WEBVTT 1 00:00:01.520 --> 00:00:05.599 Welcome to the quart Side Quantum Physics Podcast, an exploration 2 00:00:05.679 --> 00:00:10.880 of the fundamental structure of reality, where quantum laws govern matter, energy, 3 00:00:11.199 --> 00:00:16.239 and information. Here, uncertainty is a feature, not a flaw, 4 00:00:16.719 --> 00:00:19.239 and understanding begins at the smallest scales. 5 00:00:27.640 --> 00:00:30.760 I think everyone has that moment eventually. Oh yeah, you're 6 00:00:30.800 --> 00:00:33.039 sitting in traffic maybe, or you're just staring at a 7 00:00:33.039 --> 00:00:35.399 ceiling fan at three in the morning, and your brain 8 00:00:35.479 --> 00:00:37.320 just starts playing the what if game? 9 00:00:37.640 --> 00:00:38.880 Right, the what if game? 10 00:00:38.960 --> 00:00:40.920 What if I'd taken that job in Chicago, what if 11 00:00:40.920 --> 00:00:42.359 I hadn't ordered the shellfish? 12 00:00:42.560 --> 00:00:45.600 What if I'd turn left instead of right ten years ago. 13 00:00:45.719 --> 00:00:47.560 It's the road not taken exactly. 14 00:00:47.640 --> 00:00:49.880 It's the road not taken. And it's I think it's 15 00:00:49.920 --> 00:00:55.880 fundamental to how we process regret or relief or just 16 00:00:56.079 --> 00:00:57.719 you know, simple curiosity. 17 00:00:57.759 --> 00:01:00.799 It's part of the human condition, it is, and usually we. 18 00:01:00.759 --> 00:01:04.519 Treat these as fantasies, their mental exercises. In fiction, they're 19 00:01:04.560 --> 00:01:07.799 plot devices, you know, the mirror universe where everyone is 20 00:01:07.799 --> 00:01:09.000 evil and has a goateee. 21 00:01:09.079 --> 00:01:12.120 Of course, the goatee is the classic signifier. 22 00:01:12.280 --> 00:01:16.599 But the material we are looking at today suggests something, well, 23 00:01:16.959 --> 00:01:22.280 something pretty unsettling. It suggests that those what ifs aren't fantasies. 24 00:01:21.799 --> 00:01:23.000 At all, not even close. 25 00:01:23.159 --> 00:01:26.920 They might be the literal, mathematical architecture of our reality. 26 00:01:27.159 --> 00:01:28.599 It's a concept that You're right. 27 00:01:28.599 --> 00:01:30.359 It sounds like it belongs in a comic book, but 28 00:01:30.400 --> 00:01:33.439 it's actually one of the most rigorous, I would even 29 00:01:33.439 --> 00:01:37.680 say conservative, and definitely hotly debated ideas in modern physics. 30 00:01:38.000 --> 00:01:40.920 We are talking, of course, about the many world's interpretation 31 00:01:41.040 --> 00:01:43.640 of quantum mechanics. MWI and I want to set the 32 00:01:43.680 --> 00:01:47.159 table properly here because we've gone through a massive stack 33 00:01:47.200 --> 00:01:52.000 of sources, from Hugh Everett's original nineteen fifty seven thesis 34 00:01:52.439 --> 00:01:55.439 all the way to modern analysis by philosophers of science 35 00:01:55.760 --> 00:01:56.599 like David Wallace. 36 00:01:56.680 --> 00:01:58.359 It's a huge body of work. 37 00:01:58.280 --> 00:02:00.959 It is. And the goal today isn't to just say 38 00:02:01.079 --> 00:02:03.239 whoa parallel universes are cool? 39 00:02:03.359 --> 00:02:06.359 Right, that's the popside version. We aren't here for the 40 00:02:06.480 --> 00:02:09.919 sci fi tropes. Yeah, We're here to understand why why 41 00:02:10.000 --> 00:02:13.919 a serious sober physicist would look at the fundamental math 42 00:02:13.919 --> 00:02:17.080 of the universe and conclude that the only logical explanation 43 00:02:17.159 --> 00:02:19.560 is that everything that can happen does happen. 44 00:02:19.360 --> 00:02:21.479 And that is the crux of it, This theory wasn't 45 00:02:21.520 --> 00:02:23.439 invented to write a good story. 46 00:02:23.199 --> 00:02:23.879 No, not at all. 47 00:02:23.919 --> 00:02:27.280 It was invented to fix a problem, a massive glaring 48 00:02:27.360 --> 00:02:30.159 hole in our understanding of how the world actually works. 49 00:02:30.520 --> 00:02:33.360 That's the most important context. You have to start there. 