1 00:00:03,000 --> 00:00:07,240 We live in a world of matter. 2 00:00:07,240 --> 00:00:11,800 A realm of tiny particles far smaller than atoms, 3 00:00:11,800 --> 00:00:14,680 that build the universe that we know. 4 00:00:21,440 --> 00:00:23,400 But there is a mystery. 5 00:00:28,080 --> 00:00:31,480 Of all the particles scientists have discovered... 6 00:00:32,520 --> 00:00:37,600 ..the strangest and most elusive of them all seemed to defy our 7 00:00:37,600 --> 00:00:40,440 understanding of how the universe works. 8 00:00:43,040 --> 00:00:46,320 They're called neutrinos. 9 00:00:51,160 --> 00:00:53,920 Neutrinos are really remarkable particles. 10 00:00:53,920 --> 00:00:57,320 There are trillions and trillions of them streaming through our bodies, 11 00:00:57,320 --> 00:00:58,880 and we don't even notice. 12 00:00:58,880 --> 00:01:02,240 They are kind of ghostlike, and yet they're everywhere. 13 00:01:02,240 --> 00:01:04,880 Everywhere and nowhere. 14 00:01:04,880 --> 00:01:09,680 Neutrinos are so ghostly, they can pass through solid matter 15 00:01:09,680 --> 00:01:13,640 as if it didn't exist, and yet they hold the secrets 16 00:01:13,640 --> 00:01:18,040 to why the stars shine and what our universe is made of. 17 00:01:19,200 --> 00:01:23,280 The reason we care about these elusive particles is because they do 18 00:01:23,280 --> 00:01:28,000 play a fundamentally important role in the universe, in the nature 19 00:01:28,000 --> 00:01:31,960 of matter, in some of the most violent cosmic phenomena. 20 00:01:33,640 --> 00:01:36,880 Today, the quest to detect neutrinos has triggered 21 00:01:36,880 --> 00:01:40,400 multimillion-dollar experiments all over the globe. 22 00:01:40,400 --> 00:01:43,360 Even as someone who builds these experiments for a living, 23 00:01:43,360 --> 00:01:46,360 it just seems mind-blowing that they ever work. 24 00:01:47,520 --> 00:01:52,120 Now, tantalising new evidence suggests neutrinos could be a link 25 00:01:52,120 --> 00:01:55,360 between our familiar world of matter... 26 00:01:55,360 --> 00:01:59,880 ..and an unknown world of hidden particles waiting to be discovered. 27 00:02:02,440 --> 00:02:04,800 It would be a game changer. 28 00:02:04,800 --> 00:02:06,960 What exactly are these particles? 29 00:02:08,120 --> 00:02:11,400 The quest for answers has driven scientists to the edge of what 30 00:02:11,400 --> 00:02:14,160 is experimentally possible... 31 00:02:14,160 --> 00:02:18,160 ..to reveal a universe we've never seen before. 32 00:02:32,320 --> 00:02:36,800 Fermilab - world-renowned physics laboratory. 33 00:02:38,880 --> 00:02:42,600 Here, thousands of scientists build vast experiments to probe 34 00:02:42,600 --> 00:02:46,440 the very smallest particles that make-up our universe. 35 00:02:49,800 --> 00:02:52,800 Leading one of the teams is Sam Zeller. Hey, team. 36 00:02:54,400 --> 00:02:57,120 My interest in physics started when I signed up for a field trip 37 00:02:57,120 --> 00:03:00,400 to come to Fermilab in high school, that just blew my mind. 38 00:03:00,400 --> 00:03:03,480 From that point on, I was a particle physicist. 39 00:03:08,520 --> 00:03:11,840 Turns out that the universe can be described by a small number 40 00:03:11,840 --> 00:03:13,480 of subatomic particles. 41 00:03:15,840 --> 00:03:19,840 Today, scientists have discovered 17 basic particles 42 00:03:19,840 --> 00:03:21,880 that make up our universe. 43 00:03:23,600 --> 00:03:26,120 Some are the building blocks of atoms. 44 00:03:27,680 --> 00:03:31,040 Others are the things that hold matter together. 45 00:03:32,040 --> 00:03:35,920 It's an understanding of our world that physicists call 46 00:03:35,920 --> 00:03:38,160 the Standard Model. 47 00:03:38,160 --> 00:03:41,680 The Standard Model of particle physics describes the most 48 00:03:41,680 --> 00:03:43,680 fundamental constituents of matter 49 00:03:43,680 --> 00:03:46,320 and how they interact with each other. 50 00:03:46,320 --> 00:03:50,240 It is, in fact, THE most mathematically well-defined physical 51 00:03:50,240 --> 00:03:53,400 theory we as humans have ever written down. 52 00:03:54,680 --> 00:03:59,520 For 50 years, the Standard Model has withstood test after test, 53 00:03:59,520 --> 00:04:03,480 confirming the hierarchy of all the fundamental particles. 54 00:04:07,200 --> 00:04:10,400 But at Fermilab, the team are hunting for a particle 55 00:04:10,400 --> 00:04:13,520 that may turn the entire theory on its head. 56 00:04:15,000 --> 00:04:19,600 They're searching for a new and strange type of neutrino. 57 00:04:22,320 --> 00:04:26,920 A neutrino is a type of elementary particle, a basic fundamental 58 00:04:26,920 --> 00:04:29,840 building block of the universe. 59 00:04:29,840 --> 00:04:33,600 And they come in three different flavours. 60 00:04:33,600 --> 00:04:35,400 Neutrinos are everywhere. 61 00:04:35,400 --> 00:04:37,680 They are produced in the sun, 62 00:04:37,680 --> 00:04:42,400 there are neutrinos that were left over after the Big Bang. 63 00:04:42,400 --> 00:04:44,960 Humans make neutrinos. 64 00:04:46,480 --> 00:04:49,280 Neutrinos have got no electric charge. 65 00:04:49,280 --> 00:04:51,800 They've almost got no mass at all. 66 00:04:51,800 --> 00:04:54,440 They're as near to nothing as you can imagine. 67 00:04:54,440 --> 00:04:57,760 They're so reluctant to interact with stuff, they pass through 68 00:04:57,760 --> 00:05:00,240 the Earth as if it wasn't there. 69 00:05:02,160 --> 00:05:06,480 Yet despite their elusive behaviour, scientists at Fermilab 70 00:05:06,480 --> 00:05:09,320 are constructing an elaborate experiment to find 71 00:05:09,320 --> 00:05:12,440 an even stranger kind of neutrino... 72 00:05:12,440 --> 00:05:14,840 ..one that's never been detected before. 73 00:05:18,920 --> 00:05:21,800 One of the big goals here at Fermilab is to try to search 74 00:05:21,800 --> 00:05:25,760 for possibly a new type of neutrino that no-one has yet observed. 75 00:05:26,720 --> 00:05:30,880 This mysterious fourth type of neutrino would lie 76 00:05:30,880 --> 00:05:35,680 outside the three already known to exist, and could be a link 77 00:05:35,680 --> 00:05:38,320 to an unknown realm of new particles. 78 00:05:39,760 --> 00:05:44,720 If it's real, it would rewrite the equations of the Standard Model 79 00:05:44,720 --> 00:05:48,600 and transform our understanding of what the universe is made of. 80 00:05:48,600 --> 00:05:51,600 If we found evidence of a new type of neutrino, 81 00:05:51,600 --> 00:05:53,960 that would be really astounding. 82 00:05:53,960 --> 00:05:56,280 That's what gets me excited in the morning, that's what 83 00:05:56,280 --> 00:05:57,840 gets me coming into work. 84 00:05:57,840 --> 00:06:00,120 It would be a major and massive discovery. 85 00:06:05,840 --> 00:06:09,880 In the experiment's first stage, a powerful ring of magnets 86 00:06:09,880 --> 00:06:14,400 accelerates particles called protons to colossal speeds, 87 00:06:14,400 --> 00:06:17,920 sending them smashing into a graphite target. 88 00:06:19,040 --> 00:06:22,280 This collision creates a shower of new particles, 89 00:06:22,280 --> 00:06:25,960 including a powerful beam of neutrinos. 90 00:06:25,960 --> 00:06:29,880 Trillions pass directly through the solid ground 91 00:06:29,880 --> 00:06:34,320 at nearly the speed of light, racing towards the second stage... 92 00:06:34,320 --> 00:06:36,800 ..three giant neutrino detectors. 93 00:06:42,800 --> 00:06:45,760 This is the direction the neutrinos go. 94 00:06:45,760 --> 00:06:48,280 They're produced back behind us. 95 00:06:48,280 --> 00:06:53,240 Fermilab produces roughly about 150 trillion neutrinos per second, 96 00:06:53,240 --> 00:06:56,600 and only about one of them every minute will make its presence known 97 00:06:56,600 --> 00:06:58,200 in one of our detectors. 98 00:07:00,360 --> 00:07:04,840 Neutrinos interact with atoms so rarely, to increase the odds 99 00:07:04,840 --> 00:07:07,880 of catching them, the team have to build big. 100 00:07:09,320 --> 00:07:11,840 This is ICARUS, this is the ICARUS detector. 101 00:07:11,840 --> 00:07:15,000 We're about to deploy it in the world's most-intense neutrino beam 102 00:07:15,000 --> 00:07:17,080 at Fermilab, and the entire community 103 00:07:17,080 --> 00:07:18,840 is super-excited about this. 104 00:07:20,480 --> 00:07:24,920 Soon, this colossal tank packed with a web of electronics 105 00:07:24,920 --> 00:07:28,360 will be filled with 600 tons of cryogenic liquid. 