WEBVTT 1 00:00:00.080 --> 00:00:02.799 I want you to just imagine you're lying on a 2 00:00:02.839 --> 00:00:04.000 gentle hillside at night. 3 00:00:04.519 --> 00:00:06.040 Oh that sounds lovely right now. 4 00:00:05.960 --> 00:00:08.560 Yeah, it really does. The grass is, you know, cool 5 00:00:08.599 --> 00:00:10.919 and soft beneath you, right, and you're just looking up 6 00:00:10.919 --> 00:00:14.400 at this vast, quiet canopy of stars. 7 00:00:14.080 --> 00:00:15.759 Just breathing in that cool night air. 8 00:00:16.039 --> 00:00:20.199 Exactly. I want you to take a slow, deep breath 9 00:00:20.239 --> 00:00:25.160 with us, because tonight this isn't a lecture, no, not 10 00:00:25.239 --> 00:00:25.519 at all. 11 00:00:25.559 --> 00:00:28.839 It's more of a like a slow comforting exhale. 12 00:00:28.920 --> 00:00:32.399 Yeah, quiet exploration. We're just two friends whispering about the 13 00:00:32.479 --> 00:00:34.799 strangeness of existence under the night sky. 14 00:00:35.200 --> 00:00:37.280 And you know, we really want you to just let 15 00:00:37.320 --> 00:00:39.280 go of all those racing thoughts from the day. 16 00:00:39.320 --> 00:00:41.640 Just let them drift away, because for our time together 17 00:00:41.719 --> 00:00:46.880 today I have gathered this gentle stack of notes and sources. 18 00:00:46.479 --> 00:00:48.399 Some really beautiful mysteries in those nights. 19 00:00:48.359 --> 00:00:50.759 You really are We're going to dive into a deep 20 00:00:50.840 --> 00:00:53.920 exploration of a mystery that has just baffled the greatest 21 00:00:53.960 --> 00:00:55.439 minds for over a century. 22 00:00:55.479 --> 00:00:57.840 We're talking about the building blocks of reality itself, right, 23 00:00:57.960 --> 00:01:01.600 we are the very bottom of everything. And what we're 24 00:01:01.600 --> 00:01:04.959 going to ponder tonight is something called the double slit experiment. 25 00:01:04.560 --> 00:01:07.480 Which sounds a bit technical, but it's really just a 26 00:01:07.560 --> 00:01:11.599 story about the strange dual nature of light and matter. 27 00:01:12.040 --> 00:01:15.400 Yeah, and it leads to this really beautiful kind of 28 00:01:15.439 --> 00:01:19.640 haunting question, like is the universe truly there? When no 29 00:01:19.680 --> 00:01:20.879 one is looking right? 30 00:01:21.120 --> 00:01:23.519 And I know, you know, sometimes physics can feel so. 31 00:01:23.560 --> 00:01:28.040 Heavy, oh, absolutely overwhelming, full of dense math and equations. 32 00:01:28.120 --> 00:01:31.760 Yeah exactly. But tonight, for this deep dive, we are 33 00:01:31.799 --> 00:01:33.599 completely leaving the math behind. 34 00:01:33.799 --> 00:01:36.400 That's a relief. Just simple ideas tonight. 35 00:01:36.239 --> 00:01:39.400 Just simple pictures, things you can hold softly in your mind, 36 00:01:39.560 --> 00:01:43.319 like a spinning coin or a pebble dropped in a pond. 37 00:01:43.439 --> 00:01:45.799 I love that using those simple images to understand the 38 00:01:45.799 --> 00:01:46.560 whole universe. 39 00:01:46.760 --> 00:01:49.760 So let's start there. Let's set the baseline of everyday reality. 40 00:01:49.840 --> 00:01:51.439 Let's do it. We're going back to eighteen oh one. 41 00:01:51.480 --> 00:01:54.200 Actually, right, Thomas Young, he did this classic setup. I 42 00:01:54.200 --> 00:01:56.640 want you to picture just a solid wall, a simple 43 00:01:56.799 --> 00:01:59.840 dark wall, and in this wall, someone has cut two 44 00:02:00.000 --> 00:02:04.000 two narrow vertical slits, just two little doors, right side 45 00:02:04.040 --> 00:02:06.920 by side, and behind that wall, a little ways back, 46 00:02:07.000 --> 00:02:10.680 there's a blank screen like canvas resting in the dark. 47 00:02:10.879 --> 00:02:12.479 Okay, I have that picture in my mind. 48 00:02:12.560 --> 00:02:15.199 So imagine standing in front of this wall and you 49 00:02:15.319 --> 00:02:19.840 have like a handful of tiny physical objects, let's say, 50 00:02:19.840 --> 00:02:21.680 tiny pellets or little. 