WEBVTT 00:00:00.000 --> 00:00:02.919 All right, so you know how like, if we always 00:00:02.919 --> 00:00:05.940 talk about these big physics ideas, but the world 00:00:05.940 --> 00:00:09.460 just feels so normal, right? Like you were listening 00:00:09.460 --> 00:00:11.259 to this and it's like a thing you were doing. 00:00:11.339 --> 00:00:14.519 It is. But what if this was like just one possibility? 00:00:14.599 --> 00:00:16.899 Okay. And there are a bunch of other realities 00:00:16.899 --> 00:00:19.420 where like all sorts of other crazy stuff is 00:00:19.420 --> 00:00:21.699 happening right now. Right. Kind of wild. Yeah, 00:00:21.719 --> 00:00:23.359 I mean, it sounds like science fiction. Totally. 00:00:23.739 --> 00:00:26.730 But this all comes from like... the most basic 00:00:26.730 --> 00:00:29.589 stuff about how the universe works at the smallest 00:00:29.589 --> 00:00:32.109 level. Oh yeah, quantum mechanics. Right, quantum 00:00:32.109 --> 00:00:35.969 mechanics. And it is so different than the physics 00:00:35.969 --> 00:00:38.289 we see in the world around us. Exactly. Like 00:00:38.289 --> 00:00:40.990 if I throw this pen, I can like kind of tell 00:00:40.990 --> 00:00:43.670 where it is gonna land. But if you look at like 00:00:43.670 --> 00:00:47.810 an electron, all bets are off. Totally unpredictable. 00:00:47.990 --> 00:00:50.890 Yeah, and so the big question is like when and 00:00:50.890 --> 00:00:55.719 how does that quantum weirdness become the normal 00:00:55.719 --> 00:00:57.880 world that we see. Yeah, that's the big mystery. 00:00:58.100 --> 00:01:02.219 And one of the craziest answers to that is the 00:01:02.219 --> 00:01:06.079 many -worlds interpretation. And it starts with 00:01:06.079 --> 00:01:09.519 this thing called superposition, which is already 00:01:09.519 --> 00:01:12.840 a pretty wild idea. Yeah, and many -worlds really 00:01:12.840 --> 00:01:15.420 just takes it to the extreme. Okay, so superposition, 00:01:15.540 --> 00:01:18.920 for folks who haven't gone deep into the quantum 00:01:18.920 --> 00:01:23.349 world, imagine a tiny particle. Okay. And it's 00:01:23.349 --> 00:01:27.450 not in one specific place. It's like it's kind 00:01:27.450 --> 00:01:29.370 of everywhere at once. Yeah, all at the same 00:01:29.370 --> 00:01:32.109 time. Or at least it's in all the places it could 00:01:32.109 --> 00:01:34.310 be until we look at it. Until you try to measure 00:01:34.310 --> 00:01:36.890 it exactly. One of our sources describes it as 00:01:36.890 --> 00:01:40.150 like probability clouds of where the particle 00:01:40.150 --> 00:01:42.359 might be. That's a great way to think about it. 00:01:42.599 --> 00:01:45.040 Yeah, it's wild. It is. So it's like all these 00:01:45.040 --> 00:01:47.159 possibilities are just kind of hanging out. Just 00:01:47.159 --> 00:01:49.599 swirling around. And the best way to see this 00:01:49.599 --> 00:01:51.879 is with the double slit experiment. Right. Actually, 00:01:51.980 --> 00:01:54.219 we did a whole deep dive on that a while back. 00:01:54.219 --> 00:01:56.019 Yeah, that was a good one. If you want all the 00:01:56.019 --> 00:01:57.459 details on that, you should definitely check 00:01:57.459 --> 00:01:59.719 it out. Yeah. But the main point is you have 00:01:59.719 --> 00:02:02.459 these particles and you... shoot them at a wall 00:02:02.459 --> 00:02:04.579 with two slits. Too little open. And they don't 00:02:04.579 --> 00:02:06.719 just go through one or the other. Right, they 00:02:06.719 --> 00:02:09.699 act like waves. Yeah. And create this interference 00:02:09.699 --> 00:02:11.639 pattern. It's like they went through both slits 00:02:11.639 --> 00:02:13.379 at the same time. Yeah, it's like they took every 00:02:13.379 --> 00:02:16.020 path at once. Exactly. And quantum mechanics 00:02:16.020 --> 00:02:18.900 describes that by saying that each particle is 00:02:18.900 --> 00:02:21.719 a superposition of all those possible paths. 