WEBVTT 1 00:00:01.480 --> 00:00:05.599 Welcome to the Core Side Quantum Physics Podcast, an exploration 2 00:00:05.679 --> 00:00:10.919 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 00:00:27.559 --> 00:00:30.719 Hello everyone, and welcome back to the show. Today, we 6 00:00:30.920 --> 00:00:35.039 are embarking on a journey, an exploration really that wraps 7 00:00:35.119 --> 00:00:38.280 up a cliffhanger that has been dangling over the scientific 8 00:00:38.320 --> 00:00:42.039 world for exactly one hundred years. It's a big one, 9 00:00:42.119 --> 00:00:46.600 it is. It's a story of rivalry, of absolute genius, 10 00:00:47.039 --> 00:00:49.560 and of the very fabric of reality itself. 11 00:00:49.640 --> 00:00:52.719 It really is. It's arguably the ultimate intellectual face off. 12 00:00:52.840 --> 00:00:55.479 We're talking about a debate that defined the twentieth century 13 00:00:55.719 --> 00:00:58.159 and now finally in the twenty first century, we have 14 00:00:58.240 --> 00:00:59.039 a conclusion and. 15 00:00:59.079 --> 00:01:01.799 Has written in hard not just on a chalkboard exactly. 16 00:01:01.840 --> 00:01:05.079 This isn't just about equations or you know, dusty lab equipment. 17 00:01:05.280 --> 00:01:08.439 It is a genuine clash of titans about how we 18 00:01:08.519 --> 00:01:09.079 understand the. 19 00:01:09.079 --> 00:01:11.879 Universe and the cast of characters you literally couldn't write 20 00:01:11.920 --> 00:01:15.640 better protagonists. In one corner, we have the father of relativity, 21 00:01:15.640 --> 00:01:17.760 the man with the hair, the man who changed how 22 00:01:17.799 --> 00:01:20.519 we see space and time. Albert Einstein, of course, and 23 00:01:20.560 --> 00:01:23.640 in the other corner the godfather of quantum mechanics, Great 24 00:01:23.719 --> 00:01:25.120 Dane Neil's. 25 00:01:24.799 --> 00:01:29.200 Bore two absolute heavyweights, and for a long time this 26 00:01:29.319 --> 00:01:33.640 rivalry was just it was frozen in history. Specifically, there's 27 00:01:33.680 --> 00:01:36.879 that iconic image from the nineteen twenty five Solve conference. 28 00:01:36.920 --> 00:01:38.120 Well, I know what I'm sure you do. 29 00:01:38.400 --> 00:01:40.840 If you've ever googled physics history, you've seen it. It's 30 00:01:40.840 --> 00:01:44.920 that grainy, kind of sepia toned photo where you have 31 00:01:45.120 --> 00:01:47.719 basically the smartest people who ever lived, all sitting on 32 00:01:47.719 --> 00:01:48.239 one porch. 33 00:01:48.439 --> 00:01:51.560 I love that photo. Yeah, it looks so civilized, doesn't it. 34 00:01:51.799 --> 00:01:55.599 Just a bunch of guys in like wool coats and hats, relaxing, 35 00:01:55.640 --> 00:01:58.200 maybe a little slumped in their chairs, deep in conversation. 36 00:01:59.239 --> 00:02:02.959 You see Marie looking stoic, schroding or looking thoughtful. But 37 00:02:03.079 --> 00:02:06.480 right in the center there they are Einstein and Borr. 38 00:02:06.680 --> 00:02:11.919 And that civilized exterior was hiding a philosophical war. Einstein 39 00:02:12.000 --> 00:02:15.560 notoriously was deeply, deeply skeptical of the new quantum mechanics 40 00:02:15.560 --> 00:02:16.520 that bor was championing. 41 00:02:16.639 --> 00:02:17.360 He couldn't stand it. 42 00:02:17.479 --> 00:02:19.360 He really couldn't He had that famous line which I 43 00:02:19.400 --> 00:02:21.520 think even people who have no interest in physics know 44 00:02:22.120 --> 00:02:24.719 God does not play dice with the universe, which was. 45 00:02:24.639 --> 00:02:27.919 His way of saying that reality shouldn't be random. Right. 46 00:02:28.360 --> 00:02:30.280 He believed that if you knew enough, if you had 47 00:02:30.360 --> 00:02:32.199 enough data, you could predict everything. 48 00:02:32.360 --> 00:02:37.479 Yes, the universe should be orderly, clockwork, cause and effect exactly. 