WEBVTT 1 00:00:03.399 --> 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 --> 00:00:26.920 slumber under the night sky. 7 00:00:27.000 --> 00:00:31.559 If we launched our fastest, like absolutely most advanced chemical 8 00:00:31.640 --> 00:00:35.039 rocket today and we aimed it straight at the nearest star. 9 00:00:34.920 --> 00:00:36.799 System, Alpha Centauri. 10 00:00:36.520 --> 00:00:41.520 Right Alpha Centauri, it wouldn't actually arrive for another seventy 11 00:00:41.560 --> 00:00:42.719 thousand years, which is just. 12 00:00:42.679 --> 00:00:45.280 An incomprehensible amount of time for a human mind. 13 00:00:45.439 --> 00:00:48.280 Exactly. It's essentially an eternity. So if we want to 14 00:00:48.320 --> 00:00:52.159 actually survive the cosmos, or you know, even just explore 15 00:00:52.200 --> 00:00:56.280 our immediate stellar neighborhood within a single human lifespan, we 16 00:00:56.399 --> 00:00:59.079 have to completely stop relying on controlled explosion. 17 00:00:59.159 --> 00:01:01.679 We really do. Chemical rockets are a dead end for 18 00:01:01.799 --> 00:01:02.759 interstellar travel. 19 00:01:02.920 --> 00:01:05.840 Yeah, and that means we have to fundamentally rethink how 20 00:01:06.000 --> 00:01:09.760 mass moves through a vacuum, which invariably forces us to 21 00:01:09.799 --> 00:01:11.079 look at beamed energy. 22 00:01:10.840 --> 00:01:14.359 Propulsion, specifically laser driven light sales. 23 00:01:14.079 --> 00:01:18.159 Right, But pushing a macro scale object to relativistic speeds 24 00:01:18.200 --> 00:01:21.040 with photons, I mean, it isn't just a matter of 25 00:01:21.079 --> 00:01:22.719 building a bigger laser, No. 26 00:01:22.480 --> 00:01:25.159 Not at all. It's so much more complicated than that. 27 00:01:25.319 --> 00:01:29.239 Because the universe actually has built in aerodynamic limits even 28 00:01:29.280 --> 00:01:32.760 in a complete vacuum. So we are going to explore 29 00:01:32.920 --> 00:01:37.760 the granular you know, the quantum level mechanics of photon momentum. 30 00:01:37.120 --> 00:01:40.760 And really fascinating newly mapped threshold. 31 00:01:40.840 --> 00:01:44.280 Yeah, this newly mapped threshold at exactly seventy five percent 32 00:01:44.280 --> 00:01:46.879 of the speed of light, where the fundamental laws of 33 00:01:46.920 --> 00:01:51.200 relativistic physics just they mutate, They absolutely do. 34 00:01:51.280 --> 00:01:55.159 They turn our own propulsion systems into an active drag force. 35 00:01:55.000 --> 00:01:57.840 Which is wild. But before we get to that wall, 36 00:01:57.840 --> 00:01:59.680 we should probably talk about why we're abandoning rockets in 37 00:01:59.680 --> 00:02:00.359 the first place. 38 00:02:00.599 --> 00:02:03.799 Right, Well, the whole problem with chemical propulsion is rooted 39 00:02:03.840 --> 00:02:07.319 in the Siolkowsky rocket equation. It's basically an exponential trap. 40 00:02:07.359 --> 00:02:10.039 An exponential trap, yeah, yeah, because you have to carry 41 00:02:10.039 --> 00:02:11.520 the fuel exactly. 42 00:02:11.120 --> 00:02:14.360 You carry mass to produce thrust, right, but then you 43 00:02:14.400 --> 00:02:17.240 need more thrust to move that extra mass you're carrying it. 44 00:02:17.680 --> 00:02:19.759 It's like packing for a road trip, but the extra 45 00:02:19.800 --> 00:02:22.879 suitcases require you to tow a full other car just 46 00:02:22.960 --> 00:02:24.759 to carry the gas for the extra suitcases. 47 00:02:24.960 --> 00:02:26.319 That's a great way to put it. And when you 48 00:02:26.400 --> 00:02:29.439 run the numbers to reach even ten percent of the 49 00:02:29.479 --> 00:02:34.400 speed of light using conventional chemical propellants, the mass of 50 00:02:34.439 --> 00:02:38.199 the fuel required would actually exceed the mass of the 51 00:02:38.199 --> 00:02:39.319 observable universe. 52 00:02:39.400 --> 00:02:41.719 Wait, literally the entire universe. 53 00:02:41.800 --> 00:02:45.919 Literally, the map just completely breaks down. Is physically impossible. Wow. 54 00:02:46.159 --> 00:02:50.159 And even if we scale up to say theoretical fusion 55 00:02:50.240 --> 00:02:52.360 drives or you know, antimatter. 