50 00:02:34.039 --> 00:02:37.199 Many world creates a lot of let's say, philosophical headaches, 51 00:02:37.240 --> 00:02:41.159 a few, Yeah, but it solves a physics crisis, a 52 00:02:41.280 --> 00:02:43.479 genuine textbook breaking crisis. 53 00:02:43.560 --> 00:02:45.400 So let's get right into that. I want to start 54 00:02:45.439 --> 00:02:47.960 with the big headline claim, just to get my own 55 00:02:48.000 --> 00:02:51.840 head around it. Again. When we talk about many worlds 56 00:02:52.280 --> 00:02:55.319 or MWI, we are effectively saying that if I had 57 00:02:55.319 --> 00:02:57.879 a choice between coffee and tea this morning, and I 58 00:02:58.000 --> 00:03:02.120 picked coffee, Okay, there's a version of me physically just 59 00:03:02.199 --> 00:03:04.639 as real as I am right now, who picked. 60 00:03:04.400 --> 00:03:06.560 T Yes, that's the claim. 61 00:03:06.319 --> 00:03:09.400 And that version of me is existing right now, in 62 00:03:09.439 --> 00:03:10.280 this moment. 63 00:03:10.199 --> 00:03:13.000 Existing right now in a branch of what's called the 64 00:03:13.159 --> 00:03:16.240 universal wave function that is, for all intents and purposes, 65 00:03:16.520 --> 00:03:18.719 causally disconnected from the one you and I are currently 66 00:03:18.759 --> 00:03:19.199 recording it. 67 00:03:19.319 --> 00:03:21.639 Okay, that is the claim, and it's a big one. 68 00:03:21.800 --> 00:03:24.560 So to understand how we could possibly get there, you're right. 69 00:03:24.599 --> 00:03:26.280 We have to start with the villain of the story. 70 00:03:26.360 --> 00:03:28.680 The villain, yes, the thing that many worlds is trying 71 00:03:28.719 --> 00:03:31.439 to defeat. And in our notes this is referred to 72 00:03:31.479 --> 00:03:33.360 as the quantum measurement problem. 73 00:03:33.400 --> 00:03:33.759 Correct. 74 00:03:33.759 --> 00:03:37.520 You really can't appreciate the elegance of the solution, Everett's 75 00:03:37.560 --> 00:03:41.439 solution unless you understand just how broken and frankly, how 76 00:03:41.520 --> 00:03:43.680 cluggy the standard explanation was. 77 00:03:44.000 --> 00:03:47.360 So let's go back to basics quantum mechanics. We know 78 00:03:47.439 --> 00:03:51.800 that at the subatomic level, electrons photons, the absolute building 79 00:03:51.840 --> 00:03:55.360 blocks of matter. Things do not behave like billiard balls. 80 00:03:55.520 --> 00:03:59.080 Right, It's a completely different rule book. In our macroscopic world, 81 00:03:59.159 --> 00:04:02.800 a billiard ball has a definite position and a definite speed. 82 00:04:02.960 --> 00:04:04.919 It is either here or it is there. 83 00:04:05.039 --> 00:04:06.560 You can point at it, you can point at it. 84 00:04:06.800 --> 00:04:09.360 But in the quantum world, before you measure a particle, 85 00:04:09.520 --> 00:04:12.319 it doesn't have those definite properties. It exists in something 86 00:04:12.360 --> 00:04:13.840 called a superposition. 87 00:04:14.120 --> 00:04:16.759 I've always visualized this as a haze or a cloud 88 00:04:16.800 --> 00:04:20.079 of possibility. The electron isn't a single point, it's a 89 00:04:20.879 --> 00:04:22.319 smear of probability. 90 00:04:22.639 --> 00:04:24.319 That's a good heuristic, It's a really good way to 91 00:04:24.319 --> 00:04:27.560 think about it Mathematically, it's described by something called a 92 00:04:27.600 --> 00:04:30.759 wave function, right, and this wave function assigns a number 93 00:04:31.040 --> 00:04:35.240 technically a complex number and amplitude to every possible location 94 00:04:35.399 --> 00:04:36.560 the electron could be found in. 95 00:04:36.800 --> 00:04:40.480 So the electron is effectively everywhere within that wave at. 96 00:04:40.360 --> 00:04:43.600 Once in a very real physical sense. Yes, it's not 97 00:04:43.639 --> 00:04:45.240 that we don't know where it is. Is that it 98 00:04:45.240 --> 00:04:46.279 doesn't have a single wear. 99 00:04:46.519 --> 00:04:49.920 Okay. So we have this wave, this cloud of possibility, 100 00:04:50.279 --> 00:04:53.480 and we have a law that governs how that wave 101 00:04:53.759 --> 00:04:55.240 moves and changes over time. 102 00:04:55.399 --> 00:04:57.879 We do the Schrodinger equation. 