00:02:22.139 --> 00:02:24.259 Yeah, like all those paths are real possibilities. 00:02:24.340 --> 00:02:26.099 But here's the thing. If you try to figure out 00:02:26.099 --> 00:02:28.620 which slit the particle actually went through. 00:02:28.800 --> 00:02:31.439 Right. that interference pattern goes away? It 00:02:31.439 --> 00:02:33.639 just disappears. And it looks like the particle 00:02:33.639 --> 00:02:36.500 just picked one path. Yeah. So what's up with 00:02:36.500 --> 00:02:38.900 that? Well, this is where the Copenhagen interpretation 00:02:38.900 --> 00:02:41.199 comes in. Ah, the Copenhagen interpretation. 00:02:41.240 --> 00:02:44.479 It says that when you measure something, you 00:02:44.479 --> 00:02:46.979 force the wave function to collapse. In your 00:02:46.979 --> 00:02:50.319 cuff shoes. So that probability cloud becomes 00:02:50.319 --> 00:02:54.009 just one definite. outcome one reality and that 00:02:54.009 --> 00:02:56.330 was like the bridge between the quantum world 00:02:56.330 --> 00:02:59.370 and the normal world exactly from all those possibilities 00:02:59.370 --> 00:03:02.550 to the one we actually experience but a lot of 00:03:02.550 --> 00:03:04.789 physicists even like the people who came up with 00:03:04.789 --> 00:03:07.189 quantum mechanics oh yeah weren't really happy 00:03:07.189 --> 00:03:09.789 with this idea of like collapsing wave functions 00:03:09.789 --> 00:03:11.889 just because someone is watching yeah it does 00:03:11.889 --> 00:03:14.629 seem a little strange doesn't it yeah and Schrodinger 00:03:14.860 --> 00:03:16.860 He came up with this thought experiment with 00:03:16.860 --> 00:03:20.060 a cat to show why? Oh yes, the famous Schrodinger's 00:03:20.060 --> 00:03:23.840 cat. So imagine you have a cat in a box. In a 00:03:23.840 --> 00:03:26.439 sealed box. And in the box with the cat, there's 00:03:26.439 --> 00:03:30.400 a vial of poison, and it's connected to a device 00:03:30.400 --> 00:03:33.139 that's triggered by a radioactive atom. Okay, 00:03:33.400 --> 00:03:36.520 so if the atom decays, the poison is released. 00:03:36.659 --> 00:03:38.719 Yeah, and here's where it gets really weird. 00:03:39.360 --> 00:03:42.219 Quantum mechanics says that until we look at 00:03:42.219 --> 00:03:46.020 it, That atom is in a superposition of states. 00:03:46.240 --> 00:03:48.800 Both decayed and not decayed. At the same time. 00:03:48.800 --> 00:03:52.219 Yeah. So if that tiny quantum event is in this 00:03:52.219 --> 00:03:54.800 blurry state, doesn't that mean the cat is too? 00:03:55.000 --> 00:03:57.240 The cat whose life literally depends on the atom. 00:03:57.560 --> 00:04:00.199 Is the cat both alive and dead until we open 00:04:00.199 --> 00:04:02.800 the box? It seems absurd when you think about 00:04:02.800 --> 00:04:05.680 it on a larger scale. Yeah, it is like totally 00:04:05.680 --> 00:04:08.060 crazy. And that was Schrodinger's point. Yeah, 00:04:08.120 --> 00:04:10.439 he was saying, like, why does it have to be a 00:04:10.439 --> 00:04:13.319 human observer? Right, what's so special about 00:04:13.319 --> 00:04:15.539 us? And what about all the other stuff happening 00:04:15.539 --> 00:04:18.410 in the universe that no one is watching? Exactly, 00:04:18.629 --> 00:04:20.569 it raises a lot of questions. But the stuff we've 00:04:20.569 --> 00:04:23.430 been looking at says that there are some newer 00:04:23.430 --> 00:04:26.470 interpretations of the Copenhagen interpretation. 