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 --> 00:00:26.879 slumber under the night sky. 7 00:00:27.000 --> 00:00:30.320 So I was actually walking through the Museum of Modern 8 00:00:30.399 --> 00:00:31.359 Art a few months ago. 9 00:00:31.440 --> 00:00:33.399 Oh nice, Yeah, trying to get. 10 00:00:33.240 --> 00:00:35.640 Some culture, you know it is, And I found myself 11 00:00:35.679 --> 00:00:38.200 just kind of standing in front of that small, almost 12 00:00:38.280 --> 00:00:40.799 unassuming canvas by Salvat or. 13 00:00:40.840 --> 00:00:43.240 Dolly the persistence of memory. 14 00:00:43.039 --> 00:00:46.439 Exactly, the melting clocks, right, you know, the pocket watches 15 00:00:46.520 --> 00:00:50.280 draped over dead tree branches and sliding off tables looking like, 16 00:00:50.600 --> 00:00:52.600 I know, slices of cheese left out in the sun. 17 00:00:52.759 --> 00:00:55.000 It is a very striking visual. 18 00:00:54.719 --> 00:00:57.280 It really is. And usually when you look at that painting, 19 00:00:57.359 --> 00:01:00.759 you think about the psychology of time. You think about 20 00:01:00.799 --> 00:01:04.120 how an hour in a dentist's chair feels. 21 00:01:03.840 --> 00:01:05.599 Like a week, Oh yeah, absolutely, and. 22 00:01:05.560 --> 00:01:07.560 An hour with a good friend feels like a minute. 23 00:01:08.200 --> 00:01:10.359 We did to think of memory and time as these 24 00:01:10.760 --> 00:01:13.120 purely internal fluid. 25 00:01:12.760 --> 00:01:14.799 Experiences, completely subjective, right. 26 00:01:14.799 --> 00:01:18.359 Completely subjective. But standing there looking at those warped watches, 27 00:01:19.680 --> 00:01:23.280 I started wondering, well, I started wondering if Dolly was 28 00:01:23.319 --> 00:01:26.200 accidentally stumbling onto a literal truth about physics. 29 00:01:26.280 --> 00:01:29.000 That is a fascinating leap, because we tend to think 30 00:01:29.040 --> 00:01:31.599 of the physical universe, you know, the hard vacuum of 31 00:01:31.640 --> 00:01:33.959 space as this rigid container. 32 00:01:34.640 --> 00:01:37.040 A clock is a clock, a meter is a meter. 33 00:01:37.239 --> 00:01:39.959 The stage where things happen exactly the stage. 34 00:01:40.599 --> 00:01:43.200 But what if the contenter itself? What if the stage 35 00:01:43.239 --> 00:01:46.280 actually melts? What if the universe has a memory and 36 00:01:46.319 --> 00:01:48.680 that memory looks like a permanent physical distortion. 37 00:01:48.920 --> 00:01:51.879 That is honestly the perfect mental anchor for what we 38 00:01:51.920 --> 00:01:52.879 are unpacking today. 39 00:01:52.959 --> 00:01:54.079 I thought it fit pretty well. 40 00:01:54.120 --> 00:01:57.319 It does because in the world of general relativity, specifically 41 00:01:57.319 --> 00:02:00.920 in the study of gravitational waves, there is a phenomenon 42 00:02:01.120 --> 00:02:05.120 literally called memory capital M memory capital M, and it 43 00:02:05.200 --> 00:02:07.879 suggests exactly what you just said. It suggests the universe 44 00:02:07.920 --> 00:02:11.120 isn't just a static stage that events happen on. It 45 00:02:11.240 --> 00:02:13.759 suggests the stage itself is recording the play. 46 00:02:14.120 --> 00:02:15.960 And just so you listen, are clear, we aren't talking 47 00:02:15.960 --> 00:02:18.520 about metaphor here, No, not at all. We aren't talking 48 00:02:18.520 --> 00:02:21.759 about history in the sense of writing it down in 49 00:02:21.800 --> 00:02:25.360 a physics textbook. We are talking about the actual fabric 50 00:02:25.520 --> 00:02:30.400 of space time getting bent by a catastrophic event and then, 51 00:02:30.680 --> 00:02:33.120 and this is the crazy part, never bending back. 52 00:02:33.319 --> 00:02:36.240 That is the core concept. It is a permanent scar 53 00:02:36.439 --> 00:02:38.479 on the geometry of reality. 54 00:02:38.080 --> 00:02:40.639 A permanent scar. So for this deep dive, we are 55 00:02:40.639 --> 00:02:43.240 looking at a brand new study. This is published in 56 00:02:43.280 --> 00:02:46.520 Physical Review Letters in February twenty twenty. 