51 00:02:21.439 --> 00:02:24.000 Bullets, just solid everyday things. 52 00:02:23.800 --> 00:02:27.199 Right, and you start firing them one by one towards 53 00:02:27.199 --> 00:02:28.360 the wall with the slicks. 54 00:02:28.520 --> 00:02:31.039 Well, what happens is exactly what you'd expect, right, They 55 00:02:31.080 --> 00:02:31.599 travel in. 56 00:02:31.560 --> 00:02:33.479 Straight lines, right straight through the air. 57 00:02:33.599 --> 00:02:35.719 Yeah, and most of them bounce off the solid wall. 58 00:02:35.960 --> 00:02:38.280 But the ones that go through the two slits, they 59 00:02:38.319 --> 00:02:40.400 just keep going straight until they hit the backscreen. 60 00:02:40.479 --> 00:02:42.840 So you just end up with two distinct bands, two 61 00:02:42.879 --> 00:02:44.520 piles of marks directly behind the. 62 00:02:44.439 --> 00:02:47.759 Little doors, exactly two stripes on the backscreen. Because that 63 00:02:47.879 --> 00:02:52.520 is how solid matter behaves. It's simple, it's predictable. 64 00:02:52.159 --> 00:02:55.280 It makes sense. But now let's change the picture. Let's 65 00:02:55.319 --> 00:02:57.919 take that same wall with the two slits and place 66 00:02:57.919 --> 00:02:58.919 it in a quiet pond. 67 00:02:59.000 --> 00:03:01.319 Well, I like this one, just still dark water. 68 00:03:01.639 --> 00:03:04.039 Yeah, and you drop a pebble in the water. 69 00:03:04.199 --> 00:03:07.439 You create a wave, a gentle ripple, spreading out. 70 00:03:07.439 --> 00:03:11.080 Right, and that wave hits the wall and it passes 71 00:03:11.120 --> 00:03:13.120 through both of those slits at the same time. 72 00:03:13.319 --> 00:03:16.400 And when it does that, it creates two new sets 73 00:03:16.439 --> 00:03:18.159 of ripples on the other side. 74 00:03:17.879 --> 00:03:19.960 Two new waves spreading out from the slits. 75 00:03:20.039 --> 00:03:22.879 Yeah, exactly. And here's the beautiful part. Those two new 76 00:03:22.919 --> 00:03:25.520 sets of ripples they spread out and they start to 77 00:03:25.599 --> 00:03:28.800 overlap in the water. They weave together, they do, and 78 00:03:28.840 --> 00:03:32.199 where two peaks of the waves meet, the water grows taller. 79 00:03:32.919 --> 00:03:34.719 But where a peak meets a valley. 80 00:03:34.520 --> 00:03:36.400 Oh, they cancel out right exactly. 81 00:03:36.439 --> 00:03:37.919 The water goes perfectly flat. There. 82 00:03:38.000 --> 00:03:40.439 So on the back screen or the back wall of 83 00:03:40.479 --> 00:03:43.280 the pond, you don't just get two stripes, No. 84 00:03:43.319 --> 00:03:47.360 You get this beautiful repeating pattern. It's called an interference pattern. 85 00:03:47.039 --> 00:03:51.280 An alternating sequence of like strong waves and completely still water. 86 00:03:51.439 --> 00:03:55.599 Yes, multiple bands spreading across the back. It's a signature 87 00:03:55.639 --> 00:03:56.919 of waves overlapping. 88 00:03:57.080 --> 00:04:00.000 So we have this great divide in our everyday experience. 89 00:04:00.319 --> 00:04:02.919 Things are either solid objects like the tiny. 90 00:04:02.599 --> 00:04:04.840 Bullets, which give you two stripes. 91 00:04:04.520 --> 00:04:08.840 Right, or they are spread out waves like the pawn ripple. 92 00:04:08.599 --> 00:04:10.919 Which give you that beautiful overlapping pattern. 93 00:04:11.120 --> 00:04:15.080 And in our normal world, these things don't mix. A 94 00:04:15.120 --> 00:04:16.199 bullet isn't a ripple. 95 00:04:16.480 --> 00:04:18.600 No bullet is a bullet. A wave is a wave. 96 00:04:18.879 --> 00:04:23.199 So this brings us to this incredibly gentle but baffling question. 97 00:04:23.920 --> 00:04:28.240 What happens if we take the absolute smallest pieces. 