49 00:02:38.400 --> 00:02:42.039 Einstein was a determinist. He believed in a direct line 50 00:02:42.080 --> 00:02:45.039 from A to B. Borr, on the other hand, was 51 00:02:45.120 --> 00:02:48.680 leading what became known as the Copenhagen interpretation, right, and 52 00:02:48.719 --> 00:02:52.840 he argued that at a fundamental level, the universe, well, 53 00:02:52.919 --> 00:02:56.840 it is probabilistic. Things don't have definite states like a 54 00:02:56.879 --> 00:03:00.919 specific location or a specific speed until you measure them. 55 00:03:00.960 --> 00:03:04.000 Before that measurement, they're just a cloud of possibilities, a smear. 56 00:03:04.199 --> 00:03:05.960 A smear is a great way to put it, a 57 00:03:06.000 --> 00:03:09.919 wave of potential. And for a century the textbooks have 58 00:03:10.000 --> 00:03:13.680 told us bore one. The narrative has always been Einstein 59 00:03:13.759 --> 00:03:15.879 was the old guard. He was brilliant, but he couldn't 60 00:03:15.960 --> 00:03:17.680 keep up. He was wrong, but he didn't. 61 00:03:17.479 --> 00:03:19.199 Go down without a fight, Oh not at all. 62 00:03:19.400 --> 00:03:23.199 He kept throwing these brain twisters, these kidankin experiments or 63 00:03:23.199 --> 00:03:26.000 thought experiments at Boor to try and break quantum theory. 64 00:03:26.159 --> 00:03:28.719 He was looking for a crack, a contradiction. 65 00:03:28.960 --> 00:03:32.240 He wanted to find a logical inconsistency to prove that 66 00:03:32.319 --> 00:03:35.719 quantum mechanics was incomplete. And that is where our story 67 00:03:35.759 --> 00:03:40.479 gets incredibly fresh because on December thirty first, twenty twenty five, 68 00:03:40.639 --> 00:03:44.039 just a couple of months ago, scientists in China published. 69 00:03:43.639 --> 00:03:45.080 A paper in Physical Review Letters. 70 00:03:45.199 --> 00:03:48.400 That's the one, and they didn't just discuss Einstein's argument, 71 00:03:48.439 --> 00:03:51.919 they actually built it. They built the machine Einstein designed 72 00:03:51.919 --> 00:03:53.759 in his head to prove bor wrong. 73 00:03:53.879 --> 00:03:56.000 That's amazing. After one hundred years. 74 00:03:55.759 --> 00:03:59.319 One hundred years, they took this thought experiment which was 75 00:03:59.319 --> 00:04:03.319 intended to destroy the principle of complementarity, and they turned 76 00:04:03.319 --> 00:04:04.759 it into real hardware. 77 00:04:04.879 --> 00:04:08.280 So our mission today is to unpack this specific experiment. 78 00:04:08.280 --> 00:04:11.479 We're going to look at the recoil slit paradox, understand 79 00:04:11.479 --> 00:04:14.840 why Einstein thought he had bore cornered, and reveal how 80 00:04:15.120 --> 00:04:18.279 yet again, Einstein might have lost the battle, but in 81 00:04:18.360 --> 00:04:21.160 doing so, he helped us win the war for understanding 82 00:04:21.199 --> 00:04:22.160 the quantum world. 83 00:04:22.319 --> 00:04:24.360 That is a beautiful way to put it. This isn't 84 00:04:24.399 --> 00:04:26.439 just about who was right or wrong. It's about how 85 00:04:26.439 --> 00:04:29.560 the debate itself sharpened our tools to prob reality. 86 00:04:29.839 --> 00:04:31.560 Okay, so let's set the stage. We need to go 87 00:04:31.639 --> 00:04:35.720 back to nineteen twenty seven. The theoretical battleground. The core 88 00:04:35.800 --> 00:04:39.920 of the disagreement was something called complementarity. Now this sounds 89 00:04:39.959 --> 00:04:42.720 like relationship advice. You complete me, but I have a 90 00:04:42.720 --> 00:04:45.240 feeling it's a lot more complicated, just a bit. 91 00:04:45.360 --> 00:04:50.160 So the principle of complimentarity was Nils Bor's baby. Basically, 92 00:04:50.319 --> 00:04:54.079 in classical physics, the world of baseballs, planets, and cars, 93 00:04:54.480 --> 00:04:56.879 you can know everything about an object at once. Right, 94 00:04:57.000 --> 00:04:58.720 you can know exactly where a car is on the 95 00:04:58.759 --> 00:05:00.879 highway and exactly how bast it is moving. 96 00:05:01.079 --> 00:05:04.759 My GPS knows my location and my spidometer knows my speed. 