57 00:02:46.319 --> 00:02:48.120 Six rash off the presses, right. 58 00:02:48.560 --> 00:02:52.400 It's titled the Persistence of Gravitational Wave Memory, and it's 59 00:02:52.479 --> 00:02:54.919 led by Antonio Soccaros and his team. 60 00:02:54.719 --> 00:02:56.960 Who are out of the University of Illinois at Urbana 61 00:02:57.039 --> 00:02:59.680 Champagne and the Academy of Athens I believe, Yeah. 62 00:02:59.520 --> 00:03:02.039 Along with with collaborators for the University of Valencia and 63 00:03:02.080 --> 00:03:06.400 Montclair State. And they've basically been simulating what happens when 64 00:03:06.479 --> 00:03:09.039 two of the most violent objects in the cosmos slam 65 00:03:09.080 --> 00:03:09.599 into each. 66 00:03:09.479 --> 00:03:11.400 Other binary neutron stars. 67 00:03:11.520 --> 00:03:14.120 Right. And our mission today for this deep dive is 68 00:03:14.120 --> 00:03:17.319 to really understand how the universe records its own history. 69 00:03:17.520 --> 00:03:18.479 It is quite the mission. 70 00:03:18.680 --> 00:03:21.159 It is we need to figure out why neutron stars 71 00:03:21.159 --> 00:03:24.039 are so much more complicated than black holes, what role 72 00:03:24.120 --> 00:03:27.840 ghost particles like Neutrino's playing all this, and ultimately why 73 00:03:27.879 --> 00:03:30.800 this new study is considered such a massive stress test 74 00:03:30.840 --> 00:03:32.240 for Einstein's theory. 75 00:03:32.280 --> 00:03:35.680 It is the ultimate stress test, but it takes something 76 00:03:35.680 --> 00:03:39.319 we thought we understood, gravitational waves, and adds a layer 77 00:03:39.360 --> 00:03:43.360 of messiness that is frankly terrifying to simulate the mess. 78 00:03:43.439 --> 00:03:45.080 We love the mess, But before we get to the 79 00:03:45.080 --> 00:03:48.039 MESSI stress test, let's set the baseline good idea, because 80 00:03:48.280 --> 00:03:50.199 we need to understand the difference between a ripple and 81 00:03:50.240 --> 00:03:53.719 a scar. Most people listening have probably heard of gravitational 82 00:03:53.719 --> 00:03:57.240 waves by now. Ligo detected them back in twenty fifteen. 83 00:03:57.000 --> 00:03:58.439 Nobel prizes were handed. 84 00:03:58.199 --> 00:04:02.199 Out exactly we know that and big things crash space ripples. 85 00:04:03.080 --> 00:04:06.000 But to understand the new stuff, we need to know 86 00:04:06.080 --> 00:04:10.280 what exactly we are crashing together in this specific study. 87 00:04:10.520 --> 00:04:13.719 Right, so, we are looking at binary neutrons stars, two 88 00:04:13.919 --> 00:04:15.599 neutron stars orbiting each other. 89 00:04:16.120 --> 00:04:19.800 Give me the physical reality check here. I know they 90 00:04:19.800 --> 00:04:22.879 are dense, but help me visualize what we're actually dealing with. 91 00:04:22.959 --> 00:04:25.680 Sure, so imagine a star that started out much much 92 00:04:25.759 --> 00:04:28.560 larger than our own Sun. It burns through all its 93 00:04:28.639 --> 00:04:32.120 nuclear fuel over millions of years, and then the core collapses. 94 00:04:32.720 --> 00:04:36.319 It goes supernova, the big boom, the biggest. The outer 95 00:04:36.439 --> 00:04:39.319 layers blow off into space, but the core itself gets 96 00:04:39.439 --> 00:04:43.160 crushed inwards by its own gravity. It is squeezing so 97 00:04:43.279 --> 00:04:47.120 hard that the electrons and protons inside the atoms are 98 00:04:47.160 --> 00:04:49.879 literally forced together to become neutrons. 99 00:04:50.040 --> 00:04:52.920 So it's basically just a city sized atomic nucleus. 100 00:04:53.000 --> 00:04:55.839 That's the classic analogy, and it holds up perfectly. You 101 00:04:55.879 --> 00:04:59.480 have a perfect sphere, maybe twelve to fifteen miles across, 102 00:05:00.040 --> 00:05:02.720 the size of Manhattan or London. Yeah, but it contains 103 00:05:02.800 --> 00:05:04.920 up to two times the mass of our entire Sun. 104 00:05:05.120 --> 00:05:07.040 That density is just it's hard to even wrap your 105 00:05:07.040 --> 00:05:07.439 head around. 