103 00:04:57.600 --> 00:04:58.759 The Shroding your equations. 104 00:04:58.759 --> 00:05:02.000 This is the bedrock, This is the absolute foundation. It 105 00:05:02.079 --> 00:05:06.639 is to quantum mechanics what Newton's laws of motion are 106 00:05:06.759 --> 00:05:08.560 to you throwing a baseball. 107 00:05:08.680 --> 00:05:10.600 And what's the key thing to understand about it? 108 00:05:10.680 --> 00:05:13.600 The key thing, the absolutely critical thing, is that the 109 00:05:13.639 --> 00:05:16.879 Schrodinger equation is deterministic and it is linear. 110 00:05:17.040 --> 00:05:20.519 Okay, let's break that down. Deterministic meaning if I know 111 00:05:20.639 --> 00:05:22.759 the state of the wave right now, I can predict 112 00:05:22.839 --> 00:05:24.720 exactly what it will look like in five. 113 00:05:24.519 --> 00:05:25.959 Minutes, exactly to the letter. 114 00:05:26.040 --> 00:05:29.959 It evolves smoothly, the wave ripples out. It might interfere 115 00:05:29.959 --> 00:05:32.519 with itself. It's changes shape, but it does so in 116 00:05:32.560 --> 00:05:34.680 a perfectly predictable, continuous way. 117 00:05:34.839 --> 00:05:37.000 It never jumps, it never snaps, never. 118 00:05:37.160 --> 00:05:38.560 The math is smooth as glass. 119 00:05:39.000 --> 00:05:40.439 And this is the big butt. 120 00:05:40.600 --> 00:05:42.240 Here's the problem, here's the crisis. 121 00:05:42.439 --> 00:05:45.120 When we actually look at an electron, when we hit 122 00:05:45.120 --> 00:05:48.199 it with a photon, when we measure it with a detector, 123 00:05:48.680 --> 00:05:51.199 we don't see a wave. We don't see a smear. 124 00:05:51.480 --> 00:05:54.079 We see a dot. We see it at one specific location. 125 00:05:54.480 --> 00:05:56.879 And that is a measurement problem in a nutshell. How 126 00:05:56.879 --> 00:05:59.439 do you get from smear to dot? The math says 127 00:05:59.480 --> 00:06:01.959 it should all be a smear our. I say it's 128 00:06:02.000 --> 00:06:02.720 always a dot. 129 00:06:02.920 --> 00:06:07.600 The math is smooth, but reality looks chunky. 130 00:06:07.839 --> 00:06:10.000 Chunky is a great word for it quantized. 131 00:06:10.839 --> 00:06:12.639 So the standard answer, the one that's been taught in 132 00:06:12.680 --> 00:06:17.959 textbooks for nearly a century now, is called the Copenhagen interpretation. 133 00:06:17.600 --> 00:06:22.399 Developed by the giants Nils Bohr, Werner Heisenberg and their colleagues. 134 00:06:22.000 --> 00:06:23.000 In the nineteen twenties. 135 00:06:23.199 --> 00:06:24.079 And what was their solution. 136 00:06:24.560 --> 00:06:27.759 Their solution was to introduce a new rule, a second rule, 137 00:06:27.879 --> 00:06:31.000 completely separate from the Schrodinger equation. They said that when 138 00:06:31.000 --> 00:06:35.040 a measurement is made by an observer, the wave function collapses. 139 00:06:35.240 --> 00:06:37.560 Collapses. It's such a violent word it is. 140 00:06:37.639 --> 00:06:40.560 It goes instantly from that spread out wave of possibilities 141 00:06:40.600 --> 00:06:43.920 to a single definite spike at one location, and all 142 00:06:43.959 --> 00:06:45.680 the other possibility they vanished. 143 00:06:45.759 --> 00:06:47.439 They vanished from reality instantly. 144 00:06:47.680 --> 00:06:50.879 This has always felt incredibly clumsy to me. It's just 145 00:06:51.560 --> 00:06:54.959 it's not elegant. It basically says the laws of physics 146 00:06:55.079 --> 00:06:58.279 work one way when no one is looking, this beautiful 147 00:06:58.720 --> 00:07:02.600 smooth wave like evolution, right, and then they work in 148 00:07:02.600 --> 00:07:06.839 a completely different, random, instantaneous way the moment I opened 149 00:07:06.839 --> 00:07:07.279 my eyes. 150 00:07:07.480 --> 00:07:11.199 It is clumsy. It's what physicists call ad hoc. It's 151 00:07:11.240 --> 00:07:13.879 a rule tacked onto the theory for the sole purpose 152 00:07:13.920 --> 00:07:17.680 of explaining why we don't see superpositions in our daily lives. 