56 00:02:51.759 --> 00:02:53.599 Engines, which we don't even have yet. 57 00:02:53.439 --> 00:02:55.960 Right, But even if we did, you are still carrying 58 00:02:56.039 --> 00:02:59.439 the fuel. You are still suffering that logarithmic penalty of 59 00:02:59.599 --> 00:03:01.360 pushing the mass of the thing that's supposed to be 60 00:03:01.400 --> 00:03:01.759 pushing you. 61 00:03:01.879 --> 00:03:03.879 So we're just stuck in that trap exactly. 62 00:03:03.879 --> 00:03:07.520 So the only physical loophole we have left really is 63 00:03:07.639 --> 00:03:10.639 decoupling the engine from the spacecraft, entirely decoupling it. 64 00:03:10.680 --> 00:03:12.719 So you just you leave the engine behind. 65 00:03:12.840 --> 00:03:15.159 Yes, you leave the heavy propulsion source, the mass of 66 00:03:15.240 --> 00:03:18.439 power plant, and the energy emitter anchored safely in our 67 00:03:18.479 --> 00:03:21.840 fullar system, and you just transmit the momentum across the void. 68 00:03:21.879 --> 00:03:24.520 Which brings us to radiation pressure. And I think the 69 00:03:24.560 --> 00:03:27.800 concept of the solar sale is it's generally pretty well 70 00:03:27.879 --> 00:03:29.000 understood by most people. 71 00:03:29.280 --> 00:03:31.280 Yeah, it's a popular sci fi concept. 72 00:03:31.000 --> 00:03:34.919 Right, This idea of a massive, ultra thin sheet catching sunlight, 73 00:03:35.120 --> 00:03:37.400 sort of like a galleon catching the trade winds on 74 00:03:37.439 --> 00:03:37.960 the ocean. 75 00:03:38.080 --> 00:03:41.599 It's a beautiful image, but sunlight disperses way too quickly. 76 00:03:41.360 --> 00:03:42.800 Because of the inverse square law. 77 00:03:42.879 --> 00:03:45.599 Right precisely, the inver square law dictates that by the 78 00:03:45.599 --> 00:03:50.120 time a solar sail reaches say, Jupiter, the Sun's actual 79 00:03:50.199 --> 00:03:53.120 pushing force is just a tiny, tiny fraction of what 80 00:03:53.159 --> 00:03:54.159 it was when it was at Earth. 81 00:03:54.360 --> 00:03:57.360 It's basically useless for getting to another star completely. 82 00:03:57.479 --> 00:04:01.439 For true interstellar transit, you can't just rely on ambient sunlight. 83 00:04:01.719 --> 00:04:04.439 You need a highly colimated beam like a laser, like 84 00:04:04.479 --> 00:04:08.000 a massive array of lasers, have a phased array, perhaps spanning, 85 00:04:08.240 --> 00:04:10.639 you know, hundreds of kilometers across the lunar surface. 86 00:04:10.800 --> 00:04:15.159 Just firing a tightly focused beam of Terrawak class continuous 87 00:04:15.240 --> 00:04:17.160 light directly at a sale. 88 00:04:16.879 --> 00:04:21.680 Exactly, and the spacecraft itself carries zero fuel none at all. 89 00:04:21.759 --> 00:04:24.639 It's entirely just payload and a reflective surface, right. 90 00:04:24.920 --> 00:04:26.680 And the physics of this relies on the fact that 91 00:04:26.720 --> 00:04:31.279 while photons possess zero rest mass, they do actually carry momentum. 92 00:04:30.920 --> 00:04:34.160 Which always trips people up. No mass, but they have momentum. 93 00:04:34.319 --> 00:04:38.360 It's counterintuitive, I know, But their momentum is dictated by 94 00:04:38.399 --> 00:04:41.000 their energy divided by the speed of light. So when 95 00:04:41.000 --> 00:04:44.079 a photon interacts with the atomic lattice of a reflective sail, 96 00:04:44.519 --> 00:04:48.040 it actually transfers that momentum. Is it a physical push, Yes, 97 00:04:48.720 --> 00:04:52.040 it is an infinitesimally small kick, of course. But in 98 00:04:52.079 --> 00:04:55.560 the near perfect vacuum of deep space, devoid of any 99 00:04:55.639 --> 00:04:57.240 real aerodynamic. 100 00:04:56.680 --> 00:04:59.680 Friction, you can just accumulate that acceleration continuously. 101 00:05:00.000 --> 00:05:03.600 Exactly if you maintain that multi taro watt laser focus 102 00:05:03.639 --> 00:05:07.680 for days or months or even years, the acceleration compounds 103 00:05:08.160 --> 00:05:11.199 this sale goes from a total standstill to a significant 104 00:05:11.199 --> 00:05:12.240 fraction of light speed. 