106 00:05:07.439 --> 00:05:10.959 It is. Try this. If you took a teaspoon of 107 00:05:11.000 --> 00:05:14.639 neutron star material, literally just a sugar cubesworth, it would 108 00:05:14.680 --> 00:05:17.519 weigh about a billion tons. 109 00:05:17.240 --> 00:05:19.480 A billion tons, Yeah, in a teaspoon. 110 00:05:19.560 --> 00:05:22.319 That's roughly the weight of Mount Everest compressed into a spoon. 111 00:05:22.480 --> 00:05:25.319 Okay, wow, So we are taking two of these Everest 112 00:05:25.519 --> 00:05:28.560 spoon stars, spinning them around each other at a fraction 113 00:05:28.639 --> 00:05:31.079 of the speed of light and just smashing them. 114 00:05:31.000 --> 00:05:34.000 Together exactly, And as they spiral in towards each other, 115 00:05:34.160 --> 00:05:37.040 they churn up the fabric of space time. They create 116 00:05:37.079 --> 00:05:38.199 gravitational waves. 117 00:05:38.399 --> 00:05:40.519 Now, the standard explanation, you know, the one I always 118 00:05:40.519 --> 00:05:43.040 see in headlines, is the ripple in the pond analogy, right, 119 00:05:43.079 --> 00:05:44.920 the classes You throw a rock into the water and 120 00:05:45.000 --> 00:05:47.959 ripples move outwards. If I'm a rubber duck floating on 121 00:05:48.000 --> 00:05:50.079 that pond, I just bob up and down as the 122 00:05:50.160 --> 00:05:51.000 ripple goes by. 123 00:05:51.079 --> 00:05:53.920 Right, And in this analogy, the rubber duck represents a 124 00:05:53.920 --> 00:05:57.600 detector here on Earth, like Lego or maybe two astronauts 125 00:05:57.600 --> 00:06:01.480 floating in space. Okay, As the gravitude wave passes through them, 126 00:06:01.879 --> 00:06:05.800 the actual space between them stretches and squeezes. They get 127 00:06:05.839 --> 00:06:10.040 closer than further, than closer than further, Bobby, exactly. That 128 00:06:10.199 --> 00:06:13.240 is the oscillatory part of the wave, the back and forth. 129 00:06:13.360 --> 00:06:15.759 And this is the key thing I want to focus on. 130 00:06:15.759 --> 00:06:19.199 Once the wave passes, the bond goes flat again, the 131 00:06:19.319 --> 00:06:22.680 rubbert duck ends up exactly where it started. The astronauts 132 00:06:22.680 --> 00:06:24.160 return to their original distance. 133 00:06:24.519 --> 00:06:27.720 That is what happens with a Spandard wave. Yes, it's temporary. 134 00:06:28.000 --> 00:06:31.040 The universe is assumed to be elastic, it snaps back. 135 00:06:31.800 --> 00:06:35.600 But the memory effect that this paper's about, yeah, says well, 136 00:06:35.639 --> 00:06:36.399 it says that's wrong. 137 00:06:36.600 --> 00:06:40.560 It says it's mostly right, but not entirely right. General 138 00:06:40.600 --> 00:06:43.519 relativity actually predicts that, in addition to that back and 139 00:06:43.519 --> 00:06:45.800 forth oscillation, there is a DC. 140 00:06:45.800 --> 00:06:48.759 Component DC like direct current. 141 00:06:48.959 --> 00:06:51.079 Yes, a direct current, a permanent offset. 142 00:06:51.160 --> 00:06:53.199 So to go back to the astronauts in space. 143 00:06:53.120 --> 00:06:56.000 The wave passes, they bob back and forth. But when 144 00:06:56.000 --> 00:06:59.160 the wave is completely gone and space is quiet again, 145 00:06:59.560 --> 00:07:01.439 they look at each other and realize they are now 146 00:07:01.639 --> 00:07:05.160 slightly further apart or slightly closer together than they were 147 00:07:05.199 --> 00:07:05.759 before the. 148 00:07:05.720 --> 00:07:07.560 Wave hit, and they stay that way forever. 149 00:07:07.879 --> 00:07:10.319 They do not snap back. The ruler itself has been 150 00:07:10.439 --> 00:07:13.759 lengthened or shortened. Space time has been permanently displaced. 151 00:07:13.959 --> 00:07:17.040 That is haunting. I mean, it implies that every single 152 00:07:17.040 --> 00:07:20.560 time two black holes or neutron stars merge anywhere in 153 00:07:20.560 --> 00:07:24.279 the universe, the geometry of the entire cosmos gets shifted 154 00:07:24.279 --> 00:07:24.959 a tiny bit. 155 00:07:25.000 --> 00:07:27.800 It does. It's a very very small shift, usually undetected 156 00:07:27.800 --> 00:07:30.839 by our current instruments because it's so subtle. But mathematically 157 00:07:30.879 --> 00:07:33.759 it has to be there the universe literally remembers the 158 00:07:33.759 --> 00:07:35.319 event by changing its shape. 