00:04:26.829 --> 00:04:29.310 Yeah. Like this thing called decoherence. Right, 00:04:29.449 --> 00:04:31.569 decoherence. So it's like saying that quantum 00:04:31.569 --> 00:04:34.069 superposition doesn't really last that long? 00:04:34.149 --> 00:04:36.170 Not at the macroscopic level. Especially when 00:04:36.170 --> 00:04:38.470 you're talking about like a cat which is interacting 00:04:38.470 --> 00:04:40.589 with its environment. Right, all the atoms in 00:04:40.589 --> 00:04:42.389 the cat in the box are constantly interacting 00:04:42.389 --> 00:04:43.970 with each other. Yeah, so it's like all that 00:04:43.970 --> 00:04:46.389 quantum fuzziness gets kind of smoothed out by 00:04:46.389 --> 00:04:49.230 the everyday world. And those different possibilities 00:04:49.230 --> 00:04:52.129 become more like definite outcomes. So it's like 00:04:52.129 --> 00:04:54.170 the universe kind of settles on an answer like 00:04:54.170 --> 00:04:58.689 the cat being alive or dead based on which possibilities 00:04:58.689 --> 00:05:02.110 were more likely. But even with decoherence, 00:05:02.769 --> 00:05:04.790 the Copenhagen interpretation is still saying 00:05:04.790 --> 00:05:09.350 that the choice of which outcome happens is random, 00:05:09.629 --> 00:05:12.009 right? Yeah, fundamentally random. Like the universe 00:05:12.009 --> 00:05:14.889 is rolling dice. Yeah. OK, so let's talk about 00:05:14.889 --> 00:05:18.170 something even wilder. OK. The many -worlds interpretation. 00:05:18.589 --> 00:05:21.889 Right. What if the wave function never collapses? 00:05:22.230 --> 00:05:24.610 Never ever. What if all those possibilities are 00:05:24.610 --> 00:05:27.569 always real? Love them. If we can imagine a cat 00:05:27.569 --> 00:05:30.470 being both alive and dead? Yeah. Why not just 00:05:30.470 --> 00:05:32.670 go all the way with it? All the way. What if 00:05:32.670 --> 00:05:35.550 all those possibilities extend to us? To the 00:05:35.550 --> 00:05:37.529 observers. And to the whole universe? The entire 00:05:37.529 --> 00:05:39.949 universe, yeah. So if we open the box and the 00:05:39.949 --> 00:05:43.670 cat is alive... Many worlds says it's not because 00:05:43.670 --> 00:05:45.790 the wave function collapsed, it's because we 00:05:45.790 --> 00:05:49.029 are now in a timeline. A branch of reality. Where 00:05:49.029 --> 00:05:52.350 the atom didn't decay, but there's another reality 00:05:52.350 --> 00:05:55.009 out there where it did decay and another us is 00:05:55.009 --> 00:05:56.769 dealing with that. With a very different outcome. 00:05:56.930 --> 00:05:59.829 It sounds crazy. I know. But it's based on the 00:05:59.829 --> 00:06:02.370 math of quantum mechanics. And it was actually 00:06:02.370 --> 00:06:05.509 proposed back in 1957 by this guy named Hugh 00:06:05.509 --> 00:06:08.649 Everett. Right, in his PhD thesis. And it's this 00:06:08.649 --> 00:06:11.350 idea of branching realities that we call the 00:06:11.350 --> 00:06:14.149 many worlds interpretation. Exactly. So going 00:06:14.149 --> 00:06:17.670 back to the double slit experiment, instead of 00:06:17.670 --> 00:06:20.790 those multiple paths merging into one, when we 00:06:20.790 --> 00:06:24.110 look at the particle, many worlds says all those 00:06:24.110 --> 00:06:26.490 paths keep going in separate realities. They 00:06:26.490 --> 00:06:28.490 all exist. And we just happen to be in one of 00:06:28.490 --> 00:06:30.730 them. Right. And those bright bands in the interference 00:06:30.730 --> 00:06:33.529 pattern, those are just the realities where more 00:06:33.529 --> 00:06:35.870 of the particles ended up. More paths leading 00:06:35.870 --> 00:06:38.889 to the same outcome. So more populated realities. 00:06:39.069 --> 00:06:41.040 More versions of Are you seeing that outcome? 00:06:41.180 --> 00:06:43.579 It's a lot to think about. It is mind boggling. 00:06:43.720 --> 00:06:46.019 I mean, it seems kind of extreme to say that 00:06:46.019 --> 00:06:48.220 the universe splits every time something quantum 00:06:48.220 --> 00:06:50.060 happens. Yeah, one of our sources said it was 00:06:50.060 --> 00:06:51.920 like building a whole new house just to avoid 00:06:51.920 --> 00:06:54.300 doing the dishes. Yeah, compared to just having 00:06:54.300 --> 00:06:57.160 the wave function collapse. Yeah, but even the 00:06:57.160 --> 00:06:59.459 Copenhagen interpretation has all those possibilities 00:06:59.459 --> 00:07:02.040 before the collapse. That's true. So they both 00:07:02.040 --> 00:07:04.519 have to deal with those multiple realities in 00:07:04.519 --> 00:07:07.230 some way. But the Copenhagen interpretation merges 00:07:07.230 --> 00:07:09.930 them all into one. Right, through the collapse. 