105 00:05:12.720 --> 00:05:15.800 But the actual mechanical interaction there, like the exact moment 106 00:05:15.879 --> 00:05:19.279 the photon strikes the sail material, is far more complex 107 00:05:19.319 --> 00:05:20.920 than just the simple elastic collision. 108 00:05:20.920 --> 00:05:23.560 Isn't well massively more complex. It's not just a billiard 109 00:05:23.560 --> 00:05:27.279 ball hitting another earlier ball. Physicists Chaoshen and Jazzy Lee 110 00:05:27.279 --> 00:05:31.439 at the Harbin Institute of Technology recently deconstructed this whole interaction, right, 111 00:05:31.720 --> 00:05:35.360 and they broke the thrust down into three distinct measurable 112 00:05:35.399 --> 00:05:38.079 forces that are actually all occurring simultaneously. 113 00:05:38.439 --> 00:05:41.160 Yeah, looking at shennon LA's breakdown of this, I was 114 00:05:41.199 --> 00:05:44.120 really struck by how they split up those thrust vectors, 115 00:05:44.639 --> 00:05:47.160 because you know, you start with just the baseline force, right, 116 00:05:47.199 --> 00:05:48.160 the incident light. 117 00:05:48.160 --> 00:05:50.000 Right, the raw arrival of the folkus Kay. 118 00:05:50.279 --> 00:05:53.879 The photon arrives from the laser array, it strikes the 119 00:05:53.959 --> 00:05:57.279 atoms of the saale material, and it just transfers its 120 00:05:57.279 --> 00:05:59.720 initial forward momentum like a kinetic slab. 121 00:06:00.000 --> 00:06:02.759 It's the most straightforward part of the physics, the wave 122 00:06:02.800 --> 00:06:04.079 packet just hitting the surface. 123 00:06:04.199 --> 00:06:06.680 But the real efficiency, the thing that actually acts as 124 00:06:06.680 --> 00:06:09.199 a multiplier effect, comes from the second force, which is 125 00:06:09.519 --> 00:06:10.519 specular reflection. 126 00:06:10.759 --> 00:06:13.480 That's the golden ticket right there, specular reflection. 127 00:06:13.319 --> 00:06:17.439 Because if the material acts as a perfect mirror momentum, 128 00:06:17.480 --> 00:06:21.279 conservation demands that when the photon completely reverses its direction 129 00:06:21.360 --> 00:06:26.319 when it bounces off, it imparts a secondary equal kick 130 00:06:26.360 --> 00:06:27.720 of momentum to the sale. 131 00:06:27.839 --> 00:06:31.600 Yes, you are essentially double dipping, which is great. It's ideal. 132 00:06:32.040 --> 00:06:34.759 Specular reflection is exactly what you want because all of 133 00:06:34.800 --> 00:06:36.839 the momentum from that clean you know, one hundred and 134 00:06:36.879 --> 00:06:41.800 eighty degree balance is directed perfectly backward toward the emission source, meaning. 135 00:06:41.759 --> 00:06:46.040 One hundred percent of the reaction force pushes the sale perfectly. 136 00:06:45.639 --> 00:06:49.160 Forward right along your target vector. And if we could 137 00:06:49.240 --> 00:06:54.040 engineer a mathematically perfect mirror, the calculations for interstellar travel 138 00:06:54.040 --> 00:06:55.600 would be incredibly straightforward. 139 00:06:55.600 --> 00:06:56.319 But we can't. 140 00:06:56.560 --> 00:07:00.480 No real materials introduce the third force, and this is 141 00:07:00.480 --> 00:07:04.759 where all the complications begin. And that force is diffuse. 142 00:07:04.240 --> 00:07:08.720 Scattering, right, diffuse scattering because no macroscopic material is actually 143 00:07:08.720 --> 00:07:10.079 perfectly smooth. 144 00:07:09.879 --> 00:07:13.439 Or perfectly reflective. At the atomic level, a certain percentage 145 00:07:13.480 --> 00:07:15.959 of the incoming photons just won't bounce cleanly. 146 00:07:16.160 --> 00:07:17.000 What happens to them? 147 00:07:17.000 --> 00:07:21.839 Instead, they are absorbed by the material's atoms. This briefly 148 00:07:21.879 --> 00:07:24.959 elevates the electrons to a higher energy state for just 149 00:07:25.000 --> 00:07:28.079 a microscopic fraction of a second. Then the atom drops 150 00:07:28.120 --> 00:07:30.199 back to its ground state and it has to re 151 00:07:30.279 --> 00:07:31.120 emit a photon. 152 00:07:31.399 --> 00:07:34.480 Right, And because of the nature of spontaneous emission. That 153 00:07:34.600 --> 00:07:37.680 re emitted photon doesn't actually remember where. 