106 00:07:30,800 --> 00:07:35,000 All in the hope of catching just one neutrino each minute. 107 00:07:37,840 --> 00:07:39,640 Inside the control room, 108 00:07:39,640 --> 00:07:43,480 one of the experiment's other detectors, called MicroBooNE, 109 00:07:43,480 --> 00:07:45,360 is already online. 110 00:07:47,160 --> 00:07:50,040 This is a live snapshot of what's occurring in the MicroBooNE 111 00:07:50,040 --> 00:07:52,080 detector as we speak. 112 00:07:52,080 --> 00:07:56,640 We expect about one neutrino interaction every few minutes. 113 00:07:56,640 --> 00:07:59,480 This, for example, might actually be a neutrino interaction. 114 00:08:01,320 --> 00:08:05,360 When a neutrino interacts with the liquid in the detector, 115 00:08:05,360 --> 00:08:08,000 it creates a telltale scattering of particles 116 00:08:08,000 --> 00:08:10,320 that the computers will record. 117 00:08:13,560 --> 00:08:16,440 Hidden in this data may be the signature 118 00:08:16,440 --> 00:08:18,880 of the mysterious fourth neutrino. 119 00:08:23,160 --> 00:08:27,680 What brought them to this moment and possibly to the brink of upending 120 00:08:27,680 --> 00:08:30,400 one of the bedrocks of modern physics? 121 00:08:32,760 --> 00:08:36,000 That story begins almost 100 years ago... 122 00:08:37,120 --> 00:08:39,080 ..half a world away. 123 00:08:48,000 --> 00:08:52,400 Physicist and historian Professor David Kaiser has travelled 124 00:08:52,400 --> 00:08:56,880 to Rome, where, in the 1930s, scientists were investigating 125 00:08:56,880 --> 00:08:59,480 the inner workings of the atom. 126 00:09:01,440 --> 00:09:05,680 For millennia, for thousands of years, people had come to believe 127 00:09:05,680 --> 00:09:08,840 that the world is made of atoms, and those atoms were 128 00:09:08,840 --> 00:09:10,680 the smallest thing there was. 129 00:09:10,680 --> 00:09:13,800 In fact, the word atom even means unbreakable or indivisible, 130 00:09:13,800 --> 00:09:15,400 the smallest piece. 131 00:09:16,760 --> 00:09:20,440 But by the early 1900s, scientists had revealed 132 00:09:20,440 --> 00:09:22,280 a deeper hidden structure. 133 00:09:23,760 --> 00:09:27,280 If you think about an atom, it's about a nanometre, 134 00:09:27,280 --> 00:09:30,960 about a billion times smaller than a metre, roughly. 135 00:09:30,960 --> 00:09:34,560 The inside, the deep core of an atom, the nucleus, 136 00:09:34,560 --> 00:09:38,000 is about 100,000 times smaller than that. 137 00:09:38,000 --> 00:09:41,560 So we're really zooming in powers of ten, powers of ten, getting 138 00:09:41,560 --> 00:09:44,040 to unimaginably tiny scales. 139 00:09:45,680 --> 00:09:49,560 In the early 20th century, scientists had discovered the atom's 140 00:09:49,560 --> 00:09:53,760 tiny nucleus contained protons - particles with a positive 141 00:09:53,760 --> 00:09:56,360 electric charge. 142 00:09:56,360 --> 00:10:01,200 These protons held in place negatively charged electrons 143 00:10:01,200 --> 00:10:03,760 that formed the atom's outer limit. 144 00:10:08,360 --> 00:10:12,640 It's seen that protons and electrons were the only two components 145 00:10:12,640 --> 00:10:15,600 of all atoms, permanent and fixed. 146 00:10:17,920 --> 00:10:21,360 But scientists have also found something shocking. 147 00:10:21,360 --> 00:10:24,800 Some types of atoms seemed to break apart. 148 00:10:26,240 --> 00:10:28,480 That was just jaw dropping, literally. 149 00:10:28,480 --> 00:10:30,600 It contradicts the name of the thing itself. 150 00:10:30,600 --> 00:10:33,920 Atoms are supposed to not break down. 151 00:10:33,920 --> 00:10:37,520 It was as though certain atoms had too much energy. 152 00:10:38,680 --> 00:10:41,680 The nucleus would spontaneously transform... 153 00:10:42,760 --> 00:10:44,600 ..and spit out an electron. 154 00:10:46,800 --> 00:10:51,600 This was a type of radioactivity known as beta decay. 155 00:10:53,360 --> 00:10:56,240 It appeared to be this sort of mysterious energy, 156 00:10:56,240 --> 00:11:00,160 leaking from or emanating from certain atoms. 157 00:11:02,040 --> 00:11:06,160 This process was remarkable in itself, but when scientists 158 00:11:06,160 --> 00:11:09,520 measured the energy of the electrons from beta decay, 159 00:11:09,520 --> 00:11:11,240 something was wrong. 160 00:11:12,400 --> 00:11:16,360 One of the basic principles in all sciences is that energy 161 00:11:16,360 --> 00:11:19,360 can change from one form to the other, 162 00:11:19,360 --> 00:11:22,720 but the total sum must be conserved. 163 00:11:25,640 --> 00:11:29,520 This is the principle of conservation of energy. 164 00:11:29,520 --> 00:11:34,480 From collisions in the macro world to the behaviour of tiny particles, 165 00:11:34,480 --> 00:11:38,240 the principle states that energy should never disappear. 166 00:11:40,000 --> 00:11:44,400 But when scientists measured the electrons coming from beta decay, 167 00:11:44,400 --> 00:11:47,520 that's exactly what seemed to happen. 168 00:11:47,520 --> 00:11:51,640 So every time, rather than having energy conserved, 169 00:11:51,640 --> 00:11:55,640 what they were seeing is that some amount of energy would be missing. 170 00:11:56,720 --> 00:11:58,880 Where was the energy going? 171 00:11:59,880 --> 00:12:03,080 It seemed that the particles themselves were breaking 172 00:12:03,080 --> 00:12:05,240 the fundamental rules of physics. 173 00:12:11,200 --> 00:12:17,160 In 1926, a young Italian physicist called Enrico Fermi was working 174 00:12:17,160 --> 00:12:20,440 at the University of Rome's Physics Institute. 175 00:12:22,280 --> 00:12:25,480 It was here that Fermi began to probe into the developing field 176 00:12:25,480 --> 00:12:27,280 of nuclear physics. 177 00:12:30,160 --> 00:12:33,480 Enrico Fermi was really a towering figure of 20th-century physics, 178 00:12:33,480 --> 00:12:36,640 by any measure, one of the greatest physicists of the 20th century. 179 00:12:36,640 --> 00:12:39,040 This is the site where Fermi built 180 00:12:39,040 --> 00:12:43,200 what became an absolutely world-class group of researchers. 181 00:12:43,200 --> 00:12:47,080 They were known as the Via Panisperna boys. 182 00:12:47,080 --> 00:12:49,080 This is really an iconic photograph. 183 00:12:49,080 --> 00:12:51,320 It captures them in the middle of what would become 184 00:12:51,320 --> 00:12:52,840 world-changing research. 185 00:12:52,840 --> 00:12:54,920 Fermi himself was remarkably young. 186 00:12:54,920 --> 00:12:58,600 He was just 26 years old and already had been made the big senior 187 00:12:58,600 --> 00:13:02,080 professor around which this young group would come together. 188 00:13:02,080 --> 00:13:05,520 They referred to Fermi as The Pope - he was the great leader. 189 00:13:05,520 --> 00:13:08,920 Rasetti was next in line, he was The Cardinal. 190 00:13:08,920 --> 00:13:12,640 The person taking the photograph, the very young Bruno Pontecorvo, 191 00:13:12,640 --> 00:13:15,240 the youngest member of the group, they called him The Puppy. 192 00:13:18,400 --> 00:13:22,600 The group's ideas would go on to change the course of history. 193 00:13:27,240 --> 00:13:31,920 In October 1931, they invited a group of the world's greatest 194 00:13:31,920 --> 00:13:35,200 physicists to a conference held at the institute. 195 00:13:37,280 --> 00:13:42,440 High on the agenda was the problem of the missing radioactive energy. 196 00:13:45,000 --> 00:13:47,480 The conference itself was really quite remarkable. 197 00:13:47,480 --> 00:13:51,200 Seven people who agreed to come had already received the Nobel Prize. 198 00:13:51,200 --> 00:13:53,440 14 others would go on to win the prize in the years 199 00:13:53,440 --> 00:13:56,520 after the meeting. It was really just a kind of constellation 200 00:13:56,520 --> 00:13:58,120 of extraordinary talent. 201 00:13:58,120 --> 00:14:01,760 It included Marie Curie, Werner Heisenberg and Niels Bohr. 202 00:14:01,760 --> 00:14:04,600 Niels Bohr, already by this point, a sort of grand 203 00:14:04,600 --> 00:14:06,240 figure in the field. 204 00:14:06,240 --> 00:14:10,000 Bohr suggested that if these puzzles persist in trying to understand 205 00:14:10,000 --> 00:14:12,760 the behaviour of nuclei, then maybe we have to just break 206 00:14:12,760 --> 00:14:14,680 the laws of physics. 207 00:14:14,680 --> 00:14:17,320 Maybe in nuclear physics, the rules must change. 