WEBVTT 1 00:00:03.439 --> 00:00:07.719 Welcome to Bedtime Astronomy. Explore the wonders of the cosmos 2 00:00:07.759 --> 00:00:12.279 with our soothing Bedtime Astronomie podcast. Each episode offers a 3 00:00:12.359 --> 00:00:16.320 gentle journey through the stars, planets, and beyond, perfect for 4 00:00:16.399 --> 00:00:20.239 unwinding after a long day. Let's travel through the mysteries 5 00:00:20.239 --> 00:00:22.440 of the universe as you drift off into a peaceful 6 00:00:22.480 --> 00:00:23.800 slumber under the night sky. 7 00:00:27.000 --> 00:00:31.000 When you close your eyes and picture space exploration, what 8 00:00:31.160 --> 00:00:34.240 is the very first image that pops into your head? 9 00:00:34.399 --> 00:00:37.799 Oh, it is definitely something blindingly bright, you know. 10 00:00:38.000 --> 00:00:41.240 Right, if you are like most of us, it is 11 00:00:41.320 --> 00:00:44.920 always this extreme illumination. You see those gleaming white space 12 00:00:44.920 --> 00:00:48.600 suits mm hmm, like the Apollo astronauts bouncing around in 13 00:00:48.600 --> 00:00:50.719 the harsh, unfiltered glare of the sun. 14 00:00:50.880 --> 00:00:53.119 Or you picture the gold foil of a lunar lander, 15 00:00:53.240 --> 00:00:56.079 just you know, catching that intense solar radiation against the 16 00:00:56.119 --> 00:00:56.600 black sky. 17 00:00:56.759 --> 00:01:01.079 Exactly. Our entire visual vocabulary for exploring the cosmos is 18 00:01:01.079 --> 00:01:05.519 defined by extreme almost violent light. I mean light is power. 19 00:01:05.640 --> 00:01:06.879 Yeah, light is visibility. 20 00:01:07.000 --> 00:01:09.680 It's how we survive in an environment that is constantly 21 00:01:09.719 --> 00:01:10.439 trying to kill us. 22 00:01:10.560 --> 00:01:13.640 It makes complete sense from a biological standpoint and honestly 23 00:01:13.680 --> 00:01:16.359 from an engineering one too. We rely on the sun 24 00:01:16.480 --> 00:01:19.159 to charge the batteries on our solar panels. Oh yeah, 25 00:01:19.200 --> 00:01:23.560 we need bright thruster plumes to visibly confirm trajectory corrections 26 00:01:23.560 --> 00:01:26.280 in the void. And frankly, we just need the heat 27 00:01:26.319 --> 00:01:29.120 of that light to keep our instruments from freezing solid. 28 00:01:29.400 --> 00:01:32.599 Yeah. Our instinct is a human species is basically to 29 00:01:32.640 --> 00:01:35.400 follow the light. We seek it out and we build 30 00:01:35.400 --> 00:01:37.640 our entire exploratory architecture around it. 31 00:01:37.719 --> 00:01:38.120 We do. 32 00:01:38.280 --> 00:01:41.040 But then you travel to the very bottom of the Moon. 33 00:01:41.319 --> 00:01:43.400 If you look at the lunar South Pole and you 34 00:01:43.560 --> 00:01:46.319 hike to the edge of one of these massive ancient 35 00:01:46.519 --> 00:01:49.599 impact craters, places like Shackleton craters. 36 00:01:49.640 --> 00:01:51.200 Oh yeah, those are fascinating. 37 00:01:51.319 --> 00:01:54.439 You look down over the rim and suddenly that light 38 00:01:54.560 --> 00:01:56.079 is gone completely. 39 00:01:56.200 --> 00:01:56.840 It really is. 40 00:01:57.000 --> 00:01:59.000 We are talking about regions of the Moon that have 41 00:01:59.200 --> 00:02:03.560 literally not seen a single stray photon of direct sunlight 42 00:02:03.879 --> 00:02:05.120 for billions of years. 43 00:02:05.319 --> 00:02:06.640 It's incredible to think about. 44 00:02:06.719 --> 00:02:09.000 It's not just dark in the way a room gets 45 00:02:09.080 --> 00:02:12.919 dark when you turn off a lamp. It is a permanent, freezing, 46 00:02:13.120 --> 00:02:14.120 absolute shadow. 47 00:02:14.240 --> 00:02:16.319 It is the ultimate void. If you were to drop 48 00:02:16.360 --> 00:02:18.439 down into one of those crators. You are entering an 49 00:02:18.520 --> 00:02:23.400 environment that is incredibly hostile to the kind of exploratory hardware. 50 00:02:23.039 --> 00:02:24.719 We usually build because of the cold. 51 00:02:24.919 --> 00:02:28.319 Yeah, the temperatures plunge to levels that shatter conventional metals. 52 00:02:28.400 --> 00:02:30.759 There is no solar power to be harvested down there. 