153 00:07:18.040 --> 00:07:18.959 But it's not in the math. 154 00:07:19.079 --> 00:07:20.399 It is absolutely not in the math. 155 00:07:20.480 --> 00:07:23.560 The Schrodinger equation doesn't have a collapse term. There's no 156 00:07:23.920 --> 00:07:26.079 variable for observers looking now. 157 00:07:26.240 --> 00:07:27.920 It effectively requires magic. 158 00:07:28.279 --> 00:07:28.680 It does. 159 00:07:28.800 --> 00:07:32.600 It requires the act of observation, maybe even consciousness, to 160 00:07:32.680 --> 00:07:37.000 fundamentally alter physical reality instantaneously and in a way that 161 00:07:37.160 --> 00:07:39.839 literally violates the primary equation of the theory. 162 00:07:39.920 --> 00:07:43.560 And it creates this infamous Heisenberg cut problem, which is 163 00:07:43.560 --> 00:07:45.439 what it's a question of where you draw the line, 164 00:07:45.480 --> 00:07:48.000 where does the quantum world stop and the classical world begin. 165 00:07:48.199 --> 00:07:50.959 Is an atom quantum, yes, obviously, is a molecule made 166 00:07:51.000 --> 00:07:54.319 of atoms? Quantum is a virus, is a bacterium? Is 167 00:07:54.319 --> 00:07:57.680 a cat? Is a human observer? At what point does 168 00:07:57.720 --> 00:08:00.240 an object stop being a wave and become a thing 169 00:08:00.439 --> 00:08:02.120 that causes a wave to collapse? 170 00:08:02.360 --> 00:08:03.959 And Copenhagen can't answer that. 171 00:08:04.079 --> 00:08:05.720 It has no answer. It just says the cut is 172 00:08:05.759 --> 00:08:08.000 somewhere between the small thing and the big thing. It's 173 00:08:08.000 --> 00:08:09.079 completely arbitrary. 174 00:08:09.399 --> 00:08:11.800 So we have this situation where the core math is 175 00:08:12.000 --> 00:08:16.439 beautiful and continuous and predictive, but our interpretation of it 176 00:08:16.519 --> 00:08:21.439 requires this ugly, unexplained, arbitrary snap. Every time we decide 177 00:08:21.439 --> 00:08:22.800 to measure something exactly. 178 00:08:22.839 --> 00:08:23.480 It's a patch. 179 00:08:23.720 --> 00:08:26.959 It's a software fix for a bug in our understanding 180 00:08:26.959 --> 00:08:29.600 of reality, and a lot of physicists, even back then, 181 00:08:29.759 --> 00:08:31.120 we're deeply unhappy with it. 182 00:08:31.399 --> 00:08:34.879 So enter Hugh Everett the Third. It's nineteen fifty seven. 183 00:08:34.919 --> 00:08:37.679 He's a graduate student at Princeton studying under the famous 184 00:08:37.759 --> 00:08:40.000 John Wheeler. He looks at this whole mass. 185 00:08:39.879 --> 00:08:42.360 This whole Frankenstein's monster of a theory. 186 00:08:42.200 --> 00:08:45.080 And he proposes something just breathtakingly radical. 187 00:08:45.200 --> 00:08:48.519 He asks a question that is almost childlike in its simplicity, 188 00:08:48.519 --> 00:08:51.759 it's so powerful. He just asks, what if the Schrodinger 189 00:08:51.759 --> 00:08:54.679 equation is just true all the time, all the time, 190 00:08:54.720 --> 00:08:56.080 for everything, no exceptions. 191 00:08:56.399 --> 00:09:01.080 Meaning, what if the wave function never collapses? 192 00:09:01.320 --> 00:09:03.360 Right? What if we just take that second role, the 193 00:09:03.399 --> 00:09:06.159 collapse postulate, and we just delete it, We throw it 194 00:09:06.159 --> 00:09:08.519 in the trash. What if we assume that the wave 195 00:09:08.679 --> 00:09:11.960 evolves smoothly forever, even during a measurement. 196 00:09:12.480 --> 00:09:15.559 Okay, but that's the whole problem, right, Yeah, if the 197 00:09:15.559 --> 00:09:18.759 waves never collapses, then the superposition never goes away. 198 00:09:18.879 --> 00:09:19.440 It doesn't. 199 00:09:19.600 --> 00:09:22.639 The electron is still in multiple places at once, but 200 00:09:22.759 --> 00:09:25.000 I see it in one place. My detector gives me 201 00:09:25.080 --> 00:09:28.960 one click. How did Everett reconcile the math? Says it's 202 00:09:29.000 --> 00:09:31.440 everywhere with my eyes say it's here. 203 00:09:31.720 --> 00:09:32.679 This is his genius. 