208 00:14:20,240 --> 00:14:24,320 The conservation of energy, one of the most fundamental principles 209 00:14:24,320 --> 00:14:26,480 in science, was at stake. 210 00:14:29,280 --> 00:14:33,600 But one scientist at the conference, the famous Wolfgang Pauli, 211 00:14:33,600 --> 00:14:36,760 proposed an even more radical idea. 212 00:14:38,920 --> 00:14:42,200 Wolfgang Pauli had written a letter to colleagues, and he put forward 213 00:14:42,200 --> 00:14:45,640 what he called a desperate remedy, a verzweifelten Ausweg. 214 00:14:45,640 --> 00:14:48,760 It was just ridiculous, and he says so in his letter. 215 00:14:48,760 --> 00:14:51,720 It's a really quite strange-sounding idea. 216 00:14:52,880 --> 00:14:55,960 What if there was a new type of particle in the world 217 00:14:55,960 --> 00:14:58,680 that no-one had ever seen or detected before? 218 00:15:01,600 --> 00:15:05,360 Pauli suggested that instead of just an electron, 219 00:15:05,360 --> 00:15:09,120 perhaps there was an unknown particle that was carrying 220 00:15:09,120 --> 00:15:11,360 away the missing energy. 221 00:15:12,880 --> 00:15:15,280 Very few people seem to have been convinced 222 00:15:15,280 --> 00:15:17,200 that this is the right way to go. 223 00:15:17,200 --> 00:15:20,920 At that time, physicists were quite confident there existed two basic 224 00:15:20,920 --> 00:15:23,880 kinds of particles, electrons and protons. 225 00:15:23,880 --> 00:15:27,920 But Pauli was suggesting, let's make this enormous leap. 226 00:15:27,920 --> 00:15:32,040 A new particle of matter seemed a step too far. 227 00:15:35,400 --> 00:15:39,320 But Enrico Fermi, The Pope of Via Panisperna, 228 00:15:39,320 --> 00:15:42,880 took the wacky idea and ran with it. 229 00:15:42,880 --> 00:15:46,160 Fermi dedicated the next two years of his life 230 00:15:46,160 --> 00:15:49,040 to describe the obscure ghost particle. 231 00:15:50,080 --> 00:15:51,600 It would be neutral 232 00:15:51,600 --> 00:15:53,920 and carry no electric charge. 233 00:15:53,920 --> 00:15:57,800 It would be tiny - far smaller than an electron, 234 00:15:57,800 --> 00:16:02,560 and it would pass through atoms as if they weren't there at all. 235 00:16:02,560 --> 00:16:07,800 He named the particle the neutrino - Italian for "little neutral one". 236 00:16:11,440 --> 00:16:14,240 This was a really quite remarkable step. 237 00:16:14,240 --> 00:16:17,880 But many physicists, Fermi included, thought that it should be nearly 238 00:16:17,880 --> 00:16:21,520 impossible, perhaps impossible forever, to detect such a particle, 239 00:16:21,520 --> 00:16:23,320 even if it really exists. 240 00:16:29,320 --> 00:16:32,760 Outside the intellectual fervour of the lab, 241 00:16:32,760 --> 00:16:37,240 fascism was about to cast a shadow over the neutrino mystery. 242 00:16:38,560 --> 00:16:42,520 In 1939, Fermi immigrated to the USA, 243 00:16:42,520 --> 00:16:45,040 and was quickly put to work. 244 00:16:45,040 --> 00:16:48,560 He helped to develop the first operational nuclear reactor... 245 00:16:50,160 --> 00:16:52,960 ..that led eventually to the atomic bomb. 246 00:16:57,040 --> 00:17:01,200 But not everybody had forgotten about the elusive neutrino. 247 00:17:05,000 --> 00:17:09,240 Bruno Pontecorvo, The Puppy of the Via Panisperna boys. 248 00:17:10,920 --> 00:17:14,040 Upon moving to England after the Second World War, 249 00:17:14,040 --> 00:17:17,560 he continued to think about neutrinos until his life took 250 00:17:17,560 --> 00:17:19,160 a shocking turn. 251 00:17:21,200 --> 00:17:24,200 Pontecorvo was a man who created big ideas. 252 00:17:25,320 --> 00:17:29,040 The work that he did on neutrinos alone 253 00:17:29,040 --> 00:17:33,000 could have won him certainly one Nobel Prize 254 00:17:33,000 --> 00:17:36,480 and been a candidate maybe for two. 255 00:17:36,480 --> 00:17:38,920 But it wasn't to be. 256 00:17:38,920 --> 00:17:44,440 In 1950, in the midst of the Cold War, Pontecorvo and his family 257 00:17:44,440 --> 00:17:46,360 mysteriously went missing. 258 00:17:47,400 --> 00:17:51,720 Bruno Pontecorvo disappeared through the Iron Curtain in 1950. 259 00:17:52,920 --> 00:17:57,040 And for five years disappeared off the face of the planet. 260 00:17:58,280 --> 00:18:02,400 Only after five years of silence did he reappear, 261 00:18:02,400 --> 00:18:04,400 in the Soviet Union. 262 00:18:09,440 --> 00:18:11,600 So what happened? 263 00:18:11,600 --> 00:18:13,360 Was he kidnapped? 264 00:18:13,360 --> 00:18:15,040 Was he a spy? 265 00:18:18,120 --> 00:18:22,360 Professor Frank Close has spent years researching Pontecorvo 266 00:18:22,360 --> 00:18:24,440 and his mysterious disappearance. 267 00:18:24,440 --> 00:18:28,600 He has come to the National Archives in London. 268 00:18:29,920 --> 00:18:33,280 Earlier in his life, Pontecorvo had been a member 269 00:18:33,280 --> 00:18:38,720 of a communist party, and there are now MI5 files under his name. 270 00:18:40,680 --> 00:18:42,560 You're looking at these old folders. 271 00:18:42,560 --> 00:18:46,560 They're worn down the sides, they have red stamps, top secret. 272 00:18:47,880 --> 00:18:52,640 The case of Pontecorvo, it is dripping with intrigue. 273 00:18:53,800 --> 00:18:58,600 After the war, while working for the UK's atomic energy programme, 274 00:18:58,600 --> 00:19:03,080 Pontecorvo devised a method to try and detect neutrinos. 275 00:19:04,840 --> 00:19:09,440 He reasoned that nuclear reactors, which derive energy from splitting 276 00:19:09,440 --> 00:19:13,080 atoms, should produce neutrinos in vast quantities. 277 00:19:14,360 --> 00:19:18,080 But the government classified his paper. 278 00:19:18,080 --> 00:19:22,840 Now, I conjecture that this paper was a classified secret 279 00:19:22,840 --> 00:19:28,600 because if you could indeed detect neutrinos coming from a nuclear 280 00:19:28,600 --> 00:19:32,040 reactor, you would be able to work out how powerful 281 00:19:32,040 --> 00:19:33,680 the nuclear reactor was. 282 00:19:33,680 --> 00:19:35,280 So they classified it. 283 00:19:38,720 --> 00:19:43,760 As the Cold War escalated, the USA became paranoid of atomic espionage. 284 00:19:45,840 --> 00:19:50,000 In 1950, the Rosenberg spy ring was uncovered, 285 00:19:50,000 --> 00:19:53,120 and it triggered a Communist witch hunt. 286 00:19:56,040 --> 00:20:01,040 A secret letter reveals the FBI wrote to MI5 about Pontecorvo. 287 00:20:02,520 --> 00:20:07,160 The FBI now ask if we can send them any information which would indicate 288 00:20:07,160 --> 00:20:11,040 that Pontecorvo may be engaged in Communist activities. 289 00:20:12,240 --> 00:20:16,160 The letter was received in London on July 19th. 290 00:20:16,160 --> 00:20:20,800 Five days later, Pontecorvo goes off to Europe and never returns. 291 00:20:22,480 --> 00:20:24,680 Pontecorvo had vanished. 292 00:20:24,680 --> 00:20:27,440 Nobody knew where he'd gone. 293 00:20:27,440 --> 00:20:29,640 "Dearest Bruno," this is from his father, 294 00:20:29,640 --> 00:20:33,800 "We have had no more news and we imagined all sorts of mishaps. 295 00:20:33,800 --> 00:20:37,240 "Hoping to get the best news soon. Love to you all, Papa." 296 00:20:43,080 --> 00:20:45,960 Pontecorvo and his family had travelled to Rome, 297 00:20:45,960 --> 00:20:49,760 when suddenly they flew across Europe to Helsinki, 298 00:20:49,760 --> 00:20:52,920 alongside two suspected KGB agents. 299 00:20:54,320 --> 00:20:58,160 Pontecorvo's son, just 12 years old at the time, 300 00:20:58,160 --> 00:21:01,120 revealed they were then driven to Moscow, 301 00:21:01,120 --> 00:21:03,280 with Bruno hidden in the boot. 302 00:21:06,960 --> 00:21:09,000 So why did Pontecorvo leave? 303 00:21:10,560 --> 00:21:13,920 The secret MI5 letter suggests the influence of an infamous 304 00:21:13,920 --> 00:21:15,400 Soviet mole. 305 00:21:16,600 --> 00:21:19,280 The SIS representative is the interesting mention in here, 306 00:21:19,280 --> 00:21:21,960 because that is none other than Kim Philby, 307 00:21:21,960 --> 00:21:25,680 who really was perhaps the most dangerous traitor of them all. 308 00:21:25,680 --> 00:21:29,640 And Philby's task primarily was any information that came 309 00:21:29,640 --> 00:21:32,120 across his desk was sent to Moscow. 310 00:21:34,560 --> 00:21:39,120 Frank believes Philby tipped off the Soviets about the MI5 letter, 311 00:21:39,120 --> 00:21:42,640 who then may have pressured Pontecorvo to defect. 