154 00:07:37.480 --> 00:07:39.879 It came from, It has no memory of the laser beam. 155 00:07:39.959 --> 00:07:42.480 It just gets shot out in a completely random direction 156 00:07:42.560 --> 00:07:45.199 across the rear hemisphere of the sale exactly. 157 00:07:45.480 --> 00:07:48.800 And this is inherently inefficient because you are introducing vector cancelation. 158 00:07:49.079 --> 00:07:53.959 Vector cancelation, So if a photon is randomly re emitted, 159 00:07:54.240 --> 00:07:55.399 say sharply to the. 160 00:07:55.399 --> 00:07:58.360 Left, the reaction force kicks the sale to the right right. 161 00:07:58.680 --> 00:08:01.600 But then statistically another photon is being scattered to. 162 00:08:01.519 --> 00:08:03.680 The right, which kicks the sale back to the left. 163 00:08:03.759 --> 00:08:07.120 So all that lateral momentum is just completely canceling itself out. 164 00:08:07.279 --> 00:08:10.680 Yes, it's generating heat, it's causing internal stress on the 165 00:08:10.680 --> 00:08:14.959 sale material, but it is contributing absolutely zero net forward thrust. 166 00:08:15.040 --> 00:08:20.759 You're just bleeding potential energy into chaotic multidirectional noise, you 167 00:08:20.839 --> 00:08:21.279 really are. 168 00:08:21.879 --> 00:08:25.360 You lose a massive percentage of your pushing power to 169 00:08:25.399 --> 00:08:27.199 that geometric inefficiency alone. 170 00:08:27.240 --> 00:08:28.680 That seems like a huge problem. 171 00:08:28.759 --> 00:08:32.080 Well, under normal Newtonian mechanics, you know, at low speeds, 172 00:08:32.559 --> 00:08:36.600 this diffuse scattering is simply modeled as a constant fractional loss. 173 00:08:37.080 --> 00:08:38.720 It's an annoyance, but manageable. 174 00:08:38.919 --> 00:08:40.440 You just calculate it and move on. 175 00:08:40.879 --> 00:08:45.559 Exactly, You calculate the ratio of specular reflection to diffuse 176 00:08:45.840 --> 00:08:49.639 scattering based on whatever material you're using. You subtract the 177 00:08:49.639 --> 00:08:53.559 wasted lateral vectors, and boom, you have your baseline acceleration curve. 178 00:08:53.600 --> 00:08:56.039 So you just build a slightly more powerful laser to 179 00:08:56.120 --> 00:08:57.600 compensate for the inefficiency. 180 00:08:57.679 --> 00:08:59.679 Right, you just brute force it with more gigwats. But 181 00:09:00.159 --> 00:09:03.279 and this is the crux of Shannon Li's model. It 182 00:09:03.320 --> 00:09:05.080 doesn't stop at low speeds. 183 00:09:04.919 --> 00:09:06.679 Because the whole point of a light sale is to 184 00:09:06.679 --> 00:09:07.720 go incredibly fast. 185 00:09:07.799 --> 00:09:12.240 Yes, the goal is to achieve relativistic velocity. And as 186 00:09:12.279 --> 00:09:14.799 you push past roughly ten or twenty percent of the 187 00:09:14.799 --> 00:09:17.600 speed of light, classical mechanics completely. 188 00:09:17.080 --> 00:09:18.879 Break down and the weirdness starts. 189 00:09:18.960 --> 00:09:23.320 The weirdness starts, relativistic effects begin to heavily, heavily distort 190 00:09:23.759 --> 00:09:25.000 the incoming laser beam. 191 00:09:25.200 --> 00:09:29.440 Right, so we immediately run into the relativistic Doppler effect. Yes, now, 192 00:09:29.480 --> 00:09:32.679 I think everyone understands standard Doppler shifts like a siren 193 00:09:32.720 --> 00:09:35.039 passing by, So we can probably skip the acoustic analogies 194 00:09:35.080 --> 00:09:38.440 here wear enough. The real issue here is severe redshift 195 00:09:38.480 --> 00:09:40.320 from the sales frame of reference. 196 00:09:40.360 --> 00:09:44.480 Very severe. As the sale accelerates to say zero one 197 00:09:44.600 --> 00:09:47.559 two CE or point three CE, it is literally running 198 00:09:47.639 --> 00:09:49.840 away from the incoming electromagnetic waves. 199 00:09:49.919 --> 00:09:52.679 So from the perspective of the sale itself, the wavelength 200 00:09:52.720 --> 00:09:55.200 of the laser light is just being violently stretched out. 201 00:09:55.360 --> 00:09:58.320 It is the frequency drops and because a photon's energy 202 00:09:58.440 --> 00:10:00.919 is directly proportional to its frequency. 203 00:10:00.559 --> 00:10:02.559 Lanks constant times frequency rates. 