53 00:02:30.800 --> 00:02:33.520 It is, by almost every traditional metric we use in 54 00:02:33.560 --> 00:02:36.400 aerospace engineering, a totally dead zone. 55 00:02:36.560 --> 00:02:39.879 And yet here is the wild twist at the heart 56 00:02:39.879 --> 00:02:45.360 of our conversation today, that horrifying pitch black nothingness. Yeah, 57 00:02:45.840 --> 00:02:50.280 it turns out to be the absolute perfect environment, arguably 58 00:02:50.280 --> 00:02:53.280 the most valuable real estate in the entire Solar system 59 00:02:53.800 --> 00:02:57.280 for humanity's most precise, cutting edge technology. 60 00:02:57.360 --> 00:02:59.400 It really is a massive paradigm shift. 61 00:02:59.400 --> 00:03:03.400 We are literally planning to plunge into the darkest, coldest 62 00:03:03.439 --> 00:03:07.240 abyss imaginable just to build a machine of perfect. 63 00:03:07.000 --> 00:03:09.680 Light, which is such a profound inversion of how we 64 00:03:09.719 --> 00:03:10.960 normally operate in space. 65 00:03:11.039 --> 00:03:11.280 I know. 66 00:03:11.439 --> 00:03:14.199 And this isn't just some speculative science fiction concept either. 67 00:03:14.639 --> 00:03:18.080 What we are exploring today is a highly practical, rigorously 68 00:03:18.199 --> 00:03:23.080 detailed blueprint for the actual future of lunar exploration. Physicists 69 00:03:23.120 --> 00:03:25.599 have realized that the very things that make these craters 70 00:03:25.599 --> 00:03:28.960 seem so terrifying and inhospitable, you know, the absolute cold, 71 00:03:29.000 --> 00:03:32.599 the profound emptiness. Right, those are exactly the conditions required 72 00:03:32.639 --> 00:03:36.560 to stabilize a piece of quantum technology called an ultrastable laser. 73 00:03:36.639 --> 00:03:41.080 An ultrastable laser exactly, And by placing this specific instrument 74 00:03:41.199 --> 00:03:45.319 in this specific dark place, we fundamentally change how we 75 00:03:45.400 --> 00:03:48.479 navigate through space, how we measure time, and even how 76 00:03:48.520 --> 00:03:49.960 we listen to the universe itself. 77 00:03:50.080 --> 00:03:52.479 Okay, I am so ready to get into this. Yeah, 78 00:03:52.479 --> 00:03:55.000 But if I am sitting here listening to this and 79 00:03:55.080 --> 00:03:59.199 picturing a perfectly dark, perfectly cold crater, the very first 80 00:03:59.199 --> 00:04:01.599 thing I think of is a vacuum of space. Sure, 81 00:04:01.680 --> 00:04:03.400 and we know space is already a vacuum, so I 82 00:04:03.479 --> 00:04:04.960 had to push back right out of the gate, so 83 00:04:05.039 --> 00:04:08.080 for it. If the entirety of outer space is already 84 00:04:08.120 --> 00:04:12.599 this giant empty vacuum, why are physicists so obsessed with 85 00:04:12.680 --> 00:04:17.160 the extra stillness of these specific lunar craters. What makes 86 00:04:17.240 --> 00:04:19.959 the vacuum inside a dark hole on the Moon any 87 00:04:20.000 --> 00:04:22.720 better than the vacuum floating halfway between Earth and Mars. 88 00:04:22.879 --> 00:04:25.800 Well, we have to define what stillness actually means. When 89 00:04:25.800 --> 00:04:28.680 we step into the realm of high precision physics, because 90 00:04:28.720 --> 00:04:31.079 when we casually throw on the word vacuum, we usually 91 00:04:31.120 --> 00:04:32.399 just mean an environment with no. 92 00:04:32.519 --> 00:04:33.920 Air, right, like an empty void. 93 00:04:34.199 --> 00:04:37.759 But the physical world, especially at a microscopic level, is 94 00:04:37.800 --> 00:04:42.000 incredibly chaotic everywhere, even in the so called vacuum of 95 00:04:42.040 --> 00:04:45.399 deep space. Space is not perfectly empty, and it is 96 00:04:45.480 --> 00:04:47.000 certainly not perfectly cold. 97 00:04:47.160 --> 00:04:47.519 Really. 98 00:04:47.639 --> 00:04:50.360 No, When a satellite is orbiting the Earth or drifting 99 00:04:50.399 --> 00:04:54.160 between planets, it is constantly being bombarded. 100 00:04:53.839 --> 00:04:57.120 Bombarded by what like micro meteorites. 101 00:04:56.639 --> 00:04:59.120 Sometimes, yeah, but I'm talking about things much smaller and 102 00:04:59.199 --> 00:05:03.120 much more pervasive. Solar radiation, the solar wind, which is 103 00:05:03.160 --> 00:05:06.360 basically a stream of charged particles spewed out by the 104 00:05:06.399 --> 00:05:11.040 sun cosmic rays, and most importantly, fluctuating thermal energy. A 105 00:05:11.079 --> 00:05:14.120 satellite in space heats up dramatically when it's facing the Sun, 106 00:05:14.480 --> 00:05:16.959 and then it cools down rapidly when it passes into 107 00:05:17.040 --> 00:05:18.120 a planetary shadow. 