312 00:21:45,040 --> 00:21:48,720 There's no clear evidence that he had been a spy. 313 00:21:48,720 --> 00:21:53,640 But whatever his reason for leaving, Bruno's time in the West was over. 314 00:21:54,880 --> 00:21:56,560 Was he a spy or not? 315 00:21:56,560 --> 00:21:58,280 We don't yet know. 316 00:21:58,280 --> 00:22:01,480 In any event, it was clear the Pontecorvo was a top-quality 317 00:22:01,480 --> 00:22:05,040 scientist who had taken his brain to the Soviet Union. 318 00:22:13,320 --> 00:22:17,480 By 1950, the USA and the Soviet Union were engaged 319 00:22:17,480 --> 00:22:19,600 in a nuclear arms race. 320 00:22:20,640 --> 00:22:25,200 With it came a new opportunity to hunt for neutrinos. 321 00:22:27,800 --> 00:22:30,240 When a nuclear bomb goes off... 322 00:22:31,960 --> 00:22:36,280 ..there's this huge cascade of particles 323 00:22:36,280 --> 00:22:40,120 that spews out protons, electrons, 324 00:22:40,120 --> 00:22:43,240 a lot of light particles carrying off energy. 325 00:22:44,400 --> 00:22:47,680 And along with these particles spewing out, 326 00:22:47,680 --> 00:22:51,040 lots and lots of neutrinos come out for free. 327 00:22:52,160 --> 00:22:56,800 If neutrinos were real, could a nuclear weapon finally 328 00:22:56,800 --> 00:22:58,920 be the key to detecting them? 329 00:23:01,640 --> 00:23:05,880 In 1951, a young American called Fred Reines was working 330 00:23:05,880 --> 00:23:10,320 on the US nuclear programme at Los Alamos National Laboratory. 331 00:23:10,320 --> 00:23:14,680 It was here that Reines, along with his colleague Clyde Cowan, 332 00:23:14,680 --> 00:23:17,920 decided to pursue some fundamental physics 333 00:23:17,920 --> 00:23:22,040 and hunt for the missing neutrino. 334 00:23:22,040 --> 00:23:25,800 Reines went back to a question that had been kind of abandoned 335 00:23:25,800 --> 00:23:28,200 in the decades before the Second World War. 336 00:23:28,200 --> 00:23:30,880 The question of could physicists ever actually detect 337 00:23:30,880 --> 00:23:33,760 these very strange, elusive ghost-like particles? 338 00:23:34,960 --> 00:23:38,640 They called their mission Project Poltergeist. 339 00:23:40,840 --> 00:23:44,000 For detecting the neutrino, the good news was you could calculate 340 00:23:44,000 --> 00:23:48,120 the chance of doing it, and the bad news was it was almost zero! 341 00:23:48,120 --> 00:23:52,640 Reines and Cowan needed to tip the odds in their favour, 342 00:23:52,640 --> 00:23:55,920 and thought a nuclear bomb test could be the key. 343 00:23:58,800 --> 00:24:03,640 An atom bomb should produce thousands of times more neutrinos 344 00:24:03,640 --> 00:24:06,680 than even the biggest nuclear reactor. 345 00:24:06,680 --> 00:24:09,960 But it also created a problem. 346 00:24:09,960 --> 00:24:12,800 If they'd bolted the detector in place, the nuclear bomb 347 00:24:12,800 --> 00:24:15,600 would have just smashed into smithereens. 348 00:24:15,600 --> 00:24:18,040 So instead, the proposal, which I assume he gave 349 00:24:18,040 --> 00:24:20,680 with a straight face, he was being very serious, 350 00:24:20,680 --> 00:24:24,400 was to dig a shaft about 150 feet deep, right near 351 00:24:24,400 --> 00:24:28,400 where the bomb would eventually be detonated above ground. 352 00:24:28,400 --> 00:24:32,280 The team planned to drop a detector down the shaft to avoid 353 00:24:32,280 --> 00:24:34,720 the shock wave of the bomb. 354 00:24:34,720 --> 00:24:37,840 Inside that shaft, they would pad the bottom with foam 355 00:24:37,840 --> 00:24:40,960 and feathers and kind of like mattress cushions. 356 00:24:42,840 --> 00:24:48,120 It was, I mean, a creative, ambitious and maybe slightly crazy 357 00:24:48,120 --> 00:24:51,240 kind of idea to try to catch these neutrinos in the midst 358 00:24:51,240 --> 00:24:54,080 of this very dramatic, very worldly set of events 359 00:24:54,080 --> 00:24:56,440 in the early years of the Cold War. 360 00:24:57,680 --> 00:25:00,520 Work digging the shaft had begun. 361 00:25:00,520 --> 00:25:03,880 But the head of physics at Los Alamos was concerned 362 00:25:03,880 --> 00:25:07,320 that the experiment couldn't be repeated. 363 00:25:07,320 --> 00:25:10,680 He urged the team to find another way. 364 00:25:10,680 --> 00:25:13,760 Couldn't they use a nuclear reactor instead? 365 00:25:15,360 --> 00:25:19,400 Late one evening, Reines and Cowan had a brainwave. 366 00:25:22,560 --> 00:25:26,400 In the same way that the nucleus of an atom could decay and release 367 00:25:26,400 --> 00:25:31,840 a neutrino, they knew, in theory, the process should be reversible. 368 00:25:34,440 --> 00:25:38,640 On the rare occasion a neutrino could interact with a nucleus, 369 00:25:38,640 --> 00:25:43,920 it should produce two new particles called a neutron and a positron, 370 00:25:43,920 --> 00:25:47,160 and if they travelled through the right liquid, 371 00:25:47,160 --> 00:25:51,520 those two telltale particles should produce two distinctive 372 00:25:51,520 --> 00:25:53,200 flashes of light. 373 00:25:55,280 --> 00:25:59,520 So Reines and Cowan built a detector. 374 00:25:59,520 --> 00:26:03,960 Essentially a big tank filled with a solvent 375 00:26:03,960 --> 00:26:10,800 that could pick up these two...coincident signal...blips. 376 00:26:10,800 --> 00:26:16,080 This characteristic signal deep under a nuclear reactor. 377 00:26:22,080 --> 00:26:27,040 After five years of experiments, in 1956, 378 00:26:27,040 --> 00:26:29,680 finally they got their answer. 379 00:26:34,120 --> 00:26:37,560 They recorded the two telltale flashes of light. 380 00:26:39,480 --> 00:26:45,200 For the first time, they saw evidence of the elusive neutrino. 381 00:26:45,200 --> 00:26:48,040 What they had done was a remarkable achievement, 382 00:26:48,040 --> 00:26:51,000 one that seemed impossible. 383 00:26:52,640 --> 00:26:56,000 Neutrinos exist, they're real and a part of the world, 384 00:26:56,000 --> 00:26:57,840 they're not only a clever idea. 385 00:27:03,240 --> 00:27:05,040 Knowing neutrinos exist 386 00:27:05,040 --> 00:27:07,840 put a whole extra set of investigations 387 00:27:07,840 --> 00:27:09,720 on a kind of firmer path. 388 00:27:22,120 --> 00:27:26,560 If neutrinos were pouring from nuclear reactors on Earth, 389 00:27:26,560 --> 00:27:30,880 then surely they would be generated in abundance in the largest 390 00:27:30,880 --> 00:27:32,920 nuclear furnaces of all... 391 00:27:34,680 --> 00:27:36,320 The stars. 392 00:27:38,280 --> 00:27:41,600 For a long, long time, scientists had been wondering 393 00:27:41,600 --> 00:27:43,240 what makes the stars shine? 394 00:27:44,600 --> 00:27:47,640 What drives that enormous output of energy? 395 00:27:50,360 --> 00:27:56,240 People theorise that our sun is like a giant nuclear reactor, 396 00:27:56,240 --> 00:28:02,240 except, rather than heavier elements breaking down into smaller ones 397 00:28:02,240 --> 00:28:06,560 and releasing energy, you have lighter elements that fuse 398 00:28:06,560 --> 00:28:08,760 together through nuclear fusion. 399 00:28:11,040 --> 00:28:15,120 In the heart of the sun, tremendous heat and pressure force 400 00:28:15,120 --> 00:28:19,760 hydrogen nuclei to fuse together to make helium. 401 00:28:21,720 --> 00:28:27,360 And in theory, vast quantities of neutrinos that pass freely 402 00:28:27,360 --> 00:28:29,920 through the sun and out into space. 403 00:28:33,400 --> 00:28:37,920 So if we could detect neutrinos from the sun, 404 00:28:37,920 --> 00:28:42,680 we could learn about the processes that fuel it. 405 00:28:42,680 --> 00:28:46,680 We could peek inside the core of our sun. 406 00:28:50,840 --> 00:28:53,280 ARCHIVE: There was gold in them thar hills. 407 00:28:55,120 --> 00:28:58,360 Gold on the mountains, in the rivers and in the dark depths 408 00:28:58,360 --> 00:29:00,720 far below the surface of the Earth. 409 00:29:05,000 --> 00:29:10,160 In the historic gold-mining town of Lead, people still descend 410 00:29:10,160 --> 00:29:12,440 into the depths of the Earth... 411 00:29:13,480 --> 00:29:16,040 ..but no longer to mine precious metal. 412 00:29:18,040 --> 00:29:20,280 They are hunting for neutrinos. 413 00:29:22,960 --> 00:29:27,040 It was here in 1965 that an experimentalist 414 00:29:27,040 --> 00:29:31,960 called Ray Davis came to try and prove what makes the sun shine. 