98 00:04:27.839 --> 00:04:31.000 Of the universe, like tiny specks of light or pieces 99 00:04:31.000 --> 00:04:31.639 of atoms? 100 00:04:31.800 --> 00:04:34.480 Right, what if we send those through the two slits? 101 00:04:34.839 --> 00:04:37.120 Well, this is where the universe starts to look very, 102 00:04:37.279 --> 00:04:38.480 very strange. 103 00:04:38.040 --> 00:04:39.399 Because scientists did this right. 104 00:04:39.759 --> 00:04:42.680 They fired light at the slits, they did, but to 105 00:04:42.800 --> 00:04:45.879 really understand it, they had to isolate it. They dimmed 106 00:04:45.920 --> 00:04:48.519 the light source down so low that it was only 107 00:04:48.560 --> 00:04:51.199 firing one single tiny speck of light at. 108 00:04:51.120 --> 00:04:54.360 A time, a photon. Right, just one solitary little piece 109 00:04:54.399 --> 00:04:54.920 of reality. 110 00:04:55.040 --> 00:04:57.839 Yes, exactly, just one speck traveling alone in the dark. 111 00:04:57.879 --> 00:05:00.560 But wait, if it's just one single speck traveling completely 112 00:05:00.560 --> 00:05:03.399 on its own, it has to choose, right, you would think. So, 113 00:05:03.560 --> 00:05:05.639 I mean, it has to go through either the left 114 00:05:05.639 --> 00:05:08.439 slit or the right slit, just like a solitary little bullet. 115 00:05:08.480 --> 00:05:09.879 It can't overlap with itself. 116 00:05:10.000 --> 00:05:12.519 That is exactly what common sense tells us. And at first, 117 00:05:12.519 --> 00:05:14.480 when they ran the experiment, it looked like common sense 118 00:05:14.560 --> 00:05:14.839 was right. 119 00:05:14.959 --> 00:05:16.600 Oh really, what did it look like? 120 00:05:16.759 --> 00:05:20.480 Well, a single spike is fired, ping it hits the backscreen, 121 00:05:21.120 --> 00:05:24.000 there's one tiny dot, like a single bullet mark right, 122 00:05:24.040 --> 00:05:26.920 and then another speck is fired, ping it lands somewhere 123 00:05:26.920 --> 00:05:29.279 else do It just looks random, completely random, like a 124 00:05:29.399 --> 00:05:32.639 scatter of dots in the dark. But as they kept 125 00:05:32.680 --> 00:05:36.399 firing them one by one, dot by dot, taking all 126 00:05:36.480 --> 00:05:37.079 the time in the. 127 00:05:37.040 --> 00:05:39.040 World, a pattern starts to emerge. 128 00:05:39.120 --> 00:05:44.199 Yes, a very specific pattern. Slowly, those individual dots build 129 00:05:44.240 --> 00:05:47.639 up into the exact same alternating pattern of waves. 130 00:05:47.360 --> 00:05:49.079 The interference pattern, the pond. 131 00:05:48.920 --> 00:05:52.600 Ripples, exactly the pond ripples made out of individual bullets. 132 00:05:52.639 --> 00:05:56.120 Oh wow, So each individual speck, even though it's traveling 133 00:05:56.279 --> 00:05:59.439 entirely on its own, it's interfering with itself. 134 00:05:59.600 --> 00:06:03.720 That's the staggering implication. For that beautiful alternating pattern to 135 00:06:03.759 --> 00:06:06.560 appear on the back screen, there has to be an overlap. 136 00:06:06.639 --> 00:06:08.199 But there's only one spec I know. 137 00:06:08.839 --> 00:06:11.199 It means that the single particle cannot be traveling as 138 00:06:11.240 --> 00:06:12.120 a solid little deed. 139 00:06:12.279 --> 00:06:14.319 It can't just be going through one door, right. 140 00:06:14.360 --> 00:06:16.560 It must be traveling as a wave of possibilities. It's 141 00:06:16.600 --> 00:06:18.720 moving through both slits at the exact same time. 142 00:06:19.079 --> 00:06:22.959 Let that idea just linger softly for a second. 143 00:06:23.079 --> 00:06:24.319 It's hard to hold in your mind. 144 00:06:24.360 --> 00:06:27.439 It really is a single piece of reality. Spreading out 145 00:06:27.439 --> 00:06:28.480 like a ghost. 