204 00:10:02.600 --> 00:10:06.080 Exactly because of that, the red shift means the incoming 205 00:10:06.159 --> 00:10:09.960 light literally holds less energy and less momentum by the 206 00:10:10.000 --> 00:10:11.679 time it finally catches up to the sale. 207 00:10:11.720 --> 00:10:15.039 That is wild. It creates the severe power drain effect. 208 00:10:15.080 --> 00:10:16.720 It's like you're out running your own fuel supply. 209 00:10:16.840 --> 00:10:19.480 Okay, let's use a visual here. Let's assume the lunar 210 00:10:19.559 --> 00:10:23.519 laser array is firing in the high energy ultraviolet spectrum. 211 00:10:23.120 --> 00:10:25.320 Right, a good example UV lasers. 212 00:10:25.440 --> 00:10:28.679 When the sails just starting out moving slowly, it receives 213 00:10:28.759 --> 00:10:31.399 the full impact of those high energy UV. 214 00:10:31.360 --> 00:10:33.279 Photon full power maximum thrust. 215 00:10:33.399 --> 00:10:36.200 But as the sail crosses zero point three C, the 216 00:10:36.320 --> 00:10:40.120 Lorentz factor begins to aggressively stretch that light. So from 217 00:10:40.120 --> 00:10:42.480 the perspective of the atoms on the sale, the incoming 218 00:10:42.559 --> 00:10:44.559 laser isn't ultraviolet anymore. 219 00:10:44.840 --> 00:10:47.799 No, it has downshifted. It's probably in the visible spectrum now, 220 00:10:47.960 --> 00:10:49.440 maybe yellow or even red light. 221 00:10:49.720 --> 00:10:53.039 And as it approaches half the speed of light zero 222 00:10:53.080 --> 00:10:53.799 point five c. 223 00:10:54.120 --> 00:10:57.159 The light downshifts even further into the infrared or maybe 224 00:10:57.159 --> 00:10:58.320 even microwave bands. 225 00:10:58.759 --> 00:11:01.480 So the fast are the sale goes, the weaker the 226 00:11:01.480 --> 00:11:04.360 push from each individual photon becomes. 227 00:11:04.159 --> 00:11:06.759 Which means your thrust curve flattens out. 228 00:11:06.720 --> 00:11:09.759 Dramatically, which means the engineers back on Earth have to 229 00:11:09.799 --> 00:11:14.759 continuously pump exponentially more gigawatts into the laser array just 230 00:11:14.840 --> 00:11:18.679 to maintain even a tiny fraction of the original acceleration. 231 00:11:18.480 --> 00:11:22.080 Exactly, or, and this is more likely, they actually have 232 00:11:22.120 --> 00:11:25.519 to design the laser array to actively tune its mission 233 00:11:25.559 --> 00:11:27.240 frequency over time, wait. 234 00:11:27.080 --> 00:11:29.679 Like changing the color of the laser as the ship flies. 235 00:11:29.399 --> 00:11:33.279 Away, exactly, starting with something extreme like X rays, so 236 00:11:33.320 --> 00:11:36.720 that as the sail reaches relativistic speeds and runs away 237 00:11:36.720 --> 00:11:39.720 from the beam, the light down shifts into a wavelength 238 00:11:40.120 --> 00:11:43.039 that the sales material is actually optimized to reflect. 239 00:11:43.360 --> 00:11:46.679 That's incredibly complicated. You are just constantly fighting the fact 240 00:11:46.720 --> 00:11:49.039 that your propellant is basically losing its punch. 241 00:11:49.360 --> 00:11:53.159 You are, But as brutal as this Doppler redshift is 242 00:11:53.240 --> 00:11:56.600 on your energy budget, it is still just in an efficiency, right. 243 00:11:56.639 --> 00:11:58.919 It just slows down the rate of acceleration exactly. 244 00:11:59.320 --> 00:12:02.879 The really crucial revelation in shen In Lee's research is 245 00:12:02.960 --> 00:12:06.720 what happens when you push the sale to exactly seventy 246 00:12:06.720 --> 00:12:07.480 five percent of the. 247 00:12:07.440 --> 00:12:10.159 Speed of light the point seventy five sea wall. 248 00:12:10.080 --> 00:12:12.240 The point seven to five sea wall. Because right at 249 00:12:12.240 --> 00:12:15.480 that specific velocity, the physics of that third force we 250 00:12:15.559 --> 00:12:20.039 discussed the messy inefficient diffuse scattering, it stops being just 251 00:12:20.039 --> 00:12:21.639 a simple energy loss. 252 00:12:21.440 --> 00:12:23.919 And it actively mutates into a mechanical break. 253 00:12:24.080 --> 00:12:25.799 Yes, it becomes a drag force. 