97 00:05:05.040 --> 00:05:07.199 No conflict there. I don't have to choose between knowing 98 00:05:07.240 --> 00:05:08.319 where I am and knowing when. 99 00:05:08.240 --> 00:05:11.600 I'll arrive exactly. But in the quantum world, Borr argued 100 00:05:11.639 --> 00:05:14.920 that nature imposes a hard limit. There are pairs of 101 00:05:15.000 --> 00:05:18.120 properties they're called conjugate variables that simply cannot be known 102 00:05:18.160 --> 00:05:20.199 simultaneously with perfect precision. 103 00:05:20.399 --> 00:05:23.079 And the most famous pair is position and momentum. 104 00:05:23.120 --> 00:05:28.199 That's the one location and speedish momentum is mass times 105 00:05:28.199 --> 00:05:31.160 of velocity. Bor said, you can design an experiment to 106 00:05:31.240 --> 00:05:35.519 measure an electron's position perfectly, or you can design one 107 00:05:35.560 --> 00:05:39.439 to measure its momentum perfectly. But you cannot, under any 108 00:05:39.519 --> 00:05:41.800 circumstances do both at the same time. 109 00:05:42.000 --> 00:05:44.800 And not because your tools are bad, not because you're clumsy. 110 00:05:45.160 --> 00:05:49.240 No, that's the key. It's because nature itself forbids it. 111 00:05:49.839 --> 00:05:54.120 The very act of measuring one with precision fundamentally blurs 112 00:05:54.160 --> 00:05:55.600 the other one out of existence. 113 00:05:55.759 --> 00:05:59.120 I always try to visualize this because it feels so counterintuitive. 114 00:05:59.120 --> 00:06:02.759 It's like, Okay, imagine you're trying to understand a piece 115 00:06:02.759 --> 00:06:03.160 of music. 116 00:06:03.240 --> 00:06:04.480 Okay, I like where this is going. 117 00:06:04.879 --> 00:06:07.240 If you pause the song, you can say, okay, this 118 00:06:07.360 --> 00:06:09.920 exact millisecond, the note is a C sharp. You have 119 00:06:10.040 --> 00:06:12.360 the position of the song perfectly. 120 00:06:12.000 --> 00:06:13.879 Right, a frozen snapshot in time. 121 00:06:14.000 --> 00:06:17.279 But in that frozen moment, there is no melody, there's 122 00:06:17.279 --> 00:06:20.199 no tempo. You lose the momentum of the music. 123 00:06:20.279 --> 00:06:22.879 That is actually a brilliant analogy. There's no rhythm. 124 00:06:22.759 --> 00:06:26.040 Exactly, But if you let the song play, you hear 125 00:06:26.079 --> 00:06:28.319 the melody, you feel the rhythm, and the speed of 126 00:06:28.319 --> 00:06:30.920 the momentum, but you can no longer pin it down 127 00:06:30.959 --> 00:06:33.959 to a single frozen instant. You can have the flow 128 00:06:34.399 --> 00:06:36.680 or you can have the snapshot. You can't have both. 129 00:06:36.920 --> 00:06:39.759 That captures the essence of it perfectly. And in the 130 00:06:39.839 --> 00:06:42.800 quantum realm, it's not just a limitation of your hearing 131 00:06:42.959 --> 00:06:46.079 or your pause button. The particle itself doesn't have a 132 00:06:46.079 --> 00:06:48.759 defined position and momentum simultaneously. 133 00:06:48.879 --> 00:06:49.199 Wow. 134 00:06:49.600 --> 00:06:54.879 This concept undergirds everything in quantum mechanics, including Heisenberg's uncertainty principle, 135 00:06:54.920 --> 00:06:56.759 and of course, wave particle duality. 136 00:06:56.839 --> 00:07:00.480 Okay, wave particle duality. This is the other big beasts 137 00:07:00.519 --> 00:07:03.800 we need to wrangle before getting to Einstein's trap. This 138 00:07:03.959 --> 00:07:06.839 brings us to the double slit experiment. 139 00:07:06.399 --> 00:07:08.959 The classic but one experiment that if you can wrap 140 00:07:08.959 --> 00:07:12.040 your head around it, you're halfway to understanding quantum mechanics. 141 00:07:12.160 --> 00:07:14.720 It was first done with light by Thomas Young way 142 00:07:14.759 --> 00:07:15.800 back in eighteen oh one. 143 00:07:15.879 --> 00:07:19.800 Right with light, yes, But the really mind bending version 144 00:07:19.839 --> 00:07:23.439 was in nineteen twenty seven with electrons. Because an electron, 145 00:07:23.480 --> 00:07:25.519 we think of it as a particle, a little ball, 146 00:07:25.800 --> 00:07:28.800 or a thing, a thing, and that's where our intuition breaks. 