146 00:06:28.240 --> 00:06:30.639 Just taking every possible path through the dark. 147 00:06:30.720 --> 00:06:34.560 Yeah, going left and going right simultaneously, until it finally 148 00:06:34.639 --> 00:06:35.360 lands on the screen. 149 00:06:35.480 --> 00:06:38.480 And when it lands it becomes a solid dot again. Yeah, 150 00:06:38.519 --> 00:06:42.680 but while it was traveling, it was everywhere it could be. 151 00:06:42.800 --> 00:06:46.399 It's so beautiful. But also, you know, my mind immediately 152 00:06:46.480 --> 00:06:47.160 wants to solve it. 153 00:06:47.240 --> 00:06:49.399 Oh, of course, we want to know how the trick 154 00:06:49.480 --> 00:06:50.600 is done, right. 155 00:06:50.800 --> 00:06:52.920 I think, well, if it's going through both slits at once, 156 00:06:52.920 --> 00:06:54.240 why don't we just look closely? 157 00:06:54.360 --> 00:06:55.519 Why not just watch it happen. 158 00:06:55.639 --> 00:06:58.160 Yeah, let's just put a tiny little camera or you know, 159 00:06:58.279 --> 00:07:00.560 a detector right next to the doors to see which 160 00:07:00.560 --> 00:07:01.480 way it actually goes. 161 00:07:01.600 --> 00:07:04.279 And that is exactly what the scientists thought too. They 162 00:07:04.319 --> 00:07:05.439 wanted to peek behind. 163 00:07:05.240 --> 00:07:08.160 The curtain, so they added a detector right right at 164 00:07:08.199 --> 00:07:08.720 the slits. 165 00:07:08.839 --> 00:07:11.800 They did. They set up a way to gently watch 166 00:07:11.839 --> 00:07:15.560 and see does this speck go left or does it 167 00:07:15.600 --> 00:07:16.000 go right? 168 00:07:17.000 --> 00:07:17.759 And what happened? 169 00:07:18.000 --> 00:07:20.279 Well, when the detector is turned on, we do see 170 00:07:20.279 --> 00:07:22.879 this back. We see it go through just one slit 171 00:07:23.399 --> 00:07:23.879 or the other. 172 00:07:24.160 --> 00:07:26.680 Okay, so it does choose a door, it does. 173 00:07:26.759 --> 00:07:30.399 When we look, But the moment we look something heartbreaking 174 00:07:30.439 --> 00:07:33.879 happens on the backscreen. The pattern changes, the beautiful wave pattern, 175 00:07:33.920 --> 00:07:36.639 the ripples, it completely disappeared. 176 00:07:36.720 --> 00:07:38.240 Wait, really, it just vanishes. 177 00:07:38.360 --> 00:07:41.879 It vanishes. The dots stop forming the wave pattern. They 178 00:07:41.920 --> 00:07:46.199 collapse into just two plain, boring bands, exactly like the 179 00:07:46.240 --> 00:07:47.000 solid bullets. 180 00:07:47.240 --> 00:07:50.639 Wow. So just the act of watching it, yeah, forces 181 00:07:50.680 --> 00:07:52.319 it to act like a solid object. 182 00:07:52.480 --> 00:07:55.639 It's as if the universe knows we are paying attention. 183 00:07:55.800 --> 00:07:58.959 It's like it's shy. It was doing this beautiful, ghostly 184 00:07:59.120 --> 00:08:02.279 dance through both doors, but the second we look, it 185 00:08:02.439 --> 00:08:05.120 just drops to the floor and pretends it's a regular bullet. 186 00:08:05.199 --> 00:08:07.720 Exactly. The magic vanishes, the mom bites observed. 187 00:08:07.800 --> 00:08:11.920 That is just it's wild. Let's try to picture this 188 00:08:12.000 --> 00:08:15.040 with an analogy, because the physics terms for this can 189 00:08:15.079 --> 00:08:15.839 be so heavy. 190 00:08:15.920 --> 00:08:16.959 Yeah, let's keep it grounded. 191 00:08:17.199 --> 00:08:20.120 In the notes, there's this concept called quantum superposition. But 192 00:08:20.240 --> 00:08:21.800 let's just imagine a spinning coin. 193 00:08:22.199 --> 00:08:24.879 A coin spinning rapidly on a tabletop. 194 00:08:25.040 --> 00:08:28.160 Right, imagine looking down at it while it's spinning. It's 195 00:08:28.199 --> 00:08:29.319 just a blur, isn't it. 196 00:08:29.639 --> 00:08:31.000 Yeah, it's just a hum of motion. 