204 00:09:32.720 --> 00:09:35.240 This is where we have to talk about entanglement, right, 205 00:09:35.320 --> 00:09:37.919 This is the engine that drives the whole many world's idea. 206 00:09:38.159 --> 00:09:41.279 Usually when we hear entanglement, we think of that spooky 207 00:09:41.320 --> 00:09:44.000 action at a distance between two particles on our part. 208 00:09:44.000 --> 00:09:45.279 Einstein's famous phrase. 209 00:09:45.360 --> 00:09:47.799 Yeah, but ever it applied it in a new way, right. 210 00:09:47.840 --> 00:09:49.000 He applied it to the observer. 211 00:09:49.159 --> 00:09:49.559 He did. 212 00:09:49.799 --> 00:09:54.039 He took the Copenhagen interpretation's biggest problem, the special status 213 00:09:54.039 --> 00:09:57.240 of the observer, and turn it into his solution. He 214 00:09:57.360 --> 00:10:00.919 just treated the observer, the scientist, the camera as just 215 00:10:01.000 --> 00:10:02.320 another quantum system. 216 00:10:02.600 --> 00:10:04.919 Which I mean that just makes sense. We are made 217 00:10:04.919 --> 00:10:09.279 of atoms. Of course, atoms obey the laws of quantum mechanics. 218 00:10:09.720 --> 00:10:12.679 Therefore we obey the laws of quantum mechanics exactly. 219 00:10:13.120 --> 00:10:15.559 There is no cut, there is no classical world. 220 00:10:15.600 --> 00:10:17.960 It's all just quantum mechanics from the bottom all the 221 00:10:18.000 --> 00:10:18.600 way to the top. 222 00:10:18.759 --> 00:10:21.200 So let's trace the chain of events in Everetts view. 223 00:10:21.440 --> 00:10:22.519 Walk me through a measurement. 224 00:10:22.639 --> 00:10:26.039 Okay, you have an electron in a superposition. Let's say 225 00:10:26.039 --> 00:10:28.120 it's fifty percent chance of being at point A and 226 00:10:28.200 --> 00:10:29.360 fifty percent chance. 227 00:10:29.200 --> 00:10:30.120 Of being at point B. 228 00:10:30.519 --> 00:10:31.879 A simple two state system. 229 00:10:31.960 --> 00:10:33.080 Simple two state system. 230 00:10:33.279 --> 00:10:36.639 Yeah, Now, you the observer, want to find out where 231 00:10:36.679 --> 00:10:39.600 it is. So you set up a detector. Maybe it's 232 00:10:39.639 --> 00:10:41.879 a device that flashes a red light if the electron 233 00:10:41.960 --> 00:10:44.919 is at A and a green light if it's at bay. 234 00:10:45.000 --> 00:10:48.279 The electron interacts with your detector at that moment they 235 00:10:48.320 --> 00:10:51.720 become entangled. Now the detector is also in a superposition. 236 00:10:51.879 --> 00:10:54.600 So it's not just the electron anymore. The detector is 237 00:10:54.639 --> 00:10:58.559 now in a state of gonna flash red, gonna flash green. 238 00:10:58.759 --> 00:11:02.840 Precisely, position has grown to include the detector. Now the 239 00:11:02.919 --> 00:11:04.840 light from that flash travels to your eye. 240 00:11:05.039 --> 00:11:06.399 The photon hits my retina. 241 00:11:06.840 --> 00:11:09.480 Now your eye is entangled with the detector, which is 242 00:11:09.600 --> 00:11:13.480 entangled with the electron. Your retina is in a superposition 243 00:11:13.600 --> 00:11:16.960 of registered a red flash and registered a green flash, and. 244 00:11:16.919 --> 00:11:18.600 That signal goes to my brain. 