415 00:29:33,040 --> 00:29:35,960 Ray Davis got very excited that there is this new thing 416 00:29:35,960 --> 00:29:37,880 in the world called a neutrino. 417 00:29:37,880 --> 00:29:41,240 He began realising that other kinds of nuclear reactors that occur 418 00:29:41,240 --> 00:29:45,000 throughout the universe like stars, they should be spewing out 419 00:29:45,000 --> 00:29:48,040 these neutrinos all the time. 420 00:29:48,040 --> 00:29:50,560 But catching them wouldn't be easy. 421 00:29:51,640 --> 00:29:55,920 Calculations show that neutrinos from the sun would be so faint, 422 00:29:55,920 --> 00:29:59,360 background radiation would overwhelm any detector 423 00:29:59,360 --> 00:30:01,960 near the Earth's surface. 424 00:30:01,960 --> 00:30:05,800 His only option was to go to the bottom of a mine. 425 00:30:08,200 --> 00:30:12,320 Shielded beneath almost a mile of solid rock, Ray Davis 426 00:30:12,320 --> 00:30:17,000 built his detector - a steel tank the size of a house, 427 00:30:17,000 --> 00:30:21,960 filled with 100,000 gallons of dry-cleaning fluid. 428 00:30:24,320 --> 00:30:28,960 In theory, if a neutrino from the sun collided with a chlorine 429 00:30:28,960 --> 00:30:32,520 atom inside the tank, it would cause a reaction 430 00:30:32,520 --> 00:30:34,480 that Ray Davies could detect. 431 00:30:37,160 --> 00:30:39,640 Here was something that was completely fresh, 432 00:30:39,640 --> 00:30:41,800 nobody knew anything about it. 433 00:30:41,800 --> 00:30:45,400 But the key thing was that if neutrinos hit chlorine, 434 00:30:45,400 --> 00:30:49,280 which you could get in cleaning fluid, it would turn the atoms 435 00:30:49,280 --> 00:30:52,400 of chlorine into a radioactive form of argon. 436 00:30:52,400 --> 00:30:56,160 And that's when Davis got excited, because he was a radiochemist. 437 00:30:56,160 --> 00:31:02,800 And for him, detecting radioactive forms of argon was easy street. 438 00:31:04,800 --> 00:31:09,960 All Davis needed now was a target number to test the theory 439 00:31:09,960 --> 00:31:11,840 of what powered the sun. 440 00:31:13,080 --> 00:31:16,560 There were very compelling theoretical calculations 441 00:31:16,560 --> 00:31:20,640 that suggested that stars should shine in a particular way. 442 00:31:20,640 --> 00:31:24,080 And that meant there should be a predictable number of neutrinos 443 00:31:24,080 --> 00:31:27,880 streaming toward the Earth from the sun. 444 00:31:27,880 --> 00:31:32,040 Scientists had calculated that around a million trillion 445 00:31:32,040 --> 00:31:36,920 neutrinos from the sun should pass through Davis's tank each minute. 446 00:31:36,920 --> 00:31:40,640 But the probability of the neutrinos hitting the fluid and making 447 00:31:40,640 --> 00:31:46,040 an argon atom was so small, Ray Davis could only expect to find 448 00:31:46,040 --> 00:31:49,560 ten individual atoms of argon from ten neutrino 449 00:31:49,560 --> 00:31:51,520 collisions per week. 450 00:31:52,880 --> 00:31:55,280 His task was almost impossible. 451 00:31:55,280 --> 00:31:59,960 Many of his own physicist colleagues doubted this experiment 452 00:31:59,960 --> 00:32:01,440 would ever work. 453 00:32:03,000 --> 00:32:05,680 He was having to convince people that out of these millions 454 00:32:05,680 --> 00:32:08,360 and millions and millions and millions of atoms inside this tank, 455 00:32:08,360 --> 00:32:11,560 he could identify the collisions of one or two 456 00:32:11,560 --> 00:32:15,200 and convince you that these were neutrinos coming from the sun. 457 00:32:16,560 --> 00:32:19,880 Around each month, Davis flushed out the giant tank 458 00:32:19,880 --> 00:32:21,720 to extract the argon atoms. 459 00:32:23,320 --> 00:32:25,080 To everybody's amazement... 460 00:32:26,120 --> 00:32:27,640 ..he found them. 461 00:32:33,480 --> 00:32:35,720 But there was a problem. 462 00:32:35,720 --> 00:32:38,840 Instead of detecting the number of atoms that theory 463 00:32:38,840 --> 00:32:41,960 predicted, his measurements fell short. 464 00:32:43,280 --> 00:32:46,480 They knew the target number based on the nuclear physics 465 00:32:46,480 --> 00:32:49,200 theoretical explanation of how stars shine, 466 00:32:49,200 --> 00:32:52,120 and that led to a very particular target number, 467 00:32:52,120 --> 00:32:55,160 and Davis's remarkable experiment kept coming in, 468 00:32:55,160 --> 00:32:59,760 not close to it, not 80%, but only at one third of that target number. 469 00:33:01,440 --> 00:33:02,800 What happened? 470 00:33:02,800 --> 00:33:04,720 Had the experiment gone wrong? 471 00:33:05,760 --> 00:33:09,640 Another scientist carried out a blind trial to test the accuracy 472 00:33:09,640 --> 00:33:11,680 of Ray's atom detection. 473 00:33:11,680 --> 00:33:17,120 A colleague put in 500 kind of rogue atoms without telling Davis 474 00:33:17,120 --> 00:33:20,520 the number, and Davis was able to go through the whole process, 475 00:33:20,520 --> 00:33:23,640 sifted through, and he counted exactly the number 476 00:33:23,640 --> 00:33:25,520 that had been put in. 477 00:33:25,520 --> 00:33:29,160 If the experimental results were accurate, then perhaps 478 00:33:29,160 --> 00:33:32,600 scientists had gotten their theory about the sun wrong. 479 00:33:34,080 --> 00:33:36,480 Everybody was blaming everybody else. 480 00:33:36,480 --> 00:33:40,480 There were even suggestions, has the sun already burnt out in the core? 481 00:33:40,480 --> 00:33:42,320 And it was just an enormous puzzle. 482 00:33:42,320 --> 00:33:46,080 All these advances in understanding how stars shine and then hitting 483 00:33:46,080 --> 00:33:48,760 this kind of brick wall where theory and experiment 484 00:33:48,760 --> 00:33:51,400 just would not agree with each other. 485 00:33:53,440 --> 00:33:57,520 The puzzle became known as the solar neutrino problem. 486 00:34:02,600 --> 00:34:07,720 1970. 20 years since Bruno Pontecorvo defected 487 00:34:07,720 --> 00:34:10,760 to the Soviet Union. 488 00:34:10,760 --> 00:34:14,760 Even after all that time, his life behind the Iron Curtain 489 00:34:14,760 --> 00:34:16,960 remained shrouded in secrecy. 490 00:34:18,520 --> 00:34:23,200 But in a government lab outside Moscow, Pontecorvo worked 491 00:34:23,200 --> 00:34:26,840 tirelessly to explain the puzzling behaviour of neutrinos. 492 00:34:31,480 --> 00:34:36,160 He suggested that instead of just one, there may be two or even 493 00:34:36,160 --> 00:34:38,600 three different types of neutrinos. 494 00:34:40,000 --> 00:34:43,120 These were known as different flavours, 495 00:34:43,120 --> 00:34:48,040 called the electron, muon and tau neutrinos. 496 00:34:50,200 --> 00:34:53,920 If this wasn't strange enough, he calculated that something 497 00:34:53,920 --> 00:34:58,000 peculiar might happen as they travelled through space. 498 00:35:02,000 --> 00:35:07,080 A neutrino would always be born as one definite flavour. 499 00:35:08,680 --> 00:35:11,440 But over time, it would change its identity. 500 00:35:13,040 --> 00:35:17,080 It would transform, mixing back and forth 501 00:35:17,080 --> 00:35:19,720 between the three different types. 502 00:35:21,920 --> 00:35:24,560 This was called neutrino oscillation. 503 00:35:27,360 --> 00:35:31,440 Pontecorvo's idea really is...it's sort of delicious. 504 00:35:31,440 --> 00:35:36,160 These neutrinos could be not taking one identity, dropping that, 505 00:35:36,160 --> 00:35:40,360 adopting another one, dropping that, but going into this even stranger 506 00:35:40,360 --> 00:35:44,200 mixture, where they're in neither and both states at once. 507 00:35:45,760 --> 00:35:50,440 It was a bold idea. No other fundamental particle seemed 508 00:35:50,440 --> 00:35:53,320 to spontaneously change its identity. 509 00:35:54,600 --> 00:35:57,680 But if neutrinos were transforming into flavours 510 00:35:57,680 --> 00:36:01,240 that Ray Davis' detector couldn't see, 511 00:36:01,240 --> 00:36:05,200 it might explain why two-thirds of the neutrinos from the sun 512 00:36:05,200 --> 00:36:06,960 appeared to be missing. 513 00:36:09,480 --> 00:36:14,960 There's been the thought that maybe the neutrino 514 00:36:14,960 --> 00:36:18,360 isn't behaving quite the way we thought. 515 00:36:18,360 --> 00:36:23,040 Some Soviet scientists had pointed out that when neutrinos 516 00:36:23,040 --> 00:36:27,200 come from the sun, they can be converted into another 517 00:36:27,200 --> 00:36:31,120 kind of neutrino that we would not be able to detect. 