147 00:07:28.920 --> 00:07:31.040 Okay, let's do a quick recap for anyone who hasn't 148 00:07:31.040 --> 00:07:33.720 thought about high school physics in a decade. You have 149 00:07:33.759 --> 00:07:37.399 a barrier with two vertical slits in it. You fire particles, 150 00:07:37.480 --> 00:07:41.480 let's say electrons, one by one at the barrier behind it. 151 00:07:41.519 --> 00:07:42.639 There's a detection screen. 152 00:07:42.839 --> 00:07:46.600 Correct. Now, if particles were just like little tennis balls 153 00:07:46.680 --> 00:07:49.480 or bullets, you would expect them to pass through the 154 00:07:49.560 --> 00:07:52.160 left slit or the right slit and pile up in 155 00:07:52.240 --> 00:07:55.000 two distinct bands on the screen behind them. 156 00:07:55.040 --> 00:07:58.040 Two slits, two piles. Simple, that's what happens if I 157 00:07:58.079 --> 00:07:59.279 throw sand through a fence. 158 00:07:59.439 --> 00:08:03.079 But that is absolutely not what happens with quantum particles. Instead, 159 00:08:03.160 --> 00:08:05.920 you get an interference pattern. You see a series of 160 00:08:06.000 --> 00:08:10.000 alternating bright and dark stripes fringes spread across. 161 00:08:09.680 --> 00:08:11.600 The screen, And that's a wave phenomena. 162 00:08:11.759 --> 00:08:14.120 It's only a wave phenomenon. And this is exactly what 163 00:08:14.160 --> 00:08:17.519 water waves do. When two waves meet, the peaks add 164 00:08:17.600 --> 00:08:20.160 up to make bigger peaks, and a peak meeting a 165 00:08:20.279 --> 00:08:21.519 trough cancels out. 166 00:08:21.600 --> 00:08:24.360 So this proves that these tiny particles are actually acting 167 00:08:24.480 --> 00:08:27.319 like waves. The electron leaves the gun as a particle, 168 00:08:27.920 --> 00:08:30.040 but it seems to spread out like a wave go 169 00:08:30.160 --> 00:08:33.639 through both sless at once, interfere with itself with itself, 170 00:08:33.720 --> 00:08:35.799 and then it hits the screen as a particle again 171 00:08:35.879 --> 00:08:38.159 in one specific spot exactly. 172 00:08:38.360 --> 00:08:41.679 This demonstrates the wave nature of matter. But here is 173 00:08:41.759 --> 00:08:44.480 the kicker, and this is where complimentary comes roaring back. 174 00:08:45.080 --> 00:08:48.000 If you try to sneak a peak, if you put 175 00:08:48.000 --> 00:08:50.480 a little detector by the slits to see which one 176 00:08:50.559 --> 00:08:52.240 the particle actually went. 177 00:08:52.120 --> 00:08:54.720 Through, to force it to reveal its particle nature. 178 00:08:54.879 --> 00:08:58.080 Yes, the moment you do that, the wave pattern disappears. 179 00:08:58.159 --> 00:08:59.879 You just get the two piles of ten. 180 00:09:00.559 --> 00:09:02.519 That is the part that always freaks people out. It's 181 00:09:02.519 --> 00:09:05.440 like the universe is gaslighting us. Oh you're watching, fine, 182 00:09:05.480 --> 00:09:07.679 I'll behave Normally you're not watching, I'll get weird. 183 00:09:08.000 --> 00:09:11.000 It's the ultimate observer effect. You can see the wave 184 00:09:11.440 --> 00:09:14.320 the interference, or you can see the particle which path 185 00:09:14.360 --> 00:09:18.399 it took, but you absolutely cannot see both. That's complementarity 186 00:09:18.519 --> 00:09:19.639 in action, and that. 187 00:09:19.720 --> 00:09:21.639 Is exactly what Einstein hated. 188 00:09:21.879 --> 00:09:22.960 He detested it. 189 00:09:23.080 --> 00:09:27.559 He hated the idea that the act of measuring changed reality, 190 00:09:28.000 --> 00:09:31.080 or that the particle didn't have a defined path until 191 00:09:31.120 --> 00:09:34.159 we looked. He felt there were holes and inconsistencies in 192 00:09:34.200 --> 00:09:34.919 this whole picture. 193 00:09:35.000 --> 00:09:38.200 He called it spooky action at a distance. He wanted 194 00:09:38.200 --> 00:09:40.720 to find a contradiction. He wanted to catch nature in 195 00:09:40.759 --> 00:09:41.320 a lie. 196 00:09:41.600 --> 00:09:44.559 He wanted to show that you could cheat, that you could, 197 00:09:44.720 --> 00:09:48.480 in theory, know which path the particle took without destroying 198 00:09:48.480 --> 00:09:49.519 the interference pattern. 