108 00:05:18.439 --> 00:05:19.959 The temperature swings are wild. 109 00:05:19.800 --> 00:05:23.399 Exactly, and the materials on that satellite, the aluminum, the titanium, 110 00:05:23.399 --> 00:05:26.920 the circuitry, they all expand and contract with those temperature swings. 111 00:05:27.279 --> 00:05:30.720 This creates microvibrations throughout the entire physical structure. 112 00:05:30.920 --> 00:05:34.120 So even though there's no physical wind blowing against the satellite, 113 00:05:34.240 --> 00:05:37.240 the fluctuating key to the Sun is basically beating it up. 114 00:05:37.240 --> 00:05:39.279 On a microscopic level. It's jittering. 115 00:05:39.560 --> 00:05:42.439 That is a very good way to visualize it. Heat 116 00:05:42.560 --> 00:05:46.120 is ultimately just atomic motion. It's kinetic energy. If something 117 00:05:46.199 --> 00:05:51.319 is warm, its constituent atoms are vibrating violently. And if 118 00:05:51.360 --> 00:05:54.279 you are trying to build an instrument that relies on 119 00:05:54.439 --> 00:05:58.160 absolute perfect precision measuring things down to the width of 120 00:05:58.160 --> 00:06:01.639 an atom, even a fraction of a degree of heat 121 00:06:02.040 --> 00:06:06.319 creates a violent, jittery disruption. Now look at the Moon. Generally, 122 00:06:06.720 --> 00:06:10.120 the Moon is fantastic for certain types of science because 123 00:06:10.160 --> 00:06:12.120 it doesn't have the seismic activity of. 124 00:06:12.120 --> 00:06:14.360 Earth because there are no tectonic plates. 125 00:06:14.079 --> 00:06:16.959 Exactly, no tectonic plates crashing together. There are no oceans 126 00:06:17.000 --> 00:06:20.560 pounding against coastlines, no semi trucks driving down the highway 127 00:06:20.560 --> 00:06:21.600 to create ground noise. 128 00:06:21.720 --> 00:06:23.120 Yeah, the Earth is super noisy. 129 00:06:23.199 --> 00:06:25.560 It is. The Moon also lacks an atmosphere, so there 130 00:06:25.600 --> 00:06:28.480 are no wind or sound waves, which are massive disruptors 131 00:06:28.480 --> 00:06:32.000 of precision instruments. But the sunlit lunar surface still experiences 132 00:06:32.040 --> 00:06:33.519 brutal temperature swings. 133 00:06:33.279 --> 00:06:35.040 Right, It goes from something like two hundred and fifty 134 00:06:35.040 --> 00:06:38.160 degrees fahrenheit in the direct sunlight down to negative two 135 00:06:38.240 --> 00:06:40.879 hundred degrees in the lunari yet does a massive thermal 136 00:06:40.920 --> 00:06:41.639 shock every two. 137 00:06:41.560 --> 00:06:46.240 Weeks precisely that thermal cycling would completely destroy the alignment 138 00:06:46.240 --> 00:06:50.439 of an ultra stable laser. But step down into those 139 00:06:50.480 --> 00:06:52.959 permanently shadowed craters at the South. 140 00:06:52.759 --> 00:06:55.079 Pole where the sun never shines right. 141 00:06:55.439 --> 00:06:57.720 Because the sun never crests the rim to hit the 142 00:06:57.720 --> 00:07:02.160 crater floor, the ambient temperature there rests steadily around fifty 143 00:07:02.199 --> 00:07:05.240 degrees above absolute zero. That is fifty kelvin. 144 00:07:05.360 --> 00:07:08.680 Wow, And for context, absolute zero zero kelvin is the 145 00:07:08.680 --> 00:07:12.360 theoretical limit where all atomic motion essentially stops entirely right. 146 00:07:12.480 --> 00:07:16.519 Yes, so at fifty kelvin, the random microscopic jitter of 147 00:07:16.560 --> 00:07:20.759 physical materials is drastically reduced. You are entering a realm 148 00:07:20.759 --> 00:07:25.040 of thermodynamic stillness that is incredibly difficult and very expensive 149 00:07:25.120 --> 00:07:26.959 to achieve artificially on Earth. 150 00:07:27.040 --> 00:07:27.800 I can imagine. 151 00:07:27.800 --> 00:07:31.279 Furthermore, the vacuum inside these specific craters is actually of 152 00:07:31.319 --> 00:07:33.639 a higher quality than the general lunar surface. 153 00:07:33.720 --> 00:07:35.720 Okay, wait, how can a vacuum be of a higher quality. 154 00:07:35.759 --> 00:07:37.759 It's on the same moon, doesn't the whole Moon share 155 00:07:37.800 --> 00:07:39.600 the exact same lack of atmosphere. 