254 00:12:25.879 --> 00:12:30.039 Okay, this is the phenomenon of relativistic light aberration. And 255 00:12:30.120 --> 00:12:32.639 I know this requires us to strictly separate our frames 256 00:12:32.639 --> 00:12:33.759 of reference to really get it. 257 00:12:33.799 --> 00:12:35.679 Oh, we have to. It's the only way relativity makes 258 00:12:35.720 --> 00:12:36.279 any sense. 259 00:12:36.399 --> 00:12:38.759 So let's return to the perspective of an astronaut riding 260 00:12:38.759 --> 00:12:41.240 on the sale in their local frame of reference. Nothing 261 00:12:41.279 --> 00:12:44.799 has changed, right, The diffuse scattering behaves exactly as we 262 00:12:44.840 --> 00:12:46.200 described earlier. 263 00:12:45.960 --> 00:12:49.360 Exactly to the astronaut, the laser hits the saal, the 264 00:12:49.399 --> 00:12:52.399 atoms absorb some photons and they randomly re emit them 265 00:12:52.399 --> 00:12:56.519 in a standard uniform distribution across the entire back half 266 00:12:56.519 --> 00:12:57.000 of the sale. 267 00:12:57.279 --> 00:13:00.240 So to the astronaut looking out the back window, the 268 00:13:00.320 --> 00:13:02.720 light is still leaving the rear of the ship, just 269 00:13:02.759 --> 00:13:05.600 scattering harmlessly into the void behind them. 270 00:13:05.840 --> 00:13:09.919 It looks perfectly normal. But the reality of special relativity 271 00:13:10.240 --> 00:13:13.480 means the universe simply doesn't agree with the astronaut's perspective. 272 00:13:13.559 --> 00:13:15.399 Right, we have to look at the geometry of that 273 00:13:15.440 --> 00:13:18.679 scattered light from the stationary frame of reference. 274 00:13:18.320 --> 00:13:21.639 From Earth or from the laser array the origin point 275 00:13:21.720 --> 00:13:22.799 of the coordinate system. 276 00:13:23.120 --> 00:13:25.759 Because the sail is moving forward at point seventy five C, 277 00:13:26.279 --> 00:13:29.279 the Lorentz transformation suddenly applies to the angle of the 278 00:13:29.320 --> 00:13:30.000 emitted light. 279 00:13:30.279 --> 00:13:33.960 Yes, is commonly referred to in astrophysics as the headlight effect. 280 00:13:34.080 --> 00:13:36.279 The headlight effect. Okay, how does that work well? 281 00:13:36.279 --> 00:13:40.440 When an object emits radiation isotropically, meaning equally in all 282 00:13:40.480 --> 00:13:42.240 directions in its own rest frame. 283 00:13:42.159 --> 00:13:44.120 Like our sail doing the diffuse scattering. 284 00:13:44.279 --> 00:13:47.440 Exactly when it does that, an observer in a stationary 285 00:13:47.440 --> 00:13:52.519 frame will actually see that radiation beamed forward, tightly concentrated 286 00:13:52.720 --> 00:13:55.559 in the direction of the object's motion. Wow. And the 287 00:13:55.600 --> 00:13:59.519 faster the object moves, the narrower and more forward directed 288 00:13:59.519 --> 00:14:01.159 that cone of radiation becomes. 289 00:14:01.519 --> 00:14:04.799 So this applies directly to the diffusely scattered light on 290 00:14:04.840 --> 00:14:05.440 our sale. 291 00:14:05.480 --> 00:14:09.559 It does. As the sail accelerates, the stationary observer back 292 00:14:09.559 --> 00:14:15.039 on Earth sees those randomly scattered rear word photons essentially folding. 293 00:14:14.639 --> 00:14:16.320 Forward, folding forward. 294 00:14:16.519 --> 00:14:19.360 Their angles of emission are literally swept into the direction 295 00:14:19.440 --> 00:14:21.360 of travel by the relativistic speeds. 296 00:14:21.399 --> 00:14:24.399 Okay, so, if I'm tracking the math here, even though 297 00:14:24.399 --> 00:14:27.120 the light is physically emitted from the back surface of 298 00:14:27.159 --> 00:14:31.600 the sale, the extreme forward velocity of the sale itself 299 00:14:31.799 --> 00:14:36.360 just completely overpowers the rear word trajectory of those photons. Yes, 300 00:14:36.840 --> 00:14:39.799 to the stationary universe, that light is no longer moving 301 00:14:39.799 --> 00:14:43.399 backward at all. It is hurdling forward ahead of the sale. 302 00:14:43.559 --> 00:14:47.200 That's exactly what's happening. And because momentum absolutely must be 303 00:14:47.240 --> 00:14:49.200 conserved in all reference frames. 