199 00:09:49.679 --> 00:09:51.799 And if he could do that, he would prove that 200 00:09:51.879 --> 00:09:55.399 complimentarity was wrong and that quantum mechanics was at best 201 00:09:55.720 --> 00:09:56.919 an incomplete theory. 202 00:09:57.200 --> 00:10:01.120 So entered the gadankin experiment. The thought, this is the 203 00:10:01.159 --> 00:10:04.159 setup that the Chinese team finally built in twenty twenty five. 204 00:10:04.720 --> 00:10:09.159 Walk us through Einstein's brilliant, sneaky design. How did he 205 00:10:09.279 --> 00:10:10.840 plan to outsmart the universe? 206 00:10:11.200 --> 00:10:13.960 So Einstein looks at the double slit experiment and says, okay, 207 00:10:14.000 --> 00:10:16.600 I see the problem. Putting a detector right at the 208 00:10:16.639 --> 00:10:20.159 slits that disturbs the particle. That's too invasive. So I 209 00:10:20.200 --> 00:10:22.000 won't touch the double slits. I won't put a. 210 00:10:21.919 --> 00:10:24.039 Detector there, Okay, hands off approach. Right. 211 00:10:24.320 --> 00:10:26.519 Instead, he proposes adding a third. 212 00:10:26.320 --> 00:10:29.120 Slit, a third slit where before. 213 00:10:28.759 --> 00:10:30.960 The double slit. So imagine a first barrier with just 214 00:10:31.080 --> 00:10:34.159 one single slit. The particles have to pass through this 215 00:10:34.320 --> 00:10:38.000 single slit first before they travel onto the double slits 216 00:10:38.039 --> 00:10:39.080 and then to the screen. 217 00:10:39.240 --> 00:10:41.080 Okay, so it's like a funnel. They go through the 218 00:10:41.120 --> 00:10:44.120 single slit, then fan out to the double slits. How 219 00:10:44.159 --> 00:10:46.600 does that help, Einstein? It just seems like an extra step. 220 00:10:46.799 --> 00:10:51.120 Ah. But here's the genius twist. Einstein said, Let's make 221 00:10:51.200 --> 00:10:54.799 this first barrier, the one with the single slit moveable, movable. 222 00:10:54.879 --> 00:10:58.759 How Let's mount it on very sensitive, essentially friction free 223 00:10:58.879 --> 00:11:03.279 springs can move up and down springs. Okay, why springs 224 00:11:03.320 --> 00:11:07.840 momentum Newton's third law. For every action there is an 225 00:11:07.879 --> 00:11:09.279 equal and opposite reaction. 226 00:11:09.360 --> 00:11:10.799 Okay, bring this down to earth for me. 227 00:11:11.000 --> 00:11:14.840 Imagine a skateboarder standing on a floating raft in a pool. 228 00:11:15.279 --> 00:11:18.360 If the skateboarder jumps forward off the raft to get 229 00:11:18.360 --> 00:11:18.639 to the. 230 00:11:18.600 --> 00:11:20.159 Pool deck, the raft kicks backward. 231 00:11:20.200 --> 00:11:24.759 It recoils exactly the raft recoils. Now imagine the skateboarder 232 00:11:24.799 --> 00:11:27.759 is our particle and the raft is the single slit 233 00:11:27.799 --> 00:11:31.600 on springs. Yea, the particle passes through that single slit. 234 00:11:31.960 --> 00:11:33.960 For it to reach the top slit of the double 235 00:11:33.960 --> 00:11:37.600 slit barrier, it has to be deflected slightly upwards. Well, 236 00:11:37.679 --> 00:11:40.080 to go up, it has to push off of something. 237 00:11:40.399 --> 00:11:43.279 It imparts a tiny kick downwards on the single slit. 238 00:11:43.399 --> 00:11:44.799 It pushes the single slit down. 239 00:11:44.919 --> 00:11:46.679 Oh, I see, It's like a swimmer pushing off the 240 00:11:46.679 --> 00:11:48.720 side of a pool. Uhh. To go forward, you have 241 00:11:48.759 --> 00:11:50.840 to push the wall backward exactly. 242 00:11:51.080 --> 00:11:54.600 And if the particle goes downwards towards the bottom slit, 243 00:11:54.639 --> 00:11:56.039 what does it do to the single slit? 244 00:11:56.200 --> 00:11:58.000 It kicks the single slit out precisely. 245 00:11:58.639 --> 00:12:01.799 So Einstein's logic was, I don't need to look at 246 00:12:01.799 --> 00:12:04.240 the particle itself. I just need to watch the single slit. 247 00:12:05.039 --> 00:12:08.159 If the slit recoils down, I know the particle went up. 248 00:12:08.399 --> 00:12:11.000 If the slit recoils up, I know the particle went down. 249 00:12:11.600 --> 00:12:13.080 So he gets the witch path information. 