156 00:07:39.720 --> 00:07:42.600 It does, but the general lunar surface actually suffers from 157 00:07:42.600 --> 00:07:44.160 a very faint exosphere. 158 00:07:44.240 --> 00:07:45.199 Ooh yeah. 159 00:07:45.759 --> 00:07:48.959 Solar radiation hits the lunar dirt and causes a tiny 160 00:07:48.959 --> 00:07:52.519 bit of outgassing. It releases trace amounts of helium, ar 161 00:07:52.600 --> 00:07:56.519 gone and other particles. Plus the solar wind is constantly 162 00:07:56.560 --> 00:07:58.240 impacting the day side, so it's a. 163 00:07:58.240 --> 00:08:01.160 Bit dusty and gassy on a microscope level exactly. 164 00:08:01.800 --> 00:08:06.079 But inside the deep, permanently shadowed craters, the walls physically 165 00:08:06.120 --> 00:08:10.079 block the solar wind, and because it is so profoundly cold, 166 00:08:10.600 --> 00:08:13.439 any stray gas molecule that happens to wander into the 167 00:08:13.439 --> 00:08:17.120 crater instantly freezes solid and sticks to the crater floor. 168 00:08:17.360 --> 00:08:20.199 Oh wow, it's called a cold trap. So the environment 169 00:08:20.240 --> 00:08:23.759 inside the crater acts like a natural ultra high vacuum pumpy, 170 00:08:24.000 --> 00:08:28.199 just continuously scrubbing its own space empty. It effectively eliminates 171 00:08:28.199 --> 00:08:32.080 any stray acoustic vibrations and drastically reduces the rogue particles 172 00:08:32.080 --> 00:08:33.720 that could bump into delicate instruments. 173 00:08:33.759 --> 00:08:36.320 It's almost a paradox. As humans, we usually look for 174 00:08:36.399 --> 00:08:38.440 warm sennni spots to set up camp because that's what 175 00:08:38.519 --> 00:08:41.039 keeps our machines running in our bodies from freezing. Right. 176 00:08:41.200 --> 00:08:44.679 But here the dead zones, the places where absolutely nothing happens, 177 00:08:44.879 --> 00:08:47.360 where there is no ambient energy, no light, no heat, 178 00:08:47.840 --> 00:08:49.960 those are the most crucial places to be for this 179 00:08:50.039 --> 00:08:52.360 specific science. The really are It makes me think about 180 00:08:52.360 --> 00:08:56.080 how the upcoming Artemis missions are already targeting these exact 181 00:08:56.159 --> 00:08:59.799 South Pole regions because they contain water ice frozen in 182 00:08:59.799 --> 00:09:03.080 the shadows. We are heading to the darkest places for 183 00:09:03.159 --> 00:09:07.000 survival resources, and the physics community is basically saying, hey, 184 00:09:07.039 --> 00:09:09.559 while you're down there in the dark mining ice, it 185 00:09:09.679 --> 00:09:11.759 just so happens to be the perfect spot for our 186 00:09:12.080 --> 00:09:13.600 ultimate precision laser. 187 00:09:13.960 --> 00:09:17.720 It is the ultimate cosmic coincidence. The conditions required for 188 00:09:17.840 --> 00:09:21.519 long term human survival, finding ancient ice trapped in these 189 00:09:21.519 --> 00:09:24.399 cold traps to turn into drinking water and rocket fuel, 190 00:09:25.399 --> 00:09:28.279 Those are precisely the same conditions required to push the 191 00:09:28.279 --> 00:09:32.080 boundaries of quantum measurements. The stillness is the currency here 192 00:09:32.720 --> 00:09:34.799 in physics. If you want to measure a signal with 193 00:09:34.919 --> 00:09:39.320 unimaginable precision, you have to first silence all the background noise, right, 194 00:09:39.480 --> 00:09:42.159 and there is literally nowhere quiet or nowhere more still 195 00:09:42.240 --> 00:09:45.399 than a fifty kelvin shadow that hasn't seen the sun 196 00:09:45.440 --> 00:09:46.840 since the solar system was young. 197 00:09:47.120 --> 00:09:52.360 Okay, so we have established this beautifully dead, quiet, freezing environment. 198 00:09:52.480 --> 00:09:55.720 Let's talk about the actual machine that desperately needs to 199 00:09:55.759 --> 00:09:58.679 live down there. Yeah, the heart of this entire concept 200 00:09:58.799 --> 00:10:02.639 is something called an optical silicon cavity. Yes, and I 201 00:10:02.679 --> 00:10:04.960 want to make sure we really visualize this for everyone listening, 202 00:10:04.960 --> 00:10:07.759 because the name sounds a bit like, I don't know, 203 00:10:07.840 --> 00:10:11.519 an abstract math problem. But it is a very tangible 204 00:10:11.559 --> 00:10:16.120 physical object. It is. It is essentially a block of 205 00:10:16.240 --> 00:10:19.440 crystalline silicon, and on each end of this block there 206 00:10:19.519 --> 00:10:22.080 is a mirror. That's the basic anatomy. 