159 00:07:35.399 --> 00:07:38.639 Okay, so that's the what. It's a permanent scar on reality. 160 00:07:39.240 --> 00:07:41.639 But I really want to get into the why, because 161 00:07:41.680 --> 00:07:43.800 looking at the history of this theory and the sources, 162 00:07:44.160 --> 00:07:46.959 it seems like physicists have been arguing about this for 163 00:07:47.000 --> 00:07:47.759 like fifty years. 164 00:07:47.759 --> 00:07:50.920 Oh easily. It's definitely not a new idea. Even if 165 00:07:50.959 --> 00:07:54.920 these specific supercomputer simulations are new. The foundation goes all 166 00:07:54.959 --> 00:07:56.879 the way back to the nineteen seventies. 167 00:07:56.600 --> 00:07:59.120 Nineteen seventy four with Zoldovich and. 168 00:07:59.120 --> 00:08:02.920 Polmer f yes Yakov Zeldovitch and Alexander palm Reft, two 169 00:08:03.360 --> 00:08:07.560 brilliant Soviet physicists. They were working with what we call 170 00:08:07.759 --> 00:08:09.199 linearized gravity. 171 00:08:09.360 --> 00:08:13.600 Linearized gravity, which is basically the light version of Einstein's equation. 172 00:08:13.519 --> 00:08:17.279 Exactly the light version, the simpler approximation, and even in 173 00:08:17.319 --> 00:08:19.600 that leet version, they realized that if you have a 174 00:08:19.680 --> 00:08:22.879 system of stars that permanently changes its arrangement, say a 175 00:08:22.920 --> 00:08:27.240 cluster of superdense stars suddenly flying apart, the gravitational field 176 00:08:27.279 --> 00:08:29.439 at a great distance creates this prominent shift. 177 00:08:29.600 --> 00:08:31.639 It was just a theoretical prediction at that. 178 00:08:31.560 --> 00:08:35.360 Point, purely theoretical The logic was simple, though. If masses 179 00:08:35.360 --> 00:08:38.120 move away to infinity, the metric of space time has to. 180 00:08:38.120 --> 00:08:40.639 Change, so that's what we call the linear memory. The 181 00:08:40.679 --> 00:08:44.559 stars moved, so the gravity changed basic cause and effect. 182 00:08:44.720 --> 00:08:49.639 Right, But then came the nineties and things got profoundly weird. 183 00:08:50.159 --> 00:08:51.840 Entered Demetrio's Christa d'lieu. 184 00:08:52.240 --> 00:08:55.399 Yes. In nineteen ninety one, he published a paper that 185 00:08:55.480 --> 00:08:58.840 is still considered a massive landmark in the field. He 186 00:08:58.879 --> 00:09:01.600 didn't use the light version. He looked at the full, 187 00:09:02.000 --> 00:09:05.240 unadulterated nonlinear Einstein equations. 188 00:09:05.360 --> 00:09:07.559 I actually want to pause on that word. Non linear. 189 00:09:07.639 --> 00:09:09.720 We hear it all the time, right, non linear dynamic 190 00:09:09.759 --> 00:09:13.039 is non linear storytelling? In this context? Does it just 191 00:09:13.159 --> 00:09:14.159 mean complicated? 192 00:09:14.320 --> 00:09:17.399 It means that the output of a system literally becomes 193 00:09:17.440 --> 00:09:18.919 an input for the system. 194 00:09:18.960 --> 00:09:19.919 Okay, give me an example. 195 00:09:19.960 --> 00:09:22.639 In a linear system, you have a choir singing. That's 196 00:09:22.720 --> 00:09:25.240 the source, and the sound waves are the output. The 197 00:09:25.279 --> 00:09:27.480 sound waves travel through the air to your ear. They 198 00:09:27.480 --> 00:09:28.879 don't really interact with each other. 199 00:09:29.039 --> 00:09:29.159 Right. 200 00:09:29.200 --> 00:09:31.639 If the choir sing's louder, the sound is just louder. 201 00:09:31.639 --> 00:09:34.000 It's a straight line from cause to effect exactly. 202 00:09:34.120 --> 00:09:37.639 But in a nonlinear system, imagine the choir sings so 203 00:09:37.799 --> 00:09:41.320 incredibly loud that the sound waves actually compress the air 204 00:09:41.440 --> 00:09:44.559 enough to heat it up. That heat changes the density 205 00:09:44.600 --> 00:09:46.720 of the air in the room, which then changes how 206 00:09:46.720 --> 00:09:50.039 the sound travels, which might create entirely new sound frequencies. 