245 00:11:18.480 --> 00:11:22.399 And now you are entangled your brain state. The specific 246 00:11:22.519 --> 00:11:26.240 pattern of neurons firing enters a superposition. There is a 247 00:11:26.279 --> 00:11:29.240 state of you that saw the red light and thinks ah, 248 00:11:29.279 --> 00:11:31.120 the electron was at A in a state of U 249 00:11:31.559 --> 00:11:33.600 that saw the green light and thinks ah, it was 250 00:11:33.679 --> 00:11:34.039 at B. 251 00:11:34.240 --> 00:11:38.360 So the superposition didn't disappear, It didn't collapse, It just grew, 252 00:11:38.559 --> 00:11:39.360 It spread, it. 253 00:11:39.320 --> 00:11:42.240 Spread, it engulfed you. The wave function of the entire 254 00:11:42.399 --> 00:11:46.679 universe now contains two distinct components, two branches, and. 255 00:11:46.600 --> 00:11:48.759 In one component there is the U who saw a 256 00:11:49.679 --> 00:11:51.200 and in the other a U who. 257 00:11:51.039 --> 00:11:53.120 Saw B both physically real. 258 00:11:53.120 --> 00:11:57.279 And because the schroding Your equation is linear, these two components, 259 00:11:57.320 --> 00:12:01.120 these two branches, they just they continue to evolve independently 260 00:12:01.120 --> 00:12:01.799 from that point on. 261 00:12:02.200 --> 00:12:02.840 That's the key. 262 00:12:03.360 --> 00:12:06.759 Once they split, for all practical purposes, they don't interact anymore. 263 00:12:06.879 --> 00:12:09.600 This is why it's called the many world's interpretation. It's 264 00:12:09.639 --> 00:12:12.120 not that the universe physically splits apart like a log 265 00:12:12.200 --> 00:12:13.559 being chopped by an axe. 266 00:12:13.679 --> 00:12:15.320 That's the sci fi image it is. 267 00:12:15.440 --> 00:12:18.799 It's more like the universal wave function differentiates into non 268 00:12:18.879 --> 00:12:23.120 interacting branches, each of which contains a consistent version of reality, 269 00:12:23.399 --> 00:12:26.759 each of which feels from the inside like a complete universe. 270 00:12:27.080 --> 00:12:30.919 This brings us inevitably to the cat. I feel like 271 00:12:30.919 --> 00:12:33.840 we are legally required by the laws of physics podcasting 272 00:12:33.879 --> 00:12:35.559 to discuss Schrodinger's cat. 273 00:12:35.919 --> 00:12:38.480 We can't avoid the cat. It's the most famous thought 274 00:12:38.480 --> 00:12:39.639 experiment in history. 275 00:12:39.799 --> 00:12:42.799 So the classic setup a cat in a box. There's 276 00:12:42.799 --> 00:12:45.919 a single radioactive atom, there's a Geiger counter. If the 277 00:12:45.919 --> 00:12:48.000 atom decays, it triggers the. 278 00:12:47.960 --> 00:12:51.799 Counter, which releases a hammer, which smashes a vial of poison, 279 00:12:52.000 --> 00:12:52.360 and the. 280 00:12:52.240 --> 00:12:55.240 Cat dies A bit morbid but okay, if the atom 281 00:12:55.279 --> 00:12:56.919 doesn't decay, nothing happens. 282 00:12:57.080 --> 00:12:58.960 Cat lives, right, and you leave it in there for 283 00:12:59.000 --> 00:13:01.320 exactly one half life, So there's a perfect fifty to 284 00:13:01.320 --> 00:13:02.399 fifty chance of decay. 285 00:13:02.679 --> 00:13:05.360 And since the atom is a quantum system, it's not 286 00:13:05.480 --> 00:13:09.399 decayed or not decayed. It's in a quantum superposition of 287 00:13:09.480 --> 00:13:11.240 decayed ind not decayed. 288 00:13:10.919 --> 00:13:13.919 Which means the entire system in the box becomes entangled. 289 00:13:14.600 --> 00:13:17.600 The Geiger counter is in a superposition of triggered and 290 00:13:17.679 --> 00:13:21.120 not triggered. The hammers fallen and not fallen, the poison 291 00:13:21.200 --> 00:13:23.399 is released and not released. 292 00:13:23.000 --> 00:13:25.320 And therefore the cat ends up in a superposition of 293 00:13:25.360 --> 00:13:25.759 dead and. 294 00:13:25.720 --> 00:13:29.519 Alive, which is the absurd conclusion Schrodinger was trying to 295 00:13:29.519 --> 00:13:31.919 point out. He was trying to show how ridiculous the 296 00:13:31.960 --> 00:13:34.320 Copenhagen view was when you scaled it up right. 297 00:13:34.360 --> 00:13:36.519 He was saying, look, if you take your math seriously, 298 00:13:36.879 --> 00:13:39.879 you get this nonsense. A cat that's both dead and alive. 299 00:13:40.240 --> 00:13:42.759 And in the Copenhagen view, the absurdity is resolved. 300 00:13:42.759 --> 00:13:43.720 When you open the box. 301 00:13:44.159 --> 00:13:48.480 The active observation forces the universe to choose snap, the 302 00:13:48.519 --> 00:13:52.759 wave function collapses, the cat is definitively dead. Or are 303 00:13:52.879 --> 00:13:54.120 definitively alive. 