00:07:10.529 --> 00:07:12.670 And many worlds just says they all continue. 00:07:12.829 --> 00:07:15.389 They keep branching off. It's like these infinite 00:07:15.389 --> 00:07:17.670 parallel timelines all happening at the same 00:07:17.670 --> 00:07:19.889 time. Yeah, it's a lot. And one of the things 00:07:19.889 --> 00:07:22.569 our sources pointed out was that many worlds 00:07:22.569 --> 00:07:26.470 might actually be a simpler explanation mathematically. 00:07:26.629 --> 00:07:29.970 Oh, also. Because the equations of quantum mechanics 00:07:29.970 --> 00:07:33.329 don't have anything about wave function collapse. 00:07:33.509 --> 00:07:35.430 Right, it's just something we added. So many 00:07:35.430 --> 00:07:37.430 worlds is kind of just taking the math at face 00:07:37.430 --> 00:07:39.470 value. And letting the universe do its thing. 00:07:39.589 --> 00:07:42.310 But Everett's idea wasn't exactly popular when 00:07:42.310 --> 00:07:44.110 he first proposed it. Yeah, why do you think 00:07:44.110 --> 00:07:46.089 that was? Well, he was just a grad student at 00:07:46.089 --> 00:07:47.930 the time. Yeah, that doesn't help. And then he 00:07:47.930 --> 00:07:51.069 went into military research. So he wasn't really 00:07:51.069 --> 00:07:53.050 in the physics world anymore. Plus the whole 00:07:53.050 --> 00:07:55.870 idea of infinite copies of yourself living out 00:07:55.870 --> 00:07:58.329 every possible choice you could have made. That's 00:07:58.329 --> 00:08:00.689 a lot to process. Yeah, it can kind of freak 00:08:00.689 --> 00:08:03.610 people out. An existential crisis for sure. And 00:08:03.610 --> 00:08:07.170 remember, we still haven't actually proven many 00:08:07.170 --> 00:08:09.189 worlds. Right. It's still just an interpretation. 00:08:09.250 --> 00:08:11.629 It's a very popular one because it fits the math 00:08:11.629 --> 00:08:14.529 so well. And it doesn't need any extra assumptions. 00:08:14.649 --> 00:08:17.209 We can't just like hop between universes to check 00:08:17.209 --> 00:08:19.910 though. Not yet, at least. And many worlds isn't 00:08:19.910 --> 00:08:21.790 completely figured out either. Yeah. There's 00:08:21.790 --> 00:08:23.629 still a lot of research being done on how these 00:08:23.629 --> 00:08:26.009 branching realities work. Like how they interact 00:08:26.009 --> 00:08:28.670 or don't interact. And how the probabilities 00:08:28.670 --> 00:08:31.269 that we see actually come from this constantly 00:08:31.269 --> 00:08:34.889 branching universe. Right. But there's one really 00:08:34.889 --> 00:08:37.090 important difference between many worlds and 00:08:37.090 --> 00:08:41.929 Copenhagen. OK, what's that? Many worlds is deterministic. 00:08:42.169 --> 00:08:45.690 Deterministic. So in any one branch of reality, 00:08:46.289 --> 00:08:48.970 everything happens according to the laws of physics. 00:08:49.230 --> 00:08:51.009 Cause and effect. It's like a movie reel that 00:08:51.009 --> 00:08:53.809 just keeps playing out. but with infinite branches. 00:08:54.269 --> 00:08:56.690 So all that randomness we see in quantum mechanics. 00:08:56.870 --> 00:08:59.450 Yeah, like when atom decays. It's not actually 00:08:59.450 --> 00:09:02.090 random in many worlds. No, it's just that all 00:09:02.090 --> 00:09:03.950 possibilities happen. But we only see the one 00:09:03.950 --> 00:09:06.269 in our branch. We are just along for the ride 00:09:06.269 --> 00:09:09.610 in our particular timeline. Wow. Yeah. But then 00:09:09.610 --> 00:09:12.409 that brings us back to the whole free will question. 