197 00:08:31.160 --> 00:08:34.679 It's not heads and it's not tails. It's a beautiful 198 00:08:34.720 --> 00:08:36.840 mixture of both possibilities at once. 199 00:08:36.960 --> 00:08:40.159 I love that the unobserved particle is exactly like that 200 00:08:40.200 --> 00:08:45.960 spinning coin. It exists in this fluid state of pure possibility, its. 201 00:08:45.840 --> 00:08:48.279 Heads and tails, left door and right door, all at 202 00:08:48.279 --> 00:08:48.840 the same. 203 00:08:48.639 --> 00:08:51.240 Time, right until we decide to look. 204 00:08:51.320 --> 00:08:54.360 And looking looking at the particle with our little camera 205 00:08:54.480 --> 00:08:56.960 or detector, that's like slamming your hand down on the 206 00:08:57.000 --> 00:08:57.600 spinning cord. 207 00:08:57.679 --> 00:08:59.799 No, that's exactly what it's like. You slam your hand. 208 00:08:59.600 --> 00:09:03.080 Out stops instantly. The soft fluid wave is gone. 209 00:09:03.120 --> 00:09:05.759 The beautiveruse is forced to make a choice, heads or tails, 210 00:09:06.399 --> 00:09:07.639 left slit or right slit. 211 00:09:07.960 --> 00:09:10.600 Just the act of touching it, of measuring it, collapses 212 00:09:10.600 --> 00:09:14.200 all those beautiful possibilities into one single, cold fact. 213 00:09:14.320 --> 00:09:16.440 The gentle wave is gone, just because we asked it 214 00:09:16.440 --> 00:09:17.440 for a definitive answer. 215 00:09:17.639 --> 00:09:19.759 It's so wild to think about. It brings up these 216 00:09:19.759 --> 00:09:23.399 massive philosophical questions, and the notes here talk about these 217 00:09:23.440 --> 00:09:26.039 philosophers of the quantum world back in the nineteen twenties. 218 00:09:26.240 --> 00:09:30.440 Ah, Yes, Neils Bohorr and Werner Heisenberg, right, And. 219 00:09:30.399 --> 00:09:33.120 They came up with something called the copen Hagen interpretation, 220 00:09:33.360 --> 00:09:34.720 which sounds very. 221 00:09:34.559 --> 00:09:36.320 Formal, very academic Yeah. 222 00:09:36.080 --> 00:09:38.200 But really, we can just picture them as a group 223 00:09:38.240 --> 00:09:41.360 of physicists who dared to ask a very poetic question. 224 00:09:41.559 --> 00:09:42.200 Would they ask? 225 00:09:42.519 --> 00:09:44.759 They dared to ask if the moon is there when 226 00:09:44.799 --> 00:09:45.600 nobody looks? 227 00:09:45.840 --> 00:09:48.519 That is such a profound way to put it, because 228 00:09:48.519 --> 00:09:51.759 what they were really suggesting is that reality, at its deepest, 229 00:09:51.879 --> 00:09:55.519 quietest level isn't made of solid things just waiting around 230 00:09:55.519 --> 00:09:56.120 to be found. 231 00:09:56.759 --> 00:09:58.919 So if it's not solid things, what is it made of? 232 00:09:59.240 --> 00:10:01.840 It's made a problem. It's just a blur of maybes, 233 00:10:02.080 --> 00:10:06.360 like the spinning coin exactly. They argue that reality itself 234 00:10:06.440 --> 00:10:08.960 does not take a definite shape until it interacts with 235 00:10:09.000 --> 00:10:12.120 something else, until the hand slams down. 236 00:10:12.320 --> 00:10:14.480 But wait, I have to ask, because this is where 237 00:10:14.519 --> 00:10:16.440 my mind goes, and I think a lot of people's 238 00:10:16.440 --> 00:10:17.159 minds go here. 239 00:10:17.440 --> 00:10:17.879 Where's that? 240 00:10:18.399 --> 00:10:20.159 Well? Does that mean the universe needs us? 241 00:10:20.879 --> 00:10:22.600 Ah? I see what you're saying, Like. 242 00:10:22.679 --> 00:10:25.399 Does it need a human brain or a conscious mind 243 00:10:25.559 --> 00:10:28.080 to look at it to make it real? If I 244 00:10:28.159 --> 00:10:31.039 close my eyes, does the room I'm in just dissolve 245 00:10:31.080 --> 00:10:32.639 into a wave of probability? 246 00:10:32.960 --> 00:10:35.799 It's such a common thought, and it's a very romantic idea, 247 00:10:35.879 --> 00:10:36.320 isn't it. 248 00:10:36.320 --> 00:10:37.559 It is it centers us? 249 00:10:37.600 --> 00:10:41.039 But it's also maybe just a little bit arrogant of 250 00:10:41.120 --> 00:10:42.679 us to think we're that important. 