250 00:12:13.200 --> 00:12:16.080 He gets the particle information. Yes, he's using the conservation 251 00:12:16.120 --> 00:12:19.000 of momentum to measure the path indirectly. He called it 252 00:12:19.039 --> 00:12:20.039 a recoil slit. 253 00:12:20.799 --> 00:12:24.120 And the beauty of this trap. The gotcha that Einstein 254 00:12:24.159 --> 00:12:27.159 thought he had was that the slit is just a 255 00:12:27.279 --> 00:12:30.080 dumb mechanical object. Right, It's just a wall on springs 256 00:12:30.159 --> 00:12:33.320 way over here. He argued that measuring the wiggle of 257 00:12:33.360 --> 00:12:35.919 the wall shouldn't bother the particle which is now flying 258 00:12:36.000 --> 00:12:36.600 way over there. 259 00:12:36.720 --> 00:12:37.399 That was his bet. 260 00:12:37.519 --> 00:12:39.960 So the particle should still act like a wave and 261 00:12:40.039 --> 00:12:41.840 create those interference fringes on the screen. 262 00:12:42.039 --> 00:12:44.240 That was his argument. He said, Look, I have path 263 00:12:44.279 --> 00:12:47.600 information from the recoil, and I should still see interference 264 00:12:47.600 --> 00:12:51.840 on the screen. Therefore, I have observed particle and wave simultaneously. 265 00:12:52.000 --> 00:12:57.720 Checkmate or checkmate complimentary is broken. Quantum mechanics is incomplete. 266 00:12:58.240 --> 00:12:59.480 God does not play dice. 267 00:13:00.120 --> 00:13:03.519 It sounds so solid. I mean, from a classical perspective, 268 00:13:03.840 --> 00:13:06.960 it's flawless. If I throw a ball and it pushes 269 00:13:06.960 --> 00:13:10.440 a swing, measuring how the swing moves doesn't magically change 270 00:13:10.480 --> 00:13:11.399 where the ball lands. 271 00:13:12.000 --> 00:13:15.559 That's the intuition Einstein was banking on. He presented this 272 00:13:15.559 --> 00:13:18.039 to Bor at the Salve conference, probably with a bit 273 00:13:18.039 --> 00:13:20.919 of a smirk, thinking he had finally won. He thought 274 00:13:20.960 --> 00:13:21.960 he had found the loophole. 275 00:13:22.120 --> 00:13:26.039 But bor Bor was like the Jedi master of counter arguments. 276 00:13:26.440 --> 00:13:29.399 You didn't just say no. He supposedly spent a sleepless 277 00:13:29.480 --> 00:13:31.080 night on it and came back the next day. 278 00:13:31.279 --> 00:13:34.360 He did, and this rebuttal is legendary. It's one of 279 00:13:34.360 --> 00:13:37.039 the great moments in the history of science. Borr looked 280 00:13:37.039 --> 00:13:38.720 at the setup and said, okay, Ahbert, let's look at 281 00:13:38.720 --> 00:13:39.840 your springs. Let's look at your wall. 282 00:13:39.919 --> 00:13:39.960 No. 283 00:13:40.519 --> 00:13:43.159 Borr pointed out that to know which way the particle went, 284 00:13:43.320 --> 00:13:45.759 you need to measure the recoil the change in momentum 285 00:13:45.799 --> 00:13:48.600 of the slit very very precisely. You need to be 286 00:13:48.679 --> 00:13:51.360 sure that the slit moved down just a tiny bit 287 00:13:51.679 --> 00:13:53.639 and not, you know, just randomly jiggled right. 288 00:13:53.679 --> 00:13:56.080 Otherwise you don't actually know the path. If you can't 289 00:13:56.120 --> 00:13:59.480 measure the kick accurately, the whole experiment is pointless. 290 00:13:59.759 --> 00:14:04.519 But Bore reminded Einstein the slit itself is a physical object. 291 00:14:04.799 --> 00:14:07.639 It is made of atoms. It is therefore subject to 292 00:14:07.639 --> 00:14:09.120 the laws of quantum mechanics too. 293 00:14:09.320 --> 00:14:12.799 He turned the quantum weirdness back on the measuring device itself. 294 00:14:12.960 --> 00:14:15.919 He did, he said, Albert, if you measure the momentum 295 00:14:16.000 --> 00:14:20.279 of the slit with extreme precision, what happens according to 296 00:14:20.320 --> 00:14:22.799 your friend Heisenberg's uncertainty principle. 297 00:14:23.080 --> 00:14:25.919 Oh wait, we're back to the music analogy. If you 298 00:14:26.000 --> 00:14:30.159 know the momentum perfectly, the position becomes uncertain Bingo. 