199 out a little bit. It doesn't just become as narrow 0:18:18.320 --> 0:18:20.720 as the gap it's shining through. It spreads out a 0:18:20.720 --> 0:18:24.280 little bit. This is a wave effect. It happens anytime 0:18:24.359 --> 0:18:27.000 a wave passes through a slit that's narrow compared to 0:18:27.040 --> 0:18:29.800 its wavelength. We could do a deep dive under fraction, 0:18:29.840 --> 0:18:31.840 and maybe we will in a future podcast, but it's 0:18:31.880 --> 0:18:34.440 not critical to understand here. The only thing you need 0:18:34.480 --> 0:18:37.800 to understand is at anytime a wave meaning sound or 0:18:37.880 --> 0:18:42.040 light or bathtub splashes or wave functions pass through a 0:18:42.119 --> 0:18:44.920 narrow slit, they spread out a little bit. You can 0:18:44.960 --> 0:18:47.639 do the same kind of experiment in a bathtub. If 0:18:47.680 --> 0:18:50.040 you send water waves through a very narrow gap, you 0:18:50.119 --> 0:18:51.800 notice that when they come out of that gap they 0:18:51.840 --> 0:18:54.800 spread out. Then we'll just shoot out in a narrow column, 0:18:55.119 --> 0:18:57.359 all right. So now we have a very narrow gap, 0:18:57.800 --> 0:18:59.439 and on the backscreen we have a sort of a 0:18:59.520 --> 0:19:02.240 spray of results. We have a spray, not just a 0:19:02.320 --> 0:19:05.360 very narrow geometric shadow. We have a spray of results 0:19:05.359 --> 0:19:08.119 with a light can land, all right, and now we're 0:19:08.160 --> 0:19:10.879 going to make a second narrow gap. So we have 0:19:10.960 --> 0:19:13.960 sources of light in the back maybe lasers, maybe flashlights, 0:19:13.960 --> 0:19:16.520 it doesn't matter. And they shoot light out, and then 0:19:16.520 --> 0:19:19.080 we have two narrow gaps for the light to fly through. 0:19:19.680 --> 0:19:22.000 And so we have light coming through both of those 0:19:22.040 --> 0:19:25.240 gaps towards the wall. What do you see on the 0:19:25.280 --> 0:19:28.200 back screen? You might think, oh, I have two copies 0:19:28.359 --> 0:19:30.200 of what I saw when I had one narrow gap, 0:19:30.240 --> 0:19:32.680 because now I have two narrow gaps, So how hard 0:19:32.720 --> 0:19:35.840 can this be? That's not quite true. What you see 0:19:36.119 --> 0:19:39.800 is an interference pattern. An interference pattern is what happens 0:19:39.840 --> 0:19:44.120 when two waves collide. Because remember that waves are oscillation. 0:19:44.560 --> 0:19:47.880 For example, if you have waves in your bathtub, those 0:19:47.920 --> 0:19:50.239 waves are just the motion of the water moving up 0:19:50.240 --> 0:19:52.560 and down. They're not their own thing. There a motion 0:19:52.640 --> 0:19:55.560 of the water. Same way sound waves, These sounds that 0:19:55.600 --> 0:19:57.840 I'm making into the microphone that you're hearing in your 0:19:57.840 --> 0:20:01.640 earbuds or wherever you're listening, those are oscillations of the air. 0:20:02.000 --> 0:20:04.760 The air itself is shaking, and because it's the shaking 0:20:04.760 --> 0:20:07.320 of a medium, if something else comes along and shakes 0:20:07.359 --> 0:20:10.639 it in another way, those that shaking can interfere, and 0:20:10.640 --> 0:20:12.840 you can have two kinds of interference. You can have 0:20:12.920 --> 0:20:16.040 constructive interference two things are shaking it the same way, 0:20:16.040 --> 0:20:18.239 so you cant sort of double shaking. Or you can 0:20:18.280 --> 0:20:21.439 have destructive interference where two things are shaking it in 0:20:21.440 --> 0:20:25.240 the opposite way and they cancel each other out. For example, 0:20:25.480 --> 0:20:28.920 this is how noise canceling headphones work. They very rapidly 0:20:29.000 --> 0:20:32.000 hear the noises that are around you and admit exactly 0:20:32.040 --> 0:20:35.840 the right sound to cancel out the sound. You can 0:20:35.880 --> 0:20:39.320 create sound which will negate the other sound by shaking 0:20:39.320 --> 0:20:42.760 it the opposite direction. So when the two shakings add up, 0:20:42.960 --> 0:20:45.800 they add up to zero. So this is an interference 0:20:45.840 --> 0:20:48.680 pattern places where things add up to be stronger in 0:20:48.720 --> 0:20:51.560 place where things can cancel each other out. If you 0:20:51.560 --> 0:20:53.440 look on that screen, what you'll see is a very 0:20:53.440 --> 0:20:56.399 bright band in the middle where the two sources of 0:20:56.480 --> 0:20:58.480 light from the two narrow gaps are adding up on 0:20:58.560 --> 0:21:01.240 top of each other. And to the left of it 0:21:01.320 --> 0:21:03.920 is a dark band, a band where the things are 0:21:03.960 --> 0:21:06.560 canceling each other out. And as you move along the 0:21:06.600 --> 0:21:08.880 screen you get bands of light and bands of dark, 0:21:09.040 --> 0:21:11.640 bands of light and bands of dark, and the exact 0:21:11.720 --> 0:21:13.959 width of those things depends on the wavelength of the 0:21:13.960 --> 0:21:18.320 thing that's interfering with itself. Now you're probably thinking, oh, well, 0:21:18.480 --> 0:21:20.960 the light is a wave, right, and if light is 0:21:20.960 --> 0:21:23.920 a wave, it makes perfect sense for light to interfere 0:21:23.920 --> 0:21:26.320 with itself the same way sound does, in the same 0:21:26.320 --> 0:21:29.919 way waves in your bathtub night. That intuition makes sense, 0:21:30.119 --> 0:21:33.600