251 00:10:42.799 --> 00:10:43.559 Yeah, that's fair. 252 00:10:44.120 --> 00:10:47.320 Physics actually has a really gentle grounding correction for this. 253 00:10:48.200 --> 00:10:50.759 There's a famous joke by the physicist John Bell. 254 00:10:51.080 --> 00:10:52.159 Oh what did he say? 255 00:10:52.240 --> 00:10:56.559 He was mocking this idea that consciousness is required. He asked, 256 00:10:57.200 --> 00:10:59.799 did the wave wait billions of years for a single 257 00:11:00.039 --> 00:11:02.840 celled creature to appear? Or did it wait for a 258 00:11:02.919 --> 00:11:05.120 highly qualified measure with a PhD? 259 00:11:05.799 --> 00:11:08.799 That's brilliant? Did the universe stay blurry until someone with 260 00:11:08.840 --> 00:11:09.879 a degree looked at it? 261 00:11:10.080 --> 00:11:12.879 Right? It's funny, but it makes a deep point. We 262 00:11:12.960 --> 00:11:14.679 need to redefine what looking means. 263 00:11:14.919 --> 00:11:17.080 Okay, so what does it mean to look at a particle? 264 00:11:17.360 --> 00:11:20.639 Looking just means a physical interaction. It doesn't require eyes 265 00:11:20.759 --> 00:11:21.600 or a mind. 266 00:11:21.360 --> 00:11:22.840 So a camera counts, yes. 267 00:11:23.159 --> 00:11:25.759 Or even just a speck of light hitting a photographic plate. 268 00:11:26.360 --> 00:11:27.159 That plate is an. 269 00:11:27.039 --> 00:11:30.039 Observer, or a rocketting another rock exactly. 270 00:11:30.600 --> 00:11:33.799 The universe doesn't need our human eyes to exist. You 271 00:11:33.960 --> 00:11:35.799 just need pieces of itself to. 272 00:11:35.600 --> 00:11:36.759 Touch, to interact. 273 00:11:36.960 --> 00:11:41.120 Yes, any physical interaction is a measurement. That's what slams 274 00:11:41.120 --> 00:11:43.799 the hand down on the spinning coin. The universe is 275 00:11:43.919 --> 00:11:47.639 constantly measuring itself, forcing itself to be real even when 276 00:11:47.639 --> 00:11:48.399 we aren't around. 277 00:11:49.440 --> 00:11:52.240 That is a comfort, actually, to know the universe can 278 00:11:52.279 --> 00:11:53.159 hold itself together. 279 00:11:53.399 --> 00:11:55.480 It really is. We're just part of it. We aren't 280 00:11:55.480 --> 00:11:56.639 holding it up with our minds. 281 00:11:56.639 --> 00:11:59.960 So reality is solidifying all on its own through these 282 00:12:00.080 --> 00:12:03.639 tiny interactions. Yes, but what about time, because our notes 283 00:12:03.679 --> 00:12:05.799 here take a really strange turn into time. 284 00:12:05.879 --> 00:12:07.919 Oh, we are entering the territory of John Wheeler. 285 00:12:07.960 --> 00:12:11.320 Now right. This is from nineteen seventy eight. Wheeler had 286 00:12:11.320 --> 00:12:14.200 this haunting thought experiment. We talked about how the particle 287 00:12:14.279 --> 00:12:16.200 acts like a wave if we don't look, and acts 288 00:12:16.200 --> 00:12:19.240 on a bullet if we do look. Right, Well, Wheeler said, what. 289 00:12:19.279 --> 00:12:21.000 If we wait wait for what? 290 00:12:21.000 --> 00:12:23.159 What if we let the particle pass through the two 291 00:12:23.200 --> 00:12:26.799 slits in the dark and we don't decide if we 292 00:12:26.840 --> 00:12:29.120 want to look at it until after it has already 293 00:12:29.120 --> 00:12:29.679 gone through. 294 00:12:30.159 --> 00:12:32.759 Oh, the delayed choice experiment. This is where it gets 295 00:12:32.840 --> 00:12:33.840 really mesmerizing. 296 00:12:34.080 --> 00:12:35.799 It's like playing a trick on reality. 297 00:12:36.120 --> 00:12:39.480 It really is. Imagine a particle coming from a distant star, 298 00:12:40.360 --> 00:12:43.080 let's say millions of light years away. 299 00:12:43.200 --> 00:12:46.240 Okay, a tiny speck of starlight traveling through the cold 300 00:12:46.320 --> 00:12:47.919 dark for millions of years. 