207 00:09:50.399 --> 00:09:53.519 The sound itself is changing the medium it travels through. 208 00:09:53.440 --> 00:09:56.240 The output messages of the input. So apply that to gravity. 209 00:09:56.399 --> 00:09:59.600 Well. Einstein's great insight was that mass equals energy E 210 00:09:59.600 --> 00:10:03.720 equals mc squared. You have mass, you have gravity. But 211 00:10:03.840 --> 00:10:06.720 if you have energy, you also have gravity. 212 00:10:06.759 --> 00:10:09.320 And gravitational wave they carry energy exactly. 213 00:10:09.480 --> 00:10:11.919 We know they carry energy because we can detect them. 214 00:10:12.240 --> 00:10:15.919 They physically move the heavy mirrors in the Lego detectors. 215 00:10:16.159 --> 00:10:20.000 That takes actual work. It takes energy. So Chris Dedooley realized, 216 00:10:20.320 --> 00:10:24.000 if gravitational waves carry energy, then gravitational waves must have 217 00:10:24.039 --> 00:10:26.600 their own gravitational feel Waitt. 218 00:10:26.759 --> 00:10:27.799 The gravity has gravity. 219 00:10:27.840 --> 00:10:28.919 The gravity has gravity. 220 00:10:29.000 --> 00:10:31.320 That sounds like a recursive loop that would just immediately 221 00:10:31.320 --> 00:10:32.159 crash a computer. 222 00:10:32.440 --> 00:10:36.559 It makes the math incredibly difficult, which is exactly why 223 00:10:36.600 --> 00:10:39.320 it took from nineteen seventy four until nineteen ninety one 224 00:10:39.399 --> 00:10:42.480 to mathematically prove it. Christ to Doos showed that as 225 00:10:42.519 --> 00:10:45.600 these waves ripple out from a collision, the massive energy 226 00:10:45.600 --> 00:10:49.399 in the waves themselves creates a secondary gravitational distortion. 227 00:10:50.039 --> 00:10:52.759 So it's not just the physical stars smashing together that 228 00:10:52.840 --> 00:10:56.240 creates the memory scar it's the waves themselves piling up 229 00:10:56.240 --> 00:10:56.879 as they travel. 230 00:10:57.000 --> 00:11:00.639 Yes, this is called the nonlinear memory, where the Christad memory, 231 00:11:01.039 --> 00:11:04.440 the waves effectively gravitate, They pull on space time as 232 00:11:04.519 --> 00:11:07.720 they travel through it, and this effect accumulates. It builds 233 00:11:07.799 --> 00:11:10.840 up over the entire duration of the wave signal. 234 00:11:11.080 --> 00:11:12.960 So we have the linear memory, which is just the 235 00:11:12.960 --> 00:11:16.279 stars moved, and the nonlinear memory, which is the energy 236 00:11:16.320 --> 00:11:18.120 of the scream scarred the air. 237 00:11:18.240 --> 00:11:20.480 That is a fantastic way to put it, the energy 238 00:11:20.519 --> 00:11:22.639 of the scream scarred the air. And for a long time, 239 00:11:22.639 --> 00:11:24.720 that combined picture was all we had, and we modeled 240 00:11:24.720 --> 00:11:25.960 this mostly using. 241 00:11:25.759 --> 00:11:27.679 Black holes because black holes are clean. 242 00:11:27.919 --> 00:11:30.960 They are the simplest macroscopic objects in the universe. A 243 00:11:31.000 --> 00:11:34.399 black hole has mass, it has spin, and it can 244 00:11:34.480 --> 00:11:38.159 have an electric charge. That's it. No surface, no atmosphere, 245 00:11:38.200 --> 00:11:41.919 no chemistry, just pure curved space time. So when you 246 00:11:42.000 --> 00:11:45.679 program a computer to simulate two black holes merging, you 247 00:11:45.679 --> 00:11:48.799 don't have to worry about fluid dynamics or magnetic fields 248 00:11:48.840 --> 00:11:52.360 or particle physics. It's pure gravity. 249 00:11:52.039 --> 00:11:55.360 But the universe isn't always clean, and that brings us 250 00:11:55.399 --> 00:11:58.879 to the actual meat of the new study by Socaros 251 00:11:58.879 --> 00:12:01.399 in his team. They didn't want the clean version. They 252 00:12:01.399 --> 00:12:05.759 wanted the full messy reality of neutron stars. 