207 00:10:22.000 --> 00:10:25.360 That is the core physical architecture. Just imagine a highly refined, 208 00:10:25.879 --> 00:10:30.039 beautifully engineered spacer block made of single crystal silicon. It's 209 00:10:30.120 --> 00:10:33.600 usually quite small, maybe a couple of dozen centimeters. 210 00:10:33.080 --> 00:10:34.759 Long, so pretty compac very. 211 00:10:34.559 --> 00:10:37.440 Compact, and bonded to each end of the silicon spacer 212 00:10:37.519 --> 00:10:40.320 is a highly reflective mirror. But these aren't the kind 213 00:10:40.360 --> 00:10:43.039 of mirrors you have in your bathroom, I would hope not. Right. 214 00:10:43.120 --> 00:10:47.879 There are sophisticated, multi layered dielectric coatings designed to reflect 215 00:10:48.000 --> 00:10:51.440 light with a perfection that borders on the absolute, and. 216 00:10:51.480 --> 00:10:55.039 The entire purpose of this silicon block is to bounce light. 217 00:10:55.919 --> 00:10:59.039 You introduce light into the cavity through one of the mirrors, 218 00:10:59.279 --> 00:11:02.759 and it bounces back and forth endlessly between these two 219 00:11:02.879 --> 00:11:06.919 perfectly facing mirrored surfaces exactly. But it's not just any light, 220 00:11:06.960 --> 00:11:10.279 and it doesn't just bounce randomly. The distance between those 221 00:11:10.360 --> 00:11:14.240 two mirrors dictates exactly which frequencies, basically, which colors of 222 00:11:14.320 --> 00:11:16.840 light are allowed to exist inside that space. 223 00:11:16.960 --> 00:11:17.399 That's right. 224 00:11:17.600 --> 00:11:20.759 If the distance between those mirrors changes by even the 225 00:11:20.840 --> 00:11:25.120 width of a single atom, the entire system fails. Let's 226 00:11:25.120 --> 00:11:27.879 talk about why the physical distance is so strictly tied 227 00:11:27.919 --> 00:11:28.840 to the color of the light. 228 00:11:29.120 --> 00:11:31.639 Well, it comes down to the fundamental wave nature of 229 00:11:31.720 --> 00:11:35.200 light and a principle known as resonance. When you shine 230 00:11:35.200 --> 00:11:38.480 a beam of light into this cavity, the photons bounce 231 00:11:38.559 --> 00:11:41.840 back and forth between the two mirrors. Because light travels 232 00:11:41.879 --> 00:11:44.720 as a wave, it has peaks and troughs. Okay, Now 233 00:11:44.799 --> 00:11:47.559 imagine those waves overlapping as they travel back and forth. 234 00:11:47.799 --> 00:11:50.320 If the physical distance between the mirrors is an exact, 235 00:11:50.440 --> 00:11:54.519 perfect multiple of the light's wavelength, the overlapping waves will 236 00:11:54.559 --> 00:11:55.720 perfectly align. 237 00:11:55.519 --> 00:11:58.440 So they sort of stack on top of each other exactly. 238 00:11:58.879 --> 00:12:02.080 Every time a wave bounceounces back, its peak aligns exactly 239 00:12:02.120 --> 00:12:04.559 with the peak of the next wave coming in. Its 240 00:12:04.639 --> 00:12:08.080 trough aligns perfectly with the trough. They reinforce each other. 241 00:12:08.120 --> 00:12:12.120 The amplitude builds up, creating a powerful, highly stable, standing 242 00:12:12.120 --> 00:12:14.080 wave of light trapped inside the block. 243 00:12:14.279 --> 00:12:16.799 Now I want to make sure I'm grasping the alternative here. 244 00:12:17.480 --> 00:12:20.480 What happens if the distance is slightly off. Let's say 245 00:12:20.480 --> 00:12:23.120 the mirror moves away by a tiny, tiny fraction of 246 00:12:23.120 --> 00:12:23.639 a millimeter. 247 00:12:23.799 --> 00:12:27.720 If the distance shifts even slightly, the geometry is broken. 