518 00:36:33,360 --> 00:36:35,000 But there was a catch. 519 00:36:37,160 --> 00:36:42,920 The Standard Model, the most precise scientific theory in human history, 520 00:36:42,920 --> 00:36:47,200 made one important prediction that stood in the way. 521 00:36:47,200 --> 00:36:49,960 The Standard Model anticipated neutrinos 522 00:36:49,960 --> 00:36:51,880 would be completely massless. 523 00:36:51,880 --> 00:36:55,520 They would have no mass at all, much like the photon of light. 524 00:36:55,520 --> 00:37:00,160 And if they had no mass, that meant that they could not oscillate. 525 00:37:02,000 --> 00:37:06,960 If neutrinos had no mass, one of Albert Einstein's most important 526 00:37:06,960 --> 00:37:12,200 theories predicted that neutrinos could not possibly oscillate. 527 00:37:13,360 --> 00:37:16,160 There's this mind-boggling phenomenon from Einstein's 528 00:37:16,160 --> 00:37:20,400 relativity that says that a clock that is moving closer and closer 529 00:37:20,400 --> 00:37:24,680 to the speed of light will tick at a slower and slower rate. 530 00:37:24,680 --> 00:37:27,400 If that clock were moving literally at the speed of light, 531 00:37:27,400 --> 00:37:29,000 it would never tick at all. 532 00:37:29,000 --> 00:37:31,960 No time would pass for that object that moves at exactly 533 00:37:31,960 --> 00:37:33,600 the speed of light. 534 00:37:35,440 --> 00:37:39,400 According to Einstein, the faster a particle travels, 535 00:37:39,400 --> 00:37:42,240 the more its internal clock slows down. 536 00:37:43,280 --> 00:37:46,760 The laws of physics state a particle with no mass 537 00:37:46,760 --> 00:37:51,000 can only travel at the speed of light, which is where time stops. 538 00:37:53,640 --> 00:37:58,560 So if a neutrino had no mass, it would not experience the passage 539 00:37:58,560 --> 00:38:02,000 of time and would never be able to change. 540 00:38:03,600 --> 00:38:07,840 If a particle has zero mass, what that means is that its internal 541 00:38:07,840 --> 00:38:09,880 clock is not ticking. 542 00:38:09,880 --> 00:38:13,720 There's no way for that particle to experience time. 543 00:38:13,720 --> 00:38:16,160 If there's no passage of time, 544 00:38:16,160 --> 00:38:20,480 then how could they change over time from one identity to another? 545 00:38:22,520 --> 00:38:27,520 If neutrino oscillation was real, neutrinos must have some mass. 546 00:38:29,120 --> 00:38:32,480 But could the Standard Model really be wrong? 547 00:38:43,680 --> 00:38:48,880 At CERN in the 1950s and '60s, clues from experiments performed 548 00:38:48,880 --> 00:38:51,800 here helped to devise the Standard Model. 549 00:38:54,360 --> 00:38:58,320 What they found revolutionised our understanding of the particles 550 00:38:58,320 --> 00:39:01,080 that make-up our universe. 551 00:39:01,080 --> 00:39:04,600 ARCHIVE: By means of this machine, it is possible to see the tracks 552 00:39:04,600 --> 00:39:09,200 of subnuclear particles, the smallest particles known to man. 553 00:39:09,200 --> 00:39:14,680 The electron, the positron, the photon and the neutrino. 554 00:39:18,320 --> 00:39:22,320 The work at CERN led to ground-breaking new technologies, 555 00:39:22,320 --> 00:39:25,440 medical advances like PET scans, 556 00:39:25,440 --> 00:39:28,120 even the birth of the World Wide Web. 557 00:39:32,600 --> 00:39:36,880 Perhaps CERN's biggest success came in 2012. 558 00:39:36,880 --> 00:39:41,280 Nearly 50 years after the Standard Model was proposed, 559 00:39:41,280 --> 00:39:45,400 physicists detected the final particle it predicted. 560 00:39:45,400 --> 00:39:47,960 The Higgs boson. 561 00:39:47,960 --> 00:39:49,720 I think we have it. 562 00:39:49,720 --> 00:39:52,360 APPLAUSE 563 00:40:03,840 --> 00:40:07,920 The Higgs boson had been found, and that was just electrifying. 564 00:40:09,120 --> 00:40:13,600 It was the last remaining particle that had been hypothesised 565 00:40:13,600 --> 00:40:17,200 to be part of the Standard Model, a new piece that helps explain 566 00:40:17,200 --> 00:40:19,080 how and why we're here. 567 00:40:19,080 --> 00:40:22,280 And that news just rippled really far and wide. 568 00:40:24,280 --> 00:40:28,520 Today, the Standard Model describes every fundamental particle 569 00:40:28,520 --> 00:40:30,520 in the detectable universe. 570 00:40:32,160 --> 00:40:36,400 Along with the Higgs boson, there are force carriers, 571 00:40:36,400 --> 00:40:38,960 like the photon of light. 572 00:40:38,960 --> 00:40:43,200 Quarks, which form the nuclei of atoms. 573 00:40:43,200 --> 00:40:48,000 Leptons, including the electron, muon and tau. 574 00:40:48,000 --> 00:40:51,400 And three corresponding flavours of neutrinos. 575 00:40:52,960 --> 00:40:55,480 It is a map of what's out there, what we're made of 576 00:40:55,480 --> 00:40:58,040 and how we fit, all of us, 577 00:40:58,040 --> 00:41:00,560 we are made of these things. 578 00:41:00,560 --> 00:41:03,600 And that is a kind of basic understanding of nature 579 00:41:03,600 --> 00:41:08,360 of our own world that I think is just a remarkable human achievement. 580 00:41:11,600 --> 00:41:16,640 And yet, for all its success, the Standard Model could not explain 581 00:41:16,640 --> 00:41:19,920 how or why the neutrinos would have mass 582 00:41:19,920 --> 00:41:22,120 and therefore might oscillate. 583 00:41:25,240 --> 00:41:28,720 For Ray Davis and his missing solar neutrinos, 584 00:41:28,720 --> 00:41:31,760 it seemed an unsolvable paradox. 585 00:41:35,000 --> 00:41:37,800 For decades, Davis persisted. 586 00:41:37,800 --> 00:41:41,440 But he still only found one third of the neutrinos 587 00:41:41,440 --> 00:41:45,560 that were supposed to be coming from the sun. 588 00:41:45,560 --> 00:41:50,480 Well, we've been carrying on this experiment for about 20 years 589 00:41:50,480 --> 00:41:55,680 right here, but we're still observing a low flux of neutrinos. 590 00:41:57,560 --> 00:42:00,680 Eventually, the problem was too big to ignore. 591 00:42:02,080 --> 00:42:07,280 In the 1990s, scientists in Canada and Japan constructed 592 00:42:07,280 --> 00:42:11,640 a new generation of supersized neutrino detectors to finally 593 00:42:11,640 --> 00:42:13,400 settle the mystery. 594 00:42:17,360 --> 00:42:21,240 One of them lies deep beneath Japan's Ikeno Mountain. 595 00:42:21,240 --> 00:42:25,800 Scientists fit 11,000 light detectors to the inside 596 00:42:25,800 --> 00:42:30,680 of a gigantic container and fill it with 50,000 tons 597 00:42:30,680 --> 00:42:32,640 of ultra-pure water. 598 00:42:35,080 --> 00:42:39,360 This $100 million experiment is called Super-K. 599 00:42:40,960 --> 00:42:44,920 The Super-K experiment ended up being a game changer. 600 00:42:44,920 --> 00:42:49,000 Here we had a brand-new, shiny experiment, which had the special 601 00:42:49,000 --> 00:42:53,960 ability of being able to tell not only where the neutrinos 602 00:42:53,960 --> 00:42:57,920 were coming from, which direction they were coming from, 603 00:42:57,920 --> 00:43:02,560 but also whether they were muon type neutrinos, 604 00:43:02,560 --> 00:43:04,560 or electron type neutrinos. 605 00:43:06,840 --> 00:43:10,480 When neutrinos collide with the water in Super-K, 606 00:43:10,480 --> 00:43:15,880 the reaction produces a trail of light which the sensors can pick up. 607 00:43:15,880 --> 00:43:19,840 The signal allows scientists to calculate which type of neutrino 608 00:43:19,840 --> 00:43:22,600 is hit and the direction it came from. 609 00:43:26,000 --> 00:43:29,600 To test the theory of neutrino oscillation, 610 00:43:29,600 --> 00:43:33,480 Super-K focused on catching them from a new source - 611 00:43:33,480 --> 00:43:35,640 the Earth's atmosphere. 612 00:43:39,320 --> 00:43:44,400 Theory suggests that when radiation from space hits the atmosphere, 613 00:43:44,400 --> 00:43:48,440 it creates neutrinos that travel directly through the Earth. 614 00:43:50,520 --> 00:43:52,600 Some travel a short distance. 615 00:43:53,760 --> 00:43:56,480 But others will come from the other side of the planet 616 00:43:56,480 --> 00:43:58,640 to reach the detector. 617 00:44:01,040 --> 00:44:05,640 If the neutrinos are not changing as they travel, the combination 618 00:44:05,640 --> 00:44:09,360 of neutrino types they record coming from a short distance 619 00:44:09,360 --> 00:44:12,600 should be the same as those coming from afar. 620 00:44:13,880 --> 00:44:17,440 If they ARE changing as they travel, the combination of types 621 00:44:17,440 --> 00:44:19,240 will be different. 