253 00:12:05.840 --> 00:12:09.279 And neutron stars are distinct because they are actual physical 254 00:12:09.320 --> 00:12:12.440 objects made of ultra dense matter. They bring extra ingredients 255 00:12:12.480 --> 00:12:15.080 to the party. They have magnetic fields, they emit light, 256 00:12:15.559 --> 00:12:18.360 and crucially they emit neutrinos. 257 00:12:17.840 --> 00:12:21.320 And so Kros is essentially asked, do these extra ingredients 258 00:12:21.360 --> 00:12:24.360 actually contribute to the permanent space time scar? Yes, which 259 00:12:24.399 --> 00:12:27.759 is a really fair question. Yeah, because if energy equals gravity, 260 00:12:27.960 --> 00:12:32.080 and neutrinos carry energy and magnetic fields hold energy, shouldn't 261 00:12:32.080 --> 00:12:33.000 they leave a mark too? 262 00:12:33.279 --> 00:12:36.879 Exactly? And to be fair, previous researchers had laid the 263 00:12:36.879 --> 00:12:41.600 groundwork for this, people like Lydia Bieri, Poning, Chen Shingtong Yao, 264 00:12:41.679 --> 00:12:44.240 and Dave Garfinkel. They had done the pen and paper 265 00:12:44.279 --> 00:12:48.000 math to prove that theoretically electromagnetic fields and neutrinos should 266 00:12:48.000 --> 00:12:49.399 contribute to the memory. 267 00:12:49.159 --> 00:12:52.159 I saw in the sources. This is sometimes called null memory. 268 00:12:52.600 --> 00:12:55.879 Why null Oh? It relates to radiation traveling along what 269 00:12:55.919 --> 00:12:59.960 physicists call null infinity, which is basically the mathematical boundary 270 00:13:00.279 --> 00:13:03.720 or path that light, rays and massless particles take to 271 00:13:03.720 --> 00:13:05.639 get infinitely far away from a source. 272 00:13:05.759 --> 00:13:08.440 Nold infinity has to say, physics has the best names. 273 00:13:08.480 --> 00:13:09.879 It sounds like an indie rock band. 274 00:13:09.919 --> 00:13:12.399 It really does. But the point is, while the theory 275 00:13:12.440 --> 00:13:15.559 for all memory was there, no one had successfully run 276 00:13:15.639 --> 00:13:20.039 a full scale numerical relativity simulation of a binary neutron 277 00:13:20.120 --> 00:13:22.519 star merger that actually quantified this. 278 00:13:22.720 --> 00:13:24.240 No one, if we put numbers to it, right. 279 00:13:24.120 --> 00:13:27.519 No oneted tract the neutrinos and the complex magnetic fields 280 00:13:27.559 --> 00:13:30.320 and said, okay, here's exactly how much of the memory 281 00:13:30.320 --> 00:13:31.879 they create until now. 282 00:13:32.559 --> 00:13:35.720 So let's get into the simulation. They built a digital universe, 283 00:13:36.080 --> 00:13:39.440 They put two massive neutron stars in it. They gave 284 00:13:39.480 --> 00:13:43.799 them realistic magnetic fields and neutrino emissions. What did they find? 285 00:13:44.240 --> 00:13:47.159 They found that the mess matters a lot. 286 00:13:47.360 --> 00:13:48.440 Put a number on it for me. 287 00:13:48.840 --> 00:13:52.919 The simulations suggest that the contributions from the extra ingredients, 288 00:13:52.960 --> 00:13:56.639 the magnetic fields, the neutrinos, and the buryonic ejecta, which 289 00:13:56.679 --> 00:13:59.320 is the physical matter thrown out of the crash. Account 290 00:13:59.559 --> 00:14:02.200 for anyone where from fifteen percent to fifty percent of 291 00:14:02.240 --> 00:14:03.879 the total gravitational wave memory. 292 00:14:03.960 --> 00:14:06.399 Fifty percent up to half of the scar on the 293 00:14:06.480 --> 00:14:08.279 universe is caused by the extras. 294 00:14:08.519 --> 00:14:11.360 Yes, if you were to simulate this event and just 295 00:14:11.399 --> 00:14:14.639 treat the neutron stars like black holes light, completely ignoring 296 00:14:14.679 --> 00:14:17.120 the neutrinos in the matter, you could be wrong by 297 00:14:17.120 --> 00:14:19.759 a factor of two. You'd be missing half the story. 298 00:14:20.080 --> 00:14:23.720 That completely changes the picture. It means you absolutely cannot 299 00:14:23.799 --> 00:14:25.639 just look at the gravity of the masses. You have 300 00:14:25.679 --> 00:14:26.919 to look at the particle physics. 301 00:14:26.960 --> 00:14:30.080 It forces a total marriage between gravity and nuclear physics. 