304 00:14:48.919 --> 00:14:51.799 We have a catastrophic inversion of Newton's third. 305 00:14:51.639 --> 00:14:55.720 Law exactly think about it. To generate forward thrust you 306 00:14:55.799 --> 00:14:59.320 must expel energy or mass opposite to your direction of. 307 00:14:59.240 --> 00:15:01.440 Travel, throw something backward to go forward. 308 00:15:01.639 --> 00:15:05.639 Yes. When the diffuse photons were scattering backward at lower speeds, 309 00:15:06.039 --> 00:15:10.639 they provided a tiny, albeit inefficient, net forward push. But 310 00:15:10.720 --> 00:15:14.080 as the Sale reaches point seventy five C, the relativistic 311 00:15:14.159 --> 00:15:18.240 aberration becomes so incredibly extreme that the net momentum vector 312 00:15:18.279 --> 00:15:21.840 of the diffusely scattered light actually points forward in the 313 00:15:21.879 --> 00:15:23.120 stationary frame. 314 00:15:22.960 --> 00:15:26.320 Meaning the Sale is now effectively expelling photon momentum in 315 00:15:26.360 --> 00:15:27.519 its direction of travel. 316 00:15:27.639 --> 00:15:30.679 Yes, and the reaction force to a forward emission. 317 00:15:30.399 --> 00:15:32.000 Is a backward push exactly. 318 00:15:32.399 --> 00:15:35.440 Diffew scattering flips from a weak thruster into an active 319 00:15:35.480 --> 00:15:36.399 drag parachute. 320 00:15:36.440 --> 00:15:38.879 That is just it's mind blowing. The universe is taking 321 00:15:38.960 --> 00:15:41.559 the gigawatts of energy that we're actively pumping into the 322 00:15:41.559 --> 00:15:45.159 Sale from Earth, and it's using the Sale's own microscopic 323 00:15:45.159 --> 00:15:47.360 imperfections to break the spacecraft. 324 00:15:47.440 --> 00:15:50.240 It is an incredible, highly counterintuitive speed trap. 325 00:15:50.360 --> 00:15:52.320 It really is. The faster you try to push pass 326 00:15:52.360 --> 00:15:55.000 point seventy five C, the more forcefully your own scattered 327 00:15:55.080 --> 00:15:57.559 light shoves you backward. Right, Which I mean that raises 328 00:15:57.600 --> 00:16:00.600 the immediate question of whether this represents a hard physical limit, like, 329 00:16:00.879 --> 00:16:03.799 does the sale hit this relativistic wall and just completely 330 00:16:03.799 --> 00:16:05.200 stop accelerating entirely? 331 00:16:05.519 --> 00:16:08.919 And that is a vital distinction to make based on 332 00:16:09.080 --> 00:16:12.440 Shen and Lye's paper, It does not represent a hard stop. 333 00:16:12.600 --> 00:16:15.679 It doesn't. No, the net force on the sale remains 334 00:16:15.720 --> 00:16:17.279 mathematically positive. 335 00:16:16.960 --> 00:16:19.080 Okay, So it keeps moving faster. 336 00:16:19.480 --> 00:16:23.320 Yes, the forward thrust generated by the incident light plus 337 00:16:23.360 --> 00:16:27.480 whatever remaining specular reflection you have, even heavily weakened by 338 00:16:27.519 --> 00:16:30.159 the Doppler redshift we talked about earlier, all of that 339 00:16:30.279 --> 00:16:34.480 still manages to just barely overpower the backward drag generated 340 00:16:34.519 --> 00:16:36.320 by the aberrated diffuse scattering. 341 00:16:36.480 --> 00:16:39.600 So the sale will technically continue to accelerate pass point 342 00:16:39.600 --> 00:16:40.240 seven five c. 343 00:16:40.639 --> 00:16:45.000 It will, However, the energy economics become completely untenable. 344 00:16:44.600 --> 00:16:46.720 Right because you're fighting yourself exactly. 345 00:16:46.759 --> 00:16:49.919 You are pouring staggering, mind boggling amounts of energy into 346 00:16:49.960 --> 00:16:52.360 the propulsion system, and the vast majority of it is 347 00:16:52.399 --> 00:16:55.440 being consumed simply to fight the drag force created by 348 00:16:55.440 --> 00:16:57.120 your own messy photon emissions. 349 00:16:57.240 --> 00:16:59.960 So the cost of benefit ratio of pushing a spacecraft 350 00:17:00.120 --> 00:17:02.080 from say zero point seven to five c to point 351 00:17:02.159 --> 00:17:04.640 eight c. That might require more energy than pushing it 352 00:17:04.680 --> 00:17:06.480 all away from zero too point seven to five c 353 00:17:06.599 --> 00:17:07.079 in the first. 