622 00:44:23,840 --> 00:44:28,400 After two years of taking data, the team finally have an answer. 623 00:44:30,600 --> 00:44:34,720 What they were seeing was that one type of neutrinos 624 00:44:34,720 --> 00:44:39,960 was depleting when travelling through the Earth. 625 00:44:39,960 --> 00:44:44,440 So that was an indication that neutrinos can change 626 00:44:44,440 --> 00:44:46,880 from one type to the other. 627 00:44:46,880 --> 00:44:49,120 For that to happen, 628 00:44:49,120 --> 00:44:52,320 you must have non-zero neutrino mass. 629 00:44:53,560 --> 00:44:56,080 The results were ground-breaking. 630 00:44:56,080 --> 00:45:01,280 Neutrinos change their identity - neutrinos have mass, after all. 631 00:45:03,040 --> 00:45:06,520 And the Standard Model's prediction of the nature of neutrinos 632 00:45:06,520 --> 00:45:08,040 must be wrong. 633 00:45:09,120 --> 00:45:13,400 With the new input, the evidence that neutrinos really oscillate, 634 00:45:13,400 --> 00:45:15,920 they really change their identities, therefore, they really, 635 00:45:15,920 --> 00:45:17,440 really have a mass, 636 00:45:17,440 --> 00:45:20,440 this long-standing, decades-long challenge to understand 637 00:45:20,440 --> 00:45:23,960 the solar neutrino problem finally fell into place. 638 00:45:25,360 --> 00:45:29,320 Nuclear fusion in the sun produces one type of neutrino. 639 00:45:31,480 --> 00:45:35,520 But on the journey through space, the neutrinos oscillate 640 00:45:35,520 --> 00:45:38,920 and turn into a mixture of all three. 641 00:45:40,520 --> 00:45:45,760 On Earth, Ray Davis's detector could only pick out 642 00:45:45,760 --> 00:45:48,200 one of the three types. 643 00:45:48,200 --> 00:45:51,680 His results had been accurate all along. 644 00:45:55,520 --> 00:45:59,040 37 years after the experiment began, 645 00:45:59,040 --> 00:46:01,960 Ray Davis was awarded the Nobel Prize. 646 00:46:01,960 --> 00:46:04,320 APPLAUSE 647 00:46:07,880 --> 00:46:11,480 For Bruno Pontecorvo and his theory of oscillations, 648 00:46:11,480 --> 00:46:14,520 sadly, the discovery came too late. 649 00:46:15,920 --> 00:46:19,560 Nobel Prizes aren't everything, but by the time the oscillations 650 00:46:19,560 --> 00:46:23,400 had been sorted out and the whole thing finally understood, 651 00:46:23,400 --> 00:46:25,680 Pontecorvo was dead. 652 00:46:25,680 --> 00:46:28,560 So that's the final tragedy of his life. 653 00:46:34,280 --> 00:46:39,680 After almost 100 years of research and discovery, today neutrino 654 00:46:39,680 --> 00:46:43,160 physicists face perhaps their biggest puzzle yet. 655 00:46:46,560 --> 00:46:50,400 The Standard Model's equations cannot explain why neutrinos 656 00:46:50,400 --> 00:46:53,760 have mass or why they oscillate. 657 00:46:53,760 --> 00:46:58,400 It's a sign that our understanding of matter is still incomplete. 658 00:47:01,240 --> 00:47:06,040 Today, neutrino experiments costing over £3 billion 659 00:47:06,040 --> 00:47:08,920 are in overdrive, hunting for clues. 660 00:47:08,920 --> 00:47:11,720 We're in the midst of really a neutrino bonanza. 661 00:47:11,720 --> 00:47:14,040 I mean, they're popping up all over 662 00:47:14,040 --> 00:47:15,920 the field of physics. 663 00:47:17,080 --> 00:47:21,560 In Germany, scientists are trying to pin down exactly how much 664 00:47:21,560 --> 00:47:23,600 a neutrino weighs. 665 00:47:24,680 --> 00:47:28,680 In the mines of South Dakota, neutrino experiments 666 00:47:28,680 --> 00:47:33,560 hope to reveal how matter itself emerged from the Big Bang. 667 00:47:36,080 --> 00:47:40,600 And all over the globe, scientists now use neutrinos 668 00:47:40,600 --> 00:47:45,320 as a tool to probe the deepest corners of the cosmos. 669 00:47:47,000 --> 00:47:51,640 Even space filled with big, thick, dense matter like planets and stars, 670 00:47:51,640 --> 00:47:54,640 these neutrinos can zip through them as if there's nothing 671 00:47:54,640 --> 00:47:56,120 there at all. 672 00:47:56,120 --> 00:47:59,000 They become a kind of cosmic, you know, telegram for what 673 00:47:59,000 --> 00:48:02,320 the universe might have been like at the time of the Big Bang, 674 00:48:02,320 --> 00:48:05,840 or in the furthest reaches of space, as far from our own planet Earth 675 00:48:05,840 --> 00:48:07,960 as we can imagine. 676 00:48:07,960 --> 00:48:10,360 This is neutrino astronomy. 677 00:48:11,720 --> 00:48:15,760 And in the quest for cosmic neutrinos, scientists adventure 678 00:48:15,760 --> 00:48:17,920 to the ends of the Earth. 679 00:48:20,640 --> 00:48:23,840 At the South Pole, scientists have built the largest 680 00:48:23,840 --> 00:48:26,200 neutrino detector on the planet. 681 00:48:27,680 --> 00:48:32,560 It's made of 5,000 sensors drilled into a cubic kilometre 682 00:48:32,560 --> 00:48:34,520 of Antarctic ice. 683 00:48:35,600 --> 00:48:37,880 It's known as Ice Cube. 684 00:48:39,760 --> 00:48:43,000 Ice Cube is in this this huge field around me, I'm sitting, kind of 685 00:48:43,000 --> 00:48:45,880 standing in the middle of Ice Cube. 686 00:48:45,880 --> 00:48:50,000 It's kind of amazing to think that we were able to haul 687 00:48:50,000 --> 00:48:53,360 something like five million pounds of cargo down to the South Pole. 688 00:48:53,360 --> 00:48:57,440 This is instrumentation, cables, drill equipment, fuel. 689 00:49:01,320 --> 00:49:05,160 Like the Super-K detector, when a neutrino strikes an atom 690 00:49:05,160 --> 00:49:08,400 in the ice, it produces a cone of light which the sensors 691 00:49:08,400 --> 00:49:10,200 can pick up. 692 00:49:10,200 --> 00:49:14,440 This reveals the neutrino's cosmic origin, making Ice Cube 693 00:49:14,440 --> 00:49:16,280 a neutrino telescope. 694 00:49:17,360 --> 00:49:20,400 We're able to conceptually kind of put pushpins where these 695 00:49:20,400 --> 00:49:22,240 neutrino events came from, 696 00:49:22,240 --> 00:49:26,240 and that's exactly why we call it a neutrino telescope. 697 00:49:26,240 --> 00:49:30,360 Today, Ice Cube has detected neutrinos from billions 698 00:49:30,360 --> 00:49:32,720 of light years away. 699 00:49:32,720 --> 00:49:37,640 Some launched by giant black holes in the hearts of distant galaxies. 700 00:49:40,000 --> 00:49:43,160 This is the universe that has really only been open to our eyes 701 00:49:43,160 --> 00:49:45,240 for the last 50 years. 702 00:49:45,240 --> 00:49:48,960 There's all kinds of discoveries that are waiting out there. 703 00:49:50,200 --> 00:49:54,800 With new experiments like Ice Cube, neutrinos may reveal discoveries 704 00:49:54,800 --> 00:49:56,560 beyond the Standard Model. 705 00:49:59,920 --> 00:50:04,160 Neutrinos could even help unlock one of the biggest mysteries 706 00:50:04,160 --> 00:50:05,600 in physics today. 707 00:50:06,720 --> 00:50:10,560 It seems that most of what our universe is made of... 708 00:50:10,560 --> 00:50:12,000 ..is missing. 709 00:50:15,320 --> 00:50:18,600 The whole quest of particle physics is to explain 710 00:50:18,600 --> 00:50:21,320 the matter content of the universe. 711 00:50:21,320 --> 00:50:25,600 And we seem to be doing this phenomenally good job. 712 00:50:25,600 --> 00:50:28,440 You crank through the math of the Standard Model 713 00:50:28,440 --> 00:50:30,960 and everything makes sense. 714 00:50:30,960 --> 00:50:34,720 And yet it only describes some very small fraction 715 00:50:34,720 --> 00:50:36,960 of what the universe is made out of. 716 00:50:40,040 --> 00:50:45,280 Looking into space, cosmologists can see the gravitational influence 717 00:50:45,280 --> 00:50:49,360 of a material that binds entire galaxies together. 718 00:50:49,360 --> 00:50:53,000 But that is completely invisible to their detectors. 719 00:50:54,920 --> 00:50:59,880 Scientists call this material dark matter because nothing 720 00:50:59,880 --> 00:51:03,160 in the Standard Model can describe what it is. 721 00:51:05,040 --> 00:51:08,040 And yet it seems to be what most of the matter 722 00:51:08,040 --> 00:51:10,040 in the universe is made of. 723 00:51:11,320 --> 00:51:15,040 The Standard Model is very good at describing 724 00:51:15,040 --> 00:51:18,040 about 5% of the universe. 725 00:51:18,040 --> 00:51:21,200 95% of the stuff is an utter, complete mystery, 726 00:51:21,200 --> 00:51:24,360 made of dark stuff, whether it's dark matter or dark energy. 