00:09:12.509 --> 00:09:15.710 Right. If everything is deterministic and all 00:09:15.710 --> 00:09:18.129 choices are made in some reality. Yeah. Does 00:09:18.129 --> 00:09:20.549 it really matter what we choose here? It's a 00:09:20.549 --> 00:09:22.649 deep philosophical question. That's something 00:09:22.649 --> 00:09:25.210 that physicists and philosophers are still arguing 00:09:25.210 --> 00:09:27.529 about. Yeah. But you know, one of the sources 00:09:27.529 --> 00:09:30.450 we read actually had a fun way of putting it. 00:09:30.509 --> 00:09:33.049 Oh, yeah. They said that many worlds is like 00:09:33.049 --> 00:09:36.309 a giant choose your own adventure book. OK, I 00:09:36.309 --> 00:09:39.809 like that. So you, the listener, you get to choose 00:09:39.809 --> 00:09:42.450 which branch of reality you experience. You get 00:09:42.450 --> 00:09:45.080 to pick your adventure. Yeah. So choose wisely. 00:09:45.279 --> 00:09:48.240 And speaking of making complex physics accessible. 00:09:48.460 --> 00:09:51.159 Uh -huh. Hold on, did you hear that? What was 00:09:51.159 --> 00:09:53.460 that? That sounded like Diana from Physics Girl. 00:09:53.820 --> 00:09:58.220 No way. It is. She's beamed in via PBS Space 00:09:58.220 --> 00:10:00.740 Time. That's awesome. And she has some challenges 00:10:00.740 --> 00:10:03.519 for Matt. Matt from PBS Space Time. Yeah, okay. 00:10:03.779 --> 00:10:06.080 First she wants him to prove that the Earth is 00:10:06.080 --> 00:10:08.679 round. Okay. with an experiment that he can do 00:10:08.679 --> 00:10:10.919 in a year. Wow, that's a tough one. And then 00:10:10.919 --> 00:10:13.080 her second challenge is even more interesting. 00:10:13.299 --> 00:10:15.799 What is it? She wants him to explain five of 00:10:15.799 --> 00:10:19.080 the most complicated physics terms using only 00:10:19.080 --> 00:10:22.159 the 1 ,000 most common words in the English language. 00:10:22.299 --> 00:10:24.940 Wow, that's going to be hard. Yeah, but he accepted 00:10:24.940 --> 00:10:27.799 both challenges. That's amazing. It shows how 00:10:27.799 --> 00:10:30.059 important it is to be able to communicate science 00:10:30.059 --> 00:10:32.840 clearly. Yeah, so that everyone can understand 00:10:32.840 --> 00:10:34.820 it. That's what we try to do here on the Deep 00:10:34.820 --> 00:10:37.440 Dive. Exactly. Okay, so let's wrap up. Yeah. 00:10:37.700 --> 00:10:39.379 Today we've been talking about the transition 00:10:39.379 --> 00:10:43.740 from the quantum world to the normal world. The 00:10:43.740 --> 00:10:46.480 classical world. And we've looked at two interpretations. 00:10:46.960 --> 00:10:49.120 Two ways of understanding what's going on. The 00:10:49.120 --> 00:10:52.519 Copenhagen interpretation, which says that the 00:10:52.519 --> 00:10:55.759 wave function collapses when we measure something. 00:10:55.820 --> 00:10:58.039 And we end up with one outcome. And the many 00:10:58.039 --> 00:11:01.039 -worlds interpretation, which says that all possibilities 00:11:01.039 --> 00:11:03.679 continue to exist in separate realities. And 00:11:03.679 --> 00:11:06.340 we just experience one of them. And many -worlds 00:11:06.340 --> 00:11:09.919 leads to this idea of a multiverse. An infinite 00:11:09.919 --> 00:11:12.159 number of universes. Where everything that could 00:11:12.159 --> 00:11:15.159 happen does happen. It's a wild idea. But it's 00:11:15.159 --> 00:11:18.090 supported by the math of quantum mechanics. It 00:11:18.090 --> 00:11:20.850 is. So here's a final thought for you. If every 00:11:20.850 --> 00:11:24.649 possibility plays out in some reality, how does 00:11:24.649 --> 00:11:26.789 that change the way you think about the choices 00:11:26.789 --> 00:11:29.970 you make in this reality? It's something to ponder. 00:11:30.629 --> 00:11:31.870 Thanks for listening. See you next time.