354 00:17:06.960 --> 00:17:09.640 Place, easily could. It's a wall of diminishing returns. 355 00:17:10.039 --> 00:17:14.880 But we also have to acknowledge the boundaries of the 356 00:17:14.880 --> 00:17:19.319 theoretical physics models being used here, right, Because Schen and 357 00:17:20.119 --> 00:17:23.480 Lye are relying on the concept of an idealized mirror 358 00:17:23.559 --> 00:17:25.039 interacting with pure radiation. 359 00:17:25.279 --> 00:17:29.519 Right, they map the absolute theoretical limits of photon momentum transfer. 360 00:17:29.279 --> 00:17:33.920 But to isolate those specific relativistic variables, they necessarily had 361 00:17:33.960 --> 00:17:38.400 to strip away the environmental reality of the actual interstellar medium, which. 362 00:17:38.240 --> 00:17:42.160 Is a huge factor because the vacuum of space isn't actually. 363 00:17:41.839 --> 00:17:44.119 Empty, right, what's out there. 364 00:17:44.039 --> 00:17:47.920 Well, it contains trace amounts of interstellar gas, primarily hydrogen 365 00:17:47.960 --> 00:17:51.720 and helium, along with microscopic dust greens scattered about. 366 00:17:52.079 --> 00:17:54.839 Now, when a spacecraft is moving at slow speeds like 367 00:17:54.880 --> 00:17:58.079 or voyager probes, the interstellar medium is basically a. 368 00:17:58.079 --> 00:18:01.759 Non issue, completely negligible. But kinetic energy scales with the 369 00:18:01.839 --> 00:18:05.079 square of velocity, and at relativistic speeds you have to 370 00:18:05.079 --> 00:18:07.480 factor in the Lorentz multiplier on top of that, so 371 00:18:07.559 --> 00:18:10.240 it gets bad, very bad. Yeah. Traveling at seventy five 372 00:18:10.240 --> 00:18:12.640 percent of the speed of light, a collision with a 373 00:18:12.680 --> 00:18:16.799 single stationary hydrogen atom is essentially a high energy particle 374 00:18:16.799 --> 00:18:21.000 physics event. You are essentially flying a giant particle accelerator 375 00:18:21.400 --> 00:18:23.759 directly into a cloud of ambient gas. 376 00:18:24.119 --> 00:18:27.000 So the physical drag from just slamming into that gas 377 00:18:27.079 --> 00:18:28.359 would be immense on its. 378 00:18:28.240 --> 00:18:31.440 Own, immense, and beyond just the kinetic drag, you have 379 00:18:31.720 --> 00:18:35.039 the thermal nightmare of bremstralling radiation. 380 00:18:35.720 --> 00:18:39.799 Bremstrallng radiation. What is that in this context. 381 00:18:39.359 --> 00:18:44.359 It's breaking radiation. Those relativistic collisions with the hydrogen atoms 382 00:18:44.519 --> 00:18:47.759 are going to generate a massive bowshock of X rays 383 00:18:47.759 --> 00:18:48.799 and gamma rays. 384 00:18:48.720 --> 00:18:52.359 Meaning you are dumping unimaginable amounts of heat directly into 385 00:18:52.400 --> 00:18:53.799 the sale itself exactly. 386 00:18:53.839 --> 00:18:57.440 The theoretical paper treats the sale as an intantly resilient 387 00:18:57.480 --> 00:19:00.839 geometric plane, which is necessary for the math, but in 388 00:19:00.880 --> 00:19:04.279 reality you have a multi taro watte laser pumping immense 389 00:19:04.319 --> 00:19:07.240 heat into the back of the sale and relativistic hydrogen 390 00:19:07.319 --> 00:19:09.599 atoms blasting gamma ray heat into the front. 391 00:19:09.680 --> 00:19:12.559 So just sustaining the physical integrity of this ultra thin, 392 00:19:12.799 --> 00:19:15.279 highly reflective material under those thumbal. 393 00:19:15.079 --> 00:19:17.079 Lows without it instantly flashing into plasma. 394 00:19:17.200 --> 00:19:20.319 Yeah, without it vaporizing. That is an engineering hurdle that 395 00:19:20.440 --> 00:19:23.519 honestly dwarfs the math of relativistic light operation. 396 00:19:23.960 --> 00:19:27.279 It really does. The material science required to actually survive 397 00:19:27.359 --> 00:19:32.599 the journey dictates that we cannot rely on simple reflective. 398 00:19:32.000 --> 00:19:35.680 Metals like mylar or aluminum foils, right, which were the 399 00:19:35.720 --> 00:19:38.279 staples of all the early solar sale concepts. 400 00:19:38.559 --> 00:19:42.200 Right, Those would melt in a microsecond. The solutions being 401 00:19:42.279 --> 00:19:46.160