299 00:14:30.559 --> 00:14:33.519 The more precisely you measure the recoil the momentum of 300 00:14:33.559 --> 00:14:35.679 the wall. The less you know about where the wall 301 00:14:35.759 --> 00:14:37.240 actually is in space. 302 00:14:37.120 --> 00:14:41.320 So the slit starts what being fuzzy. 303 00:14:40.879 --> 00:14:43.440 In a quantum sense. Yes, the uncertainty in the slit's 304 00:14:43.480 --> 00:14:46.159 position gets larger and larger. And here's the kill shot. 305 00:14:46.639 --> 00:14:49.600 If the slit's position is uncertain, if it's effectively wiggling 306 00:14:49.639 --> 00:14:53.679 around or smeared in space, that uncertainty gets transferred to 307 00:14:53.679 --> 00:14:55.240 the particle as it passes through. 308 00:14:55.399 --> 00:14:57.120 So if the source the ways is wiggling all over 309 00:14:57.200 --> 00:14:57.919 the place. 310 00:14:57.840 --> 00:15:00.799 The pattern washes out, the peaks and Trotsky all jumbled up. 311 00:15:01.200 --> 00:15:04.440 More calculated that the uncertainty in the slit's position would 312 00:15:04.480 --> 00:15:09.000 constitute a blurring effect exactly large enough to completely wipe 313 00:15:09.000 --> 00:15:10.799 out the interference fringes on the screen. 314 00:15:11.159 --> 00:15:15.159 That is incredible. So Bor's comeback was go ahead, Einstein, 315 00:15:15.799 --> 00:15:19.159 measure the recoil. But the moment you measure it accurately 316 00:15:19.240 --> 00:15:22.120 enough to know the path, the slit itself will jitter 317 00:15:22.240 --> 00:15:24.600 enough to destroy the wave pattern you're looking for. 318 00:15:24.840 --> 00:15:27.360 That's it. You can know the path recoil, but the 319 00:15:27.399 --> 00:15:31.960 cost is the interference pattern. Complimentarity is saved. Nature covers 320 00:15:32.000 --> 00:15:33.240 her tracks it's. 321 00:15:33.080 --> 00:15:35.639 Like nature has a built in privacy setting. You cannot 322 00:15:35.639 --> 00:15:36.000 hack it. 323 00:15:36.360 --> 00:15:38.720 So in nineteen twenty seven, Bor won the debate on 324 00:15:38.759 --> 00:15:42.519 the chalkboard. Einstein conceded the point, though he never really 325 00:15:42.559 --> 00:15:45.519 gave up on his philosophical objection. But physics is an 326 00:15:45.519 --> 00:15:46.759 experimental science, right. 327 00:15:46.919 --> 00:15:49.080 You can argue math all day, but until you build it, 328 00:15:49.120 --> 00:15:49.759 you don't really know. 329 00:15:50.000 --> 00:15:52.039 And that brings us to the present day, or rather 330 00:15:52.440 --> 00:15:56.039 December thirty first, twenty twenty five. Why on earth did 331 00:15:56.039 --> 00:15:58.480 it take a century to actually build this. We've been 332 00:15:58.519 --> 00:16:01.120 to the moon, we have iPhone. Why couldn't we put 333 00:16:01.120 --> 00:16:02.120 a slit on a spring? 334 00:16:02.279 --> 00:16:04.440 It sounds so simple when you say it like that. 335 00:16:05.000 --> 00:16:09.200 Well, it's because putting a tiny slit on momentum sensitive 336 00:16:09.200 --> 00:16:11.960 springs that can feel the kick of a single electron 337 00:16:12.080 --> 00:16:16.399 or photon is hard. Hard is an understatement. Yeah, it's 338 00:16:16.399 --> 00:16:17.720 technologically nightmare. 339 00:16:17.759 --> 00:16:19.799 How small is the kick we're talking about? What's the scale? 340 00:16:20.120 --> 00:16:23.799 Imagine a single mosquito crashing into an aircraft carrier. Do 341 00:16:23.840 --> 00:16:26.879 you think the captain of the carrier feels the ship recoil? 342 00:16:27.200 --> 00:16:29.200 Definitely not, No way, That's. 343 00:16:29.039 --> 00:16:31.639 The scale of the problem. A photon or an electron 344 00:16:31.679 --> 00:16:35.120 has almost zero mass. A mechanical spring made of metal 345 00:16:35.200 --> 00:16:38.919 or even silicon is just astronomically too heavy. It wouldn't 346 00:16:38.960 --> 00:16:41.399 even register the impact. It's lost in the noise. 