248 00:12:28.039 --> 00:12:30.480 The wave bounces back, but now it has traveled a 249 00:12:30.519 --> 00:12:34.399 slightly different distance. When it overlaps with the next incoming wave, 250 00:12:34.559 --> 00:12:37.159 they are out of sync. A peak of one wave 251 00:12:37.200 --> 00:12:39.679 will align with the trough of another wave. When a 252 00:12:39.679 --> 00:12:42.039 peak and a trough meet, they cancel each other out. 253 00:12:42.159 --> 00:12:42.720 Oh, I see. 254 00:12:42.759 --> 00:12:46.679 This is called destructive interference. The energy doesn't disappear from 255 00:12:46.720 --> 00:12:49.279 the universe, of course, but the light wave itself cannot 256 00:12:49.279 --> 00:12:53.840 sustain its structure inside that specific physical boundary, it scatters 257 00:12:54.000 --> 00:12:56.960 or is transmitted out and the standing wave collapses. 258 00:12:57.320 --> 00:12:59.399 Okay, I think I have an analogy for this, but 259 00:12:59.440 --> 00:13:01.679 correct me if I stray too far. Think about a 260 00:13:01.720 --> 00:13:05.320 perfectly tuned acoustic instrument. Let's use a guitar. Okay, Sure, 261 00:13:05.559 --> 00:13:08.600 when you pluck a guitar string, the string is anchored 262 00:13:08.600 --> 00:13:12.720 at two incredibly fixed points, the nut at the top 263 00:13:12.759 --> 00:13:14.559 of the neck and the bridge down on the body. 264 00:13:15.320 --> 00:13:19.960 That physical distance between those two anchor points is completely 265 00:13:20.000 --> 00:13:24.440 non negotiable. Because of that exact fixed distance, When you 266 00:13:24.440 --> 00:13:27.840 pluck the string, only one specific pure note let's say 267 00:13:27.840 --> 00:13:31.519 an A can sustain itself and ring out loudly. All 268 00:13:31.559 --> 00:13:34.480 the other chaotic vibrations, all the scraping noise of your 269 00:13:34.480 --> 00:13:37.799 finger hitting the string, quickly die out because those random 270 00:13:37.840 --> 00:13:41.039 frequencies just don't fit the physical geometry of the string. 271 00:13:41.480 --> 00:13:44.960 The physical distance ensures the purity of the note that is. 272 00:13:44.960 --> 00:13:47.679 On the right track, absolutely, but we have to take 273 00:13:47.679 --> 00:13:49.919 it a step further to truly capture what the optical 274 00:13:50.000 --> 00:13:53.000 cavity is doing. A guitar string is one dimensional and 275 00:13:53.039 --> 00:13:56.440 it's vibrating matter. Our cavity is three dimensional, and we 276 00:13:56.519 --> 00:14:00.759 are dealing with an electromagnetic field photons instead of physical 277 00:14:00.759 --> 00:14:05.600 strength exactly, But the underlying principle of boundary conditions is identical. 278 00:14:05.960 --> 00:14:08.440 The distance between the guitar's bridge and the nut is 279 00:14:08.480 --> 00:14:11.120 the exact equivalent of the distance between our two mirrors. 280 00:14:11.840 --> 00:14:14.039 The silicon block acts as the neck of the guitar, 281 00:14:14.440 --> 00:14:17.159 holding the mirrors perfectly rigidly apart. 282 00:14:17.320 --> 00:14:18.240 Okay, I'm checking. 283 00:14:18.480 --> 00:14:22.600 The optical cavity acts as the ultimate unforgiving gatekeeper. It 284 00:14:22.720 --> 00:14:26.200 dictates the physics of the space. It essentially says, only 285 00:14:26.320 --> 00:14:30.240 light of this exact specific frequency, this exact wavelength is 286 00:14:30.279 --> 00:14:34.399 allowed to resonate and build power here. Any other frequency 287 00:14:34.480 --> 00:14:36.519 will destroy itself through interference. 288 00:14:36.840 --> 00:14:39.200 So if we want to keep our optical guitar perfectly 289 00:14:39.240 --> 00:14:41.840 in tune, the neck of the guitar, the silicon block 290 00:14:42.159 --> 00:14:46.200 can never ever bend, warp, expand, or shrink. Never if 291 00:14:46.240 --> 00:14:48.440 the distance between the mirrors changes by even a fraction 292 00:14:48.519 --> 00:14:52.559 of an atomic diameter, are perfectly cure color of light shifts, 293 00:14:53.039 --> 00:14:55.799 the standing wave collapses. It basically goes out of tune. 294 00:14:55.559 --> 00:14:58.240 And that introduces the terror of thermodynamics that we were 295 00:14:58.240 --> 00:14:59.080 discussing earlier. 296 00:14:59.120 --> 00:15:00.720 Oh right, because if he yes. 297 00:15:01.120 --> 00:15:04.240 In the physical universe, almost everything expands when it gets 298 00:15:04.279 --> 00:15:07.639 warm and contracts when it gets cold, even a highly 299 00:15:07.720 --> 00:15:11.960 engineered block of single crystal silicon. If you put this 300 00:15:12.080 --> 00:15:15.519 silicon cavity on a table in a normal laboratory room 301 00:15:15.559 --> 00:15:19.480 on Earth, just the microscopic fluctuations in the room's ambient 302 00:15:19.519 --> 00:15:23.639 air temperature, maybe the air conditioning turns on, or a scientist. 