302 00:14:30.320 --> 00:14:32.639 Take the neutrinos for example. I usually think of them 303 00:14:32.639 --> 00:14:35.840 as these ghostly little things that don't interact with anything, 304 00:14:36.480 --> 00:14:38.000 like there are millions passing through. 305 00:14:37.799 --> 00:14:40.519 My thumb right now, trillions actually trillions. 306 00:14:40.080 --> 00:14:43.440 And normally they are totally gravitationally insignificant, right. 307 00:14:43.879 --> 00:14:47.159 But a neutron star merger is not a normal environment. 308 00:14:47.840 --> 00:14:51.679 When these stars crash together, the temperature spikes to billions 309 00:14:51.679 --> 00:14:55.639 of degrees. The nuclear matter is literally being crushed and 310 00:14:55.679 --> 00:14:56.879 boiled at the same time. 311 00:14:57.159 --> 00:14:57.759 What does that do. 312 00:14:58.240 --> 00:15:01.000 It releases a flood of neutrinos that is frankly hard 313 00:15:01.000 --> 00:15:04.679 to comprehend. The paper mentions a total energy release of 314 00:15:04.720 --> 00:15:06.799 around ten to the fifty three ergs. 315 00:15:07.120 --> 00:15:09.200 Yeah, I saw that, and I know ergs is a 316 00:15:09.279 --> 00:15:11.879 unit of energy, but I have zero intuition for that 317 00:15:12.120 --> 00:15:13.440 is ten to the fifty three a lot. 318 00:15:13.559 --> 00:15:17.440 It's an astronomical number. For context, our Sun over its 319 00:15:17.600 --> 00:15:22.240 entire ten billion year lifespan will emit something like ten 320 00:15:22.320 --> 00:15:24.080 to the fifty one ergs of energy. 321 00:15:24.279 --> 00:15:27.360 Wait, so this single crash releases one hundred times more 322 00:15:27.440 --> 00:15:30.440 energy in just a few seconds just in neutrinos than 323 00:15:30.480 --> 00:15:33.519 the Sun does in its entire ten billion year life. 324 00:15:33.840 --> 00:15:37.279 Exactly. It is a sudden explosion of energy that briefly 325 00:15:37.320 --> 00:15:42.240 outshines the entire galaxy in neutrino radiation. And because E 326 00:15:42.240 --> 00:15:46.200 equals mc squared, that massive energy has a mass equivalence. 327 00:15:46.320 --> 00:15:49.600 So as these trillions of neutrinos stream out of the 328 00:15:49.639 --> 00:15:53.720 crash site in all directions, they're effectively carrying an immense 329 00:15:53.720 --> 00:15:55.120 amount of mass away with them. 330 00:15:55.200 --> 00:15:58.639 Right, imagine the crash site is losing weight incredibly rapidly. 331 00:15:59.200 --> 00:16:02.120 That sudden chain in the mass distribution that lightning of 332 00:16:02.159 --> 00:16:05.720 the central load causes a massive shift in the gravitational 333 00:16:05.720 --> 00:16:08.000 field that is the neutrino memory. 334 00:16:08.399 --> 00:16:10.519 It's like if you were standing on a trampoline holding 335 00:16:10.519 --> 00:16:13.799 a heavy bowling ball. The trampoline is curved down around you. 336 00:16:14.080 --> 00:16:16.840 If you suddenly throw the bowling ball away, the trampoline 337 00:16:16.879 --> 00:16:19.000 snaps to a new shallower shape. 338 00:16:19.039 --> 00:16:21.279 That's a very good analogy for the linear part of it. 339 00:16:21.320 --> 00:16:23.600 The mass has moved, so the curve changes. But there's 340 00:16:23.600 --> 00:16:26.720 also the nonlinear part we talked about. The neutrinos themselves 341 00:16:26.720 --> 00:16:29.559 are dragging space time as they fly outward, and the 342 00:16:29.600 --> 00:16:32.679 study showed this combined effect is a huge component of 343 00:16:32.679 --> 00:16:33.960 the final memory signal. 344 00:16:34.320 --> 00:16:37.320 And there's a specific visualization in the paper they mentioned 345 00:16:37.360 --> 00:16:40.120 that really caught my eye. A blue rectangle. Can you 346 00:16:40.159 --> 00:16:41.679 describe what we're looking at there? 347 00:16:41.919 --> 00:16:45.000 Sure? So this is a graph showing the strain, basically, 348 00:16:45.000 --> 00:16:48.320 the memory accumulation over time. On the horizontal axis you 349 00:16:48.399 --> 00:16:51.519 have time in milliseconds. On the vertical axis, you have 350 00:16:51.600 --> 00:16:52.559 the memory building up. 351 00:16:52.639 --> 00:16:54.759 And what exactly does the blue rectangle highlight? 