301 00:12:48.039 --> 00:12:51.480 Yes, And on its way to Earth, it passes around 302 00:12:51.519 --> 00:12:52.960 a massive galaxy, so. 303 00:12:52.919 --> 00:12:55.080 It could go around the left side of the galaxy 304 00:12:55.200 --> 00:12:55.399 or the. 305 00:12:55.440 --> 00:12:57.759 Right side exactly. The galaxy is like the wall with 306 00:12:57.799 --> 00:13:00.919 the two slits. The starlight is the particle. 307 00:13:00.679 --> 00:13:03.240 And it has been traveling for millions of years. It 308 00:13:03.240 --> 00:13:06.159 already passed the galaxy a long, long time ago. 309 00:13:06.120 --> 00:13:09.960 Right, and it finally reaches us here on Earth tonight. Okay, 310 00:13:10.120 --> 00:13:13.679 And we decide today, right now in our observatory whether 311 00:13:13.720 --> 00:13:15.799 we want to measure it as a wave looking for 312 00:13:15.840 --> 00:13:19.000 the pond ripples, or measure it as a particle, the 313 00:13:19.080 --> 00:13:19.799 solid bullet. 314 00:13:19.960 --> 00:13:23.000 But wait, if it has already been traveling for millions 315 00:13:23.039 --> 00:13:26.840 of years, how can a choice we make tonight on 316 00:13:26.919 --> 00:13:30.000 Earth change how it behaved millions of years ago when 317 00:13:30.039 --> 00:13:31.159 it passed that galaxy. 318 00:13:31.240 --> 00:13:33.159 It feels like it should be possible. It feels like 319 00:13:33.200 --> 00:13:34.559 we're reaching back into the past. 320 00:13:34.840 --> 00:13:36.919 Right if we look for a wave today, does the 321 00:13:37.039 --> 00:13:39.000 universe go back in time and make sure it went 322 00:13:39.039 --> 00:13:40.960 around both sides of the galaxy. 323 00:13:40.720 --> 00:13:42.799 For a long time. This was just a quiet thought 324 00:13:42.799 --> 00:13:45.919 in Wheeler's head, But recently we've actually been able to 325 00:13:45.960 --> 00:13:47.159 test this in modern labs. 326 00:13:47.480 --> 00:13:49.759 This is the part that just absolutely blows my mind, 327 00:13:49.759 --> 00:13:51.960 the fact that we can test this. We have the 328 00:13:52.080 --> 00:13:56.480 twenty fifteen experiment in Australia at the Australian National University, Yes. 329 00:13:56.360 --> 00:13:59.039 The a and U experiment, And they didn't just use 330 00:13:59.080 --> 00:14:02.759 a speck of light, used an actual atom, a single 331 00:14:03.080 --> 00:14:05.559 relatively massive helium atom. 332 00:14:05.399 --> 00:14:09.240 So a real physical piece of matter exactly, and they 333 00:14:09.320 --> 00:14:12.440 dropped it through crossroads of lasers. The lasers act like 334 00:14:12.480 --> 00:14:13.480 the two slits. 335 00:14:13.200 --> 00:14:16.399 Right, yes, exactly, they split the atom's possibilities, and then 336 00:14:16.399 --> 00:14:19.440 at the very last microsecond, while the atom is already falling, 337 00:14:19.879 --> 00:14:20.679 already past. 338 00:14:20.519 --> 00:14:22.639 The crossroad, the machine makes a choice. 339 00:14:22.320 --> 00:14:26.000 A random number generator. It decides at the absolute last 340 00:14:26.039 --> 00:14:27.480 instant whether to look or not. 341 00:14:27.559 --> 00:14:29.919 Whether to slam the hand down on the coin or 342 00:14:30.039 --> 00:14:31.559 let it stay ablur exactly. 343 00:14:31.799 --> 00:14:34.519 And then ten years later, in twenty twenty five, at 344 00:14:34.639 --> 00:14:38.440 MIT scientists did the exact same thing, but with ultra 345 00:14:38.480 --> 00:14:41.200 cold atoms trapped in this beautiful cage of light called 346 00:14:41.200 --> 00:14:42.639 an optical lattice. 347 00:14:42.440 --> 00:14:44.600 Just floating there in the dark, and they tested the 348 00:14:44.600 --> 00:14:47.200 same delayed choice they did, and what did they find 349 00:14:48.159 --> 00:14:52.320 in both places? Australia and Mit. What happens when we 350 00:14:52.399 --> 00:14:54.080 make the choice at the last second. 