303 00:15:23.159 --> 00:15:25.159 Walks past, just a person walking past. 304 00:15:25.080 --> 00:15:27.679 Oh yeah, body heat is more than enough. That would 305 00:15:27.720 --> 00:15:31.399 cause the silicon block to expand and contract by microscopic amounts. 306 00:15:31.720 --> 00:15:35.039 The mirrors would constantly be moving closer together and further apart. 307 00:15:35.440 --> 00:15:38.480 The resonant frequency of the light would drift endlessly. The 308 00:15:38.559 --> 00:15:41.200 instrument would be completely useless for the kind of extreme 309 00:15:41.200 --> 00:15:42.519 precision we are chasing. 310 00:15:42.399 --> 00:15:45.840 Which brings us directly back to our dark, freezing lunar crater. 311 00:15:46.799 --> 00:15:49.000 We know the Shackleton crator is sitting at a very 312 00:15:49.080 --> 00:15:52.559 chilly fifty kelvin. That is incredibly cold. It slows down 313 00:15:52.600 --> 00:15:55.000 a massive amount of that thermal jitter. It does, But 314 00:15:55.120 --> 00:15:58.559 fifty kelvin is actually not cold enough for the absolute 315 00:15:58.639 --> 00:16:01.679 perfection we are aiming for here. The physicists need to 316 00:16:01.720 --> 00:16:05.480 reach a very specific magic number sixteen. 317 00:16:05.000 --> 00:16:06.720 Kelvin exactly sixteen kelvin. 318 00:16:07.000 --> 00:16:10.600 Wait, if the ambient environment of the crater floor itself 319 00:16:10.639 --> 00:16:13.919 is fifty kelvin, how does the machine get colder than 320 00:16:13.960 --> 00:16:16.960 the dirt it's sitting on. Are we plugging in some giant, 321 00:16:17.039 --> 00:16:20.240 power hungry cosmic refrigerator down there, because that seems like 322 00:16:20.240 --> 00:16:22.120 it would defeat the whole purpose of going to a 323 00:16:22.200 --> 00:16:23.480 dead quiet zone. 324 00:16:23.759 --> 00:16:27.200 You've hit on one of the most brilliant engineering solutions 325 00:16:27.200 --> 00:16:31.240 in this entire architecture. It raises a fascinating point about 326 00:16:31.279 --> 00:16:34.679 how we engineer things for the deep space environment compared 327 00:16:34.720 --> 00:16:36.559 to how we do it on Earth. Okay, if you 328 00:16:36.600 --> 00:16:39.440 were sitting in a lab in Colorado or Germany and 329 00:16:39.480 --> 00:16:42.159 you want to cool a silicon block down to exactly 330 00:16:42.200 --> 00:16:46.519 sixteen kelvin, you have to build a massive, incredibly complex 331 00:16:46.600 --> 00:16:48.320 piece of machinery called a cryostat. 332 00:16:48.480 --> 00:16:50.559 Right. I've seen pictures of those. They look like giant 333 00:16:50.559 --> 00:16:52.519 metal barrels covered in wires and tubes. 334 00:16:52.639 --> 00:16:56.159 They are absolute beasts. They require huge amounts of electrical power, 335 00:16:56.240 --> 00:16:59.480 constant maintenance, and they often rely on circulating liquid helium 336 00:16:59.519 --> 00:17:02.320 as a coolant. And here is the brutal irony of 337 00:17:02.360 --> 00:17:05.359 the whole setup. What's that the machinery itself, the pumps, 338 00:17:05.400 --> 00:17:09.960 the compressors, pushing the liquid helium around creates severe mechanical vibration. 339 00:17:10.119 --> 00:17:13.359 Of course, so you cool the silicon down to stop 340 00:17:13.400 --> 00:17:17.279 thermal jitter. But in doing so you introduce mechanical shaking 341 00:17:17.400 --> 00:17:21.720 from the refrigerator. You then have to build elaborate actively 342 00:17:21.759 --> 00:17:25.759 controlled isolation tables just to float the cavity and protect 343 00:17:25.759 --> 00:17:28.559 it from the very machine that is cooling it. It 344 00:17:28.680 --> 00:17:32.440 is an engineering nightmare of cascading complexities. 345 00:17:32.519 --> 00:17:33.880 That sounds exhausting it is. 346 00:17:34.319 --> 00:17:38.119 But on the Moon, inside this permanently shadowed crater, we 347 00:17:38.200 --> 00:17:41.440 don't need active machinery. We don't need liquid helium pumps. 348 00:17:41.720 --> 00:17:44.119 We can use the geometry of the universe passively. 349 00:17:44.519 --> 00:17:49.319 So no power cords, no liquid coolants vibrating the table, just. 350 00:17:49.279 --> 00:17:50.359 Physics, just physics. 