304 00:13:54.320 --> 00:13:55.360 But in everts view. 305 00:13:55.480 --> 00:13:58.440 In everts view, the universe makes no choice. The math 306 00:13:58.519 --> 00:14:01.200 is never absurd. The math is the reality. The atom 307 00:14:01.279 --> 00:14:04.679 is both. The poison is released and not released. 308 00:14:04.759 --> 00:14:06.200 The cat is dead and alive. 309 00:14:06.320 --> 00:14:07.600 And when I open the lid. 310 00:14:07.639 --> 00:14:10.480 You become part of the system. You become entangled with 311 00:14:10.519 --> 00:14:14.440 the contents of the box. The superposition just expands one 312 00:14:14.519 --> 00:14:16.120 final time to include you. 313 00:14:16.519 --> 00:14:19.000 So the universe now contains a branch. 314 00:14:18.879 --> 00:14:21.759 A branch with a dead cat and a very sad 315 00:14:21.799 --> 00:14:23.399 observer who is now looking at. 316 00:14:23.320 --> 00:14:25.320 A dead cat in another branch, and. 317 00:14:25.320 --> 00:14:27.960 A completely separate branch with a live cat and a 318 00:14:28.120 --> 00:14:29.759 very happy, relieved observer. 319 00:14:30.000 --> 00:14:32.240 And the crucial point here, the thing that's so hard 320 00:14:32.240 --> 00:14:35.519 to grasp, is that both of those observers are me. 321 00:14:36.000 --> 00:14:39.200 Yes, they're both you. They both share your exact memories 322 00:14:39.279 --> 00:14:41.759 up to the very moment of the split. They both 323 00:14:41.799 --> 00:14:43.720 feel like the one and only real you. 324 00:14:44.120 --> 00:14:46.720 The you looking at the live cat feels like the 325 00:14:46.720 --> 00:14:50.360 only you. He has no idea his counterpart. 326 00:14:49.879 --> 00:14:52.399 Exists, and the you looking at the dead cat feels 327 00:14:52.399 --> 00:14:54.879 like the only you. He is now living in a 328 00:14:54.879 --> 00:14:57.120 world where the cat died and he has no access 329 00:14:57.159 --> 00:14:58.360 to the reality where it lived. 330 00:14:58.559 --> 00:15:01.240 This solves the measurement problem because there is no measurement problem. 331 00:15:01.279 --> 00:15:03.200 There's no collapse. The math just keeps running. 332 00:15:03.320 --> 00:15:04.080 It's seamless. 333 00:15:04.200 --> 00:15:08.679 It creates a perfectly seamless deterministic physics. The entire wave 334 00:15:08.720 --> 00:15:12.080 function of the universe evolves according to one single rule. 335 00:15:12.840 --> 00:15:15.519 But the cost, the cost is that we have to 336 00:15:15.559 --> 00:15:18.960 accept that all these other outcomes, all these other branches, 337 00:15:19.360 --> 00:15:23.960 actually exist. They are not possibilities, they are realities, which. 338 00:15:23.759 --> 00:15:27.759 Brings up the immediate subjective objection, the thing everyone asks. 339 00:15:27.440 --> 00:15:29.159 First, I know exactly what you're. 340 00:15:29.039 --> 00:15:31.279 Going to say. I don't feel like I'm splitting. If 341 00:15:31.320 --> 00:15:35.279 I'm constantly branching into literally billions of copies every second, 342 00:15:35.639 --> 00:15:40.360 shouldn't I feel I don't know, stretched or diluted or something. 343 00:15:40.440 --> 00:15:43.360 It's a very common intuition, a very natural one. But 344 00:15:43.440 --> 00:15:47.039 the physics actually explains precisely why you don't feel it. 345 00:15:47.039 --> 00:15:48.960 It's a phenomenon called decoherence. 346 00:15:49.120 --> 00:15:50.879 We see this term a lot in the sources. It's 347 00:15:50.919 --> 00:15:53.799 a more modern addition to Everett's original idea. Right, he 348 00:15:53.840 --> 00:15:54.759 didn't have the full picture. 349 00:15:54.840 --> 00:15:58.279 That's right, the full mathematical theory of decoherence was really 350 00:15:58.320 --> 00:16:00.840 developed in the seventies and eighties, and it's what makes 351 00:16:01.080 --> 00:16:05.480 MWI a truly robust modern theory. It's the wall between 352 00:16:05.559 --> 00:16:06.120 the worlds. 