352 00:16:54.919 --> 00:16:57.679 It highlights the time duration over which ninety percent of 353 00:16:57.720 --> 00:17:01.440 the post merger memory is accumulated. For a black hole merger. 354 00:17:01.679 --> 00:17:04.480 The memory happens in a literal snap. It's a step 355 00:17:04.519 --> 00:17:07.880 function on a graph. Nothing nothing s and app full memory, 356 00:17:08.039 --> 00:17:11.759 instant scarring. But for neutron stars, the blue rectangle shows 357 00:17:11.759 --> 00:17:15.799 that the memory builds up gradually. It takes tens of milliseconds, 358 00:17:15.880 --> 00:17:18.799 which sounds fast, but is an absolute eternity in this 359 00:17:19.079 --> 00:17:22.839 specific physics context for the memory to reach its full height. 360 00:17:23.319 --> 00:17:25.799 Why the delay? Why doesn't it just snap like a 361 00:17:25.799 --> 00:17:26.400 black hole. 362 00:17:26.640 --> 00:17:29.440 Because the neutrinos take time to leak out of the 363 00:17:29.519 --> 00:17:33.640 ultra dense core. The ejected matter, these massive clouds of 364 00:17:33.759 --> 00:17:38.279 gold and platinum dust, takes time to expand outward, and 365 00:17:38.319 --> 00:17:42.440 the remnant itself, the hyper massive blob of matter left 366 00:17:42.440 --> 00:17:45.240 over from the crash, wobbles and slashes around for a 367 00:17:45.279 --> 00:17:47.440 while before finally settling down. 368 00:17:47.519 --> 00:17:49.799 So it's a slow motion scar exactly. 369 00:17:50.079 --> 00:17:53.079 And that slope how fast or slow the memory builds 370 00:17:53.200 --> 00:17:56.599 up tells you immense amounts about the internal physics of 371 00:17:56.640 --> 00:17:57.279 the explasion. 372 00:17:57.400 --> 00:17:59.920 Now, speaking of the internal physics, we have to talk 373 00:18:00.000 --> 00:18:02.799 about the magnetic fields because this is where my own 374 00:18:02.920 --> 00:18:04.839 naive assumption totally failed me. 375 00:18:04.880 --> 00:18:06.759 While I was reading what was your assumption. 376 00:18:06.480 --> 00:18:09.319 Okay, so my logic was pretty straightforward. I know these 377 00:18:09.359 --> 00:18:12.400 stars are giant magnets. Magnetars are some of the scariest 378 00:18:12.440 --> 00:18:16.000 things in space, and I know energy equals gravity. Magnetic 379 00:18:16.000 --> 00:18:17.039 fields contain energy. 380 00:18:17.200 --> 00:18:18.680 Solid logic, so far right. 381 00:18:18.720 --> 00:18:22.039 So if I take two stars with super strong magnetic 382 00:18:22.039 --> 00:18:24.759 fields and smash them together, I'm adding more energy to 383 00:18:24.759 --> 00:18:27.599 the whole system than if they were unmagnetized. More energy 384 00:18:27.599 --> 00:18:30.079 should mean more gravity, which should mean a much bigger 385 00:18:30.079 --> 00:18:30.680 memory scar. 386 00:18:31.039 --> 00:18:33.640 That is entirely logical. It is what we call the 387 00:18:33.759 --> 00:18:36.400 naive expectation. And I don't mean that as an insult 388 00:18:36.440 --> 00:18:39.240 at all. It's exactly what many physicists expected too before 389 00:18:39.240 --> 00:18:40.079 they ran the numbers. 390 00:18:40.279 --> 00:18:41.599 But the simulation said no. 391 00:18:42.000 --> 00:18:45.640 The simulation showed that in some cases, adding incredibly strong 392 00:18:45.680 --> 00:18:49.640 magnetic fields actually reduced the final gravitational wave memory. 393 00:18:49.720 --> 00:18:52.480 How was that possible? Where did all that magnetic energy go? 394 00:18:52.759 --> 00:18:56.680 The energy is absolutely there, but it's not making gravitational waves. 395 00:18:56.720 --> 00:18:59.480 You have to remember a key rule. Gravitational waves are 396 00:18:59.519 --> 00:19:00.960 generated by asymmetry. 397 00:19:01.119 --> 00:19:02.599 Asymmetry, yes, they are. 398 00:19:02.480 --> 00:19:05.960 Generated by what we call the quadrupole moment. Basically, you 399 00:19:06.039 --> 00:19:10.240 need a lumpy, uneven wobbling distribution of mass to shake 400 00:19:10.319 --> 00:19:10.960 space time. 401 00:19:11.200 --> 00:19:13.039