727 00:51:24,360 --> 00:51:28,040 And what either of those are, we don't know. 728 00:51:28,040 --> 00:51:31,840 All we really know about dark matter is that it creates gravity, 729 00:51:31,840 --> 00:51:35,080 but it's not interacting with the instruments 730 00:51:35,080 --> 00:51:37,760 that we have used to observe the universe. 731 00:51:38,720 --> 00:51:41,160 Whatever is filling space... 732 00:51:41,160 --> 00:51:44,400 ..much more of it than the ordinary matter that makes up us 733 00:51:44,400 --> 00:51:48,240 and our planet and our stars, it's some other, other kind of particle. 734 00:51:50,400 --> 00:51:54,400 Whatever dark matter particles are, scientists must look 735 00:51:54,400 --> 00:51:57,600 beyond the Standard Model to find them. 736 00:51:57,600 --> 00:52:00,200 Neutrinos might be the key. 737 00:52:08,040 --> 00:52:12,440 At Fermilab, for over 20 years, scientists have been 738 00:52:12,440 --> 00:52:15,000 investigating neutrino oscillations. 739 00:52:16,520 --> 00:52:18,800 What they've found doesn't add up. 740 00:52:21,040 --> 00:52:25,560 The first observation that something was amiss was in the late 1990s. 741 00:52:26,600 --> 00:52:30,320 Something we don't quite understand is going on. 742 00:52:32,280 --> 00:52:36,560 At Fermilab, scientists fire the beam of mostly muon type 743 00:52:36,560 --> 00:52:39,680 neutrinos just 500 metres to their detector. 744 00:52:41,480 --> 00:52:45,680 Neutrinos oscillate too slowly for the detector to see them change 745 00:52:45,680 --> 00:52:49,680 over such a short distance, at least according to theory. 746 00:52:51,040 --> 00:52:54,720 But the data showed electron type neutrinos mysteriously 747 00:52:54,720 --> 00:52:57,680 appearing in the detector. 748 00:52:57,680 --> 00:53:02,320 Neutrinos seem to oscillate faster than is theoretically possible. 749 00:53:03,880 --> 00:53:09,920 The strange thing that we're seeing is that neutrinos seem 750 00:53:09,920 --> 00:53:13,600 to be changing from one type to the other, 751 00:53:13,600 --> 00:53:16,640 much faster than expected. 752 00:53:16,640 --> 00:53:21,440 In order for that to happen, we think it's possible 753 00:53:21,440 --> 00:53:24,200 that there are extra neutrinos out there. 754 00:53:26,360 --> 00:53:29,120 In addition to the three flavours of neutrino 755 00:53:29,120 --> 00:53:32,240 that the Standard Model predicts, 756 00:53:32,240 --> 00:53:35,680 there could be a fourth neutrino that affects them, 757 00:53:35,680 --> 00:53:37,960 making them oscillate faster. 758 00:53:39,800 --> 00:53:43,000 Scientists call it a sterile neutrino. 759 00:53:44,280 --> 00:53:46,920 And it's never been directly detected. 760 00:53:49,960 --> 00:53:52,400 So we call it a sterile neutrino. 761 00:53:52,400 --> 00:53:55,880 In essence, just because it interacts even less 762 00:53:55,880 --> 00:53:59,080 with other particles than the regular neutrinos do. 763 00:54:00,720 --> 00:54:05,080 A sterile neutrino would be the ultimate ghost particle. 764 00:54:05,080 --> 00:54:08,440 It would never collide with atoms in our world. 765 00:54:08,440 --> 00:54:11,080 No detector could ever see it. 766 00:54:11,080 --> 00:54:14,480 But it may reveal itself through its effects 767 00:54:14,480 --> 00:54:17,800 with the neutrinos we can see. 768 00:54:17,800 --> 00:54:22,240 The only way that we can tell they exist 769 00:54:22,240 --> 00:54:26,200 is through their effects on neutrino isolation. 770 00:54:28,000 --> 00:54:32,680 If sterile neutrinos exist, it would break the neat symmetry 771 00:54:32,680 --> 00:54:37,160 of the Standard Model that organises particles in groups of three. 772 00:54:37,160 --> 00:54:40,200 What if there is a fourth kind of neutrino, 773 00:54:40,200 --> 00:54:42,520 a so-called sterile neutrino? 774 00:54:42,520 --> 00:54:45,200 Well, where would you put that on our map? 775 00:54:45,200 --> 00:54:49,680 There's no room to kind of shoehorn in, to squeeze in a fourth neutrino. 776 00:54:49,680 --> 00:54:52,560 So I think there really is a lot riding on this. 777 00:54:55,280 --> 00:54:59,880 If they're real, sterile neutrinos would have mass, 778 00:54:59,880 --> 00:55:04,400 but not interact with our detectors, just like dark matter. 779 00:55:06,120 --> 00:55:10,600 They could be the first particle of dark matter ever discovered, 780 00:55:10,600 --> 00:55:14,000 and through their effects on the neutrinos we can see, 781 00:55:14,000 --> 00:55:18,120 they could give scientists a window into an unknown world. 782 00:55:19,720 --> 00:55:24,000 Now, the neutrino might be a kind of link, almost a kind of messenger 783 00:55:24,000 --> 00:55:28,440 or portal to this whole other possible kind of stuff out there. 784 00:55:34,000 --> 00:55:37,760 At Fermilab, scientists are edging towards the truth. 785 00:55:39,160 --> 00:55:42,640 Physicist Angela Fava is making the finishing touches 786 00:55:42,640 --> 00:55:44,960 to the ICARUS detector. 787 00:55:44,960 --> 00:55:47,200 Trigger in five hertz... 788 00:55:47,200 --> 00:55:49,440 OK, on now... 789 00:55:51,040 --> 00:55:53,760 We are starting what we call the cold commission, 790 00:55:53,760 --> 00:55:56,320 so the cooling-down phase, in a couple of weeks. 791 00:55:56,320 --> 00:55:59,600 Then we will need to get it filled with liquid argon. 792 00:56:00,880 --> 00:56:03,600 Fingers crossed, then you start seeing 793 00:56:03,600 --> 00:56:06,160 your neutrino tracks immediately. 794 00:56:08,080 --> 00:56:12,240 Once complete, ICARUS will join MicroBooNE and eventually 795 00:56:12,240 --> 00:56:15,800 a third detector called SBND. 796 00:56:15,800 --> 00:56:19,560 A sterile neutrino would act like a catalyst, 797 00:56:19,560 --> 00:56:22,760 speeding up how ordinary neutrinos can transform 798 00:56:22,760 --> 00:56:25,480 from one flavour to another. 799 00:56:25,480 --> 00:56:29,680 If the team see these fast oscillations, it could prove 800 00:56:29,680 --> 00:56:32,440 the sterile neutrino is real. 801 00:56:35,160 --> 00:56:38,560 This would be really a great discovery. 802 00:56:38,560 --> 00:56:44,840 I mean, game changing, it opens up perspective for knowing 803 00:56:44,840 --> 00:56:47,080 the universe in a different way. 804 00:56:48,680 --> 00:56:52,520 Once we'll have all the detectors up and running, it will take still 805 00:56:52,520 --> 00:56:56,240 at least three years, but definitely we are confident that we can 806 00:56:56,240 --> 00:56:58,240 answer this question. 807 00:57:00,080 --> 00:57:02,280 Let's wait for the data and see. 808 00:57:07,400 --> 00:57:09,480 I think we're getting a lot closer. 809 00:57:09,480 --> 00:57:12,040 Neutrino physicists are incredibly patient. 810 00:57:12,040 --> 00:57:14,440 It takes a long time for us to collect our data 811 00:57:14,440 --> 00:57:16,560 and we really want to be sure in what we're seeing 812 00:57:16,560 --> 00:57:19,680 before we potentially make a very important discovery. 813 00:57:21,720 --> 00:57:25,680 We're trying to answer some of THE biggest questions in physics. 814 00:57:25,680 --> 00:57:28,720 And I think it's really unique that neutrinos may hold all the answers. 815 00:57:30,240 --> 00:57:33,960 What began as a hypothetical particle that no-one 816 00:57:33,960 --> 00:57:36,240 thought possible to detect 817 00:57:36,240 --> 00:57:40,240 could now be a key to unlock what most of our universe 818 00:57:40,240 --> 00:57:44,120 is made of and how it works. 819 00:57:44,120 --> 00:57:48,280 Every time we look up, there seem to be these very curious neutrinos. 820 00:57:48,280 --> 00:57:52,600 They are constantly bedevilling our mental maps of how we carve 821 00:57:52,600 --> 00:57:54,600 up nature and try to dig in and study. 822 00:57:54,600 --> 00:57:56,960 And that's just amazingly exciting. 823 00:57:56,960 --> 00:58:00,200 So they've gone from "maybe they exist, maybe they don't, 824 00:58:00,200 --> 00:58:01,920 "we might never know..." 825 00:58:01,920 --> 00:58:05,800 to being our surest ticket to the next step. 826 00:58:05,800 --> 00:58:10,400 History has shown that with every little bit of progress, 827 00:58:10,400 --> 00:58:15,680 we've learned huge, surprising things about our cosmos. 828 00:58:15,680 --> 00:58:19,680 To me, that's really exciting, and I'm curious to know 829 00:58:19,680 --> 00:58:21,960 where else could we go?