353 00:16:06.399 --> 00:16:07.679 So what is it? How does it work? 354 00:16:07.840 --> 00:16:10.159 Think of it this way. Why do we see weird 355 00:16:10.240 --> 00:16:14.919 quantum effects like superposition and interference in tiny isolated atoms 356 00:16:15.200 --> 00:16:17.960 but not in big things like bowling balls, Because they're 357 00:16:18.200 --> 00:16:21.799 it's not just the size, it's the interaction. Big things 358 00:16:21.840 --> 00:16:24.440 are constantly interacting with the environment, right. 359 00:16:24.399 --> 00:16:29.360 Air molecules, photons from the lights, thermal radiation. They're constantly 360 00:16:29.399 --> 00:16:30.440 bouncing off us. 361 00:16:30.879 --> 00:16:33.960 Billions and billions of them every nanosecond, and all those 362 00:16:34.000 --> 00:16:38.279 tiny interactions carry away information. They scramble the phases of 363 00:16:38.320 --> 00:16:42.480 the different parts of the wave function. This process is decoherence. 364 00:16:42.759 --> 00:16:45.720 Okay, scramble the phases. I like the analogy of the 365 00:16:45.840 --> 00:16:47.960 radio stations that was used in one of the papers. 366 00:16:48.399 --> 00:16:49.200 Can we unpack that? 367 00:16:49.440 --> 00:16:52.240 It's a perfect analogy. Imagine you have two radio stations. 368 00:16:52.279 --> 00:16:54.799 If they are broadcasting on frequencies that are very very 369 00:16:54.799 --> 00:16:57.840 close together, say one on one point five FM and 370 00:16:57.879 --> 00:16:58.799 one on one point. 371 00:16:58.600 --> 00:17:00.480 Five to one FM, You'll get in inference. 372 00:17:00.559 --> 00:17:03.759 You'll get interference you'll hear a staticky mess. You'll hear 373 00:17:03.840 --> 00:17:07.200 bits of both at once. That's like a quantum superposition 374 00:17:07.319 --> 00:17:10.599 where the branches are still coherent. They can still interact 375 00:17:10.599 --> 00:17:12.200 and interfere with each other, like. 376 00:17:12.160 --> 00:17:15.599 In the famous double flit experiment, where an electron wave interferes. 377 00:17:15.200 --> 00:17:16.640 With itself exactly. 378 00:17:17.039 --> 00:17:20.000 But as the system you the cat, the detector interacts 379 00:17:20.000 --> 00:17:23.319 with the environment, the branches get kicked by all those 380 00:17:23.359 --> 00:17:26.839 air molecules and photons, and it causes them to drift 381 00:17:26.880 --> 00:17:28.200 apart in frequency. 382 00:17:28.279 --> 00:17:32.039 They become mathematically orthogonal. Orthogonal is the technical term. It's 383 00:17:32.039 --> 00:17:34.440 a fancy word from geometry that just means they're at 384 00:17:34.480 --> 00:17:37.920 right angles to each other in this abstract mathematical space. 385 00:17:37.720 --> 00:17:38.839 But they have zero overlap. 386 00:17:38.960 --> 00:17:43.720 Zero overlap. Yeah. Once decoherence happens, which for a macroscopic 387 00:17:43.759 --> 00:17:46.839 object like a cat or a person, takes an unthinkably 388 00:17:46.920 --> 00:17:49.839 tiny fraction of a second, the live cat branch and 389 00:17:49.880 --> 00:17:52.799 the dead cat branch are completely out of phase with 390 00:17:52.839 --> 00:17:55.039 each other. They can no longer affect each other in 391 00:17:55.079 --> 00:17:57.480 any way. So if I am the live cat observer, 392 00:17:57.640 --> 00:17:59.359 the reason I don't see that goes to the dead 393 00:17:59.359 --> 00:18:02.000 cat is this reason I don't hear ninety eight point 394 00:18:02.000 --> 00:18:03.960 five FM when my radio is tuned to one on 395 00:18:04.039 --> 00:18:05.000 one point five FM. 396 00:18:05.160 --> 00:18:07.559 It's a perfect analogy. The radio waves for that other 397 00:18:07.640 --> 00:18:10.400 station are in the room. They're passing through your body 398 00:18:10.480 --> 00:18:12.319 right now. They're physically present. 399 00:18:12.160 --> 00:18:15.119 But my receiver, my reality isn't in tuned to them. 400 00:18:15.400 --> 00:18:18.559 You are dynamically isolated from them. They can't interfere, they 401 00:18:18.559 --> 00:18:21.759