347 00:16:41.639 --> 00:16:43.679 So how did janway Pan in his team at the 348 00:16:43.759 --> 00:16:46.600 University of Science and Technology of China crack this? They 349 00:16:46.679 --> 00:16:49.519 didn't invent some impossibly tiny metal springs, did. 350 00:16:49.440 --> 00:16:51.879 They No, they did something much more clever. They realized 351 00:16:51.919 --> 00:16:54.440 they needed a substitution. They needed a slit that was 352 00:16:54.519 --> 00:16:56.840 light enough that a single photon could push it around. 353 00:16:56.879 --> 00:16:58.799 What's lighter than a piece of metal foil and. 354 00:16:58.879 --> 00:17:00.519 Adam, a single rubidi atom? 355 00:17:01.559 --> 00:17:03.240 Pause, the slit is an atom? 356 00:17:03.399 --> 00:17:06.240 Yes, this is the brilliance of the modern apparatus. They 357 00:17:06.319 --> 00:17:10.319 used a single rubidium eighty seven atom as the movable slit. 358 00:17:10.640 --> 00:17:13.279 Okay, my brain just stuttered. How does an atom act 359 00:17:13.319 --> 00:17:17.480 as a slit, a slit? As a hole? An atom is? Well, 360 00:17:17.759 --> 00:17:19.359 it's a thing, a ball of matter. 361 00:17:19.599 --> 00:17:22.119 Think of it less as a physical hole and more 362 00:17:22.200 --> 00:17:26.559 as a scattering center, or like a traffic circle for 363 00:17:26.599 --> 00:17:27.200 a photon. 364 00:17:27.559 --> 00:17:28.680 A roundabout right. 365 00:17:28.880 --> 00:17:31.680 The photon comes in and interacts with the atom. In 366 00:17:31.720 --> 00:17:35.160 this particular experimental setup, the atom acts as an ultra 367 00:17:35.240 --> 00:17:39.119 light beam splitter. The photon offentively bounces off it and 368 00:17:39.160 --> 00:17:41.519 can go one of two ways, left or right. Okay, 369 00:17:41.640 --> 00:17:43.960 But because the atom itself is so light and is 370 00:17:44.000 --> 00:17:46.799 free to move, when the photon bounces off it, it 371 00:17:46.920 --> 00:17:49.039 imparts momentum to it. It gives it a kick. 372 00:17:49.160 --> 00:17:52.680 So the atom is the wall on springs. But where 373 00:17:52.720 --> 00:17:55.319 are the springs. You can't just have an atom floating 374 00:17:55.319 --> 00:17:57.400 in a vacuum. It would fly away or fall down 375 00:17:57.440 --> 00:17:58.039 with gravity. 376 00:17:58.160 --> 00:18:01.519 The springs are lasers, These remarkable piece of technology called 377 00:18:01.519 --> 00:18:02.519 an optical tweezer. 378 00:18:02.680 --> 00:18:04.839 I love that term, optical tweezer. It sounds like a 379 00:18:04.839 --> 00:18:05.920 tool doctor who would use. 380 00:18:06.000 --> 00:18:08.319 It's exactly what it sounds like. By focusing laser beams 381 00:18:08.480 --> 00:18:12.119 very very tightly, you can create an electromagnetic potential, well, 382 00:18:12.240 --> 00:18:15.799 an energy trap trap made of light. Yes, the atom 383 00:18:15.960 --> 00:18:18.039 is held right in the center of the beam. If 384 00:18:18.039 --> 00:18:20.160 it tries to move away from the center, the light 385 00:18:20.200 --> 00:18:24.119 itself pushes it back. It behaves mathematically exactly like a 386 00:18:24.160 --> 00:18:27.240 particle in a harmonic potential, which is just the fancy 387 00:18:27.240 --> 00:18:29.720 physics way of saying it's sitting on a spring that. 388 00:18:29.799 --> 00:18:32.400 Is so cool. So we have a single rubidium atom 389 00:18:32.920 --> 00:18:36.079 trapped in a laser beam acting as the slit, and 390 00:18:36.119 --> 00:18:37.599 we're firing a photon at it. 391 00:18:37.640 --> 00:18:40.680 And crucially, they had to cool this atom down, way 392 00:18:40.960 --> 00:18:44.240 way down to its ground state of motion within the trap. 393 00:18:44.480 --> 00:18:45.920 Why does it need to be so cold. 394 00:18:45.839 --> 00:18:48.839 Because temperature is just jiggling, it's random thermal motion. If 395 00:18:48.839 --> 00:18:51.440 the atom is hot, it's already bouncing around wildly inside 396 00:18:51.440 --> 00:18:54.200 the laser trap. You'd never be able to detect the tiny, 397 00:18:54.240 --> 00:18:57.000 tiny kick from the photon against all that background noise. 398 00:18:57.200 --> 00:18:58.759 It'd be like trying to hear a pin drop at 399 00:18:58.759 --> 00:18:59.799 a heavy metal concert. 400 00:19:00.079 --> 00:19:02.599 Perfect analogy. They had to cool it until it was 401 00:19:02.599 --> 00:19:08.480