351 00:17:50.400 --> 00:17:52.200 How exactly do we get it from fifty down to 352 00:17:52.240 --> 00:17:53.240 sixteen without a. 353 00:17:53.160 --> 00:17:58.759 Refrigerator by utilizing radiative cooling, specifically through deployable radiation panels. 354 00:17:58.839 --> 00:18:01.640 The housing of the optical cavity is equipped with thermal 355 00:18:01.680 --> 00:18:04.799 panels that literally unfold once the rover places the instrument 356 00:18:04.839 --> 00:18:07.400 on the crater floor. And what these panels do is 357 00:18:07.400 --> 00:18:10.799 they look up up. Yes, they point away from the 358 00:18:10.839 --> 00:18:13.920 fifty kelvin lunar dirt, and they look directly out into 359 00:18:13.960 --> 00:18:16.400 the infinite deep abyss of outer space. 360 00:18:16.640 --> 00:18:18.839 And space is colder than the greater. 361 00:18:18.920 --> 00:18:22.119 Vastly colder. Deep space has a background temperature of about 362 00:18:22.160 --> 00:18:25.799 two point seven kelvin. That is the cosmic microwave background, 363 00:18:26.240 --> 00:18:30.240 the fading residual radiation left over from the Big Bang itself. 364 00:18:30.359 --> 00:18:30.680 Wow. 365 00:18:30.960 --> 00:18:34.000 Now, the laws of thermo dynamics dictate that heat naturally 366 00:18:34.039 --> 00:18:37.960 flows from warmer objects to colder objects through the Steffon 367 00:18:38.000 --> 00:18:41.240 Boltzmann law. An object will radiate its thermal energy away 368 00:18:41.240 --> 00:18:43.880 as infrared light. Because the panels are looking at a 369 00:18:43.920 --> 00:18:47.400 two point seven kelvin sky, the residual heat inside the 370 00:18:47.400 --> 00:18:50.200 fifty kelvin silicon block travels up to the panels and 371 00:18:50.279 --> 00:18:53.680 is radiated away, lost forever into the infinite heat sink 372 00:18:53.720 --> 00:18:54.400 of the universe. 373 00:18:54.559 --> 00:18:57.480 That is, honestly mind bending. The machine cools itself simply 374 00:18:57.519 --> 00:19:00.000 by exposing its heat to the infinite cold of deep space. 375 00:19:00.640 --> 00:19:02.839 It's like opening a window in winter to cool down 376 00:19:02.839 --> 00:19:05.880 a hot room. But the window opens into the cosmic void. 377 00:19:06.000 --> 00:19:07.000 That's a great analogy. 378 00:19:07.119 --> 00:19:08.880 But wait, if it's looking at a two point seven 379 00:19:08.920 --> 00:19:11.200 kelvin's guy, why isn't just keep cooling down until it 380 00:19:11.240 --> 00:19:13.559 hits two point seven kelvin? Why does it stop at sixteen? 381 00:19:13.759 --> 00:19:16.720 Well, that is where the careful thermal engineering comes in. 382 00:19:17.119 --> 00:19:19.599 The silicon block isn't just floating in space. It is 383 00:19:19.680 --> 00:19:23.799 still physically near the fifty kelvin lunar surface. Even with 384 00:19:23.920 --> 00:19:27.000 heavy thermal shielding around the bottom and sides of the instrument, 385 00:19:27.440 --> 00:19:30.079 A tiny amount of heat from the lunar dirt will 386 00:19:30.119 --> 00:19:33.200 slowly conduct or radiate into the device, So. 387 00:19:33.200 --> 00:19:35.480 It's leaking a tiny bit of heat in from. 388 00:19:35.359 --> 00:19:39.359 The ground exactly. So the engineers design the surface area 389 00:19:39.440 --> 00:19:43.279 of the radiation panels very precisely. The panels dump heat 390 00:19:43.319 --> 00:19:46.400 into space at a specific rate, while the shielding allows 391 00:19:46.440 --> 00:19:49.279 a microscopic trickle of heat in from the ground. When 392 00:19:49.319 --> 00:19:52.599 those two rates perfectly balance each other out, the system 393 00:19:52.640 --> 00:19:54.079 reaches thermal equilibrium. 394 00:19:54.119 --> 00:19:54.680 I see. 395 00:19:54.799 --> 00:19:57.000 By designing the panels correctly, they can ensure that this 396 00:19:57.079 --> 00:20:00.920 equilibrium point lands exactly at sixteen kelvin. It is a natural, 397 00:20:01.039 --> 00:20:03.720 free vibration free cryogenic freezer. 398 00:20:03.839 --> 00:20:06.279 Okay, I understand how we get to sixteen calvin now, 399 00:20:06.440 --> 00:20:09.240 but why sixteen kelvin? Specifically, why go through all the 400 00:20:09.279 --> 00:20:12.039 trouble of unfolding panels and balancing heat laws. Why not 401 00:20:12.119 --> 00:20:14.200