351 00:14:54.879 --> 00:14:58.000 The result is always the same, and it is breathtaking. 352 00:14:59.000 --> 00:15:01.360 Even if the choice is made the very last second 353 00:15:01.440 --> 00:15:05.120 after the particle has crossed the slits, the particle perfectly 354 00:15:05.200 --> 00:15:06.399 matches our choice. 355 00:15:06.480 --> 00:15:08.360 It matches it perfectly perfectly. 356 00:15:08.559 --> 00:15:12.080 If the random machine decides to look, the particle lands 357 00:15:12.120 --> 00:15:14.840 as a solid bullet, it acts like it only ever 358 00:15:14.879 --> 00:15:15.679 went through one door. 359 00:15:15.879 --> 00:15:17.960 But if the machine decides not to look. 360 00:15:17.960 --> 00:15:21.159 Then the particle lands as part of a beautiful wave pattern. 361 00:15:21.480 --> 00:15:23.559 It acts like it went through both doors. 362 00:15:23.720 --> 00:15:25.879 Even though the choice was made after it already went 363 00:15:25.879 --> 00:15:28.480 through the doors. Yes, how I mean, how do we 364 00:15:28.559 --> 00:15:30.679 make sense of that without breaking our brains? Are we 365 00:15:31.039 --> 00:15:31.919 changing the past? 366 00:15:32.200 --> 00:15:34.159 No, and that's the gentlest way to understand it. We 367 00:15:34.200 --> 00:15:35.159 aren't changing the past. 368 00:15:35.399 --> 00:15:37.200 Okay, so what is happening? 369 00:15:37.279 --> 00:15:40.120 It's just that the history of the particle simply wasn't written, 370 00:15:40.639 --> 00:15:42.720 not until the very moment we measured it. 371 00:15:42.720 --> 00:15:44.759 It didn't have a past exactly. 372 00:15:44.840 --> 00:15:48.559 The spinning coin was just spinning, floating in time. It 373 00:15:48.600 --> 00:15:50.759 wasn't heads and it wasn't tails, and it didn't have 374 00:15:50.799 --> 00:15:53.840 a solid history until the hand came down. 375 00:15:54.000 --> 00:15:57.559 Wow, it just hovered impossibility. 376 00:15:56.840 --> 00:16:00.879 It did. The universe leaves things undefined until they absolutely 377 00:16:00.919 --> 00:16:03.879 have to be defined. It's incredibly efficient in a way. 378 00:16:04.039 --> 00:16:07.639 It's beautiful. It's like the universe is a blank canvas 379 00:16:07.759 --> 00:16:10.159 and it only paints in the details right where you 380 00:16:10.200 --> 00:16:10.519 touch it. 381 00:16:10.639 --> 00:16:11.799 Yes, exactly that. 382 00:16:12.000 --> 00:16:14.480 I want to gently bring you, the listener, back to 383 00:16:14.519 --> 00:16:16.080 that hillside we started. 384 00:16:15.720 --> 00:16:18.000 On, just feeling the soft grass again. 385 00:16:17.759 --> 00:16:20.320 Looking up at those stars. How does it feel to 386 00:16:20.440 --> 00:16:22.440 know that you are part of a universe that is 387 00:16:22.519 --> 00:16:23.840 so fluid. 388 00:16:23.799 --> 00:16:24.600 So responsive. 389 00:16:24.840 --> 00:16:28.799 Yeah, so wonderfully undefined until it touches you, until you 390 00:16:28.840 --> 00:16:29.679 interact with it. 391 00:16:29.679 --> 00:16:31.720 It makes you feel very connected to it all, doesn't it. 392 00:16:31.720 --> 00:16:34.639 It really does. I want you to take another deep, 393 00:16:35.399 --> 00:16:36.559 slow breath. 394 00:16:36.799 --> 00:16:38.159 Just let the air out slowly. 395 00:16:38.320 --> 00:16:40.240 There is no need to figure it all out tonight. 396 00:16:40.360 --> 00:16:43.320 The universe is just a soft, spinning blur of. 397 00:16:43.240 --> 00:16:45.840 Possibilities, and right now all you need to do is rest. 398 00:16:46.120 --> 00:16:47.639 You don't have to observe anything. 399 00:16:47.879 --> 00:16:50.960 You can just let the waves wash over you. But 400 00:16:51.159 --> 00:16:56.320 before we drift off completely, there's one final quiet thought 401 00:16:56.360 --> 00:16:57.320